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Original:Slow sand filtration water treatment plants

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INTRODUCTION

The limited amounts invested in the treatment of safe water in rural areas are partly explained by the high cost of water treatment systems. Consequently, the majority of rural communities are still drinking superficial water that does not meet the required standard of quality, causing serious health problems.

In many cases, the high cost of water treatment systems and the poor quality of water deter investment in even the simplest systems of untreated water conveyance by gravity, further aggravating the health situation and forcing the population – particularly women and children – to walk long distances to fetch water of a worse quality than that which they could obtain by conveyance from the headwaters of nearby rivers.

Slow sand filtration systems are a technically viable water treatment solution. Nevertheless, there are still a number of difficulties involved in the implementation of the technology and the operation of the system. In addition, the direct cost of the construction is relatively high. A large percentage of these systems have been abandoned for the following reasons:

  • Inappropriate designs, as the variations in the quality of water at

different times of the year were not taken into account.

  • The people in charge of operating them are usually members of the

community who have not been adequately trained to operate the system.

  • The institutions responsible do not monitor the installations adequately.
  • Spare parts are not locally available.
  • The sand on the filter bed is not replaced when the minimum thickness

has been reached, after several layers have been scraped off.

The main characteristic of slow sand filtration is that, due to the effect of biological activity, it efficiently removes pathogenic organisms from raw water, particularly the bacteria and viruses responsible for transmitting water-related diseases. Furthermore, no chemical products are required, nor highly qualified, continual supervision.

Slow filtration is undoubtedly the most adequate technology for rural areas. In order to avoid some of the problems described, however, it is necessary to apply solutions that take into account local technical and economic capacities, so that the system can actually achieve its purpose to supply drinking water to rural populations.

The proposal contained in this handbook is a low-cost alternative that is technically adequate and easily managed by the community. It consists of modified gravel pre-filtration, slow sand filtration and disinfection units adapted

DESIGN

Before embarking on a treated water supply project, it is essential to evaluate and define the level of organisation in the community. If it is insufficient or non- existent then the operation and maintenance of the system will most likely be neglected and eventually abandoned.

The community must participate in every stage of the installation of the water supply service, including the selection of the technical option, the construction and supervision of the works and the management of the service.

In the design stage, the community must study the proposed solutions and decide upon the best technical alternative and the most appropriate service. Consequently, they must be aware of the costs of the system – including their own contribution to the construction and the rates to be paid, the advantages and disadvantages of each alternative and their operation and maintenance responsibilities (UNDP 1998).

The following factors must be borne in mind for a preliminary study to define possible water treatment solutions:

  • Weather conditions : The temperature has a substantial influence on

the performance of water treatment systems and the intensity and duration of the rain influences the quantity and quality of the source.

  • Watershed characteristics : Human and natural factors, such as

discharges of residual water or run-off from chemically treated farmland, can seriously affect the quality of the water.

  • Quality of the water : A physical, chemical and bacteriological analysis

of the water is necessary in order to determine its quality and the level of treatment required.

  • Location of the plant : The land must be easily accessible, having a

natural slope of between 5 and 10% and having no exposure to natural hazards or to subterranean water nearby. A written agreement with the owner must be signed.

  • Characteristics of the community : It is necessary to know the

customs and beliefs that could affect the acceptance of the system, the characteristics of existing organisations, the availability of natural materials and human resources, and the level of schooling.

  • Existence of water-related diseases.

The selection of water treatment processes as a possible solution for supplying drinking water to rural communities depends on the quality of the raw water.

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