This odorous air is then pumped through a soil filter where the odorous compounds in the air stream are absorbed on to soil particles and then destroyed by naturally occurring soil bacteria. Sludge treatment Raw sludge from the primary sedimentation tanks and the biological solids from the secondary sedimentation tanks is pumped to the three digesters. These digesters have a floating cover to exclude air, and are built half above ground and half below ground and the upper half surrounded by a brick veneer for heat insulation. Each digester holds 5000m 3 of sludge and each is heated to 37 o C. Bacteria that thrive under these conditions break down the sludge over a period of days releasing carbon dioxide and methane, and in the process change the highly putrescible sludge into a practically odourless free draining material that is then stored in open lagoons before being used on land as a soil conditioner and fertiliser. The methane produced is used as a fuel in engines for driving pumps and in engines driving alternators to provide electric power for the plant.
Heat is recovered from the engines and used to heat the digesters.
The methane is also used to fuel boilers when extra heat is required for the digesters or for building heating. Some of the methane is purified and compressed for use as a vehicle fuel, very similar to compressed natural gas. Oxidation ponds The 230 hectares of ponds serve the dual purpose of removing a portion of the remaining pollution load and greatly reducing the number of potentially disease causing micro-organisms. Most of the remaining load is removed by the action of helpful micro-organisms operating in both the overlying water and in the bottom mud. Oxygen is maintained in the ponds by the combination of wind induced surface aeration and the photosynthetic action of naturally occurring algae.