Treatment of Anaerobic Digestate Using Microfiltration and Ultrafiltration for Pretreatment and Nanofiltration and Reverse Osmosis for Polishing

Sammanfattning

Many studies on anaerobic digestion mainly focus on biogas production, while fewer studies focus on the management of the residual material left after the anaerobic digestion which is digestate. Digestate treatment is important to promote nutrient recovery and support circular economy. Membrane filtration can be used for digestate treatment however, the high solids content of digestate causes severe membrane fouling, reducing membrane performance. Therefore, this study aimed to investigate pre-treatment methods before membrane filtration in order to reduce membrane fouling during ultrafiltration and subsequent polishing using nanofiltration and reverse osmosis membranes. Screening and coagulation were investigated as pre-treatment methods, although only the screened digestate was used during the membrane filtration experiments. For ultrafiltration (UF), two membranes with pore sizes of 0.04 µm and 1.2 µm were tested. The 0.04 µm membrane was operated at transmembrane pressures of 1, 1.5, and 2 bar, while the 1.2 µm membrane was operated at 1 and 2 bar. The Vibro-Lab system (Sani Membranes), in order to investigate whether vibration-assisted filtration could help reduce membrane fouling and improve filtration performance. The filtered digestate obtained from UF was then used as feed for nanofiltration (NF) and reverse osmosis (RO). NF and RO experiments were carried out using a DuPont FilmTec NF270 membrane and an Alfa Laval RO99 membrane, respectively. The filtration experiments were conducted using a laboratory-scale dead-end filtration cell. NF experiments were performed at pressures of 5, 10, and 15 bar, while RO experiments were conducted at 15 and 18 bar. The results showed that increasing transmembrane pressure increased permeate flux but also increased membrane fouling and reduced membrane permeability. UF achieved high rejection of phosphorus and dry matter, while nitrogen and potassium rejection remained lower. NF showed low rejection of nitrogen and phosphorus. In contrast, RO achieved higher rejection of COD and phosphorus, although nitrogen rejection remained lower. Despite the improved permeate quality obtained with RO, the final permeate concentration still exceeded the targeted discharge standards, indicating that additional post-treatment or additional RO stages would likely be required, especially for nitrogen removal.