Revision history [back]
If the water source is an open well then it is likely that there will be bacteriological contamination unless it is in a very remote area. Pauline has suggested some good precautions of capping the well and building a concrete apron around it. This may not be feasible if other people are using the well for water supply, especially if they are using buckets to draw water. This would cut down on the risks of bacteriological contamination significantly but there may be sub-surface flows to the well and you need to be very careful if there are any latrines or leaking sewage pipes nearby.
Water could be tested for faecal coliform bacteria at a public health laboratory if there is one nearby or using a portable kit such as the Oxfam Del Agua kit. There is a school of thought that says there is little point in testing water if you can do little to change the situation if the water fails the test. A simple inspection of the water source can identify with some accuracy the likelihood of bacteriological contamination without the need for testing. Shallow wells are likely to be contaminated, especially without the protection described above but deeper boreholes and natural springs are likely to be better, though this comes with no guarantees.
Biological contamination in most places is a much higher risk than chemical and causes most water borne diseases; diarrhea, cholera, dysentery etc. Globally the World Health Organisation standards are considered the minimum but in many countries these are adapted into local standards. There are a lot of different parameters that can be measured and for which there are standards and for some chemicals the standard is the minimum detectable limit. In the UK the public water supply is tested against some 200 parameters whereas on every bottle of water you will find a shorter list of perhaps 12 of the key ones, sometimes compared against the WHO standard. Testing is expensive and requires a specialised laboratory though portable kits do exist for some things. Chemical contamination may come from the water itself or from pollution from mining, industry or agro-chemicals. If the area is remote and the local water authorities have not identified any chemical contamination of the water in the area then it is probably OK.
Treatment of bacteriological contamination is generally cheaper and easier than chemical contaminants. Often a 3 stage process is used in municipal treatment plants; sedimentation, filtration and disinfection though my experience with small scale treatment plants for single institutions such as schools is not good as they are often not operated correctly. If the water is relatively clear then you could just use disinfection and chlorine is the most common way of doing this and I give more details below. Other common methods of treating small quantities of water for drinking purposes include filtration through ceramic candle filters and boiling.
Chlorination is one of the most effective ways of killing pathogens and making water safe from biological contamination. It should be noted that sediment in the water reduces the effectiveness of chlorine. Chlorine comes in many different forms, gas, liquid and solid. With a small school tank the simplest form to use, if they are available, would be high volume chlorine tablets sufficient for the amount of water in the tank. It would also be possible to use High Test Hypochlorite granules (HTH) which are often used in swimming pools or a liquid domestic bleach or disinfectant. Each form has a different available chlorine content and it is useful to make up a standard 1 per cent chlorine solution (10g of chlorine/litre of water). This also ensures that solid forms of chlorine are fully dissolved before being added to the tank. The quantities of common forms of chlorine needed for a 1% solution are shown below.
Chlorine Source Available Chlorine % Q Required Approximate measure High test hypochlorite (HTH) granules 70 14g 1 heaped tablespoon
Bleaching powder 34 30g 2 heaped tablespoons
Stabilised tropical bleach 25 40g 3 heaped tablespoons
Liquid household disinfectant e.g. Parazone Domestos 10 100ml 7 tablespoons
Liquid laundry bleach 5 200ml 14 tablespoons
Antiseptic solution e.g. Milton, Javal 1 1 litre 1 litre
Chlorine tablets e.g. Aquatab. Various strengths, easy to use. 60-70 14g Tablets specify the quantity of water
Enough chlorine must be provided to disinfect the water completely and to leave a free residual chlorine level in the water of 0.2-0.5mg/l. The chlorine residual level can be accurately measured with a simple colour comparator device but as a rough guide the free residual level is in the right range when chlorine becomes detectable in the water by taste or smell. The dose of chlorine to add to achieve the desired free residual level depends on the quality of the water. If there is a high load of suspended solids, more chlorine will be needed. It is however usual that chlorine should be added in the range of 1–5 mg/litre. The amount of the 1% chlorine solution described above needed to achieve this range of chlorination in different sized water tanks is shown below.
Level (mg/l) 1,000l tank 2,000 l tank 5,000 l tank 6,000 l tank 8,000 l tank
1 0.1 litres 0.2 litres 0.5 litres 0.6 litres 0.8 litres
2 0.2 litres 0.4 litres 1.0 litres 1.2 litres 1.6 litres 3 0.3 litres 0.6 litres 1.5 litres 1.8 litres 2.4 litres 4 0.4 litres 0.8 litres 2.0 litres 2.4 litres 3.2 litres 5 0.5 litres 1.0 litres 2.5 litres 3.0 litres 4.0 litres
This chlorine should be allowed to run through the pipes so that they are also disinfected and then left to stand for at least 30 minutes as this contact time is necessary for it to kill all of the pathogens. Chlorine evaporates from a free water surface so water that is left in the tank will gradually reduce in chlorine content.
While disinfection will probably be needed to make the water 100% safe, it may place unrealistic demands on the school authorities in terms of management or long term procurement of chemicals. If people at the school are already drinking from the well and are likely to continue to do so at home after the system is installed you will gain little from chlorination.