Susanville CSD Wastewater Treatment Facility - Executive Summary

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The Susanville Consolidated Sanitary District (SCSD) uses chlorine at the Wastewater Treatment Facility for water treatment, for bacterial and viral destruction, and to control slime and algae growth.  Multiple feed points are provided to control some of the biological activity as well as to achieve disinfection goals.  Disinfection is achieved by withdrawing gaseous chlorine from 1-ton cylinders and feeding through chlorinators (with integral vacuum regulators),  rotameters, and injectors. 
Normally, one cylinder rests on the scale and feeds the system through one pipe manifold.  Up to two cylinders can feed simultaneously through the manifold.  Typically, an additional cylinder is stored at the facility.  An underhung bridge crane is used for handling cylinders.  The chlorine hoist and trolley has a two-ton capacity. 
Feed equipment consists of chlorinators with integral vacuum regulators to meter the gas under vacuum and injectors to create the vacuum and mix the  
gas with water for transport and application.  From the ton cylinder to the vacuum regulator, the system is pressurized. 
Two chlorine leak detectors are provided in the chlorine storage room and two in the chlorination room.  In addition to the alarm annunciator and rotating beacons, warning lights are installed near each entry to the chlorination room and the chlorine storage room. 
SCSD uses chlorine at the Wastewater Treatment Facility in quantities large enough to trigger the Accidental Release Prevention Program.  Chlorine is transported to the facility by truck, stored onsite in one-ton cylinders, and used for treated wastewater disinfection.  Typically one 1-ton chlorine cylinder is in use, and two 1-ton cylinders are stored at the facility. 
The SCSD facility has not had any releases during the past five years that resulted in any injuries, off-site evacuations, or pr 
operty damage. 
The SCSD Wastewater Treatment Facility is subject to OSHA's Process Safety Management (PSM) regulations for all equipment and operations associated with chlorine storage and disinfection systems.  SCSD's PSM programs include the following to minimize the potential for an accidental release: 
  7 Review of the design of all new or significantly modified equipment and controls for the chlorine 
    storage and disinfection systems to ensure they are properly designed and installed. 
  7 Updating of standard operating procedures to include specific information on safety procedures. 
    All procedures must be reviewed and certified annually. 
  7 Initial safety training and 3-year refresher training for all operators and maintenance staff. 
  7 Procedures to ensure that all contractors receive the same safety training that SCSD provides for 
    its own employees. 
  7 Regular inspection of all equipment, monitoring sys 
tems and controls, including documentation of 
    all inspections. 
  7 Prompt corrective action for any non-conforming items identified by the regular inspections. 
  7 Safety reviews conducted prior to system startup, if any equipment or operations are modified. 
  7 Investigation of any incidents that have the potential to have caused chlorine releases. 
  7 Periodic evaluation of the safety records of all outside contractors who work on the Risk 
    Management Plan (RMP)-regulated systems. 
  7 Development of an effective emergency response program. 
  7 Implementation of an employee participation program to ensure that all facility-wide staff are 
    aware of the PSM/RMP program, and are actively consulted regarding safety issues. 
  7 Independent audits of the entire PSM/RMP program every three years. 
Emergency procedures are described in SCSD's Safety Manual.  In the unlikely event of an accidental release of chlorine, the key elements of SCSD's 
emergency response program are as follows: 
  7 All facility staff members are trained in the specific elements of the program. 
  7 Operators are trained and equipped for emergency operations to repair small accidental releases. 
  7 The facility uses a combination of audible alarms and auto-dialer to alert the staff of a potential 
    accident and to conduct in-house communications. 
  7 In the event of a large release, the facility would immediately contact the City of Susanville Fire 
    Department and the Lassen County Office of Emergency Services (LCOES). 
Based on recent safety reviews that were conducted as part of the RMP evaluations, SCSD is considering the following actions to either reduce the likelihood or severity of potential chemical releases: 
  7 Purchase drift pins and punch for fusible plugs on the chlorine cylinders. 
  7 Install tie downs for the active chlorine cylinders. 
The Risk 
Management Plan must assess the downwind impacts of hypothetical accidental releases.  EPA requires facilities to model the distance that a plume of released gas would travel before it dispersed to an ambient concentration equal to the "Toxic Endpoint Concentration."  The Toxic Endpoint Concentrations for various compounds were specified by EPA, and are generally concentrations that would cause no physical harm but could interfere with people's ability to leave the area.  The Toxic Endpoint Concentration for chlorine is 3 parts per million (ppm).  
SCSD conducted a process hazard analysis (PHA), which included an evaluation of potential chlorine release scenarios.  In accordance with EPA's rule, two general types of hypothetical accidental release scenarios were developed: 
  7 The "Administrative Worst-Case Release" that arbitrarily assumes the entire contents of the largest 
    container of chemical is released to the atmosphere in 10 minutes.  SCSD is unaware of any 
e event that could actually cause such a catastrophic release at the facility. 
  7 "Alternate Release Scenario," which is a release that the PHA team concluded has a realistic (but 
    small) chance of actually occurring at the facility. 
Anhydrous liquid chlorine (chlorine gas that is stored as a liquid under pressure at ambient temperature) is imported to the site by truck and stored in a one-ton cylinder for use in the disinfection process.  The Administrative Worst-Case Release Scenario assumes that the entire ton of chlorine is emitted as a gas cloud in 10 minutes, during a period of exceptionally calm winds and stagnant atmospheric conditions (1.5 meter/second wind speed and an atmospheric stability class F) that would result in minimal dispersion of the gas cloud.  The thermodynamic properties of anhydrous chlorine indicate that such a large instantaneous gas release is nearly impossible.  The release of a large quantity of gaseous or  
liquid chlorine from a cylinder would result in rapid chilling of the chlorine, and the formation of liquid or frozen chlorine.  The liquid or frozen chlorine would take much longer than 10 minutes to vaporize into a gas cloud.  Nevertheless, the RMP rule dictates that the Worst-Case Scenario assumes the release of one ton of gaseous chlorine.  
The formulas in EPA's RMP guidance for wastewater treatment plants were used to estimate the downwind impacts for the one-ton chlorine release.  The formulas for chlorine apply to water treatment facilities as well as wastewater treatment facilities.  Urban surface roughness conditions were used to account for buildings and trees in the vicinity of the facility.  The model indicated that the gas chlorine cloud would travel 1.3 miles before it dispersed to the 3 ppm Toxic Endpoint Concentration.  The estimated residential population within a radius of 1.3 miles from the facility is 300 people. 
The followin 
g hypothetical accident was selected as the Alternate Release Scenario: the pigtail tubing on the pressure side of the chlorine cylinder fails, resulting in the release of chlorine through a 3/8-inch diameter hole.  The release is either observed or detected by the chlorine leak sensor.  The valve is closed and cylinder disconnected, and the leak is stopped within 10 minutes.  The release is partially contained by the building, resulting in a reduction of the calculated unconfined release rate. 
Using EPA guidance, the calculated gaseous chlorine release rate is 7.4 pounds per minute (lb/min).  This calculated release rate is conservative because according to the Handbook of Chlorination, for a one-ton cylinder at 68 0F, a chlorine withdrawal rate in excess of 0.28 lb/min (400 lb/day) will lead to a cooling of the system and potential freezing of the cylinder.  Most of the released chlorine will freeze in a pool beneath the leak, and then be released at a much lower rate to the atmosphe 
re.  However, for the purpose of this evaluation, the calculated release rate of 7.4 lb/min was used. 
The formula in EPA's RMP guidance for wastewater treatment plants was used to estimate the downwind impacts for the alternative release scenario.  Urban surface roughness conditions were used.  The model indicated that the gas chlorine cloud could travel 0.1 miles before it dispersed to the 3 ppm concentration limit.  The estimated residential population within a radius of 0.1 miles from the facility is 1 person.
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