City of Spokane AWTP - Executive Summary
EXECUTIVE SUMMARY |
The US EPA's Clean Air Act Amendments of 1990 included provisions that require facilities that use certain substances to develop a plan to reduce the likelihood of an accidental release of the substances to the atmosphere and reduce the likelihood of serious harm to the public and the environment. The plan must also include mitigation measures to reduce the potential impact on the public in the unlikely event of a release. The requirements of this plan, commonly referred to as a Risk Management Plan (RMP), are detailed and codified in 40 CFR Part 68.
The City of Spokane's Advanced Wastewater Treatment Plant (SAWTP) stores Chlorine, Sulfur Dioxide and Digester gas in quantities above regulatory thresholds at which a Risk Management Plan (RMP) is required. Chlorine is used to disinfect treated effluent from the plant and Sulfur Dioxide is used to dechlorinate the effluent stream before it is discharged to the Spokane river. Digester gas is generated
as a byproduct of solids stabilization (anaerobic digestion) and mainly composed of Methane and Carbon Dioxide.
SAWTP is subject to program three of the RMP, the strictest level because a worst case release could affect the public and the environment.
The RMP for SAWTP consists of three parts:
1) Hazard Assessment (assessment of the potential effects of a toxic chemical release or gas explosion),
2) Accidental Release Prevention Program (a process safety program that lessens the likelihood of a toxic chemical release or explosion), and
3) SAWTP Emergency Response Program (a planned response to minimize consequences of a toxic chemical release or explosion).
A Hazard Assessment was performed to determine the effects a release would have on the public. For chlorine and sulfur dioxide, the distance a set endpoint concentration of the gas would travel must be determined. For digester gas, the effects of an explosion must be determined. In addition, an e
stimate of the population that could be affected by a release of chlorine, sulfur dioxide, or digester gas was determined, and sensitive receptors such as hospitals, schools, and nursing homes were identified. The Hazard Assessment considers two release scenarios-a "worst case" and an "alternative case."
The regulations require the development of a worst-case release scenario based on conservative assumptions. For example, it is required to assume that the entire contents of the largest single container of chlorine or sulfur dioxide will be released over 10 minutes. This is unlikely to occur since the properties of chlorine and sulfur dioxide would cause a freeze and thaw cycle to occur at the leak, which would slow the release. In addition, only "passive" mitigation methods such as buildings or dikes can be considered when determining the distance the release could travel. Passive mitigation, as defined, requires no mechanical, electrical, or human input.
However, in many scenarios mitigating the release by isolating the process could reduce the amount released. In addition, the worst-case scenario requires that conservative atmospheric conditions be assumed which results in a large area of impact.
The hazard assessment requires that the "toxic endpoint" or distance from the point of release to a location at which the chemical concentration equals or exceeds a certain concentration must be determined. That concentration is defined as the maximum airborne concentration below which individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects, or symptoms that could impair an individual's ability to take protective action. Because the cloud from a chlorine or sulfur dioxide leak would disperse relatively quickly, an individual at a toxic endpoint would be exposed to the exposure limit concentration for much less than the 1 hour assumed by the limit. In addition, the exp
osure limit concentrations result in relatively minor health effects. Therefore, an individual at the toxic endpoint would be affected less than the results of the worst-case scenario may imply.
Flammable worst case for Digester Gas (Methane)
The "worst case" scenario for flammable substances assumes an explosion occurs under the most conservative of assumptions. The entire contents of an anaerobic digester and the methane gas sphere which contain above the threshold of 10,000 pounds of methane are assumed to simultaneously be ignited and explode. This is very unlikely to occur since the proper mixture of methane and oxygen required for an explosion would be difficult to achieve. In most cases, the anaerobic digester is at least one half full of liquid which would reduce the quantity of methane and there are passive mitigation devices such as flame arresters, condensate traps, and regulators which would greatly reduce the likelihood of an explosion. As in the case of the toxic
"worst case" scenario, the distance to the area affected has to be established. The impact zone for a digester gas explosion is defined by "1-psi over pressure." The 1-psi over pressure is outside the explosion shock wave, along which structural damage and broken glass is possible.
The RMP rule also requires that at least one alternative release scenario be evaluated for sulfur dioxide, digester gas, and chlorine. The alternative scenarios reflect a type of release that is more likely to occur compared to the worst-case scenario. Unlike the worst-case scenario, the alternative release scenario may consider "active" mitigation such as shutoff valves and a more realistic release quantity and release rate. Active mitigation is defined as requiring mechanical, electrical, or human input. Lastly, it assumes local, typical meterology, which is more realistic than the conservative meteorological conditions that must be assumed for the worst-case scenari
o. These alternative release scenarios are considered to be more representative of the effects likely in the event of a release.
Toxic alternative scenario for Chlorine and Sulfur Dioxide
The "alternative" release for Chlorine and Sulfur Dioxide at SAWTP is the failure of a fusible plug on a Chlorine or Sulfur Dioxide cylinder or a break in the line connected to the tank close to the tank creating a 3/8" hole for escape of contents.
Flammable alternative scenario for Digester Gas (Methane)
The "alternative" scenario for digester gas is a flame out of one of the methane fired boilers, used to heat the digesters and subsequent ignition causing an explosion of the digester gas sphere. This scenario although only remotely possible, is considered to be more likely than the "worst case" scenario. The quantity of digester gas involved and the weather conditions present are also considered to be more likely than the "worst case" scenario.
Accidental Release Prevention Program
The City of Spokane has developed a comprehensive approach to chemical safety that has resulted in few onsite incidents and no off site consequences. Employees receive annual training in areas such as employee right to know, respiratory protection, confined space entry, personal protective equipment, material handling, fire prevention, hazardous substances, emergency response and lock out tag out. All SAWTP Operators and Senior Maintenance Mechanics have been trained to Operations Level Hazardous Materials Awareness (OSHA 29 CFR 1910.120(e)) and all other personnel at SAWTP have been trained to Awareness Level. SAWTP management will train all its employees in accordance with regulatory requirements to ensure that training is current and complete including refresher process operation and emergency response training.
In addition to the mandatory training listed above, SAWTP has written operating procedures for Chlorination, Dechlorination and Digeste
r gas processes. Operating procedures include safe work practices in order to startup, shutdown, operate under emergency conditions to prevent accidental releases and manage hazardous situations if they occur. Operating procedures that provide for safe operation of processes and caution of health hazards are also used to train new employees and to provide refresher training for existing staff.
Hot work permits are a good example of safe work practices to be performed in on or around the three processes that are the focus of the RMP. Hot work is defined as the use of torches , welding equipment, grinding, cutting, brazing or other spark or flame producing work. Hot work permits will be required for SAWTP personnel and contractors working on affected equipment on site. During the process of completing a hot work permit, the applicant must identify the hazard, determine appropriate safeguards, and then initiate and maintain the safeguards throughout the project.
employees have operated the affected existing processes for several years, they will become thoroughly familiar with all Risk Management Plan elements and comply with terms listed therein.
Safety Information/Hazard Analysis
One of the major reasons facilities such as SAWTP develop RMPs is to keep Local Emergency Planning Committees(LEPCs) appraised concerning process chemicals, technology and equipment stored and used onsite. LEPCs use the information to develop training programs and response procedures or as resource materials. The information will also be supplied to employees and contractors working onsite.
Process Hazard Analysis (PHA) was conducted to evaluate the potential causes and consequences of an accidental release of toxic materials such as Chlorine and Sulfur Dioxide or a flammable material such as digester gas (Methane). Information gained from PHA is used to improve safety procedures and potentially reduce the consequences of a hazardous release. Changes to proc
edures will be incorporated into the overall process safety management program.
Onsite Contractors and Vendors
Contractors and vendors performing work at the treatment plant will be made aware of the hazards of Chlorine, Sulfur Dioxide and digester gas. They will also be made aware of the emergency response plan and the correct emergency response procedures in the event of an emergency.
SAWTP Emergency Response Policies
The Emergency Response Program develops a plan for dealing with a release. OSHA Process Safety Management regulation 29 CFR 1910.119(n) and EPA RMP regulation 40 CFR 68 Subpart E require that an Accidental Release Emergency Response Plan be prepared. The plan must be prepared in accordance with the provisions of another overlapping OSHA regulation-Employee Emergency Plans (29 CFR 1910.38(a)). In addition, provisions of the OSHA hazardous waste and emergency response standard, 29 CFR 1910.120(q), must also be considered. The Emergency Planning and Response
Plan described in this section complies with the requirements of 40 CFR 68.95, 29 CFR 1910.38(a), and 29 CFR 1910.120(q).
The Emergency Planning and Response plan provides specific emergency response procedures for accidental releases of chlorine, sulfur dioxide, or digester gas. The emergency response procedures cover a release from the initial alarm stage through either leak stoppage or hazmat assistance. As part of the emergency response procedures there are plans for victim rescue, leak investigation, and communication with additional support agencies. In addition, critical plant operations are identified to insure that, if possible, the critical SAWTP functions are kept operational. The Emergency Planning and Response plan also indicates the level of training need to carry out the emergency response procedures. Only personnel trained at the Hazardous Materials Technician Level can attempt to stop a leak. The emergency response procedures refers to "The City of Spokane Adva
nced Wastewater Treatment Plant Emergency Response Plan" in cases when plant evacuation is necessary.
Information regarding self-contained breathing apparatus is also provided in the Emergency Planning and Response plan. It also addresses plant site communication, emergency response equipment, first aid and medical treatment, medical surveillance and consultation, and emergency response drills.
Emergency response policies were developed to serve as a framework for organized reaction to events that cause emergencies such as hazardous chemical releases and explosions. Emergency procedures in effect at SAWTP are in compliance with EPA RMP Accidental Release Emergency Response Planning and OSHA Employee Emergency Response Plans.
Emergency response policies in effect at SAWTP detail steps in response to an accidental release of Chlorine and Sulfur Dioxide or a chemical explosion involving digester gas. Policies and plans cover the response steps from initial alarm and evacuation, to s
ite control when waiting for hazmat assistance. Other information included in the emergency response program is site communication, inventory and location of emergency response equipment, equipment needed for medical treatment and first aid, methods to obtain medical assistance and emergency response practice drills.