Ideally, the filter should contain 20 to 80 coliform colonies and the number of all types of bacterial colonies should not exceed 200. This technique is considered more statistically reliable than the Most Probable Number procedure. A lab technician divides the water sample into five to ten test tubes which contain a coliform nutrient along with an agent that reacts with the waste products of coliform bacteria to produce a distinctive color. After incubating the bacteria, the technician counts the number of tubes showing the reaction color. This index represents the number of coliform bacteria that would likely produce the observed results.
Labs use both types of analyses.
The MF technique is good for large numbers of samples and produces results more rapidly. As organic degrada tion takes place, bacteria and other decomposers use the oxygen in the water for respiration. Since bacteria decompose organic material, water with a high BOD level also generally has a high bacterial count. Although some waters are naturally organic-rich, a high BOD often indicates polluted or eutrophic waters. The standard BOD test is a simple means of measuring the uptake of oxygen in a sample over a predetermined period of time.