Pilot-scale Evaluation of AirPrex® for Digestate Treatment

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 staff conducted an in-depth pilot-scale investigation of AirPrex® (CNP-Technology Water and Biosolids Corp., Kenosha, WI) as a possible approach to mitigate undesirable effects of Bio-P on the facility’s solids treatment and handling processes. Results from the evaluation concluded that pretreatment of digestate by an AirPrex® type technology to promote in-vessel struvite formation was effective at sequestering phosphorus from the dewatering recycle stream and reducing struvite formation potential in downstream equipment. In addition, this pretreatment resulted in a 15 to 20 percent reduction in polymer required for dewatering and a notable increase in dry cake solids leading to an overall 7 to 10 percent reduction in biosolids hauling requirements depending on the degree to which struvite is recovered as a separate fertilizer stream or commingled with the biosolids.