196_influence of ph in boron removal

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Influence of pH on boron removal by reverse osmosis in secondary

urban effluent

M. J. Sasa*, M. Folch*, E. Huertas *, M. Salgot* and G. Oron**

* Soil Science Laboratory, Faculty of Pharmacy, University of Barcelona, Joan XXIII s/n, 08028, Barcelona,Spain.

(E-mail: [email protected] ; [email protected] ; [email protected] ; [email protected] )

** Ben Gurion University of the Negev, Environment Water Resources, Blaustein Institutes for Desert Research,84990, Midreshet, Ben-Guiron, Israel.

(E-mail: [email protected] )

AbstractSpanish Law, establish a maximum boron concentration, of 0.5 mg/L in reclaimed water to be

reuse in agriculture purposes. For reclaimed water with higher values of boron concentrations, it isnecessary to add an additional treated. One alternative is the use of membrane technology, as forexample reverse osmosis.

The aim of the research was to study and compare the retentate of boron by reverse osmosis underdifferent pH conditions (from 7 to 10), from diverse reclaimed water, in order to fix the SpanishLaw. The pH behavior of the permeate and retentate water were study from three secondary

effluents: activated sludge, modified infiltration-percolation, and constructed wetland .Boron removal by reverse osmosis depends on the pH. The best boron retentate is achieved whenpH of the feed water is above 9.24. Better boron removal is also obtained by using secondarymunicipal wastewater instead of distilled water, due to the ionic species presents.

The research demonstrates that pH behavior in the permeate flow depends on the treatment systemused to generate the reclaimed water. Thus the regeneration of water from natural treatmentsystems (as for example: modified infiltration-percolation or constructed wetlands systems), is a

better alternative than with conventional treatment systems (as activated sludge), due to a lowerpermeate pH value obtained after boron removal.

KeywordsBoron, pH, Reverse Osmosis, Reclaimed Water

INTRODUCTION

The reuse of reclaimed water in agriculture is an alternative in areas affected by water shortages (in

terms of quantity and quality). Reclaimed water quality should be established in order to prevent

contamination or even crop damage. One of the chemical parameters regulated by the Spanish Law

(Royal Decree, RD 1620/2007) is the boron concentrations in reclaimed water. The RD 1620/2007

set up a maximum boron concentrations of 0.5 mg/L in reclaimed water for being reused in

agriculture.

Boron is an essential element for plant growth and it is one of the most important micro-nutrients

for the plants. As micro fertilizers, it occupies the first place in its effectiveness among other micro-

elements. But it is beneficial to plants only in small quantities, and its excessive amounts areinjurious and even lethal to plants. Regular use of irrigation water with more than 1 mg B/L isharmful for most of the plants (Melnyk et al., 2005).

Boron normally occurs in natural water at concentrations lower than 1 mg B/L (Harp, 1997),

although sea water average concentration is 5 mg B/L (Riza, 2004). Boron is often considered as an

indicator of sanitary pollution in wastewaters with concentrations around 3 mg B/L. As an

anthropogenic source, boron has different origins: products of agricultural(microfertilizers);

domestic (soaps, detergents, laundry powders); glass manufacturing; and industrial production

(insecticides, fire retardants, anti-freezing formulations, insulation and textile grade fibber, neutron

absorber, mild antiseptics, cosmetics, medicines, pesticides) (Harp, 1997; Magara et al., 1998; Fox

et al., 2002; Sahin, 2002; Yilmaz et al., 2005).

Boron concentration in wastewater can reach over 60 mg/L at ceramic industries (Ripolles et al., 1992), or it can be found in around 1.2 mg/L in urban sewage (where industrial and domestic

wastewater is mixed) (Esteller, 1994).

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Currently at Spain, a high proportion of the industrial sector treats their own wastewater at onsitetreatment plants, before discharging their wastewater or reusing it for irrigation. Conventional

sedimentation and biological treatment remove little, if any, boron from wastewater, and chemicalscommonly used in the water treatment also have little or non-effect on the boron levels in water

(Sahin, 2002). Technologies used to remove boron from wastewater are different and include ionexchange, coagulation, electrocoagulation, electrodialysis, reverse osmosis (RO) and nanofiltration

(Sahin, 2002; Yilmaz et al., 2005; Dydo et al., 2005; Cengeloglu et al., 2008).

Reverse osmosis (RO) equipment is able to separate pollutants or particles until a size of 0.0001

µm. Therefore, this technology can lead to remove salts, hardness, pathogens, turbidity, pesticides,

synthetic organic compounds, and most drinking water pollutants (American Water Works

Association, 1998).

In recent years, membrane manufactures have developed RO membranes with boron rejections of

91 96 % of boron retentate. However, most of the current desalination plants have to implement

the additional treatment steps such as pH adjustment of feed water, post-treatment of RO permeatewith ion exchange or several pass stage of permeate in order to improve boron rejection.(Cengeloglu et al., 2008).

The objective of this paper was to investigate and compare the retentate of boron by RO technology

under different pH conditions. In order to carry out this work, different types of secondary effluents

were studied: activated sludge, modified infiltration-percolation and constructed wetlands systems.

METHODS

Reverse Osmosis System

The RO system used in this study (GE model E2-1400-5) is installed over a mobile platform that

includes two polyethylene tanks: one of 25 L of capacity (feed tank) and another one of 85 L

(cooling tank). Feed tank is connected to a stainless steel rotary vane pump that feeds twopolyamide RO membranes (Desal model AG2540TF) in series. The RO system was modified and

several valves were installed to allow self operation of membranes. During this research only oneRO membrane was used. Feed water went through the rotary vane pump and, as a consequence,

increases the inlet operation pressure that allows to obtain the permeate.

Figure 1. Schematic diagram of the RO system used in this study.






Retentate increased its temperature and was sent to the cooler tank. Retentate and permeate were

mixed up at feed tank. Figure 1 presents an schematic diagram of the described RO system.

Membranes characterizationA commercial RO membrane (GE Osmonics, Desal model AG2540TF) was used in this research.

The characterization of this polyamide membrane, initially thought for brackish water desalination,is summarized in Table 1.

Table 1. Characterization of RO membrane.

Parameter Characteristic

Configuration Spiral wound

Activated area (m2) 2.6

Typical operation pressure (kPa)

Maximum pressure (kPa)

Maximum pressure drop (kPa)

1,379

3,103

69 (per element)

345 (per vessel)

Chlorine tolerance (mg/L) 1,000Optimum retentate pH

Operating pH

Cleaning pH

7.0 7.5

4.0 11.0

2.0 11.5

Feed turbidity (NTU) <1

Feed silt density index (SDI) < 3

Water permeability (cm/s · atm) 10-11 x 10-5

Experimental proceduresThe operational conditions of RO equipment were: inlet pressure of 1,200 kPa, feed water

temperature from 22 to 30 °C, conversion factor of RO was 0.27, and nominal inlet flow rate 270L/h.

This research was conducted through two main experiments. The first one was carried out with

distilled water supplemented with boric acid (3 mg B/L) and a pH range between 6.5 to 10.0. The

second one was performed with different secondary effluents with the addition of boric acid (boron

concentration between 4 and 5 mg B/L) and a pH from 7.5 to 10.0.

Tested secondary effluents came from different wastewater treatment plants which include non-

conventional (modified infiltration-percolation and horizontal subsurface constructed wetlands) and

conventional technologies (activated sludge). Effluents were always filtered with a 5 micron filter

before starting the operation of the system. The RO system worked 5 hours/day. Samples from feed water, retentate and permeate were taken

each hour. Boron concentration, pH, electrical conductivity and temperature were measured in each

sample. Pressure of the system (inlet and outlet of membrane) was monitored each hour too. After

samples were taken, pH was adjusted by adding NaOH (2.5M) at feed water tank.

Cleaning of membrane was necessary each time that inlet pressure increased to 1,450 kPa. This

process was carried out with different commercial products from CIBA (IRGATREAT AS 2203,

IRGATREAT MF 2252 and IRGATREAT BCO8), 1 mL of each product was added at the feedtank and they were mixed with 25 L of distilled water. Afterwards, the RO system worked from 60

to 90 minutes. Finally, the system was rinsed with distilled water for 90 minutes before starting a

new cycle. After the cleaning process, pressure dropped to 1,200 kPa.






Parameters Analysis

The determination of boron in feed, retentate and permeate water were performed by the CarmineMethod (Standard Method 4500B; Standard Methods for the Examination of Water and

Wastewater, 2005). The absorbance of the boron-carmine complex at the wavelength of 550 nmwas measured using a Cecil 7200 model spectrophotometer.

The electrical conductivity and the pH were measured by a Crison conductimeter model 525 and

Crison pHmeter model 2002, respectively.

RESULTS AND DISCUSSION

Effect of pH on boron retentate at distilled waterThe effect of pH on boron retentate working with distilled water is presented in Figure 2. Results

showed an important influence of pH on boron retentate. At pH values next to neutrality, boron

retentate was around 50%, whereas a pH of 9.5 revealed an increase in boron retentate that reached

80%. The results obtained in this study are in accordance to those of other authors (Rodrigo and

Peñate, 2007; Huertas et al., 2008).

Figure 2. Representation of boron retentate (%) in relation with pH value of distilled water

The value of the dissociation constant of boric acid (pKa) is 9.24, which means that boric acid is the

molecular species predominant when pH is below 9.24. Boric acid permeates easily through the RO

membrane, which results in low boron retentate. At pH above 9.24, borate ion becomes thedominant species and the retentate of boron is increased (Rodríguez et al., 2001). Previous studies

have shown that retentate of borate ions by RO membranes is high due to size and chargeexclusions of the charged, hydrated ionic species (Seidel et al., 2001; Sagiv and Semiat, 2004; Bick

and Oron, 2005). On the other hand, boric acid species are neutrally charged and are not hydrated in

aqueous solutions. Hence, this species can pass readily through RO membranes because of its

relatively small size and lack of electrical charge (Huertas et al., 2008).

Effect of pH on boron retentate at secondary municipal wastewater effluentThree different types of secondary effluents from municipal wastewater treatment plants were

study. Table 2 presents the average and the standard deviation of studied parameters of secondary

effluents (modified infiltration-percolation, activated sludge and constructed wetlands).






Table 2. Characteristics of secondary effluent

Parameters

Infiltration-

Percolation

(IP)

Activated

Sludge

(AS)

Constructed

Wetland

(CWL)

pHE.C. (µS/cm)

COD (mg/L)Suspended solids (mg/L)

NH4+

- N (mg/L)NO3

- - N (mg/L)

Total Coliforms (Ulog/100 mL)

7.1 ± 0.11262 ± 55

99 ± 2413 ± 6

18 ± 310 ± 5

4.5 ± 0.3

7.6 ± 0.12140 ± 440

108 ± 3611 ± 4

17 ± 158 ± 7

5.4 ± 0.6

7.78 ± 0.023670 ± 550

188 ± 2610 ± 3

NRNR

5.8 ± 0.2

Figure 3 presents boron removal working with RO treating at different pH ranges. Results show thesame tendency explained when distilled water was used as feeding water. An improvement on

boron retentate is observed when pH is between 9.00 and 9.50 (75%) for all secondary effluents.

Figure 3. Representation of boron removal of different secondary effluents (modified infiltration-

percolation, activated sludge and constructed wetlands) in relation with pH

At higher values of pH (from 9.50 to 10.00), boron retentate increase above 85% for all tested

effluents. After the increase of pH value (up to 9) at feed water, it is necessary a post-treatment in

order to neutralize pH at permeate water if reclaimed water is going to be reused in agriculture.

Results demonstrate that retentate pH is proportional to feed water pH, independently on the type of secondary effluent. This is due to the conversion factor of RO (0.27).

Figure 4 shows the behavior of the pH in different permeates. The continuous line represent

retentate pH vrs feed water pH. Data above the line means that the pH of permeate is higher than

the pH of the feed or retentate (see figure 4a and 4c). Meanwhile for the activated sludge effluent,

the permeate pH shows a higher pH value than the feed pH (Figure 4b).

Once a RO treatment system is applied for boron removal, the permeate pH is alkaline, so a post-treatment is necessary in water reuse for irrigation purposes. Thus secondary effluent rendering

lower pH values after boron removal should be chosen. Consequently, less chemicals products will

be required to adjust the pH and, therefore, the cost of the treatment will be reduce (savings inpurchasing them).







Feed water pH

7,5 8,0 8,5 9,0 9,5 10,0 10,5

6

7

8

9

10

11

Retentate pH vrs Feed water pH

Permeate pH vrs Feed water pH

Permeate pH = -59.5704 + 14.8593 (Feed water pH) 0.799 (Feed water pH)2

r2 = 0.5740

P < 0.0001

Feed water pH

7,5 8,0 8,5 9,0 9,5 10,0

6

7

8

9

10

11



Permeate p H= -104.4546 + 24.4346 (Feed water pH) 1.2976 (Feed water pH)2

r2

= 0.9521

P < 0.0001

A. Modified infiltritation-percolation B. Activated Sludge

Feed water pH

7 8 9 10

5

6

7

8

9

10

11



Permeate pH = -28.9803 + 6.9249 (Feed water pH) 0.3049 (Feed water pH)2

r2

= 0.9535

P < 0.0001

C. Constructed Wetland

Figure 4. pH behavior of permeate flow as a function of pH feed flow, in secondary effluent. Permeate Salinity

Salinity was also monitoring during this research, by measuring the electrical conductivity. Figure 6shows the evolution of this parameter during RO operation when treating the effluents of modified

infiltration-percolation, activated sludge and constructed wetlands. Permeate from non-conventional

technologies (modified infiltration-percolation and constructed wetlands) presents a permeate

salinity lower than activated sludge effluent. Water composition of different effluents could be themost probable explanation for this phenomenon. Natural technologies are capable to remove part of

the anions and cations of wastewater, whereas conventional treatment is not able to affect the

concentration of these ions. Further studies should be carried on in order to analyze the composition

of these effluents.






Time (hours)

0 20 40 60

Salinityatpermeate(mg/L)

0

10

20

30

40

50

Modified Infiltration-Percolation

(Salinity) = 16.9019 + 0.4259 (Time) 0.0093 (Time)2

r2

= 0.2419

P = 0.0002

Time (hours)

0 10 20 30 40


0

20

40

60

80

100

120

Activated Sludge

(Salinity) = 38.3717 - 0 .5980 (Time) + 0 .0301 (Time)2

r2

= 0.1343

P = 0.1235

6A. Modified Infiltration-Percolation 6B. Activated Sludge

Time (hours)

0 10 20 30


0

10

20

30

40

50

60

70

Constructed Wetlands

(Salinity) = 36.3433 3.1706 (Time) + 0.1045 (Time)2

r2 = 0.2741

P = 0.0214

6C. Constructed Wetlands.

Figure 6. Electrical conductivity evolution of permeate of different secondary effluents (modified

infiltration-percolation, activated sludge and constructed wetlands)

CONCLUSIONS- Boron can be removed from both distilled water and secondary municipal wastewater

effluents by RO system. Boron retentate is improved with pH values above 9.24. If pH isincreased, a post-treatment (neutralization) is necessary in water reuse for agriculture.

- Boron removal in distilled water is lower than in secondary municipal wastewater effluent,due to the presence of ionic species at the SMW that improve boron retentate.

- The behaviour of pH at the permeate flow in RO system, depends on the treatment systemused to treated the wastewater. The regeneration of water from natural treatment systems (as

infiltration-percolation, or constructed wetlands), are a better alternative than those from

conventional treatment systems (as activated sludge) due to a lower permeate pH after

removal the boron. This fact means that less chemicals products are going to be used as a

post-treatment for neutralize the permeate flow.

ACKNOWLEGEMENTS

This work has been possible because of the collaboration of CICYT (2006-13523-C02-01).












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196_influence of ph in boron removal

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