Duncan Mara

Guidelines for the microbiological quality of treated wastewater used in agriculture: recommendations for revising WHO guidelines.

Three different approaches for establishing guidelines for the microbiological quality of treated wastewater that is reused for agriculture are reviewed. These approaches have different objectives as their outcomes: the absence of faecal indicator organisms in the wastewater, the absence of a measurable excess of cases of enteric disease in the exposed population and a model-generated estimated risk below a defined acceptable risk. If the second approach (using empirical epidemiological studies supplemented by microbiological studies of the transmission of pathogens) is used in conjunction with the third approach (using a model-based quantitative risk assessment for selected pathogens) a powerful tool is produced that aids the development of regulations. This combined approach is more cost-effective than the first approach and adequately protects public health. The guideline limit for faecal coliform bacteria in unrestricted irrigation (< or = 1000 faecal coliform bacteria/ 100 ml) is valid, but for restricted irrigation < or = 10(5) faecal coliform bacteria/100 ml is recommended when adult farmworkers are exposed to spray irrigation. A limit of < or = 10(3) faecal coliform bacteria/100 ml is recommended if flood irrigation is used or children are exposed. The guideline limit for nematode eggs for both types of irrigation is adequate except when conditions favour the survival of nematode eggs and where children are exposed; in these cases it should be reduced from < or = 1 egg/l to < or = 0.1 egg/l.

Sanitation and Health

As one article in a four-part PLoS Medicine series on water and sanitation, David Trouba and colleagues discuss the importance of improved sanitation to health and the role that the health sector can play in its advocacy.

The bacteriological quality of rainjar water in rural Northeast Thailand

Rainjars are popular in northeast Thailand because of the time and effort saved in water collection resulting from their use. Bacteriological analysis showed rainjar water to be much purer than drinking water from alternative sources (P < 0.001). In rainwater samples from one village, Escherichia coli was absent in 60% of rainjars compared with 46% of other vessels used for storing drinking water in the home (P < 0.01). The use of a tap and the physical height of the jar served to protect the water by limiting water handling. Moreover, mosquito netting fitted to the top of the rainjar significantly improved its water quality (P < 0.05), probably by preventing the access of small lizards. It is suggested that any systematic attempt to improve the quality of rainjar water further, in order to achieve the WHO guidelines, would be both problematic and expensive whilst the expected health benefits remain negligible. The problems of using drinking water quality guidelines for discrete supplies and the role of water analysis are discussed.

Estimation of norovirus infection risks to consumers of wastewater-irrigated food crops eaten raw

A quantitative microbial risk analysis-Monte Carlo method was used to estimate norovirus infection risks to consumers of wastewater-irrigated lettuce. Using the same assumptions as used in the 2006 WHO guidelines for the safe use of wastewater in agriculture, a norovirus reduction of 6 log units was required to achieve a norovirus infection risk of approximately 10(-3) per person per year (pppy), but for a lower consumption of lettuce (40-48 g per week vs. 350 g per week) the required reduction was 5 log units. If the tolerable additional disease burden is increased from a DALY (disability-adjusted life year) loss of 10(-6) pppy (the value used in the WHO guidelines) to 10(-5) pppy, the required pathogen reduction is one order of magnitude lower. Reductions of 4-6 log units can be achieved by very simple partial treatment (principally settling to achieve a 1-log unit reduction) supplemented by very reliable post-treatment health-protection control measures such as pathogen die-off (1-2 log units), produce washing in cold water (1 log unit) and produce disinfection (3 log units).

Towards an agenda for improving wastewater use in agriculture

This paper sets out the trends and challenges of wastewater use in agriculture; identifies the risks and benefits of wastewater irrigation; describes the risk-assessment and management framework adopted by the World Health Organization, the Food and Agriculture Organization of the United Nations and other international and national organizations; and proposes measures for applying the framework to reduce health risks by moving from unplanned to a planned, integrated, approach to wastewater use for irrigation.

Estimation of norovirus and Ascaris infection risks to urban farmers in developing countries using wastewater for crop irrigation.

A quantitative microbial risk analysis-Monte Carlo method was used to estimate norovirus and Ascaris infection risks to urban farmers in developing countries watering their crops with wastewater. For a tolerable additional disease burden of <or=10(-4) DALY loss per person per year (pppy), equivalent to 1 percent of the diarrhoeal disease burden in developing countries, a norovirus reduction of 1-2 log units and an Ascaris egg reduction to 10-100 eggs per litre are required. These are easily achieved by minimal wastewater treatment-for example, a sequential batch-fed three tank/pond system. Hygiene improvement through education and regular deworming are essential complementary inputs to protect the health of urban farmers.

Stabilization ponds: Design and operation

This review will initially consider the reasons for employing stabilization ponds and will discuss their advantages and disadvantages as reflected by their present widespread usage. The differentiation of ponds into specific types will then be reviewed with possibly a conclusion being drawn which is to be followed by a consideration of the mode of operation of the defined pond types and will include the concepts of loadings, oxygen sources, stratification and surface streaming, seasonal feedback, the treatment of strong industrial wastewaters, and the removal of bacteria, nutrients and biodegradable organic material. The subject of process design will be reviewed at some length with all the advocated approaches being considered. The use of one or more of these techniques will be supported. Finally, the effects, advantages, disadvantages, potentials, and currently suggested harvesting and removal techniques of algae in ponds will be reviewed at length.

Wastewater irrigation and health: challenges and outlook for mitigating risks in low-income countries

In Drechsel, Pay; Scott, C. A.; Raschid-Sally, Liqa; Redwood, M.; Bahri, Akissa (Eds.). Wastewater irrigation and health: assessing and mitigating risk in low-income countries. London, UK: Earthscan; Ottawa, Canada: International Development Research Centre (IDRC); Colombo, Sri Lanka: International Water Management Institute (IWMI).In most developing countries wastewater treatment systems have very low coverage or function poorly, resulting in large-scale water pollution and the use of poor-quality water for crop irrigation, especially in the vicinity of urban centres. This can pose significant risks to public health, particularly where crops are eaten raw. Wastewater Irrigation and Health approaches this serious problem from a practical and realistic perspective, addressing the issues of health risk assessment and reduction in developing country settings. The book therefore complements other books on the topic of wastewater which focus on high-end treatment options and the use of treated wastewater. This book moves the debate forward by covering also the common reality of untreated wastewater, greywater and excreta use. It presents the state-of-the-art on quantitative risk assessment and low-cost options for health risk reduction, from treatment to on-farm and off-farm measures, in support of the multiple barrier approach of the 2006 guidelines for safe wastewater irrigation published by the World Health Organization. The 38 authors and co-authors are international key experts in the field of wastewater irrigation representing a mix of agronomists, engineers, social scientists and public health experts from Africa, Asia, Europe, North America and Australia. The chapters highlight experiences across the developing world with reference to various case studies from sub-Saharan Africa, Asia, Mexico and the Middle East. The book also addresses options for resource recovery and wastewater governance, thus clearly establishes a connection between agriculture, health and sanitation, which is often the missing link in the current discussion on حaking wastewater an assetٮ

Estimation of Ascaris infection risks in children under 15 from the consumption of wastewater-irrigated carrots

Ascaris lumbricoides, the large human roundworm, infects approximately 1,200 million people, with children under the age of 15 being particularly at risk. Monte Carlo quantitative microbial risk analyses were undertaken to estimate median Ascaris infection risks in children under 15 from eating raw carrots irrigated with wastewater. For a tolerable additional disease burden of 10(-5) DALY (disability-adjusted life year) loss per person per year (pppy), the tolerable Ascaris infection risk is approximately 10(-3) pppy, which can be achieved in hyperendemic areas by a 4-log unit Ascaris reduction. This reduction can be easily achieved by wastewater treatment in a 1-day anaerobic pond and 5-day facultative pond (2 log units) and peeling prior to consumption (2 log units).

Waste Stabilization Ponds: A Highly Appropriate Wastewater Treatment Technology for Mediterranean Countries

This chapter describes waste stabilization pond (WSP) systems for wastewater treatment. WSP systems comprise a series of anaerobic and facultative ponds and sometimes maturation ponds. Rock filters can be used instead of matura- tion ponds and they can be aerated to remove ammonia and to improve biochemi- cal oxygen demand and suspended solids removals. Effluent quality is high, and properly designed and well maintained WSP systems produce effluents that can be safely used for both restricted and unrestricted crop irrigation.