Innovative multiple resource recovery pathways from EBPR wastewater treatment–derived sludge

Wastewater treatment-derived sludge is a growing concern. Environmental issues, rising sludge production rates, and stringent regulations create the necessity to seek for treatment and valorization alternatives. Sludge is a potential source of high-value materials which can be recovered and transformed into new products such as animal feeds; bioplastics; biofuels, biostimulants; or biobased fertilizers. Considering the current legal constraints hindering the use of certain waste streams, the objective of this work is to show the technical viability for obtaining multiple valuable products from sludge. The emphasis is placed on novel valorization pathways, such as microalgae and purple bacteria cultures growing over sludge. The obtained products are benchmarked against traditional methods for resource recovery such as direct land application and P recovery from ashes. Our results show, besides the nutrient (TKN 7.38, TP 4.41; K 0.47 g 100 g TS-1) and energy content (HHV 22.53 MJ Kg(-1) TS), that sludge could be employed to produce a suitable growing medium for microalgae and purple bacteria cultures obtaining, in the latter, remarkable high contents of high-quality proteins (64.50 % dw) for potential valorization as animal feed ingredient. We also obtained nutrient rich microalgae biomass (TKN 7.10, TP 8.10; K 0.40 g 100 g TS-1) which could be used as inputs for biobased fertilizers or biostimulants preliminarily complying with the nutrient requirements in EU 2019/1009. Current global scenario, showing economic and supply risk uncertainties regarding food production inputs, generates the urgent need to find feasible pathways for obtaining recovered products such as the ones presented in this study.