WATER QUALITY

 

How to test water quality? Low-cost, low-tech options for microbial testing.

Microbial testing is arguably the most important factor when monitoring the quality of drinking water.  Without noticing anything strange, a single glass of cool, refreshing water could contain billions of microorganisms. Just a few dozen of these organisms could be enough to make you very sick.[1] According to estimates from the World Health Organization, diarrheal disease from contaminated drinking water causes more than half a million deaths annually.[2]

There are a large number of microbial testing options available today, but they are not perfect. Selective media and enzyme-based assays that detect E. coli’s beta-glucuronidase enzyme are often well-suited for low-resource settings.[4] Overall, these microbial water quality tests take one of three approaches:

• Presence-absence (P-A): P-A tests don’t provide quantitative information about microbial water quality. Instead, they change color to tell you whether or not microbial contamination has been detected. Test kits are comparatively inexpensive, but often involve adding a powdered nutrient mixture and allowing a 24-hour incubation period for organisms to grow. P-A tests are suitable for screening in situations where microbial contamination is not expected (e.g., deep groundwater).
• Most probable number (MPN): MPN tests are semi-quantitative. Several samples of the same water are tested in tubes, plastic bags, or small plastic plates with multiple “wells.” The user adds a nutrient solution (“culture media”) and waits 12-48 hours for organisms to grow before counting the number of positive samples, indicated by a color change. The user then converts that number of positives to a statistical estimate of bacterial concentration, as per the instructions for the particular test.
• Membrane filtration: Membrane-based tests are the most quantitatively accurate. In general, a 100 mL water sample is forced or vacuumed through a small, round filter paper (the membrane) using a little hand pump. All the bacteria in the sample are caught on the filter as the water passes through. The filter is then incubated with some sort of culture media. Each bacterium caught on the filter will multiply into a little colony. After the incubation, the user counts the colonies – possibly with the aid of a magnifying glass – to determine how many “colony-forming-units” were present in the original 100 mL sample. Due to the filtration step, membrane-based tests are more difficult when water samples contain a lot of suspended material, and they can take a bit of time.

When selecting a test, it is helpful to consider not only the quantification needs and the cost, but also factors like the sample volume, the format and stability of the culture media (e.g., powder, liquid, agar, films, absorbent pads), the incubation time, and the ease of reading results. Incubation temperature is also an important practical consideration: Some tests require a warm incubator, while others can handle ambient (warm climate) temperatures. A few small-volume tests are even designed to be incubated with the user’s own body heat.

          

    salvatory eustace