Wastewater Re-use and Groundwater Quality (Proceedings of symposium HS04 held duriim IUGG2003 al Sapporo. July 2003). IAI IS Publ. 285. 2004." 5 Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment JULES B. VAN LIER Subdepartment of Environmental Technology, Wageningen University, PO Box 8129, 6700 EV Wageningen, The Netherlands [email protected] FRANS P. HUIBERS Irrigation and Water Engineering Group, Wageningen University, Nienwe Kanaal If 6709 PA Wageningen, The Netherlands Abstract Shortages in irrigation water in and around urban areas call for alter­ native sources, particularly in the (sub)tropical zones. Domestic sewage represents such an alternative source and its nutritional value has been known by farmers for a long time. In many situations even raw (diluted) domestic sewage is used for agricultural purposes, especially in those areas that cannot afford extensive sewerage and treatment systems. However, discharge or re­ use of non-treated effluents gives rise to serious environmental problems, including threats to human health. Whenever agricultural use of urban effluents is considered, an integrated approach should be pursued, taking into account the agricultural requirements as well as the possible technological solutions for cost-effective sanitation and treataient. Such an integrated set-up questions the existing paradigms in sanitation and treatment and will call for a more decentralized approach, minimizing the requirements for large-scale infra-structural investments, such as sewerage systems. Also, with respect to the available treatment techniques, economic sustainability is often disregar­ ded in making the final choices. Amongst the available compact technologies for wastewater treatment, the anaerobic (pre-)treatment is seen as an appropriate technology, but so far, its potential has hardly been utilized. Most interestingly, anaerobic treatment is ideal for implementation in a decentral­ ized mode. The products of anaerobic Ueatment consist of nutrient-rich effluents, stabilized digested sludge and energy rich biogas. In particular, the former two can be used beneficially by local fanners, whereas the biogas produced can be used on the site if it is produced in sufficient quantities. The present paper discusses the prospects of anaerobic (pre-)treatment, embedded in centralized and decentralized treatment and re-use concepts. Its cost- effectiveness may lead to a more rapid implementation of productive sewage reclamation, while the environmental problems are concomitantly addressed. Key words agricultural re-use; anaerobic treatment; cosl-elïecliveness; decentralization; sanitation paradigm; source separation; wastewater reclamation INTRODUCTION Appropriately treated domestic sewage can be regarded as ideal for irrigation and fertilization purposes, particularly in the (semi)arid climate region. In addition to an increased availability of an additional source of irrigation water, treated sewage contains valuable plant nutrients, such as nitrogen, phosphorous, and potassium (NPK) at levels of interest (e.g. Feigin et al, 1991; Darwish et al, 1999). Feigin et al (1991) 6 Jules B. Van Lier & Frans P. Huibers Table 1 Benefits and constraints of effluent use in irrigated agriculture. Benefits Constraints Increased availability of irrigation water resources Health hazards through water contact (farm Secure and year around supply labourers) Reduced need for fertilization Environmental risks if not properly managed Increased crop yield Soil degradation in case of high salinity Effluent can be marketed (cost optimization in Crop quality hazards treatment and re-use chain) Irrigation operational hazards (clogging, supply- Alleviation of high-quality water scarcity (increase demand difference) of regional self-sufficiency) Negative economic impact (consumer's Alternative tertiary wastewater treatment perception, governmental crop restriction regulations) Social and cultural norms and values also commented on the positive effects of the various micronutrients on crop produc­ tion. From the environmental engineering point of view it is of interest that by using treated effluents in irrigated agriculture, the agricultural field can be considered as a tertiary treatment step, while non-controlled environmental pollution is prevented if well managed (Asano, 1998). The fact that domestic sewage is validated as a resource with an economic value means that treated effluents can potentially be marketed. The latter will increase the financial feasibility of a treatment and re-use scenario, since the overall costs can then be divided over the polluters (community) and beneficiaries (fanners). Once treated effluents are directed to agricultural fields, the pressure on high-quality water resources decreases, contributing to solutions related to water availability problems. Another important advantage is the alleviation of scarcity high- quality water, in those areas where clean water is difficult to obtain. As such, the use of domestic effluents has interesting economic benefits as listed in Table 1. The general constraints in irrigation with treated wastewater (Table 1) focus on human and environmental health hazards, which include: (a) the fate of pathogens; (b) the fate of excess nutrients in the environment (e.g. outside the season); and (c) the fate of micro-pollutants. Also crop productivity and quality hazards should be consid­ ered; these are related to bio-accumulation of pollutants and/or vegetative growth induction owing to excess dosage of nitrogen. Problems of soil salinization and sodification may occur in the long term causing loss of productivity (Abo Gobar, 1993). When working with treated wastewater, irrigation systems should be amended and potential risks must be minimized, which could lead to higher costs for the farmer, as explained below. Another constraint hampering a direct link between wastewater production and agricultural use is the gap between treated wastewater supply and agricultural demand with respect to day and night flow, but more importantly to seasonal variations. Waste­ water will be produced continuously while the demand for irrigation water depends on the season and the crop growth stage. Social constraints include a negative perception on the side of the consumers, possibly leading to lower market values of the crops produced, while socio-cultural norms might simply restrict the official use of treated effluents. Last but not least, governmental crop restriction regulations might affect the economic sustainability of the farmers that use the treated sewage for crop irrigation. Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment 1 Although some constraints are well documented, it must be noticed that in various countries and/or regions, the common practices already overrule the constraints, and farmers even use raw sewage to meet their water and nutrient demands. Therefore, it is recognized that there is a strong need to develop cost-effective treatment technologies for the reclamation of domestic wastewater. The treatment systems to be developed must be complemented with appropriate agricultural engineering and proper manage­ ment in order to address some of the above-mentioned constraints, since the complete solution cannot be found in the treatment technology alone. For example, crop choice could be amended when there is only a limited removal of pathogens in the treatment system. Various countries (e.g. in the Middle East and northern Africa) apply restricted irrigation criteria making a division between: (a) crops eaten raw; (b) crops that need cooking; (c) animal fodder; (d) industrial crops (e.g. cotton); and (e) tree crops (FAO, 1992). Such an approach suppoits the acceptability of wastewater irrigation by consumers. In addition to crop choice, fanners could also anticipate the irrigation water quality by applying the proper irrigation technology and water management. For instance, with pathogen-rich wastewater the use of sprinkler irrigation is discouraged, while (sub­ surface) drip irrigation, although expensive and only applicable to certain crops, clearly has advantages, since it prevents contacts with the pathogen-rich wastewater (Oron et al, 1991). On the other hand, suspended solids-rich treated wastewater might cause clogging problems at the water emitters of these micro-irrigation systems (Ravina et al, 1997). Although risk minimization can be attained by using proper irrigation techniques, care still should be taken when dealing with pathogen rich wastewater, since risks are not eliminated but mitigated. The same is true for potential salts accumulation in the soil that incontrovertibly requires periodic leaching to prevent an irreversible loss of the agricultural land. It is also clear that the change in irrigation technology will increase the financial burden to the farmers and require higher skills and will not likely take place on a voluntary basis. The above options for intervention illustrate that the topic "treated sewage for agri­ cultural use" needs an interdisciplinary approach, which has not yet been developed, or has only been developed to a limited extent. Most likely, by recognizing this, the status quo of "non-controlled sewage disposal" vs "lack of irrigation water and nutrients" will be more quickly addressed. EXISTING PARADIGM Long term application of wastewater treatment, reclamation and re-use has been experienced at various locations in the arid and