By combining a sequential P-extraction with batch experiments the presented investigations clarify the mechanisms of (enhanced) biological phosphate elimination and especially to answer the following questions: are any physicochemical mechanisms involved in the biological phosphate elimination? are there any interactions between biological and physicochemical unit operations? If so, are these of synergistic or antagonistic nature?

The following main results were found:

Results of batch experiments:

– calcium decreases under anaerobic conditions; this indicates Ca-P-precipitation.

– magnesium increases under anaerobic conditions and decreases under aerobic conditions just as phosphate does; there is a high correlation between dissolved magnesium and dissolved phosphorus. Therefore magnesium seems to participate in biological mechanisms (counter-ion of polyphosphates).

– concerning the total P elimination efficiency both unit processes seem to have a synergistic effect (biologically mediated P-precipitation).

Results of P-fractionations:

– there is a certain amount of particulate physicochemically bound phosphorus, which should not be neglected.

– although the biologically bound phosphorus dominates (as a rule), this amount depends on the concentration of the readily biodegradable COD significantly.

– during the cyclic P-release/P-uptake not only transitions from soluble phase to particulate phase but alsotransfers within the particulate phase take place.

– the total P-content of the activated sludge and the sum of the Non Reactive Phosphorus-fractions (without Phase Separation-Non Reactive Phosphorus) seem to be suitable to characterize the capacity of the biological phosphate elimination.

Conclusions:

Physicochemical mechanisms take part in the enhanced biological phosphate removal and they should be taken into consideration. Therefore they should be included into the deterministic model development.