Emergency treatment of drinking-water at the point of use
Technical note 5 on drinking-water, sanitation and hygiene in emergencies
Pre-treatment, storage and settlement of water
Normally, drinking water supplies need to be treated during and after an emergency to make them safe and acceptable to the user. Treatment at the point of use is generally quicker and less expensive to implement than a centralized system, but it can be more difficult to manage. Only water used for drinking and preparing food needs to be treated. Nevertheless, this still amounts to about five litres per person per day. This technical note describes some of the most common and simple treatment options suitable for use during an emergency.
There are a wide variety of technologies for treating water at the point of use. The methods described below will remove physical and microbiological pollution, but not chemical contamination. Water is made safe to drink through a process of disinfection. To be effective, most disinfection processes require the water to be pre-treated first so that it is free from suspended particles.
Aeration brings water into close contact with air which increases the oxygen content of the water. This will:
- remove volatile substances such as hydrogen sulphide and methane which affect taste and odour;
- reduce the carbon dioxide content of the water; and
- oxidize dissolved minerals such as iron and manganese so that they can be removed by sedimentation and filtration.
Water can be aerated in a number of ways. One simple method for householders is to rapidly shake a container part-full of water for about five minutes (Figure 5.1), leave it standing the water for a further 30 minutes to allow any suspended particles to settle.
Storage and settlement
When water is allowed to stand undisturbed and in the dark for a day, more than 50 per cent of most harmful bacteria die. Additionally, the suspended solids and some of the pathogens will settle to the bottom of the container, removing further risk. Storage for two days reduces contamination further still, and also reduces the number of organisms which act as intermediate hosts for diseases such as Guinea worm infection (dracunculiasis). Households can maximize the benefits of storage and settlement by using a simple three-pot system as illustrated in Figure 5.2.
Figure 5.2: The three-pot system for storing water
Always draw water for drinking from Pot 3. This water has been stored for at least two days, and the quality has improved. Periodically this pot will be washed out and may be sterilized by scalding with boiling water. Each day when new water is brought to the house:
- Step a: Slowly pour water stored in Pot 2 into Pot 3, wash out Pot 2.
- Step b: Slowly pour water stored in Pot 1 into Pot 2, wash out Pot 1.
- Step c: Pour water collected from the source (Bucket 4) into Pot 1. Strain it through a cloth.
Using a flexible pipe to siphon water from one pot to another disturbs the sediment less than pouring.
A filter removes contamination by physically blocking particles while letting the water pass through.
Straining is a simple method of filtration. Pouring water through a clean piece of cotton cloth will remove some of the suspended silt and solids (Figure 5.3). It is important that a clean cloth is used as a dirty cloth may introduce additional contaminants into the water. Especially-made monofilament filter cloths may be used in areas where Guinea worm disease is prevalent. Cloths should be cleaned using soap and clean water.
Household filters may be assembled 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 upwards or downwards through the filter. Figure 5.4 shows a simple upward rapid flow filter.
Water passes slowly through a ceramic or ‘candle’ filter (Figure 5.5). In this process, suspended particles are mechanically filtered from the water. Some filters, for example, are impregnated with silver which acts as a disinfectant and kills bacteria, removing the need for boiling the water after filtration. Ceramic filters can be manufactured locally, but are also mass-produced. They have a long storage life so can be stored in preparation for future emergencies.
Impurities retained by the surface of the candle need to be brushed off under running water at regular intervals.