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.