ARSENIC REMOVALCase History

Milos Markovic

Arsenic removal

35000 m3/day

Plant in Subotica-SERBIA

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x, %H3AsO3

H2AsO3-

HAsO3--

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Arsenic is a common, naturally occurring drinking water contaminant thatoriginates from arsenic-containing rocks and soil and is transported to natural waters through erosion and dissolution

Arsenate exists in four forms in aqueous solution based on pH: H3AsO4,

H2AsO4, HAsO42-, and AsO4

3-. Similarly, arsenite exists in five forms:

H4AsO3+, H3AsO3, H2AsO3-, HAsO3

2-, and AsO33-

As shown in Figure, which contains solubility diagrams for As(III) and As(V), ionic forms of arsenate dominate at pH >3, while arsenite is neutral at pH <9 and ionic at pH >9

Conventional coagulation/filtration is a common water treatment methodology used to remove suspended and dissolved solids from source water. Alum and iron (III) salts, such as sulphates, are the most common coagulants used for drinking water treatment.

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Fe2(SO4)3

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As can be seen on the Figure, removal efficiency is greater in the case of iron salt for larger pH range

Iron removal can be used to remove arsenic from drinking water

This process involves two major steps: (1) oxidation of reduced iron, Fe(II), to the relatively insoluble Fe(III) in order to form precipitates; and (2) filtration of the water to remove the precipitated iron hydroxides.

Two primary removal mechanisms exist: adsorption and coprecipitation. The following major steps occur when using iron removal for arsenic treatment: (1) the soluble iron and any As(III) are oxidized; (2) As(V) attaches to the iron hydroxides through adsorption and/or coprecipitation; and (3) the particle/precipitate subsequently is filtered from the water.

According to the inlet Arsenic and Iron concentrations, it can be used one and two stage filtration process. At lower Arsenic concentration in raw water, preoxidation and one stage filtration is in most cases sufficient; Iron content in raw water is sufficient for arsenic removal. At greater arsenic concentration, two stage filtration and iron salt dosing is necessary.

Water treatment plant in Subotica,Serbia

Plant capacity: 400 l/s

Inlet flow: 315-400 l/s

Key contaminants:

As: 71-115 μg/l Fe: 0.62 mg/l

NH4: 0.64 mg/l Turbidity: 3.80 NTU

Coagulant: Fe(SO4)2 in front of second stage

Oxidant: chlorine in front of first stage

Filtration: Culligan industrial filters, four leaf clover system, made of made of steel and protected by anti-corrosion coatings, a heavy layer of epoxy resin in the inside and synthetic paint on the outside. Multilayer filter has a selective mineral within filtering layers, for iron and manganese removal.

Filtration rate: 12 m/h

Influence of arsenic concentration in raw water on removal efficiency

On the next diagram, As concentration in the treated water versus inlet As concentration and Fe2SO4 dose is presented:

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1 1.5 2 2.5 3 D, mg Fe/l

[As], ng/l[As] in raww ater, ng/l[As] in treat.w ater, ng/lMCL=10 ng/l

Condition for these doses is that concentration of Fe in raw water has to be not less than 0.6 mg/l.Optimal doses of iron were in the range from 1.4-1.6 mg/l, if arsenic concentration is not higher than 100 μg/l.In case of extreme inlet As concentration, up to 130 μg/l, it become necessary to apply much higher doses of iron, up to 2.8 mg/l.

Influence of the iron content in raw water

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0.5 0.6 0.7 0.8 [Fe] in raw water, mg/l

[As], ng/l[As] in raww ater, ng/l[As] outlet fromII phase, ng/lDose in IIphase, 10 mg/lMCL=10 ng/l

When the content of iron in raw water was under 0.6 mg/l, the concentration of As at the outlet of the II stage exceeded maximal concentration level. In this case , it is necessary to add some quantity of iron before I stage filter.

Fe2SO4 dosing before both stage filters

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[As], ng/l[As] outlet fromI phase, ng/l[As] outlet fromII phase, ng/lMCL=10 ng/l

Dose in II phase

As iron dose before I stage increased from 0.7-2.0 mg/l, the efficiency of arsenic removal in I phase filters didn’t change much – arsenic concentration decreased from 43 to 39 μg/l. The dose of iron before II phase was constant, 1.4 mg/l.

It can be concluded that in case of iron content in raw water of 0.6 mg/l, was not necessary to dose Fe2SO4 before I stage filter.

Optimal Fe2SO4 dose

Optimal dose of ferric sulphate was examined for inlet arsenic concentration up to 100 μg/l. The salt doses were in the range of 1.3-2.8 mg/l.

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[As], ng/l[As] outlet fromII phase, ng/l

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As, μg/l By decreasing of iron dose from 2.8 to 1.3 mg/l, there was minimum on the As outlet concentration curve.

This implies that the optimum dose of ferry sulphate is 1.5-1.6 mg/l, if content of arsenic in raw water is not higher than 100 μg/l.

Filtration cycleI stage filters

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[Fe], mg/l[Fe] outlet from Iphase, forQ=50%, mg/l[Fe] outlet from Iphase, fromQ=70%, mg/lMCL=0,3 mg/l

Under previous conditions, iron ”break through” not happened. After 48 h, outlet concentration of iron from I phase filters were on the detection limit - 0,01 i 0,02 mg/l.

During 48 hours, we investigated iron concentration on outlet from I phase filters, under operating capacities of 50 and 70 l/s.

II stage filters

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[Fe] outlet from II

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For the II phase filters, we investigated iron concentration on outlet from II phase filters, under operating capacities from 50 and 100 l/s

On the previous diagram, it can be seen that the arsenic outlet concentration from II phase “breaks through” MCL level, when iron on outlet increases to 0.10-0.14 mg Fe/l. In this way, it is possible to established criteria for determining of the end of filtration cycle ( II phase).

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[Fe] outlet from II phase,

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The treshold

The following diagram shows filtration cycle duration (for 50 % and 100 % of nominal flow per line) by using previous criteria.

Duration of filtration mode is 18.5 hours, if operation flow is 50 l/s per line, and only 13 hours for capacity of 100 l/s. The optimum filtration cycle for II phase filters is 12-13 hours.

Conclusions

Inlet arsenic concentrations are not much higher than

130 μg/l (max. contract value)

Ferry sulphate dose is in front of the second line is 1.40 -

1.60 mg/l,

Dosing in front of the I phase is not necessary if the inlet

iron concentration is stable

Backwashing is performed once in 48 hours for I phase

filters, and once in 12 hours for II phase filters

Removal of arsenic from raw water is achieved below the desired value(and in most cases below limits of detection) if:

New plants – under construction

Kanjiža 5200 m3/d

Horgoš 3500 m3/d

Subotica II 17500 m3/d

Vršac 32000 m3/d

Apatin10500 m3/d

Indjija13000 m3/d