Demonstrating the CalPrex System for High-Efficiency Phosphorus Recovery

Sustainable phosphorus management is needed on local, national and global scales to help ensure global food security and the mitigation of eutrophication caused by point and non-point sources of nutrient loading. This challenge can be met in part with increased availability and adoption of cost-effective technologies that significantly reduce or remove phosphorus from treated water and sludge at water resource recovery facilities (WRRFs). The precipitation and capture of the struvite mineral (MgNH4PO4·6H2O) has become the most widely adopted solution to concentrate phosphorus in WRRF streams as a recoverable mineral to date, with expected total phosphorus removal of around 15-25% in most systems. Yet, persistent struvite precipitation within these systems, and increasingly stringent regulatory requirements on nutrient loading in effluents, require more efficient technologies for phosphorus extraction. This report summarizes the results of the CalPrex® phosphorus recovery pilot and the implications for full-scale implementation. CalPrex is a patented calcium-based phosphorus recovery technology yielding high-rate phosphorus extraction by using a process influent of thickened fermented sludge or acid digest. This increases the amounts of soluble and reactive species of phosphorus to approximately 65% of the total phosphorus and allows CalPrex to recover a higher percentage of the phosphorus, as a mineral called brushite (CaHPO4·2H2O). As part of this Water Research Foundation (WRF) study, the CalPrex system operated as an 8-12 gallon per minute pilot unit at the Madison Metropolitan Sewerage District’s Nine Springs Facility in Madison, Wisconsin throughout the year of 2018. Results from collected data demonstrate the average total phosphorus removal by CalPrex was 46.7-49.9%, and soluble phosphorus removal in the reactor was up to 90.6-93.1%. Detailed mass balance results are presented, as well as cost-benefit analyses of potential full-scale CalPrex installations at WRF project participating WRRFs. Additional impacts for WRRF biosolids and for brushite fertilizer production are also addressed.