City of Arcata Wastewater Treatment Facility - Executive Summary |
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The Arcata Wastewater Treatment Facility, located on the north-eastern shore of Humboldt Bay, California, serves city of Arcata residents (population 15,880) and portions of the unincorporated communities of Fieldbrook and Bayside. The wastewater facility was designed to process an Average Dry Weather Flow of 2.3 MGD and a maximum hydraulic capacity through the primary system of 5.0 MGD. Wastewater treatment is accomplished through solids removal, primary and secondary treatment (oxidation ponds/treatment marshes/enhancement marshes). Secondarily treated wastewater effluent is disinfected using chlorine and dechlorinated using sulfur dioxide prior to discharge to Outfall 001- Humboldt Bay and Outfall 002 - Arcata Marsh and Wildlife Sanctuary. The Wastewater Treatment Facility has been handling chlorine for over 25 years and has never experienced a release. On average, there are 15 (1 ton) chlorine containing cylinders on site. Typically six (6) cylinders are on-line and nine (9 ) in storage. There are also six (6) sulfur dioxide containing cylinders, with two (2) cylinders on-line and four (4) in storage. Following a chemical demand evaluation, staff determined that due to an annual average chlorine consumption of 1.5 - 2.0 tons per week, chemical supplier location (500 miles), delivery delays due to road conditions and effluent disinfection requirements (public health and off-shore commercial shellfish harvesting) the amount of on-site chemical is necessary. Disinfection is accomplished through treated wastewater being inducted with chlorine gas and detained in the chlorine contact basin. Following detention, sulfur dioxide is inducted into the chlorinated wastewater for dechlorination and the effluent is discharged to Outfall 001 (Humboldt Bay) and Outfall 002 (Arcata Marsh and Wildlife Sanctuary). The vaccum operated induction units contain sensors that provide notification upon high vaccum, low vaccum and/or power failure status sensors and ar e connected to an alarm system located at the 'operator's control panel' located within the Plant Operations Building. There are also chlorine/sulfur dioxide leak detector sensors that alarm at the 'operators control panel'. If these alarms are not acknowledged within six (6) minutes, the Arcata Police Dispatch is autodialed and notified, Police Dispatch then page Operator's on-duty for response. The on-call operators are within 30 minute response time of the wastewater treatment plant. As additional backup, there is on-site automatic audible alarm that alerts employees within the Corporation Yard of a gas leak or release and a manually activated audible alarm to instruct employees and visitors to evacuate the premises ~ this alarm is scheduled to be connected to the automated system by September 1999. All alarms have, or will have, UPS's installed by September 1999. Equipment inspections are regularly conducted and recorded by operational staff. In the event of a leak or rel ease, chlorine response trained employees will don SCBA suits prior to entering the staging area. Four (4) SCBA suits are stored in the Plant Operations Building, and two (2) SCBA suits are stored at location .5 mile north of the treatment plant. Prior to engaging in response activities the Arcata Fire Department will be notified for backup, and a minimum of three employees must be present prior to entering the area. Two employees, wearing radios, will respond to the leak and a third employee will stand by to conduct monitoring and external communications with other emergency responder personnel. A windsock and spotlight, visible from a two (2) miles radius, is located on the primary digester top to provide a visible indicator of wind direction and wind intensity. The predominate wind direction flows out of the north west towards Humboldt Bay and agricultural lands. The population center of town is located north and upgradient of the wastewater treatment facility. The Humboldt Bay region typically experiences moderate to cool, wet weather. The EPA has defined the worst-case scenario as a 10 minute release of the largest quantity of a regulated substance from a single vessel or process line failure that results in the greatest distance to an endpoint. The distance to the endpoint is the distance a toxic vapor cloud will travel before dissipating to the point that serious injuries from short-term exposures will no longer occur. Assuming a release takes place at ground level, the toxic endpoint for chlorine is defined as 3 parts per million. This airborne concentration is the maximum airborne concentration below which it is believed that nearly all individuals may be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individuals ability to take protection action. Due to its sharp odor characteristic, chlorine is noticeable even when the amount in the air is small. It is usually possible for persons to leave an area before serious harm is suffered. Inhalation of chlorine causes general restlessness, panic, severe irritation of the throat, and difficulty breathing. Effect Parts Per Million (Chlorine) Detectable Odor 3 PPM Noxiousness 5 PPM Throat Irritation 15 PPM Coughing 30 PPM Death after a few breaths 1000 PPM Results for the worst-case scenario are derived through the use of the EPA's 'RMP Guidance for Wastewater Treatment Plant Reference Table'. Use o f this table indicates that a 10 minute release of a 1 ton chlorine containing cylinder in an urban setting results in a 3 PPM endpoint in a 1.3 mile radius. It should be noted that topography, atomospheric conditions and other site specific variables are not taken into consideration in the endpoint result. Estimated affected population, based upon 1,536 dwelling units multiplied by 2.4 persons/household, is 3,686 persons. Although limited by similar restrictions (topography, atmospheric conditions), the alternative release scenario results are based upon a 60 minute release from a vessel leak releasing 19.6 pounds chlorine per minute. EPA's 'RMP Comp' distance to endpoint result 0.1 miles. There are no residential structures within the wastewater treatment plants' proximity for 0.5 miles. Estimated affected population is 0. In 1995 staff investigated and evaluated disinfection alternatives to chlorination/dechlorination. Methods evaluated included ultraviolet (UV) disinfection , ozonation and sodium hypchlorite. Due to wastewater effluent characteristics, NPDES permit requirements and financial costs for system design, modification and operations, disinfection utilizing chlorine was deemed the most viable. Per direction of the Arcata City Council, the 1999-2000 City budget allocates moneys to conduct an alternative disinfection study. The City's Injury Illness and Prevention Plan (IIPP) was adopted in May of 1996, in accordance with the California Code of Regulations and California Labor Code. The IIPP established policies and procedures for, including, but not limited to Confined Space, Hazard Communication, Asbestos, Respiratory Protection, Hearing Conservation, Accident/Incident Reporting, Emergency Evacuation, Medial Emergencies, Code of Safe Practices, General Safety Rules, Tuberculosis Control Program and Bloodborne Pathogens, Contractor Safety Requirements and Inspections. It the policy of the City to establish guidelines and procedures for the maintenance of an on-going IIPP. To fulfill the goal of providing a safe working environment, the IIPP is administered by the City Manager who reviews federal and state statutes and/or regulations relating to safety and risk management, coordinates and implements a comprehensive Safety and Loss Prevention Program. This is accomplished through safety and health inspections, accident investigations, employee training, and designated departmental safety representatives. Specific to chlorine response, the City coordinates and participates in twice per year chlorine response training drills with the Arcata Fire Department and neighboring wastewater facilities. Following training and review, operational and response procedures are revised where necessary. Hot work permits, pre-start up review and revision of the Process Hazard Analysis procedures will be conducted by September 1999. In addition to the federal RMP, the City of Arcata participates in the California Accidental Releas e Program, administered through the Humboldt County Department of Environmental Health (707) 445-6215. |