# **Chapter 4. Assessing and Managing Microbial Risk to Public Health**

## **4.1 Risk assessment and management framework of the 2006 WHO Guidelines**

The assessment and management of the microbial risks to human health caused by wastewater use in agriculture is essential to avoid any excessive additional burden of disease for those who work in wastewater-irrigated fields or consume wastewater-irrigated foods.

The excreta-related pathogens and related diseases most commonly associated with wastewater are detailed in Box 3.1 and Table 3.1. However, a list of these pathogens and diseases says nothing about the risks they may pose to human health. Epidemiological studies of the types reviewed by Shuval _et al_ . (1986) and Blumenthal and Peasey (2002) (section 3.1.1) provide good information on “actual” rather than “potential” risks (Box 3.2), but they are complicated and expensive to perform and are unlikely to be done, especially in low- and middle-income countries, on the scale required to provide information on local risks. Moreover, results from areas where diseases are endemic are not comparable to those where they are not endemic. In many high-income countries, health concerns are mainly with regard to enteric viruses associated with treated wastewater. The concentrations of enteric viruses are normally so small that it is difficult to establish an epidemiological endpoint; thus, the respective guidelines are largely based on the estimation of potential risk.

The 2006 WHO _Guidelines for the Safe Use of Wastewater, Excreta and Greywater_ (WHO, 2006) have taken a radically different approach to the ones used in the 1973 and 1989 WHO Guidelines (WHO 1973, 1989). The 2006 WHO Guidelines are based on a risk assessment and management approach that follows the Stockholm Framework (Fewtrell and Bartram, 2001)—the same risk management framework that is now applied to all decisions about drinking-water and sanitation interventions—rather than specifying the required quality of treated wastewater, as was done in the 1973 and 1989 guidelines.

The 2006 WHO approach for microbial risks is (i) to define a tolerable maximum additional burden of disease, from which it is possible (ii) to derive tolerable risks of disease and infection, (iii) to determine the required pathogen reduction(s) to ensure that the tolerable disease and infection risks, and hence a tolerable maximum additional burden of disease, are not exceeded, (iv) to determine how the required pathogen reductions can be achieved, and (v) to put in place a system for verification monitoring (Table 4.1). This approach became possible only after the DALY (disability-adjusted life years) metric was developed and introduced by WHO and the World Bank in 1993 (World Bank, 1993a; see also Murray and Lopez 1996), allowing for the definition of a tolerable additional burden of disease and for the comparison of disease burdens resulting from different health risks (Box 4.1).<sup>16</sup> Using these concepts and approaches, the Australian _National Guidelines for Water Recycling: Managing Health and Environmental Risks_ (EPCH/NRMMC/AHMC, 2006) and the quantitative microbial risk analysis (QMRA) procedures used by Mara _et al_ . (2007) were developed.

The required degree of pathogen reduction (step iii above) can only be determined by QMRA (see Box 4.2)<sup>17</sup> which was first applied to wastewater use in agriculture by Asano _et al_ . (1992), Shuval _et al_ . (1997) and Tanaka _et al_ . (1998). QMRA is the keystone of a rational management framework for microbial risk assessment and control in wastewater irrigation based on a ‘multiple-barrier’ approach to risk management (Figure 4.1).

> 16 A fuller explanation of DALYs is given in WHO (2010).

> 17 A more detailed description is given in WHO (2006) and in Annex A.

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