Ecoli and liquid Chloride in a local water system
A villages water reservoir is contaminated with Ecoli. The water comes from a river and is "filtered" in 3 pools before it comes into the reservoir. The reservoir is 22 feet by 22 feet with a height of 12 feet. Water is constantly beeing taken and refilled without us being able to measure how much is taken and how much is filled up. The community has liquid chloride but they dont know how much to put in. I don't know what to tell them given that chloride has to be continuously be present. Ecoli makes the 400 children and 150 families of the village sick. What shall I tell them? How to measure when they have to add chloride? We could install a constant drip but again I am unsure about the amount given that we can not measure how much water is taken/refilled in the reservoir.
Ideally you would want:
1) a flow meter,
2) a metering pump dosing just before a static mixer,
3) an online dual validated chlorine monitor monitoring the chlorine level after the static mixer but before the reservoir,
4) a controler system that adjusts the dose based on the flow and chlorine level,
5) a second dual validated chlorine monitor after your reservoir that can adjust the setpoint of the control system to take account for slower chlorine demand across the reservoir,
6) the reservoir should have some sort of weir that ensures a minimum residence time, and baffles, so all water spends the same amount of time passing through it - you want at least 30 minutes residence time.
You then would use this system to ensure a minimum Ct value (chlorine concentration x time) - that would typically be around 15 mg.min/l for groundwater or 30 mg.min/l for surface water.
However, budget may be an issue...
As a minimum you need to know the flow rate. From that and an understanding of your reservoir you need to determine the shortest time that water can spend in the reservoir. Hopefully that value is at least 30 minutes.
Take that time in minutes and divide 15 or 30 by it to determine the chlorine residual that you need to achieve at the outlet of the reservoir. So if your reservoir is holding water for 30 minutes, you would want 0.5 mg/l chlorine residual if you are treating groundwater or 1 mg/l if surface water (from a river or lake).
Does that make sense?
Chlorination is pretty serious stuff, so you I would suggest that you engage a competent consultant/contractor with lots of experience of similar projects. They should already know everything I've mentioned above, and more...
You don't say how they collect water from the reservoir is it a piped outlet to taps? Is it a closed tank? Does it enter the tank via a pipe? What is the pressure head at the inlet to the tank.
There are various devices that can be used on a pipe that is flowing full to dose chlorine solution such as water-powered pumps (e.g. Dosatron) and venturi suction systems. Unlike the drip system, these only work when the water is flowing and the dose can be made to be proportional to the flow.
There are other devices that may also be suitable. Look at the publication Chlorinating Small Water Supplies – A review of gravity-powered and water-powered chlorinatorsthat you can find at https://wedc-knowledge.lboro.ac.uk/de...
If it is a surface water source it is likely to have a variable chlorine demand even if it is pre-settled, depending on recent rainfall runoff that will affect the turbidity and organic content. If this is the case then having a set dose (even id proportional to the flow) will be problematic since the water quality (and hence the chlorine demand) can vary from day to day.
First consider if it necessary to treat all the water they use. If people can be trained and motivated, it may be more cost effective to dose the chlorine solution at household level and to dose just the water used for drinking and food preparation. If you do this it also deals with potential problems of post-treatment pollution of chlorinated water at or after it is collected from public water points. However, users still need to store it hygienically in the home and to allow 30 minutes contact time for the disinfection to take place.
Brian Skinner WEDC
Hi. I assume that the 3 pools are more settling than filtering ponds. I would suggest that the first step is to inspect these ponds and see if there are any obvious sources of Ecoli; perhaps they are seen as quiet places to openly defecate? These would need to be addressed before looking at chlorination. Turning to the reservoir is the water turbidity at entry and exit low enough for chlorination? If not, you may be wasting your time. Assuming satisfactory, is the reservoir covered or open? If the latter, you might have greater problems with any chlorine you dose being lost and the residual being unsatisfactory. I would suggest that you check the chlorine demand of the water coming into the reservoir and then do a one off dose of the reservoir and check that the residual is satisfactory. I suggest that you put this chlorine in at or near the water entry point and explore how to disperse over the tank and avoid short circuiting to the exit. If there is a lot of silt in the tank, this will absorb the chlorine if you disturb it. If you really cannot measure the amount of water, I would urge you to make an estimate by asking a representative number of families how much they take each day. You could then add the requisite amount of chlorine each day and test the residual each time as well. You could then adjust the daily dose to attain a satisfactory residual. Beware, if the water is heavily contaminated and you add a lot of chlorine, the taste might encourage people to drink less safe water.
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. http://www.who.int/water_sanitation_h... . 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 http://www.sphereproject.org/resource.... 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 ... (more)
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