PREDICTION OF RECYCLE PHOSPHORUS LOADS FROM ANAEROBIC-DIGESTION

In wastewater treatment plants with enhanced biological phosphorus rem oval, recycle phosphorus loads from sludge digestion have to be consid ered. Dissolved phosphorus from polyphosphate hydrolysis and degradati on of organic solids is partly precipitated and adsorbed during sludge stabilization. Calcium inns and magnesium ions as well as aluminosili cates (zeolite A) from detergents art mainly. involved in these proces ses. Experiments in a laboratory digester with pure excess sludge and mixed sludge show the extent of the fixation and indicate that recycle loads ate minimized at a level of below 10%, if primary sludge is pre sent. Even the use of prefermented primary sludge did not significantl y influence this fraction. In stabilization of pure excess sludge, the dissolved phosphorus fraction comes to 20-25%. A mathematical model i s presented which combines the involved processes of phosphorus releas e and refixation. The availability of calcium for precipitation proces ses is an influential parameter and may change under different wastewa ter conditions (hardness, zeolite content) and operation modes of the activated sludge plant (sludge age). Iron is only available for phosph orus precipitation in digestion after deducting the amount needed for the fixation of sulphur. The model allows the prediction of phosphorus recycling under different operation conditions and the comparison of process alternatives, For this purpose it is necessary to have detaile d information about the fresh sludge composition with emphasis on the content of polyphosphate and inorganic phosphorus in the excess sludge . (C) 1997 Elsevier Science Ltd.