Introduction
Recirculating aquaculture systems (RAS) are optimised for efficient water usage and minimum discharge. The main output in such systems, besides evaporation, is the backwash water required to wash off retained solids from drum filters. In order to increase the water reuse efficiency further, it is possible to treat the backwash water and reintroduce parts of the liquid fraction back into the RAS. This leads to the generation of thickened sludge that can eventually be used as raw material for a series of biorefinery processes such as biogas production and alike.
A chemical process that is commonly used for such water treatment tasks is the application of flocculants and coagulants. Metal-based flocculants and polymer-based coagulants have been extensively studied for their effectiveness in aquaculture systems (Ebeling et al., 2005, Ebeling et al., 2003, Heiderscheidt et al., 2020). However, within today's circular economy paradigm, it is also of interest how such substances affect any follow-up process that utilises the generated sludge. Prior studies in the field of wastewater treatment found that the choice of flocculant or coagulant affects the performance of biological sludge treatment processes (Cainglet et al., 2023).
Against the outlined background, this study assessed the effect of two flocculants (polyaluminum chloride, iron chloride) and two coagulants (starch-based, chitosan-based) on the biomethane yield and sludge stabilisation performance under aerobic conditions.
Material and Methods
The solid fraction of drumfilter backwash water was collected at a commercial RAS farm () producing African catfish (Clarias gariepinus). Sampling took place over six hours during a single day, filtering the backwash water through a net with a mesh size of 300 µm, yielding approximately 10 kg of sludge.
Reaching the laboratory, the sludge was mixed with water to reach a dry matter content of 30 g/L. Eventually, the individual flocculants/coagulants were added at dosages determined in a series of pre-trials, mixed at 200 rpm for 1 min and then at 20 rpm for 30 min. Eventually, the treated sludge was left to settle, the supernatant was removed and the sludge designated for biomethane yield tests frozen and shipped.
Biomethane tests were conducted batch-wise according to standard methods (Drosg et al., 2013). In brief, sludge was inoculated with digestate from a biogas plant at a ratio of 3:7 (sludge – inoculum). 500 mL of the mixture was transferred into a 1 L bottle that was sealed airtight. The emerging biogas was stripped of CO2 by passing it through a gas scrubber filled with NaOH. The purified CH4 entered a third bottle filled with water, which was displaced into a measuring cylinder according to the produced volume of CH4.
A second fraction of the sludge was diluted to a dry matter content of 10 g/L, from which 400 mL was filled into each of three 500 mL reagent bottles per treatment, with a control that was not treated with flocculant/coagulant. All bottles were placed in water baths at 20°C and 30°C and continuously aerated for 21 d. The pH and dissolved oxygen concentration were determined daily. In addition, total organic carbon (TOC), total nitrogen (TN), ammonium (NH4+), nitrite (NO2-), nitrate (NO3-), orthophosphate (PO43-), calcium, magnesium, and iron concentrations were determined in intervals of three days.
Results and Discussion
The experiments were concluded but data analysis is incomplete at the time of abstract submission. Nevertheless, the outcomes of this study are of relevance for RAS farm managers who consider the application of flocculation/coagulation technology in order to meet discharge levels and use of the resulting sludge for biorefinery, e.g., to partly recover energy from by-products.
Acknowledgements
The project leading to these results has received funding from the Interreg project "AquaCycle" (ATCZ00002). We would furthermore like to thank HappyFish s.r.o. for providing access to their facilities in order to obtain a sufficient quantity of sludge for this experiment.
References
Cainglet, A., Kujala K., Liimatainen M., Prokkola H., Piippo S., Postila H., Ronkanen A.K., Heiderscheidt E., 2023. The influence of coagulant type on the biological treatment of sewage sludge. Science of the Total Environment 869, 161706.
Drosg, B., Braun R., Bochmann G., Saedi T.A., 2013. 3 - Analysis and characterisation of biogas feedstocks. in: Wellinger A., Murphy J. and Baxter D. (Eds.), The Biogas Handbook. Woodhead Publishing, pp. 52-84.
Ebeling, J.M., Rishel K.L., Sibrell P.L., 2005. Screening and evaluation of polymers as flocculation aids for the treatment of aquacultural effluents. Aquacultural Engineering 33, 235-249.
Ebeling, J.M., Sibrell P.L., Ogden S.R., Summerfelt S.T., 2003. Evaluation of chemical coagulation–flocculation aids for the removal of suspended solids and phosphorus from intensive recirculating aquaculture effluent discharge. Aquacultural Engineering 29, 23-42.
Heiderscheidt, E., Tesfamariam A., Pulkkinen J., Vielma J., Ronkanen A.-k., 2020. Solids management in freshwater-recirculating aquaculture systems: Effectivity of inorganic and organic coagulants and the impact of operating parameters. Science of the Total Environment 742, 140398-140398.