Mareeba Shire Council recently engaged Amiad Water Systems to design, construct and commission a 6-10L/s arsenic filtration plant for the township of Chillagoe in Far North Queensland.
The town sources its water from a local bore field which suffers from arsenic levels in the range of 0.010-0.020mg/L, exceeding the Australian Drinking Water Guidelines.
The contract specified that the plant must achieve a target level of arsenic of less than 0.005mg/L in the product water. To achieve the target Amiad proposed a system of chlorination, ferric chloride dosing, DMI-65 catalytic media filtration and cartridge filter polishing. The supplied plant is fully automatic and controlled by an Allen Bradley PLC with a Schneider PC/touch screen loaded with Citect software, for local and remote operation.
Treatment process
Chlorination: in groundwater, arsenic occurs predominantly as arsenite As (III), and requires conversion to arsenate As (V) by chlorination to enhance the effectiveness of the filtration process. Chlorine also acts as a catalyst for the DMI-65 media, and is required for its regeneration to re-establish the oxidizing environment on the surface of the media.
Ferric chloride dosing: arsenic can bond with iron salts in the water and with metal based coagulants such as ferric chloride. Ferric chloride is dosed such that there is a sufficient reservoir of iron for arsenic to form complexes and precipitants with the iron salts via the chemical processes of precipitation, co-precipitation and adsorption, which can then be filtered.
DMI-65 catalytic media filtration: DMI-65 is a manganese dioxide (MnO2) infused media whose surface acts as a good oxidant and is effective in removing both arsenite and arsenate, as well as iron/arsenic complexes and precipitants. The DMI-65 media filters are periodically backwashed and rinsed based on either pressure differential across the media filters, or on time, whichever occurs first.
Cartridge filters: 1 micron cartridge filters were installed to polish the product water and provide a final barrier to the precipitated arsenic.
The arsenic filtration plant, designed and constructed by Amiad utilising DMI-65 media, was commissioned in March, 2015, and has been successfully reducing the arsenic to 0.001 mg/L, making it suitable for consumption by the community of Chillagoe, Queensland.
