Introduction
Aquaculture systems generate sludge continuously and inevitably. This sludge consists of indigestible feed components, uneaten feed, and microbial biomass and contains high amounts of nutrients. It can thus be treated as valuable side stream under the circular economy paradigm, being useful for e.g., aquaponics to reduce reliance on inorganic fertilizer. In any case, it must be regarded as a residual material whose disposal entails costs. Within this context, aerobic sludge stabilization is a fast and simple process that offers high nutrient recovery efficiency and nitrogen availability. While the process is more energy-intensive than anaerobic digestion (Chen et al., 1997), it offers the advantage of a lower hydraulic retention time (HRT), making it potentially more cost-effective as bioreactors of large volume require high additional investment. However, there are no studies that assessed the optimization potential and the economic side of aerobic sludge stabilization in aquaculture, let it be as a means of end-of-pipe treatment or a method for nutrient mobilization, targeting aquaponic systems.
Against the outlined background, this study aimed at optimizing aerobic sludge stabilization by investigating the influence of pH, temperature, duration, and presence of PSB on the reduction of organic matter and nutrient leaching. The obtained results were then validated on pilot-scale to assess scale-effects.
Material and Methods
A series of experiments was conducted to identify the maximum solids load, hydraulic retention time, and the effect of PSB and commercial enzymatic products for sludge treatment. Sludge for the experiments originated from the drum filter backwash of a RAS stocked with Clarias gariepinus and fed with Skretting Meerval throughout sludge collection. 1 L glass bottles equipped with a cap and aerated through PVC pipes served as bioreactors. The bottles were placed in a water bath with temperature regulation.
The maximum solids load was determined by mixing sludge with a known amount of total solids (TS) with deion. H2O to achieve TS loads from 5 to 25 mg L-1. The reactors were then aerated over seven days and the oxygen saturation determined daily. Eventually, the effect of the maximum solids load on sludge reduction performance with and without addition of PSB was assessed in batch experiments. The respective amount of TS was added and the reactor kept under continuous aeration over 14 days.
The optimum hydraulic retention time (HRT) for continuous sludge reduction was determined at two different temperatures (20°C and 30°C) and four HRT levels (2 d, 5 d, 10 d, 20 d), with a batch treatment as control. Water exchanges were carried out daily with exchange water set to the pre-determined maximum TS concentration. Experiments were run until at least 3 times HRT was reached.
Abiotic parameters were determined daily in the batch experiments and in increasing time intervals in the continuous experiment. Water and sludge samples for C, N, and P analyses were analysed according to standard methods.
Results and Discussion
The maximum TS load under which the reactor remained aerobic was 10 g L-1. This is five to ten times lower than the loads reported in aquaponics literature (Goddek et al., 2018; Monsees et al., 2017). As expected, temperature had a significant effect on the remineralization process in aquaponic sludge treatment. In batch experiments, the pH reached its minimum after approx. 6 days at 30°C, compared to 14 days at 20°C, indicating accelerated microbial activity. Similarly, the degradation of total organic carbon (TOC) showed a decline around day 4–5 across most treatments, except at higher HRT (10 and 20 d) at 30°C. While TOC concentrations were lowest (~50 mg L-1) in treatments with shortest HRT, they increased with longer HRT, ranging from 75 to 150 mg L-1. Total nitrogen concentrations in solution increased over time, reaching their inflection point faster at 30°C (~7 days) compared to 20°C (~14 days). Higher nitrogen concentrations were associated with longer HRT and warmer conditions. A 5-day HRT appears optimal, balancing pH reduction and system feasibility. At this duration, pH dropped to 5.2 at 20°C and 4.5 at 30°C, which is sufficient to mobilize plant nutrients due to shifts in speciation.
In practical terms, a small recirculating aquaculture system (RAS) with a total volume of 5 m³ and a daily sludge production of about 3 L/d (50 g/L DM) would require a 75 L tank (based on the 5-day HRT and 10 g/L TS target) for sludge stabilization. However, an improved solid-liquid separation is necessary to achieve optimum performance.
Acknowledgements
The project leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 871108” (AQUAEXCEL3.0). This output reflects only the author’s view and the European Union cannot be held responsible for any use that may be made of the information contained therein. The authors further acknowledge Interreg project “AquaCycle” (ATCZ00002).
References
Chen S.L., Coffin D.E., Malone R.F., 1997. Sludge production and management for recirculating aquacultural systems. Journal of World Aquaculture Society 28, 303-315.
Goddek S., Delaide B., Joyce A., Wuertz S., Jijakli M.H., Gross A., Eding E.H., Bläser I., Keizer L.C.P., Morgenstern R., Körner O., Verreth J., Keesman K.J., 2018. Nutrient mineralisation and organic matter reduction performance of RAS-based sludge in sequential UASB-EGSB reactors. Aquaculture Engineering 83, 10-19.
Monsees H., Keitel, J., Paul, M., Kloas W., Wuertz S., 2017. Potential of aquacultural sludge treatment for aquaponics: Evaluation of nutrient mobilization under aerobic and anaerobic conditions. Aquaculture Environment Interactions 9, 9-18.