Effects Of Organic Carbon Source And Air-exposure Time On Biological Filter Purification In Reciculating Aquaculture System

Biological filter as the core unit of water treatment in the recirculating aquaculture system（ RAS）, its purification effect is directly related to the breeding efficiency. Therefore, at present, more and more researchers studied how to improve the purification ability of biological filter. Carbon source is one of the necessary nutrition elements in the process of bacterial growth. Its form and content in the water not only affect the structure of the biofilm, but also influence affect the metabolic activity of the bacteria, thus affecting the water purification effect of the bio- filter. The length of biofilm air-exposure time affect the number of the main function flora, and affect the biofilter purification capacity. This paper systematically studied the effects of organic carbon and air-exposure time on biological filter purification. The main results are as follows:1、The preliminary screening results showed that with the same carbon source, when the rate of C /N increased from 0 to 6, the removal rate of ammonium nitrogen increased at first and then decreased. When C/N ratio was lower, the removal rate of nitrite had no significant difference（P > 0.05） among different carbon sources groups. While with the continued increasing of C/N ratio, the removal rate of nitrite reduced significantly（P < 0.05）. Except for ethanol group, the other three groups with C/N ratio increased, the removal rate of COD_Mn increased rapidly first and then stabilized. The removal rates of nitrate and total nitrogen increased first and then decreased in each group with significant variation（p <0.05）, and both reached the maximum when the value of C/N ratio was four. The rescreening results of carbon sources showed that when the value of C/N ratio was four, the removal rates of ammonium nitrogen, nitrate, total nitrogen and COD_Mn of each treatment group were all significantly higher than the control group（P < 0.05）. The removal rate of nitrite was the highest in the control group, which was 93.59%. When the additional carbon sources was ethanol, the removal effect of TAN, NO₂^--N, NO₃^--N and TN of the biological aerated filter was better than the other three carbon sources. Comprehensive consideration showed that, choosing ethanol as the additional carbon source, and making the C/N rate of 4:1,could well improve the purification efficiency of the biofilter.2、The study based on previous research, using recirculating aquaculture systems（RAS） simulator, compared the growth of flounder juveniles in different experimental groups. The results showed that the flounder juveniles after 40 d feeding, the growth rate of the control group was lowest, was 104.0%, and the alcohol group and the glucose group was significantly higher than the control group（P<0.05）; The survival rates in all groups were >95%. The feed coefficient of the brown sugar group was slightly larger than the control group, and the other three groups were significantly less than the control group. Ammonia nitrogen concentration was less than 0.3mg/L during the culture process in each group. N itrate concentrations first increased and then stabilized in all experimental groups, and at steady stage, the control group were higher than the other four groups. Adding a small amount o f soluble organic carbon to RAS would lead to contents of organic substances in the water increased rapidly after microbial decomposition and utilization returns to normal within a short time, to some extent, this environment was beneficial to the healthy growth of flounder juveniles in RAS. The results of this study ha ve important significance for the organic carbon in the actual aquaculture production application.3、The research aimed to provide theoretical help for establishing a scientific bio- filter cleaning method in the culture production. By using recirculating aquaculture systems（RAS） simulator, stud ying the changes of the b iofilm nitrification effect and the number of the main function flora in the biofilm after different air-exposure time treatment. The results showed that When the air-exposure time was 3 hours, after 21 hours running, the removal rate of ammonium nitrogen and nitrite were best, 80.1% and 93.5%, respectively. With air-exposure time became longer, the biofilm removal ability of COD_Mn became weaker. Each group after corresponding treatment, the numbers of heterotrophic bacteria, Nitrosomonas and Nitrobacter on biofilms were significantly reduced. In addition to the 9h group and 15 h group, the numbers of the main function flora among the other groups are extremely significant difference（P<0.01）. To sum up, in the actual production, in the case of bio-filter needs to be washed, the wash time should be controlled at about 3h.