What it is
The colony count, also called total viable count or heterotrophic plate count, measures how many bacteria of all kinds grow from a sample when it is kept at 22°C for three days. Most of them are harmless environmental organisms. It is a measure of general hygiene and stability rather than a test for any particular harmful bacterium.
Where it comes from
Every untreated water supply has a natural background count. It rises when water stands for a long time in tanks or pipes, when the water is warm, when there is biofilm in the system, after work on the pipework, or when a filter or UV unit is overdue for servicing.
Why it matters
There is no health-based limit, which is why the guideline is written as no abnormal change. A high or rising count is a sign that something in the system has changed and is worth investigating, and it can make UV disinfection less effective. On its own it does not mean the water is unsafe.
What can be done about it
Most often the answer is housekeeping: flush little-used outlets, clean and disinfect storage tanks, and service filters and UV lamps on schedule. Where counts stay high, a Pozzani engineer will look for dead legs, warm sections of pipework and the condition of the tank.
Information last reviewed 5 October 2026 by Pozzani Pure Water.