P recovery from nightsoil-treated sludge using chemical and biological methods

Abstract

The study aimed to comprehensively enhance the techno-economic feasibility of phosphorus (P) recovery from nightsoil-treated sludge (NTS) or primary settled-nightsoil sludge (PSNS) using acid leaching (a chemical process) and anaerobic co-digestion (a biological process). Results obtained from the chemical process showed that 92.7% of P could be effectively extracted from PSNS of 20,000 mg/L with 0.5 M of H2SO4​ at a reaction time of 45 min (optimum condition). Moreover, the pseudo second-order was fit to describe the dynamics of the P, Fe, Mg, and Ca leaching processes. Kinetic studies showed that the 99% leaching of the maximum extracted P concentration at equilibrium (9.6673 mg/g) took place in less than 1 h. Regarding P recovery, the highest phosphorus yields (more than 98%) were obtained at pH 11 for all cases. In addition, 15.2% struvite (magnesium ammonium phosphate, MAP) with the highest P content of 9.7% in precipitates could be achieved under pH 9 with sea salt addition. For the biological process, anaerobic co-digestion (AcoD) was inhibited by the accumulation of undigested feedstock due to the higher PSNS found in AD4 (25FW:75PSNS) and AD5 (100PSNS). A more stable process was found in AD2 (75FW:25PSNS) under HRT 15 days, in which COD removal efficiency and P release were 97.2 and 80.2%, respectively. This recommended condition allowed a high organic loading rate (OLR) at 12 gVS/L/d, resulting in the highest biogas yield of 0.93 L/L/d. P recovery efficiency from effluents and %P in solids decreased with increasing total suspended solids (TSS), indicating the adverse effect of TSS on struvite quality. The precipitates formed from AD2 effluents had the highest MAP of 46% with 10.94% of P in solids. The operational costs assessment demonstrated that the chemical method generated a higher average cost of more than 100 USD kg P −1 . However, the AcoD system under AD2 and AD3 had a profit margin of more than 8 USD kg P −1 when the product’s market price was greater than 13.5 USD kg P −1 . Thus, the P recovery process using AcoD of PSNS and FW could be used as a pragmatic way to utilize PSNS as an alternative resource for P.