Newburyport Wastewater Treatment Plant - Executive Summary |
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The City of Newburyport's accidental release prevention policy involves a unified approach that integrates technologies, procedures, and management practices. All applicable procedures of the U.S. Environmental protection Agency (EPA) Prevention Program are adhered to. The Newburyport Wastewater Treatment Facility (WWTF) treats wastewater from most residential, commercial, and industrial establishments in the City of Newburyport, Massachusetts. The facility includes an operations building, a headworks building, various process tanks and structures, and a chlorination building, which contains liquid chlorine containers, chlorinators and various safety equipment. The average amount of chlorine typically stored at the facility is 6,000 pounds. The facility is operational 24 hours a day. Wastewater treatment operators are at the facility from 7 a.m. to 6 p.m. on weekdays, but operators are on-call to respond to any emergency situations that may arise. The offsite consequence analys is includes consideration of two chlorine release scenarios, identified as a "worst-case release" and an "alternative release". The worst-case release scenario is defined by the EPA as the "maximum quantity in the largest vesselreleased as a gas over 10 minutes", due to an unspecified failure. The alternative release scenario is defined as "more likely to occur than the worst-case release scenario". The toxic "endpoint" concentration for chlorine is 3 parts per million (ppm). This level was selected by the EPA and is the Emergency Response Planning Guideline Level 2 (ERPG-2) limit. This is defined by the American Industrial Hygiene Association (AIHA) as the "maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action". The residential population within a circl e with a radius corresponding to the toxic endpoint distance (i.e. the distance from the source at which the concentration of chlorine gas is at or below its toxic endpoint concentration) is defined so as to "estimate the population potentially affected". The worst-case release scenario at the Newburyport WWTF involved the failure of a single one-ton container (2,000 pounds) of chlorine. EPA-mandated meteorological conditions such as F stability class, a wind speed of 1.5 m/sec (3.4 miles/hour), ambient temperature of 770F, and 50% humidity were used in the scenario. The EPA's RMP*Comp model estimated a distance to the toxic endpoint of 0.9 miles, which would potentially affect a residential population of 5,500. The EPA model used may be unrealistic because: 7 Only a portion (approximately 26%) of the compressed liquefied chlorine released to the atmosphere flashes as a vapor. The remaining unflashed liquid forms liquid-droplet aerosols. This results in the formation of a very de nse chlorine cloud consisting of vapor and liquid droplets, with dispersion characteristics significantly different than for a cloud consisting only of chlorine gas. 7 Only the population within an elliptical plume extending downwind of the release point is potentially affected. The prevailing winds at the facility are towards the ocean. The plume area, or footprint, is approximately one-quarter of the area of the full circle, therefore only approximately 1,400 of the 5,500 residents would be exposed to chlorine gas. The alternative release scenario involved a <-inch hole in the flexible tank hosing that connects the tank to the treatment system. This could potentially result in a release of approximately 1,566 pounds of chlorine over one hour at an average rate of 26.1 lb./min. The EPA's RMP*Comp model estimated a distance to the toxic endpoint of 0.1 miles, potentially affecting a residential population of approximately 600. The Newburyport WWTF has used chlorine disinfection f or the past 40 years without accidental releases. The Newburyport WWTF's accidental release prevention program includes the following elements: ' Emergency training for operators ' Preventive maintenance program ' State-of-the-art process and safety equipment ' Use of accurate and effective operating procedures, written with the participation of the operators Chlorine-specific prevention steps include the availability of self-contained breathing apparatus (SCBA) for emergency repair work, chlorine safety training, and chlorine detectors. The facility also has an emergency response program, which is coordinated with the City of Newburyport Fire Department. This program includes an emergency response decision tree and a notification plan. |