Dietary carbohydrate composition can change waste production and biofilter load in recirculating aquaculture systems

This study investigated the effect of dietary carbohydrate composition on the production, recovery and degradability of fecal waste from rainbow trout (Oncorhynchus mykiss) in recirculating aquaculture systems (RAS). Dietary carbohydrate composition was altered by substituting starch with non-starch polysaccharides (NSP) while keeping the diets isonitrogenous and isoenergetic. We tested a high starch, low NSP (LNSP) and a low starch, high NSP (HNSP) diet in six identical, small-scale RAS (V = 460 L). Each diet was tested in three independent systems over a period of six weeks. Shifting dietary carbohydrates from starch to NSPs resulted in a 50% increase in the production of chemical oxygen demand (COD) based on digestibility. Fecal waste recovery showed a 40% increase in HNSP treatments when compared with LNSP. Consequently, the COD output from HNSP systems doubled from 91 g to 194 g of COD per kg feed when compared with LNSP. Although COD production was higher in HNSP systems, the COD load on the biofilters was significantly lower when compared with LNSP systems. COD-to-nitrogen (COD/N) ratios in the biofilter load were 1.7 ± 0.2 and 2.2 ± 0.2 g COD/g N for HNSP and LNSP, respectively. Shifting the dietary carbohydrate composition from starch to NSPs decreased the biodegradability of fecal COD from 66.3% to 43.7% (P <0.001). Fiber analyses revealed that approximately 40% of the COD in HNSP feces came from cellulose and hemicellulose. The increased COD production of HNSP diets could be exploited by using fecal COD as an internal carbon source in denitrification. Full denitrification would be theoretically possible with a measured COD/N ratio of 7.2 in the waste stream of HNSP systems. However, it is not clear if the low COD bioavailability of HNSP feces could be a limiting factor. This study shows that COD/N ratios in the biofilter load and system output can be manipulated by changing dietary carbohydrate composition. Although an increased dietary NSP content increased COD production, it also increased COD recovery, decreased COD load on the biofilters and generated sufficient carbon for denitrification on internal sources.