Membrane filtration for valorization of digestate from the anaerobic treatment of distillery stillage
Magdalena Zielińska, Wioleta Mikucka
Abstract
ABSTRACT Currently, nutrient-rich digestates generated in anaerobic processing of distillery stillage are mainly used as fertilizers, which is a seasonal application. The objective of this study was to valorize the liquid phase of digestate with ultrafiltration (UF) membranes, depending on their cutoff and pressure. UF (150 kDa) resulted in the chemical oxygen demand (COD) rejection of 89%– 93%, independently of the pressure (0.2–0.4 MPa). The pre-treatment with microfiltration (MF) and post-treatment with UF (5 kDa) increased the COD removal to 98%. The rejection of total nitrogen (TN) and total phosphorus (TP) reached 81 and 87%, respectively, in the 3-stage system (MF-UF150 kDa-UF5 kDa). Because permeates were particle-free and had low color and COD concentration, their quality was assessed in terms of suitability for algae production; low COD/N (below 0.7) may support the growth of autotrophic algae. The concentrations of phosphorus and microcomponents (iron and potassium) also indicated the potential of reusing these permeates for algae production. Nutrient-concentrated retentate (about 51 g COD, 13 g TN and 1.3 g TP from 10 L of the feed) can be used as fertilizers. The MF pre-treatment increased permeate flux in UF more than 10 times, which allows for energy saving and thus reduces the costs of digestate valorization.