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as this is the most common reason given for requiring sewerage. The objective measurement of each of the criterion is stressed. |
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The second section suggests a mechanism for objectively prioritizing communities for sewerage, using the selection criteria discussed in section one. A weighted matrix approach is suggested. Each community is awarded a score for each of the criteria and that score is multiplied by the weight assigned to that topic. Criteria considered to have most influence on the prioritizing process receive the highest weight. The weighted scores of each community are then totalled and the communities having the highest total score are most favoured for sewerage. The paper uses a worked example to explain the approach. The results of the matrix are finally tested with a sensitivity analysis to asses the confidence of the predictions. |
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3.2
Criteria Affecting the Selection of Sewerage Schemes |
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The list of factors affecting the prioritization of sewerage schemes is long and will vary from place to place and over time, depending on the conditions prevailing when the analysis is being done. The criteria described below are the most common, but planners should feel free to add or subtract criteria to suit local conditions. As an example, the author undertook a selection analysis in which pollution of coastal lagoons was considered a critical factor. Such a consideration would only be necessary for communities adjacent to lagoons and reefs of national importance. In the majority of situations such a criterion would have little relevance. |
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As with most design activities, current conditions are only of importance as a guide to what is likely to happen in the future. Sewerage schemes are designed to meet the needs of a community for many years (called the design life) and must be capable of handling the maximum sewage flow, which usually comes at the end of the design life. In general, selection criteria should relate to the conditions expected at the end of the design life rather than those currently experienced. If, however, there are current conditions that are relevant to the prioritization process (such as heavy surface water pollution or schemes proposed for built-up areas, which have already reached optimum density) then they should be included. |
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