| Recirculating aquaculture system (RAS) is one of the potential technology to solve the problem caused by contraditional aquaculture. However, marine biofilter is found to have problems on start-up stage, which exhausts long time on biofilm growth. Moreover, common method of biofilter design is based on the mass balance, which calculate the biofilter size for the average TAN production rate. Consequently, in practice, fish always suffers the peak TAN which can not be removed by the biofilter immediately. Therefore researches on biofilm formation and recirculating flow rate are conducted in this study, and the results are presented below:(1) Marine ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) suffered inhibition of nitrite nitrogen when concentration exceeding 100mg/L and 12 mg/L, respectively. However, in freshewater reactor, it was observed that AOB did not suffer inhibition under experimental condition when NOB was inhited by nitrite nitroten more than 50mg/L. Correspondingly, complete inhibition concentration of free nitrous acid (FNA) on marine and freshwater NOB was more than 0.05mg/L and 0.21 mg/L, respectively. Marine NOB was sensitive to environmental factors, resulting in nitrite accumulation and biofilm growth delay. The dominant NOB in marine environment was Nilrobacter while Nitrospira dominated freshewater. Besides, marine biofilm biomass and protein content of Extracellular Polymeric Substances (EPS) was surprisingly higher that freshwater biofilm confirmed by the analysis of total organic carbon and three-dimension excitation emission matrix fluorescence spectroscopy (3D-EEM), but contrast with the distribution of nitrification abilities. The discrepancy may result from the acclimation mechanism of bacteria to salinity.(2) Complete nitrification was reached on day 25 in biofilm on modified carriers, while it was day 35 on new carriers. Modified carriers can accelerate NOB colonization during the initial stage of biofilm formation, reducing nitrite accumulation and inhibitory effects of FNA on nitrifiers. Moreover, it was observed mainly proteins and polysaccharides remained on the modified carriers during biofilm formation, indicating the remains might play the role as organic substrate and spatial protection for the biofilm.(3) Biofilter was inoculated with mature biofilm carriers, complete nitrification was reached earlier when more inoculation provided (such as 10～15% inoculation ratio). However, new biofilm formation may be inhibited by mature biofilm, resulting from fierce substrate competition between new biofilm and mature biofilm.3 acyl-homoserine lactones (AHLs) (C4-HSL、C8-HSL ' 3OC8-HSL) were found in biofilm formation, the latter two may accelerate biofilm formation, therefore, synthetic AHL addition may provide the possilities of quick biofilm growth when sufficient substrate provided.(4) Total ammonia nitrogen (TAN) releasing peak (11～12mg/(h*kg)) was reached during 4-8h during 12h after tilapia feeding, while 4.2-4.7mg/(h*kg)TAN releasing rate was maintained during 0-4h and 8-12h. Furthermore, inorganic nitrogen concentrations under 375L/h (TAN:3.26-3.37mg/L, NO2--N:1.32～1.45mg/L) were higher than those under 750L/h, exceeding the threshold valued of tilapia growth (TAN≤3mg/L, NO2--N≤1mg/L). Varied flow operation through increasing recirculating flow rate can lower inorganic nitrogen at releasing peak stage (TAN:2.03-2.24mg/L, NO2-N:0.56-0.62mg/L). Besides, it was flexible and cost effective and provided support for water qualities adjustment and low-cost aquaculture. |
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