| A fine-bubble OD was an economical and efficiency process due to uptakingoxygen with high efficient and then has been widely used in municipal wastewatertreatment plant (WWTP). However, the conventional fine-bubble aeration oxidationditch with the same flow velocities in the aeration zone and anoxic zone leads to highenergy consumption. Therefore, a novel process of fine-bubble aeration oxidation ditchprocess is proposed in this research. The change of channel flow velocities in aerobicand anoxic zone is achieved by changing the channel section size in fine-bubbleaeration oxidation ditch.A novel pilot-scale variable velocity oxidation ditch process with fine-bubbleaeration (VODF) equipped with an anaerobic selector was established in Xi’an fourthWWTP. It was used to treat50m3/d raw domestic wastewater during two year. Themain results are listed as follows：(1) The VODF is tested as a feasible method. Computational Fluid Dynamics(CFD) simulation, tracer method and ADV velocity measurement instrument wereapplied to analyze and determined the flow field of mixed liquor in the VODF process.The results show that the three means of velocity distribution could coincide with eachother in the VODF. The channel width ratio is nearly close to the inverse ratio of thecorresponding flow velocities.(2) When the flow rates (cross-sectional mean approximate velocities) of the nonaeration (plug flow) and aeration zones average was controlled at0.15m/s and0.07m/s,respectively, no sludge accumulation in VODF was observed. The velocities are muchless than oxidation ditch critical0.3m/s. It contributes to increase the size of floc,comparing with traditional OD process.Therefore, the microenvironment in the floc wascreated,and it was more favorable for simultaneous nitrification/Denitrification (SND).(3) When the operational condition of VODF process is SRT of15d,HRT of14h,the average organic load of0.07kgCOD/(kgMLSS.d) and the temperature rang from15℃to28℃, the concentrations COD,NH4--N and TN concentrations meet dischargeone-class A requirement of the new discharge standards of China (GB18918-2002). Inaddition, TP concentrations in the effluent meet discharge one-class B requirement ofthe new discharge standards of China (GB18918-2002)，even achived dischargeone-class A requirement part of time.(4) A/O (anoxic zone/oxic zone) volume partition in circle OD can be controlledby DO concentrations by automatic control system at end of fine-bubble aeration zone.When the dissolved oxygen concentration and the corresponding A/O ratio werestabilized at1.5mg/L and0,1.2mg/L and0.3,0.9mg/L and1.8,0.6mg/L and4.0,0.3mg/L and9.7respectively, and consequently nitrate average concentrations about25.33,19.22,9.23,6.11and1.07mg/L and the effluent NH4+-N concentrations0.21,0.16,0.25,7.33and17.15mg/L respectively in effluent can be obtained in secondary clarificationtank effluent. The effluent total nitrogen (TN) concentration was less than15mg/L andconsequently, nitrogen removal rate by SND was36%When the DO concentration was0.9mg/L at the aeration zone end and A:O volume ratio was1.8.(5) Microthrix Parvicella in activated sludge has adverse effects on nitrogen andphosphorus removal. The maximum specific nitrification(ammonia oxidation and nitrateoxidation) rates, maximum specific denitrification rates, maximum specific release ofphosphorus rates(SRPR) and maximum specific uptake phosphorus rates(SUPR)ofscum containing much Microthrix Parvicella (scum)and sludge mixture containing alittle Microthrix Parvicella (mixture) were analyzed separately. The maximum specificammonia oxidation and nitrate oxidation rates of scum were1.15mgNH4+-N/gVSS·hand0.87mgNO2--N/gVSS·h respectively.However, the rates of sludge mixture were2.05mgNH4+-N/gVSS·h and1.65mgNO2--N/gVSS·h respectively.The maximumspecific denitrification (readly, slowly and endogenous) rates of scum were8.37,2.49and0.53mgNO3--N/gVSS·h respectively, while the rates of sludge mixture were4.98,1.94and1.07mgNO3--N/gVSS·h respectively.In addition, the scum SRPR and SUPRwere0.64and1.06mgPO43--P/gVSS·h respectively while the mixture SRPR and SUPRwere2.86and2.63mgPO43--P/gVSS·h respectively. The results show that thenitrification ability of scum, which contains a great deal of Microthrix Parvicella, wasweaker than that of sludge mixture. The denitrification ability of Microthrix Parvicellain scum on utilizing readly and slowly degradable organics was better than that of in the mixture. In addition, available carbon source for PAOs (phosphorus accumulatingorganisms) in anaerobic tank was decreased because of Microthrix Parvicella. As aresult,the the phosphorus uptake was affected in VODF tank.(6) The composition of VFA has important effects on phosphorus release anduptake. The SRPRs, SUPRs and SDNUPRs (specific denitrifying uptake phosphorusrates) were determined by acetic acid and propionic acid taken as substrates respectively.The results administrated that the SRPRs, SUPRs and SDNUPRs of acetic acid were4.29,3.66and1.9mgPO43--P/gVSS·h, while the rates of propionic acid were2.60,1.44and0.83mgPO43--P/gVSS·h respectively. In addition, the ratios of△P/△HAC(P-release/HAC-consume) and△P/△HPr(P-release/HPr-consume) were0.29and0.006during anaerobic phase respectively. Concusion can be drawn from the abovecomparisons that the substrate of acetic acid is more conducive to release and uptakephosphorus by PAOs.(7) Different electron acceptors have important effects on phosphorus removal. TheSUPRs for Oxygen, nitrate and nitrite as electron acceptors were9.72,4.32and3.98mgPO43--P/gVSS·h in accordance with the electron acceptors of oxygen, nitrate andnitrite. There is a good linear relationship between nitrate and nitrite consumption withphosphorus uptake.The ratios of phosphorus uptake to N of nitrate and nitrite are0.96,0.49mgP/mgN respectively.In view of the above research,Multiple A:O patterns of oxygen supply are helpfulto improve effluent quality. The diagnosis of the fine-bubble OD in Guilin Qi-lidianWWTP was performed.The recommendation was an existing A:O operation mode couldbe transformed to two A: O oxygen supply modes and the effluent quality would beimproved by increasing DO concentration to0.5mg/L at the oxidation ditch effluentand transferrring the A:O volume ratio to1:1.2.This modified OD could inhibit nitrogenand phosphorus release, as well as improve effluent quality of the secondary settlementtank, saving aeration fluxes by efficient utilization of oxygen. |
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