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

Abstract

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.

Popular Abstract

Anaerobic digestion is a common method used to produce biogas from organic waste, but it also generates a leftover material called digestate. Although digestate contains useful nutrients such as nitrogen, phosphorus, and potassium, it can also cause environmental problems such as water pollution, nutrient runoff into rivers and lakes, and unpleasant odors if not properly treated.

One possible treatment method is membrane filtration, which is a process where water is passed through very fine filters (membranes) that allow water molecules to pass through while retaining particles, contaminants, and dissolved substances depending on the membrane type. However, the high amount of solids in digestate can block the membranes and reduce their efficiency over time.

In this study, different membrane filters were tested to clean digestate. The treatment was done in several stages, and each stage removed smaller and smaller contaminants from the water. First, a screen removed large pieces of material such as straw, fibers, and other solid particles that were floating in the liquid. Then the liquid passed through an ultrafiltration membrane, which removed smaller particles such as fine suspended solids, bacteria, and small organic particles. After that, nanofiltration was used to remove even smaller unwanted substances. Finally, reverse osmosis was used, which is the strongest cleaning step and removes almost everything from the water, including dissolved salts, nutrients, and very small contaminants, allowing mainly water molecules to pass through. Each step made the liquid cleaner by removing smaller and smaller particles, resulting in much cleaner water at the end.

The study showed that when higher pressure was used, more water could pass through the filters, but the filters also became clogged faster. Microfiltration membranes clogged more easily than ultrafiltration membranes because their larger pores allowed more particles to get stuck inside. Ultrafiltration was good at removing particles and phosphorus from the water. However, nutrients like nitrogen and potassium were harder to remove because they were dissolved in the water. Nanofiltration removed only a small amount of these dissolved substances. Reverse osmosis worked best and removed most of the remaining pollution, especially phosphorus and organic material. However, nitrogen levels in the final treated water were still higher than the discharge limits, indicating the need for additional treatments to meet environmental requirements.

• author: Machatine, Gonçalo Simão
• supervisor: Mikael Sjölin
• organization: Chemical Engineering (M.Sc.Eng.)
• course: METM01 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Anaerobic digestio, Digestate, Ultrafiltration, Nanofiltration, Reverse osmosis, Membrane fouling, Nutrient recovery, Circular economy, Membrane technology
• language: English
• id: 9238292
• date added to LUP: 2026-06-22 09:30:19
• date last changed: 2026-06-22 09:30:19