Heterotrophic Plate Counts and Drinking-water Safety The Significance of HPCs for Water Quality and Human Health

Heterotrophic Plate Counts and Drinking-water Safety provides a critical assessment of the role of the Heterotrophic Plate Count (HPC) measurement in drinking water quality management. It was developed from an Expert workshop of 32 scientists convened by the World Health Organization and the WHO/NSF International Collaborating Centre for Drinking Water Safety and Treatment in Geneva, Switzerland. The workshop sponsors were the U.S. Environmental Protection Agency, Health Canada, U.S. Centers for Disease Control and Prevention, and the American Waterworks Association Research Foundation. Heterotrophs are organisms, including bacteria, yeasts and moulds, that require an external source of organic carbon for growth. The HPC test (or Standard Plate Count), applied in many variants, is the internationally accepted test for measuring the hetrotrophic microorganism population in drinking water, and also other media. It measures only a fraction of the microorganisms actually present and does not distinguish between pathogens and non-pathogens.Although most, if not all, bacterial pathogens are heterotrophs, most of the microorganisms detected by the HPC test conditions are not human pathogens, thus the colony counts obtained do not alone normally correlate with the presence of pathogens, in the absence of other indicators of faecal contamination. High levels of microbial growth can affect the taste and odor of drinking water and may indicate the presence of nutrients and biofilms which could harbor pathogens, as well as the possibility that some event has interfered with the normal production of the drinking water. HPC counts also routinely increase in water that has been treated by an in-line device such as a carbon filter or softener, in water-dispensing devices and in bottled waters and indeed in all water that has suitable nutrients, does not have a residual disinfectant, and is kept under sufficient conditions. However, there is no firm evidence that non-pathogenic bacterial growth as measured by HPC is accompanied by increased risk of illness among consumers.On the other hand there is some evidence that the presence of the indigenous non-harmful bacteria may challenge the survival of pathogens that may be present in biofilms and on surfaces. There is concern that some immuno-compromised persons may be at risk from exposure to otherwise harmless bacteria if exposure is excessive. There is debate among health professionals as to the need, utility or quantitative basis for health-based standards or guidelines relating to HPC-measured regrowth in drinking water. The issues that were addressed in this work include: the relationship between HPC in drinking water (including that derived from in-line treatment systems, dispensers and bottled water) and health risks for the general public; the role of HPC as an indirect indicator or index for pathogens of concern in drinking water; the role of HPC in assessing the efficacy and proper functioning of water treatment and supply processes; the relationship between HPC and the aesthetic acceptability of drinking water.Heterotrophic Plate Counts and Drinking-water Safety provides valuable information on the utility and the limitations of HPC data in the management and operation of piped water systems as well as other means of providing drinking water to the public. It is of particular value to piped public water suppliers and bottled water suppliers, manufacturers and users of water treatment and transmission equipment and inline treatment devices, water engineers, sanitary and clinical microbiologists, and national and local public health officials and regulators of drinking water quality.