NUTRIENT RECOVERY FROM ANAEROBIC DIGESTATE BY DIFFERENT COMBINATION OF PRESSURE DRIVEN MEMBRANES

Anaerobic digestate, the main by-product from anaerobic digestion process, will increase in the next years arriving at 177 Mt dry matter from the current level of 31 Mt by 2050. This huge amount cannot be directly applied on the soil as the EU “Directive Nitrate” limits this option at 170 kgN/ha for year. Considering the high content of nitrogen, phosphorous and potassium compounds in agricultural digestate, innovative and green methods have been investigated by the scientific community to exploit the digestate for bio-fertilizers production. This work tested different combinations of pressure driven membrane steps (micro, ultra, nanofiltrations and reverse osmosis) to maximize the nutrients and water recovery from agricultural digestate. The microfiltration (MF) step was ineffective in fully separating nutrients and removing colloids and suspended solids. As a result, it can be skipped to prevent nutrient loss in the reverse osmosis (RO) concentrate. Nanofiltration (NF) also underperformed due to rapid membrane fouling caused by its small mesh size. Conversely, ultrafiltration (UF) successfully removed fine and colloidal particles while allowing ammonium and potassium compounds to pass through. Consequently, the nutrients recovery in the concentrate of the RO and the total water recovered in the combination including the UF step was 40–50% w/w and 32.7% w/w, respectively. Moreover, the UF concentrate was also adopted as inoculum for biogas production tests, whose performances were compared to the conventional inoculum represented by the liquid fraction of the agricultural digestate. The performances were similar, but the kinetics were higher with the UF concentrate, as an effect of the higher microorganisms and nutrient concentrations.