Refining Phosphorus Recovery: Practical Improvements for Water Resource Recovery Facilities

Introduction Metro Water Recovery, based in Denver, Colorado, serves 2.2 million population equivalents through the Robert W. Hite Treatment Facility (RWHTF) and the Northern Treatment Plant. RWHTF has a design capacity of 220 MGD and treats an average daily flow of 130 MGD. The facility's solids handling complex includes gravity thickening, grease processing, dissolved air flotation thickening of waste activated sludge, two-phase mesophilic anaerobic digestion, centrifuge dewatering, phosphorus recovery, and biosolids management for storage, hauling, and land application. In 2016, Metro piloted the MagPrex post-digestion phosphorus recovery system (PREX) to address challenges posed by enhanced biological phosphorus removal (EBPR). EBPR results in high concentrations of soluble phosphorus in the digesters, which combines with magnesium and ammonia to form nuisance struvite, causing scaling in equipment, lowering dewatering efficiency, and upcycling phosphorus through the treatment process. In response to stricter effluent TP limits imposed by the Colorado Department of Public Health and Environment (CDPHE), Metro commissioned the world's largest PREX reactor in October 2020. This system allows RWHTF to reliably meet a TP target of 0.7 mg/L, with fewer operational disruptions associated with nuisance struvite formation. Phosphorus Recovery System Overview The PREX reactor is a completely mixed system receiving 600-800 GPM of digester effluent with orthophosphate (OP) concentrations between 350-450 mg/L. PREX quickly transforms OP into struvite by adding magnesium chloride and raising pH through air stripping via coarse air bubble diffusers . The reactor can harvest struvite as a reusable product, with large crystals settling in the reactor cone for separation, Figure 1. Since startup, the reactor has achieved >90% OP conversion, enabling Metro to maintain effluent TP concentrations below 0.4 mg/L. Optimization Objectives While the PREX system has successfully achieved the required OP conversion, opportunities for further optimization quickly became apparent. Initially, only 2% of the struvite formed in the PREX reactor was captured for reuse, and maintenance costs to address nuisance struvite downstream remain high (Figure 2), estimated at over $9,000 per month in labor alone. This study aims to enhance the system design and assess the downstream impacts on centrifuge dewatering, struvite scaling in PREX effluent, and centrate piping. The optimization efforts are guided by three key objectives: 1. Maximize Struvite Recovery: Increase recovery beyond the initial average of 300 lbs/day to meet the reactor's estimated potential of 2,000 lbs/day. This effort investigates changes to process operation and configuration to increase struvite recovery. 2. Minimize Downstream Struvite Accumulation: Address scaling in the PREX effluent and centrate system. Although anerobic digestors have observed a reduction of nuisance struvite, portions of PREX effluent system require a high pressure cleaning every three weeks to remove struvite accumulation. 3. Optimize Residual Magnesium for Dewatering: Evaluation of residual magnesium from the PREX reactor to improve dewatering efficiency by reducing polymer demand and lowering chemical costs. Methods The PREX reactor was operated under various conditions, including adjustments to hydraulic retention time (HRT), pH, and Mg:OP ratios. A 3D multi-phase turbulent model was developed to refine operational settings and predict particle settling efficiency. To assess struvite scaling, ion mass balances were performed on centrate samples collected at two points: upstream at the centrifuge discharge and downstream at the outlet to the sidestream deammonification reactor. These samples were analyzed for OP, TP, magnesium (Mg), and pH. Additionally, soluble and total magnesium concentrations were measured in the PREX effluent and centrate. The relationship between these concentrations, polymer demand, and dewatering performance was explored. Conductivity and zeta potential measurements were collected as potential proxies for cation concentrations, enabling real-time monitoring of dewatering performance. Results 1.Struvite Recovery Optimization The 3D CFD model revealed that only 10% of struvite crystals under 250 μm in size could be captured under typical airflow conditions (Figure 3). Particles with a diameter >700 um had an average recovery rate of 80%. However, over 50% of the struvite produced by PREX was less than 250 μm, significantly limiting recovery. Improvements included adjusting classifier operation such as classifier bed height , introducing dilution water, and recycling classifier overflow. These changes increased struvite recovery from 300 to 1,200 lbs/day. 2.Minimizing Downstream Struvite Accumulation While the PREX system significantly reduced nuisance struvite formation in the digesters, accumulation persisted in the PREX effluent piping due to the presence of small struvite crystals and turbulent flow, which caused localized pH shifts and the precipitation of residual OP, Mg, and NH4. Polyvinylidene fluoride (PVDF) liners are being installed in the effluent lines to reduce turbulence and mitigate scaling. Ongoing ion mass balance sampling will provide further insights, with results to be presented at the conference. 3.Residual Magnesium and Dewatering Performance Maintaining residual magnesium concentrations of 50-60 mg/L reduced normalized dewatering polymer usage per flow by 39%, resulting in chemical cost savings of $800,000 annually (from $2.9 million to $2.1 million) (Figure 4). Conductivity and zeta potential measurements strongly correlated with cation concentrations, indicating their potential use for real-time monitoring and future optimization. Conclusions The optimization of Metro's PREX system demonstrates the potential for substantial improvements in phosphorus recovery and operational reliability. Struvite recovery increased fourfold, highlighting the importance of fine-tuning operational parameters and integrating recycling strategies. Addressing scaling in effluent lines through material upgrades and improved chemistry management will further enhance long-term system reliability. Optimizing magnesium levels improved dewatering performance and delivered substantial chemical cost savings. Potential real-time monitoring using conductivity and zeta potential as proxies for cation concentrations offers an innovative approach to operational control and performance optimization. As phosphorus regulations become more stringent, these advancements position Metro as a leader in sustainable resource recovery. These findings provide valuable insights for other utilities aiming to enhance phosphorus recovery, minimize operational disruptions, and reduce chemical usage, advancing the next generation of phosphorus management for water resource recovery facilities.