Unlike bacteria, compliance with the protozoa criteria of the DWSNZ does not require the water supplier to monitor the organism directly. Testing for protozoa is expensive. Compliance is based on the ability of the treatment plant to remove protozoa, and more particularly, to remove Cryptosporidium which is a more difficult task than removing Giardia. Giardia is a larger organism than Cryptosporidium and its physical removal by particle removal processes is easier. The level of chlorine required to inactivate Cryptosporidium is too high for use in water treatment, but Giardia can be inactivated by chlorine during water treatment. However, the necessary chlorine concentration or the contact time of chlorine with the water, or both, are somewhat higher than those required for inactivation of E. coli.
The approach to basing protozoal compliance on the treatment process and its performance requires knowledge of:
- the concentrations of Cryptosporidium in the source water
- the efficiency of the treatment plant processes at removing or inactivating Cryptosporidium.
Comparing i) with ii) shows whether the treatment plant can remove or inactivate enough of the protozoa in the source water to produce safe drinking water.
Chlorine is used as a disinfectant by most water treatment plants in New Zealand, because of its effectiveness against bacteria and viruses. However, it is relatively ineffective against Cryptosporidium . Treatment plants must include other treatment processes to achieve compliance with respect to protozoa.
6.2 Treatment processes for protozoa
Two types of treatment processes protect against protozoa:
processes designed to remove particles from the water – because Cryptosporidium is just another particle
disinfection processes that inactivate the organism.
Processes that physically remove Cryptosporidium have varying degrees of effectiveness, and include various types of filters.16 Their effectiveness is often increased by a preceding coagulation/flocculation stage, which clumps small particles together and assists in their sedimentation or filtration.
Disinfectants that inactivate Cryptosporidium at an acceptable rate are chlorine dioxide, ozone and UV radiation. The percentage of the organism that they inactivate depends on the concentration of the disinfectant (or intensity, in the case of UV radiation) and the time that the Cryptosporidium is exposed to the disinfectant.
A water supply that has a groundwater source that is classified as secure does not need any additional treatment to achieve compliance with the DWSNZ with respect to protozoa.
6.2.1 Log credits
The capacity of a treatment process to reduce the number of infectious. Cryptosporidium oocysts in water is specified by the number of log credits17 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 the log credits of the individual processes together.18
The ability of a treatment process to remove or inactivate Cryptosporidium depends on how well it is operated. A poorly run process may achieve very little removal, despite it being capable of scoring a substantial number of log credits. The assessment of compliance therefore depends on the water supplier being able to show satisfactory operation of each process.
Turbidity is one performance parameter used to show satisfactory operation for several processes that remove particles from water. Turbidity is measured by the water supplier after treatment. Clear, treated water shows the process is working well and that if there was Cryptosporidium in the raw water it will have been reduced to a safe level. Turbid water leaving the treatment process does not necessarily contain Cryptosporidium, because there may have been no Cryptosporidium in the raw water. However, the poor performance of the process does make it more likely that any Cryptosporidium that was in the raw water will not have been removed.
NES note: Catchment activities that affect the levels and variability of raw water turbidity increase the difficulty for a treatment plant to comply with the DWSNZ. Full conventional treatment19 can handle high levels of turbidity, but fluctuations in turbidity levels make producing good-quality water difficult. Where other treatment processes are in use, an increase in raw water turbidity may exceed the treatment plant’s design specifications.
Turbid water leaving the filters also threatens the efficacy of the following disinfection process, whether it is intended to inactivate protozoa and bacteria, or just the bacteria.
6.3 Cryptosporidium in the source water
Cryptosporidium concentrations in source waters are very variable, even over short time scales. Increased Cryptosporidium concentrations are often associated with increased turbidity of untreated water. Note, however, clear raw water is not guaranteed to be free of the organism.
To determine whether the number of log credits accrued by the treatment plant is enough 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.
The water supplier is responsible for monitoring Cryptosporidium , or undertaking a catchment risk assessment, to assess the log credits required to treat its source water.20 Supplies serving more than 10,000 people are required to take source water samples for direct Cryptosporidium measurements (fortnightly samples over 12 months). Supplies serving 501–10,000 people may use the catchment risk assessment option.21
NES note: Catchment activities that are likely to increase the concentration of Cryptosporidium in a source water could lead to an increase in the log credits a water supply requires, to achieve compliance with the DWSNZ with respect to protozoa.
16 Particle removal processes take Cryptosporidium out of the water. Inactivation by disinfection does not take the Cryptosporidium out of the water, but renders the organisms incapable of causing infection.
17 Log credits are a measure of the level of removal of oocysts by a treatment process. It is a logarithmically based scale. 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.
18 Although this is generally true, some combinations of processes are exceptions. These are specified in the DWSNZ.
19 Treatment using coagulation/flocculation, clarification, filtration and chlorination.
20 The supplier may seek information from its regional council to assist in undertaking the catchment risk assessment.
21 A survey to assist in linking the results of the risk assessment to the expected Cryptosporidium concentration in the water is being planned at the time of preparing this guide.