The World Health Organization (WHO) has recognized both the potential and risk of untreated wastewater use and so has developed guidelines for policy makers attempting to legislate permission for the safe use of wastewater. In the 1989 guidelines (see table 4), the WHO acknowledged that most previous standards were unnecessarily high for public health protection and do not reflect reality of wastewater use on the ground. The WHO is currently revising their guidelines on wastewater reuse. Publication of the revised version is expected in 2004.

The main features of the 1989 WHO guidelines for wastewater reuse in agriculture are as follows:

• Wastewater is considered as a resource to be used, but used safely.

• The aim of the guidelines is to protect against excess infection in exposed populations (consumers, farm workers, populations living near irrigated fields).

• Fecal coliforms and intestinal nematode eggs are used as pathogen indicators.

• Measures comprising good reuse management practice are proposed alongside wastewater quality and treatment goals; restrictions on crops to be irrigated with wastewater; selection of irrigation methods providing increased health protection, and observation of good personal hygiene (including the use of protective clothing).

• The feasibility of achieving the guidelines is considered alongside desirable standards of health protection.

Many countries have welcomed the guidance from WHO standards and guidelines. France, for example, used a similar approach in setting guidelines, which were published in 1991. These are similar to those of WHO in defining analogous water categories (called A, B and C in the WHO guidelines; table 4) and microbiological limits, but complement them with strict rules of application. For example, for category A in the French guidelines, the quality requirement must be complemented by the use of irrigation techniques that avoid wetting fruit and vegetables, and for irrigation of golf courses and open landscaped areas, spray irrigation must be performed outside public opening hours.

As noted above, the WHO guidelines continue to be the benchmark target for decision makers in developing the wastewater recycling sector, however, as demonstrated, goals need to be in line with the capabilities of the country in question. Some countries have modified the microbiological criteria to suit local epidemiological and economic circumstances, as, for example, Mexico (see chapter 3.4)

FAO Guidelines for agricultural use (1985)

In contrast to the WHO guidelines that focus mainly on the protection of human and public health, the FAO has developed a field guide for evaluating the suitability of water for irrigation. Guideline values given identify potential problem water based on possible restrictions in use related to 1) salinity, 2) rate of water infiltration into the soil, 3) specific ion toxicity, or 4) to some other miscellaneous effects. The guide is intended to provide guidance to farm and project managers, consultants and engineers in evaluating and identifying potential problems related to water quality. It discusses possible restrictions on the use of the water and presents management options which may assist in farm or project management, planning and operation. Guiding values for salinity and other characteristics of wastewater are given in table 4. However, the FAO guidelines must be seen as orientation values that are in no way intended to replace case-to-case assessments.

² ECw means electrical conductivity, a measure of the water salinity, reported in deciSiemens per metre at 25°C (dS/m) or in units millimhos per centimetre (mmho/cm). Both are equivalent. TDS means total dissolved solids, reported in milligrams per litre (mg/l).

³ SAR means sodium adsorption ratio. SAR is sometimes reported by the symbol RNa. At a given SAR, infiltration rate increases as water salinity increases. Evaluate the potential infiltration problem by SAR as modified by ECw. Adapted from Rhoades 1977 and Oster and Schroer 1979.

⁴ For surface irrigation, most tree crops and woody plants are sensitive to sodium and chloride.

⁵ NO₃ -N means nitrate nitrogen reported in terms of elemental nitrogen (NH₄ -N and Organic-N should be included when wastewater is being tested).

me/l = milli equivalent per litre (mg/l ÷ equivalent weight = me/l); in SI units, 1 me/l= 1 milli mol/litre adjusted for electron charge. mg/l = milligram per litre ≈ parts per million (ppm).

Table 4: Guidelines for Interpretations of Water Quality for Irrigation (adapted from University of California Committee of Consultants 1974)