Dallas Water Utilities Southside WWTP - Executive Summary
Risk Management Plan |
The City of Dallas Water Utilities maintains a policy of safe water and wastewater treatment plant operation, reducing to the greatest extent possible any hazards associated with the treatment processes, and reducing any subsequent risk to the surrounding community, their employees, and the environment. They cooperate with the surrounding community and local emergency response agencies to promote effective contingency planning in the unlikely event that a process incident occurs.
The primary activity at the Southside Wastewater Treatment Plant (WWTP) is municipal wastewater treatment.
Use of Regulated Substances
7 Chlorine is used primarily as a disinfectant, but may also be used for odor control, and to assist in settling of solids in the clarification process.
7 Sulfur Dioxide is used in the dechlorination process to remove residual chlorine from the treated water prior to discharge to the Trinity River.
7 Digester Gas is a bypr
oduct of anaerobic digestion process, which destroys pathogens and reduces the volume of solids produced during the wastewater treatment process. The digester gas contains methane, which is flammable, and is recycled in the process to provide the heat that is required for the anaerobic digesters. In addition, 75 percent of the buildings at the plant are heated utilizing digester gas. Excess digester gas is flared.
Quantities Handled or Stored
7 Chlorine: 150,000 pounds
7 Sulfur Dioxide: 86,000 pounds
7 Digester Gas: 14,000 pounds
Components of the Risk Management Plan
The Risk Management Plan (RMP) consists of three major parts. The first part is the Hazard Assessment. The Hazard Assessment is done to determine the effects that a release of a regulated substance could have on the public. The second part is a Prevention Program that consists of seven elements designed to improve the system safety and decrease the likelihood of a release. The third part is the Emergency Response Program,
which develops a plan for dealing with a release in the unlikely event that one would occur.
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 concentration of the gas would travel must be determined. For digester gas, the effects of an explosion must be determined. The Hazard Assessment considers two release scenarios-a "worst case" and an "alternative release." The Hazard Assessment also includes a five-year accident history of Southside WWTP.
The Hazard Assessment must also estimate the population outside of the plant boundary that could be impacted and identify sensitive receptors such as hospitals, schools, and parks. If a release were ever to occur, the gas would migrate primarily in the direction of the prevailing wind. But the regulations require that the release be treated as though it would spread in all directions. In other words, the distance determined by
the Hazard Assessment is only valid for the direction of the wind. Someone located the same distance upwind would not be affected by the release.
The regulations require the development of a worst-case release scenario based on conservative assumptions. For example, the Plan must assume that the entire contents of the largest single container of chlorine would be released over 10 minutes. This is unlikely to occur because the physical properties of chlorine would cause the chlorine to actually freeze over. The frozen chlorine would then have to thaw before further chlorine would be released to the atmosphere. This freeze-thaw cycle would slow the release. Because of the freeze-thaw cycle, the distance the chemical would travel is shorter than the Plan predicts. The same situation applies to sulfur dioxide. In addition, the regulations require that active efforts or "active mitigation" to stop the release cannot be considered. In many scenarios, active efforts
to stop the release would reduce the amount released. However, "passive mitigation" methods such as buildings or dikes can be considered. Passive mitigation, as defined, requires no mechanical, electrical, or human input. The regulations also require that the worst-case scenario assume conservative atmospheric conditions that result in a larger area of impact.
The hazard assessment requires the determination of the "toxic endpoint." The toxic endpoint is the distance from the point of release to a location at which the chemical concentration is equal to a certain concentration. That concentration is defined as the maximum airborne concentration below which individuals could be exposed for up to one 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 leak would disperse relatively quickly, an individual at the toxic endpoint woul
d be exposed to the exposure limit concentration for much less than the one hour assumed by the regulations. In addition, the exposure limit concentrations at the toxic endpoint result in relatively minor health effects. Therefore, an individual at the toxic endpoint would be less affected than the results of the worst-case scenario would imply. Applying these assumptions to the Southside WWTP, computer modeling estimated that the distance to the toxic endpoint would be 3.10 miles.
As in the regulations for chlorine, a worst case scenario for sulfur dioxide must determine the distance to the toxic endpoint if the largest vessel released its entire contents over 10 minutes. However, sulfur dioxide and associated process piping and feed equipment at Southside WWTP is stored in fixed tanks located inside a building. The regulations allow a mitigation factor of 0.55 to be applied to release of chemicals stored within buildings. This mitigation factor assumes that the buildi
ng housing the sulfur dioxide would suppress the release rate by 55 percent. Computer modeling estimated that the distance from Southside WWTP to the toxic endpoint would be 2.30 miles.
For the digester gas worst-case scenario, it must be assumed that all the gas in the vessel containing the largest quantity of digester gas is catastrophically released, and that the gas is ignited and explodes. This is very unlikely to occur since the proper mixture of methane and oxygen required for an explosion would be difficult to achieve. The distance to the end of the impact zone for a digester gas explosion is defined by "1-psi overpressure." The 1-psi overpressure is the outside of the explosion shock wave, along which broken glass and other similar structural damage is possible. Calculations for digester gas at Southside WWTP showed that even the impacts of a worst-case scenario explosion would only travel 475 feet.
Five-year Accident History
Under the RMP regulations, a five-year
accident release history must be reported for regulated substances exceeding the threshold quantity. The release must have caused at least one of the following: on-site or off-site death(s), injuries or significant property damage, off-site environmental damage, evacuations, or sheltering in place.
There was one reportable accident within the past five years at Southside WWTP. A pressure relief valve vented, exposing a plant staff member to chlorine gas. There were no off-site injuries, damage, or evacuations resulting from the incident. Although the RMP submit form illustrates a 1 pound release occurring over 1 minute, the release was less than indicated. A value less than one cannot be entered in these fields.
The RMP regulations defines three Program levels based on the processes' relative potential for public impacts, the level of effort needed to prevent accidents, and coverage of other regulations. To qualify for Program 1, a worst-case release must have no
public receptors within the distance to the toxic endpoint and must have had no accidents with offsite consequences within the past five years. Program 1 imposes limited hazard assessment and emergency response requirements. Any process not eligible for Program 1 and is subject to OSHA's PSM standard is subject to Program 3, which imposes OSHA's PSM standard as the prevention program as well as additional hazard assessment, management, and emergency response requirements. Process not eligible for Program 1 or subject to Program 3 are placed in Program 2, which imposes prevention program requirements, as well as additional hazard assessment, management, and emergency response requirements.
Southside WWTP chlorine and sulfur dioxide facilities would result in off-site consequences in a worst-case release and are not subject to OSHA's PSM standard. Therefore, the chlorine and sulfur dioxide processes fall under Program 2. However, the digester gas worst-case release would not result in of
f-site consequences and has not had a reportable accident within the past five years and therefore, is eligible for Program 1. Because the digester gas is subject to Program 1, the process is not subject to an alternative release scenario or prevention program elements.
The RMP regulations require that at least one alternative release scenario be evaluated for each regulated toxic substance (chlorine and sulfur dioxide). 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 actions to stop or contain the release and a more realistic release quantity and release rate. Lastly, the alternative release scenario assumes local, typical meteorology, which is more realistic than the conservative meteorological conditions that must be assumed for the worst-case scenario. The alternative release scenarios are considered more rep
resentative of the effects likely in the event of a release
Chlorine and Sulfur Dioxide
Because the chlorine and sulfur dioxide systems are very similar, the same scenario was used for both substances. The scenario assumes liquid is isolated in the evaporator pressure container and subsequently heated by the evaporator bath. The liquid in the pressure container heats up and the pressure rises. The excess pressure is released through a pressure relief valve and vented to the roof. Because the releases are vented out of the buildings, no building mitigation was considered for chlorine or sulfur dioxide.
Although the RMP submit form, section 3, part 3.7, states the releases occurred over 1 minute, the releases only lasted approximately 5 seconds each. The RMP submit software will not allow a value less than one minute entered. Computer modeling predicted a toxic endpoint distance of 0.61 miles and 0.52 miles for chlorine and sulfur dioxide, respectively.
Program consists of seven elements designed to improve the system safety and decrease the likelihood of a release.
The RMP regulations require that information concerning process chemicals, technology, and equipment be compiled as part of an RMP program. Emergency response planners can use such information to develop training programs and procedures, or as a general resource. The information will be supplied to contractors who will work in the chlorine process area. All the required process safety information was compiled as required by the RMP regulations.
A hazard review was conducted systematically to evaluate potential causes and consequences of accidental releases. This information was used by Southside WWTP staff to improve safety and reduce the consequences of accidental releases. The chlorine and sulfur dioxide Hazard Review was conducted by CH2M HILL with the assistance of Southside WWTP staff familiar with the process operation and maintenanc
e and plant management. The review was done using the "What-if" method. Based on the results of the review, numerous changes in operating, maintenance, and other process safety management procedures that would improve the overall safety of the chlorine and sulfur dioxide systems were identified. The changes that most effectively help prevent an accidental release have been or will in the future be implemented.
Operating procedures for the chlorination and dechlorination processes have been developed. Written operating procedures assure continuous, efficient, and safe operation of the facility. The goal of the operating procedures is to provide clear instructions for the safe operation of the process. Operating procedures are also used to train new employees and to provide refresher training for existing staff.
The detailed operating procedures include startup, shutdown, and emergency operating procedures. The procedures describe how the system should be operated i
n order to minimize the chances of an accidental release. The procedures also emphasize safety considerations during operation and address hazardous situations that can occur and how to correct them.
The U.S. Environmental Protection Agency (EPA) requires that training be included in the RMP. An effective training program can significantly reduce the number and severity of accidental release incidents. A program has been developed with guidelines on how to conduct regular and structured plant training. Training record forms are included with the program for documentation purposes. Elements of the program include training in safety information, operating procedures, maintenance, and emergency response procedures.
Maintenance ensures equipment used to process, store, or handle regulated substances is functioning properly and installed to minimize the risk of releases. An effective maintenance program is one of the primary lines of defense against a release and addres
ses equipment testing and inspection, preventive maintenance schedules, and personnel training. When implemented, a regular maintenance program ensures process equipment performs properly and documentation exists to support safe and reliable operation and maintenance of the process.
Each incident that resulted in or could reasonably have resulted in a catastrophic release of a regulated substance must be investigated. A process to identify the underlying causes of incidents and implement procedures for preventing similar events has been developed. To investigate an incident, an investigation team will be established. As part of the investigation, an incident report will be prepared to recommend system changes to prevent the incident from occurring in the future.
The investigation team should ask questions such as what equipment failed, which people failed, and which material leaked, reacted, or exploded? As part of the incident review, staff actions that may have
contributed to the incident will also be reviewed. A determination will be made as to whether it is necessary to institute additional training for the employees to prevent a similar incident from occurring in the future. The incident investigation report and any changes resulting from the report will be reviewed with all staff members who operate and maintain the applicable system.
The Southside WWTP will complete a compliance audit for the RMP program plan at least once every 3 years. The primary goals of conducting a compliance audit are to verify compliance with RMP requirements and good process safety practices, identify process safety deficiencies and develop corrective actions, and increase safety awareness among plant staff.
Emergency Response Program
The Emergency Response Program develops a plan for dealing with a release. 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 regulated substances. The emergency response procedures cover a release from the initial alarm stage through Hazmat assistance. As part of the emergency response procedures, there are plans for victim rescue, leak investigation, and communication with additional support agencies.
The Emergency Planning and Response plan also addresses plant site communication, emergency response equipment, first aid and medical treatment, medical surveillance and consultation, and emerg
ency response drills.
Planned Changes to Improve Safety
The following recommendations were made as a result of this Risk Management Program. Some of the recommendations are currently under evaluation, design, and construction. Others are being prioritized and budgeted accordingly.
Chlorine and Sulfur Dioxide Systems
7 A once-through emergency scrubber capable of treating the largest container was recommended for the chlorine and sulfur dioxide systems. In addition, the evaporator and chlorine feed rooms must be connected to the emergency treatment system.
7 To minimize the potential volume of accidentally released chlorine or sulfur dioxide, only one tank should be online at a time.
7 Management should contact the Local Emergency Planning Committee (LEPC), in addition to the Fire Department Hazmat team to prepare and coordinate a plan for community evacuation, should it ever become necessary.
7 A fire-sprinkler system, smoke detectors, and additional alarms were recommended. Additional
alarms should be added inside and outside the evaporator rooms at each system.
7 Buildings should be hard-wired to an area of the plant that is staffed 24 hours a day. Leak alarms must be distinct from other alarms.
7 Continuous normal ventilation must be included in the chlorine and sulfur dioxide systems.
7 The storage-tank manifold design for both the chlorine and sulfur dioxide storage tanks should allow discharge-line couplings to be easily disconnected. A lowered grating is recommended.
7 All vent lines on evaporators and chlorine feeders must be connected to the emergency treatment system.
7 Exit doors should have panic hardware installed.
7 Floors should be sloped to a sump to contain a potential liquid leak. Floors drains should be directed to a location within the emergency treatment system that could treat any collected liquid chlorine.
7 More stringent control of site access should be implemented.