Lexington Water Treatment Plants - Executive Summary
The City of Lexington (N. C.) Water Treatment Plants operates and maintains a vacuum chlorination system for disinfection purposes. The Water Treatment Plants use a combination of training, preventive maintenance and experience to minimize the risk of exposure to city personnel as well as the community. Water Treatment Plant management recognizes mandated programs, such as Hazard Communication/ Right-To-Know(RTK) and Process Safety Management (PSM), as means of commitment to safety and implementation of safe procedures. |
The City of Lexington maintains two (2) separate water treatment facilities on the same site off of Business-85 north of Lexington, N. C. Although separate facilities, the two (2) water treatment plants do share the same chlorine facility. The Lexington Water Treatment Plants chlorine facility consists of an open-sided structure designed to accommodate up to twelve (12) ton chlorine containers; historically no more than five (5) ton chlorine containers have been on
hand at the water treatment plants, but due to Process Safety Management (PSM) regulations inventory control measures have been instituted and no more than four (4) ton chlorine containers are on site at any given time. The original chlorination system was a pressure system where nuisance leaks were inherent in the system; in 1990 the chlorination system was changed over to a vacuum system which has reduced the number of nuisance leaks to zero.
Chlorine gas under pressure enters a vacuum regulator and the gas pressure is reduced to less than atmospheric pressure. The chlorine gas is then pulled through Schedule 80 PVC piping by a vacuum created by water flowing through chlorinator injectors located at their respective locations. At the point of injection the chlorine gas is converted into a chlorine solution by mixing with the injector water; the chlorine solution flows through Schedule 80 PVC piping to submerged application points.
The City of Lexington Water Treatment Plants' w
orst-case release scenario as well as its alternative release scenario were modeled with RMPComp. The worst-case scenario was a full ton container release (2,000 lbs.) in a release duration of ten (10) minutes. The release was modeled in urban surroundings with an assumed wind speed of 3.4 mph with the most stable atmospheric conditions and an air temperature of 77 degrees F; the only mitigating circumstance considered in this model was that the release was in an enclosed space, in direct contact with the outside air. The distance to endpoint was 1.3 miles with mainly residences being in the affected area although there is a nursing home less than 0.4 miles from the facility.
The alternative release scenario was a 0.5-inch pipe leak with a duration of 120 minutes. The release rate was 0.2 pounds per minute with a total quantity release of 24 lbs. This release was also modeled in an urban environment with an assumed wind speed of 6.7 mph with neutral atmospheric conditions and an
air temperature of 77 degrees F; again the only mitigating circumstance considered in this model was that the release was in an enclosed space, in direct contact with the outside air. The distance to endpoint was 0.4 miles with mainly residences being in the affected area and again there is a nursing home less than 0.4 miles from the facility. This scenario was selected as an alternative because it represents the most likely release that will occur.
The City of Lexington Water Treatment Plants fall under OSHA's Process Safety Management (PSM) rule and as a result has established managerial guidelines to control the chemical inventory. In addition, employees are trained annually by the chemical supplier in safe handling practices. The employees are also trained annually in-house on Hazard Communication/Right-To-Know (RTK) topics specifically regarding chlorine and on chemicals in general; "new" employees are provided this training within thirty (30) days. Only experienced employee
s are alllowed to connect full or dissconnect empty ton chlorine containers; those employees attend a twenty-four (24) hour "Hazardous Materials Technician for Water Resources" class setup up through the Davidson County Community College.
There have been two (2) accidental releases in the past five years. The first incident involved a chlorine solution release in which an employee was affected by the off-gassing of the chlorine solution and was treated at the Lexington Memorial Hospital Emergency Room. There were no off-site consequences. The second incident involved a leaking 0.5-inch black iron pipe which was identified upon opening the valve on the ton chlorine container; the ton container valve was immediately closed and the chlorine released was not measurable. This incident involved no on-site injuries and no off-site consequences.
The City of Lexington Water Treatment Plant emergency response plan calls for the current chlorine supplier, which is established on a calendar
year basis through a bid process, to respond to a chemical release. Chlorine leak detectors would notify water treatment plant personnel of a release and those personnel would then notify the current chlorine supplier and the Davidson County Emergency Management Office. Public notification would be handled through the Davidson County Emergency Management Office.
Currrently plans are under way to convert the City of Lexington Water Treatment Plants disinfection process to one that uses a sodium hypochlorite solution in lieu of chlorine gas. Plans are to construct the system and have it operational within FY 1999-2000.