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John Cody gravatar image

Hi You do not say in your question how you have confirmed that the source is contaminated with E.Coli? E. Coli are naturally present in water and soils, so in your case it would be adviable to confirm that the coliform bacteria are of faecal origin (i.e. using a test for Thermotolerant coliforms) Providing that the water exiting the pond has a turbidity below 5 NTU (easily measured with a turbidity tube). A potentially straight forward solution would be to connect a reservoir using a 10,000 litre plastic tank, via a cut off valve) to the reservoir. Measuring the time that it takes for the tank to fill would then give you a measure of the supply flow volume, adjusting for frction losses in the pipeline, using the Hazen Williams formula for example. Providing a sanitary tapstand from reservoir to the tank would potentially reduce the risk of contamination post collection. In addition, according to the WHO Drinking Water Quality Guidelines you can expect a significant reduction in pathogen loads with 24 hours of storage. . The storage time would also make it easier to estimate chlorine doses, as you would be treating a known volume of water, rather than a variable flow rate. Unfortunately if the turbidity is above 5 NTU the effectiveness of chlorination is reduced. The max limit of 5 NTU is from the Sphere standard for water supply, see This limit is deemed acceptable in addressing immediate needs in humanitarian contexts, though where pre-treatment is included a target limit of 1 NTU is recommended by the WHO guidelines. Providing a reservoir will allow suspended solids to settle out of the water, but is unlikely to reduce turbidity, To reduce the turbidity coagulation and flocculation will almost certainly be required. If the Turbidity is below 5 NTU you can chlorinate directly into the tank. To achieve sufficient mixing you need to ensure that water enters the tank with sufficient turbulence. Alternatively you could connect a flexible bladder between reservoir and tap stand. This would increase retention time in the system, reducing the risk of turbulence re-suspending settled solids, and facilitating chlorination and mixing. As your water source is open, the presence of organic humic substances is highly likely (normally testable by checking for green or brown color in the water). These substances react with chlorine forming THM's, which can impart an objectionable odour and taste to the water, at quite low concentrations of chlorine. The concentration of organics will be variable, and this means that your chlorine demand is unlikely to be stable if you are treating water directly form the pond. The only practical way of establishing chlorine demand is to carry out a jar test. This simple procedure involves treating samples of increasing volume (e.g 0.25L;0.5 L; 0.75 L; and 1 L) with chlorine doses and measuring the residual chlorine to determine which dosage provides a residual chlorine concentration above 1 mg/L. Constructing a graph of residual v dose allows interpolation of results. An alternative to chlorination would be to provide ceramic candle filters at the household level. Provided the filter candles are treated with silver nitrate these will provide microbiologically safe water.

Hope this helps.