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poor design, construction or operation and in such cases, renovation of on-site systems will nearly always be more appropriate than their replacement by sewerage. During a visit to Mauritius the author was asked to comment on a proposal to provide sewerage to a number of communities where on-site sanitation was said to have failed (STWI, 1993). Site investigation showed that the reason the pit latrines were no longer working was that the pits were full. A programme of pit emptying or replacement was a much cheaper alternative to sewerage. Sewerage should only be considered in communities where on-site sanitation can be proved to have failed irreparably or is likely to fail within the design life (Reed, 1994). |
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3.2.4
Industrial Pollution. |
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Although it is rare for a public sewerage scheme to be constructed purely for the treatment of industrial pollution, it is often a significant contributory factor in deciding which community should be sewered. Industrial effluent can be much more concentrated than domestic sewage and it can produce serious pollution of water sources. Some industries produce large volumes of effluent and its disposal to a communal sewerage network may require sewer pipe diameters to be increased. |
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If some of the communities being considered for sewerage are industrialized, it will be necessary to find a method for including the impact of those industries on the selection process. A comparison of the organic load (in terms of kilograms of BOD per day) produced by industry is one method, but this is more important where sewage treatment is to be included in the scheme. Effluent volume and discharge times may well be more relevant because of their effect on sewer pipe sizes. |
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Some measure of the cost of a scheme must be included in the selection procedure. A comparison of total capital cost (assuming all the schemes are designed using the same criteria) will identify which schemes are the cheapest and which are within the budget available. It will not show how effectively the money is being |
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