| Aquaculture industry is developing rapidly,resulting in increasing volumes of wastewater,which have a significant impact on the environment and prevent the pursuit of ecological civilization.Recirculating aquaculture systems are a new industrial farming technology that fosters environmental sustainability by reducing water consumption and improving sewage discharge management.These systems feature a high degree of control of the aquaculture environment,thereby not only decreasing risks(including natural disasters,environmental pollution,and disease outbreak),but also resulting in optimal growth conditions for the breeding varieties throughout the year while being economically sustainable.As a result,the industrialization of recirculating aquaculture has received increasing attention and support.The core of water treatment processes in recirculating aquaculture systems is represented by biofilters,whose stability and efficiency directly affect the quality and performance of the whole aquaculture system.Designing biofilters that are suitable for aquaculture production and operation is therefore a key step.During the operation of a circulating aquaculture system p H values tend to decrease,thereby affecting the performance of biofilters and their associated microbial communities.To improve biofilter nitrification efficiency,we investigated the effect of different p H values on the nitrification performance of the microbial community growing in biological filters.The main findings of our study are as follows:1.A designed recirculating aquaculture system had a target yield of 7000 kg and a stocking density of 35 kg/m3,with feeding rate to 1%,water temperature to 14 °C,salinity to 30‰ and p H to 7.2.A moving bed biofilm reactor(MBBR)was specifically designed for our aquaculture system.Four identical biofilters(dimensions: 4 m x 4 m x 2.5 m;water level: 2.2 m)were used to treat wastewater.The influent flow rate was 286 m3/h,the hydraulic retention time was 29 min,the water exchange rate was 1.16 m3/h and the number of cycles per day was 20.The diameters of the total inlet pipe,water inlet branch,and water supplement pipe were 300 mm,200 mm,and 20 mm,respectively.A JGR150 Roots blower with an air volume flow rate of 1716 m3/h supplied aeration.Our system provided new scientific insight for the design of biofilters and MBBRs applied to recirculating aquaculture systems.2.The MBBRs were started with blank carriers under the following conditions: ammonia nitrogen concentration of 2 mg/L,COD concentration of 4–5 mg/L,salinity of 30‰,dissolved oxygen concentration of 6–7 mg/L,and p H of 8.0.The results showed that ammonia oxidation capacity developed quickly in the biofilters within the first 36 days of operation.However,nitrite oxidation capacity developed slowly and we observed nitrite accumulation,with a peak on day 36.The Shannon,Chao1 and ACE diversity indices for the biofilter microbial community showed a decreasing trend,while biofilm function and stability increased after day 29.The relative abundance of ammonia-oxidizing bacteria(AOB)was directly proportional to total ammonia nitrogen(TAN)removal efficiency,with a Pearson correlation coefficient of 0.624.In the early stages of MBBR operations,the ratio of AOB to nitrite-oxidizing bacteria(NOB)at five periods were 20:1,6:1,4:1,2:1,and 1:1,respectively.The observed decrease in the AOB/NOB ratio,which approached a value of 1 as time progressed,was likely due to the slower colonization rates of NOB relative to AOB.The values of the AOB/NOB ratio provided a basis for explaining the trend in nitrite concentration observed during the start-up phase.These results showed that microbial community composition could be predicted dynamically during the MBBR start-up period,thereby providing critical information to address the issue of nitrite accumulation in the start-up stages.3.Four groups of MBBRs were operated at four different influent p H values: 7.0(G1),7.5(G2),8.0(G3),and 8.5(G4),respectively.After a stable nitrification phase,the ammonia conversion rate of the four MBBR groups reached approximately 80% after 36 days,with no significant difference among groups,indicating that p H had no significant effect on TAN removal.However,the OTUs that characterized the AOB community were sensitive to different influent p H values(p < 0.05).We did find significant differences in nitrite removal performance,with the four treatment groups showing decreasing removal efficiency in the following order: G2 > G3 > G1 > G4.Different p H values showed an asymmetric inhibitory effect on bacteria,with inhibition at high p H values being stronger than at low p H values,as also suggested by the AOB/NOB ratio.Principal component analyses,Venn maps,and variance analyses showed significant differences in microbial community,AOB,and NOB under different p H conditions.Overall,different influent p H values had a significant influence on the nitrite oxidation performance and microbial community of the biofilters,with the effect of p H on bacteria being asymmetrical.The p H values should be closely monitored during system operations and low p H values should be adjusted to ensure safe culture conditions and high nitrification efficiency of the microbial community. |
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