Solid-liquid separation

This report reviews various approaches for processing of biogas plant digestate for the purpose of nutrient recovery. It covers both established and emerging technologies and assesses technical performance and where possible economics. Techniques for nutrient recovery from digestate are developing rapidly and aim to improve nutrient management in agriculture and in waste treatment systems.

The report is aimed at biogas plant developers and operators as well as agriculture policy makers and was produced by IEA Bioenergy Task 37. IEA Bioenergy Task 37 addresses challenges related to the economic and environmental sustainability of biogas production and utilisation.

Read the full report here

This review examines how membrane technologies can be used to process nutrient-rich manure and digestate liquids, which often cause membrane fouling due to their high content of organic matter, solids, and mineral compounds. It summarizes the key physico-chemical properties of these effluents and explains how different solid–liquid separation methods interact with these complex variables. The paper outlines the major factors influencing membrane fouling, along with approaches for inspecting membranes and analyzing foulants. It also discusses how feed characteristics, operating conditions, pH control, flocculation–coagulation, and cleaning strategies affect system performance. Finally, it highlights techniques for recovering nutrients such as ammonia-nitrogen and phosphorus, as well as removing heavy metals from farm effluents.

Sustainable fertilizer production—especially for phosphorus—will be a major challenge in the coming decades, making nutrient recovery from agricultural, urban, and industrial organic wastes increasingly important. This study evaluated five full-scale technologies used to recover nutrients from anaerobic digestate in farm-scale plants, including drying with acid capture, stripping with acid capture, and membrane-based separation. All systems achieved strong recovery performance, with average nitrogen and phosphorus yields exceeding 50%. The techno-economic analysis showed notable differences among the technologies: membranes produced high-quality water and reduced digestate volume, drying was limited by available heat, and stripping faced issues with suspended solids. Overall, the systems showed comparable costs, similar to those reported in other European applications, highlighting their practical feasibility.