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he 1996 Mexican environmentalregulation NOM-001-ECOLestablishes the maximum amount of

contaminants permitted in wastewaterdischarged into public water bodies ornational property. This restriction aims toreduce wastewater disposal into the riverand the negative impacts for health andthe environment, through a fine of US $0.25 per cubic metre of untreatedwater that exceeds the permitted limits.

However, the regulation also leads to areduction in the nutrient values andforms a constraint to irrigation by waste-water. In accordance with this regulation,the public water-supply company of Guanajuato city, called SIMAPAG(2)

constructed an activated sludgewastewater treatment plant, whichstarted to operate in June 2002. In thisarticle, the plants benefits are reviewed.The aim is to give some preliminaryanswers to the research question: Howdoes the water-treatment plant influence the benefits of the use of wastewater for cropproduction?

The need to assess these effects appearsessential for future, since current nationalenvironmental laws and local policies willincrease the volume of treated wastewaterand actual will change the conditions of raw wastewater irrigation. In GuanajuatosHydraulic Plan for 2000-2025, anadditional 47% of wastewater is planned

to undergo treatment, which togetherwith the 16% of wastewater alreadytreated, makes for a total of 53% of treatedwastewater in the near future.

The volume of residual waters generatedin the 46 municipalities of Guanajuatototal 207.13 million cubic meters per year.If this water could be used directly foragricultural purposes, it could irrigatearound 20,500 ha, which is almost 5% of

the actual irrigated land in Guanajuato(416,690 ha). There are 16 treatment plantsin the urban areas and another 26wastewater-treatment systems in the ruralareas. The lack of technical andadministrative capacity prevents the watertreatment programmes from being carriedout satisfactorily.

SIMAPAG AND THE TREATMENTPLANT PROJECT Water supply for the city of Guanajuato(with a total population of around106,000) is provided by SIMAPAG. Thereare 31 water-supply operator agencieslike SIMAPAG in the state of Guanajuato.They act as financially autonomouspublic utilities with an independentadministration. Only ten of these have ahealthy financial status, but SIMAPAGhad an outstanding performance in termsof financial surplus and overall efficiency(CEAG, 2001). The potable water-supplycoverage is 95%, and sewer coverage82%. Domestic connections represent

almost 94% of the total number and thereare very few commercial and industrialconnections. The average production perconnection is 27.7 m 3 and the average feeis US $0.59/m3 (CEAG, 2001).

SIMAPAG constructed an activated sludgewith chlorine treatment plant. The federalgovernment contributed 24% of the funds,the local government 40%, and SIMAPAGthe remaining 36%. In , the planttreatment design parameters aredescribed. According to the averageproduction per connection, the expectedsewage effluent (3) from Guanajuato city isaround 0.14 m 3/s, this is a volume of 6.3million cubic metres. Until the treatment

plant started to operate, this effluent wasdischarged into the Guanajuato River.Nowadays, 70% is treated, while thewastewater produced by the communityof Marfil, representing the remaining 30%of untreated wastewater, is sewerageddownstream of the treatment plant outlet.Currently, SIMAPAG has to pay annuallyUS $472,500 for this remaining 30% of untreated wastewater.

WATER AND NUTRIENT VALUEThe treatment plant gives SIMAPAG theopportunity to sell the treated water. Nocommercial transaction has taken place asof yet, but there will be greatercompetition among the different sectors.Moreover, raw wastewater irrigation willhave to compete against the use of treatedwater, since every cubic meter of untreated wastewater disposed in theGuanajuato River costs SIMAPAG a fine of US $0.25.

Thus the farmers request to use raw

wastewater will only arouse the interest of SIMAPAG if the farmers are willing to paythe fine expenses, which they cannot.The expected water productivity in small-scale irrigation systems is only around

_________________Paula Silva-Ochoa , Mexico

polsil@hotmail.comChristopher A. Scott , IWMI India

In 1999, IWMI explored the advantages and risks ofthe use of urban wastewater for crop production

along the Guanajuato river. At least 140 hectares ofland downstream of the city of Guanajuato (1), farmers

in two periurban communities (San Jose de Cerveraand Santa Catarina) irrigate with raw wastewater.

The benefits from wastewater irrigation includeadditional water, nutrients and the benefit obtained

for the treatment (Scott, et al., 2000).

The Impact of a Treatment Planton Wastewater Irrigation in Mexico

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An overviewof theTreatmentPlant inGuanajuato,Mexico

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US $0.15/m 3 (Silva,et al., 2000). A higher

productivity could be reached, even up toUS $0.50/m3, if more profitable crops likevegetables are cultivated. But thesevegetables are consumed raw, which isrestricted under regulation NOM-001-ECOL-1996. The operation cost of onecubic metre of treated water is US $0.11. Bymeans of a 10% charge for sanitationservices, SIMAPAG recovers US $0.04/m3

from the domestic users and US $0.08/m 3

from industrial and commercial users. Inorder to be profitable, the selling price forthe treated wastewater should be at leastUS $0.07/m3. Industrial customers couldpay up to US $0.50/m 3, which would givea surplus of US $0.43/m 3.

The existing concentration of nitrogenphosphorous in the effluent is enough tomeet the nutrient requirements for alfalfa,the most common cultivated crop.Farmers show very little concernregarding the reduction of nutrients dueto the water treatment process upstream,since the treated water still has a high

content of nutrients. Water users are moreafraid about water-level reduction in theriver than about nutrient reduction in theriver effluent. The sludge would representanother important source of nutrients.

The storage and elimination of this

material is one of the major operationalproblems, while the area that couldbenefit from the waste-treatment plant isaround 20 to 30% of the total study area.Unfortunately for the moment, the solidwaste is taken to a landfill.

FOREGONE TREATMENT COSTIMPACTIt is obvious that wastewater irrigationwas not considered as an alternativemethod for wastewater treatment. Theselection of the water-treatment processwas entirely based on environmentalregulation NOM-001-1996. The reasonbehind this is the great percentage of irrigated land without water entitlement.SIMAPAG only recognises the land thathas a regular water right. Annually, this islegally granted to 300,000 to 500,000 m 3,which represents just 30 to 50 ha.

Theoretically, the water-treatment plant inGuanajuato city would produce treatedwater for all types of landscape irrigation,

including for example, golf courses andparks (though with higher maximumlimits than for agriculture). However,there is presently no reuse process in placeexcept for irrigation for agriculture.

Nevertheless, if the produced treatedwater is not sold; the capital investmentwill not be justified. The cost and difficultyin operating and maintainingconventional treatment plants to meet thespecified guidelines means that they arenot recommended where waste-stabilisation ponds and wastewaterstorage and treatment reservoirs can beused (Blumenthal, et al , 2000).

CONCLUSIONS ANDRECOMMENDATIONSThe major potential impact of the water-treatment plant is the possible reductionin wastewater discharge in the river, if thetreated water is sold to an industrialconsumer outside the Guanajuato River

sub-basin. However, this would lead tocompetition over the water. The positionof the farmers is weak because only 30 to40 ha have proper water entitlement. Thisimpact is not affected yet, because of additional sources of urban wastewaterentering into the river downstream of thetreatment plant.

Further research is needed to identifyconditions under which the substantialbenefits of wastewater irrigation can be

captured while financial sustainability of the water-supply utilities is maintained.There are several aspects that need to beanalysed regarding the relationship of theurban production of treated water andwastewater irrigation such as:

a water market for treated effluent andits commercial feasibility in irrigation(comparison between the use of treatedwater and raw wastewater); water rights conflicts; hydrological impact of selling thetreated water outside the sub-basin; water-quality assessment at the finaluse point (e.g., farm level for irrigation);and an accounting of nutrients lost fromraw wastewater.

Notes(1) Very often the state and its capital city have thesame name in Mexico. Unless it is mentioned,Guanajuato refers to the province of Guanajuatoand not to the city.(2) SIMPAG stands for Sistema de Agua Potable y Alcantarillado de Guanajuatoin Spanish, or theGuanajuato Water Supply System.(3) This figure comes from the assumption that70% of the total produced water per outlet will beseweraged.

Table: Plant treatment design parameters

Parameter Unit Influent Effluent

Discharge design Lps 140 140Total Suspended Solids (TSS) Mg/l 217