Slab Creek Wastewater Treatment Plant - Executive Summary
EXECUTIVE SUMMARY |
SLAB CREEK WWTP
BOAZ, MARSHALL COUNTY, ALABAMA
DESCRIPTION OF FACILITY AND REGULATED SUBSTANCES HANDLED:
The Board of Water and Sewer Commissioners of the City of Boaz, Alabama, owns and operates an activated sludge waste water treatment plant under National Pollutant Elimination Discharge (NPDES) Permit No. AL0049603. This facility is designed for an average daily flow of 4.1 million gallons per day (MGD) and a peak flow of 6 MGD. The treatment stream consists of influent aeration, grit removal, primary clarification and scum removal, trickling filters, final clarification, disinfection, aerobic and anaerobic sludge digestors, and effluent discharge to a tributary of Slab Creek. Slab Creek is on the Locust Fork branch of the Black Warrior River.
Disinfection is accomplished by injecting chlorine gas through a diffuser in the waste stream. In accordance with the facility's NPDES permit the feed rate is adjusted to maintain a residual of 0.5 parts per million (
PPM) at the end of the required contact time. Dechlorination is accomplished by injecting sulfur dioxide gas into the waste stream at the effluent end of the chlorine contact chamber at a rate to reduce the chlorine residual to 0.1 PPM prior to discharging to the receiving stream.
There are five regulated substances which are most likely to be encountered at a waste water treatment facility: chlorine, sulfur dioxide, methane, propane, and ammonia in either anhydrous or liquid form. Only the first three are found at the Slab Creek Plant and only one, chlorine, above the threshold amount of 2,500 pounds. The Slab Creek facility presently uses chlorine from two (2) one ton cylinders.
WORST-CASE RELEASE SCENARIO AND ALTERNATIVE RELEASE SCENARIO:
The RMPComp program was used to evaluate the offsite consequence analysis for worst-case and alternate-case scenarios. The endpoint for a chlorine release in each case was one and three-tenths (1.3) miles and seven-tenths (0.7) miles respe
ctively. Public receptors in these areas included single and multi-family housing, a community college and associated dormitories, an elementary school, churches, and business and industrial areas.
GENERAL ACCIDENTAL RELEASE PREVENTION PROGRAM AND CHEMICAL-SPECIFIC PREVENTION STEPS:
The Board's personnel responsible for operations and maintenance of the treatment facility have procedures in place for prevention of chlorine releases. These procedures include regularly scheduled maintenance of the process equipment and training of personnel on use of the process equipment. Documentation of the training classes and maintenance scheduled is kept at the facility office. Written procedures are in place for the isolation and repair of leaking chlorine cylinders. These procedures also cover how to treat different kinds of leaks or releases.
The personnel are also trained regularly in the use of personal protective equipment and self-contained breathing apparatus.
FIVE YEAR ACCIDENT HI
There has been only one accidental chlorine release at the Slab Creek facility within the preceding five (5) years. The release occurred when one of the plant operators was removing a vacuum regulator from a one ton chlorine cylinder. A small amount of chlorine gas which was trapped in the regulator escaped into the atmosphere. The process equipment is housed in a concrete block and brick building with exhaust fans and louvers for ventilation of the building. The only person affected was the operator working on the process equipment and that was due to his proximity to the valve when the release occurred. This accidental release had no offsite consequences and was below the limit for public notification. As a result of this accidental release the Board adopted a policy of returning the regulators to the manufacturer each year for refurbishing. No other accidental releases have occurred since adoption of this policy.
EMERGENCY RESPONSE PROGRAM:
The City of Boaz, Alabama
, has an emergency response plan. Marshall County also has an Local Emergency Response Committee. The Board's personnel will not respond to a chlorine release or other emergency at the plant but will coordinate with local Emergency Management Agency (EMA), law enforcement, and fire department personnel who will respond. In the event public notification is required this will be handled through the EMA. In the event of a worst-case release evacuation of populated areas within the area of influence will be coordinated by the EMA.
PLANNED CHANGES TO IMPROVE SAFETY:
The management of the facility is currently exploring the possibility of reducing the amount of chlorine stored on-site to a quantity below the regulated threshold. A lower chlorine inventory would mean less chlorine that could be accidentally released. If an inventory reduction is possible an amendment to this Risk Management Plant will be filed at that time.
Another change being investigated at this time is the insta
llation of automatic shutoff valves to be installed on each chlorine cylinder in use at the site. Upon detection of 1 to 3 PPM chlorine the existing chlorine detectors would transmit a signal to the automatic shutoff valves and the cylinder valves would be closed within seconds.
Regular maintenance of equipment used to load and unload the chlorine cylinders at the facility is also planned as a preventive measure. Failure of the wire rope, hoist or trolley during unloading of a full cylinder could result in a worst-case release - the entire contents of the one ton cylinder released outdoors without the building which houses the process equipment to serve as a passive mitigation feature.