## 10 Focus on Sewage Treatment  
The use of MCDA in Sewage treatment mainly concerned three main sectors where decisions are required: (a) choice of treatment technology or its upgrade, (b) methods for disposal of the resulting sludge and (c) methods of disposing of treated wastewater liquids. A review of decision support systems in wastewater treatment plants documented the progression from life-cycle assessment, cost-benefit ratios, mathematical models through to the use of MCDA methods from about the year 2000 but did not focus on individual MCDM methods [92]. A method of validating similar DSS using case studies was reported by [93]. Looking at the various sectors individually, the choice of sewage treatment technology involves many decision criteria. The AHP method has been used for this in China with TOPSIS in a fuzzy number setting and ten key indicators (initial cost, recurring cost, land cost, reliability, durability, stability, resource recovery, simplicity, effect on ecosystems, and risk) [94]. The PROMETHEE method was applied to a related problem, improving the treatment processes as described in [95]. A similar problem was addressed by [96] using 10 criteria (capital cost, operational cost, water quality improvement, land required, ease of operation, maturity, reliability, public acceptability, employment, and government support). A bespoke environmental decision support system for selecting treatment technologies was described by [97] using criteria under three main headings, economic, environmental and technical. Options for remote villages converting from septic tank systems to a centralized treatment plant were analyzed using PROMETHEE [98] with 28 criteria divided between 4 groups (environmental, technical, financial and social) that were determined following stakeholder consultation. The same MCDA technique was used in Australia by [99] to choose upgrade options for sewage treatment plants in towns, this time with 17 criteria in five groups (financial, environmental, functional, health and social). In Surat City, [100] used a weighted average technique with 18 criteria to select sewage treatment options. The question of sustainable disposal of sludge from sewage treatment plants is also important and a number of technologies for this have been assessed. For instance, various scenarios for using sludge for energy generation (incineration) has been examined using fuzzy MCDA techniques and twelve criteria (climate change, acidification, human toxicity, initial cost, recurring cost, life-cycle cost, social acceptance, health and safety implications, land requirement, reliability, efficiency of resource conversion, and operability) [101]. When sludge is dried, approximately 80% of the moisture can be removed and this can be helped with an additive and by controlling the temperature. MCDA was used to select the best combinations of these options using TOPSIS and MOORA with weights selected using CRITIC [17; 102]. The selection of areas suitable for land-spreading of sludge was demonstrated in Spain by [103] who used a PROMETHEE type MCDA method, with 12 criteria, built into a GIS system. PROMETHEE was also used to identify soils suitable for locating on-site sewage treatment (septic tanks), using 8 criteria, mostly physical properties of the soils or porewater [104].