The production of 100,000 tonnes per year of synthetic rutile from the processing of ilmenite, coal and copperas (iron sulphate) produces 200 cubic metres per hour of effluent water. A 2.0 hectare biological filter, comprising natural and man- made wetlands, has been built into the last stage of the effluent water treatment system. It is an area of shallow water (<30cm), densely vegetated with indigenous aquatic plants. normal effluent water meets stringent quality limits with respect to ph and heavy metals. however, equipment failure and operating errors can allow partially treated effluent to flow out of the final storage dam._x000d_>
In these cases, pH can range from 2.5 to 10.8, with high Fe, Mn, Zn and other metal contents. The biological filter accepts all liquid discharges from the final storage dam. It has a residence period of 10-12 hours and a flow length of approximately 1400m._x000D_
After 16 months of operation, its effective pH buffering and metal removal capacities have been proven to last for more than 4 days. The effectiveness of the biological filter is beyond dispute, although the mechanisms involved are not well understood. It is an efficient, low cost emergency back-up to the engineered water treatment systems but, like any filter, it may need to be cleaned during its lifetime. The use of biological filters in effluent water treatment systems is strongly recommended to provide a natural back-up in case of human or mechanical/chemical system failure.