Study On Efficiency And Affecting Factors Of SBR-BAF Process For The Treatment Of Municipal Wastewater

With the deterioration and eutrophication of water environment, it is essential to treat and reuse the municipal wastewater due to the increasingly severe water resource shortage in China. There are several limitations with the suspended growth and attached growth single-sludge system respectively. Moreover, the effluent quality can't meet the wastewater reuse standard by means of gravity sedimentation in suspended growth sludge system. Under this background, an innovative two-sludge SBR-BAF process, combining the advantages of suspended growth and attached growth process, was developed to provide new brainstorm for the upgrade of built-up WWTPs (wastewater treatment plant) and the design of new WWTPs.In this study, the mechanism of organic substances, nitrogen and phosphorus removal is explored using two-sludge SBR-BAF system. In order to investigate the operation characteristics and removal efficiency of the two-sludge SBR-BAF process under the influences of all kinds of factors, a series of Lab-scale experiments with synthetic municipal wastewater as influent were carried out. The main aim of this paper is to find out the following objects: First, the influence of variations of factors (anaerobic and aerobic duration, MLSS and COD/TP ratio )on the removal efficiency of COD and TP in the SBR process; Second, the performance of BAF reactor under the impacts of anaerobic/aerobic volume ratio, reflux ratio, ratio of gas to liquid, C/N ratio, temperature and NH4+-N volume load; Finally, the influence of backwash, period of backwash and backwash parameters on effluent quality.Through the above experiments, conclusions reached are as follows:Firstly, the SBR-BAF system wholly performs well in disposing the organic substances, nitrogen and phosphorus of municipal wastewater. The mean removal efficiency of COD, NH4+-N, TN ,TP respectively is 95.98%,96.20%,68.91%,97.81%, and the corresponding concentration of effluent respectively is 19.79 mg/L, 1.45 mg/L,11.83 mg/L,0.15 mg/L , which means every index can meet the first class (A) of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant.Secondly, in SBR reactor, the investigation of anaerobic and aerobic duration indicates that TP-release has almost been accomplished when the anaerobic stage reached 45 minutes, then after 90 minutes of aerobic reaction, the removal efficiency of TP can reach 99%, and COD removal efficiency reaches 85.36%. So the anaerobic duration, aerobic duration is set as 45 minutes, 90 minutes, respectively. The study on the factor of MLSS which also has a great impact on the CODand TP removal efficiency shows that, under the conditions of unchanged TP in influent, the removal efficiency of COD and TP declines sharply as the concentration of MLSS declines.Another factor influencing P-release is C/P ratio .The experimental results show that, the P-release of sludge is more with higher C/P ratio. And the higher the ratio of C/P, the faster the rate of phosphorus accumulation in the aerobic stage. But the change of P-release becomes moderate after C/P ratio surpasses 40.Lastly, the results of experiments with BAF show that: there's no big difference for the disposal efficiency of COD and NH4+-N, when the anaerobic/aerobic volume ratio is in two different level, 2:1 and 1:1. However, the removal efficiency of TN is better when the anaerobic volume is augmented.'Causes the denitrification efficiency is improved. As for the factor of reflux ratio, similar trend can be found. When the reflux ratio is increased from 100% to 200%, there's little impact on the COD and NH4+-N disposal efficiency, but the removal efficiency of TN is improved, in consideration of improved denitrification efficiency with higher reflux ratio. The effects of ratio of gas to liquid on COD, NH4+-N and TN removal need to be described respectively due to different results. With the ratio of gas to liquid decreasing first from 10:1 to 5:1, then to 2:1, the COD removal has little change, while the NH4+-N removal efficiency has a downtrend. The TN removal efficiency reaches the greatest level at the ratio of 5:1. For the process of both decreasing from 5:1 to 2:1 and increasing from 5:1 to 10:1, TN removal efficiency falls down. The difference between the two process is that the latter falls much more slightly. TN removal efficiency raises correspondingly with the C/N ratio increasing from 3 to 5.5. The removal efficiency of NH4+-N changes little with the increase of NH4+-N volume load, while its concentration in the effluent have an uptrend. The COD removal efficiency increases slowly with the increase of water temperature, while the temperature has a great effect on the removal of NH4+-N. Under the temperature of 13℃, NH4+-N removal efficiency is only 65.25%, which is improved into 96.13% when temperature increases into 27℃.The results of backwash experiment with BAF indicate that: the period of backwash of the BAF reactor designed in this experiment is long. Generally speaking, BAF needs to be backwashed when the headloss increases to 10 cm every 17~20 days. At the same time, the study shows the reactor can renew wastewater efficiently again about 2 hours after backwash.The two-sludge SBR-BAF process as an innovative technics, with high sludge loadings, short HRT (hydraulic retention time) and optimally cultivated nitrobacteria, PAOs (phosphate accumulating organisms), solves series of conflictions of denitrification and dephosphorization among sludge age, carbon source and loadings etc.In conclusion, the two-sludge SBR-BAF system has a high ability of removal of organisms, nitrogen and phosphorus to municipal wastewater treatment. And another advantage of the reactor is saving energy and convenient management. So it is an integrative reactor with high efficiency and low consumption and can be widely used.