10Irrigation with raw, partially and treated wastewater is a widespread practice in many 11 arid and semi-arid zones. The importance of wastewater for agriculture has 12 increasingly been recognised not only as a valuable water resource but also for its 13 nutrient value. However, inappropriate management of irrigation with wastewater can 14 pose substantial risks to public health and the surrounding environment as a result of 15 its microbial and toxic components. In this review, we summarise recent research 16 and provide a broad overview of the potential risks associated with the chemicals in 17 wastewater used for irrigation including their environmental, and health impacts, 18 factors that may affect the fate of these chemicals, and available mitigation methods 19 and management options to reduce their impacts. A primary aim of this review is to 20 construct a generalised ranking of the risks from the chemical constituents of 21 wastewater used for irrigation in arid and semi -arid zones. 22 . 23
Keywords: irrigation with wastewater, environmental24 risks, chemical constituents, wastewater 25 management, arid zones 26 to human health and environmental quality as a result of the microbial and toxic 56components is a serious obstacle for wastewater reuse in agriculture. Most of the 57 existing research has tended to focus on the microbial risks regarding the use of 58 wastewater and guidelines for the safe use of wastewater in agriculture. This may be 59 due to the immediate effects of microbiological components on public health 60 compared to the longer term risks posed by chemical exposure ((WHO, 2006, Bos et 61 al., 2010. Generally, using wastewater in agriculture is unlikely to contribute to direct 62 health impacts from chemicals hazards unless the wastewater is heavily 63 contaminated with discharges from industrial sources. Another explanation may be 64 the difficulty in assessing the health impacts of toxic chemicals in wastewater as it 65 usually has a long latency period (Bos et al., 2010). 66Inappropriate management of wastewater irrigation can contribute to serious 67 environmental problems especially in arid and semi-arid zones where wastewater 68 could be the predominant water supply for agriculture (Pescod, 1992, Ayers and 69 Westcot, 1985, WHO, 2006, Simmons et al., 2010. Wastewater irrigation could lead 70 to negative impacts on soil properties and fertility, crop yields, groundwater and 71 surface water quality, and the aquatic ecosystem. The magnitude of the potential 72 impacts will depend on the concentration of the chemicals, their solubility and 73 inherent toxicity. Other important factors the rate and frequency of wastewater 74 application, the type of crop, and target yields, inhertent soil properties and condition, 75 the vulnerability of the aquifer, climatic conditions, and technology level and the 76 social-economic status of the farmers. In order to ensure good crop yields and 77 minimise the environmental risks associated with the chemical constituents in 78wastewater, a r...
