Potential of aquacultural sludge treatment for aquaponics: evaluation of nutrient mobilization under aerobic and anaerobic conditions

In recirculating aquaculture systems (RAS), mechanical removal of suspended solids by clarifiers or drum filters provides an organic mixture rich in nutrients. Still, in most traditional RAS, this sludge is discharged directly or following dewatering. Here, the potential recycling of nutrients from sludge is assessed, comparing aerobic and anaerobic mobilization of nutrients experimentally, ultimately aiming at an application in aquaponic systems. Nutrient mobilization processes were studied, monitoring soluble nutrients photometrically in the treatment tanks (NO3--N, NO2--N, total ammonia nitrogen, soluble reactive phosphorus [SRP], K+, Mg2+ and Fe2+), the nutrient composition of the sludge (total phosphorus, Fe, Mn, Al, S, Mg, Ca) by inductively coupled plasma optical emission spectrometry, as well as C: N ratio, total solids (TS) and total suspended solids (TSS). Aerobic treatment (aerated, AT) resulted in a 3.2-fold increase in mean (+/- SD) SRP from 9.4 (+/- 0.7) to 29.7 (+/- 2.1) mg l(-1), most likely owing to a decrease in pH. In contrast, in the anaerobic treatment (unaerated, UT), SRP remained unchanged between 9.4 (+/- 0.7) and 9.3 (+/- 0.4) mg l(-1). Both treatments resulted in increased K+ concentrations from 28.1 (+/- 1.5) to 36.8 (+/- 2.3) mg l(-1) in AT and to 32.2 (+/- 2.3) mg l(-1) in UT. AT revealed best mobilization of P and K+ without major losses of NO3--N. Thus, aerobic treatment of water-sludge mixture has a high potential for significant improvements of nutrient recycling in aquaponics.