TheAl/A2/O-membrane Bioreactor Coupled With Nanofiltration Process For Coking Wastewater Advanced Treatment

With the increasing shortage of water resources and the growing requirement ofenvironmental quality, it is imperative to raise the reuse ratio of industrial wastewaterand make an effort to realize its "zero emissions". The Coking wastewater is one kindof refractory industrial wastewaters due to its complicated components, and ischaracterized by high COD, NH₃-N and slowly degradable organics. A popularproblem faced in most of coking plants is that effluent COD and NH₃-Nconcentrations from biological treatment processes cannot meet the needs of IndustryEffluent Discharge Standard. If the coking wastewater can be advanced treatment andreused, it will have a great social and economic benefit. Therefore, in this thesis, apilot-scale Anaerobic/Anoxic/Aerobic Membrane Bioreactor with Nanofiltrationprocess was studied.This thesis focuses on how the total HRT influences the treatment effect of theA1/A2/O-MBR and the relationship between TMP, temperature and permeate flux,and investigate the mechanism of membrane fouling during the long term operation.The research result shows the optimal HRT for coking wastewater treatment is70hand the best reflux ratio is3. When the A1/A2/O-MBR works under HRT of70h andreflux ratio of3, the average removal rates of COD, NH₃-N, TN, TP are92.0%,87.0%,59.0%and33.0%respectively. The corresponding average effluentconcentrations are171mg/L,17mg/L,129mg/L and1.6mg/L respectively. WhenA1/A2/O-MBR operated under low membrane flux（2.1L/m²h、3.5L/m²h、6.1L/m²h）, the membrane fouling speed was lowly, the TMP also increasedslowly（average increase rate was18.7Pa/d）. When a high membrane flux wasperformed in A1/A2/O-MBR （11.9L/m²h）, the membrane fouling was deteriorate,the TMP also had a significant increasing （average rise rate was128Pa/d）.With the help of some surface analytical methods including SEM, EDX andATR-FTIR, it was found that the pollutants referring to MBR membrane fouling included organic and inorganic matters, which contained phenol, proteins andcarboxylic acid organic matters and the elements of phosphorous, calcium and iron.These pollutants lead to the pore blockage and gel layer fouling. Additionally, thecleaning modes to the MBR membrane fouling, including acid cleaning, alkalicleaning, oxidant cleaning and combination cleaning were performed during thecleaning experiments of MBR membrane. After comparative test,0.1%NaOH wasapt to remove the pollutants, and the specific membrane flux was improved from1.84L/m²h kPa to22.91L/m²h kPa. The recovery of membrane flux had achieved81.1%.It was shown that the organic fouling and gel layer fouling contribute to the MBRmembrane fouling.On the basis of A1/A2/O-MBR experiment, we studied advanced treatment ofcoking wastewater by a bench-scale nanofiltration membrane system which was usedto treat effluent from A1/A2/O-MBR. The effluent quality of nanofiltration membraneand the correlation between operational conditions and membrane performance werecarefully analyzed and discussed based on experiment. The research results showedthat the membrane flux increased with increasing operational pressure and feed flowrate. When the pressure got to a certain extent, concentration polarization becameserious and the resistance of membrane increasing, as a result, the water fluxstabilized. According to experimental results, we found that the nanofiltrationmembrane could effectively remove COD, NH₃-N and TP. The average removal ratesof COD, NH₃-N, TP are89.0%,80.7%and96.0%respectively. The correspondingaverage effluent concentrations are13.6mg/L、4.7mg/L and0.08mg/L, and reach toReusing wastewater quality standard for industrial circulation cooling water（HG/T3923-2007）.The research also showed that the optimal operating pressure of nanofiltrationtreatment of coking wastewater（normal wastewater concentration） was0.8MPa, andthe appropriate feed flow is1.13L/min.