For validation, what is the maximum allowable heterotrophic plate count (HPC) in water systems, according to AWWA C651?

The maximum allowable heterotrophic plate count (HPC) in water systems, as stated in AWWA C651, is 500 CFU/mL. This standard is crucial as it indicates that while the presence of microorganisms is expected in water systems, high counts can signify issues such as biofilm formation or the presence of pathogens that pose health risks.

Setting the HPC limit at 500 CFU/mL helps ensure that the water quality remains within safe and acceptable levels for public health. Keeping the HPC count lower than this threshold is generally a best practice in water management to minimize the risk of bacterial growth, particularly for opportunistic pathogens like Legionella.

In the context of legionella assessment and management, maintaining a low HPC is vital because it can directly impact the risk of Legionella proliferation. Higher counts could indicate potential problems in the system that may lead to conditions favorable for Legionella, which can cause serious health issues, including pneumonia. Therefore, 500 CFU/mL serves as an important benchmark for validating the safety and effectiveness of water distribution systems.