A reliable and efficient operation of a conventional water treatment plant is a demanding task. A continuous supply of different chemicals must be guaranteed, spare parts of mechanical equipment must be stocked or easily available, and the treatment plant operated by well-trained and skilled personnel. The local infrastructure should support maintenance and repair of treatment plant components. However, these criteria are hardly ever met by local conditions prevailing in rural areas of developing countries.

Wagner states in the preface of the manual "Upgrading Water Treatment Plants" [2], which is the result of a WHO working group on operation and maintenance established in the 1990s: "In the majority of plants, especially in the less developed countries, much of the expensive equipment does not operate properly due to lack of understanding or disregard of maintenance and operation recommendations". Only a few plants are designed on the basis of bench and pilot plant testing. The need for careful design is most urgent in countries with the least resources. However, design studies are in fact considered a luxury rather than a necessity in these countries. The most widely encountered deficiency in water treatment is the application of coagulants to raw water. Incorrect dilution of the solution, inadequate doses and inappropriate dosing are the most common mistakes. Difficulties are also experienced with the flocculation step. Uncontrolled energy inputs result in small floes of low settleability. Sedimentation tanks are often not well-designed; short circuiting and incorrect water abstraction lead to poor clarification and overloading of the subsequent filters. These in turn cannot be backwashed properly and produce filtered water of high turbidity. Finally, poorly or inoperative chlorination equipment is commonplace in rural water treatment plants in developing countries, as the equipment usually originates from industrialised countries and, hence, foreign exchange is required to purchase these installations and spares. The described difficulties encountered with conventional water treatment will result in the production of water of erratic quality which is often unsafe for consumption.

Objectiveness demands that earlier experienced operational difficulties with slow sand filters have to be mentioned at this point. Initially, slow sand filters were developed to combat the cholera and typhus epidemics in Europe last century. On account of its simplicity and low-cost, the slow sand filter concept was then indiscriminately exported to developing countries in the early days of technical cooperation. Slow sand filters operate perfectly well with raw water of low turbidity as generally encountered in European surface waters. However, raw water quality in tropical climates can vary considerably, especially as regards turbidity and solid matter load. Therefore, this direct transfer of technology has proved inadequate. The inability of slow sand filters to sustain adequate filter runs when subject to high turbidity loads became obvious. Worldwide practical experience revealed that the slow sand filter design concept was often misunderstood, the use of pretreatment processes, such as plain sedimentation or flocculation and sedimentation, were either inefficient or unreliable as well as inappropriate, and that operation and maintenance deficiencies contributed to the poor performance of slow sand filters. In the early 1960s in Brazil, for example, the communities were not adequately trained in slow sand filter operation, thus causing a high failure rate of the slow sand filters [3]. In Cameroon on the other hand, slow sand filters were operated adequately twenty years ago. However, due to the raw water quality deterioration caused by progressive deforestation of the catchment areas, these filters faced increasing operational difficulties which required treatment plant rehabilitation [4]. Finally, an evaluation of the performance of four slow sand filter plants carried out in India in 1993 revealed that its current design, construction and operation, including source protection, is far from being satisfactory and often leads to poor filter performance [5].

Successful projects call for a multidisciplinary approach requiring various types of inputs. Sociocultural, institutional, and natural conditions must be considered along with financial and technical aspects. The synthesis of all these inputs lead to appropriate and sustainable solutions. This manual focuses mainly on technical aspects and gives answers to perhaps the least difficult problems. From the technical viewpoint, development of the roughing filter technology has contributed towards an efficient and reliable slow sand filter operation appropriate for rural water supply schemes in developing countries.