Cost-effective testing methods for assessing the safety of drinking water
We are considering introducing the use of sand filters into our water harvesting projects in Rajasthan. Whilst we know these filters work in controlled conditions with trained users, we want to assess their effectiveness when used in rural communities in remote areas in Rajasthan. We are looking for a cost-effective and simple method of assessing water quality which can be used by trained villagers in these remote villages. All of the systems that we have found so far are expensive, bearing in mind that the villages which we support are very poor, and require careful application to be effective.
Dear Dr. Wilson,
Please have a look at the answers to a similar question about simple water testing methods. It gives several examples of suitable test kits:
I believe in terms of cost, and simplicity - one can't beat the 'presence-absence' (P/A) test vials. They can be purchased, or if one purchases the broth ingredients, they can be made by local technicians or teams. They are imprecise (as has been pointed out by others), but can be left overnight at the household level and inspected the next day.
You didn't mention what type of water harvesting - and that could influence your decision. Are these rooftop systems at household level? In which case,P/A tests are probably your best bet. If this is a community system, a higher-accuracy test might be warranted, such as what Lisa suggested, or whatever is available locally. Local availability and supply chain are very important considerations.
All the options given so far are great, but still relatively expensive.
In my opinion, the most tried and tested means of checking whether water is bacteriologically safe to drink is the presence of a chlorine residual. This has been used across the world, at every economic scale.
If you chlorinate your water using relatively cheap calcium hypochlorite tablets after filtration(1), then test using relatively cheap DPD tablets(2) comparing the colour of water + DPD with a standard chart at time of use(3), this will indicate a number of things:
A) If the water was recontaminated you will measure no chlorine(4) B) If the sand filter has failed & is letting through gross contamination then the chlorine (if correctly dosed) will be overwhelmed and you will measure no chlorine(4) C) If the anticipated chlorine dose is measured then from a bacteriological perspective, the water should be safe to drink.
This is much cheaper and less complex than any direct measures of bacteriological activity (P/A, H2S, etc.) it is in my opinion also somewhat safer as it is more broad ranging, covering many viruses and other pathogens too.
I should also say that this overcomes the main problem with sand filters - they don't necessarily disinfect the water - as pathogens can pass through. Using chlorine disinfects the water - using sand filters before it removes particulates & increases the efficiency and effectiveness of the chlorine disinfection.
(1) or calcium hypochlorite powder or granules or cheaper, but less stable sodium hypochlorite liquid, or another chlorine based disinfectant.
(2) or even cheaper but slightly more complex DPD liquid or powder - DPD is a generic chemical that turns pink in the presence of chlorine, there are others available, but it's likely the cheapest & most accurate.
(3) time of use should be at least 20 minutes after application of chlorine - or check the instructions.
(4) or at least a big reduction from normal.
It depends on how you define cost effective. I have not done any specific research but I would guess that the minimum cost per test will be in the region of GBP 2.50 to 5.00, so none of the options will be cheap in comparison to chlorine tablets, which cost less than a few pence per litre of treated water). As one of the other posters has pointed out it is cheap and easy to test for residual chlorine, and once a residual is detected you can assume that the water is free from bacterial pathogens such as vibrio cholerae or shigella. Chlorine is not as effective against protozoan or viral pathogens.
In terms of checking water that has been treated with a biosand filter I think you would need to check for thermotolerant coliforms, which will likely require a test with an incubator (hence relatively expensive kit and a power supply). I stand to be corrected but I think it is likely that the schmutzdecke in a properly functioning filter will provide a source of non-thermotolerant ecoli contamination.
I am unsure whether Presence / Absence tests (P/A) can discriminate between thermotolerant e.coli or non-thermotolerant e.coli. The former are an indication of faecal contamination, while non-thermotolerant e.coli have a number of natural sources, such as soils, decaying leaf litter etc. It would be necessary to check this requirement against the capabilities of specific products
If the purpose of your testing is to determine the effectiveness of the bio-sand filters I think it is unlikely you will find a cheap option, as you will likely need to test pre and post treatment (so approx. GBP 5.00-10.00 minimum for each run) to get credible results. Added to this there are ethical considerations associated with the event of a positive test. In essence you are proposing to provide a poor community with the equipment and training required to check if their water supply is low risk or unsafe (as you will not be able to prove it safe) hence you would need to provide similar support to the community to identify safer or lower risk options in the event of a positive test.
From previous experience the effectiveness of the bio-sand filters will be determined by human factors rather than technical process factors. If you decide to include a trial of bio-sand filters in your project I think money would be more efficiently invested in hygiene promotion and continuous support for adoption rather than testing. I recommend to plan an initial commitment of a minimum of 6 months of continued support for adoption and correct use, monitoring and evaluation.
Daniele Lantagne and Thomas Classen (MIT and the London School of Tropical Medicine & Hygiene respectively) have done quite a bit of work recently on the use of household water treatment, and the need for continued support to ensure proper utilization. Much of what I have reviewed has been on emergency contexts, so has not dealt with the ... (more)
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