Transcript Arsenic Removal from Drinking Water

ARSENIC REMOVAL
Case History
Milos Markovic
Arsenic removal
35000 m3/day
Plant in Subotica-SERBIA

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 AsO43-. Similarly, arsenite exists in five forms:
H4AsO3+, H3AsO3, H2AsO3-, HAsO32-, and AsO33100
x, %
H3AsO4
90
H2AsO4HAsO4--
80
H3AsO3
90
H2AsO3-
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
HAsO3--
0
0
4

100
x, %
5
6
7
8
9
10
11
12
13
pH,-
4
5
6
7
8
9
10
11
12
13
pH,-
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
Iron removal can be used to remove
arsenic from drinking water
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.
100
Stepen uklanjanja
As, %%
efficiency,
Removal
90
80
70
As can be seen on the
Figure, removal efficiency
is greater in the case of
iron salt for larger pH
range
60
50
Al2(SO4)3
40
Fe2(SO4)3
30
20
10
0
5
6
7
8
9
10
pH,-
 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
NH4: 0.64 mg/l
Fe: 0.62 mg/l
Turbidity: 3.80 NTU
 Oxidant: chlorine in front of first stage
 Coagulant: Fe(SO4)2 in front of second 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:
[As], ng/l
[As] in raw
w ater, ng/l
[As] in treat.
w ater, ng/l
MCL=10 ng/l
140
120
100
80
60
40
20
0
1
1.5
2
2.5
3 D, mg Fe/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
[As] in raw
w ater, ng/l
[As] outlet from
II phase, ng/l
Dose in II
phase, 10 mg/l
MCL=10 ng/l
[As], ng/l
120
100
80
60
40
20
0
0.5
0.6
0.7
0.8
[Fe] in raw
water, mg/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
[As], ng/l
[As] outlet from
I phase, ng/l
[As] outlet from
II phase, ng/l
MCL=10 ng/l
50
45
40
Dose in II phase
35
30
25
20
15
10
5
0
0.5
1.0
1.5
2.0
D, mg Fe/l
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.
[As] outlet from
II phase, ng/l
As,ng/l
μg/l
[As],
14
MCL=10 ng/l
12
10
By decreasing of iron dose
from 2.8 to 1.3 mg/l, there
was minimum on the As
outlet concentration curve.
8
6
4
2
0
1.0
1.5
2.0
2.5
3.0
DFe, mg/l
This implies that the optimum
dose of ferry sulphate is 1.51.6 mg/l, if content of arsenic in
raw water is not higher than
100 μg/l.
Filtration cycle
I stage filters
During 48 hours, we
investigated
iron
concentration on outlet
from I phase filters,
under
operating
capacities of 50 and
70 l/s.
[Fe] outlet from I
phase, for
Q=50%, mg/l
[Fe] outlet from I
phase, from
Q=70%, mg/l
MCL=0,3 m g/l
[Fe], mg/l
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20
30
40
50
Time, h
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.
II stage filters
For the II phase filters,
we investigated iron
concentration on outlet
from II phase filters,
under
operating
capacities from 50 and
100 l/s
Series1
[As] outlet
from II phase,
mg/l 14
Series2
Series3
12
MCL=10
ng/l
10
8
6
4
2
0
0.0
0.1
0.2
0.3
[Fe] outlet
from II
phase, mg/l
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).
The following diagram shows filtration cycle duration (for 50 % and 100 % of
nominal flow per line) by using previous criteria.
For Q=50%
For Q=100%
[Fe] outlet
from II phase,
mg/l 0.30
The treshold
0.25
0.20
0.15
0.10
0.05
0.00
0
3
5
8
10
13
15
18
20 Time, h
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
Removal of arsenic from raw water is achieved below the desired value
(and in most cases below limits of detection) if:
 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
New plants – under construction
 Kanjiža
5200 m3/d
 Horgoš
3500 m3/d
 Subotica II
17500 m3/d
 Vršac
32000 m3/d
 Apatin
10500 m3/d
 Indjija
13000 m3/d