Human beings intercept the natural water cycle in order to take water for their purposes, and after using the water, they return it to the cycle. During this usage, the water becomes polluted. Pollution can also occur in other ways and at other stages of the water cycle. Through various naturally occurring cleansing processes, the quality of the water is improved as it makes its way through the cycle. It depends, however, on the type and quantity of the contamination that entered the hydrologic cycle, whether the water can cleanse itself.

Some of the main factors that contribute to the ever-increasing amounts of non-degradable domestic, agricultural and industrial wastes are: overpopulation, natural catastrophies and droughts, increasing industrialization and the utilization of chemicals in agriculture. These contaminants interfere with the balance of the hydrologic cycle and disturb the complex processes of the natural breakdown of pollutants by entering that cycle in the following ways directly, via the disposal of sewage, by percolating through the ground, by aerosol dispersion due to precipitation or evaporation, or via plants. The consequence of this is that most of the fresh water available to humans is contaminated and moreso, the nearer the available water source is to the point of contamination. Infection and toxicity in men and animals can result from the intake of contaminated water=and even from external contact. They, in turn, discharge the orally ingested pathogens (disease causing agents) which then wind up in the water cycle, reproduce, reinfect, etc.

Fig. 2: Diagram patterned from the natural hydrologic cycle to demonstrate the artificial flow of water utilized for personal consumption. Water treatment is but one component of the entire water supply and disposal system. It cannot be dealt with in insolation but rather in the overall context of the artificial cycle of domestic water, and the role of the individual with his/her habits and circumstances of life

In order to break through this chain,one could begin at various points:

1. Identification and elimination of the source of contamination: waste water treatment; collection, disposal and reuse of waste and fecal matter; observance of hygiene.

2. Improving the quality of the water designated for personal use: treatment of drinking water.

3. Controlling the effects: health care, especially in the case of infectious diseases, to avoid spreading the contagion.

These measures can only be effective if they are simultaneously undertaken. The most important factor is that the water consumer understands the connections between the water cycle, water consumption' and contamination and realizes the consequences of interference.

The intention of this manual is to make a contribution towards solving the problem posed by contaminated drinking water in the Third World. Different methods for the treatment of drinking water are discussed and the conditions are highlighted under which plants and equipment could be built. The manual intends to give planners of such projects a helping hand to realize their plans and it points out the problems and risks inherent in these activities. The immediate goal of the measures we are about to look at is the improvement of the quality of the available water through treatment. A hoped-for long range objective is to achieve the following beneficial side effects:

a reduction in the occurrence and spread of water-induced diseases; reduction of health-care costs; an increase in economic productivity; employment effects, strengthening of the self-help potential, education and training of the population through their active participation.

Since the consequences of a poor supply of clean drinking water affect the rural and urban slum populations the most, we will restrict ourselves to these groups. That means:

this manual will describe techniques that can be applied to a range of settlements that goes from a single household to a community of some two thousand inhabitants; it is assumed that a water source is available.

We will pay special attention to . surface water and shallow ground water resources, which are typically severely contaminated through pathogens and turbidity.

Before choosing the appropriate techniques, the following criteria must be looked at in order to conform to the given limitations of our target group in terms of the availability of material, financial and technical resources:

the lowest possible level of complexity; construction and operations for the maximum utilization of the locally available materials and labor force; minimal usage of mechanical and automated equipment and chemicals that need to be imported; simple operation and maintenance; low costs; optimal employment of primary energy sources for construction and operation.

The starting point for establishing a drinking water treatment plant is an already existing water supply system. The spectrum of possible schemes ranges all the way from the single household water supply from wells, rain barrels or other types of water intakes(river, lakes, etc.) to a communal piped water supply system. The effort of drafting guidelines for these differing starting conditions is further complicated by different socioeconomic and infrastructural factors pertinent to the various locations.

Stating general recommendations is problematic, so this manual must confine itself to present schematically possible methods for treating drinking water in the target areas mentioned above. Furthermore, suggestions shall be made for the application of these methods, along with possible limitations. Several concrete examples will be presented. Recommendations for the planning procedure of a given project are intended to aid in the selection process.

Why is the treatment of drinking water in rural water supply projects so seldom taken into consideration? Why have so few organizations attempted to tailor known types of installations either by simplifying them, or shrinking them down to suit the capacities and resources of smaller communities in developing countries? Why do we so often see finished plants that are not in use, or poorly maintained, while the inhabitants of the area know quite well how to repair bicycles or radios?

The treatment of drinking water, indeed, seems costly, demanding and complicated, especially when measured against the standards of the industrial nations. Operation and maintenance of such installations require knowledge and experience. Insufficient care and monitoring, interruption of the operation or an incorrectly simplified design carry with them the danger that the water quality will worsen and lead to the spread of infection. Often the hygiene habits of the population for which the installation is intended are such that its purpose is not self- evident. Why then should they feel themselves responsible for the plant?

The reasons why conventional plants cannot succeed and why alternatives are rarely considered, can be summarized as follows:

The costs are too high, the benefits are difficult to quantify in monetary terms. Operation, maintenance and monitoring are problematic since specially trained personnel are needed, spare parts and chemicals have to be imported. Socio-economic factors and insufficient instructions to operators of the plant create an acceptance problem and a lessening of the feeling of responsibility. Information about feasible alternative concepts and examples is lacking. A considerable amount of time is necessary for their testing and evaluation. The organizations are lacking that would take over, locally, the responsibility for the planning, implementation and operation of the project. The administration of small projects is cumbersome. The motivation of political decisionmakers to invest in fringe areas is minimal.