Which sampling method is most effective for bacteriological tests?

Sampling from an active tap is considered the most effective method for bacteriological tests because it ensures that the water collected is representative of the water that is actually being delivered to consumers. When water is drawn from an active tap, it flows directly from the distribution system, which minimizes the potential for contamination that may occur in other sampling methods. This approach helps to capture the current conditions of the water supply, including any changes in water quality that may happen due to factors such as temperature or stagnation.

Collecting samples from a reservoir could lead to results that do not accurately reflect the quality of the water entering a distribution system since water in a reservoir can stagnate and develop higher levels of bacteria over time. The dipper method might introduce external contaminants during the collection process, affecting the accuracy of the test results. Similarly, sampling from a standpipe can cause issues as it might not provide a clear view of the distribution system's water quality due to potential sediment build-up or residual water that has been stagnant.

Overall, using an active tap emphasizes current water quality and reduces the influence of contamination, making it the preferred method for accurate bacteriological testing.