Understanding the distribution system as a bioreactor: a framework for managing heterotrophic plate count levels

It is important to manage the level of heterotrophic plate count (HPC) bacteria in drinking water. Since the vast majority of HPCs measured in the distribution system are normally actually produced there, managing HPC numbers implies managing the distribution system as a bioreactor. This paper proposes a new concept, the HPC Production Index (HPI), which is the ratio of HPC production in a given distribution system to that produced by an essentially biostable water entering that same system. HPI is related to various physical, chemical and water quality factors such as contact time, disinfectant residual and level of biodegradable organic matter, which affect the performance of the distribution system as a bioreactor. HPI therefore provides a conceptual framework for managing distribution system HPC levels.