Maple Creek Wastewater Treatment Plant - Executive Summary
The Greer CPW's Maple Creek Wastewater Treatment Plant personnel continually strive to prevent unwanted or accidental releases of hazardous materials from their facility. Policies in effect relate to operations and maintenance of equipment, ongoing employee training, emergency response planning and coordination with appropriate local response agencies. These policies are embodied in the Risk Management Plan and OSHA Process Safety Manual, which together provide the means to prevent or mitigate accidental releases. |
The Maple Creek WWTP is located off Gilliam Road to the east of the City of Greer. This facility treats domestic and industrial wastewaters for subsequent discharge into the South Tyger River under NPDES Permit No. SC0046345. Treatment processes include solids screening, extended aeration with return activated sludge, clarification and disinfection.
Disinfection is accomplished by chlorination of the plant effluent just prior to discharge. Facilities in place to acc
omplish this task include pumping and chlorinator equipment to provide a chlorine solution at rates proportional to discharge flow rates. Chlorine for this process is stored and handled on-site in ton cylinders. Normal operations include two cylinders on-line, with zero spare cylinders in reserve. The on-line cylinders have individual vacuum regulators, and are manifolded on the vacuum side with automatic switchover to the chlorination equipment.
Chlorine is the only regulated substance maintained on-site in quantities above the regulated threshold limit.
The offsite consequence analysis includes consideration of two chlorine release scenarios, identified as "worst case release" and "alternative scenario". The first scenario is defined by EPA, which states that "the Owner or Operator shall assume that the...maximum quantity in the largest vessel...is released as a gas over 10 minutes" due to unspecified failure. The alternative scenario is defined as "more likely to occur tha
n the worst-case release scenario".
The worst-case release scenario involves a failure of two ton-cylinders which are manifolded together, resulting in a release of 4000 lb of chlorine in a 10-minute period. The analysis is followed by conditions pre-defined by EPA, including use of the one-hour average ERPG-2 as the toxic endpoint, wind speed of 1.5 meters per second, atmospheric stability Class F, appropriate temperature/humidity values, ground-level release, urban topography and related variables.
Based on the above conditions, utilizing EPA's RMP Guidance for Wastewater Treatment Plants, the worst-case toxic endpoint occurs at a maximum distance of 1.9 miles from the release point, potentially affecting 4900 people.
The alternative release scenario involves the rupture of the flexible connector (pigtail) to one of the cylinders. Assuming a hole with a diameter of one inch (conservative), the chlorine release rate is approximately 150 pounds per minute. Utilizing EPA's pre
-defined conditions (wind speed 3 m/s, atmospheric stability Class D), the maximum toxic endpoint occurs at 0.2 miles from the release point, potentially affecting 20 people.
The facility is equipped with chlorine detection equipment, which is an active mitigation measure.
Both scenarios are based on prescribed assumptions. In an actual release event, the plume of gas would be more elliptical than circular and extend downwind and/or downgradient, likely affecting a smaller area and population than the models predict.
The general accidental release prevention program is a multi-faceted effort configured to comply with both EPA's RMP and OSHA's PSM rules. Key elements include a high level of employee training, an aggressive equipment maintenance routine, detailed operation procedures, hazard reviews and management controls.
Chemical-specific prevention steps include availability of self-contained breathing apparatus (SCBA), awareness of the hazardous and toxic properties of chl
orine, and the presence of chlorine detection equipment.
No accidental releases of chlorine have occurred at this facility in the past five years.
The facility has an emergency response program, which has been coordinated with the City of Greer Fire Department, which is a member of the Local Emergency Response Planning Committee (LEPC). The program includes itemized response steps including notification instructions. Emergency action drills are conducted annually.
Continual evaluation of technology and operations are performed to identify improvements that may help prevent or mitigate accidental releases.