Abstract:To avoid the harms caused by the peak ammonia and nitrite produced during the ammonia release period, and increase the productivity and reduce the cost, operations under variable flow rates were investigated. To obtain the proper parameters for variable flow rates, the characteristics of tilapia ammonia release were firstly studied. After then, the effects of different flow rates on the nitrification rates and water qualities of recirculating aquaculture system (RAS) were studied. The results showed that during feeding interval, the highest ammonia release rate (0.21~0.23g/h) was found in 4~8h, 〖JP2〗while in other stages the ammonia releasing rate was 0.08~0.09g/h. Moreover, no obvious effects of flow rates on nitrification abilities were observed and the maximum ammonia oxidizing rate (7.72g/(m3·h)) 〖JP〗was higher than the maximum nitrite oxidizing rate (7.21g/(m3·h)) in nitrifying biofilm, indicating that mass transfer did not greatly affect the nitrification process. Besides, high flow rate (750L/h) in RAS produced lower ammonia and nitrite than those of low flow rate (375L/h), which also exhibited peak values of ammonia (3.26~3.37.mg/L) and nitrite (1.32~1.45mg/L) exceeding the permissive concentrations for tilapia in 12h. Therefore, the RAS was operated at 750L/h in 4~8h and 375L/h in other time during feeding interval. Lower ammonia and nitrite concentrations (2.03~2.24mg/L and 0.56~0.62mg/L) were obtained during the peak stage of ammonia release under variable flow rates. This operation could be achieved just by providing additional pumping for 4h under 375L/h during the peak stage of ammonia release, and it could be a reference for economical RAS technique.
