To meet the WHO/UNICEF target of ‘Water & Sanitation for All by 2025’ some 4.4 billion people will\ud have to be provided with improved sanitation during 2001−2025, and around half of these are/will be in\ud ‘urban’ areas – but in reality we are talking about periurban areas. Given that most population growth\ud over the next few decades will occur in ‘urban’ (again, really periurban) areas of developing countries,\ud periurban sanitation will have to become much more important than it already is. Our current focus is on\ud achieving the sanitation target of the Millennium Development Goals, but these efforts will have to be\ud doubled if we are to meet the WHO/UNICEF 2025 sanitation target in periurban areas, and then\ud maintained for the next quarter century as we seek to meet the sanitation needs of the additional two\ud billion or so people expected in periurban areas by 2050.\ud So the Big Question is: How can we provide affordable sanitation to these very large numbers of poor\ud people in periurban areas in developing countries? The answer to this question depends in part on the\ud population density: at low population densities on-site sanitation systems are normally feasible, but (and\ud as we have known since the early 1980s), even if there is sufficient space for them, they may not\ud necessarily be the cheapest option (and, because we are attempting to serve poor and very poor people,\ud we have to consider cost); and, of course, at high population densities on-site systems become infeasible\ud as there is no space for them. In addition to being affordable, the chosen sanitation system has to be both\ud socially acceptable and institutionally feasible.\ud Consider the typical periurban situation: a high population density, one too high to permit on-site\ud sanitation systems. What are the ‘best’ solutions for sanitation? If affordable, the system of choice would\ud normally be simplified sewerage (also known as ‘condominial’ sewerage). With this sanitation system\ud we should remember that in Natal in northeast Brazil, where it was developed in the early 1980s, it\ud became cheaper than on-site sanitation at the relatively low population density of ~160 persons per ha,\ud there were no connection charges and the monthly charge for the service was only USD 1.50; and that in\ud Chisty Nagar in Orangi, Karachi, Pakistan, where Brazilian-style simplified sewerage was first installed\ud in Asia in the mid-1980s, the residents obtained their water (only ~27 litres per person per day) from\ud public standpipes, thus demonstrating that a plentiful on-plot water supply is not a sine qua non for the\ud system. Simplified/condominial sewerage is one of the components of the very successful ‘Slum\ud networking’ programme in India, and it has also been used in small villages in northeast Brazil. It is\ud socioculturally very acceptable as it appears to its users to be similar to conventional sewerage, so their\ud sanitation system is the ‘same’ as that enjoyed by the rich. It is also institutionally acceptable simply\ud because it is a sewerage system and, as such, it can be readily understood and appreciated even by very\ud conservative sewerage design engineers, especially when they realise that its hydraulic design is actually\ud more rigorous than that used for conventional sewerage
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