Evaluating the Impacts of WAS Release Pretreatment on Anaerobic Digestion and Dewatering at Robert W. Hite Treatment Facility

Waste sludge from biological phosphorus removal systems has been associated with negative impacts to anaerobic solids digestion and dewatering processes. Ahead of instituting a large-scale full plant biological phosphorus removal process, Metro Wastewater Reclamation District conducted an in-depth pilot-scale investigation of WAS release prior to anaerobic digestion as a possible approach to mitigate undesirable effects of biological phosphorus removal on the facility’s solids treatment and handling processes. Results from the evaluation concluded that pretreatment of WAS using a stripping reactor to promote the release of phosphorus and magnesium prior to anaerobic digestion was effective at reducing struvite production in the anaerobic digesters by 44 percent compared to a no-treatment alternative. This reduction would result in longer durations between digester cleaning for the removal of settled struvite and lower total biosolids mass for dewatering and hauling. Dewaterability testing indicated an approximate 7.5 percent increase in dewaterability, equating to an increase in total solids concentration of 1.6 percentage points of the dewatering biosolids. The reduction of struvite formation paired with the increase in dewaterability is anticipated to reduce the biosolids hauling requirements by approximately 8 percent.