Controlling Magnesium and Phosphorus to Reduce Struvite Precipitation in Plant Processes and Increase Revenue

The recovery of nutrients from wastewater during treatment has gained attention both as a way of removing the nutrients as required by the plant's discharge permit and to make the recovered resource available for beneficial use. The Waste Activated Sludge Stripping To Remove Internal Phosphorus process (WASSTRIP patent pending) has been proposed to facilitate the nutrient recovery as well as to minimize the formation of struvite within the solids processing system. Benchtop testing was conducted at the Durham Advanced Wastewater Treatment Facility (AWWTF) to determine the rate of phosphorus and magnesium release within the waste activated sludge (WAS) tank as part of the WASSTRIP process. Testing included both endogenous release and release with the addition of volatile fatty acids (VFA) in the form of overflow from the plant's fermenter thickener.Results of the study reveal release rates were 5 times higher with VFA addition than those of endogenous release. However, the VFA concentration had little impact in the release rates when applied at ratios higher than 0.015 mg VFA / mg VSS (volatile suspended solids). The release of magnesium versus phosphorus occurred 0.20 – 0.25 milligrams of magnesium per milligram of total phosphorus which agrees with the results previous testing.Process modeling was used to describe the results of testing. The modeling results suggested lower anaerobic hydrolysis and a lower sequestration rate of phosphorus accumulating organisms (PAO). The lower sequestration rate indicates that the rate of phosphorus and magnesium release was limited by another factor.The results of theist testing were used to assess the use of the existing tankage at the Durham AWWTF for conversion to the WASSTRIP process. The evaluation shows that this configuration is expected to be successful but will be contingent on consistent operation of the EBPR process and the fermenter during the winter.