Aerobic treatment and acidification of pikeperch (Sander lucioperca L.) sludge for nutrient recovery

Aquaponics (AP) is an integrated multi-trophic system that combines elements of recirculating aquaculture systems (RAS) and hydroponics (HP). In AP systems, waste streams from the RAS, rich in nutrients, are reused to grow plants hydroponically. Fish sludge (the solid fraction of the waste stream) contains a major part of these nutrients (i.e. the macro and microelements and especially P) and these are discharged from the AP system in some system designs. To increase the reuse of nutrients in AP systems, processes of recovery, such as the transfer of nutrients from the solid to the soluble phase (i.e. cations, anions, and chelates), should be developed. In this study, we investigated the nutrient mineralisation performance (NMP) of pikeperch (Sander lucioperca) sludge aerobically treated in a long-term experiment (121 days) and in a short-term experiment (15 days) with artificially lowered pH (pH 6.3). Mass balance equations were used to determine and compare the NMP between experiments. NMP showed a macronutrient recovery (i.e. nutrient passing from the solid phase to the soluble phase) between 0 and 40%, while the NMP of micronutrients was 2%. In the long-term experiment, P had a negative value (− 1%) indicating that its soluble forms were lowered due to wash out and precipitation or were assimilated by the microorganisms. In the short-term experiment, a quick drop in pH had an almost immediate (1 day) increasing effect on P, K, and Ca solubility while N and Mg were better mineralised in the long-term experiment. Our study demonstrated that a low pH step is an efficient way to improve the nutrient recovery from aerobically treated sludge.