Plastic Bags for Testing Water Quality

Alice Wang

Water-borne illnesses kill millions of people worldwide each year. Of these deaths, more than 1.5 million are children under the age of five who die from diarrheal diseases, caused largely by unsafe drinking water. The need for safe drinking water around the world is great, as is the need for efficient and accessible microbial water quality testing to identify safe and unsafe drinking water. With those needs in mind, researchers at the University of North Carolina at Chapel Hill's Gillings School of Global Public Health have designed a portable water quality test based on a surprisingly simple innovation – the use of plastic bags. 

This new method of water quality assessment tests for fecal indicator bacteria by using a plastic bag divided into multiple compartments. Field workers collect water in a 100 mL bottle.  When it is time to test the water, a color-changing medium is dispensed into the water sample.The field worker then pours the medium-supplemented water from the bottle into the plastic bag, manually adjusts the volume of water in each bag compartment and seals the compartments by snapping a plastic clip on the bag, essentially making it the equivalent of a portable set of test tubes. The sample incubates at ambient temperatures (between 25 and 44.5 degrees Celsius, 77 and 113 degrees Fahrenheit).  If the target bacteria – either E.  coli or hydrogen-sulphide-producing bacteria – grow in a compartment, the water turns a distinct color.  

“I’ve been working on methods of microbial analysis for most of my career, and it has been clear from my own work and interaction with others that there is a lack of something cheap, simple and useful for water quality analysis in low-resource settings,” said Mark Sobsey, PhD, Kenan Distinguished Professor in the Gillings School’s Department of Environmental Sciences and Engineering.  

Sobsey came up with the idea of using plastic bags as an inexpensive and portable water quality testing method, and he has led the effort to continue the test’s development and dissemination. 

Quantifying Bacteria in Water

In addition to its ease of use, portability and low cost, a major advantage to this new method over previously used methods is that it quantifies bacteria levels in water rather than simply detecting whether bacteria exists or whether people in a community are using water sources deemed acceptable based on the number of bacteria in the water. 

“One of the simplest ways to quantify bacteria in water is to take smaller volumes of 100 mL of water and dispense them in samples,” explained Sobsey. “After adding the medium, the samples incubate and bacteria can be detected by the distinctive color change their growth produces in the water.”

A standard metric known as the most probable number estimation (MPN) is then used to quantify bacteria levels. Until now, this process has been cumbersome and impractical for field workers. 

“You need test tubes, racks to hold the test tubes, sterile pipes, etc. So all of a sudden, that method gets complicated to utilize in low-resource settings,” Sobsey pointed out. Mulling over this dilemma eventually led Sobsey and his colleagues to what may seem like a simple idea, but one that had not been used before. “We thought, “Let’s use the same principle and do the equivalent in the simplest container possible,’” he recalled.  “And the simplest thing we could think of was a single plastic bag divided into compartments.”

Testing Water Quality in Peru

With the help of funding from USAID and the Gillings School of Global Public Health, Sobsey was able to work on the development of the test for a few years.

MEASURE Evaluation soon recognized the potential this new method could have on water quality in developing countries and decided to pilot test it in Peru, in conjunction with the country’s annual Demographic and Health Surveys (DHS.)

“We piloted the test with a subset of survey teams,” Sobsey said. “We chose to implement our water test with three DHS survey teams in different geographic regions of Peru.” These regions included the mountains, the jungle and the city of Lima. The results were very promising.

Training of field workers took less than two days. When running the tests, they collected three 100 mL bottles per household, running one bottle through the plastic-bag test in the evening and reading results the following evening. The other two bottles were sent to a reference lab, where one was tested using the plastic-bag method and the other analyzed by a standard membrane filtration test – a more complicated test that requires a lab.  

“The results for all three bottles came out to be essentially the same,” Sobsey said. “There was no difference in the amounts of bacteria detected.”

Positive feedback from field workers added to these desirable results. They rated the plastic-bag method a 9.3 three on a scale of one to 10 (10 being the easiest,) proving that the test places very little burden on survey workers.

International Recognition

So what’s next for this innovative water quality test? Sobsey said another test would be conducted in a yet-to-be-determined country in 2012. Peru has also expressed interest in scaling up the test to the national DHS program.

The test has been received very enthusiastically in the global health community. In the past few months, Sobsey and his team members have made presentations at the USAID headquarters, the Singapore International Water Week, the International Water Association Health-related Water Microbiology Symposium 2011 in New Zealand, UNC’s Water and Health Conference and the annual meeting of the American Society for Tropical Medicine and Hygiene.  

Visit Sobsey’s USAID Launch Water profile page to learn more about his work on low-cost microbial water quality test.  Learn more about the plastic-bag test.