The options suggested are quick short-term measures to provide a safe supply of drinking water from polluted sources. The options should be sustainable until a longer-term safe and cost-effective supply is available to the population.

The methods described are suitable for water taken from any source but, in general, will only remove physical and microbiological pollution. Pollution by chemicals such as after a spillage of industrial waste will not normally be removed by these processes and specialist advice should be taken.

In general terms, treatment of water at household level follows the processes shown in Fig1. However, depending on the quality of raw water, some processes may not be necessary.

Straining

Pouring water through a clean piece of cotton cloth will remove a certain amount of the suspended silt and solids. It is important that the cloth used is clean, as a dirty cloth may introduce additional pollutants. Specifically made monofilament filter cloths may be used in areas where guinea-worm disease is prevalent.

Such cloths remove organisms known as copepods, which act as intermediate hosts for the guinea-worm larvae. The cloth must always be used with the same surface uppermost. The cloth may be cleaned using soap and clean water.

Aeration

Aeration is a treatment process in which water is brought into close contact with air for the primary purpose of increasing the oxygen content of the water. With increased oxygen content:

The close contact between water and air required for aeration can be achieved in a number of ways. At a household level, rapidly shake a container part-full of water, for about five minutes and then stand the water for a further 30 minutes to allow any suspended particles to settle to the bottom.

On a larger scale, aeration may be achieved by allowing water to trickle through one or more well-ventilated, perforated trays containing small stones, as shown in Fig 2. Again, the water must be collected in a container and allowed to stand for about 30 minutes to settle suspended particles.

Storage and settlement

When water is stored for a day in safe conditions, more than 50% of bacteria die. Furthermore, during storage, the suspended solids and some of the pathogens will settle to the bottom of the container. The container used for storage and settlement should have a lid to avoid recontamination, but should have a neck wide enough to facilitate periodic cleaning. For example a bucket with a lid could be used for this purpose.

Water should be drawn from the top of the container where it will be cleanest and contain less pathogens. Storage and settlement for at least 48 hours also eliminates organisms called the cercariae, which act as intermediate host in the lifecycle of bilharziasis (schistosomoasis), a water-based disease prevalent in some countries. Longer periods of storage will lead better water quality. A household can maximise the benefit of storage and settlement by using the three-pot system illustrated in Fig 3.

Filtration

Filtration is the passage of polluted water through a porous medium (such as sand). The process uses the principle of natural cleansing of the soil.

Simple up-flow sand filter

Simple household filters may be put together inside clay, metal or plastic containers. The vessels are filled with layers of sand and gravel and pipework arranged to force the water to flow either upwards or downwards through the filter. Figure 4 shows a modified simple upward rapid flow filter.

A filter such as this could be built from a 200lt drum. It has a filter bed made up of coarse sand (of about 0.3m depth) of grain size between 3 and 4mm diameter, and supported by gravel covered by a perforated metal tray. The effective filtration rate of such a filter could be as high as 230lt per hour.

Such filters must be dismantled regularly to clean the sand and gravel and remove any settled silt. The frequency of cleaning is dependant on the level of turbidity of the raw water. Furthermore, such filters are not effective at removing the pathogens. Therefore the water must be disinfected or stored for 48 hours in order to make it safe.