project planning stage. Many of these potential disadvantages of wastewater irrigation, together with such hazards as odor, vector development, and the effects of accidental discharges of toxic substances, can be avoided by the use of properly treated wastewater. This includes adequate control of non-biodegradable and toxic industrial wastewaters, which generally require separate treatment or at least pretreatment prior to discharge to public sewers (Chapter 5).

## _Chemical risks to plant health_

Crop yields may be reduced if the physicochemical quality of the wastewater used for irrigation is unsuitable—for example by being too saline or having concentrations of boron, heavy metals and other industrial toxicants, nitrogen, and/or sodium which inhibit plant growth either directly in the case of toxicants or indirectly by reducing the plant’s ability to absorb nutrients. The principal (and still current) reference document on the physicochemical quality of water, including wastewater, used for crop irrigation is FAO’s _Water Quality for Agriculture_ (Ayers and Westcot, 1985). This may be supplemented by two later FAO publications _The Use of Saline Waters for Crop Production_ (Rhoades _et al_ ., 1992) and _Quality Control of Wastewater for Irrigated Crop Production_ (Westcot, 1997), and the World Bank publication _Salinity Management for Sustainable Irrigation: Integrating Science, Environment, and Economics_ (Hillel, 2000).

In general treated domestic wastewaters, or treated municipal wastewaters that contain little industrial effluent, present no problem; care has to be exercised as the proportion of industrial effluent in the wastewater increases. However, even for treated domestic wastewaters, there are five parameters that should be monitored during the irrigation season: 1) electrical conductivity (as a measure of total dissolved solids or “salinity hazard”), 2) the sodium adsorption ratio (as a measure of the ‘sodium hazard’),<sup>14</sup> 3) the concentrations of boron, 4) the concentrations of total nitrogen, and 5) pH.<sup>15</sup> These measurements are relatively easy to do in the case of large wastewater-use schemes, but at the smaller scale of urban/peri-urban agriculture they would not generally be possible. The environmental health departments of city and town councils should nonetheless be encouraged to conduct these five analyses reasonably regularly (for example, at least monthly) throughout the irrigation season.

# _3.2.2   Environmental benefits_

If wastewater use schemes are managed well, they can have several environmental benefits (Mara and Cairncross, 1989):

- Avoidance of surface water pollution, which would occur if the wastewater were not used but discharged into rivers or lakes. Major environmental pollution problems, such as dissolved oxygen depletion, eutrophication, foaming, and fish kills, can thereby be avoided.

- Conservation or more rational use of freshwater resources, especially in arid and semi-arid areas—i.e. fresh water for urban demand, wastewater for agricultural use.

- Reduced requirements for artificial fertilizers, with a concomitant reduction in energy expenditure and industrial pollution elsewhere.

- Soil conservation through humus build-up and through the prevention of land erosion.

- Desertification control and desert reclamation, through irrigation and fertilization of tree belts.

> 14 **1⁄2** The sodium absorption ratio is [Na] **÷** {0.5([Mg] + [Ca])} where [Na], [Mg] and [Ca] are the concentrations of sodium, magnesium and calcium, respectively, in milli-equivalents per liter.

> 15 The FAO Guidelines (Ayers and Westcot, 1985) should be consulted for details of these parameters, crop sensitivities to boron, total nitrogen and pH, and how electrical conductivity and the sodium absorption ratio interact.

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