Effects of holding time and temperature on conform numbers in drinking water

Concern has often been expressed about the reliability of coliform counts from drinking water samples held in transit before analysis. Samples collected from a municipal distribution system were split, stored at ambient (22°C) and refrigerator (5°C) temperatures, and analyzed over time for total coliform and heterotrophic bacterial densities. Coliform populations declined significantly at both temperatures after 24 hours. The rate of decline was 2.5 orders of magnitude greater at 22°C than at 5°C. Average losses in 24 hours were 34 percent at 5°C and 87 percent at 22°C. Growth and interference of heterotrophic bacteria with coliform colonies on membrane filter plates were frequently observed for samples held at ambient temperature for 30 hours. A model based on actual coliform recovery was developed to predict initial coliform counts in samples held at various times before examination. For the waters examined, samples held at 22°C for 30 hours with initial coliform counts as high as 100/100 mL could have been reported as satisfactory, thus resulting in inaccurate and misleading information.