Wastewater Treatment Plant #2 (WWTP#2) - Executive Summary
EXECUTIVE SUMMARY |
A. The City of Harlingen Waterworks System (HWWS) 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 HWWS emergency response policy involves the preparation of response plans that are tailored to each facility and to the emergency response services available in the community, and is in compliance with the EPA Emergency Response Program requirements.
B. The Wastewater Treatment Plant #2 (WWTP #2) has been installed to disinfect and treat the flow of domestic/industrial wastewater prior to discharge into the Arroyo Colorado River. Additionally, the Plant treats and recycles dye bath water for the Fruit of the Loom facility. The facility is located on Harrison Street (Hwy.106 East). The plant utilizes a chlorinating station, liquid chlorine containers, a chlorinator,
various safety equipment, an instrument room containing flow recorders, various electrical panels and additional safety equipment. The amount of chlorine on site is sixteen one-ton containers with eight on-line and eight on stand by. Sulfur dioxide is used to neutralize chlorine residuals using six one-ton containers on-line and three containers as standbys. Water treatment operators are at the facility daily and respond to any trouble alarms that may occur.
C. The offsite consequence analysis includes release scenarios for chlorine and sulfur dioxide identified as a "worst-case release" and "alternative release 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 an unspecified failure. The alternative release scenario is defined as "more likely to occur than the worst-case release scenario".
Atmospheric dispersion modeling has to be
performed to determine the distance traveled by the chlorine release before its concentration decreases to the "toxic endpoint" selected by EPA of .0087 mg/L, which is the Emergency Response Planning Guideline Level 2 (ERPG-2). 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 circle with a radius corresponding to the toxic endpoint distance has to be defined "to estimate the population potentially affected".
The worst-case release scenarios at the Waste Water Treatment Plant #2 involves the failure of the main feed lines connecting the eight one-ton chlorine containers and the six one-ton sulfur dioxide containers that are connected to t
he chlorinating process. An off-site consequence analysis for these scenarios was performed. The pre-defined conditions set by EPA, namely the release of the entire amount as a gas in10 minutes, use of the one-hour average ERPG-2 as the toxic endpoint, and consideration of the population residing within a full circle with radius corresponding to the toxic endpoint distance.
EPA - mandated meteorological conditions, namely Stability F, wind speed of 1.5 m/sec, air temperature (77?F), and average humidity (50%) were used.
Atmospheric dispersion modeling for the chlorine worst-case scenario was performed using the EPA assumptions; a distance to toxic endpoint of 2.9 miles and an estimate of residential population potentially affected of 14,300 was obtained. The sulfur dioxide worst-case scenario distance to toxic endpoint results were 2.5 miles potentially affecting a population of 8,500.
The alternative release scenario involves the rupture of the flexible connection (pigtai
l) connected to one of the one-ton containers of chlorine and sulfur dioxide. The amount of the chlorine release is 317 lbs. at an average rate over one hour (the duration of the release) at 10.5 lbs./min. to the toxic endpoint distance of the ERPG-2 level. The sulfur dioxide alternate release scenario was estimated releasing gas at 15 lbs./min. with a total of 495 lbs. released.
D. The general HWWS accidental release prevention program is based on the following key elements:
* High level of training of the operators
* Preventive maintenance program
* Use of state-of-the-art process and safety equipment
* Use of accurate and effective operating procedures
* Performance of a hazard review of equipment and procedures
* Implementation of an audit and inspection program
Chemical-specific prevention steps include availability of self-contained breathing apparatus (SCBA), worn by the operators during connection / disconnection of chlorine and sulfur dioxide supply, awareness
of the hazardous and toxic properties of the chemicals, and presence of chlorine detectors.
E. No accidental releases of chlorine or sulfur dioxide have occurred at this facility in the past five years.
F. The facility has an emergency response program that has been reviewed by the Harlingen Fire Department. This program includes an emergency response decision tree and a notification plan. Emergency response drills and drill evaluations will be conducted every six months; emergency operation and response procedures will also be reviewed at that time.
G. Changes to improve safety (recommended actions) will be reviewed upon completion of a facility Hazard Assessment and Compliance Audit.