Semen Litchi-based Natural Carbon Source Supported Denitrification In Recirculating Aquaculture Systems

Microbial filter is the core technology of Recirculating Aquaculture Systems(RAS), where ammonia was converted to nitrate by microorganism aerobic nitrification. However, low organic carbon level hampered denitrification process caused nitrate accumulation, of which was harmful to aquaculture species. In order to solve the nitrate accumulation during the recirculating aquaculture system, agricultural by product in South China, rice husk and semen litchi were chose as the natural carbon source for denitrification. The nutrients, the content of heavy metals, the microstructure, carbon releasing and denitrification performance of rice husk and semen litchi were investigated firstly. Then semen litchi was used as carbon source and blended with polyvinyl alcohol(PVA) to increase the porosity and mechanical strength of semen litchi. The morphology and the release velocity of semen litchi/PVA was studied systematically. The following work has been completed in this dissertation:(1) The nutrients, the content of heavy metals, the characteristic, and the functional group of rice husk and semen litchi were investigated firstly, and then carbon release ability in seawater was studied. The results showed that rice husk was rich in crude fiber(42.9%), and the content of total organic carbon was 58.17%; starch in semen litchi was up to 48.8% and the content of total organic carbon was 60.07%. After that, the content of copper, lead, cadmium, chromium, arsenic were analyzed. The results showed that rice husk containing 66.9 mg/kg chromium, which was six times higher of permitted content of the feed hygiene standard; whereas, all the heavy metals’ content in semen litchi was within the relative safety standards. The SEM results showed that the surface of rice husk was very rough and many porous structure were observed on cross-section; the surface of semen litchi was smooth, with many pebble-like particles inside, and a large number of gap between particles. FTIR results showed rice husk and semen litchi both were rich in hydrophilic functional group, which indicated that rice husk and semen litchi had good bioaffinity. The carbon releasing of two materials in seawater was similar. At the first 4 days, the carbon releasing velocity was fast, and then slow down at 4-8 days, at last, the total organic carbon(TOC) concentration saturated at 18-46 days.(2) Denitrification performance using shattered rice husk and semen litchi as the sole carbon source was studied. Rice husk denitrification results showed that rice husk denitrification effect was not obvious, at the first three days, only a few of nitrate nitrogen had been removed, and at other 10 days, the removal rate of nitrate nitrogen were below 50%; semen litchi denitrification results showed that its denitrification performance was excellent, in 1-8 days, nitrate nitrogen removal rate was ranged from 98.4% to 100% and the removal velocity of nitrate nitrogen was up to 192.4 mg N/(L?d), meanwhile, with no nitrite accumulation in effluent. Microbial community structure analysis showed that microbial diversity and abundance increased in rice husk and semen litchi sludge. However, there was more dominant denitrifying bacteria in semen litchi sludge. According to the first two experimental results, it confirmed that semen litchi was safer and more effective as external carbon source.(3) Semen litchi was used as carbon source and blended with PVA and sodium alginate(SA) to prepare semen litchi/PVA composite carbon source. The microstructure, pore structure and specific surface area of the composite materials were characterized. The results showed that the composite materials having a regular sphere structure, drying diameter of 3.6-4mm, density of approximately 0.43 g/cm3. Average pore size was about 5.93 nm and BET specific surface area was ranged from 13.84 to 26.6 m2/g. Carbon releasing of composite carbon source in seawater was studied and the results showed that the carbon releasing velocity was quickly at the beginning, and the semen litchi/PVA composite was broken after four days swelling in seawater system. The preparation process of semen litchi/PVA composite carbon source should be optimized.In summary, semen litchi contains about 48.8% starch and low levels of heavy metals, made it an excellent biocompatible carrier for denitrifying bacteria. The removal of nitrate nitrogen rate of semen litchi could reach to 98.4-100% in seawater, and the removal velocity could reach to 192.4 mg N/(L?d) with no nitrite nitrogen accumulation in effluent. Semen litchi was a safe and effective external carbon source. When semen litchi was blended with PVA could form good shape, moderate strength and porosity composite carbon source. However, further optimize of the preparation process was needed to solve the broken problems of the composite carbon source in seawater.