How do plant protein based diets change COD production and COD/N ratios for denitrification on internal carbon sources in RAS?

Introduction Changing diet composition from fish meal-based protein sources to plant-based protein sources will affect the amount and composition of waste produced in aquaculture operations. Plant-based diets and the inherent increase in fiber content will lead to lower digestibility and an increased production of organic solid waste, measured as chemical oxygen demand (COD). An increase in COD production can be beneficial if COD/N-ratios are limited for denitrification on internal COD sources. Denitrification is a valuable tool to control nitrate, reducing water exchange and nutrient emissions and is often limited by COD availability. However, fibers are generally considered as substrates of low degradability and could be a limiting a factor for denitrification. Current knowledge gaps in this field of research call for a closer investigation of the composition of solid waste, its fiber fractions and the resulting effects on COD/N-ratios and denitrification potential in RAS. Therefore this research assessed the impact of a plant protein-based diet on the production, composition and denitrification potential of the solid COD waste fraction of rainbow trout (Oncorhynchus mykiss) cultured in a RAS. The goal was to determine COD and N mass balance, COD/N ratios and denitrification potential. Material & Methods A 6 week experiment was conducted in 6 identical small-scale conventional RAS (V=450 l), stocked with 25 trout of an initial average body weight of 103 g. The fish were fed a high fiber diet (HF, plant protein based) and a low fiber diet (LF, fish meal based).The two diets were isoenergetic and isonitrogenous based on digestibility and each treatment was replicated three times. The feeding rate was at approximately 1.4 % BW/day. Each RAS was operated at a water exchange rate of 450 l/kg feed. Water quality, fish performance, body composition, waste production, waste recovery and composition of feed and waste was determined. Results & Discussion Water quality was not affected by the treatments, fish growth was 14% lower in LF when compared to HF. Solid COD production and COD/N ratio increased for the HF treatment when compared to the LF treatment (see tab. 1 & fig. 1). This was mainly caused by an increase of the fiber fraction in the waste but as well because of a higher recovery rate in sedimentation. Assuming that 4.4 g of COD are necessary to reduce 1 g NO3-N to N2 (including a biomass yield of 35%), full denitrification on internally generated COD sources in RAS could be theoretically realized with a HF diet when compared to a LF diet. However, if fibers are not degradable, the effective solid COD/N ratios for denitrification would drop to 3.1 and 2.8 g COD/g N/kg feed for HF and LF respectively (see fig. 1). If these predictions are true will be confirmed in bioassays using the collected solid waste samples. The role of fibers as qualitative limitation for COD bioavailability in denitrification still needs investigation.