A3.7 Consumption volume of unheated (unboiled) water per person per day

Due to the sensitivity of enteric pathogens to heat (see Information Sheet: Guidance on Issuing and Lifting Boil Water Advisories, Attachment C) only the consumption of unheated water per person per day should be considered. The amount of unheated (unboiled) water consumed per day varies between individuals and depends on many factors including lifestyle and climate. The Australian Bureau of Statistics, Australian Health Survey (2012) reported the average amount of plain water consumed by adult Australians as 1064 mL per day. This has been used to give a reference exposure volume (WHO 2016) of 1L per person per day for this QMRA (which excludes heated water and other sources of drinks including soft drink, cordial and alcohol).

Evidence from an international review of drinking water consumption data (Mons et al. 2007) reported an average consumption volume per person ranging from 718 to 964 mL per person per day citing Australian studies (Robertson et al. 2000a; Robertson et al. 2000b). The review also showed that the distribution of plain water consumed was left skewed and that means were suitably conservative point estimates for QMRA (Mons et al.2007; WHO 2016). Using a reference value of 1L per person per day for consumption of unheated (unboiled) tap water is a reasonable default.

It is acknowledged that consumption levels of unheated water may be higher in tropical and arid regions of Australia. Comparing the required LRVs using an indicative Cryptosporidium concentration typical of a Category 3 source water for both a 1L and 2L daily consumption of unheated water yields a result that differs by 0.3 LRV. With rounding and discounting for Cryptosporidium infectivity, both calculated LRVs are determined to be the same. Taking local circumstances into account appropriate values should be discussed with the relevant health authority or drinking water regulator. These should be calculated on a case-by-case basis when implementing the microbial health-based targets, as not all comparisons between 1L and 2L may yield the same required LRVs.