(94e) A Novel Three-Stage Process to Treat Sewage Sludge with High Phosphorus Recovery and Bioenergy Production

Many municipal and industrial wastewater treatment plants (WWTPs) have a biological phosphorus (P) removal process to concentration P into the activated sludge and also employ anaerobic digestion (AD) to reduce sludge volume. However, these two processes negatively affect each other, causing issues that impact performance and increase operating costs. For instance, P accumulated in the sewage sludge through a bio-P process will be released during AD, resulting in high recycled P loading and P mineral scale in reactors and pipes, which further cause poor dewaterability and require additional equipment with costly maintenance. Meanwhile, P recovery is more difficult by its mixture with a high concentration of biological solids, which requires expensive separation equipment (e.g., fluidized bed reactors) and limits its application only to a few large metro WWTPs. In this study, a novel integrated process, including a first-stage thermophilic acid AD (TAAD), a second-stage P mineralization and precipitation, and a third-stage mesophilic high-rate AD (HRAD), was proposed to promote nutrient removal/recovery and renewable bioenergy production in WWTPs. In the first-stage TAAD at 55 °C, thickened waste activated sludge (TWAS) collected from a local WWTP in Minnesota was used as the feedstock loaded to a continuous stirred-tank reactor (CSTR). After over 400-day operation at a hydraulic retention time (HRT) of 2-5 days, the P solubilization in first stage reached up to 900-2000 mg P/L, corresponding to over 80% of the total P in the TWAS. Meanwhile, whey permeate concentrate was used as an organic waste additive to TAAD with the purposes of controlling the pH around 6 and facilitating the degradation of sludge with increased volatile fatty acids (VFAs) release. Subsequently, the second-stage P precipitation from the TAAD effluent was conducted through a standardized jar test, and the maximum P removal was achieved with a molar dosage of 1:1.9 (phosphate to magnesium ions) in the solution. With this dosage, up to 80% of the soluble phosphate was recovery in the form of P-bearing mineral precipitates (e.g., struvite). These precipitates with mainly P-bearing minerals could be sold as a biofertilizer with a high P purity and create a revenue stream, thus significantly improving the economic feasibility of this process. After that, the supernatant with a high level of soluble COD and VFAs was pumped to an up-flow anaerobic sludge blanket (UASB) reactor operated at 37 °C, and most of the residual organics were converted to biogas at an HRT of 1-3 days via this third-stage HRAD process. Therefore, at an overall HRT of 3–8 days, this integrated system achieved a significant sludge treatment performance with very efficient P recovery and biogas production that outperformed a conventional wastewater sludge AD process with an HRT of 20-30 days. In conclusion, this novel three-stage process treating sewage sludge has the potential to improve the efficiency and effectiveness of P recovery and enhance the generation of bioenergy in the form of biogas with a shorter retention time, thus benefiting current WWTP operations.