Nutrient recovery from agricultural digestate by integrated ultrafiltration and reverse osmosis: effect of pretreatment strategies from laboratory to demonstrative scale

Agricultural digestate is a nutrient-rich by-product of anaerobic digestion whose sustainable management is increasingly constrained by environmental regulations and emerging criteria for mineral fertilizers, such as “REcovered Nitrogen from manURE” (RENURE). This study systematically investigates the influence of different pretreatment strategies, namely coagulation, polymeric flocculation, acidification and moderate heating, on an integrated pressure-driven membrane treatment train based on ultrafiltration (UF) and reverse osmosis (RO) applied to agricultural digestate. The pretreatments were assessed in terms of mass distribution, membrane permeability, nutrient partitioning and compliance with RENURE-related thresholds. The results demonstrate that pretreatment selection plays a decisive role in governing both filtration performance and nutrients recovery. Coagulation improved solid removal and moderately enhanced ultrafiltration stability, while polymeric flocculation increased fouling propensity without significantly improving dewatering efficiency. Thermal conditioning reduced viscosity but caused substantial ammonium losses due to volatilization (up to ∼20% of the initial nitrogen mass). In contrast, acidification at pH 4 markedly enhanced ultrafiltration permeability and improved ammonium concentration in the final RO retentate, representing the best operational compromise among the tested acidification conditions. The most effective strategy identified at laboratory scale was subsequently validated at technology readiness level 7 in a demonstrative plant operating under real biogas plant conditions. The demonstrative results confirmed the laboratory-scale trends, achieving ammonium concentrations above 17.5 g kg-1 and ammonium mass flows of around 45.9 kg d-1, and highlighted acidification as a key enabling step for achieving stable membrane operation and improved nitrogen recovery.