Demonstrating a Membrane-Based Process for the Recovery of Key Nutrients from Anaerobic Digestate Solutions

This study demonstrates a two-staged membrane separation process for the selective recovery of two critical nutrientsNH4+ and K+from a nutrient-rich anaerobic digestate (AD) collected from a wastewater treatment plant. NH4+ and K+ are both critical for fertilizer synthesis, and their recovery from wastewater sources presents a sustainable route for nutrient production while simultaneously addressing the environmental issues associated with excessive nutrient loading (eutrophication) in local water streams. For this, a nutrient-rich real anaerobic digestate feed stock from a local municipal wastewater treatment plant was selected due to its high nutrient content: NH4+ (650−850 mg/L) and K+ (70−90 mg/L). The first separation stage focuses on separating nutrients from organic micropollutants using a surface-modified ionically crosslinked nanofiltration membrane. This stage produces a nutrient-rich (NH4+ and K+) permeate stream by achieving ≥80% NH4+ and K+ passage (1 − %rejection) and ∼95% organic pollutant (TOC) rejection. These membranes also effectively rejected multivalent cations, including Mg2+ (∼92%), Ca2+ (∼89%), and Fe (100%), and improved product quality. This membrane in the first stage also entirely retained the phosphorus (P) present in the AD in the retentate stream. For the second filtration stage, a synthetic solution containing NH4+, K+, and Na+ at concentrations representative of the Stage 1 permeate was used as the feed to evaluate selective NH4+/K+ separation. For this, a covalently crosslinked polyelectrolyte-modified membrane was used to demonstrate the partitioning of NH4+ and K+ in two separate streams. In summary, this study demonstrated the membrane-based nutrient recovery technologies for treating real AD matrices, extending their applicability beyond the synthetic matrices investigated in earlier studies.