Fourche Creek Wastewater Treatment Plant - Executive Summary
Policy Statement |
It is the Little Rock Wastewater Utility's (LRWWU) policy to operate the Fourche Creek Wastewater Treatment Plant (WWTP) safely, reducing to the greatest extent possible any hazards associated with the chlorine and digester gas systems and reducing any subsequent risk to the surrounding community, personnel, and environment. Safe operation depends on proper storage of chlorine and digester gas, proper handling of the chlorination and digester gas systems, and inherent safety features in the design of the chlorination and digester gas systems. Safety in storage and handling is achieved through safe handling procedures and personnel training.
The policy includes working with the surrounding community and local emergency response agencies to promote a spirit of cooperation and teamwork, to orchestrate an effective contingency plan in the unlikely event of a process incident occurring at the Fourche Creek WWTP. The emergency response plan includes procedures for notif
ying the Little Rock Fire Department.
Stationary Sources and Regulated Substance Handled
Primary Activities: Wastewater treatment.
Use of Regulated Substances:
1) Chlorine is used primarily as a disinfectant. Other possible uses of chlorine are for odor control and to assist in settling of biosolids in the clarification process.
2) Digester gas produced as a result of anaerobic digestion is used primarily for generating power to drive the sludge pumps. Digester gas in excess of what is required for engine generators is flared through waste gas burners.
Quantities Handled or Stored:
1) Chlorine: Total Amount Stored: 16 one-ton containers, equivalent to 36,800 pounds.
2) Digester gas: Total Amount Produced or Stored on a Daily Basis: 228,115 cubic feet (ft3), equivalent to 13,714 pounds of digester gas, containing up to 72 percent by volume of methane.
Worst-case Release Scenario (WRS)
1) Toxic Substance WRS - Chlorine
Failure of one liquid chlorine container resulting
in a release of 2,300 pounds of chlorine gas in 10 minutes. Passive mitigation is not considered because the containers are stored in the open. The release rate of 1.74 kg/s is modeled using DEGADIS 2.1.
Distance to Endpoint: Under the-worst case weather conditions prescribed by the
RMP Rule, the distance to a toxic endpoint of 0.0078 mg/L (or 3 ppm) is 2.59 miles, beyond which there will be enough dispersion that a hazard to the public will no longer exist.
Population Exposed: It is estimated, based on the USCB population density for Pulaski County, that 9,600 persons would be affected within the WRS toxic endpoint circle.
Environmental Receptors: There are no environmental receptors within the WRS toxic endpoint circle.
2) Flammable Substance WRS - Digester Gas
Fourche Creek WWTP has six primary and two secondary digesters. According to the RMP rule, the greatest amount of digester gas held in any single largest vessel is 4,035 pounds in the secondary digester and the associ
ated piping. The scenario is the failure of the secondary digester gas system and the associated piping, followed by a vapor cloud expansion and catastrophic explosion. Passive mitigation is not considered because the digester is in the open.
Distance to Endpoint: The distance to the flammable endpoint of 1.0 psi overpressure, determined using the RMP Offsite Consequence Analysis Guidance, is 0.10 mile.
Population Exposed: It is estimated, based on the USCB population density for Pulaski County, that 20 persons would be affected within the WRS flammable endpoint circle. In actuality, the release effect would be confined within the facility boundary and therefore no population is affected. The facility has 30 full-time employees who would be affected by the digester gas WRS.
Environmental Receptors: There are no environmental receptors within the WRS flammable endpoint circle.
Alternative Release Scenario (ARS)
Toxic Substance ARS - Chlorine
Release of chlorine from a <-inc
h-diameter pipe connecting the chlorine container to the feed manifold. For a release duration of 20 minutes, the calculated release rate is 0.018 kg/s. The release rate calculation is based on the chlorine density of 2.899 kilograms per cubic meter (kg/m3) at 298 oKelvin (K), container pressure of 7.93 atm, the ratio of chlorine heat capacities of 1.355, and a passive mitigation of 45 percent because of the enclosed scale room. The release rate of 0.018 kg/s is modeled using DEGADIS 2.1.
Distance to Endpoint: Under realistic weather conditions of 3.0 m/s wind speed and a wind stability class of "D," chlorine would travel 0.26 mile or 0.42 kilometer (km) before dispersing enough to no longer pose a hazard to the public. The toxic endpoint of 3 ppm was used for the ARS.
Population Exposed: It is estimated, based on the USCB population density for Pulaski County, that 100 persons would be affected within the ARS toxic endpoint circle.
Environmental Receptors: There are no environm
ental receptors within the ARS toxic endpoint circle.
Flammable Substance ARS - Digester Gas
According to the RMP Rule (40 CFR 68.12b), no ARS is required for digester gas because it is an RMP Program 1 process.
Distance to Endpoint: An analysis of the distance to the endpoint is not necessary because the digester gas storage and supply system is an RMP Program 1 process.
Accidental Release Prevention Program and Chemical-specific Prevention Steps
The LRWWU's Fourche Creek WWTP is in compliance with the Risk Management Program (RMP) Rule, 40 CFR 68. The facility has conducted a comprehensive review of all of the chlorine and digester gas systems, as well as of administrative, technical, and operating and maintenance procedures, in addition to the other required program elements of the RMP Rule. Hazard reviews for chlorine and digester gas were conducted at this facility using a "What If" analysis.
Five-year Accident History
The LRWWU's Fourche Creek WWTP has never had releases of
either chlorine or digester gas resulting in offsite injury or dispersion, or in onsite injury.
Emergency Response Program
The LRWWU's Fourche Creek WWTP has an Emergency Procedures Plan, which coordinates response efforts with the Little Rock Fire Department's HAZMAT team, the police department, and the hospital. Response activities also have been discussed with the Arkansas Office of Emergency Services.
Planned Changes to Improve Safety
It was observed during the hazard review that the LRWWU's Fourche Creek WWTP has the necessary equipment and the operating and training procedures required for the safe operation of the chlorination system. The following recommendations for improving the safety of the system were made:
1. Change the set point for the chlorine detection and alarm system to 3 ppm.
2. Provide a copy of the updated emergency response procedures for chlorine to the Little Rock Fire Department.