Arsenic Treatment Plant
In 1982 the Ministry of the Environment started construction of a new water collection, storage and treatment system to remove arsenic and heavy metals from groundwater at the site. Renovations were made to the former research building. A 9,084 cubic metre clay-lined equalization storage basin was constructed, along with an 80 metre long concrete cut-off dike, pumping stations, and a tile system for the collection of water. The plant was put into service in January 1983.
How it Works
Arsenic-contaminated groundwater, from different sources and at varying concentrations, is collected by the cut-off dike and collection piping. It is then pumped to the clay-lined equalization storage basin. The equalization basin has two functions. It balances the concentration of arsenic contaminated water, creating one homogenous mix; and it regulates the rate of flow to the treatment plant, allowing for more efficient operation. The water is then pumped from the basin to the arsenic treatment plant.
The water is treated through a chemical precipitation process using ferric chloride, lime, and a polymer. Three mixing tanks and a clarifier are used in the process. In the first mixing tank, ferric chloride is added at a ratio of 2.5 parts iron to one part arsenic. In this tank the mixture separates into ferric ions and chloride ions. The mixture flows from this tank to the second tank, where powdered lime is added to raise the pH level. The higher pH causes the arsenic and heavy metals to cling to the ions, and creates a fine, brownish precipitate known as ferric arsenate. In the third tank, a polymer, a substance of high molecular weight, is added to help the ferric arsenate form larger particles.
The water then goes to the clarifier. The large particles settle out quickly, sinking to the bottom of the clarifier in the form of a brownish, ferric arsenate sludge. The sludge is pumped from the bottom of the clarifier to a 50 by 40 metre underdrained sludge drying lagoon. Leachate from the sludge drying lagoon is pumped back to the equalization storage basin for re-processing. The treated water, with 99.5 to 99.9 per cent of the arsenic removed, is returned to the Moira River.
Arsenic Treatment Plant Efficiency
A total of 73,760 cubic metres of leachate (contaminated groundwater) was processed in 2007. The annual average arsenic concentration of the leachate was 88 mg/l and of the average plant effluent was 0.112 mg/l, resulting in an average removal efficiency of 99.9 %
In 2007, the total amount of arsenic removed through treatment is calculated at 5,804 kg.
