EFFECT OF ANAEROBIC SRT ON COMPLETE PHOSPHATE REMOVAL IN A POST-DENITRIFICATION SYSTEM

A continuous flow post-denitrification (Dephanox type) lab-scale system fed with raw wastewater was operated in order to investigate the effect of the anaerobic solids retention time (anaerobic SRT) on phosphate removal. The presence of an internal settler just after the anaerobic tank enhanced the anaerobic SRT. Additional enhancement was provided by accumulation of solids into the internal settler under extreme operational conditions that seemed to be optimal for nutrients removal. Batch experiments with Dephanox sludge showed that the particulate COD fraction contributed with an additional 50% of phosphate release, thus an additional VFAs production can be inferred. In the case of the Dephanox sludge, the enhanced anaerobic SRT assisted effectively to achieve complete phosphate removal. As reference, the same batch experiments with AO and A<SUB>2</SUB>O sludge from a full scale plant fed with primary effluent were carried out. The results showed that particulate COD fraction did not contribute significantly on the VFAs production. The combination of long SRT and abundant particulate COD apparently promoted fermentative organisms and denitrifying phosphorus accumulating organisms (DN-PAOs) acting in a syntrophic association on phosphorus removal.