| Sludge is an inevitable by-product generated in the wastewater treatment process.With the improvement of both people’s living standards and environmental quality standards,wastewater discharge and sludge production are continuously increasing.Because of the complex components and instability of the sludge,it has been a top issue on how to efficiently and safely remove the sludge.There are some limitations on traditional technique,leading to an explorer of some new approaches on sludge treatment.Also,the wastewater treatment is an energy-intensive and low-efficiency process by combining bio-chemical and physical processing.Multiple reaction units and a massive investment in a variety of processing equipment are needed for the wastewater treatment process.Meanwhile,it will produce the secondary pollution sources such as excess sludge and greenhouse gases.It is still not well understood on how to develop a new better reaction system,with higher efficiency,lower energy consumption,and lower level of emissions.In this study,we firstly assessed the pyrolysis of coking sludge at the first time and selected the coking wastewater treatment process as a case for energy equivalent modeling and energy saving optimization.We studied the inherent physical and chemical properties and heat value of coking sludge,and analyzed the mass distributions of the three-phases products of sludge pyrolysis,with the emphasis on composition distributions of gaseous-phase products.On this basis,four indexes of sludge pyrolysis,including raw material cost,net energy consumption,carbon emission and adsorption capability of the sludge-derived activated carbon（SAC）were studied,and the comprehensive scoring model was established for sludge pyrolysis.This model was then applied and optimized for the SAC preparation by sludge pyrolysis.The obtained SAC,with excellent performance,was then used for a further study on adsorption of coking wastewater treatment.We then discussed the advantages and limitations of the two modes（cyclic-centralized mode（CCM）and traditional-distributed mode（TDM））of using the two different adsorbents of common active carbon（CAC）and SAC.We further developed a combined system using a one-stage aerobic biological treatment unit coupled with double activated carbon adsorption（AB-DA）for coking wastewater treatment and revealed the inherent law of energy conversion in the treatment process.Finally,we established an advanced treatment system using the complex multi-stage biological treatment units coupled with two-stage adsorption（A2BA1）for coking wastewater.This model was later created and validated using mathematical methods and software simulation.Thus,the energy saving optimization and operation low were obtained.Some key research results of this paper are listed below.（1）The element distributions of coking sludge varied sharply with municipal sewage sludge（MSS）,which was more suitable for SAC preparation.In the proximate analysis of coking sludge,the contents of volatile substances and fixed carbon were 42%and 45%higher than that of MSS,respectively,and the ash content was 45%less than that of MSS.These results implied that it was beneficial to the SAC pore-forming and energy recovery of the gaseous and liquid phases’products.In the element analysis,the C and S elements were about30%higher than that of MSS.It implied that it was beneficial to SAC preparation,but the condensation of gaseous phase products was needed to reduce the secondary pollution by sulfurous gas.Due to the high oxygen content of coking sludge and oxidizability with other elements,the combustion heat value of coking sludge was lower than that of MSS.（2）In the coking sludge pyrolysis,the mass distributions of three-phase products and the volume distributions of gaseous compositions were closely related to the pyrolysis temperature,holding time and heating rate,and the pyrolysis temperature is the most significant.As the pyrolysis temperature increased,along with the anabatic secondary reaction of volatile organic compounds,the liquid product yield decreased slowly,but the gaseous product yield increased.As one kind of the non-condensable gases,the volume rate of CO raised obviously,but that of H2 and CH4 increased slowly.With the extension of holding time and the improvement of heating rate,the yields of solid and liquid products decreased but the yield of gaseous products increased when the pyrolysis temperature exceed550 oC.The variation trends of the compositions of non-condensable gases were similar,but the variations resulted from holding time and heating rate were smaller than that caused by pyrolysis temperature.（3）Comprehensive scoring model has been established and it could be used to evaluate the sludge pyrolysis process accurately.The importance of the four evaluation index in the sludge pyrolysis were raw material cost,net energy consumption,carbon emission and adsorption capability of SAC,achieved by the expert scoring method.The data of the four evaluation index obtained from the different experimental conditions were normalized with the interval of[0,1].Then,the comprehensive scoring model was obtained,using the importance proportion of the four evaluation indexes,multipled the corresponding normalized data obtained from different experimental conditions.The higher the score,the better comprehensive benefit of sludge pyrolysis was achieved.（4）The optimization parameters of sludge carbon preparation could be obtained using the comprehensive scoring model of sludge pyrolysis.The results suggested the advantages of CCM compared with the TDM.The optimized parameters of coking sludge at the highest comprehensive score were:pyrolysis temperature of 550 oC,holding time of 60 min and heating rate of 20 oC/min.Under the optimized pyrolysis parameters,the comprehensive score of SAC preparation by coking sludge was higher than of CAC preparation by using bituminous coal.On this basis,the two different adsorption modes（i.e.,CCM and TDM）,used in the coking wastewater treatment were studied.CCM was considered as a better adsorption model for industrial wastewater treatment in the future.（5）The combination system of aerobic biological treatment unit coupling double adsorption（AB-DA）,has been designed and successfully performed.The primary model of energy equivalent was established and the best operating mode of energy-saving benefit was obtained.By the determination of system boundary and the optimization method of energy conversion evaluation,considering with the energy flows changed by the input and output material flows in the research system,the primary model of energy equivalent has been established.Using the two evaluation indexes,i.e.,net energy benefit（NEB）and benefit cost ratio（BCR）,to evaluate the energy saving of different operation modes,the M2（SAC）mode was proved as the best mode for energy-saving benefit.（6）The combination system of complex multistage biological treatment units coupling two-stage adsorption（A2BA1）has been developed successfully.The new method of energy saving assessment by using energy equivalent depth model and mathematical software simulation was established.Further the optimal operation conditions and the possible ways of the system running with zero energy cousumption were obtained.Based on the input and output material flows,a new method for energy conservation assessment has been established by integrating the depth model of energy equivalent and LINGO software simulation.The reliability of the results was later calculated using ABC classification method and the calculation of the energy-saving rate.The optimization results of the operating conditions were:influent COD concentration of raw coking wastewater of 2500-3000 mg/L,effluent COD concentration of biological treatment of around 100 mg/L,and the dosage of activated carbon addition of 1.0-2.0 g/L,and the average energy saving rate could reach up to 48.3%.Also,the correlationship between the price performance ratio of adsorbent and the operation with zero energy consumption in wastewater treatment has been obtained.SAC preparation by sludge pyrolysis is a possible technology that enables a reduction,stabilization,and recycling process for sludge disposal.The short-range reuse of SAC to wastewater system can be achieved by applying the prepared SAC into the adsorption process of wastewater treatment.The innovative combination of treatment units enables the parameter optimization,energy saving and emission reduction in wastewater treatment process. |
