FERMENTATION OF RETURN ACTIVATED SLUDGE TO ENHANCE BIOLOGICAL PHOSPHORUS REMOVAL

Biological phosphorus removal (BPR) activated sludge systems are configured to create conditions favorable for the growth of phosphorus accumulating organisms (PAOs). The generation of a sufficient quantity of PAOs is also contingent on the presence of volatile fatty acids (VFAs) in the anaerobic zone - the greater the amount of VFAs, the more the PAO growth (and the higher the BPR efficiency). In plants where significant fermentation occurs in the collection system, the incoming wastewater can have a relatively high proportion of VFAs, and will be well suited for BPR. However, if the influent wastewater does not have a sufficiently high VFA content, then supplemental addition of VFAs may be required for efficient BPR. Traditionally, pre-fermenters have been used for this purpose, generating the supplemental VFAs through fermentation of primary sludge. The pre-fermentation step has typically utilized primary sludge. This paper describes the successful experimentation with the fermentation of return activated sludge (RAS) to enhance BPR at the 40-mgd Truckee Meadows Water Reclamation Facility (TMWRF) in Reno, Nevada. This facility currently utilizes a Phostrip process with lime addition to remove P. Fermentation of a portion of the RAS stream was proposed as a method for augmenting the VFA content of the influent and making BPR viable. A full-scale study was conducted to investigate the feasibility of this proposal. Results of this study have been promising and suggest that RAS fermentation can be used to allow the plant to operate in the BPR mode. The resulting elimination of chemicals will save the plant approximately 250,000 annually.