The Integration Analysis Of Technical System For Upgrading Wastewater Treatment Plants In Taihu Lake Basin

Due to the increasingly stringent discharge standards,the upgrading of urban wastewater treatment plants（WWTPs）has been paid widespread attention.Taihu Lake Basin（TLB）as one of China’s most economically active regions,the Chinese government successively established the National Special Water Project（NSWP）to develop new technologies to retrofit existing wastewater treatment processes during the11th to 13th Five-Year Plans.However,there is a lack of systematic sorting of the existing technologies,and thus hinders the application and promotion of the upgrade technologies.Based on the outcomes of the NSWP and a field survey,this research analyzed the current status of wastewater treatment in the TLB and systematically integrated the retrofitting measures of WWTPs in terms of achieving the Grade IA of the national standard and local stricter discharge standards（DB 32/1072-2018 and DB33/2169-2018）.In particular,the boundary conditions,design parameters,specific recommendations of the technologies,and some typical engineering cases were provided accordingly.Finally,LCA was used to quantify the influence of the operating phase of WWTPs.The present work will hopefully assist in retrofitting and constructing WWTPs in the best way.The main results are as follows:（1）Among the 312 existing urban WWTPs in TLB（data up to 2020）,A²/O is the dominant wastewater treatment process,accounting for 55.1%;The WWTPs in TLB are mainly small and medium-sized（<100,000 m³/d）,accounting for 84.94%;The discharge standards of WWTPs in TLB mainly include Grade IA and more stringent local standards（DB 32/1072-2018 of Jiangsu Province and DB 33/2169-2018 of Zhejiang Province）.Some WWTPs should be upgraded to implement Quasi-IV standard.In addition,the average unit land occupancy of WWTPs in TLB is 1.46±0.80m²/m³·d,which is generally lower than the national average（1.52 m²/m³·d）,indicating that the wastewater treatment in TLB tends to develop the model of intensive and efficient.（2）The retrofitting measures of WWTPs in TLB for meeting the Grade IA standard primarily include pretreatment units optimization（PUO）,secondary biological treatment units enhancement（SUE）,and secondary effluent advanced treatment technologies（SEA）.The PUO is mainly proposed for the WWTPs characterized with high inorganic fraction and low C/N of influent.By setting fine and/or membrane grille to further remove the smaller SS from the effluent and optimize the operation mode of the primary sedimentation tank to avoid premature depletion of the internal carbon source.The SUE is mainly for upgrading WWTPs with high TN effluent requirements,including A²/O-MBBR,A²/O-A/O and A²/O-A-MBR.The A²/O-MBBR technology is mainly applied to upgrade the WWTPs without reserved land.Adding suspended filler in the secondary biological treatment system can effectively strengthen the ability to remove nitrogen and phosphorus.A²/O-A/O is often used to upgrade WWTPs with reserved land,and the nitrification and denitrification capacity of the biological treatment system can be strengthened by post-anaerobic and aerobic.A²/O-A-MBR technology is often applied to upgrade WWTPs with low influent carbon sources.Through the post-anoxic tank to strengthen the use of internal carbon sources in the denitrification process,the post-MBR process can improve the denitrification capacity of low C/N ratio wastewater and the shock loading of low temperature.The SEA is aimed at enhanced removal of P,SS,COD and chromaticity in the secondary effluent and mainly includes high-efficiency sedimentation tank（HEST）,magnetic coagulation high-efficiency sedimentation tank（MHEST）,filter cloth filter tank（FFT）and V/D filter tank（VFT/DFT）.MHEST is often used to upgrade the WWTPs with tight land and high hydraulic load.FFT is often applied to the WWTPs in which the land is tight and the hydraulic elevation is limited;VFT/DFT is applied to the WWTPs in which the land is sufficient and the operation and management level is limited.Based on the outcomes combing and field survey,the two recommended processes to achieve the Grade IA standard are i)coarse/fine grille→aeration sedimentation tank→A²/O-A/O→MHEST→FFT→disinfection（Chengshi WWTP in Changzhou）and ii)coarse/fine grille→aeration sedimentation tank→membrane grille→A²/O-A-MBR→disinfection（Shuofang WWTP in Wuxi）.Through a one-year monitoring of the water quality of Chengshi WWTP and Shuofang WWTP,it was found that the annual average values of COD,NH₃-N,TN and TP were 24.74±2.88,0.57±0.16,8.51±0.67,0.15±0.05mg/L and 14.4±4.2,0.62±0.78,7.7±3.0,0.23±0.11 mg/L,respectively,,which all could meet the Grade IA standard.（3）The retrofitting measures of WWTPs in TLB for meeting the stricter local standards primarily include advanced treatment technologies of denitrification filter（DNF）and constructed wetlands（CWs）.DNF is mainly used to upgrade the WWTPs in TLB that have difficulty meeting TN limits,including denitrification biofilter and the deep-bed denitrification filter（DBDF）.The DBDF is suitable for the treatment of secondary effluent with a low nitrate-nitrogen concentration.Due to the shortage of carbon sources in the secondary effluent,the denitrification performance of DNFs often relies on additional carbon sources such as sodium acetate and glacial acetic acid,and the development of sulfur-and iron-based filter media to strengthen the autotrophic denitrification process is important in the context of peaking carbon emissions and carbon neutrality.CWs is often applicable to upgrade small and medium-scale WWTPs with reserved land or abandoned ponds nearby.Wetland plants and substrate fillers as the core elements of CWs.The recommended plants of CWs in TLB include Ceratophyllum demersum and Hydrilla verticillata,Phragmites australis,Canna indica and Oenanthe javanica,Eichhornia crassipes Nymphoides peltata.Substrate fillers can choose natural minerals such as gravel,coarse sand,volcanic rock and zeolite.Based on the outcomes combing and field survey,the two recommended processes to achieve the stricter local standards are i)DNDF（DNBF）with quartz sand（biotite）as filter media（Chengbei WWTPs in Changzhou）and ii)surface flow CWs+horizontal subsurface flow CWs+post-stabilization pond（Zaisheng WWTPs in Jiaxing）.Through a one-year monitoring of the water quality of Chengbei WWTP and Zaisheng WWTP,it was found that the annual average values of COD,NH₃-N,TN and TP were 18.19±0.49、0.28±0.11、6.27±0.78、0.09±0.02 mg/L and 11.84、0.029、11.02、0.141mg/L,respectively,which all could meet the stricter local standards.（4）The environmental impacts in the operation phase of the A²/O process（S1）and the representative upgrading processes（A²/O+DNF（S2）,A²/O-A/O+DNF（S3）and A²/O-MBBR+DNF（S4））were evaluated by simapro9.0 software.Compared with the S1,the eutrophication indexes of S2,S3,and S4 decreased by 10.7%,13.2%and 21.6%,respectively.However,the indexes of Ozone layer depletion,Global warming and Human toxicity of upgrading processes were increased,indicating that the effluent quality of WWTPs has been improved after the upgrading,but the overall environmental load has increased.Combined with the environmental impact and environmental benefits,the S4 system with suspended filler is better than S2 and S3.