THE INFLUENCE OF COAGULANTS AND PRECIPITANTS ON THE BEHAVIOUR OF SEWAGE SLUDGE ON AGRICULTURAL LAND
During the waste water process, coagulants are used to remove phosphorus and BOD and to stabilise the process. The coagulants will accumulate in the sludge mainly as metal phosphates and a minor portion as metal hydroxides. These elements will influence the behaviour of the sludge when it is disposed of on agricultural land.
The availability of phosphorus in soils for plant uptake is determined by chemical equilibrias in the soil, the solubility of the products and the pH of the soil. Phosphorus in sludge as iron or aluminium phosphates, will enter the soil system as precipitated phosphorus. Since the soil is supersaturated with aluminium hydroxide over aluminium phosphates, the sludge phosphorus will be released and enter the plant-soil system as ordinary fertilisers.