Phosphorus recovery from sewage sludge using wet elution

• Main Author: Wong, Eunice Weilin
• Other Authors: Lim Teik Thye
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/168220
• Institution: Nanyang Technological University

Phosphorus (P) plays a crucial role in many biological processes, including energy metabolism and DNA and RNA synthesis, thus making it an indispensable resource for all living organisms. However, P is also a non-renewable resource that is primarily obtained from phosphate rock, a finite resource. Hence, sewage sludge (SS) is introduced as a renewable phosphorus-rich resource that can be developed as a sustainable method for P recovery. Alkaline leaching of P from SS directly has reported low efficiencies, but amendment by alum sludge has shown an increase in P recovery. Therefore, this study aims to further elevate the alkaline leaching efficiency of P in alum-amended sewage sludge using pyrolysis and optimizing alkaline leaching conditions. Two sewage sludge (SS) samples (DKS, DUS) and alum sludge (DAS) were collected and subsequently processed by drying, crushing, sieving, and pulverizing. Processed samples (labelled KS, US and AS respectively) then underwent characterization inclusive of moisture and ash content, pH, density, conductivity, TDS, and calorific value. An optimization study on acidic elution of sludge samples was also conducted on DKS and DUS as individual samples as well as a mixture of DKS and DUS with AS (labelled as M and UA respectively) to determine the optimal pH for acid pretreatment. In this experiment, a mixture of DKS and alum sludge was prepared with an Al:P molar ratio of 3:1 (labelled M) before being subjected to acid pretreatment at optimal pH (3.5~4) using 6M HCl for 2 hours. The remaining solid residue was processed into powder before pyrolysis at 400°C and alkaline treatment was conducted on the resulting char (MAC) at two different NaOH concentrations (0.1M and 1.0M) and leaching times (0.5h and 2h) to optimize the process. Alkaline treatment was also conducted on KS char (KC) to provide a baseline. The analyte concentration in the leachates was determined using ICP-OES. The optimization study on acid pretreatment showed that the Al ions from alum sludge was successful in minimizing P loss through the acidic leachate. The elemental P loss during acid pretreatment at the optimal pH was 41.4% for KS while it was 7.9% for M (subsequently known as M during experiment). Overall, the highest P recovery efficiency (86.4%) was achieved for MAC with alkaline leaching conditions at 1.0M NaOH and 0.5h leaching time. Further analysis found that the independent variables (conc. of NaOH and leaching time) were more significant when recovering P from KC instead of MAC.