ARSENIC REMOVAL Case History. Milos Markovic. Arsenic removal. 35000 m3/day Plant in Subotica -SERBIA.
Plant in Subotica-SERBIA
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
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.
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.
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 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.
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.
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).
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.
Removal of arsenic from raw water is achieved below the desired value
(and in most cases below limits of detection) if: