Development And Evaluation Of Biological Treatment Processes For Coking Wastewater Treatment And The Effectiveness In The Removal Of Typical Pollutants

Coking wastewater,as a representative and typical industrial wastewater,is difficult to be treated due to its high toxicity,unbalanced nutrient composition and the coexistence of high concentration and complex pollutants.Effective treatment of coking wastewater has been a worldwide research hotspot for nearly a century.In the past 30 years,coking wastewater treatment processes(from pretreatment to water reuse)has been continuously developed and innovated.Among these processes,biological treatment is a key because it achieves the removal of more than 95%of the pollutants in the wastewater.Process configuration is the key factor determining the overall process performance and the removal efficiencies of specific pollutants in coking wastewater treatment.At the same time,the identities and concentrations of pollutants in the wastewater also affect the operational stability of the biological system.The complexity brought about by the compatibility of biological treatment processes and pollutants have greatly increased the difficulty for decision-makers to select process combinations in the construction of new coking wastewater treatment plants or upgrading existing systems.Unmatched process combinations resulted in highly costs and difficulties in meeting the emission standards.A majority of literatures to-date focus on improvements in process removal efficiencies.Limited number of research has been performed on the development of schemes for the evaluation and selection of biological processes for coking wastewater treatment taking into account for the coupling effect of multiple factors and multiple parameters.Therefore,this dissertation focuses on the establishment of relationships between the biological treatment processes and its effectiveness in removing nitrogen-containing compounds and heavy metals with an in-depth discussion on the degradation,transformation and fate of COD in the coking wastewater.Firstly,by using a pilot-scale O/H/O process system as a platform,4 operating strategies were designed including straight flow,bypass,nitrification liquor reflux,and bypass+nitrification liquor reflux modes.Taking into account the design parameters of the reactors and the water quality indexes,the COD removal and nitrogen-containing compound loading rates in each unit in the O/H/O process was evaluated to study the versatility in the removal of nitrogen-containing compounds and total nitrogen.16s r RNA gene amplicon sequencing was used to further analyze the correlation between the microbial community compositions and total nitrogen removal efficiencies in the O/H/O process.By comparing to the A/A/O process(e.g.reactors and operating modes),the reasons why the three-sludge O/H/O process can achieve multiple nitrogen removal modes was elucidated.Secondly,the fate,removal and distribution of 8 heavy metals in the coking wastewater treatment A/O/H/O process system were studied.The interacting mechanisms of coexisting components affecting the mobility of different heavy metals were explored.Thirdly,a stepwise process including‘membrane filtration→coagulation→adsorption separation→ozone oxidation’was developed to achieve fractionation of the residual COD in the biologically treated coking wastewater.Particle size distribution and spectroscopy techniques were used to characterize the properties of the COD-contributing components in the effluent at each fractionation stage.The correlation between the water quality parameters and the COD is established.The Nemerow index model was used to evaluate the environmental risk of the biologically treated effluent.Fourthly,the Analytic Hierarchy Process(AHP)model was used to evaluate different coking wastewater treatment processes A/O,A/A/O,A/O/O,A/A/O/O,O/A/O,and O/H/O.Two groups were defined according to COD loading rates.The low-loading group is defined as systems with a COD loading rate below 0.8 kg COD/(m~3·d),and the high-loading group is that above 1.6 kg COD/(m~3·d).The technological,economic,environmental and administrational aspects were determined as a subset of the criteria level,18 factors were determined as a subset of the sub-criteria level,calculated according to the AHP model.The comprehensive benefits of the six processes are expressed as the number for comparison.The main conclusions are as follows:(1)The O/H/O process shows superiority in nitrogen removal from coking wastewater,especially for the total nitrogen.The operation mode showed high flexibility including bypass,reflux and DO control,which resulted in a variety of nitrogen removal operational modes.Among them,the O1 reactor mainly responsible for COD removal and ammoniation.The H reactor mainly allowed hydrolysis and denitrification reactions.And the O2 reactor conferred stable and reliable nitrification.(2)Directional control of migration of heavy metals and their enrichment in the H reactor were achievable in the A/O/H/O process.The phase distribution and sludge enrichment behaviors of 8 heavy metal ions,e.g.,Cu,Pb,Ni,and Zn,have been verified in the full-scale.With the emergence of heavy metal risk management technologies,the A/O/H/O process showed revolutionary advantages compared to traditional technologies.Therefore,A/O/H/O processes may be developed into novel pollution control technologies for the joint management of wastewater and sludge in the future.(3)Selection of advanced treatment technology for biological treated coking wastewater determines the overall efficiency and benefit.The step-wise fractionation method comprising membrane filtration,coagulation,adsorption and ozone oxidation can achieve fractionation of residual COD in biologically treated coking wastewater.After these substances were removed separately,the residual COD can be reduced to close to zero(from 168.8 mg/L to 5.2 mg/L),which meets the requirements of water reuse.Among them,membrane filtration effectively removed suspended solids(particle size>0.45μm).The efficiency of coagulation largely depended on the hydrophobicity of the colloids.The adsorption process effectively removed the unsaturated dissolved organic matter that produces fluorescence.And,the ozone oxidation process effectively removed the residual and reducing inorganic substances.(4)Multi-component pollutants in the coking wastewater present complex environmental risks,making the optimal biological process for wastewater treatment a multi-standard decision-making problem.AHP results indicated that the A/A/O process is the best process for low-loading coking wastewater treatment.The O/H/O process showed the highest score for high-loading coking wastewater treatment.The O/H/O process that uses a three-phase fluidized bed for in-situ sludge separation overturned traditional anaerobic pretreatment.Its outstanding advantages include ensuring the efficiency and stability of nitrification by effectively eliminating biologically toxic substances.It does not require additional carbon sources for subsequent denitrification and does not require sludge return.A variety of nitrogen removal pathways may be achieved by the selection of control methods and conditions,showing superiorities in carbon removal,total nitrogen removal,and heavy metal fate regulation,making it a promising alternative technology for coking wastewater treatment.The above research findings show that the selection of optimal wastewater treatment technologies need to be based on the understanding of the nature of the wastewater characteristics and an accurate prediction of changes in water quality caused along the treatment process.The understanding of the basics such as physics,chemistry,biology,biochemistry,physical chemistry and their innovative applications help in the selection of reasonable process combinations.For specific wastewater,its main water quality characteristics and the removal of core pollutants needs to be identified.Future research should be devoted to the establishment of a database for the processes for wastewater treatment.Some advanced theory or technologies could be selected with the aid of group theory and artificial intelligence platform.