Wastewater and wastewater residuals contain pathogens that pose an unacceptable risk to human health unless measures are taken to control the hazard. Risk reduction is based on the principle of multiple barriers that, when applied in concert, act to reduce the risk to an acceptable level. This approach to risk reduction is enshrined in the United States (USEPA, 1993) and European regulations (European Commission, 1986) governing the land application of biosolids. The individual elements of the multiple barrier approach are (i) using a sludge treatment process capable of inactivating pathogens, (ii) prohibiting the application of biosolids to land used for the production of certain categories of food crops or with public access, and (iii) imposing minimum periods between application and harvesting of food crops or grazing by livestock animals during which time surviving pathogens will be reduced further in numbers. The reduction in pathogen numbers from sludge production to the point of human exposure is the combination of that achieved in (i) and (iii) above. Hence, the degree of pathogen reduction achieved during sludge treatment is augmented by the duration of post-application controls.
To address concerns raised about the robustness of risk reduction measures in programs where historically reliance had been placed on post-application controls, utilities are considering additional treatment to produce Class A or enhanced biosolids. This puts operators of sludge treatment facilities in control of the greater part of the overall pathogen reduction goal. It is important, therefore, that operators understand the ability of the process to inactivate pathogens, the associated control points, and the critical limits. Carrington et al. (1982) observed that destruction of salmonellas in full-scale operational anaerobic digesters was consistently lower than that achieved during laboratory experiments using sludge from the same works. Factors affecting the efficiency of anaerobic digestion include poor mixing, short circuiting, uneven heating, and reduced hydraulic retention time due to accumulated debris in the base of the digesters. The efficiency of full-scale operational sludge treatment plants also varies over time as shown by Soares et al. (1994) when they investigated the removal of enteroviruses as measured in 12 monthly sampling events. Operational efficiency (in terms of pathogen inactivation) can significantly affect the net pathogen reduction, particularly for a Class A or enhanced treatment process. The effect of operational efficiency on human health risks associated with growing crops of land receiving biosolids has been modeled (United Kingdom Water Industry Research, 2003; Gale, 2004). Modeling showed that a process achieving >6 log destruction (of pathogens) for 99% of the time, but with a 0 log reduction for the remaining 1% of the time (e.g., due to short circuiting), achieves a net reduction of just 2 log, giving rise to a 10000-fold increase in the relative risk. The relative effect on less efficient processes (Class B, Treated) of operational underperformance was markedly smaller. Consider a process capable of achieving 2 log reduction under ideal conditions. Inefficiency resulting in 1% of the sludge receiving no pathogen inactivation is equivalent to net destruction of 1.7 log. The modeling demonstrates the crucial importance of characterizing the sludge treatment process, identifying the critical control points, and establishing critical limits linked to monitoring. Thereafter, routine measurement of the CCPs may be used to demonstrate that the process is operating efficiently and achieving the necessary level of pathogen destruction.
ACKNOWLEDGMENTS
The views and opinions expressed are those of the authors and do not necessarily reflect those of United Utilities Water Ltd.
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