International guidelines for water recycling: microbiological considerations C.P. Gerba* and J.B. Rose** * Dept. of Soil, Water, and Env. Science, Univ. of Arizona, Tucson, AZ 85721, USA (E-mail: gerba@ag.arizona.edu) ** College of Marine Sci., Univ. of South Florida, St. Petersburg, FL 33701, USA Abstract The World Health Organization and others have proposed international guidelines for the safe reuse of domestic wastewater. Treatment and microbial standards have been suggested with varying microbial water quality targets to suit local circumstances of affordability and health risks. However, it is important to recognize that universal application of bacterial standards for water quality assessment will not yield the same risks from microbial pathogens in reclaimed wastewater around the world. The concentration and types of pathogens in raw wastewater varies from one region of the world to another depending on the incidence in the community. This is influenced by the general hygiene, season, and per capita water use. Also, different treatment processes may be more effective in removing indicator bacteria and certain groups of pathogens than others. Proposed guidelines have also neglected the importance of water in the transmission of viral and bacterial diseases in developed countries and their impact on morbidity and mortality. Additional research is needed to better reduce the uncertainly of proposed guidelines for water recycling and to better define the risks associated to the exposed populations. Keywords Effluent reuse; guidelines; international standards; microbial standards; pathogens; water recycling Introduction Wastewater reuse or recycling of domestic wastewater is an essential part of water management in many regions of the developed and developing world. The wastewater may be recycled for industrial purposes (cooling towers), food or non-food crop irrigation, recreational waters (contact or non-contact), or as a supplement to potable supplies. Contained in domestic wastewater are a variety of disease causing bacteria, viruses, proto- zoa, and helminths that can be transmitted by ingestion and/or inhalation. The transmission of these agents by ingestion of contaminated food crops, swimming, and drinking water is well documented. Since some of these pathogenic microorganisms will always be present in untreated domestic wastewater they must be reduced to levels which minimize the risk to the population exposed to the recycled water. For this reason various water quality and treatment standards have been purposed depending on the intended reuse application. The World Health Organization Guidelines were derived from the review of epidemio- logical studies of wastewater reuse, along with what was achievable by wastewater treat- ment processes (Havelaar et al., 2000). Studies were available on disease risk for some pathogens and illnesses on the risk of exposure to raw wastewater and excreta, and on the risk to farm workers in developing countries (Blumenthal et al., 2001). However, there was little data available on the effect of the use of treated wastewater to the general public, par- ticularly in relation to consumption of vegetable crops. Where epidemiological evidence was not sufficient to allow the defining of a level (microbiological quality) at which no excess risk of infection would occur, data on pathogen removal by wastewater treatment processes, pathogen die-off in the field, and pervading guidelines for water quality were taken into account. These guidelines are shown in Table 1. Water Science and Technology: Water Supply Vol 3 No 4 pp 311–316 © IWA Publishing 2003 311