A conceptual, stoichiometry-based model for single-sludge denitrification in recirculating aquaculture systems

In the near future, the discharge of nitrate from recirculating aquaculture systems (RAS) to receiving water bodies is expected to be constrained by environmental regulations. Following wastewater treatment terminology, nitrate removal that makes use of the organic solid wastes generated within the RAS as the energy source for denitrification, may be termed 'single-sludge denitrification'. In this approach, the costs associated with the addition of an external carbon source, the treatment of solid-wastes generated in the RAS, and the supplementation of alkalinity are reduced. The simple and economical operation of such a process can be realized by adopting the conventional activated sludge (AS) methodology. Organic solid-wastes taken from the solids-filter of a RAS growing gilthead seabream were characterized for their chemical and biodegradation properties. The results were used to generate a conceptual model to predict the performance of single-sludge denitrification in RAS. The model was run under typical operational conditions, employing the mean solids retention time (SRT) as the key operational parameter. Results indicated that in order to attain high denitrification rates, the ratio of the flux of organic solids (as COD) supplied to the denitrification reactor to the flux of NO₃^- reduced should be between 4.0 and 6.0 g COD (g NO₃-N)^{- 1} and the SRT values should be lower than 10 d. At these conditions, sludge production was estimated to be between 40% and 60% of the solids feed mass (in terms of COD), and NH₃-N production as a result of ammonification was estimated to be less than 10% of the NO₃^--N removed. Empirical verification of the model is presented elsewhere. The model can be used as a design tool and for predicting the performance of the process at any operational conditions.