Testing a pool for organisms of the coliform group indicates whether or not intestinal bacteria is present. The EPA requires that pool water disinfectants limit the probable number of coliform backteria to less than 2.2 organisms per 100 milliliters. If a membrane filter test is used, there should be no more than 1 entercoccal organism per 50 milliliters. No more than 15 percent of samples taken can fail to meet this bacterial standard.
Standard Plate Count
The standard plate count, sometimes known as the agar plate count or total viable bacteria count, often supplements a coliform test. It is most useful and appropriate for indoor pools that are not subject to the same amount of foreign objects an outdoor pool often is. The APHA Joint Committee states that no more than 15 percent of the samples taken over a period of time should contain more than 200 bacteria per millimeter.
Field Test Standards
The EPA and Michigan State University's publication, "Bacteriological Analysis of Pool Water" both cover ideal sample collection methods. The water samples used for bacteriological analysis should be taken on opposite sides of the pool in the shallow area as far as possible from the inlets. Samples should be taken slightly below the surface of the water. Also, samples should preferably be taken when the number of pool users during the hour preceding sampling was at least 50 percent of the pool's maximum bather load and the number of pool users during the time of sampling is at least equal to 25 percent of the maximum bather load.
Additional Considerations During Sampling
Although an interpretation of pool water's bacteriological data is necessary to determine whether or not a pool is suitable for use, it is not the only information about the water that should be held to a standard or examined. The bacterial test results should be interpreted with other conditions of the pool water at the time of sampling like water clarity, amount of disinfectant, pH and bather load. This will help point to any changes in the pool operating procedure that may be affecting bacteria levels or public health.