What Are Indicator Organisms?

Common indicators include forms of coliform bacteria and E. coli. The exact test depends on the permit and local rules. A result above the permit limit does not explain the cause by itself. It tells the operator that the process and the sample need careful review.

For example, a high result might follow low disinfectant feed, poor mixing, high flow, dirty UV sleeves, a weak lamp, a sampling mistake, or a lab problem. The operator should confirm facts before making a large process change. Check flow, equipment status, recent trends, sample location, and test quality.

Indicator results are only one part of the picture. Chlorine residual, UV intensity, dose settings, contact time, turbidity, and maintenance records can help explain what happened. Good records connect the lab result to plant conditions.