View all publications

Appendix 7: Log Credits

The capacity of a treatment process to reduce the number of infectious25Cryptosporidium oocysts26 in water is specified by the number of log credits27 it is assigned. The greater the number of log credits assigned to a treatment process, the larger the percentage of oocysts the process is able to remove or inactivate. The DWSNZ specify the number of log credits each treatment process can earn.

Treatment plants often have more than one treatment process that can remove or inactivate Cryptosporidium. The overall effectiveness of the treatment plant (ie, the total contribution made by all treatment processes) is calculated by adding together log credits of the individual processes.28 (For more information, see the Guide to the Ministry of Health Drinking-water Standards for New Zealand, chapter 6 (Compliance requirements for Protozoa) here)

To determine whether the number of log credits accrued by the treatment plant is sufficient to produce safe water, the water supplier needs to know the average concentration of Cryptosporidium in the source water. Once this has been measured directly, or estimated from a risk assessment of activities in the catchment, the minimum number of log credits required to treat the water can be determined. The DWSNZ provide a table that specifies the number of log credits required to treat a source water based on the results of the monitoring or catchment risk assessment.

For example, the risk assessment or source water monitoring undertaken by a water supplier in accordance with DWSNZ may have shown that the concentration of protozoa in the source water is approximately 1 oocyst per 10 litres. This means that testing (for the purposes of Regulation 4(1)(c)) has been undertaken. The drinking water treatment plant needs a minimum of 4 log removal capacity to deliver water that complies with health quality criteria in terms of concentrations of protozoa.29 If this plant has 4 or more log credits, it can be assumed to have complied with the maximum acceptable values for protozoa. Water from the plant is therefore assessed as satisfying Regulation 4(1)(c). Therefore, consent applications for activities that could affect source water for this plant will be assessed under Regulation 7.

25 Particle removal processes take Cryptosporidium out of the water. Inactivation by disinfection does not take Cryptosporidium out of the water, but renders the organisms incapable of causing infection.

26 An (oo)cyst is defined in the Drinking-water Standards for New Zealand as a thick-walled structure within which Cryptosporidium zygotes develop and which serve to transfer the organism to new hosts. A cyst is the non-motile dormant form of Giardia, which serves to transfer the organism to new hosts.

27 Log credits are a logarithmically based scale used to measure the level of removal of oocysts by a treatment process. For example, 1 log credit means there is a 101 (10-fold) reduction in the oocyst concentration; 2 log credits is a 102 (100-fold) reduction, and so on.

28 Although this is generally true, some combinations of processes are exceptions. These are specified in the Drinking-water Standards for New Zealand.

29 Refer to Table 5.1, Drinking-water Standards for New Zealand 2005.