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NPDES Permits in New England

Discharge Permit for MWRA Outfall Questions and Answers
May, 1999

What is an NPDES permit, and why does the MWRA need one?

NPDES stands for "National Pollutant Discharge Elimination System." The federal Clean Water Act requires that such a permit be obtained by anyone who discharges pollutants to surface waters through a "point source" (a conveyance such as a pipe or ditch). The MWRA must obtain such a permit for its discharge from the new Deer Island wastewater treatment plant.

NPDES permits set limits on the amount of pollutants which can be discharged, and include many other conditions designed to protect water quality.

Who issues the MWRA's permit?

The MWRA must receive permits from both the federal Environmental Protection Agency (EPA) and the state Department of Environmental Protection (DEP). EPA and DEP are issuing a single document which serves as a joint state and federal permit.

How does the public participate in the permit process?

EPA and DEP released the draft permit for public comment in March, 1998. The draft permit was distributed at public meetings; was placed in public libraries in Worcester, Natick, Framingham, Boston, Falmouth, and Nahant; was posted on EPA's internet site at http://www.epa.gov/region1; and was mailed to members of the public on request. Public meetings and hearings were held in Boston, Nahant, and Barnstable.

All comments were carefully evaluated, and appropriate changes were made to the permit. Along with the final permit, EPA and DEP are issuing a written summary of all comments received and a response to each.

What does the permit require?

The permit sets limits on specific pollutants, at levels which ensure that water quality standards are met at the site of the discharge. The permit also requires extensive monitoring of the effluent, as well as a comprehensive program to monitor the health of Massachusetts Bay.

In addition, the permit requires implementation of a "contingency plan" to ensure that any unexpected problems are dealt with quickly.

Other permit requirements include an industrial pretreatment program; a water conservation program; and pollution prevention efforts. These requirements and others are described in the MWRA Permit Overview.

What goes into the Deer Island treatment plant, and what kind of treatment is provided?

The MWRA's Deer Island treatment plant is designed to treat sewage and other wastewater from 43 cities and towns in the Greater Boston area, including 2.5 million people and 5,500 businesses.

The treatment process includes "primary treatment," where wastewater passes through large settling tanks (typically removing 50-60% of the solid material suspended in the water). This is followed by "secondary treatment," where bacteria are used to consume pollutants and form clumps of waste material, which then falls to the bottom of a second set of settling tanks (typically removing 85-90% of the solid material). Finally, the treated effluent is disinfected before it is discharged.

What are "combined sewer overflows," and how are they treated by the permit?

In modern times, municipalities typically construct separate sewage and storm water drainage systems. However, parts of Boston, Cambridge, Somerville, and Chelsea are still served by older "combined sewers," which are pipes designed to carry both sewage and storm water. These pipes have overflow points which discharge during wet weather. The MWRA has constructed a number of disinfection facilities which chlorinate some of these discharges.

The permit sets limits on bacteria and chlorine levels in these combined sewer overflows. The permit also prohibits any discharges which would cause or contribute to an exceedance of state water quality standards.

The MWRA is required by a federal court order to construct facilities which will greatly reduce these overflows, and to eliminate them in certain critical areas (for example, near Dorchester Bay beaches).

What are "technology-based limits"?

The Clean Water Act requires that municipal wastewater facilities must provide "secondary treatment" before discharging. EPA regulations translate this general requirement into a set of specific effluent limits (for example, no more than 30 milligrams of suspended solids per liter of effluent).

These limits are known as "technology-based limits" because they are based on what can be achieved by a particular technology, i.e. a well-functioning secondary treatment plant. Technology-based limits are uniform across the country.

What are "water quality-based limits"?

For each permit, EPA evaluates whether technology-based limits are adequate to ensure compliance with state water quality standards. If they are not, EPA establishes additional "water quality-based limits" designed to ensure that such standards are met.

For each possible contaminant in the effluent, EPA calculates the level of that pollutant which would cause or contribute to an exceedance of state water quality standards. This calculation takes into effect the background levels of pollution that already exist in the receiving water, as well as the dilution which will occur when the effluent mixes with that water.

When there is no background concentration of a particular pollutant, the calculation is simple. For example, if the water quality standard for pollutant X were five parts per million, and the effluent will be diluted by a factor of 100:1, then the water quality standard is multiplied by the dilution to arrive at a limit of 500 parts per million. If pollutant X already existed in the background, the limit would be reduced to account for that fact and ensure that the water quality standard would not be exceeded.

Numeric limits are set for all pollutants which have a reasonable potential to cause or contribute to an exceedance of a water quality standard.

What is a "whole effluent toxicity (WET) test?"

Whole effluent toxicity tests measure the toxicity of the effluent to living organisms. Species known to be sensitive to contaminants are exposed to various concentrations of effluent, and the effects are observed. A 48-hour test measures acute toxicity; a 7-day test measures chronic toxicity. The permit requires that the MWRA conduct toxicity tests each month.

What is a "pretreatment program"?

A pretreatment program is designed to prevent four major problems, caused by pollutants from industrial facilities:

  1. interference with the treatment facility operations
  2. pass-through of pollutants into the treatment facility's effluent
  3. contamination of municipal sewage sludge
  4. exposure of the treatment facility to hazardous chemicals.

The NPDES permit requires the MWRA to monitor and control the discharge of pollutants into the MWRA sewer system from industrial and commercial sources. The permit also includes extensive pollution prevention requirements for these sources.

Does the permit include sludge monitoring requirements?

Yes, the permit requires monitoring and other requirements for each possible sludge disposal option, including:

  1. land application
  2. use as fertilizer
  3. solid waste land fill disposal.

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