Membrane Fractioning of Pre-Treated Waste Activated Sludge for the Recovery of Valuable Biocompounds

The recovery of added-value biocompounds from waste activated sludge (WAS) is a promising alternative to its current management. In this study, WAS was partially wet oxidised producing a highly complex stream mainly composed of humic acids, proteins and carbohydrates. This stream was ultrafiltered to assess the influence of membrane material and molecular weight cut-off (MWCO) on its fractioning based on the different sizes of the biomolecules contained in the oxidised WAS. Flat-sheet polyethersulfone (PES), permanently hydrophilic polyethersulfone (PESH), and polyacrylonitrile (PAN) membranes with a MWCO of 50 KDa were evaluated. The best performance was obtained with PES membrane, achieving high retention values (around 70 % for proteins and carbohydrates, and 47 % for humic acids) and high differences in selectivity between proteins and carbohydrates with humic acids (around 23 %). For the size fractioning experiments, PES membranes of 10 and 3 kDa (PES10 and PES3) were used, obtaining the best results when filtering the PES50 membrane permeate with the PES3 membrane, retaining 83 % of carbohydrates, 87 % of proteins and 69 % of humic acids. These results open the possibility of separating carbohydrates, proteins and humic acids through an integrated membrane process. Besides, membranes were characterized by atomic force microscopy, infrared spectroscopy, and contact angle measurements. Multiple fouling models were assessed, and the main fouling in PAN membrane, and to a lesser extent in PES, was reversible. Conversely, PESH membrane fouling had a strongly irreversible character. Cake filtration can be considered the main fouling mechanism in all experiments.