Sunday, January 27, 2008

Simple Field Tests for Water Quality

We’ve been performing some quick and inexpensive tests on the water sources around Pun Pun Farm to assess water quality. Three parameters of concern when designing the water treatment system are turbidity, dissolved iron concentration, and microbial contamination.


Turbidity (cloudiness owing to suspended solids and/or organic matter) in the source water may cause clogging of sand and charcoal filtration units. Turbidity also reduces transmittance of UV light through water and thus markedly reduces the effectiveness of UV disinfection.

There are several ways of quantifying turbidity in water, the most common being some measure of the attenuation (reduction in strength) of light as it passes through a column of water. Electronic turbidity meters are the most precise; however, they are expensive. A turbidity tube is inexpensive and easy to construct, and provides an approximate measurement of turbidity with sufficient accuracy for field water quality assessments.

To measure turbidity, sample water is slowly added to the tube and the depth (in cm) recorded when the target disk at the bottom of the tube becomes invisible.

Dissolved iron

Excess iron in water used for drinking, cooking and washing imparts unappealing color and taste, and reduces the effectiveness of UV disinfection by impairing transmittance of UV light through water. If the amount of iron in a water supply is above the US EPA and World Health Organization guideline of 0.3 mg/l (0.3 ppm), the water can cause staining of laundry or discoloration of faucets and basins. However, even high concentrations of iron are not considered a health problem.

The water treatment system at Pun Pun Farm will incorporate a slow sand filter to precipitate excess iron from solution.

Microbial contaminants

We’re using the simple hydrogen sulfide (H2S) test to detect the presence of fecal coliforms in water samples from around the farm. Hydrogen sulfide-producing microorganisms are often associated with fecal coliforms, and thus indicate that a water source may be contaminated with human or animal feces.

The test is inexpensive and simple to administer – a microbial growth medium and iron source are added to the water sample and the sample is incubated at around 30 Celsius for 24 – 48 hours. If the hydrogen sulfide-producing indicator organisms are present, then the H2S they produce will combine with iron to form iron sulfide, an insoluble black precipitate. If no H2S-producing microbes are present the sample will remain clear yellow over the 48-hour incubation period.

This test does not detect fecal coliforms directly but responds simply to the presence of H2S. There are a variety of biological and geochemical processes that may result in the presence of H2S to the sample, giving false positive results for the test. Also, the test does not distinguish between fecal coliforms originating from the digestive tracts of animals versus humans. However, given the broad scope of the test and its sensitivity, a negative test result lends strong confidence that the water source in question is free of fecal contamination.

The water treatment system at Pun Pun Farm will use a combination of a slow sand filter and disinfection by germicidal ultraviolet light to neutralize the threat of biological contamination of the community’s drinking water.

You can find more detail about these methods for DIY water quality testing on the Aqueous Solutions website, where our technical paper Simple Field Tests for Water Quality is available for download from the Resources page.

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