Pulse Point Newsletter for October 30, 2001
Published by Alliance Consulting International
Partners in Environmental and Occupational Health and Safety
By Enrique Medina, MS, CIH

Mexico's renewed emphasis on water quality, which the Fox administration has labeled a "national security issue", means increased enforcement of wastewater discharge regulations by the federal National Water Commission. Municipal wastewater treatment plants are under pressure to bring their discharges into compliance with Official Mexican Standard NOM-001-ECOL-1996, which establishes the Maximum Permissible Limits for contaminants in wastewater discharged to National Waters and Properties, such as rivers, lakes, marshes, estuaries and oceans. The result is that local agencies are monitoring and regulating facilities within their service networks to bring them into compliance.

Industrial facilities, which generate and discharge wastewater to municipal sewer systems, are required to register with the state environmental or municipal wastewater agency and obtain an industrial discharge registration. The federal standard regulating industrial (as well as commercial, and residential subdivisions) wastewater discharge to sewer municipal networks is NOM-002-ECOL-1996.

This Official Mexican Standard establishes the Maximum Permissible Limits (MPLs) of contaminants in wastewater discharges to urban and municipal sewer systems. Its main objective is preventing and controlling contamination in national waters and properties, as well as protecting the infrastructure of sewerage systems.

This Official Mexican Standard in not applicable to discharges from independent storm water systems, domestic sewer discharges, or non-process related industrial discharges, if conveyed through separate sewer networks. Process wastewater, which results from production of commercial goods or services, is regulated. In practice, most industrial operations are required to comply with this standard in their sanitary discharges from employee showers, etc., under the premise that these may potentially carry residual process materials or waste.

These NOMs (for the Spanish acronym) regulate wastewater quality by means of Particular Discharge Conditions, which are defined as physical, chemical, and biological parameters and their maximum permissible limits (MPL) allowed in wastewater discharged into urban or municipal sewer systems (or national waters in the case of NOM-001).

Materials or waste considered hazardous must not be discharged or deposited into the urban or municipal sewer system. The MPLs listed in NOM-002 are:

Biochemical Oxygen Demand (BOD)
Total Suspended Solids (TSS)
Floating Matter
Grease and oil
Settleable solids

The MPLs are indicated in Table 1 of the Standard. The table contains values for instantaneous, daily average and monthly average MPLs. The Daily Average is the value resulting from the analysis of a compound sample taken on a day representative of the discharge-generating process. A Monthly Average is the value resulting from calculating a weighted average as a function of the proportional flow rate, of the values resulting from analysis of at least two representative compound samples, collected in a one-month period. The MPLs in the instantaneous column are for reference only. In the event the value of any analysis exceeds the instantaneous value, the facility must present to the competent authority the daily and monthly averages as well as the analytical laboratory results, within the time limit and in the form established by the local ordinances.

The maximum permissible limits for the parameters of BOD and TSS are established in Table 2 of NOM-001-ECOL-1996, or in the Particular Discharge Conditions with which the municipal wastewater treatment plant must comply.

The standard allows a "Competent authority", which is defined as Federal, State, and Municipal agencies or their public water-supply organizations, to implement and place additional requirements on this standard. Such actions must be justified by a technically supported study presented either by the competent authority or by those responsible for the discharge. The competent authority may set new MPLs, or  add MPLs for parameters not contemplated in this Standard. The industrial discharge registration may require facilities to analyze their wastewater for any component, which could potentially be present in the discharge based on their knowledge of the production processes or activities.

Discharging facilities have the obligation to analyze their wastewater discharge to determine the daily average or the monthly average concentration of the parameters listed in the Standard.  They must also keep analytical records for at least three years. Reporting requirements are set by the state or municipal permitting agency in the discharge registration and may range from monthly to annually, depending on the nature and volume of the discharge.

Reported discharge concentrations must be derived from analyses of compound samples, which are mixtures of simple samples combined in volumes proportional to the volumetric discharge flow rate measured at the point and time of sampling.

Simple samples should be continuously collected at the point of discharge on a normal day of operation, and quantitatively and qualitatively reflect the process or processes most representative of the discharge-generating activities. They should be collected at least in sufficient volume to perform the necessary analyses while measuring the discharge flow rate at the point and time of sampling. The number of simple samples required is based on hours of operation of the discharge point, according to Table 2 of the standard.

If the operation of the process or discharge-generating activity does not take place continuously, the facility must present to the competent authority information describing their operating system and sampling program.

Facilities may be exempted from analyzing one or more of the parameters indicated in this Standard, when they demonstrate to the competent authority that, due to the characteristics of the production process, or the use given to water, they do not generate or concentrate the contaminants to be exempted. The competent authority may verify the information presented.

Facilities are also required to inform the competent authority of any change to their production processes or activities, when they modify the quality or volume of wastewater that were authorized in the corresponding discharge permit.

Facilities that concentrate contaminants in the discharge after introducing water conservation or recycling systems or programs, and which consequently exceed the MPLs, can request that the competent authority review the situation, and, if appropriate set site-specific Particular Discharge Conditions.

Official Mexican Standard NOM-001-ECOL-1996, which Establishes the Maximum Permissible Limits for Contaminants in the Discharge of Wastewater in National Waters and Properties requires testing for analytes similar to NOM-002, with the following additions: Total Nitrogen, and Total Phosphorus. The MPLs for this standard are different than for NOM-002 and vary based on the receiving water and beneficial uses.

NOM-002-ECOL-1996 went into effect in 1996 with a phased compliance calendar based on population size. Cities with more than 50,000 residents needed to comply by January 1, 1999. Those with populations between 20,0001 and 50,000 will need to comply by January 1, 2004, while smaller municipalities have until 2009 to meet the requirements of this standard.

If you have questions about how this article or other health, safety or environmental issues, please contact us at (619) 297-1469 or send us an email at emedina@pulse-point.com.

Alliance Consulting International
Partners in Environmental and Occupational Health and Safety
3361 28th St.
San Diego, California 92104
(fax (619)297-1023

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Pulse Point is written for the benefit of our readers with the sole intent to provide general information. The articles are not intended as specific opinions or as a substitute for professional advice in individual cases.