The Effect Of Gas-water Ratio On Removal Efficiency Of Biological Aerated Filter In Recirculating Aquaculture Systems

The most important in recircualting aquaculture system（RAS） is recycling usage foraquaculture wastewater after Purification Treatment. Biological aerated filter（BAF）plays a keyrole on removing pollutants in aquaculture wastewater, and it also is the largest energy unit. Forthe purpose of energy-saving, this paper studies the effect of gas-water ratio on removal efficiencyof BAF, to find the best gas-water ratio. The main conclusion are as follows:1. In this study, The BAF operating conditions were as follows: TAN concentratio is1.0～2.4mg/, CODMnconcentration is1.0～2.4mg/, the temperature is10～20℃, pH is7.8～8.0,gas-water ratio is6:1, hydraulic retention time(HRT) is30min. The process of natural hangingmembrane naeds75days, now,the effluent concentration of TAN and nitrite are low and lessthan0.45mg/L and0.27mg/L.2. Under different gas-water ratio there were some difference in the pollutants removal rationfor BAF. It is different in water quality. The water quality was unstable under low gas-water ratio.And with the increase of gas-water ratio, the removal ration of TAN increased to, theaccumulation of nitrite in the effluent reduced. The efflulent water quality becam better and stable.When gas-water ratio was3:1, the effluent TAN concentration fluctuated between0.125mg/L~0.716mg/L. But it fluctuated between0.228mg/L~0.348mg/L and0.210mg/L~0.345mg/L whengas-water ratio was20:1and30:1. There was a significant difference（p <0.01） between theeffluent TAN concentration when gas-water ratio was20:1and30:1to the others. There was nosignificant difference（p>0.05） between the effluent TAN concentration under other differentgas-water ratio. The effluent nitrite concentration decreased with the increase of gas-water ratio.The biggest effluent nitrite concentration was0.302mg/L under the gas-water ratio of3:1, and theaverage effluent nitrite concentration was0.169mg/L. And the biggest effluent nitriteconcentration was0.110mg/L under the gas-water ratio of5:1, and it was all lower than0.1mg/Land the other gas-water ratio. There was no much difference between CODMnremoval ration withthe change of gas-water ratio.3. The pollutants changed as the same trends under different gas-water ratio along the BAF.Most amount of TAN and CODMnwere removed in the range of0~20cm along the BAF, and than they were changed a little. Sume times the removal ration decreased, the main reason was that theerosion to the biofilm from gas was so large that the biofilm exfoliated and flowing out, performedthat the effluent CODMnconcentration increased.4. It was different between biofilm thickness under different gas-water ratio. The biofilmthickness redused with the increase of gas-water ratio. The maximum biofilm thickness was0.155g biofilm per square fillers when gas-water ratio was5:1, But it was0.054g biofilm persquare fillers when gas-water ratio was30:1.5. There was much difference in total number of heterotrophic bacteria of the biofilm underdifferent gas-water ratio, but it only a little, they all fluctuated between106～107, there was littleeffect between total number of heterotrophic bacteria from gas-water ratio. There were somedifference between AOB and NOB number under different gas-water ratio, and it also had somedifference along the BAF. Generally speaking, the biological activity was higher when gas-waterratio were5:1and30:1than the others. Considering water quality, under this experimentalconditions, the optimal gas-water ratio for BAF was30:1, onw the TNA removal rate was80%,and the accumulation of nitrite in the effluent was lower, and the biological activity was higher.Effluent water quality was batter and stable.