Performance Of The Biological Nitrifying And Denitrifying Technology And Its Influencing Factors In The Recirculating Aquaculture System

The yields of China's aquaculture has been ranked the first in the world for 24 years. However, large conventional aquaculture facilities consume large water resources, and also produce large amounts of nutrients and solid wastes. Through the technology of recirculating aquaculture system, the water is recycled and efficiently used, discharge is minimized and contamination from the external environments is reduced. The moving bed biofilm reactor (MBBR), as an efficient nitrification reactor, is often used as the biofilter for RAS. In the application of MBBR in RAS, there still have some problems existed, such as the slow start-up process, the influencing factors of ammonia oxidation rate, and the removal of accumulated nitrate nitrogen.Therefore, in' this study, the heterotrophic nitrifying bacteria were isolated to augmented the MBBR start-up process; the influencing factors of ammonia oxidation rate were clarified during MBBR operation; solid-phase denitrification (SPD) technology were used to remove the accumulated nitrate nitrogen; ultimately, the augmented MBBR was used in a pilot Koi RAS. The process of nitrogen removal from wastewater in aquaculture provides the data and theoretical basis for the further application of MBBR in freshwater recirculating aquaculture system.A total of 31 strains of the heterotrophic nitrifying bacteria were isolated and identified, and the Paracoccus sp. N74-1, which was an efficient aerobic denitrifying nitrification-aerobic denitrifying strain, was selected. The optimal growth conditions of strain N74-1 were: sodium succinate as carbon source , the temperature was 30?, the ratio of carbon to nitrogen was 6, and the pH value was 9. Under this condition, the removal rate of ammonia-nitrogen was 8.0 mg/(L h). The strain N74-1 also has the aerobic denitrification function, and the 24 h removal rate of nitrite was 94.1% while the primary nitrate-nitrogen concentration was 160 mg/L.The high density polyethylene K2 was selected as MBBR biofilm carrier through the comparison of the weight, thickness, nitrification efficiency and relative abundance of nitrifying bacteria in the biofilms of different carriers. With the bioaugmentation of N74-1 the start-up time of MBBR could shorten 10 days. It was clear that the concentration of ammonia nitrogen in MBBR effluent was less than 0.5 mg/L under 100 mg TAN/(L media d). The MBBR effluent quality was unstable when the load was higher than 200 mg TAN/(L media d). In simulated aquaculture experiments, the MBBR could be started up within 20 days by N74-1 treated, and the TAN concentration could meet the requirement of aquaculture under the condition of 12.5 kg/m3. The effect of different flow rate conditions on the start-up of MBBR in sturgeon aquaculture was studied, which indicated that the flow rate could affect the concentration of ammonia nitrogen and nitrite in MBBR of sturgeon RAS. At different water flow rates, the removal efficiency of nitrite was corresponding to the relative abundance of Nitrospira in the biofilms.The effects of temperature, water flow rate, C/N ratio and initial ammonia concentration on the ammonia oxidation rate of MBBR were studied. A predictive quadratic polynomial regression model of ammonia oxidation rate was established. The results of single-factor experiments showed that the removal rates of ammonia nitrogen and nitrite were both increased with temperature in the range of 15-30?, and the ammonia nitrogen removal rate at 25? was about 2 times higher than that at 15?.While the water flow rate was 0.5 L/min, the MBBR has the highest removal rate of ammonia nitrogen, and the removal rate of nitrite was increased with increasing flow rate. The low C/N ratio (0 and 0.5) is not conducive to the nitrifying process, especially the nitrite oxidation, and adequate carbon source is beneficial to both nitrification and aerobic denitrification. When the initial ammonia nitrogen concentration is below 15 mg/L, it can be completely removed at 24 h, and the maximum ammonia nitrogen load of the K2 carrier is between 150-200 mg TAN/(L media d).Based on the response surface methodology (RSM), the MBBR ammonia nitrogen oxidation rate regression model of ammonia nitrogen concentration, COD/TAN and water flow rate was established. The regression model was significant (p=0.0007), and the removal rate of ammonia nitrogen in the MBBR would be analyzed and predicted in RAS.The results showed that the SPD technology can effectively remove the nitrate nitrogen in RAS, and the effects of the technology on the nitrification system and the bacterial community in aquaculture water were evaluated. The nitrate nitrogen concentration in RAS were maintained below 10 mg/L by SPD reactor. The bacterial community structure of RAS was significantly affected by the SPD process and the changes included an increase in the proportion of Proteobacteria and Firmicutes and a decrease in Nitrospira abundance in RAS.In the application of bio-augmented MBBR in a pilot Brocarded Carp RAS, heterotrophic nitrifying bacteria N74-1 made MBBR to start up successfully within one month. With different culture densities, the VTR of MBBR was obviously different in the culture process of pilot system for more than 200 days, which caused the difference of ammonia and nitrite nitrogen concentrations in the culture pond with different culture densities.The water flow rate also had a certain effect on VTR, and the ammonia nitrogen concentrations at different water flow rates(2 m3/h and 1 m3/h)were significantly different. Using BiologTM ECO-plates and high throughput sequencing analysis, it was shown that the function of the microbial communities in MBBR biofilms was related to their composition.The relative abundances of Sphingobium and Hyphomicrobium with low aquaculture density were higher than those with high density, and the utilization rate of phenolic acid in low density was also higher than that in high density. With the increase of density, the relative abundance of bacteria (Rhodobacter sp.), Which can use simple organic matter such as monosaccharide, increased, and the ability of total sugar utilization in the bacterial community increased accordingly. On the other hand, the concentrations of nitrogen concentrations in water were significantly correlated with the relative abundance of Bdellovibrionaceae,Caldilineaceae, Roseiflexus, Rhodobacter and Rhodobacteraceae.