Research In Disposal Of Wastewater Under Low Dissolved Oxygen(DO) In Membrane Bio-reactor

Simultaneous nitrification and denitrification ( SND ) means nitrification and denitrification occur in the same reactor under the same operating conditions. It has the advantages of less space and oxygen, which conventional removing nitrogen systems do not have. Membrane bioreactor(MBR), which is a combined process of biological treatment with membrane separation, is a new technology of wastewater treatment. MBR is used in SND to prevent sludge bulking because of its efficient separation. The high sludge concentration in MBR is favourable to SND at the same time.The best effect and process of SND, characters of sludge and membrane fouling in different systems were reached under the operating conditions of different COD volume loads and DO in MBR, which fed with synthetic wastewater including nitrogen. The results as follows:(1) SND occured in MBR under low DO. Under COD/TN=8¹0, COD volume load =3.36kgCOD/ (m³Ã—d) ,TN volume load =0.38kgTN/ (m³Ã—d), HRT=2.5h, DO=0.2⁰.3mg/L, pH=7.0⁸.0, T=26²8℃, removal rate of COD, NH₄⁺-N, TN came to 96.25%, 91.70%, 56.71%. Repeatability experiment showed better effect: removal rate of COD, NH₄⁺-N, TN came to 96.47%, 93.73%, 65.34%.(2) The reach of process of SND showed that NH/-N declined with the time gone. NO₃^--N declined in the first hour, but in the remainder hours it rose. The reason was that carbon was enough for denitrification in the first hour. NO₂^--N accumulated in the first hour, but was translated to NO₃^--N with the time gone.(3)When MLSS was low, the kinds of microbe were rich. When MLSS was high, the kind of microbe were simplex. Sludge bulking occurred under low volume load and DO.(4)The mean size of sludge particles (279.6 μ m) in the system in which COD volume load was 1.68kgCOD/ (m³Ã—d) under the optimum operating condition was larger than those in general MBR, which was advantageous to supply micro-environments for SND. The mean size of sludge particles (65.147 μ m) in the system in which COD volume load was 3.36kgCOD/ (m³ × d) under the optimum operating condition was closed to those in general MBR. Because of high MLSS in the system, the system alsocould supply micro-environments for SND.(5)When MLSS was low, resistance of gel was the main part of filtration resistance.Flux could resume after physical cleaning. When MLSS was high, intestine resistancewhich was hard to be removed was the main resistance.(6)The operating resistance of system in which MLSS was low was lower than that ofhigh MLSS system. The operating resistance of system in which MLSS was low wasabout lOmPa for 14d. According to quality of effluent and operation management, itwas suggested that COD volume load=1.68kgCOD/ (m3 X d) , TN volumeload=0.19kgTN/ (m3 X d) ,DO=0.2⁰.3mg/L was optimum operating condition for SND in MBR. (7) Aerobic granular sludge formed in MBR under COD volume load=0.47kgCOD/(m3Xd) or 1.68kgCOD/(m3Xd),DO=0.3⁰.5mg/L,SRT=45d,T=25°C,pH=7.0 8.0.The aerobic granular sludge could be described as microbial congeries , in which filamentous bacteria were framework and extracellular polysaccharides were bond. It can be presumed that filamentous and extracellular polysaccharides play an important role in the formation of Aerobic granular sludge.