ARSENIC REMOVAL Case History

1
ARSENIC REMOVALCase History

• Milos Markovic

2

• Arsenic removal

35000 m3/day Plant in Subotica-SERBIA
3

• 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 AsO33-

• As shown in Figure, which contains solubility
  diagrams for As(III) and As(V), ionic forms of
  arsenate dominate at pH gt3, while arsenite is
  neutral at pH lt9 and ionic at pH gt9

4
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.
As can be seen on the Figure, removal efficiency
is greater in the case of iron salt for larger pH
range
5

• 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.

6
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

• 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

7
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
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.
8
Influence of the iron content in raw water
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.
9
Fe2SO4 dosing before both stage filters
As iron dose before I stage increased from
0.7-2.0 mg/l, the efficiency of arsenic removal
in I phase filters didnt 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.
10
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.
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.
11
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.
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.
12
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
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).
13
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.
14
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

15
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