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Technology for Remediation and Disposal

of Arsenic

Pornsawan Visoottiviseth(*) and Feroze Ahmed

I Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

II Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

A Passive Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

B In Situ Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

C Chemical Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

D Solar Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

III Coagulation and Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

A Bucket Treatment Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

B The Star Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

C Fill and Draw Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

D Arsenic Removal Unit Attached to Tube Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

E Iron-Arsenic (Fe-As) Removal Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

F Lime Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

IV Sorptive Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

A Activated Alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

B Granular Ferric Hydroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

C Read-F Arsenic Removal Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

D Iron-Coated Sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

E Shapla Arsenic Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

F Sono Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

G SAFI Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

H Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

I Indigenous Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

J Cartridge Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

V Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

P. Visoottiviseth

Department of Biology, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400,

Thailand, e-mail: [email protected].

F. Ahmed

Department of Civil Engineering, Bangladesh University of Engineering and Technology,

Dhaka 1000, Bangladesh, e‐mail: [email protected]

D.M. Whitacre (ed.), Reviews of Environmental Contamination Volume 197. 77

doi: 10.1007/978-0-387-79284-2_4, # Springer Science þ Business Media, LLC 2008

VI Membrane Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

A Techno-Food Water Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

B MRT-1000 and Reid System, Ltd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

C Low-Pressure Nanofiltration and Reverse Osmosis . . . . . . . . . . . . . . . . . . . . . . . . . 107

D Mobile Reverse Osmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

E Combined Sand and Nanofiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

VII Bioremediation by Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

VIII Phytoremediation of Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

IX Comparison of Arsenic Removal Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

X Conventional Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

XI Alternative Water Supply Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

A Deep Tube Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

B Dug/Ring Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

C Surface Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

D Piped Water Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

XII Operational Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

A Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

B Technology Verification and Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

XIII Disposal of Generated Arsenic Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

I Introduction

Arsenic contamination of water is a major public health problem in many countries

worldwide. Symptoms of arsenic exposure have no known effective treatment, but

drinking arsenic-free water can reduce risk to affected populations and alleviate

symptoms of arsenic toxicity. In areas where the drinking water supply contains

unsafe levels of arsenic, two main options have been identified: either find a safe

source (mitigation) and/or remove arsenic from the contaminated source (remedia-

tion). Substantial effort has been invested in developing techniques for removing

arsenic from water. Some of these techniques have been field implemented, while

others are only performed in the laboratory. For any effective technology to

be appropriate for use in affected areas of developing countries, it should ideally

be simple, low cost, versatile, transferable, and should use local resources. Most

importantly, such technologies must be accessible to local communities and espe-

cially to women. All over the world, women collect and carry water for their

families, use water for cooking and cleaning and for growing food. Therefore,

women should be at the forefront as users of arsenic treatment technologies. Das

et al. (2004) concluded that, in villages of India and Bangladesh, even a highly

successful technology may not succeed in rural areas unless it fits well with the rural

circumstances and is well accepted by the local population.

Several methods are available for removal of arsenic from water in large conven-

tional treatment plants. The most commonly used processes include oxidation and

sedimentation, coagulation and filtration, lime treatment, adsorption onto sorptive

media, ion exchange, andmembrane filtration (Cheng et al. 1994; Hering et al. 1996,

78 P. Visoottiviseth, F. Ahmed

1997; Kartinen and Martin 1995; Shen 1973; Joshi and Chaudhuri 1996). A review

of these well-established arsenic removal technologies is presented by Sorg and

Logsdon (1978). Jakel (1994) has documented several advances in arsenic removal

technologies. In view of the lowering of the drinking water standards by the United

States Environmental Protection Agency (US EPA), a review of arsenic removal

technologies was made to consider the economic factors involved in implementing

the lower standards for arsenic (Chen et al. 1999). Many arsenic removal technol-

ogies have been discussed, in detail, in the American Water Works Association

(AWWA) reference book (Pontius 1990). A comprehensive review of low-cost,

well water treatment technologies for arsenic removal, with the list of companies

and organizations fostering them, has been compiled by Murcott (2000). Arsenic

removal technologies have also been reviewed by Ahmed et al. (2000, 2001), the

World Bank (2005), and Ahmed (2003). Other low-cost technologies that have been

investigated are presented in Table 1.

Some of these technologies can be reduced in scale and can be conveniently

applied at the household and community level for removal of arsenic from

contaminated water. During the last 2–3 yr, many small-scale arsenic removal

technologies have been developed, field tested, and used in action research programs

in some Asian countries. The following review of these technologies briefly updates

what is known of the technological development in arsenic removal, and discusses

the current status of the problem, as well as prospects and limitations of different

treatment processes; this review also delineates areas needing further improvement

for successful implementation and adaptation of technologies to rural conditions.

II Oxidation

Arsenic is present in groundwater as As(III) and As(V), in different proportions.

Most treatment methods are effective in removing arsenic in pentavalent form, and

hence include an oxidation step as a pretreatment to convert arsenite to arsenate.

Arsenite can be oxidized by oxygen, ozone, free chlorine, hypochlorite, permanga-

nate, hydrogen peroxide, and Fulton’s reagent; in developing countries oxygen,

hypochloride, and permanganate are commonly used for oxidation. The following

reactions describe oxidation by oxygen, hypochloride, and permanganate:

H3AsO3 þ 1=2O2 ! H2AsO4� þ 2Hþ ð1Þ

H3AsO3 þ HClO ! HAsO4� þ Cl� þ 3Hþ ð2Þ

½H3AsO3 þ 2KMnO4 ! 3HAsO4� þ 2MnO2

þ þ 2Kþ þ 4Hþ þ H2O ð3ÞThe oxidation processes convert predominantly noncharged arsenite to charged

arsenate, which can be easily removed from water.

Aeration is the simplest means of oxidation, and many treatment processes

depend on oxidation by air. However, air oxidation of arsenic is a very slow

process, often requiring weeks (Pierce and Moore 1982). Air oxidation of arsenite

Technology for Remediation and Disposal of Arsenic 79

Table 1 Performance and cost of some specific technologies for removing arsenic from con-

taminated water

Technology Performance

(% removed)

Cost

PRECIPITATION

(Taking advantage of naturally-

occurring Fe and/or Mn

precipitation) (University of

Kalyani, West Bengal)

70%–80% Final As conc.

20–36 ppb

None

SEDIMENTATION

Used with precipitation or

coagulation

COAGULATION

Iron salts (Murcott 1999) >90% $0.06/yr (40 L/d at

15 mg/L dose)

Alum (Ahmed et al. 2000)

BUETa90% Final As conc. 30 ppb Low

‘‘Bucket’’ or ‘‘Tea-Bag’’

WHO-SEA method

80%–99% Final As conc.

50–70 ppb

$0.05/packet treats 10 L

(cost is less if mass

produced)

Iron filings (zero valance) >94%–99% $0.22/yr

(Ramaswami et al. 2001)

University of Colorado-

Denver

Other coagulants: tablets, lime,

natural or synthetic

polymers

>78% Low

CONVENTIONAL

FILTRATION

Cloth, sand, charcoal, other

native material media –

coconut husks, peanut

shells, water hyacinth,

rubber leaves, etc.

20%–75% Low

ADSORPTION

Activated alumina metal oxide 90%–96% $0.02–$0.03/20 L

(Project Earth Inc.) Final As conc. 10–25 ppb Capital cost <$100 per

unit (cheaper if

produced locally)

Iron filings and sand 90% $0.06/L

(AsRT – Univ. of Connecticut) Final As conc. <27 ppb for

>1000 pore volume of

eluent

Capital cost = $900 for

pilot unit; full-scale

unit from about $10.

(two columns

treating 3.8 L/min)

Laterite 50%–90% None or low

Other adsorbents: Bijoypur

clay, hematite (Fe2O3),

fly ash

Low


can be catalyzed by bacteria, strong acidic or alkaline solutions, copper, powdered

activated carbon, and high temperature (Edwards 1994).

Chemicals such as chlorine and permanganate can rapidly oxidize arsenite to

arsenate under a wide range of conditions. Hypochloride is readily available in rural

areas, but the potency (available chlorine) of the hypochloride declines under poor

storage conditions; chlorine escapes from hypochloride when it comes in contact

Table 1 (continued)

Technology Performance

(% removed)

Cost

Ion (anion) exchange Final As conc. <2 ppb Expensive

(Clifford et al. 1998)

Univ. of Houston

Activated alumina Final As conc. <50 ppb $1,400/unit

(West Bengal Engineering

College / Water for People)

Each unit serves

200–300 households

Ferric hydroxide [Fe(OH)3] or

ferric hydroxide-coated

newspaper pulp (Khair et al.

1999)

Final As conc. <50 ppb Low

Univ. of Dhaka

OXIDATION

Aerationa 25% Potentially very low

Photochemical oxidationa

SOLAR DISTILLATION

Solar Still (Young Associates)a Final As conc. 0 ppb $0.02 per person per day

MEMBRANE

Fe(III) coagulation þmicrofiltration

(Clifford et al. 1998) Univ.

of Houston

Final As conc. <2 ppb

depended on pH, Fe

dose

Expensive

Reverse osmosis (RO) 86% Expensive

Membrane filtration and RO

(Wanichapichart 2005)

Prince of Songkla Univ.,

Thailand

90%–93% (100–150 ppb) Moderate

Electrodialysis 80% Expensive

Nanofiltration Expensive

BIOREMEDIATION

Absorption by immobilized

green alga, Chlorellavulgaris

85%–90% Very low

(Visoottiviseth and

Lauengsuchonkul 2004)

Mahidol Univ., Thailand.

PHYTOREMEDIATION

Wetland treatment 80%–90% Very lowaBangladesh University of Engineering and Technology.

Source: Modified from Murcott (2000).


with air. A residual concentration of 0.2 mg/L free available chlorine in water is

required for oxidation of arsenite. Potassium permanganate is also readily available

in developing countries, is more stable than bleaching powder, and has a long shelf

life. Potassium permanganate effectively oxidizes arsenite and ferrous compounds.

Ozone and hydrogen peroxide are very effective oxidants but their use in develop-

ing countries is limited. Filtration of water through a bed containing solid manga-

nese oxides can rapidly oxidize arsenic, without releasing excessive manganese in

the filtered water.

A Passive Sedimentation

Passive sedimentation has received considerable attention because of the habit of rural

peoples of drinking stored water from pitchers. Routine activities associated with

collecting and storingwater in homesmay reduce arsenic concentrations. Experiments

conducted in Bangladesh showed passive sedimentation to variably reduce arsenic

concentrations in drinking water. Arsenic reduction by sedimentation appears to be

dependent on water quality, particularly on the presence of precipitating iron. Ahmed

et al. (2000) reported >50% reduction of arsenic concentrations by sedimentation in

tubewell water containing 380–480mg/L alkalinity as CaCO3. However, this process

was unreliable in reducing arsenic to levels less than the World Health Organization

(WHO) maximum contaminant level (MCL) of 10 mg/L. High alkalinity and the

presence of iron in tubewell water increase arsenic removal during storage. Most

studies showed reductions up to 25% of the initial arsenic concentrations found in

groundwater from these wells. However, passive sedimentation failed to reduce

arsenic to the desired level (50 mg/L) in Bangladesh when arsenic content of

tubewell water was high (BAMWSP, DFID, and Water Aid Bangladesh 2001).

B In Situ Oxidation

In situ oxidation of arsenic and iron in aquifers has been attempted in a Department

of Public Health Engineering–Danish International Development Agency (DPHE-

Danida)-funded Arsenic Mitigation Pilot Project in Bangladesh. The aerated tube-

well water was stored in a 500-L capacity feed water tank (Fig. 1) and released back

into the aquifer through the tube well. The dissolved oxygen in the water oxidizes

arsenite to the less mobile arsenate and ferrous to ferric iron; this results in a

reduction of the arsenic content in the tubewell water. The oxidation of arsenite

to arsenate reaction is shown in Equation 1 above. Subsequent reactions of arsenate

and ferric hydroxide are shown in Equations 7 to 9, below. Experimental results

confirm that such in situ oxidation reduces the concentration of arsenic in tubewell

water to about half of its original value. This effect results from both underground

precipitation and adsorption on ferric iron.

The study revealed that the in situ oxidation method is effective in reduc-

ing arsenic content to meet the Bangladesh standard of 0.05 mg/L when the


concentration of arsenic in tubewell water is less than 0.10 mg/L. At higher con-

centrations, such water treatment may not achieve a final concentration sufficient to

meet the required standard. Treatment efficiency improves if the dissolved oxygen

content of recharge water, or quantity of water recharged in the aquifer, is

increased. Usually, the excess water pumped from the tube well during the day is

stored in the feed water tank. It is then recharged at night back into the tube well,

where it resides undisturbed overnight. The method is simple, chemical free, and

may be well accepted by people using it; unfortunately, it is not highly effective in

removing arsenic.

C Chemical Oxidation

In situ chemical oxidation of arsenite to arsenate and ferrous iron into ferric iron by

chemical oxidation in the aquifer, and subsequent coprecipitation and adsorption,

can immobilize subsurface arsenic. Matthess (1981) injected 29 t potassium

permanganate directly into 17 contaminated wells to reduce contamination in an

aquifer containing high concentrations of arsenite and ferrous iron. The water of

this aquifer also had a low pH value. Arsenate was precipitated with ferric oxides,

and arsenic content in water was reduced from 13,600 to 60 mg/L. However, theremay be problems and uncertainties in effectiveness of in situ remediation of arsenic

by chemical oxidation. The introduction of reactive chemicals and microbes in the

aquifer may have unforeseen effects on subsurface ecology and groundwater

chemistry. Moreover, in a dynamic system of continuous recharge, water move-

ment and withdrawal in a treated aquifer may induce their own effects. Long-term

effectiveness of such remediation is, therefore, not assured.

D Solar Oxidation

Solar oxidation and removal of arsenic (SORAS) is a simple method for oxidizing

arsenic (III) by irradiating drinking water in transparent bottles with sunlight to

1400 mm

1100 mmGI sheet Tray

Top View

XX

Inlet to TW

Tray (75 mm slope)

1400 mm

1100 mm

600 mm

35 mmGI pipe

150mm Wash out

75 mm

Fig. 1 Feed water tank for in situ arsenic removal by oxidation in the aquifer


reduce dissolved arsenic (Wegelin et al. 2000). Ultraviolet radiation catalyzes the

oxidation of arsenite in the presence of oxygen (Young 1996). Experiments in

Bangladesh show that the process can reduce the arsenic content of water by about

one-third.

III Coagulation and Filtration

Coagulation and flocculation removes arsenic from solution by three mechanisms

(Edwards 1994): (1) precipitation—the formation of insoluble compounds, (2)

coprecipitation—the incorporation of soluble arsenic species into a growing metal

hydroxide phase, and (3) adsorption—the electrostatic binding of soluble arsenic to

external surfaces of the insoluble metal hydroxide. Precipitation, coprecipitation,

and adsorption by coagulation with metal salts and lime, followed by filtration, is the

method most frequently employed for removing arsenic from water. This method

can effectively remove arsenic and other suspended and dissolved solids (e.g., iron,

manganese, phosphate, fluoride, and microorganisms) from water. Moreover, it

achieves additional health and aesthetic benefits because it may improve turbidity,

color, and odor, resulting in significant water quality improvement.

Chemical coagulation is effective in removing arsenic from drinking water, but

the dose requirement is several times higher than that required for conventional

water treatment, especially for destabilization and removal of colloidal

particles. Alum (Al2(SO4)3�18H2O), ferric chloride (FeCl3), and ferric sulfate

(Fe2(SO4)3�7H2O) are common coagulants used for removing arsenic from water.

Ferric salts are more effective in removing arsenic than alum on a weight basis and

are effective over a wider range of pH. In both cases, pentavalent arsenic is more

effectively removed than is the trivalent form (Ahmed and Rahaman 2000). In the

coagulation-flocculation process, aluminum sulfate, ferric chloride, or ferric sulfate

is added to, and dissolved in, water, with stirring for 1 to a few minutes. Aluminum

or ferric hydroxide micro-flocs are rapidly formed. The water solution is then gently

stirred for several minutes to allow agglomeration of micro-flocs into larger ones

that settle more readily. During this process, microparticles and negatively charged

ions become attached to the flocs by electrostatic forces. Arsenic is also adsorbed

onto coagulated flocs. Because trivalent arsenic is nonionic, it is not significantly

removed during this treatment process. Therefore, As(III) must be oxidized to As

(V) for removal by this process, which can be achieved by the addition of bleaching

powder (chlorine) or potassium permanganate (Equations 2 and 3, above). The

chemical equations describing alum coagulation are as follows:

Alum dissolution:

Al2ðSO4Þ3 � 18H2O ! 2Al3þ þ 3SO42þ þ 18H2O ð4Þ

Aluminium precipitation (acidic):

2Al3þ þ 6H2O ! 2AlðOHÞ3 þ 6Hþ ð5Þ


Coprecipitation (nonstoichiometric, nondefined product):

H2AsO�4 þ AlðOHÞ3 ! Al� AsðcomplexÞ þ other products ð6Þ

Arsenic precipitated as Al(AsO4) or adsorbed on aluminum hydroxide flocs as the

Al–As complex is partially removed by sedimentation; filtration may be required

for complete removal of flocs. Similar reactions take place for ferric chloride and

ferric sulfate, resulting in the formation of a Fe–As complex as an end product; this

complex is then removed by sedimentation and filtration.

The reactions for arsenate with hydrous iron oxide are shown below, where

[�FeOHo] represents an oxide surface site (Mok and Wai 1994; Hering et al.1996).

FeðOHÞ3ðsÞ þ H3AsO4 ! FeAsO4 � 2H2Oþ H2O ð7Þ

�FeOHo þ AsO 3�4 þ 3Hþ ! �FeH2AsO4 þ H2O ð8Þ

�FeOHo þ AsO 3�4 þ 2Hþ ! �FeHAsO �

4 þ H2O ð9Þ

Effective immobilization of arsenic by hydrous iron oxide (Equations 7 to 9)

requires oxidation of arsenic species to As(V).

Freshly formed hydrous ferric oxide (HFO) and hydrous aluminum oxide (HAO)

have maximum arsenic adsorption capacities of approximately 0.1 M As(V)/M Fe

or Al (i.e., 46 mg As/g of ferric chloride or 23 mg As/g alum). When the sorbents

are formed in situ, adsorption capacities are much higher [(~0.5–0.6 M As(V)/M Fe

or Al]. The difference reflects effects of coprecipitation (preformed hydroxides only

remove arsenic through adsorption), while in situ formation also leads to copreci-

pitation (Edwards 1994).

Arsenic removal by coagulation is mainly controlled by pH and coagulant dose.

Adsorption is theoretically favored at a pH below the sorbent point of zero charge,

because positively charged surfaces of the sorbents attract arsenate anions. Labora-

tory tests have shown arsenate adsorption to be optimal on HFO below pH 8 and on

HAO below pH 7 (Sorg and Logsdon 1978; Edwards 1994; Hering et al. 1996). In

alum coagulation, removal is most effective at a pH 7.2–7.5. Efficient iron removal

by coagulation is achieved at a pH range of 6.0 to 8.5 (Ahmed and Rahaman 2000).

Cations and anions are very important in arsenic removal by coagulation. Anions

compete with arsenic for sorptive sites and lower the rates of removal. Manning and

Goldberg (1996) expressed the comparative theoretical affinity at neutral pH for

anion sorption on metal oxides as:

PO4 > SeO3 > AsO4 > AsO3 >> SiO4 > SO2 > F > BðOHÞ3PO4 has the highest affinity for metal oxides and is most likely to compete with

arsenic for adsorption sites. Dissolved silicates can interfere with removal of both

arsenite and arsenate. The presence of more than one anion may have a synergistic


effect on arsenic removal. Addition of either silicate or phosphate has some effect

on arsenic removal, but presence of both can reduce arsenate removal by 39% and

arsenite removal by 69% (Meng et al. 2000). Results of arsenic removal studies in

Bangladesh (Meng and Korfiatis 2001) concluded that elevated levels of phosphate

and silicate, in well water, dramatically decreased adsorption of arsenic by ferric

hydroxides. A Fe/As mass ratio greater than 40 was required to reduce arsenic

concentration to less than 50 mg/L. Elevated sulfate and carbonate levels slightly

reduce arsenite removal but have little effect on arsenate removal. Natural organic

matter can also reduce arsenic removal efficiency.

A Bucket Treatment Unit

The Bucket Treatment Unit (BTU), developed from the DPHE-Danida Project, uses

the principles of coagulation, coprecipitation, and adsorption. This unit consists of

two 20-L buckets placed one above the other. Chemicals are manually mixed with

arsenic contaminated water in the upper (red-colored) bucket by vigorous stirring

for 30–60 sec, and are then flocculated by gentle stirring for about 90 sec. After

mixing, the water is allowed to settle for 1–2 hr. Thereafter, water from the top

bucket is drained into the lower (green-colored) bucket via a plastic pipe, then

through a sand filter installed in the lower bucket. Flow is initiated by opening a

valve fitted above settled sludge in the bottom of the red bucket; thus, inflow of

sludge in the sand filter is avoided. The DPHE-Danida bucket treatment unit is

shown in Fig. 2a.

The BTU units utilize chemical doses of 200 mg/L aluminum sulfate and 2 mg/L

potassium permanganate, supplied in crushed powder form for water treatment.

Their performance in removing arsenic under field and laboratory conditions is

reported to be good (Sarkar et al. 2000; Kohnhorst and Paul 2000). The perfor-

mance of the BTU was studied (BAMWSP, DFID, and Water Aid Bangladesh

2001) with mixed results. Under rural operating conditions in Bangladesh, units

often failed to remove arsenic to the target level (0.05 mg/L). Poor mixing and

variable water quality (particularly pH) appeared to cause the poor performance.

Bangladesh University of Engineering and Technology (BUET) modified and

improved performance of the BTU by using 100 mg/L ferric chloride and 1.4 mg/L

potassium permanganate. Water so treated gave arsenic values below 20 mg/L, and

values never exceeded 37 mg/L, compared with arsenic concentrations in the

tubewell water of 375–640 mg/L. The BUET-modified BTU is depicted in Fig.

2b. Further field testing of the modified units is underway in a rural area of the

Comilla district of Bangladesh.

The modified BTUs are very effective in removing iron, manganese, phosphate,

and silica. Initially, fecal coliform bacteria were found in treated waste, probably

derived from contact with contaminated human hands. This biocontamination was

eliminated by adding bleaching powder to the chemical packet used in the BTU. The

BTU is a promising technology for economic arsenic removal at the household level.

It can be locally built using available materials and is effective if operated properly.


The Mennonite Central Committee (MCC) in Bangladesh also experimented

with a range of arsenic removal technologies (from tube water) including the

DPHE-Danida BTU. MCC replaced permanganate with bleaching powder to

achieve oxidation in the units they tested; alum was used for coagulation-sedimen-

tation of arsenic. The DPHE-Danida bucket treatment units they tested were found

to remove more than 90% of arsenic present in tubewell water.

B The Star Filter

The Star Filter developed by the Stevens Institute, USA, also uses two buckets, one

to mix chemicals (iron coagulant and hypochloride) supplied in packets and the

other to separate flocs by sedimentation and filtration. The second bucket has an

inner bucket with side slits Fig. 3) to help sedimentation and retain the sand bed.

Visible large flocs are formed after chemical packets are added and stirred into the

water. After treatment, clean water is collected after filtration through cloth to

prevent entry of sand. The sand bed is quickly clogged by flocs and requires

washing at least twice a week. An assessment showed that the technology was

FlexiblePlasticPipe

ClothScreen

SandFilter

PVCSlotted Screen

TopBucket

BottomBucket

a b

FlexiblePlastic PipeFilter

TopBucket

BottomBucket

Fig. 2 Double-bucket household arsenic treatment unit. (a) BPHE-Danida* Unit; (b) BUET**

Modified Unit. (* Department of Public Health Engineering-Danish International Development

Agency; ** Bangladesh University of Engineering and Technology.)


effective in reducing arsenic levels to less than 0.05 mg/L for 80%–95% of the

samples tested (BAMWSP, DFID and Water Aid Bangladesh 2001).

C Fill and Draw Units

The fill and draw unit is a community-type treatment unit designed and installed

during the DPHE-Danida Arsenic Mitigation Pilot Project. It comprises a 600-L

(effective) capacity tank with a slightly tapered bottom for collection and with-

drawal of settled sludge. The tank is fitted with a manually operated mixer that has

flat-blade impellers. The tank is filled with arsenic-contaminated water, and the

required quantity of oxidant and coagulant is then added. The water is then mixed

for 30 sec by rotating the mixing device at the rate of 60 rpm and is then left

overnight to allow sedimentation. The water takes some time to become completely

still, which helps flocculation. The floc formation is caused by the hydraulic

gradient of the rotating water in the tank. The settled water is then drawn through

a pipe fitted a few inches above the bottom of the tank, is passed through a sand bed,

and is finally collected through a tap (Fig. 4). The mixing and flocculation processes

in this unit are controlled to better effect higher removal of arsenic.

D Arsenic Removal Unit Attached to Tube Wells

The principles of arsenic removal by alum coagulation, sedimentation, and filtration

have been employed in a compact unit for water treatment at the village level in

West Bengal, India. The arsenic removal plant is attached to a tube well fitted with a

hand pump (Fig. 5). This unit is effective in removing 90% of the original

concentrations (300 mg/L) of arsenic from tubewell water. The treatment process

involves addition of sodium hypochloride (Cl2) and dilute aluminum alum, fol-

lowed by mixing, flocculation, sedimentation, and up-flow filtration.

Chemicalsmixingstick Main bucket

Interior bucket

Slits

Outlet withcloth filter

Plastic pipe todeliver treatedwater

Filter sand

Transfer of chemicalmixed water

Fig. 3 Star Filter developed by Stevens Institute Technology


E Iron-Arsenic (Fe-As) Removal Plants

The use of naturally occurring iron precipitates in Bangladesh groundwater is a

promising method for removing arsenic. Tubewell water, used in 65% of the area of

Bangladesh, contains iron in excess of 2 mg/L. In some areas, the concentration of

dissolved iron is higher than 15 mg/L, which is unacceptable for domestic water

supplies. Although there is no clear link between natural concentrations of iron and

arsenic, they commonly coexist as contaminants of groundwater. Most tubewell

water samples that satisfy the Bangladesh DrinkingWater Standard for Iron (1 mg/L)

also satisfy the standard for arsenic (50 mg/L). Only about half the samples with 1–5

mg/L iron satisfy the arsenic standard; 75% of the samples with iron content above

5 mg/L are unsafe because of high concentrations of arsenic.

Iron precipitates [Fe(OH)3] formed by oxidation of dissolved iron [Fe(OH)2]

present in groundwater have an affinity for adsorbing arsenic. Aeration and sedi-

mentation of tubewell water rich in dissolved iron does, indeed, remove arsenic. In

Bangladesh, iron removal plants (IRPs) constructed on the principles of aeration,

sedimentation, and filtration successfully remove arsenic without using other che-

Cover

Impeller

Tank

Sludgewithdrawal

pipe

Handle

Filtrationunit

Gear system

Treatedwater

Fig. 4 DPHE-Danida fill and draw arsenic removal unit

A - Mixing; B - Flocculation; C - Sedimentation; D - Filtration (Up-flow)

A B

C

DB

Fig. 5 Arsenic removal plant for use with tube wells (designed and constructed in India)


micals. The conventional community-type IRPs work as arsenic removal plants

(ARPs) as well. A typical plant of this sort is shown in Fig. 6. This plant was

designed by BUET and first used in rural areas about 20 yr ago for iron removal

from groundwater.

The DPHE-Danida project also installed experimental up-flow Fe-As removal

plants after the design provided by BUET. The treatment unit shown in Fig. 7 is

100 mm PVC pipe (slotted) 25 mm thick Slab

200

125

625

75

Plug

Tubewell

Outlet

Pitcher(Kalshi)

25 300 75 275 125

125

175

75

550

75

SECTION X -X

A Aeration B Initial sedimentation C Adsorption D Filtration E Final sedimentation

PLAN

A B

A

C D

E

C

DEPlatform

X X

Fig. 6 A typical iron-arsenic removal plant, designed by BUET, that also removes arsenic


attached to a tube well and is rather simple to operate. The As-Fe removal unit relies

on oxidation, precipitation, adsorption, and filtration to remove arsenic. Such units

were installed to remove arsenic from tubewell water having concentrations be-

tween 0.1 and 1.0 mg/L. The efficiency of the units depends on the arsenic and iron

content of water. Results from unit operation show them to operate reliably at 50%

efficiency (arsenic removal).

Dahi and Liang (1998) suggest that As(III) is oxidized to As(V) in the IRPs,

which augments arsenic removal in IRPs constructed in Noakhali. The relationship

between removal of iron and arsenic when using IRPs is shown in Fig. 8. Results

show that most IRPs can lower arsenic content of tubewell water by 50%–80% of

original concentrations. The efficiency of these community-type plants can be

improved by increasing contact time between arsenic species and iron flocs.

Medium-scale Fe-As removal units (capacities of 2000–3000 m3/d) have been

constructed in district towns using the aforementioned principle. Some production

wells that supply water to urban areas are also contaminated with arsenic. The

working principles of a medium-sized Fe-As unit are shown in Fig. 9. The main

processes effecting treatment are aeration, sedimentation, and rapid sand filtration,

with provision for addition of chemical if required. These units utilize the natural

iron content of water and have rather low (40%–80%) efficiency. A downside of

these plants is that the water requirement for washing the filter beds is very high.

The experience of operating small- and medium-sized Fe-As removal plants in

Filter Media

Aeration Tray

Tubewell

Washout Valve

Treated Water

Fig. 7 Fe-As removal unit fitted with an upflow filter


Bangladesh suggests that arsenic removal by coprecipitation and adsorption on

natural iron flocs has good potential, if arsenic content of water does not exceed

0.10 mg/L.

F Lime Treatment

Water treatment by addition of quicklime, CaO, or hydrated lime, Ca(OH)2, also

removes arsenic. Lime treatment is a process similar to that of coagulation with

metal salts. The precipitated calcium hydroxide [Ca(OH)2] acts as a sorbing

R2 = 0.69

20

30

40

50

60

70

80

90

100

20 30 40 50 60 70 80 90 100

Iron Removal , %

Ars

enic

Rem

ova

l ,%

Fig. 8 Correlation between Fe and As removal in treatment plants

Pump

Back washing Water supply

Overheadwater tank

Chlorination

Pump

Aeration

Inlet pipe

Filter bed

Fig. 9 Fe-As removal plant suitable for use by small towns


flocculent for arsenic. An excess of lime will not dissolve but remains as a thickener

and aid to coagulation. The excess lime, along with precipitates, must be removed

by sedimentation and filtration. Arsenic removal by lime is usually between 40%

and 70% effective. The highest removal is achieved at pHs between 10.6 and 11.4.

McNeill and Edward (1997) studied arsenic removal by water softening and found

that the main mechanism of arsenic removal was sorption onto magnesium hydrox-

ide solids that form in situ. Trace levels of phosphate were found to slightly reduce

arsenic removal below pH 12, whereas arsenic removal efficiency at lower pH is

increased by addition of iron. The disadvantage of arsenic removal by lime is that it

requires large quantities (800–1200 mg/L), which produce a large volume of

sludge. Obviously, water to which lime has been added requires secondary treat-

ment to properly adjust the pH level. Lime softening may primarily serve as a

pretreatment before the use of alum or iron coagulation.

IV Sorptive Filtration

Arsenic can be removed from water to very low levels by filtration through sorptive

media. Activated alumina, activated carbon, iron- or manganese-coated sand,

kaolinite clay, hydrated ferric oxide, activated bauxite, cerium oxide, titanium

oxide, silicon oxide, and many other natural and synthetic substances have been

used as sorptive media to remove arsenic from water. The efficiency of sorptive

media depends on the use of oxidizing agents to aid arsenic sorption. Media

eventually saturate with contaminants removed from water; the specific sorption

affinity of the medium for components present determines sorptive life. Saturation

is reached when active sites of the media are exhausted and the media cannot

remove further impurities.

A Activated Alumina

Activated alumina (Al2O3) has a good sorptive surface area, in the range of 200–300

m2/g, and is an effective medium for arsenic removal. When water passes through a

packed column of activated alumina, all impurities, including arsenic, are adsorbed

on the surfaces of the media grains. The column will eventually become saturated,

first at the top and later toward its bottom. Arsenic removal by activated alumina is

influenced by both the pH and the arsenic content of the water. Arsenic removal is

optimum at pHs between 5.5 and 6.0, when the surface of the medium is positively

charged; removal efficiency drops as the point of zero charge is approached. When

the surface of the medium becomes negatively charged at pH 8.2, the removal

capacities are only 2%–5% of the capacity at optimal pH (Clifford 1990).

Regeneration of saturated alumina is performed by exposing the medium to 4%

caustic soda (NaOH), either in a batch process or by column flowthrough. Either

process produces arsenic-contaminated caustic wastewater. The residual caustic


soda is then washed and the medium is neutralized with a 2% solution of sulfuric

acid. During the process, about 5%–10% alumina is lost and the capacity of the

regenerated medium is reduced by 30%–40%; activated alumina must be replaced

after three or four regenerations. As with coagulation processes, prechlorination

dramatically improves the column capacity. The activated alumina-based sorptive

media used in Bangladesh include BUET Activated Alumina, Alcan Enhanced

Activated Alumina, and media employed in the Apyron Arsenic Treatment Unit.

Each of these three units/processes is reviewed below.

BUET Activated Alumina

The BUET activated alumina arsenic removal unit (ARU) consists of subunits for

oxidation-sedimentation, filtration, and activated alumina adsorption. Oxidation

and sedimentation is performed in a 25-L plastic bowl. Approximately 1 mg/L

potassium permanganate is added to water in the bowl to oxidize As(III) to As(V);

the mixture is stirred vigorously with a wooden stick and then allowed to settle for

about 1 hr. The settled water is filtered through a sand bed and is then passed

through the activated alumina column. The unit is very effective in removing

arsenic and iron from tubewell water. One practical problem with the ARU is that

women have difficulty in raising water to the level required for gravity flow through

the subunits. The problem has been addressed by design modification. The modified

BUET activated alumina ARU is shown in Fig. 10.

ActivatedAlumina

OxidationSedimentation Unit

SandFiltrationUnit

Fig. 10 The BUET activated-

alumina arsenic removal unit


Alcan Enhanced Activated Alumina Unit

In this process, water from a tube well is allowed to pass through an enhanced

activated alumina bed and the treated water is collected as shown in Fig. 11. The

unit has a simple and robust design. No chemicals are added during treatment, and

the process relies entirely on the active surface of the media for adsorption of

arsenic. Other ions present in water, such as iron and phosphate, may compete for

active sites on alumina and thereby reduce the arsenic removal capacity of the unit.

Iron present at elevated levels in shallow tubewell water will eventually accumulate

in the activated alumina bed and interfere with water flow. The unit can produce

more than 3,600 L arsenic-safe drinking water per day, enough for 100 families.

Alcan’s enhanced activated alumina unit is designed for single use and therefore

saturated media must be replaced after use. Environmentally safe disposal of spent

activated alumina (~40 kg per treatment cycle) is required.

Apyron Arsenic Treatment Unit

Apyron Technologies Inc. (ATI), USA, has developed an arsenic treatment unit

(ATU) in which Aqua-Bind media is used to reduce arsenic in groundwater. The

ATU consists of a cylindrical adsorber vessel containing Aqua-Bind media. This

media consists of nonhazardous aluminum oxide (Al2O3) and manganese oxide

(Mn2O3) that can selectively remove As(III) and As(V) from water. The column

receives water under slight positive pressure from a manually operated lift pump

(Fig. 12). Water flows downward through a two-chamber housing capable of

capturing particulate iron and adsorbing arsenic. Discharge water exits into the

designated container at a rate of approximately 15 L/min. Experimental units

installed in India and Bangladesh are reported to consistently reduce arsenic in

water to less than 10 mg/L. The proponents of ATI’s Aqua-Bind media (Senapati

and Alam 2001) claim that it (1) removes both arsenic (III) and arsenic (V); (2)

successfully treats arsenic levels from 25 to more than 4,000 ppb in the presence of

Tubewell

Outlet (TreatedWater)

Inlet(Water from Tubewell)

Fig. 11 An Alcan enhanced activated alumina unit


up to 15 ppm of iron; (3) reduces contact times (ideal for point-of-use systems), (4)

operates over a wide pH range (6–8) and temperatures (0�–100�C), (5) is nonleach-able, allowing safe disposal of spent media (as per the Toxic Characteristic Leach-

ing Procedure test); (6) is NSF 61 certified for use in drinking water applications;

(7) resists microbe growth; and (8) is highly selective for As, even with competing

ions (sulfates, silica, Ca, etc.).

B Granular Ferric Hydroxide

Granular ferric hydroxide (AdsorpAs) is a highly effective adsorbent used for

removal of arsenate, arsenite, and phosphate from natural water and wastewater.

AdsorpAs treatment capacity ranges from 40,000 to 60,000 BV (bed volumes),

until adsorbed arsenic exceeds the permissible level of 0.01 mg/L. AdsorpAs has

0.2–2.0 mm grain size, 72%–77% porosity, 250–300 m2/dm3 specific surface, 1.22–

1.29 kg/dm3 bulk density, and 52%–57% active substances (Fe(OH)3 and

b-FeOOH). It has an adsorption capacity of 45 g/kg for arsenic and 16 g/kg for

phosphorus on a dry weight basis (Pal 2001). The granular ferric hydroxide reactors

are fixed-bed adsorbers that operate as conventional filters. The units require iron

removal by pretreatment to avoid clogging the adsorption bed. A typical granular

ferric hydroxide-based arsenic removal unit is shown in Fig. 13. Water containing

high dissolved iron and suspended matter is pretreated by aeration and filtration

through a gravel/sand filter bed, and is then passed through AdsorpAs in the

adsorption tower for removal of arsenic. M/S Pal Trockner (P) Ltd of India and

Sidko Limited of Bangladesh installed several granular ferric hydroxide-based

arsenic removal units in India and Bangladesh. Proponents claim AdsorpAs has

very high arsenic removal capacity, 5- to 10 fold higher than activated alumina. The

unit, therefore, produces less residual spent solids; typically the residual mass of

spent AdsorpAs is 5–25 g/m3 of water treated. The spent granular ferric hydroxide

is a nontoxic solid waste. Under normal conditions, arsenic does not leach from

spent AdsorpAs.

Aqua-BindArsenic

Aqua-BindFilter Media

Lift PumpBucket ContainingTreated Water

Deliver Hose Pipe

Raising HosePipe

Fig. 12 An Apyron arsenic treatment unit


C Read-F Arsenic Removal Unit

READ-F is an adsorbent produced and promoted by Nihon Kaisui Co. Ltd, Japan

and Brota Services International, Bangladesh for arsenic removal in Bangladesh.

Read-F is selective for arsenic ions under a range of conditions, effectively adsorb-

ing both arsenite and arsenate. Oxidation of arsenite to arsenate is not needed for

arsenic removal in this method, nor must pH be adjusted before or after treatment.

The READ-F is composed of an ethylene-vinyl alcohol copolymer (EVOH) and

hydrous cerium oxide (CeO2�nH2O), with the latter acting as the adsorbent.

The material contains 60% water, with a 0.7-mm average particle size and a 1.6

g/ml specific weight. The material contains no organic solvent or other volatile

substance and is not classified as a hazardous material. Laboratory tests at BUET

and field testing of the materials at several sites under the supervision of the

Bangladesh Arsenic Mitigation Water Supply Project (BAMWSP) showed that

the adsorbent is highly efficient in removing arsenic from groundwater. Two

units utilizing READ-F technology are available commercially in Bangladesh:

one is a household treatment unit and the second is a community treatment unit

(Fig. 14). The units remove iron by sand filtration to avoid clogging the resin bed.

The household unit has sand and resin beds arranged in one container, whereas

these two beds exist separately in the community unit. According to proponents,

READ-F is approved by the Japan Ministry of Health and Welfare for treatment of

potable water and has provided efficient and dependable arsenic removal for water

treatment facilities at Ibaraki, Japan for 3 yr. READ-F is regenerated by adding

sodium hydroxide, then sodium hypochloride, and is finally rinsed with water. The

regenerated READ-F is neutralized with hydrochloric acid and flushed before

reuse. After neutralization, wastewater is treated with a small amount of adsorbent

for safe disposal.

ContaminatedWater Inflow

TreatedWater Outflow

Gravel Filter Bed Adsorption Tower

Fig. 13 A granular ferric hydroxide-based arsenic removal unit


D Iron-Coated Sand

BUET has tested a unit that utilizes iron-coated sand for removal of arsenic from

groundwater (Fig. 15). Pretreatment for removal of excess iron, to avoid clogging of

the active filter bed, is required. Pretreatment consists of precipitating iron by air

oxidation. The water is then filtered through sand to trap excess iron. This sand

filter, about 10 cm in depth, is placed in a 15-cm-diameter PVC chamber having

perforations at its base. Water flows from the top of the bucket into the sand filter

Fig. 14 An arsenic removal unit (ARU) based on READ-F materials

Plain sand

IronCoatedSand

Fig. 15 A household arsenic

removal unit based on iron-coated

sand


via a replaceable plastic pipe. A 1- to 2-cm-thick gravel bed is placed at the bottom

to retain sand. The water then passes through a second 40-cm-deep iron-coated sand

filter that is responsible for removing arsenic. Water enters into a strainer placed in

iron-coated sand and eventually flows to the tap.

Figure 16 shows the arsenate and arsenite removal capacities of iron-coated sand

(Ali et al. 2003). Raw water containing 300 mg/L arsenic is effectively cleaned of

arsenic when filtered through iron-coated sand (Fig. 16). It was found that iron-

coated sand will process about 350 BV, each satisfying the Bangladesh drinking

water standard (50 ppb) before becoming exhausted. Saturated media is regenerated

by passing 0.2 N sodium hydroxide through the column or soaking the sand from

the column with 0.2 N sodium hydroxide, followed by flushing with distilled water.

Bed volumes, which represent the ratios of volume of water treated to volume of

media, continued to remove arsenic successfully after five regeneration cycles.

Iron-coated sand is effective in removing both As(III) and As(V).

E Shapla Arsenic Filter

The Shapla Arsenic Filter, a household arsenic removal unit, has been developed

and promoted by International Development Enterprises (IDE), Bangladesh. The

unit media constitutes iron-coated brick chips manufactured by treating such chips

with a ferrous sulfate solution; the media works on the same principles as iron-

coated sand. Water from contaminated tube wells is allowed to pass through earthen

containers filled with the filter media; the containers are fitted underneath with a

drainage system. A drawing of the Shapla Filter is shown in Fig. 17.

0

50

100

150

200

250

0 50 100 150 200 250 300 350 400 450

Bed Volumes

Ars

enic

Co

nc.

µg

/L

As (III) vs Bed Volume

As (V) vs Bed Volume

Fig. 16 Arsenic removal capacity of iron-coated sand


It has been claimed that 20 kg Shapla filter media can clean (to nondetectable

levels) up to 3,000 L tubewell water having arsenic concentrations of 0.3–0.4 mg/L.

Currently, the cost of the filter, including the reusable earthen container, is 350 Tk

(the Bangladeshi Taka equals about $US 5). IDE estimates that 20 kg filter material

can be produced at a retail price of Tk 100, and claims that the exhausted filter

media is nontoxic and can be disposed of safely. Experimental units have been

installed for field testing in rural areas of Kachua, Sonargoan, Noakhali, and

Rajshahi in Bangladesh in collaboration with UNICEF (United Nations Interna-

tional Children’s Emergency Fund), Danida, and SDC (Sustainable Development

Commission of UNICEF). Reports say the unit is effective in arsenic removal and

affordable for the majority of the rural population.

F Sono Filter

The Sono filter uses zero valent iron filings (cast iron turnings), sand, brick chips,

and wood coke for removing arsenic and other trace metals from groundwater in

Bangladesh (Munir et al. 2001; Khan et al.2000). The filtration system originally

consisted of a 3-kalshi unit (burned clay pitchers), widely used in Bangladesh for

water storage and for drinking and cooking. Three kalshis were arranged vertically

one above the other on a steel or wooden frame (Fig. 18a). The top kalshi contained

3 kg cast iron turnings, and this layer is covered with 2 kg sand. The middle kalshi

contained 2 kg sand, 1 kg charcoal, and 2 kg brick chips. Brick chips are also placed

around the holes to retain finer materials. Tubewell water is poured in the top kalshi,

filtered through the top and middle kalshis, and the treated drinking water is

collected from the bottom kalshi. Nikolaidis and Lackovic (1998) showed that

Iron CoatedCrushed BrickParticles

Cloth Filter onPerforated Plate

Flexible WaterDelivery Pipe

Treated Water in a Bucket

Suppor

Lid

Fig. 17 Shapla filter for arsenic removal in households


97 % arsenic can be removed by adsorption on a mixture of zero valent iron filings

and sand, and recommended that arsenic species can be removed through formation

of coprecipitates and mixed precipitates and by adsorption onto the ferric hydroxide

solids. The Sono 3-Kalshi unit was very effective in removing arsenic, but the one-

time-use unit is rapidly clogged if groundwater contains excessive iron. Field

observations indicated that, over time, the iron filings bond with the solid mass,

rendering cleaning and replacement of materials difficult. To overcome these

problems, the filter media are housed in two plastic buckets (Fig. 18b). The cast

iron turnings have been processed into a complex iron matrix (CIM), which is

capable of maintaining its active CIM integrity for years. Manganese in CIM

catalyzes oxidation of As(III), and all As(V) is removed by a surface-complexation

reaction between the surface of hydrated iron (FeOH) and arsenic species (Hussan

2003).

In 2007, the Sono Filter competed for and won the $1 million ‘‘Grainger

Challenge Prize’’ for sustainability; the prize was awarded for innovative solutions

in removing arsenic from drinking water.

G SAFI Filter

The SAFI filter is a type of household candle filter. The candle is made of composite

porous materials such as kaolinite and iron oxide on which hydrated ferric oxide is

deposited by sequential chemical and heat treatment. The candle filter works on the

principles of adsorption, filtration, and on chemically treated active porous com-

posite materials. The oxyhydroxides of Fe, Al, and Mn assist in the removal of

Raw Water

Filter Media 1Sand, Iron Filings& Brick Chips

Filter Media 2Sand,Charcoal &Brick Chips

Filtered Water

a b

TopBucket(Red)

BottomBucket(Green)

Filter Media 1 Coarse Sand, Composite IronMatrix(CIM) Coarse Sand Brick Chips

Filter Media 2 Coarse Sand, CharcoalFine Sand Brick Chips

FilteredWater

Fig. 18 The Sono Filters for arsenic removal from groundwater. (a) Sono three-kalshi arsenic

filter. (b) Modified Sono arsenic filter. (From Hussan 2003.)


arsenic, iron, and bacteria. Some of the relevant features claimed for the SAFI filter

are shown in Table 2.

The SAFI filter was reported to have good arsenic removal capacity initially, but

efficiency declined with time. Moreover, the filter media became clogged, and the

unit suffered rapid erosion from mechanical cleaning and was generally regarded to

show poor workmanship. The filter candle, in many cases, was found to leak at

joints and to disintegrate because of inadequate strength.

H Activated Carbon

Granular activated carbon (GAC) removes arsenic by adsorption to some extent,

depending on the pH of the raw water. Studies conducted by All India Institute of

Hygiene and Public Health (AIIH & PH), using granular activated carbon, revealed

that GAC could be used to remove arsenic from groundwater, but the process was

not economically viable.

I Indigenous Filters

There are several filters available in Bangladesh that use indigenous material as

arsenic adsorbents. Red soil, rich in oxidized iron, brick chips, clay minerals, iron

ore, iron scrap or filings, and processed cellulose materials are known to adsorb

arsenic. Some filters manufactured from these materials include Granet Home-

made Filter, Chari Filter, Adarsha Filter, Bijoypur Clay, and Processed Cellulose

filter.

The Garnet home-made filter contains relatively inert materials such as brick

chips and sand. No chemical is added to the system. Air oxidation and arsenic

adsorption on iron-rich brick chips, plus natural iron flocs (in groundwater) may be

the main mechanisms by which arsenic is removed from groundwater using this

process. The Garnet home-made filter was previously evaluated (BAMWSP, DFID

and Water Aid Bangladesh 2000). The unit produced an inadequate quantity of

water and did not show reliable results under various operating conditions in

different areas of Bangladesh.

Table 2 Some important features of SAFI filters claimed by the proponent

Characteristics of filter Claims by the proponent

Candle lifetime 2 yr (treatment capacity, 4000 L with As content 1.5 ppm)

Flow rate 40 L/d (small type), 60–80 L/d (standard type)

Leaching No leaching of arsenic from candle up to pH 11

Regeneration Can be regenerated three times (cycles) at a cost of Tk.70 per

regeneration

Clogging If clogged, flow can be restored by treatment with 5% H2SO4

Candle cost Tk 600 (standard type), Tk 250 (small type)


The Chari filter is a modified version of the three-kalshi filter, in which open

pitchers or Charis were used to allow easier access to the absorbent materials for

washing and replacement. The top Chari filter uses brick chips and iron filings,

while sand is placed in the middle Chari. The unit was developed by Dhaka

Community Hospital (DCH) and has been used extensively in their arsenic mitiga-

tion programs. It appears that the ‘‘open concept’’ design of the Chari filter renders

long life to the filter, when compared with regular three-kalshi units, which are

replaced every 3–6 mon. Open units, however, are more vulnerable to contamina-

tion. During field visits, most Chari filters have been found to exist in unhygienic

conditions. The effectiveness of this process to remove arsenic, bacteria, and other

contamination is not known.

The Adarsha filters employ clayey material to form filter candles. The Adarsha

filter participated in an assessment conducted in Bangladesh but it failed to meet the

arsenic reduction criterion (BAMWSP, DFID and Water Aid Bangladesh 2001).

Aluminum-rich Bijoypur clay and treated cellulose were also found to adsorb

arsenic from water (Khair 2000). However, no commercial unit has been con-

structed to utilize these materials for arsenic removal in Bangladesh.

J Cartridge Filters

Cartridges filled with sorptive media or ion-exchange resins are available in the

market. These units remove arsenic and other dissolved ions present in water.

Cartridge filters are unsuitable for water having high impurity levels and/or iron

because such ions have high affinity for media and can quickly saturate it, requiring

frequent regeneration or replacement. The Chiyoda Arsenic Removal Unit (Japan),

available in Bangladesh, was tested at BUET Laboratory. This Chiyoda Removal

Unit treated 800 BV and still met the WHO guideline value (10 mg/L), or 1,300 BV,and still met the Bangladesh Standard (50 mg/L), when the feed water arsenic

concentration was 300 mg/L (Ahmed et al. 2000).

V Ion Exchange

The ion-exchange process utilizes similar principles to that of activated alumina but

employs synthetic resins of enhanced ion-exchange capacity. The synthetic resin is

based on a cross-linked polymer skeleton called the matrix. The charged functional

groups are attached to the matrix through covalent bonding and fall into strongly

acidic, weakly acidic, strongly basic, and weakly basic groups (Clifford 1990). The

resins are used to remove specific undesirable cations or anions from water. The

strongly basic resins can be pretreated with anions such as Cl– and are used for

removal of negatively charged species, including arsenate.

The capacity of ion exchange to remove arsenic is dependent on the sulfate and

nitrate content of raw water because these ions have a higher affinity for ion

exchange than does arsenic. Compared to other types of media, the ion-exchange


process is less dependent on the pH of water. Arsenite is uncharged and is therefore

not removed by ion exchange; hence, preoxidation of As(III) to As(V) is required

for removal of arsenite. The oxidant must be removed from ion-exchange media to

avoid damage to the resins. Arsenic removal would be enhanced if ion-exchange

resins were developed that could selectively remove arsenic species.

Ion-exchange capacity is similar to adsorption capacity in that both are measured

by the number of active sites per unit of media, usually expressed by milliequiva-

lents (mEq) per mL. Typical theoretical exchange capacities for strong base anion-

exchange resins range from 1 to 1.4 mEq/mL (Clifford 1990) or 3.0 to 4.2 mEq/g

dry wt. The maximum sorption capacity for arsenic (molecular weight of 75) is 315

mg As/g. Actual sorption capacities under field conditions are much lower.

After they are exhausted, ion-exchange resins can be easily regenerated by

treatment with NaCl solutions. The equations that describe this regeneration are

as follows:

Arsenic exchange:

2R-Clþ HAsO �4 ! R2HAsO4 þ 2Cl� ð10Þ

Regeneration:

R2HAsO4 þ 2Nþ þ 2Cl� ! 2R-Clþ HAsO �4 þ 2Naþ ð11Þ

where R stands for ion-exchange resin.

Tetrahedron Technology

Tetrahedron (USA) promoted an ion exchange-based arsenic removal technology in

Bangladesh. The technology proved its arsenic removal efficiency, even at high

flow rates. Figure 19 shows the schematic diagram behind this technology. This

Chlorine Source

Sieve

Stabilizer

Stone Chips

Stand Resin Column(Ion Exchanger)

Column Head Tap

Fig. 19 Depiction of the Tetrahedron arsenic removal technology


process utilizes a stabilizer and an ion exchange (resin column) along with facilities

for chlorination using chlorine tablets. Tubewell water is pumped or poured into the

stabilizer through a sieve containing a chlorine tablet. The water, mixed with

chlorine, is stored in the stabilizer and subsequently flows through the resin column

when the tap is opened. The purpose of the chlorine is to kill bacteria and oxidize

arsenic and iron. The stabilizer smoothes flow pulses from the pump and traps iron

and other hydroxide precipitates formed in water. Finally, the ion-exchange media

adsorbs and cleans arsenic, sulfate, and phosphate from the water. This Tetrahedron

filter was tested in Bangladesh (BAMWSP, DFID andWater Aid Bangladesh 2001)

and demonstrated promising results. The residual chlorine minimized bacterial

growth in the media. The saturated resin can be regenerated by NaCl solution.

Liquid wastes from the process, including salt and arsenic produced during regen-

eration, require safe disposal.

VI Membrane Techniques

Synthetic membranes can remove many contaminants from water, including bacte-

ria, viruses, salts. and various metal ions. Usually, two types of membrane filtration

are used: (1) low-pressure membranes such as microfiltration (MF), and (2) high-

pressure systems, such as ultrafiltration (UF), nanofiltration (NF), and reverse

osmosis (RO). The pore size of membranes and size of materials to be separated

are shown in Fig. 20. It is apparent (Fig. 20) that RO and NF have the appropriate

0.001 0.01 0.1 1.0 10 100 1000

Aqueous salts Bacteria

Algae

Viruses

Humic acids

Metal ions

Cysts

Clay Silt Sand

Nonofiltration

Ultrafiltration

Microfiltration

Conventional Filtration ProcessesReverse Osmosis

Size,Micron

RelativeSize ofVariousMaterialsPresentIn water

SeparationProcesses

Fig. 20 Sizes of impurities present in water and pore sizes of various membranes. (From Najm

and Trussell 1999.)


pore sizes for removal of arsenic. In recent years, a new generation of RO and NF

membranes have been developed that are less expensive and operate at lower

pressures. Arsenic removal by membrane filtration is independent of pH and

presence of other solutes but can be adversely affected by presence of certain

colloids. Also, iron and manganese, if present, can lead to scaling and membrane

fouling. Once fouled by impurities, the membrane cannot be successfully back-

washed but must be replaced. Therefore, water with high suspended solids content

requires pretreatment for arsenic removal. Most membranes, however, cannot

withstand oxidizing agents. The US EPA (2002) reported that NF was capable of

more than 90% removal of arsenic, while at ideal pressures RO provided removal

efficiencies greater than 95%. Water rejection (about 20%–25% of the influent)

may be an issue in water-scarce regions (US EPA 2002).

A Techno-Food Water Technology

Techno-food (Bangladesh) Co. is a manufacturer and supplier of water deminerali-

zation units that use membrane technology to clean industrial water supplies. This

organization markets several domestic water purification systems with water treat-

ment capacities varying from 60 to 1,200 L/d. Techno-food Bangladesh have also

introduced new-generation NF and RO membranes for arsenic removal in Bangla-

desh. The Techno-Food water treatment units operate at 50–150 psi, and remove

95%–98% of total dissolved solids, including arsenic. In this method, arsenic

removal is independent of pH. In addition to arsenic, membrane filtration removes

many other impurities, including bacteria. The membrane does not utilize chemi-

cals and does not accumulate arsenic as do other adsorbing materials; hence,

disposal of used membranes is not a threat to the environment. Operation and

maintenance of these units is simple, requiring only periodic wiping to clean

membranes. The seller claims that the RO and NF membrane-separation technolo-

gy is among the safest of arsenic removal systems. Installation of small community

or large production units for public water supplies is possible at affordable costs

with this technology.

B MRT-1000 and Reid System, Ltd

Jago Corporation, Ltd promoted a household RO water dispenser (MRT-1000)

manufactured by B & T Science Co., Ltd, Taiwan. This system was tested at

BUET and demonstrated an arsenic (III) removal efficiency greater than 80%. A

wider-spectrum RO system, named Reid System, Ltd was also promoted in Ban-

gladesh. Experimental results showed that this system could effectively reduce

arsenic and other impurities in water. The capital and operational costs of the RO

system are relatively high.


C Low-Pressure Nanofiltration and Reverse Osmosis

Oh et al. (2000) applied RO and NF membrane processes to treatment of arsenic

contaminated water in which needed pressure was applied with a bicycle pump. A

NF membrane process coupled with a bicycle pump can operate under conditions of

low recovery and low pressure (from 0.2 to 0.7 MPa). The rejection rate for arsenite

is lower than for arsenate in ionized forms; hence, water containing higher arsenite

levels requires preoxidation to achieve acceptable arsenic removal. Tubewell water

in Bangladesh has an average ratio of arsenite to total arsenic of 0.25. However, RO

coupled with a bicycle pump operating at 4 Mpa can remove arsenic because of its

high rate of rejection. The study concluded that low-pressure NF with preoxidation

or RO (coupled to a bicycle pump) could successfully treat arsenic-contaminated

groundwater in rural areas (Oh et al. 2000).

D Mobile Reverse Osmosis

ThePrinceofSongklaUniversity, Thailand (Wanichapichart 2005) has tested amobile

RO machine capable of removing arsenic from water. This machine (Fig. 21) pro-

duced drinking water at a rate of 208 L/hr and could fill a 750-L container daily. It

operates by passing pipeline water through a 0.5-mm filter to an RO spiral-wound

membrane. The quality of water before and after filtration, and user attitudes toward

the project, are shown in Table 3. Houses nearest the RO machine benefited most

Fig. 21 A mobile reverse osmosis

(RO) unit


from the project; others more distantly located did not use water from the system.

Users in the community agreed that membrane technology was useful in providing

drinking water of acceptable quality during the summer months. Communities in

the district may consider a mobile RO machine as an alternative to provide arsenic-

free water, both for their community and for nearby districts. However, the mem-

brane in such units must be changed every 3 mon, which entails further expense.

E Combined Sand and Nanofiltration

The efficiency of combining oxidation, sand filtration, and nanofiltration (NF) to

remove arsenic was studied jointly by Rajshahi University, Bangladesh and Hok-

kaido University, Japan. A pilot plant with equipment for (1) oxidation by air and/or

NaClO, followed by (2) two sand filter columns, each filled with 1-m-high manga-

nese sand, and (3) a NF unit for further removal of arsenic to trace level, was

constructed. This plant was field tested for 8 mon using tubewell water contami-

nated with arsenic (98–170 mg/L), iron (2,470–9,900 mg/L), and manganese (455–

700 mg/L). Oxidation and one-stage filtration through manganese sand reduced

arsenic content of water to 44 mg/L, which is within the limits acceptable in

Bangladesh. The NF unit was installed after sand filtration to attain a high level

of arsenic removal. Arsenic content, in treated water following NF, was 0.4 mg/L or

less (Rahman and Magara 2002). The NF system gave a high water recovery rate of

60%, although the process is costly.

VII Bioremediation by Algae

Remediation of arsenic by an alga, Chlorella vulgaris, has been explored by many

investigators. Metal accumulation by algae is influenced by a number of biotic and

abiotic factors (Genter 1996). Abiotic factors include chemical speciation of As,

Table 3 Operating parameters of, and user attitudes toward, use of the reverse osmosis (RO)

technology

Elements Feed Permeate Standard Summarized (%)

from 238 families

pH 4.5–6.1 4.5–6.1 6.5–8.5 51% faced with shortage of

drinking water in

summerHardness (as CaCO3) 3.1–11 <1.0 <100

Iron 0–0.13 <0.01 <0.3

Sulfate <25 <25 <250 42% paid for bottled water

Chloride <5 <5 <250 34.6% obtained free water

from the RO machineNitrate 1.0 0.6 <4.0

Arsenic (ppb) 100-150 <10 10 97.5% in the district

requested an RO‐machine

Treated-water output of 15–

500 gal/mon/family for

cooking and drinking


metal concentration, duration of exposure, concentration of other ions (e.g., Ca,

Mg, P), pH, presence of complexing and chelating agents, redox conditions,

temperature, light, and flow rate of water. Biotic factors influencing performance

include species-specific characteristics, algal biomass, extracellular products, stage

of organism development, and cellular activity. Results indicated that Chlorellavulgaris grew better in a medium with arsenate at concentrations up to 2,000 mg/L

and accumulated arsenate at levels up to 50,000 mg As/kg dry cell wt (Maeda and

Sakaguchi 1990; Maeda and Ohki 1998). Visoottiviseth and Lauengsuchonkul

(2004) reported that the green alga Chlorella vulgaris exhibited good growth

when exposed to very high concentrations of arsenic. The authors immobilized

this alga on alginate beads and tested its efficiency. Results indicated that efficiency

depended on the numbers of beads and duration of exposure (Fig. 22). They then

constructed an arsenic filter using the immobilized Chlorella vulgaris in combina-

tion with other adsorbent materials (Fig. 23). Tests of this algal filter in arsenic-

contaminated areas of Thailand showed that it performed well, removing at least

95% of arsenic present. This filter is economic, although the alginate beads must be

changed every 3 mon. The used beads can be further used as fish food.

VIII Phytoremediation of Arsenic

Because soils are a major source of arsenic contamination, removal of arsenic from

soil should help reduce arsenic pollution of surface waters. In 2001, the arsenic

hyperaccumulating fern, Pteris vittata, was discovered by Ma et al. (2001), and

later Visoottiviseth et al. (2002) discovered Pitylogramma calomelanos, anotherspecies of arsenic hyperaccumulating fern. Both ferns have been tested for their

efficiency in removing arsenic from water and soil (Tu et al. 2004; Alkorta et al.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

0 min 5 min 10 min 15 min 30 min 1 hr 3 hr 9 hr 24 hr 7 day

Time

% A

s re

mov

al

675 beads

2025 beads

3375 beads

Fig. 22 Removal of arsenic from water using different numbers of algal beads at various exposure

times


2004; Huang et al. 2004). In Thailand, a field trial on phytoremediation using the

fern Pitylogramma calomelanos was planned in an area highly contaminated with

arsenic. However, use of this fern was rejected by the local population because they

could not discern any economic benefit to them from its use. Thus, plants selected

for phytoremediation must meet rather diverse criteria. They must accumulate high

levels of As, have high biomass, have a short life cycle, tolerate high concentrations

of arsenic, and be economically important to users. Marigolds are plants that

possess such characteristics and have been used in a field trial (Fig. 24). Each

marigold plant has at least 10 flowers and can be sold for $2.00 per 100 flowers. The

flowers can be planted, harvested, and sold in a 45-d cycle. In addition to achieving

arsenic removal, it was estimated that growers of marigold plants could each earn at

least $57,500 per ha per yr (Chintakovid et al. 2007).

IX Comparison of Arsenic Removal Technologies

Advances in using modern technology to remove arsenic from rural water supplies

have been remarkable during the last 2–3 yr. A comparison of extant processes is

shown in Table 4.

All technologies described have merits and demerits and are continually being

refined. The major objectives of these technological refinements are to (1) improve

efficiency in arsenic removal, (2) reduce capital and operating costs, (3) improve

user friendliness, (4) achieve easier maintenance, and (5) resolve arsenic sludge and

concentrate management problems. Arsenic removal technologies must be eco-

nomically sound to be accepted by users.

Fig. 23 Arsenic filter using immobilized Chlorella vulgaris and an adsorbent material


X Conventional Filtration

The performance of conventional filtration methods is variable, and the efficiency

of arsenic removal achieved can range widely (from 4.5% to 96%), depending upon

conditions and the particular adsorbent used (Jiang 2001). Criteria for selecting a

suitable adsorbent include cost of the medium; ease of operation and handling;

adsorption capacity (breakthrough point) and potential for re-use and regeneration.

In a recent review of arsenic treatment technologies published by Vu et al. (2003),

iron filings, ferric salts, granular ferric hydroxide, alumina-manganese oxide, Aqua-

bind, and kimberlite tailings are all listed as potentially low-cost adsorbents.

Properly handled, all can remove arsenic in a relatively short time, and the adsor-

bents themselves can then be removed from water by filtration. Moreover, metal-

loaded polymers are new prospective sorbents for removing As(III) and As(V) from

water and may have promise (Dambies 2004). Other sorbent materials for arsenic

removal have also been reviewed: granular activated carbon (Bissen and Frimmel

2003; Daus et al. 2004; Jiang 2001); iron-coated sands (Jiang 2001; US EPA 1999);

manganese greensand (Viraghavan et al. 1999); manganese-coated sand (Bajpai

and Chaudhuri 1999); iron hydroxide granulates (Bissen and Frimmel 2003; Daus

et al. 2004; Driehaus et al. 1998; Pal 2001); natural materials (natural zeolites,

volcanic stone, catcaceous powder) (Elizalde-Goznales et al. 2001); zero valent

iron (iron filings) (Nikolaidis et al. 2003; Su and Puls 2001, 2003), and kimberlite

tailings (Dikshit et al. 2001). Elizalde-Gonzales et al. (2001) reviewed naturally

occurring solids as agents for arsenic adsorption from water. These authors con-

clude that, although such solids are cheap—indeed, they can be obtained free of

charge—their rates of removing arsenic from water are usually low. Naturally

Fig. 24 Marigold plants grown in the high arsenic-contaminated areas of Thailand


Table 4 A comparison of advantages and disadvantages of major arsenic removal technologies

Technology Advantages Disadvantages

Oxidation and

sedimentation

l Air oxidation l Relatively simple, low-

cost but slow

l Partial removal of arsenic

l Chemical oxidation l Relatively simple and

rapid

l Used as pretreatment for other

processes

l Oxidizes other impurities

and kills microbes

Coagulation and

filtration

l Alum coagulation l Relatively low capital

cost

l Produces toxic sludges

l Iron coagulation l Relatively simple l Low removal of As(III)

l Utilize commonly

available chemicals

l Preoxidation is required

l Removal efficiencies may be

inadequate to meet strict standards

Sorption techniques

l Actvated alumina l Well known and

commercially available

l Produces arsenic-rich liquid and

solid wastes

l Iron coated sand l Well defined l Replacement/regeneration is required

l Ion exchange resin l Many possibilities and

large development effort

l High-tech operation and

maintenance

l Other sorbents l Relatively high cost

Membrane techniques

l Nanofiltration l Well defined; high

removal efficiency

l High capital and running costs

l Reverse osmosis l No toxic solid wastes

produced

l High-tech operation and

maintenance

l Electrodialysis l Capable of removing

other contaminants

l Arsenic-rich water effluent is

produced

Bioremediation

l Algal filter l Produce no toxic wastes l Light is required for photosynthesis

l Phyto-remediation l High efficiency (>95%)

l Environment friendly

l Low capital cost

l Provides income to users

l Used algal beads can be

fed to fish

Source: Based on Ahmed 2003; The World Bank 2005.


occurring solids are often employed for household-level water treatment in under-

developed countries, such as Bangladesh. Saha et al. (2001) studied a wide range of

adsorbents (mainly natural materials) for their ability to remove As(III) and As(V)

from water. They compared the following materials: Kimberlite tailings, water

hyacinth, wood charcoal, banana pith, coal fly ash, spent tea leaves, mushroom,

saw dust, rice husk ash, sand, activated carbon, bauxite, hematite, laterite, iron

oxide-coated sand, and hydrous granular ferric hydroxide. Activated alumina was

used as the reference in this study. Removal efficiencies for As(III) varied from 5%

to 92% and for As(V) from 12% to 99%. Iron oxide-coated sand and hydrous

granular ferric oxide performed best and was studied in detail. In the field portion of

the study, columns containing these two adsorbents were run for 20 d with influent

water containing 0.32 mg/L arsenic. The column with granular ferric hydroxides

supplied 45 L water/d with less than 0.01 mg/L arsenic.

XI Alternative Water Supply Options

Although shallow tube wells recently dug in South Asia to tap alluvial aquifers do

provide low-cost drinking water, this water is often contaminated with arsenic.

Such contaminated water poses a hazard to millions of people. The problem is

exacerbated because tube wells with high levels of arsenic are in the same areas

where contaminated tube wells predominate. In the absence of an alternative water

source, people drink arsenic-contaminated water without considering the conse-

quences. Alternatively, people who avoid tubewell water by drinking surface water

are at high risk of contacting water-borne diseases. Unless arsenic contaminated

water is cleaned before use, the only alternatives for access to arsenic-safe water

include deep tube well, dug/ring well, rainwater harvesting, treatment of surface

water, and piped water supply.

A Deep Tube Well

The aquifers deposited at different geological times are usually stratified. Deeper

aquifers, separated by relatively impermeable strata, are relatively free from arsenic

contamination. Study results show that only about 1% of tube wells with depths

exceeding 150 m are contaminated with arsenic levels above 50 mg/L. Similarly,

only 5% of deep tubewell water exceeds arsenic levels of 10 mg/L (BGS and DPHE

2001). Therefore, deep aquifers separated from shallow contaminated aquifers by

the prerequisite impermeable layer can be dependable sources of arsenic-safe

water. To avoid percolation of arsenic-contaminated water, annular spaces of

deep tubewell boreholes must be sealed, at least to the level of the impermeable

strata (Fig. 25). It is very difficult to seal a small-bore tube well, although techno-

logical refinements using clay as a sealant are underway. A protocol for installation

of deep tube wells adequate to achieve arsenic mitigation has been developed in

Bangladesh. Because of stratification problems, such deep tube wells may initially


yield arsenic-safe water but later see an increase of arsenic content from mixing of

contaminated and uncontaminated waters. Such mixing can be minimized by

recharging the deep aquifer with water filtered through coarse media or by replen-

ishment from horizontal movement of water.

Experience in the design and installation of tube wells showed that reddish sand

produces water with optimum amounts of dissolved iron and arsenic. The reddish

sand color results from oxidation of iron on sand grains to ferric form. Such sand

will not release arsenic or iron into groundwater; rather, ferric iron-coated sand will

adsorb arsenic from groundwater. The Dhaka water supply, although contaminated

by arsenic, is probably protected by its local red-colored soil. Hence, installation of

tube wells in reddish sand, if possible, should be undertaken to protect against

arsenic contamination.

B Dug/Ring Well

Dug wells are the oldest method of procuring groundwater. Water from dug wells is

relatively free from dissolved arsenic and iron, even in locations with contaminated

tube wells. Why such dug wells avoid high arsenic levels is not fully known. The

following are among explanations given for low arsenic content of dug well water:

(1) arsenic and iron precipitates because of oxidation from open air exposure and

Silty clay

Arsenic Contaminated Shallow Aquifer(Sandy silt )

Arsenic-safe Deep Aquifer (Fine to medium sand)

Water Table

Clay Seal

Clayey Layer

Strainer

Sand Trap

Relatively Impermeable

Manually OperatedDeep Tubewell

Fig. 25 Deep tube well diagram


agitation during water withdrawal, and (2) dilution with freshwater from seepage

into the well from replenishing rainwater, surface water, or groundwater (from the

top layer of a water table). Such water would be cleaned by percolation through

aerated soil.

Dug well collects water from surface of an aquifer, which is close to ground

surface where microbial pollution and presence of organic matters are higher. Water

from such aquifers smells bad, has high turbidity, and contains ammonia. The water

from shallow aquifers is also susceptible to bacterial contamination. Satisfactory

protection against bacteriological contamination is possible by sealing the well top

with a watertight concrete slab. Water may be withdrawn by installing a manually

operated hand pump. Completely closed dug wells have good sanitary protection,

but a dearth of oxygen can adversely affect the water quality. Construction and

operational difficulties are often encountered when dug wells are sunk into silty and

sandy soils. ‘‘Sand boiling’’ interferes both with digging and operation of dug wells

and can lead to well wall collapse. Water in the well must be chlorinated for

disinfection after construction, and limemay also be added to improve water quality.

C Surface Water Treatment

Sand Filter (SF)

One option for a community-type surface water-based system is a slow sand filter

(SSF), commonly known as a pond sand filter (PSF) in Bangladesh, where it was

originally designed for filtration of pond water. It is a package-type slow sand filter

unit developed to treat surface waters, usually low-saline pond water, for domestic

water supply in coastal areas. Water from the pond or river is manually pumped

from a tube well to the filter bed, which rests above ground. Treated water is

collected for distribution from taps. Treated water from a PSF is normally bacteria

free or within safety limits. The average period between maintenance for the PSF

unit is usually 2 mon, after which the sand in the bed must be cleaned. The

construction of a typical PSF is shown in Fig. 26. The operating conditions

necessary for slow sand filters include low turbidity (<30 nephelometric turbidity

units, NTU), low bacterial count, no algal bloom, absence of Cyanobacteria, and

free from bad smell and color. A protected surface water source is ideal for slow

sand filtration. If the foregoing conditions are broached, the following problems are

likely to be encountered: low discharge, need for frequent cleaning, and poor

effluent quality. Community involvement in proper operation and maintenance of

these small units is absolutely essential if the system is to remain operational.

The package-type SSF provides low cost and very high efficiency for removal of

turbidity and bacteria. It is preferred for use by medium-size settlements in their

water supply systems. Although PSF has very high bacterial removal efficiency,

it fails to reduce bacterial count to acceptable levels when surface water is heavily

contaminated. In such cases, treated water may require chlorination to meet


drinking water standards. Roughing (pretreatment) filters are combined with a SSF

when water turbidity exceeds 30 NTU. Roughing filters are responsible for remov-

ing turbidity and color to a level adequate for efficient operation of the SSF.

Although the cost is higher, communities may construct small-scale conventional

surface water treatment plants that utilize coagulation-sedimentation-filtration and

disinfection to cope with variable raw water quality.

Wetland Treatment

Wetland treatment can remove arsenic from surface water using aquatic or wetland

plants commonly found in a contaminated area. In Asian countries, where sunlight

is available all year, wetland treatment may be more suitable than other technolo-

gies, because it is inexpensive to establish and maintain and is environmentally

friendly. Among plants commonly used in this method are alum, phragmites,

vetiver grass, and cattail. Among these plants, alum (Colocasia esculenta) was

best at removing arsenic. Alum retains the highest concentration of As it absorbs it

in its underground stem (Aksorn and Visoottiviseth 2004). As plants grow and

absorb arsenic, they are harvested, and new plants are seeded to repeat the process

(Fig. 27). Wetland treatment in Thailand removed more than 90% of arsenic

(accumulating As at concentrations from 300 to >1000 mg As/L) from surface

water.

Rainwater Harvesting (RWH)

Rainwater harvesting (RWH) can be used as a source of clean water in South Asian

countries. The advantages and disadvantages of RWH systems are presented in

Raw water from pond

Filter sand

Coarseaggregate

Under drainagesystem

Handpump for watersupply to filter

CLEARWATER

SURFACEWATER

Fig. 26 Pond sand filter for treatment of surface water


Table 5. If one relies on a rainwater supply system, both the volume of water

required in the community and probable availability of rain (intensity and distribu-

tion) must be determined in advance. In particular, a storage tank and catchment

area of the proper size must be constructed. The unequal distribution of annual

Fig. 27 Elephant ear or alum, Colocasia esculenta, growing in a wetland

Table 5 Advantages and disadvantages of rainwater collection systems

Advantages Disadvantages

l The quality of rainwater is comparatively

good

l The initial cost may prevent a family from

installing a rainwater harvesting system

l The system is independent and therefore

suitable for scattered settlements

l Water availability is limited by rainfall

intensity and available roof area

l Local materials and craftsmanship can be

used in construction of the rainwater system

l Mineral-free rainwater has a flat taste, which

may not appeal to all persons

l No energy costs are incurred in running the

system

l Mineral-free water may cause nutrition

deficiencies in people on mineral-deficient

diets

l Easy to maintain by owner/user l Poorer segments of the population may not

have suitable roofs for rainwater harvesting

l The system can be located very near

consumption points

Source: GOB (2002).


rainfall in Asian countries is offset by constructing large storage tanks; such tanks

constitute the main cost of the system.

In Bangladesh, the most commonly used household water container is a round

concrete tank about 1.6 m high and 2 m in diameter. It contains approximately

3,200 L water and is connected via a gutter to collect rainwater from the roof. The

container can supply water to two families, comprising 10 people, for about 64 d

(assuming 5 L/person/d). Because this amount (5 L/person/d) is less than is

normally consumed, rainwater harvesting is inadequate by itself to meet the

needs of an average household.

In Thailand, water containers hold only 2,500–3,000 L, but each family may use

multiple containers. Such containers were given to residents by the Thai Govern-

ment to mitigate the arsenic pollution problem in the Ron Phibun district of

southern Thailand. Unfortunately, people living in the affected area prefer to

drink well water rather than rainwater because they like its taste better.

The quality of rainwater is relatively good, but the water is not free from

impurities. Analysis of stored rainwater has shown some bacteriological contami-

nation. The cleanliness of roofs and storage tanks is critical in maintaining good

rainwater quality. Initial runoff from the roof should be discarded to prevent entry

of impurities into the stored water. If the storage tank itself is clean, bacteria or

parasites from rainwater will tend to die off. Some devices have been offered, or

good practices suggested, that either contain or divert the first ‘‘foul’’ flush from

roofs away from the storage tank. When such flows cannot be successfully diverted,

regular roof and gutter cleaning, before the rainy season, is needed, and regular

maintenance thereafter; such cleaning enhances the quality of stored rainwater. The

storage tank must be cleaned and disinfected annually or when the tank is emptied.

The rather large tanks are difficult to clean, and to do so effectively requires

someone to actually climb inside the tank. In Thailand, women do not like to do

this because it is thought to bring bad luck to the head of the family. Another

downside to RWH is that minerals, generally regarded as essential for good tasting

drinking water, are essentially lacking in rainwater.

The roof constitutes the normal catchment area for rainwater collection. Rain-

water can be collected from any type of roof, although concrete, clay tiles, and

metal give the cleanest water. The poorer sections of Bangladesh are not well

positioned to utilize rainwater. These people lack roofs or have only small thatched

ones. A thatched roof covered with polyethylene can be used as a catchment for

rainwater but requires application of certain skills to properly guide water to the

storage tank. In coastal areas of Bangladesh, people are known to use their clothes,

fixed at four corners with a pitcher underneath, to collect rainwater. A plastic sheet

(Fig. 28) has also been tried as a catchment for rainwater harvesting by people who

have no good alternatives. Land surfaces have been used for rainwater catchment as

well, and the resultant water has been stored underground in gravel/sand-packed

reservoirs. In such cases, water is channeled toward the reservoir and allowed to

pass through a sand bed before entering the reservoir. This process is analogous to

recharge of an underground aquifer during the rainy season for utilization in the dry

season.


D Piped Water Supply

Having a supply of piped water is preferred by consumers because (1) water can be

delivered in close proximity to consumers, (2) the water is protected from external

contamination, (3) it is usually monitored for quality, (4) it is likely to benefit from

institutional operation and maintenance, and (5) the required quantity of water is

delivered. Only piped water can compete in convenience, storage, and use with

water from tube wells. Moreover, piped water is feasible for clustered rural settle-

ments and settlements in many urban fringes. The piped water can be connected to

houses or to spigots near the house, or at other standposts depending on the financial

condition of consumers. Piped water can come from sinking a deep tube well in a

arsenic-safe aquifer, or treatment of contaminated surface or tubewell water in

community treatment plants. In Bangladesh, supplying piped water to rural areas

has been examined, and a large number of pilot schemes are being implemented

there by various organizations. Piped water is a more problematic and costly option

when populations live in the scattered rural areas of Bangladesh and West Bengal.

XII Operational Issues

There are many challenges in reducing arsenic contamination to the desired levels.

Analysis of arsenic at low concentration is difficult, as is performance monitoring

of water treatment systems. Validating the claims made by sellers concerning the

Fig. 28 Plastic sheet catchment

for rainwater harvesting (RWH)


performance of treatment technologies or arsenic measurement devices is also

important, but challenging. Safe disposal of toxic sludge and spent media is a

continuing environmental concern. Technologies that rely on patented media or

processes and imported components may not be easily procured when such materi-

als and components are needed.

Many of the same issues are also important in the operation of small water

treatment facilities used in households, or at the community level. It is not possible

to make broad arrangements for operation, repair, and maintenance of small water

supply systems. The know-how and participation of people at the local level are

vital for keeping small treatment systems operational. Many small-scale systems

have failed for lack of local initiative, commitment, and ownership. The unit cost of

the small systems may be higher because of the necessity of scaling down larger

conventional water treatment system designs.

A Costs

Cost is a key factor when considering the use and sustainability of arsenic removal

technology in rural areas. Cost varies with materials used, quantity of media/

chemicals used, quality of groundwater, etc. Most technologies have only been

installed and operated during either field or pilot-scale testing. Hence, the costs of

installation, operation, and maintenance are not well known or standardized to meet

various local conditions (The World Bank 2005). Costs of alternative water supply

systems are presented in Table 6. The unit cost of water produced by different

systems, expressed as annualized capital recovery, is presented in Table 7. The cost

of arsenic mitigation depends on which technology is used.

An evaluation of comparative technology costs demonstrates that deep tube

wells can provide water at nominal cost in many cases but cannot provide

Table 6 Costs for installing, operating, and maintaining alternative technologies to secure water

with acceptable levels of arsenic

Alternative

technology

No.

household

(s)/ unit

Unit cost,

$US

Operation and

maintenance cost/

yr, $US

Comments

Rainwater

harvesting

1 200 5 Low reliability

Dug/ring wells 25 800 3 Depth about 8 m

Deep tube well 50 900 4 Depth about 300 m

Pond sand filters 50 800 10–20 Slow sand filter process

Surface water

treatment

1,000 15,000 3,780 Conventional process

Piped water supply 100 8,000 500 Systems utilize sources

of arsenic-safe

groundwater

1,000 40,000 800

Source: Based on GOB (2002).


arsenic-free water at all locations. Dug/ring wells can provide water at moderate

cost, but whether desired water quality can be maintained is unclear. Piped water

can be provided, but at a higher economic cost, though convenience and health

benefits are quite high. An important consideration for piped water is that an

increase in the local household density reduces unit cost to any single household.

A study by the World Bank suggests that the cost of having a piped water supply

will be lower than other options for a medium village of 500 households (The

World Bank 2005). The cost of installing RWHs, at the household level, is very

high. Installation of similar community-scale RWHs may be cheaper, although

effective management of such systems may be difficult.

The capital recovery/amortization factor for the computation of annual capital

recovery in Table 7 has been calculated using the following formula:

Capital Recovery=Amortization Factor ¼ ð1þ iÞNðð1þ iÞN � 1Þ=i

where i ¼ interest rate and N ¼ number of years.

Table 7 Comparison of operating and maintenance costs, water production/unit, and technical

life of each technology used to secure water safe to drink and use

Technology Tech

life

Annual capital

recovery, $US

Operation and

maintenance cost/

annum, $US

Water

production,

m3

Cost of

water/m3,

$US

Alternative water supply

Rainwater

harvesting

15 30 5 16.4 2.134

Deep tube well 20 120 4 820 0.151

4,500 0.028

Pond sand filter 15 117 15 820 0.161

2,000 0.066

Dug/ring well 25 102 3 410 0.256

1,456 0.072a

Conventional

treatment

20 2,008 3,780 16,400 0.353

Piped water 15 5,872 800 16,400 0.407

73,000 0.091

Arsenic

treatment

(no.

households)

Shapla 6 0.9 11 16.4 0.73

SONO 45-25 5 3 1 16.4 0.24

Read-F 5 1.2 29 16.4 1.84

Magic Alkan 4 3.2 36 16.4 2.39

Bucket treatment 3 3 25 16.4 1.70a At full development potential of the system.

Source: Based on Ahmed (2004); The World Bank (2005).


Verification of technologies in Bangladesh shows that their performance is

dependent on pH and the presence of phosphate and silica in natural groundwater.

Most of these technologies fall short of meeting the treatment capacity promised by

sellers. A reduction in rated capacity increases the unit cost of water treatment.

B Technology Verification and Validation

Much development work has transpired during the past 5 yr to meet the demand for

arsenic mitigation technology. Appropriate validation of the performance of these

technologies is needed to help buyers select the one that meets their requirements.

The US EPA has developed protocols for validation of arsenic treatment technol-

ogies and arsenic field test kits as part of the Environmental Technology Verifica-

tion (ETV) Program. The protocols are in use for validation of technologies in

collaboration with the ETV program in Canada and at Battelle Laboratories in the

U.S. (USEPA 2002, 2003). The WHO has developed generic validation protocols

for adoption in South East Asia regional countries (WHO 2003). The Bangladesh

Council for Scientific and Industrial Research (BCSIR) has been given the respon-

sibility for verification and validation of arsenic removal technologies under the

Bangladesh Environmental Technology Verification–Support to Arsenic Mitiga-

tion (BETV-SAM) Program. Sono, Read-F, Sidco, and Alcan filters have been

verified and provisionally certified for deployment in Bangladesh, and some are in

the process of verification. Other arsenic removal technologies are undergoing

verification in Bangladesh, presently.

XIII Disposal of Generated Arsenic Waste

Arsenic removal units produce a variety of arsenic-rich solids and semisolids, such

as arsenic-saturated hydrous ferric or aluminum oxides and other filter media.

Regeneration of activated alumina and ion-exchange resins results in various liquid

wastes that may be acidic, caustic, saline, and/or too arsenic rich for easy disposal.

Hence, disposal of sludge, saturated media, and liquid wastes rich in arsenic is of

environmental concern.

Large treatment plants must discharge contaminated brine streams, resulting

from the RO/NF technologies, into large bodies of water. Inland treatment plants

would either pretreat waste prior to discharge, or would discharge to a sanitary

sewer. Discharge to sanitary sewers may also require pretreatment to remove high

arsenic levels. The waste stream produced by ion exchange (IE)/activated alumina

(AA) technologies contains highly concentrated brine with high total dissolved

solids (TDS). These brine streams may require pretreatment before discharge to

either a receiving body of water or sanitary sewer.

Hazardous wastes are often blended into stable waste or engineering materials

such as glass, brick, concrete, or cement block. There is a possibility of air or water

pollution downwind or downstream from kilns burning arsenic-contaminated


sludge. In Hungary, experiments showed that some 30% of arsenic in coagulated

sludge was lost to the atmosphere in this way (Johnston et al. 2001). Sludge or spent

filter media with low arsenic content can be disposed of in landfills without

significant increase in the background concentration of arsenic. Wastes with high

concentrations of arsenic may need solidification or confinement before final

disposal.

Regeneration of AA columns results in a toxic waste containing very high con-

centrations of soluble arsenic. The effluent from regenerating AA columns contains

acid and caustic rinses, which can be mixed and disposed of on a prepared bed of cow

dung in shallow holes dug into the earth. The microorganisms in cow dung transform

the arsenic to gaseous arsine, which is released into the surrounding air.

The Toxic Characteristic Leaching Procedure (TCLP) test was developed by the

USEPA to identify wastes likely to be hazardous for disposal in a landfill. The

permissible level for TCLP leachate is generally 100 times higher than the maxi-

mum contaminant level (MCL) in drinking water. The TCLP test was conducted on

different wastes collected from arsenic treatment units and materials in Bangladesh

(Eriksen-Hamel and Zinia 2001; Ali et al. 2003). It has been observed that, in

almost all cases, amounts of arsenic leached from such wastes were very small.

Arsenic leaching testing was conducted at Bangladesh University of Engineering

using different extraction fluids. The results show that for all extractants, arsenic

concentration in the column effluents were initially very high, but afterward

dropped sharply (Ali et al. 2003). Several researchers also conducted TCLP tests

on sludge resulting from arsenic removal by coagulation using aluminium and

ferric salts. Results indicated that arsenic content in leachate ranged between

0.009 and 1.5 mg/L (Brewster 1992; Chen et al. 1999). Such levels are well

below those requiring classification as hazardous wastes. It appears that most

sludges would not be considered as hazardous, even if the WHO guideline value

of 0.01 mg/L for arsenic in drinking water were used.

Scientistswhoworkwith arsenicmitigation have long recognized the high capacity

of the sea as a sink for arsenic containments. The arsenic in the sea is absorbed mostly

by marine algae and marine animals. Inorganic arsenic in marine waters is trans-

formed into organic arsenic compounds by marine fauna and flora. Seaweeds, as well

as seaweed-eating animals, contain high concentrations of arsenosugarswhereas other

marine filter-feeding animals, such as shrimps and lobsters, contain high concentra-

tions of arsenocholine. These organic arsenicals are considered nontoxic and are quite

safe for marine animals. Phytoremediation studies have demonstrated that organic

arsenic residues accumulated in plants, even at high concentrations, are sufficiently

low in toxicity to allow them to be disposed in the sea.

Summary

Groundwater contaminated with arsenic must be treated to meet stringent drinking

water standards or guideline values. In recent years, several reliable, cost-effective,

and sustainable treatment technologies have been developed, although improve-


ments will continue to emerge as work continues. All treatment technologies work

by concentrating arsenic at some stage of treatment. Large-scale use of arsenic

removal systems generates arsenic-rich treatment wastes, and indiscriminate dis-

posal of these sizable wastes may lead to environmental pollution. Safe disposal of

arsenic-rich media is a growing environmental concern that needs to be addressed.

For the developing world, arsenic-contaminated water requires some form of

treatment to be sufficiently safe for consumption by local populations. Such treat-

ment is particularly important where arsenic [particularly as As(III)] levels in raw

water exceed 200 mg/L. At this level and above, >95% removal efficiency is

required to produce water that meets international standards, an unlikely result in

many locations. Alternative sources for securing safe water may also include

rainwater harvesting, use of uncontaminated (filtered) surface waters, and water

extraction from new deep tube wells and dug wells. There are disadvantages

attendant to using these alternative water sources. For example, rainwater has few

mineral salts and is subject to contamination from air pollution or by microbes,

including pathogens. Similarly, surface waters, e.g., pond waters, or water from dug

wells may require purification before use. Excessive pumping from deep tube wells

may lower the water table sufficiently to allow entry of arsenic-contaminated

waters from shallower horizons. Bioremediation and phytoremediation are more

suitable to developing countries where sunlight is plentiful. In such countries, plant

biodiversity is also great and may allow identification of plants suitable for biore-

mediation. In addition to removing arsenic from water, phytoremediation can also

provide economic benefit to the people who apply the methods.

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