In Mexico, microbiological and chemical standards for wastewater reuse in agriculture have developed considerably over the last 15 years. Existing guidelines were reviewed in 1991, 1993, and again in 1996. Particular attention was paid to (1) the cultivation of vegetables and other crops eaten raw, (2) the importance of wastewater reuse in agriculture as a form of wastewater treatment and disposal, and (3) the diversity of treatment processes available to achieve the guidelines.

The final revision of the microbiological standards occurred in 1996, resulting in the introduction of NOM-001-ECOL-1996 (see table 6) "that establishes the maximum permissible limits of contaminants in wastewater to be discharged into national waters and onto national soil". As in the WHO guidelines, fecal coliforms are used as the indicator to determine pathogenic contamination. The maximum allowable limit in wastewater discharges to national water or property, as well as wastewater application to soils (for agricultural irrigation) is 1,000 and 2,000 (most probable number, MPN) of fecal coliforms per 100 ml, for monthly average and daily average, respectively. To determine parasitic contamination, helminth eggs are used as the indicator. The maximum allowable limit in wastewater application to soils (for agricultural irrigation) is one helminth egg per liter for restricted irrigation, and five helminth eggs per liter for unrestricted irrigation, following the technique established in annex 1 of these regulations.

(m=monthly mean, d=daily mean, MPN=most probable number)

Table 6: Mexican Standard NOM-001-ECOL-1996 governing wastewater reuse in Agriculture

Note: Unrestricted irrigation is defined as permitting irrigation of all crops, whilst restricted irrigation excludes salad crops and vegetables that are eaten raw.

The new standard, with a single set of parameter limits regardless of the discharge source, was designed to be achievable with the technology and resources available at present and in the near future in Mexico and to be more realistically policed, by reducing the amount of monitoring required. The limits imposed within the standard were designed to be sufficient to protect "at-risk" groups according to currently available literature. Revision of many of the possible treatment processes resulted in the proposed microbiological standards. A stricter helminth standard would have required conventional treatment plants to use filters and this would have carried significant financial implications.

The concentration of basic contaminants, heavy metals and cyanides in wastewater discharges to national water or property, may not exceed the value indicated as the maximum allowable limit in annex tables 2 and 3 of these regulations. The allowable range for pH is 5 to 10 units.

Recommendations to review WHO standards (2000)

Blumenthal et al. recommend a review of the current WHO guidelines. They base their recommendation on their appraisal of recent research evidence based on a combined approach using empirical epidemiological studies supplemented by microbiological studies of the transmission of pathogens in conjunction with a model-based quantitative risk assessment for selected pathogens.

Their research leads to the conclusion that for unrestricted irrigation, there is no evidence to suggest a need to revise the fecal coliform guideline limit of ≤ 1000 fecal coliform bacteria/100 ml. However, there is epidemiological evidence that the guideline limit for nematode eggs (≤ 1 egg/l) is not adequate in conditions that favor the survival of nematode eggs (lower mean temperatures and the use of surface irrigation), and it needs to be revised to ≤ 0.1 egg/l in these conditions. For restricted irrigation, there is evidence to support the need for a guideline limit for exposure to fecal coliform bacteria to protect farm workers, their children and nearby populations from enteric viral and bacterial infections. The appropriate guideline limit will depend on which irrigation method is used and who is exposed. For example, if adult farm workers are exposed to spray or sprinkler irrigation, a guideline limit of ≤ 10⁵ fecal coliform bacteria/ 100 ml is necessary. A reduced guideline limit of ≤ 10³ fecal coliform bacteria/100 ml is warranted when adult farm workers are engaged in flood or furrow irrigation and when children under age 15 are regularly exposed through work or play. Where there are insufficient resources to meet this stricter guideline limit, a guideline limit of ≤ 10⁵ fecal coliform bacteria/100 ml should be supplemented by other health protection measures. The guideline limit for nematode eggs (≤ 1 egg/l) is adequate if no children are exposed, but a revised guideline limit of ≤ 0.1 egg/l is recommended if children are in contact with wastewater or soil through irrigation or play. The evidence reviewed does not support the need for a separate specific guideline limit to protect against viral infections, and there was insufficient evidence to support the need for a specific guideline limit for parasitic protozoa.

Therefore, Blumenthal et al. suggest revised microbiological guidelines for treated wastewater use in agriculture as shown in table 7.

^a In specific cases, local epidemiological, socio-cultural and environmental factors should be taken into account and the guidelines modified accordingly.

^b Ascaris and Trichurisspecies and hookworms; the guideline limit is also intended to protect against risks from parasitic protozoa.

^c During the irrigation season (if the wastewater is treated in WSP or WSTR which have been designed to achieve these egg numbers, then routine effluent quality monitoring is not required).

^d During the irrigation season (feca coliform counts should preferably be done weekly, but at least monthly).

^e A more stringent guideline limit (4200 fecal coliforms/100 ml) is appropriate for public lawns, such as hotel lawns, with which the public may come into direct contact.

^f This guideline limit can be increased to 41 egg/l if (i) conditions are hot and dry and surface irrigation is not used or (ii) if wastewater treatment is supplemented with anthelmintic chemotherapy campaigns in areas of wastewater reuse.

^g In the case of fruit trees, irrigation should stop two weeks before fruit is picked, and no fruit should be picked off the ground. Spray/sprinkler irrigation should not be used.

Table 7: Recommended revised microbiological guidelines for treated wastewater use in agriculture ^a

Wastewater treatment technologiessuitable for meeting the revised microbiological guidelines for agriculture include the use of waste stabilization ponds (WSP), wastewater storage and treatment reservoirs (WSTR), or conventional treatment processes. When using WSP, the revised guidelines usually require the use of 1 or more maturation ponds after the anaerobic and facultative ponds. Use of sequential batch-fed storage and treatment reservoirs can be designed to meet the guidelines for unrestricted and restricted irrigation. When conventional treatment processes are used secondary treatment, filtration and disinfection are often needed to meet the revised guidelines. The cost and difficulty in operating and maintaining conventional treatment plants to the level needed to meet the guidelines means that they are not recommended where WSP and WSTR can be used.