Activated sludge plants for enhanced biological phosphorus removal (EBPR) are often disturbed by short periods of low organic loading. Depending on the exact nature of the disturbance this may result in a partial or complete depletion of the internal PHB stores. PHB and phosphate measurements in a pilot-scale EBPR process show that recovery from such disturbances is slow and temporarily results in high phosphate concentrations. The measurements strongly suggest that the main reason for this slow recovery is a dependency of P-uptake on a slowly rising level of PHB. In a number of batch experiments this dependency of P-uptake on PHB was clearly shown. Also, based on the results of these batch experiments, a more detailed analysis was made of the effect of organic loading and aeration time on EBPR recovery. It is concluded that to obtain EBPR recovery, the aeration time should be carefully adjusted to the organic loading, particularly if the organic loading is low.

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