Practical studies of lignocellulose filter cake from RAS sludge

Effluent water and mechanically filtered sludge from recirculating aquaculture systems (RAS) contain significant concentrations of nitrogen (N) and phosphorus (P) and may be applied directly as fertiliser, e.g. in hydroponic systems. However, where this is not possible the nutrients must be removed and should ideally be applied as fertiliser elsewhere, but this often involves costly transportation. To increase the amount of nutrients per kg of material to be transported, e.g. to agricultural fields, the sludge may be filtered using various methods. Chemical coagulants and flocculants are commonly applied to aid particle and nutrient removal. This report describes initial trials with fish sludge amended with the natural polymers lignocellulose and purified cellulose as filter materials, conducted at Denmark Technical University (DTU) Hirtshals, and further testing of the filer cakes as substrates for composting conducted at NORSØK, Tingvoll. The optimal composition of lignocellulose and cellulose to be used for filtration of fish sludge was determined by filtering 1 L of sludge at 4.85 g TCOD/L (total chemical oxygen demand) with combinations of lignocellulose (20, 25, 30, 35 g/L) and cellulose (0.5, 0.75, 1, 2 g/L). A minimum ratio of 0.75 g of cellulose to 35 g of lignocellulose per L of sludge was required for filtration. This combination could produce a filter cake with a dry matter content of 24 - 29%, containing most of the nutrients (N, P). The weight of material containing N and P was reduced by more than 80% by the filtration. Two types of filter cake were produced at DTU Hirtshals with these natural polymers, with and without aluminium coagulant. The cakes were frozen and transported to the Norwegian Centre for Organic Agriculture, Tingvoll where they were amended with fish meal to make batches with a C/N ratio of 55, 30, and 20. The thermophilic decomposition and CO2 respiration of these batches were studied in Dewar flasks. The mixtures without chemical c