Reducing Biosolids Phosphorus by Efficient Release and Harvesting of Phosphorus

The Madison Metropolitan Sewerage District (MMSD) Nine Springs Wastewater Treatment Facility is upgrading its solids handling processes, adding acid-phase digestion and phosphorus (struvite) harvesting. Research conducted during the past ten years by MMSD and University of Wisconsin-Madison studied methods to trigger controlled phosphorus release from waste activated sludge (WAS) to permit reduction and recovery of phosphorus prior to anaerobic digestion. Recovery of phosphorus by harvesting serves to a) permit recovery and re-use of a valuable mineral resource, and b) reduce the phosphorus content of the biosolids applied to land. Releasing phosphorus and harvesting prior to anaerobic digestion adds another advantage to this list, which is c) reduction of struvite formation potential in and following anaerobic digestion.Our most recent research has built upon and refined our previous research work done, and has culminated in a unique process that is being implemented in our current upgrade. During the evaluation of this process additional methods were researched, both to better understand this process and to note any viable alternates to consider should operational needs dictate. The result is a body of work that benefits not only MMSD but may have potential applications and benefits for the burgeoning nutrient recovery and management field.Final design of MMSD's solids handling improvements is complete and the design incorporates key concepts derived from this research including acid-phase digesters, WAS blending tanks to permit phosphorus release, and struvite harvesting. While relatively few facilities use acid-phase digestion at this time, concepts developed and presented will be of interest to other facilities interested in releasing phosphorus from WAS for harvesting. Moreover, reduction of phosphorus in biosolids ultimately requires physical removal of the phosphorus from the solids process stream, and harvesting is the logical means to accomplish this.