Massaponax Wastewater Treatment Facility - Executive Summary
EXECUTIVE SUMMARY |
The Spotsylvania County Department of Utilities (SCDU) Massaponax Wastewater Treatment Facility is located at 10900 H.C.C. Drive in Fredericksburg, Virginia. The facility treats wastewater received from surrounding municipalities before discharging into the Rappahannock River. Like many wastewater treatment plants, the Massaponax Facility uses chlorine for disinfection before the water is discharged. The facility receives chlorine in one-ton (2,000-pound) cylinders and stores up to a maximum of eight cylinders at any given time.
Chlorine is a toxic substance regulated by the United States Environmental Protection Agency's Risk Management Program Rule [40 CFR Part 68]. The Massaponax Facility has prepared a Risk Management Plan (RMP) in accordance with the requirements of 40 CFR Part 68.
The Massaponax Facility has used chlorine in its disinfection system for its entire operating history. During that time, facility personnel have taken con
siderable care in the storage and handling of chlorine. The facility receives one-ton cylinders by truck, lifts them using a motorized crane designed specifically for moving one-ton cylinders, and places them on saddles in a locked building. Before accepting any chlorine cylinder, facility operators check the condition of the cylinder and inspect it for leaks. The building is used solely for chlorine storage and connection to the disinfection system and, therefore, is not subject to daily traffic.
A chlorine sensor, located within the chlorine storage area, sounds an alarm when the chlorine concentration exceeds one part per million (ppm). The alarm notifies the operators of low concentrations (greater than or equal to 1 part per million) of chlorine in the storage area so they can respond immediately and correct the situation. The alarm is tested monthly.
The USEPA requires that facilities which meet the requirements of 40 CFR Part 68 model two release scenari
os, a worst-case release scenario and an alternative release scenario. Affected facilities are required to estimate the distance that a chemical plume will travel before its concentration falls below the USEPA-established "toxic endpoint" concentration for the chemical of concern. Affected facilities are also required to estimate the population and other environmental receptors that may be impacted under such a release scenario.
The worst-case release scenario is an unlikely event, used solely for planning purposes, whereby the complete contents of the one-ton cylinder are assumed to be released over a ten-minute period. The release is directionally independent, therefore, the distance to the toxic endpoint is assumed to comprise the radius of a circle drawn around the facility. The purpose of this exercise is to prepare the facility and emergency responders for "worst-case" conditions.
The facility contracted an outside consultant with air dispersion modeling expertise to mode
l the release in accordance with USEPA requirements. The dense gas dispersion modeling program DEGADIS was used for the worst-case release scenario using actual meteorological conditions. The result was a release which impacted offsite public and environmental receptors.
The USEPA also requires facilities to model an alternative release scenario or "more-likely" release scenario which also impacts offsite receptors (if one exists). Since the facility has not experienced the release of chlorine in its operating history, the selection of an appropriate alternative release scenario was based on knowledge of the Massaponax Facility and its operations. The facility used the process hazard analysis (PHA) conducted as part of the OSHA Process Safety Management Plan in 1997 to review a number of scenarios and the measures in place to mitigate a subsequent release. The PHA did not identify any likely scenarios which, when modeled, would reach an offsite receptor. Therefore, the facility
used the unlikely scenario of a failure of the pressure relief device (fusible plug) on the cylinder in order to meet the USEPA requirement that an alternative release result in an offsite impact. The facility used the USEPA Offsite Consequence Analysis (OCA) Guidance to estimate the distance to the toxic endpoint for the alternative release scenario.
Five-Year Accident History
Through training, prevention measures, and safe operating practices, the facility has not experienced the release of chlorine which has adversely affected offsite receptors during its entire operating history.
The facility operators are trained to respond to a minor release of chlorine within the chlorine storage area. The operators wearing the proper personal protective equipment are capable of entering the storage area and repairing or replacing faulty equipment. In the event of a more significant release, the facility has a mutual aid agreement with the Virginia Department of Emergen
cy Services (VDES), which would provide HAZMAT trained personnel to assist in mitigating the release.
Planned Changes to Improve Safety
The Massaponax Facility continually reviews the existing operating procedures, process safety information, and training program to protect the safety of its employees and the surrounding population.