Wastewater reuse for Agricultural Irrigation and Its Impact on Health
It is time to review the standard NOM-001-ECOL-1996?
Spanish version: Reuso de Agua Residual Para Riego Agricola y su Impacto en la Salud
Dr. Guillermo M. Ruiz-Palacios (1)
Dr. Ursula Blumenthal (2)
Dr. Anne Peasey (1, 2)
Dr. Enrique Cifuentes (3)
- National Institute of Nutrition Salvador Zubirán (Mexico).
- London School of Hygiene and Tropical Medicine (UK).
- National Institute of Public Health (Mexico).
Risk to the health of farmers and consumers, associated with Mexican current standards on the use of wastewater in agriculture.
Dr. Guillermo M. Ruiz-Palacios; Ursula Dr. Blumenthal; Dr. Anne Peasey; Dr. Enrique Cifuentes.
The National Institute of Nutrition (Mexico) and the London School of Hygiene and Tropical Medicine (UK) conducted epidemiological studies in the Mezquital Valley, Hidalgo, between 1989 and 1997 to evaluate the validity of current regulations established by the World Health Organization (WHO) for the safe use of wastewater in agriculture and aquaculture (1989).
These studies were conducted in the Mezquital Valley, 100 km. to the north of Mexico City, in the irrigation districts 03 and 100, where more than 82,000 hectares of arable land, mainly cereals and fodder is irrigated with wastewater from Mexico City. A portion of this water passes through a sedimentation dam where water quality improves before its use; this is equivalent to a partial treatment.
The effluent from the first dam (retention time 1-7 months, depending on the season) meets WHO standards for restricted irrigation (Table l), even though a smaller volume of wastewater joins the effluent prior being used. The effluent from the second dam was withheld for 2-6 months (& 3 months of combined retention) and quality was further improved.
Part of the effluent from the first dam joins the river and below is used to irrigate a large area of crops of vegetables, many of which are eaten raw; river water is essentially partially treated waste water (quality 104 faecal coliforms/100 ml).
These crops are sold in local markets and consumed by the rural population of local communities, including those located near the second dam. In a nearby region, vegetables were irrigated with well water.
WHO standards for the safe use of wastewater in agriculture (1989)
|Irrigation||Standard of faecal coliforms (FC/100ml)||Standard of nematode eggs (eggs/liter)|
2.1 Risks to farmers
Farmers and their children who are in contact with untreated wastewater (106-108 FC/100 ml and> 100 eggs/l) through irrigation or play, have a prevalence of infection by Ascaris significantly higher than those in control group who practice seasonal agriculture (Peasey, in preparation; Blumenthal et al. 1996). The excess in the rate of infection is higher in children of 5-14 years old than in adults. The youngest children (1-4 years) also have a significantly higher rate of diarrheal disease (Cifuentes et al. 1993).
The Contact with sedimented wastewater in a dam (Endhó dam) prior to its use (104-105 FC/100 ml and <1 egg/l) produces an excess in the rate of infection by Ascaris in children compared to the rate of control group, especially in aged 5-14 years, but not so in adults (Blumenthal et al. 1996). Children aged 5-14 years also have a significantly higher rate of diarrhea (Cifuentes et al. 1993).
Residual water retention in two serial dams (Endhó and Rojo Gomez dams), with an effluent containing an average of 4×103 FC/100 ml (maximum 105 FC/100 ml) results in a minimum excess in the rate of infection by Ascaris in any age group (Cifuentes et al. 1994). However, there is a significant excess in the rate of diarrhea in children aged 5-14 years and a lesser extent in adults who are in contact with the effluent of the Rojo Gomez dam, compared to those without contact with this effluent (Blumenthal et al., in preparation). Also was detected a serological response to Calicivirus-Mx significantly higher among adults who had contact with this effluent.
Epidemiological studies on the vegetables consumption irrigated with water from the Tula River (effluent of the Endho dam, with a minimum discharge of waste water from irrigation canals and local communities, ie, 104-105 FC/100 ml and <1 egg/l) showed a minimal excess in the rate of enteric infection among consumers of all ages.
There was an excess in the rate of diarrhea among those who ate onions compared to those who consumed a small amount of this vegetable. This effect was most noticeable in adults and in children under 5 years. There was also a greater serological response to Calicivirus-Mx in children aged 5-14 years who consumed green tomato.
There was no excess in the rate of diarrhea or in the serological response to Calicivirus Mx or enterotoxigenic Escherichia coli (LT) in relation to the consumption of other vegetables (eg, squash, cauliflower, cabbage, carrot, tomato, pepper, lettuce, radish, cucumber and cilantro).
The consumption of larger amounts of chile watered with wastewater has resulted an excess of the rate of diarrhea compared to those who consumed small amounts, despite the use of irrigation bed designed to prevent contact between chili and irrigation water.
The consumption of wild vegetables, eg, spinach, purslane, quintoniles, growing in areas irrigated with the effluent of the Endho dam was an excess risk of infection by Ascaris compared with those who did not consume wild vegetables (Peasey, in preparation).
3. IMPLICATIONS FOR MEXICAN REGULATIONS
The current rules in Mexico for the use of wastewater in agriculture (NOM-001 -ECOL) were introduced in 1997 (Table 2). These rules are part of significant reorganization of the regulations on industrial and domestic discharges to national waters and lands. Previously there were over 40 different normativities, making it almost impossible to control. The new regulation is designed to be more technically and economically feasible in today’s Mexico.
Mexican standards for wastewater use in agriculture
|Irrigation||Standard of Faecal coliforms (FC/100ml)||Standard of nematode eggs (eggs/liter)|
(m = monthly average, d = daily average)
The unrestricted irrigation is defined as the risk of all crops, while the restricted irrigation excludes vegetables that are eaten raw.
The new results of these epidemiological studies about enteric infections suggests the need to review The Mexican standards for the reuse of wastewater in agriculture. It is important that the authorities responsible for defining the policies of wastewater reuse in Mexico decide which is the most suitable approach to local conditions. There are several possible scenarios: (a) That there is no potential risk of infection – faecal indicators are not detected; (b) Without excess for risk of infection – above that risk by other routes of transmission; (c) A minimum but accepted or recognized risk – when balancing economic and public health factors; and (d) Reduce the morbidity in the exposed population, ie, consumers, farmers and neighboring population of irrigated areas.
3.1 Restricted Irrigation
The WHO standards for safe reuse of wastewater in agriculture and aquaculture (1989) did not propose a standard for fecal coliforms by the lack of risk evidence of viral and bacterial infections for farmers and local residents. However, the results of our studies in (section 2) and other US studies (Camann et al. 1986) suggest that a fecal coliform standard should be included in the restricted irrigation regulations. The US data suggest that 105 FC/100 ml would protect farmers and residents of nearby areas. However, Mexican studies suggest that schoolchildren and adults who are in direct contact with wastewater may be at risk of enteric infections still at a level of 104 FC/100 ml. A lower level of 103 FC/100 ml, as required by the existing standard in (1996), would increase security in places where there are large populations at risk through field work and where children are often exposed. The authorities should decide which way to go, ie if when assessing the economic cost of such a standard against risks to public health is chosen the stage of minimal risk, then it would be acceptable a standard of 104 FC/100 ml.
The existing standard of nematode eggs (WHO 1989) is not adequate to protect the health of farmers and their families, especially children. A higher standard of < 0.5 eggs/liter, it is necessary to give this protection to farmers and their families. In the standard of nematode eggs for restricted irrigation is < 5 eggs/liter, and it is clear that this rule should be tightened. However, to what level? depends on the priorities of the Mexican authorities. To ensure minimum risk of infection, a standard of at least < 1 egg/liter is required.
3.2. Unrestricted Irrigation
Epidemiological studies here reported and other recent studies, shows no evidence to suggest that the fecal coliform standard of WHO for unrestricted irrigation (1989) 103 CF/100 ml, is not adequate to protect the health of consumers. Our studies suggest that consumption of vegetables irrigated with 104-105 CF/100 ml carries a minimal risk of enteric infection. The risk assessment studies (Shuval et al. 1997) indicate that the WHO standard for fecal coliform is stricter than necessary in areas where enteric infections are endemic. The Mexican standard of 103 CF/100ml, is clearly adequate to protect the health of consumers in irrigated areas (where enteric infections are endemic) and large urban populations of consumers (where the rate of enteric infections is lower).
There is some evidence to suggest that the standards of counting nematode eggs (WHO 1989) are inadequate to protect the health of consumers. Our studies indicate an increased risk for infection by Ascaris among consumers of wild vegetables irrigated with effluent from a dam ( Endhó dam). A higher standard of <0.5 eggs/liter It is suggested to protect consumers. As mentioned above for the standard of faecal coliforms, Mexican authorities when revising the standard of nematode eggs, in light of the results, must decide if the goal is to eliminate the excess risk, reduce risk or minimize morbidity. It is clear that proper regulation depends on the chosen public health objective.
Based on the results of our Mexican studies and other recent studies, are suggested certain changes in the regulations for wastewater reuse in Mexico (Table 3). These proposals do not require from Mexican authorities large technical or financial investment. In fact, the new rules proposed, can be achieved with stabilization ponds and retention dams. Is not required the same economic investment needed for conventional treatment plants, neither is required highly qualified personnel for its operation and maintenance.
Proposed changes to the Mexican Standard NOM-001-ECOL-1996
|Irrigation||WHO Standard||Mexican existing standard||Proposed Standard for Mexico|
(m = monthly average, d = daily average)
Blumenthal UJ, Mara DD, Ayres R, Cifuentes E, Peasey A, Stott R, Lee D y Ruiz-Palacios G. 1996. Evaluation of the WHO nematode egg guidelines for restricted and unrestricted irrigation. Water Science and Technology 33, 277-283.
Camann et al. The Lubbock Treatment System Research and Demonstration Project Volume IV.
Lubbock Infection Surveillance Study (LISS). (Project Summary): US EPA, May 1986.
Cifuentes E, Blumenthal UJ, Ruiz-Palacios G, Bennett S. 1993. Health impact evaluation of wastewater use in Mexico. Public Health Reviews 19, 243-250.
Cifuentes E, Blumenthal UJ, Ruiz-Palacios G. 1994. Escenario epidemiológico del uso agrícola del agua residual. El Valle del Mezquital. Salud Pública México, 36 (1), 3-9.
Shuval H, Lampert Y, Fattal B 1997. Development of a risk assessment approach for evaluating wastewater reuse standards for agriculture. Water Science and Technology Vol. 35, 15-20.
WHO 1989. Health guidelines for the safe use of wastewater in agriculture and aquaculture. Technical Report Series 778. Geneva: World Health Organization.