Dietary carbohydrates and denitrification in recirculating aquaculture systems

Due to overfishing of global fish stocks and increasing fish meal prices, plant ingredients are being increasingly used as an alternative source of protein in fish feeds. However, the inclusion of unpurified plant ingredients will also increase the content of fibers in feeds. Fibers are nearly indigestible and will therefore increase solid waste production in aquaculture. This solid waste can be used to as a carbon source for denitrification to control nitrate levels in recirculating aquaculture systems (RAS), thereby reducing both solid and dissolved waste production. Additionally, fibers can change the recovery characteristics and lower the degradability of fecal waste. Therefore, this study investigates how changes in the dietary carbohydrate composition can affect waste production, system performance and denitrification in RAS. Furthermore, ultrasound treatment (to decrease particle size in fecal waste) and enzymatic conditioning (to increase fiber degradability) were tested as possible means to increase the bioavailability of carbon in fecal waste for denitrification. Comparing a high fiber (HNSP) and low fiber (LNSP) diet in RAS stocked with rainbow trout confirmed that the fibers in the HNSP diet increase fecal waste production. Although the HNSP diet produced more fecal waste than the LNSP diet, both diets produced the same amount of biodegradable fecal carbon. Since feces removal was higher in RAS using the HNSP diet, the load of degradable organic matter on the biofilters was lower with the HNSP diet than with the LNSP diet. Furthermore, fecal waste produced with the HNSP diet contained larger particles than feces of the LNSP diet, which could also improve the recovery of fecal waste with microscreens. Feces produced with the HNSP diet were also less degradable than feces produced with the LNSP diet. By using fecal waste as an internal carbon source for denitrification, solid and dissolved waste emissions from RAS could be reduced by ~50% for the HNSP diet. However, only approximately half of the supplied cellulose and hemicellulose were degraded in the denitrification reactors, whereas lignin was not degraded at all. Thus, the overall degradability of organic carbon in fecal waste was limited by fibers as hemicellulose, cellulose and lignin. Ultrasound and enzymatic conditioning did not sufficiently increase the degradability of fecal waste. Nonetheless, fibers originating from unpurified plant ingredients may also have beneficial effects on RAS performance by increasing fecal recovery. A more selective choice of feed ingredients could be used to increase the recovery and degradability of fecal waste in RAS.