## _Step 4: Determine how the required pathogen reductions are to be achieved_

The log-unit pathogen reductions for restricted and unrestricted irrigation determined in Step 3 are used as follows:

- _Restricted irrigation:_ the required pathogen reduction has to be achieved solely by wastewater treatment (section 4.3.1) in order to protect the health of the fieldworkers —but see section 4.3.2 for non-treatment options.

- _Unrestricted irrigation:_ using the same wastewater treatment as for restricted irrigation (since even with unrestricted irrigation there are fieldworkers whose health needs protection), the balance of the required pathogen reduction has to be achieved by a locally appropriate selection of the post-treatment health-protection control measures shown in Table 4.2—i.e., if wastewater treatment has to achieve an _x_ -log unit reduction to protect the fieldworkers and if a total reduction of _y_ log units is required to protect the consumers, then ( _y_ – _x_ ) log units have to be achieved by a selection of post-treatment health-protection control measures. For example, if norovirus reductions of 1 log unit and 3 log units are required for restricted and unrestricted irrigation, respectively (see sections A.3 and A.5 of Annex A), then for unrestricted irrigation there is a 2-log unit reduction that has to be achieved by post-treatment health-protection control measures—for example, 1 log unit by die-off and 1 log unit by produce washing in clean water, or 2 log units by drip-irrigating low-growing crops such as lettuce.

## _Step 5: Verification monitoring_

Wastewater treatment plants (WWTP) need to perform at or above their design performance —i.e., they must achieve at least their design pathogen removals. However, due to the difficulty and expense of monitoring WWTP effluents for pathogens (here, noroviruses), especially in low- and middleincome countries, recourse must be made to assessing WWTP performance by the removal of fecal coliforms (see section B.1 of Annex B).

In the case of unrestricted irrigation, in order to ensure continuous protection of consumer health, a Hazard Analysis Critical Control Point (HACCP) system should be put in place to monitor the efficacy of post-treatment or non-treatment health-protection control measures (section B.2 of Annex B).

# _4.2.2 Helminthic risks_

The 2006 WHO Guidelines treat helminthic risks differently from viral, bacterial and protozoan risks. They make the same recommendation for human intestinal nematode eggs as was made in the 1989 WHO Guidelines (≤1 egg per liter of treated wastewater), with the proviso that, if children under the age of 15 are exposed, additional protective measures (e.g., regular deworming at home or at school) be put into place. The only difference between the 1989 and 2006 Guidelines was that the recommendation for ≤1 egg per liter of treated wastewater was based on expert opinion in the 1989 Guidelines, but on epidemiological evidence in the 2006 Guidelines.

However, dose-response data for _Ascaris lumbricoides_ have recently become available (Navarro _et al_ . 2009) and thus it is now possible to conduct QMRA-MC risk simulations for _Ascaris_ , which can therefore be considered as the ‘reference’ helminthic pathogen. It is very suitable for this role as it is generally the commonest helminth and its eggs are able to survive for very long periods of time (months−years) in the environment. Log unit reductions of _Ascaris_ can now be determined by QMRA and these can be split between wastewater treatment and post-treatment health-protection control measures, as is done for viral, bacterial and protozoan pathogens since this allows for a lower level of wastewater treatment.

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