Prediction of recycle phosphorus loads from anaerobic digestion

In wastewater treatment plants with enhanced biological phosphorus removal, recycle phosphorus loads from sludge digestion have to be considered. Dissolved phosphorus from polyphosphate hydrolysis and degradation of organic solids is partly precipitated and adsorbed during sludge stabilization. Calcium ions and magnesium ions as well as aluminosilicates (zeolite A) from detergents are mainly involved in these processes. Experiments in a laboratory digester with pure excess sludge and mixed sludge show the extent of the fixation and indicate that recycle loads are minimized at a level of below 10%, if primary sludge is present. Even the use of prefermented primary sludge did not significantly influence this fraction. In stabilization of pure excess sludge, the dissolved phosphorus fraction comes to 20–25%. A mathematical model is presented which combines the involved processes of phosphorus release and refixation. The availability of calcium for precipitation processes is an influential parameter and may change under different wastewater conditions (hardness, zeolite content) and operation modes of the activated sludge plant (sludge age). Iron is only available for phosphorus 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 detailed information about the fresh sludge composition with emphasis on the content of polyphosphate and inorganic phosphorus in the excess sludge.