RAS Fermentation to Enhance Biological Phosphorus Removal

Attaining satisfactory P removal is difficult in EBPR systems where the influent COD:TP and COD:TKN ratios are low. In-process fermentation of EBPR process mixed liquor is a means for improving P removal performance as an alternative to adding an external carbon source. One approach is to reduce mixing intensity in unaerated zones of an existing system to retain a blanket of fermenting sludge on the base of the tank. Alternatively a purpose-built RAS fermentation zone can be incorporated in the process configuration. This has been done in the upgrade of the Loughborough WWTP considered in the paper.Consideration of the stoichiometry of the processes occurring in fermentation of EBPR mixed liquor indicate that there should be a net benefit in terms of P removal. However, there are many interactions between biomass decay and the associated lysis of N and P, nitrate load to the unaerated zones, temperature, and SRT of the EBPR system. This necessitates careful optimization to derive the full benefits. In the case of Loughborough, RAS fermentation appears to improve removal by approximately 1 mgP/L. However, the full extent of the benefits of RAS fermentation at this plant will become evident only once the system is optimized.