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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 bio sand filter specifically. You should search for their recent publications, for example. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516252/. You could also contact the Public Health Team in Oxfam who have done quit a bit of work in relation to house hold water treatment, and the associated hygiene promotion requirements (West Africa, Pakistan and Haiti). Murray Burt of UNHCR also has direct recent experience of household filters in Darfur.

Apologies for the long winded answer to a question that you did not in fact pose. However I have already typed it so I feel invested. Hopefully I have provided sufficient detail to help you in your decision.

Best regards

John Cody