Nutrient recovery from pig manure digestate using electrodialysis reversal:Membrane fouling and feasibility of long-term operation
Membrane technologies are challenged by severe membrane fouling when treating animal manure digestate due to high turbidity and high concentrations of organic and inorganic matter. In this study, a bench-scale electrodialysis reversal (EDR) was carried out to assess its feasibility for nutrient recovery from pig manure digestate. Pretreatment consisted of acidification for releasing phosphorus to the liquid phase and low-speed centrifugation for removing large particles. Results showed complete removal of NH4+ and removal of phosphate up to 84% from the feed solution, with their concentrations in the product reaching 4.2 and 0.7 g/L at a volumetric ratio of 6/2, respectively. During the semi-continuous operation, particle and chemical deposition were significantly mitigated. Despite periodical HCl cleaning, tyrosine-like and humic-like substances fouled anion-exchange membranes (AM) gradually. Nevertheless, this fouling ceased after treating the digestate at a capacity of 330 L/m2 membrane when the conductivity and ion-exchange capacity of AM decreased by 18.5% and 13.3% compared with the virgin membrane, respectively. Based on infrared spectra, these organic foulants existed in the fouling area around the membrane surface, rather than migrating deeper into the interior of the membrane. This study reveals a potential of long-term operation of EDR for treating pig manure digestate.