THE PRESENCE OF pathogenic microorganisms in wastewater and sewage sludge poses risks to human, animal, and plant health. The potential routes of exposure (to pathogens) include direct contact with sewage and/or sludge, contamination of food crops irrigated with treated wastewater or surface water receiving sewage discharges, application of treated sewage sludge to land, and recreational contact with sewage-contaminated fresh or marine waters. In response, microbiological standards and/or guidelines have been developed for irrigation water (World Health Organization, 1989) and the application of sewage sludge to agricultural land (USEPA, 1993). Pathogens may also be spread by vectors (animals capable of transporting pathogens) such as insects, birds, and rodents. For this reason, it is prudent to minimize the likelihood of vector transmission by reducing the attractiveness (of sludge) to vectors or by preventing access to sludge by physical means. The U.S. regulations specify treatment goals or post-application measures designed to reduce vector attraction. In contrast, European legislation requires only that the sludge be subject to a process of stabilization before land application (European Commission, 1986).
Historically, the development of sewage treatment was driven by the need to reduce environmental contamination brought about by the uncontrolled discharge of human wastes to rivers and streams. The principal goal of the various forms of sewage treatment was (and remains) removal of gross solids and to mitigate the polluting effect by reducing the readily assimilable organic fraction of the settled sewage. Commonly used treatment processes such as anaerobic digestion were neither designed, nor operated specifically, to remove or inactivate pathogenic microorganisms, and the mechanisms of pathogen inactivation are poorly understood in some instances. For example, process objectives such as volatile solids reduction and pathogen reduction may respond to changes in process controls in different ways. This can result in practical difficulties for operators of treatment facilities trying to achieve specific microbiological standards or pathogen reduction goals where these are a relatively new requirement, as in the UK, for example.
This paper reviews the effect of the wastewater and sludge treatment on pathogens. Information about the mechanisms of pathogen inactivation obtained from laboratory and field studies can be used to prepare operating regimes designed to assure that pathogen reduction goals are achieved consistently. Impending changes to the European and United Kingdom regulations herald a move away from end product testing of microbiological content to a more proactive means of assuring product quality based on a system of quality management, predicated on hazard analysis critical control point (HACCP) principles.
Answers to all your Biology Questions
