Nitrogen Removal By Multi-stage Biological Filter Process

A three-stage (carbon oxidation stage one, ammonia oxidation stage two, denitrification stage three) biological aerated filter (BAF) process, using ceramist as filter media was operated to enhance nitrogen removal. This makes fixed microorganisms growing at different reactors under optimal conditions. Then they can work at removal rates to their maximal kinetic rate. The treat abilities comperation of a single BAF and two-stage BAF system were discussed. The performance and optimization of three-stage BAF system also been studied.Firstly, the performance of single-stage BAF reactor was studied as a basis for comparison. The impact of organic loading rate on COD and NH₄⁺-N removal efficiencies were observed for domestic wastewater treatment. The results show that, when higher organic loading rate (about 2.81kgCOD/m³Â·d) was adopted, the effluent COD concentration can meet primary A national discharge standard, but the effluent NH₄⁺-N concentration was much higher than the standard required; when lower organic loading rate (about 1.33kgCOD/m³Â·d) was adopted, the effluent COD and NH₄⁺-N concentration both can meet primary A standard. As heterotrophic bacteria and nitrifying bacteria both existed in the single-stage BAF reactor, their optimal environmental conditions are different, and backwash has apparent impact on the microorganism's activities, so it is very difficult to achieve efficient and stable COD and NH₄⁺-N removal.Then, a two-stage BAF system was operated to enhance COD and NH₄⁺-N removal in two separated reactors. This can make heterotrophic bacteria and nitrifying bacteria growth at their optimal conditions through organic loading rate and backwash controlling. The results show that, at same HRT and air/water ratio, COD and ammonia removal efficiencies in two-stage BAF were much higher than single-stage BAF. The first column has efficient COD removal (effluent COD≈50mg/L) at even higher organic loading rate (>3kgCOD/m³Â·d). Effluent NH₄⁺-N concentration of second column was lower than 5mg/L. The working period of second column can reach 5⁷ days, backwash intensity and duration can also be reduced. The nitrifying microorganism's activities in the second reactor are higher than the nitrifying area in a single-stage BAF.A post-denitrification reactor was added behind the two-stage BAF system to qimprove TN removal. The performance of three-stage BAF system processing under different hydraulic loading rate, nitrified liquid reflux, temperature conditions was studied. The results show that water temperature has strong impact on the treat abilities of three-stage BAF system. Effluent COD, NH₄⁺-N and TN concentration can meet t primary A standard at the condition of water temperature was greater than 23℃with no internal reflux and alkalinity supply. When water temperature was between 8²3℃, at lower organic loading rate and internal reflux was adopted, the effluent COD, NH₄⁺-N and TN concentration also can meet the standard above.