Preliminary Research On The Removal Mechanism Of Pollutants Using Biological Aerated Filter In Recirculating Aquaculture Systems

The wastewater treatment and reusing is always playing an important role in RAS. As the core unit of RAS, Biological Aerated Filter(BAF)plays a key role on removing pollutants in aquaculture wastewater. The purpose of this paper is to study the mechanism of pollutants removal,and the paper discussed the shock resistant ability of the BAF and the conversion of pollutants along the flow direction.Main conclusions were as follows:1. The process of natural hanging membrane charged with bamboo substrate lasts 40 days,under the influent conditions of ammonium nitrogen is 0.6～0.8mg/L,COD_Mn is 8～20 mg/L,temperature is 6～20℃,PH is 7.8～8.0,DO is 1.8～3.8mg/L,gas/water ratio is 3:1.The effluent concentration ammonium nitrogen and nitrite are low and less than 0.3 mg/L and 0.01 mg/L respectively.2. Removal efficiency of ammonia nitrogen ascended first then descended with the rising of ammonia nitrogen loading.Removal rate of ammonia nitrogen reaches up to 70% when the ammonia nitrogen loading was 0.87g/m³·h,and the trend would be decreased if continuely increased the loads.So the ammonia nitrogen loading shouldn't be more than 0.87 g/ m³·h.Removal efficiency of COD_Mn increased with the rising of organic loading,but the rate of increase was slowed when the organic loading exceeded 11.58 g/m³·h,it shows that the removal capacity of BAF was close to saturation,so the organic loading was not more than 11.58 g/m³·h .Increasing the hydraulic load appropriately did not markedly influence the efficiency of pollutants,but removal efficiency would be decreased if the organic loading exceeded 0.87 g/m³·h,so 0.87 g/m³·h is the threshold.Because of the dissolved oxygen is 1.8～3.8 mg/L of influent,and the carbonizes bacteria and nitrifying bacteria would consume oxygen when pollutants was degradated,so the DO in local area of the BAF was at low levels. In this case, biodegradation rate of ammonia nitrogen converted to nitrate nitrogen is lower than the rate of protein converted to ammonia nitrogen,so the effluent COD_Mn and ammonium nitrogen concentration increased,and the outflow water quality worsed. It should be given appropriate aeration in order to ensure the stable running of the BAF.3. The best bed height of organic removal was different from that of ammonia nitrogen removal in up-flow biological aerated filter. Along the flow direction,the ammonia nitrogen removal lagged behind the COD_Mn. It was indicated that the active layer of carbonizes bacteria was in the lower of reactor and the active layer of nitrifying bacteria was in upper the of reactor. The main removal height of COD_Mn and ammonia nitrogen was 0～10cm and 0～30cm respectively.The developing tendency of bimass is similar with bioactivity along the fluent directionwhich are all increased.The upper biomembrane still have certain bioactivity,although the upper bimass is fewer than the lower.Pollutants which can not be degradated of lower media could be removed on upper media when BAF at impact,so height of filter bed could be appropriately increased in order to make the BAF had powerful resistance to impact.The removal efficiency of COD_Mn and TSS of 100 mesh net ( particle size≤90μm) was better than 400 mesh ne(tparticle size≤25μm),and the removal rate of ammonium nitrogen are similar,when the wastewater was filtered by 100 and 400 mesh screen respectively.It shows that the removal effect of bigger particles was better than the smallers.From the removal effect, the removal efficiency of pollutants is better under the influent filtered by 100 mesh screen.However, the more particles get into the bio-filter and the larger their sizes are, the more prone to be blocked the reactor will be,and the frequency of backwash will be increased,so these should be considered comprehensively in engineering.