For all its virtues the Membrane Filtration method does have disadvantages. To obtain trustworthy results it is necessary to incubate the samples an elevated temperature of exactly 44.5 ±0.2 ºC. Thus the method requires an expensive incubator and a reliable source of electricity, and the cost per test is relatively high. It is difficult to use the method in the field and samples must be processed within six hours after being taken, making it difficult to test water in remote communities.
The Nicaraguans working with the test in the field were asking us "Isn't there something simpler?".
So to complement the Membrane Filtration method we searched for a method that could be used in rural areas to measure the quality of potable water without the need for a laboratory nor a source of electricity. We considered various presence/ absence methods such as those that use the hydrolysis of MUG to detect fecal coliforms; however the majority suffered from two of the same disadvantages as the Membrane Filtration method: they required incubation at a precise temperature, and they were too costly.
Prototype of the apparatus for the Simple Field Test, and typical results
The method seemed ideal for use in Nicaragua, so we designed and built the simple apparatus shown above. It is cheap, light and portable, easily carried by hand or on the rack of a motorcycle. We also modified the method, collecting and incubating the samples in sterile Whirlpak sample bags instead of glass bottles.
Because of this, and because some published studies had claimed that the test did not work well in some countries TASCA and ENACAL/DAR carried out a validation study before introducing the method for general use. The report is available at the links on the right.
Click below to view the Simple Field Test validation report (pdf) in <ENGLISH> or
Samples from 215 water sources were tested by both the M-FC and the Simple Field Test. The results showed a correlation of 90% between the tests, equal to that reported in the studies mentioned above.
With this validation the promoters of ENACAL/DAR began to use the test in 1991. It is now in wide use by the rural health promoters of MINSA in the Departments of Chontales, Jinotega, Madriz, Matagalpa and the RAAS (Autonomous South Atlantic Region).
The health promoters have accepted the test enthusiastically, describing it as "practical". It is also educational; the black color and hydrogen sulfide smell of the water after incubation make a strong impression, especially on those who were consuming that water.
Teaching the Simple Field Test, El Cuá, 2008
Details of the Test
After marking the Whirlpak bag so as to identify the sample and the time of sampling, a sample of water is taken according to the norms of "Standard Methods for the Examination of Water and Wastewater ", filling the bag to the 100 ml line. The bag is placed, still open, into the wire rack. A capsule of medium (Pathoscreen, Hach Co.) and the blade of a knife or scissors are sterilized with an alcohol-soaked cotton swab, the capsule is cut open and the dry growth medium poured into the water sample. The bag is carefully closed and left in the wire rack at ambient temperature with the box lid closed for 24 hours. The temperature in the box is checked occasionally to ensure that it is between 25º and 35º C. If the temperature is below 25ºC incubation is continued to at least 30 hours. If it threatens to rise above 35º C any number of means are used to cool the samples, including placing the box in the shade or wrapping it in a wet towel.
In contaminated samples, bacteria produce hydrogen sulfide from peptone and sulfates in the medium. This reacts with the iron salts in the medium to form an intense black precipitate, as shown in the picture at the top of the page. Such samples are recorded as positive. If there is no change in the light yellow color of the sample plus medium the result is negative. Samples with bacterial growth but no black precipitate are recorded as "OTB - Other types of bacteria".