Selection Of Coking Wastewater (CW) Biological Treatment Process (BTP) And The Operation Energy Consumption (EC) In CW Treatment Plant

Coking wastewater is generated from incomplete coal gasification.Since the complex constituents,long hydraulic retention time(HRT)and large amount of EC is required during the treatment of CW,it is considered as a kind of high-energy consumption and high-cost consumption industrial wastewater.Due to different understanding for CW characteristic and multiple discharging targets,many factors,such as EC,the costs of investment,running capital,treatment efficiency,secondary pollution,operation management(ect.)should be considered when we choose it.CW BTP is various.Therefore,establishing a model to select the CW BTP can help the decision-makers to reduce the blindness and subjectivity of alternatives selection.For the high-energy consumption of CWTP,wastewater characteristics,technological process,operation time and equipment status(etc.)are the main factors affecting the EC in CWTP.Thus,analyzing how these factors work systematically and looking for suitable model for the EC will help a lot for the optimization of the CWTP and provide scientific basis for energy conservation during the disposal of coking wastewater.Wastewater quality,BTP and operation in 52 CW plants were investigated in this paper.The BTP is divided into two parts according to the concentration of chemical oxygen demand(COD).The first part is low concentration(LW)(about 3000 mg·L-1)BTP,which including AO(anoxic-oxic),AOO(anoxic-oxic-oxic),AAO(anaerobic-anoxic-oxic),AAOO(anaerobic-anoxic-oxic-oxic)process.Another part is high concentration(HC)(about 5000 mg·L-1)BTP,which comprising AAO(anaerobic-anoxic-oxic),OAO(oxic-anoxic-oxic),OHO(oxic-hydrolysis-oxic)process.Based on the theoretic analysis and the experct consultation,a set of selection and evalution model including 3 layers and 18 indexes of CW BTP was established.Then,the ordering of evalution is employed using fuzzy AHP.The optimum and the most stable BTP in different concentration are employed by sensitivily analysis and variability analysis,respectively.The results show that the cost of infrastructure investment,total nitrogen(TN)removal efficiency,COD removal efficiency,capacity of handing sludge,the cost of running are the 5 key factors to select the CW BTP.The weight of 5 factors account for 54.9% of the total weight.AAO with 5 out of 10 conditions belonging to the optimum process under LC.OHO with 8 out of 10 conditions belonging to the optimum process under HC.The variability value of AAOO and OHO is 0.250 and 0.289,respectively.Therefore,AAO is the biggest proportion of optimum process under LC;AAOO is the stable process under LC.OHO is the comprehensive optimum process under HC.The study illustrate that,there are different choice of the biggest comprehensive benefits under different considered.Selection of CW BTP need consider the weight of assign in criterion layer and the actual engineering situations and so forth.According to general formual of the actual CW treatment project(pretreatment+ biological treatment+ advanced treatment),the whole treatment process was composed of eight EC systems including air flotation,wastewater transportation,sludge transportation,aeration,mixing,dehydration,dosing and utility system according to the differences of EC equipment,disposal target and unit function.Analyses the factors of each EC system,a new method is proposed to calculate the value of EC,which is the actual HRT algorithm and 24 h retention time algorithm.Then,the total EC model was obtained by weighting based on the model of each energy-consuming unit.A practical CW treatment project with the core process of OHO—the optimum process under high concentration,fluidized bed was proposed as the case study in this paper.After that,the EC value was calculated using actual HRT algorithm and 24 h retention time algorithm respectively.The accuracy of the model was evaluated by making a comparison between the calculated value and the actual electric meter readings during 24 h operation.The results indicated that the overall EC of CWTP was strongly related to wastewater inflow rate,sludge production,dissolved oxygen,nitrification liquid reflux ratio,COD,and TN of the influent.The correlation coefficients and relative errors were 0.93,4.28% ~ 19.18% for HRT algorithm,and 0.97,3.45%~8.94% for 24 h retention time algorithm,respectively,which revealed the significant positive liner correlations between calculated values and 24 h meter reading values,and proved the accuracy of the model.Therefore,the total EC model based on that of the unit system is a new systematic method which could be used to predict the EC caused by the variation of wastewater characteristic,technological process,equipment status,etc.during the project operation.