(651g) Cascading Phosphorus Recovery and Bioenergy Production from Wastewater Sludge in a Novel Three-Stage Process

Efficient phosphorus (P) recovery and sustainable sludge management are critical challenges in modern wastewater treatment plants (WWTPs). Conventional biological P removal and anaerobic digestion (AD) processes often interfere with each other, leading to operational inefficiencies such as uncontrolled mineral precipitation, equipment clogging, and reduced dewatering efficiency. This study presents a novel cascading three-stage process that sequentially integrates thermophilic acid anaerobic digestion (TAAD), phosphorus precipitation, and high-rate anaerobic digestion (HRAD) to maximize P recovery and bioenergy production from wastewater sludge while reducing process complexity and costs.

The cascading system begins with the first-stage TAAD operated at 55 °C, which solubilized over 80% of the phosphorus in thickened waste activated sludge (TWAS) within 2–5 days. Unlike conventional mesophilic digestion, thermophilic conditions significantly enhanced P release without the requirements of additional chemical pretreatments. Furthermore, variations in organic loading rates did not significantly impact nutrient solubilization, reaffirming the robustness of this TAAD process. Following P solubilization, the process cascades into the second-stage P precipitation reactor, where over 90% of dissolved P was recovered as high-purity mineral precipitates such as brushite (CaHPO₄) and struvite (NH₄MgPO₄). The highest P removal (>97%) was achieved at a Mg:P molar ratio of 1.5:1. Additionally, struvite precipitation simultaneously removed ammonium, further improving effluent quality. The third stage involves HRAD, which further converts the P-stripped liquid effluent into biogas. This process achieved an average soluble COD (sCOD) removal of 88%, with a specific biogas yield of 0.46 L/g-sCOD/day and a methane content of 62.8%. As compared to conventional AD systems which require a typical hydraulic retention time (HRT) of 20–30 days, this integrated process reduced HRT to 3–8 days while maintaining high treatment efficiency. Pilot-scale implementation of this cascading system included a 15-L thermophilic fermenter, a 2-L phosphorus precipitation reactor, and a 5-L mesophilic HRAD reactor. Although slight performance reductions were observed due to operational instabilities at a larger scale, the overall efficiency remained comparable to that of existing WWTP processes. Process refinements, such as whey permeate addition to lower and stabilize pH in the first-stage TAAD fermenter, further optimized the system performance.

This study demonstrates that a cascading integration of TAAD, phosphorus precipitation, and HRAD can provide a cost-effective, scalable solution for WWTPs. The ability to recover phosphorus as valuable mineral products while simultaneously enhancing biogas production presents significant economic and environmental benefits. By reducing operational costs, improving sludge treatment efficiency, and offering a sustainable approach to phosphorus management, this novel three-stage process holds promise for widespread adoption in current wastewater treatment facilities.