Research On Two-stage Sequencing Batch Membrane Bioreactor (TSBMBR) In Treating Municipal Wastewater

The two-stage sequencing batch membrane bioreactor (TSBMBR) process was put forward to face with the problems in biological nitrogen and phosphorous removal, like different sludge age between nitrification and phosphorous bacteria, competition for carbon during denitrification and phosphorous release process, influence of nitrate on the phosphorous release in anaerobic zone, phosphorous release during sedimentation process, sludge with high phosphorous concentration in the effluent which can cause high TP concentration in effluent and etc. The two-stage sequencing batch membrane bioreactor (TSBMBR) process was aim to optimize the removal of nitrogen and phosphorous. With the control of operation parameters, phosphorous bacteria and nitrification bacteria can be controlled in two-stage reactors. Phosphorous and large amount of COD could be removed in the first stage, and nitrogen could be removed in the second stage, which could in a whole realize the simultaneous removal of COD, nitrogen and phosphorous. The membrane fouling can also be mitigated effectively. This operation can provide references for application and design. The characteristics of two-stage sequencing batch membrane bioreactor (TSBMBR) process can be summarized as follows:â‘ Dissolve the contradiction between autotrophic nitrification bacteria and heterotrophic bacteria and enable the sludge in the reactor that provides effluent is not P-rich type, and with the effective separation of membrane, the concentration of TP in the effluent can be completely reduced. And the optimal nitrogen and phosphorous removal can be obtained;â‘¡Large amount of COD can be removed by the first stage of reactor and the refractory biodegradable organics can be furthered removed in the second stage reactor, hence the removal efficiency of COD can be improved;â‘¢Reduce the toxic shock loading to nitrification bacteria and enhance the stability of denitrification;â‘£Combine sequencing bath and intermittent suction operation modes together to mitigate membrane fouling effectively.TSBMBR process was applied to treat wastewater of Liaoquan residential area, Laoshan district, Qingdao. Experiment and mechanical analysis were combined to systematically research on the feasibility of TSBMBR in removing phosphorous and nitrogen, the removal efficiency, solution characteristics and influence on membrane fouling.1. The research on TSBMBR includes removal efficiency of phosphorous and organic pollutants in SBMBR1 (phosphorous removal stage), phosphorous removal characteristics and mechanism in one cycle and removal efficiency of nitrogen and denitrification dynamics in SBMBR2 (nitrogen removal stage). Through the control of sludge age (5~7d i n SBMBR1,25~30d in SBMBR2), phosphorous bacteria and nitrification bacteria were the dominant species in each stage. Therefore, the contradiction between phosphorous removal and nitrogen removal can be optimized in two chambers separately. After the stable operation of system, COD, TN, TP and NH3-N average concentration in the effluent was 25.75mg/L, 33 mg/L,0.46mg/L and 2.71 mg/L, which could meet the Criteria of Scenic Environment Using Water. Concerning the phosphorous release effects in sedimentation period of PAOs and sedimentation efficiency, the optimal sedimentation time in SBMBR1 was 20 min. Through the periodic experiment, the nitrification rate constant K1 equaled to 0.21 and the denitrification rate constant K2 equaled to 0.22. That is to say, the denitrification rate is higher than nitrification rate.2. Lab experiments and SEM observation were performed to analyze the sludge characteristics in SBMBR1 and SBMBR2. From biological and physical point of view, the sludge characteristics in two stages are totally different. The sludge in SBMBR1 was light yellow and with the time passed by, the sludge size gradually increased and the SV, SVI value gradually decreased. The sludge in SBMBR1 presented granular characteristics and settled very well. The sludge in SBMBR1was brown and the sludge flocs was smashed by the shear force due to high intensity of aeration. The sludge size decreased and the sludge flocs blended with water forming ropy solutions, and there was no distinguish interface between water and sludge. Low sludge age and high loading operation enabled the sludge in SBMBR1 keeping higher sludge activity and the MLVSS/MLSS ratio and SOUR values of SBMBR1 were higher than SBMBR2. Both in SBMBR1 and SBMBR2, there were large amount of zoogloea, bacillus, coryneform of bacteria, and pediococcus. However, in SBMBR1 there were also a lot of Protozoa like epistylis and etc. A certain amount of filamentous bacteria can also be found in SBMBR1. The biological species in SBMBR1 were much more abundant than that in SBMBR2.3. Membrane fouling on flat-sheets membrane was observed and analyzed with AFM, SEM and TEM. After the measurement of membrane surface EPS and membrane surface ETX analysis, the pollutants composition on the surface of flat-sheets membrane was calculated. After 110 days operation, pollutants layer formed on the membrane surface in SBMBR2, which could induce the increase of trans-membrane pressure (TMP). The roughness of fouled membrane surface was 20 times higher than the new membrane and the thickest height of membrane pollutant layer was 1695.74nm. Darcy's Law was used to analyze the resistance distribution on the membrane surface, the resistance of membrane (Rm) accounted for 38.24% of total resistance, the cake resistance (RC) accounted for 53.09% of total resistance (43.09% of which was gel polarization resistance), and the pore blocking resistance(Rp) accounted for 8.67% of total resistance. The gel lay caused by concentration polarization on the membrane surface was the main reason of the reduction of membrane permeability. The membrane pollutants were composed of organic pollutants, biological pollutants and inorganic pollutants. The organic pollutants represented by EPS on the membrane surface were 417.69EPSmg/g.MLSS, and the protein/polysaccharide ratio was 3.5. Protein was the predominant pollutants. The biological pollutants on the membrane surface are mainly bacillus and pediococcus and the inorganic pollutants were scales mainly composed of Fe, Na, Zr, Rb and Ca compounds inner and outer the membrane.4. The influential factors of membrane fouling in SBMBR2 were also investigated. Membrane fouling was mitigated through optimizing operation parameters and dosing PAC. The increase of sludge concentration could increase the membrane fouling velocity. EPS and SMP accumulated in the reactor were the main formation reason of gel layer. In reactor solutions, substances with high molecular weight accounted for 61%, which was much higher than the value in the influent and effluent. This is the main reason of membrane fouling. The Combination of sequencing bath and intermittent suction operation modes could mitigate membrane fouling effectively, the velocity of which was much lower than the non-stop MBR. After operation for more than 100 days, the TMP only increased 0.12bar. After the dosing of PAC, the sludge size increased, and the SMP in the supernatant reduced, and the membrane filterability was mitigated dramatically. With the dosage of PAC reached 1g/L, the specific membrane flux enhanced 70% and the membrane flux reduction rate in one cycle was also reduced. The protein and polysaccharide concentration in the supernatant was also reduced by 14%, and 12%, which was beneficial for the mitigation of membrane fouling. The combination of physical and chemical cleaning was performed on the cleaning of fouled membrane. After aeration for 2 hr, 400 mg/L sodium hypochlorous cleaning for 2 hr, and then 200 mg/L sodium hypochlorous marinating for 20hr, the specific membrane flux recovered to 81.51%.