| Wastewater treatment is a complex process related to many issues, including technique, economy, environment and society. It is unreasonable to use single objective optimization model, such as cost-benefit analysis method, to select the optimal wastewater treatment alternative. However, there were many problems of existing multi-objective optimization models of wastewater treatment alternatives selection, such as, the large difference between decision indicator systems, and the too subjective qualification methods of decision indicator, and the unstable calculating process of indicator-weight, and the optimization models not well matched to mathematic characteristics of the alternatives selection. Therefore, there was still not a generally accepted optimization model of the wastewater treatment alternatives selection.The dissertation was a part of the Netherlands Government's Grant Project Technical manual for environmental infrastructure in China's western small cities (the project number is MOC-NGGP-2003-3), which aimed at how to systemically, objectively and scientifically select alternatives for the WWTP of western small cities. In the dissertation, the appropriate alternatives for small towns and cities wastewater treatment were discussed, the sustainable decision indicators and their qualification methods were presented, and the way to stably and effectively calculate indicator- weight was studied. Furthermore, a new multi–criteria decision method and its sensitive analysis for optimal wastewater treatment alternatives selection were set up. The main research contents are as follows:①By field survey and literature review, it was concluded that the wastewater flux of towns or small cities was below 5×104m3/d in western China, and mostly fell into the range of (0.2~2.0)×104m3/d. Moreover, the domestic wastewater was the main component of wastewater in western small cities. For the WWTP of small cities, SBR series, Oxidation Ditch series and biofilter series were recommended, and nature treatment systems were suggested to the region abundant with land resource.②There were some shortcomings for the minuum cost and'greeness'respectively as the decision targets of optimal wastewater treatment alternatives selection, so sustainability was suggested as the decision object of optimal wastewater treatment alternatives selection. Referred to the researchs of some researchers, it was presented that the sustainability of wastewater treatment alternatives could be discribed by technique sustainability, economy sustainability, environment sustainability and society sustainability. Based on the situation of basic research data in China, the objective , reasonable and operational decision indicator systems was set up, which included technique, efficiency of energy utilization, indirect energy consumption , land occupation, annual expense, O&M,professional skill reqirement. Furthermore, the qualification methods of decision indicators were also presented. The case study showed that the sustainable decision indicators were more thorough and reasonable than economic indicator used by cost–benefit analysis method, and the qualification methods of decision indicators were easy to apply.③The Modified Genetic Algorithm based on Fuzzy System was established (FMGA) to determine the weights of decision indicators. Compared with other tranditional weight calculation methods, FMGA not only could keep the original decision information as more as possible and automatically modify the consistency of judgment matrix, but also could stably, quickly and effectively calculate the weights of decision indicators.And the scale of five parameters of FMGA was discussed by simulation test. The reslts showed that the number bits of variables' binary code was depend on the precision reqirement , the size of population N should equal to or more than 300 and the number of elitist indivadual A could be between 5 percent and 30 percent of N . Cluster level should divided N into two parts with the same scale, mutation probability p m equaled to O (L N1).FMGA and Canonical Genetic Algorithm (CGA) were compared in order to verify the advantage of FMGA. The compared results showed that FMGA could more effectively avoid premature convergence and get global optimal than CGA.④The decision indicator weights of wastewater treatment alternatives selection in Three Gorges Reservoir Region Areas were studied. In result, the rank in desending order of importance of decision indicators were, annual expense (0.2365)>technique(0.2094)>land occupation(0.1640)> O&M(0.1544)>professional skill reqiremnet (0.1383)>energy consumption(0.1014),which showed annual expense and technique were two most important decision indicators at present.⑤According to the math characteristics of wastewater treatment alternatives selection, which was fuzziness, greyness and dependency of decision indicators, the membership function was combined with grey relational analysis to construct fuzzy-grey relational analysis method. Fuzzy-grey relational analysis method was applied to multi-objective decision model of optimal wastewater treatment alternatives selection, which well reflected math character of wastewater treatment alternatives selection. The case study indicated that, fuzzy-grey relational analysis method was more reasonable and easy to operate compared with grey relational analysis method, fuzzy algorithm and TOPSIS.⑥Sensitive analysis of fuzzy-grey relational analysis method was studied. By the sensitivity analysis, the range of decision indicators could be received, in the case that the rank of alternatives be changed or kept. The study showed that sensitive analysis of fuzzy-grey relational analysis method could offer some information about improving the process for researchers, designers and managers.⑦SBR, BAF and Orbal Oxidation ditch were compared for the western small city wastewater treatment plant with capacity of 4000m3/d. The result of Fuzzy-grey relational analysis showed that BAF was the best alternative, and SBR was better than Orbal Oxidation ditch. Furthermore sensitive analysis showed that BAF and SBR were better than Orbal Oxidation ditch. For western small city wastewater treatment, the treatment process could better match with sustainability by reducing energy and material consumption, and improving professional skill level of panel.
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