Brushy Creek Regional Water System - East - Executive Summary |
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Lower Colorado River Authority and the Brazos River Authority Brushy Creek Regional Wastewater System - East The Brushy Creek Regional Wastewater System - East is an innovative and cost-effective regional sewerage system that serves the wastewater treatment needs of the urban areas in and around the city of Round Rock, Texas. The plant began expansion to a regional facility in 1995 and was completed in 1997. It has a system capacity of 11.8 million gallons per day permitted average flow; and 33.5 million gallons per day permitted peak flow. Its current average daily flow is approximately 8.0 million gallons per day. Only chlorine and sulfur dioxide are stored above the allowable Threshold quantity. Chlorine Chlorine serves to destroy disease-causing organisms in the Treatment Plant effluent. It is essential that all plant effluent have a chlorine residual of at least 1.0 mg/l after 20 minutes contact. The recommended dose for application to effluent from the ac tivated sludge process is eight mg/l. The chlorine is stored in 8 one-ton cylinders and divided into four groups of two cylinders each. One group is in use, the second is held as backup and the third and forth groups are held in reserve. The second group is engaged by an automatic switchover module when the first group is exhausted. Each group is connected to a header that is provided with a separate vacuum regulator, which reduces the gas from a varying supply pressure to a constantly regulated vacuum. Chlorine gas feeder cabinets are wall-mounted in the Chlorination Room. Each feeder is equipped with a manual rate control valve located on and adjusted from the front of the cabinet as well as an automatic control valve, operated by PLC. Sufficient valving has been provided to allow different feeders to serve different junction boxes. Induction units provide the vacuum required to pull the chlorine gas to the various points of application. These induction units create a vacuu m through a motor-driven open propeller, located 24 inches below the minimum water surface, which also provides mixing of the chlorine gas with the wastewater and feeds directly into the Chlorine Contact Chambers. Chlorine feed rates are adjusted by the automatic rotameters in response to changes in the flow rate. There are two chlorine contact chambers at the Brushy Creek Plant. Each chlorine contact chamber is capable of detaining a flow of 11.8 million gallons per day (mgd) for 20 minutes, but the chambers are able to detain a peak flow of around 34 mgd when required. Sulfur Dioxide The concentration of the chlorine residual measured at the effluent weirs of the Chlorine Contact Chambers is used to determine the amount of sulfur dioxide dosed at the Effluent Structure. The plant effluent flow is to be dechlorinated to less than 0.1 mg/l for discharge. Sulfur dioxide is stored in 4 one-ton cylinders, also connected to separate headers that are mounted on anchored support s. Almost all of the equipment used for sulfur dioxide feeding and induction, with the exception of the rotameters and some materials of construction, is identical to the equipment used for chlorine induction. Sulfur dioxide gas feeders, or manual rotameters, are wall-mounted. Each rotameter is equipped with a manual and automatic rate control valve. Three of these rotameters have been supplied, each capable of delivering the maximum recommended dose of sulfur dioxide. Sufficient valving has been provided so that either induction unit for this system may be fed through any rotameter. Sulfonation (the process of sulfur dioxide reduction of the chlorine residual) takes place almost instantly. For this reason, it is not necessary to provide a contact chamber for detention of the flow. Sulfur dioxide is introduced into the flow at the Effluent Structure. Accidental Release Prevention and Emergency Response Policies Authority policy requires a written emergency action plan, a nnual exercises and reviews of the plan, emergency response training for facility personnel, and coordination with local responders. In addition, semi-annual safety inspections are conducted by the Authority's Safety Officer and risk management consultants from our insurance company. Worst-Case Release Scenario The worst-case scenario is the release of 2,000 pounds of Chlorine (one-ton cylinder) in 10 minutes. The release rate is reduced to 110 lbs./min. because the cylinders are stored inside a brick building as a passive mitigation measure. The EPA rules specify an atmospheric stability of F and a wind speed of 1.5 meters per second. The maximum temperature in the last three years was 102 degrees F. Therefore, the distance to the end point for chlorine vapor is 0.9 miles in an Urban environment. Alternative Release Scenarios The plant did not have any reportable accidental releases of regulated chemicals in the past five years so we chose to analyze a broken 5/16" va lve/pipe connection at the vessel interface for both Chlorine and Sulfur Dioxide. A vapor release was chosen because the plant does not use liquid chlorine or sulfur dioxide. EPA's RMP Guidance for Wastewater Treatment Plants Reference Tables specify an atmospheric stability of D and a wind speed of 3 meters per second. According to Exhibit 4-15 (Chlorine Vapor), the release rate is 15 lbs./min. Chlorine is stored in a building so the release rate is reduced by 55% for a revised release rate of 8.3 lbs./min. An automatic scrubber system further reduces the release rate to nearly zero. Therefore, the distance to the end point for chlorine vapor is less than 0.01 miles in an Urban environment. The very same conditions apply to Sulfur Dioxide. According to Exhibit 4-19 (Sulfur Dioxide Vapor), applying both active and passive mitigation measures, the distance to the end point for sulfur dioxide vapor is also less than 0.01 miles in an Urban environment. Accidental Release Preven tion Program The chemical storage area is enclosed and equipped with the latest technology to contain and scrub any chemical releases. Plant personnel are trained to handle chemical containers in such a manner as to prevent leaks or spills while connecting or disconnecting containers. Personnel are also trained to shut down the system by closing the cylinder valve in the event of an emergency or anticipated emergency. Five-Year Accident History There were no reportable accidental releases of regulated chemicals in the last five years. Emergency Response Program Brushy Creek - East has the capability to respond to and mitigate the release of on-site hazardous chemicals that present a moderate health and/or safety threat to Brushy Creek - East and responding personnel, present a moderate potential for adverse environmental impact, are of limited size, and have low fire/explosion potential. If the spill/release is beyond the capability of on-site personnel, the City of R ound Rock's and Williamson County's HAZMAT teams will respond by calling 911. Training and exercises are conducted periodically. Planned Changes to Improve Safety No changes are planned at this time |