Waco Metropolitan Regional Sewerage System - Executive Summary

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  Brazos River Authority 
  Waco Metropolitan Area Regional Sewerage System 
 
 
The Waco Metropolitan Area Regional Sewerage System 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 Waco, Texas.  The plant was opened in 1971 and has a system capacity of 22.0 million gallons per day actual average flow; 37.8 million gallons per day permitted average flow; and 83.0 million gallons per day permitted peak flow.  Plant awards include the Operations and Maintenance Excellence Award in 1993 from EPA Region VI and the Texas Water Commission and the Operations and Maintenance Excellence Award in 1991 from the Texas Water Commission.    
 
Only chlorine and sulfur dioxide are stored above their established threshold quantity.  
 
Chlorine 
 
At any one time, Chlorine maybe stored in as many as 24 cylinders, each containing as much as 2000 pounds of liquid chlorine.  The chlorine is used to disinfect the t 
reated wastewater prior to its discharge to the receiving stream.  The chlorine feed equipment requires that the 24 cylinders be divided into 4 groups of six cylinders each.  Two groups are connected to the feed equipment with the remaining two groups placed in storage.  Of the two groups connected, chemical is drawn from only one group.  The other group is placed in stand-by and is activated by an automatic switchover valve.  This valve is activated upon the depletion of all available chlorine from the first group of cylinders.  Gaseous Chlorine is withdrawn from the cylinders through regulators that require a vacuum to operate.  
 
Upon the loss of vacuum the regulator will close, thus shutting off the chlorine feed. After passing through the regulator, the gas goes through the automatic switchover valve, a flow rate indicator, which can be adjusted manually or automatically.  Finally, the gas passes through the "Chlor-a-Vac" pump.  This pump produces the vacuum that is required for th 
e chlorine feed system to operate.  This pump is submerged in the treated effluent and mixes the chlorine gas with the treated effluent.  This mixing occurs within a concrete junction box prior to the chlorinated water flowing to a series of concrete basins that retain the water for a minimum of 20 minutes.  This 20-minute retention is required by federal regulations and needed to assure the water is adequately disinfected prior to being released to the environment. 
 
When the water leaves the chlorine contact chambers, sulfur dioxide is injected into the water.  The sulfur dioxide reacts with any remaining chlorine residual, dissipating both chemicals before the water is released to the receiving stream.  The sulfur dioxide feed system is essentially the same as the chlorine feed system and is describe below. 
 
Sulfur Dioxide 
 
Eight sulfur dioxide (SO2) cylinders connected to the headers at the Sulfur Dioxide Storage Facility are arranged for active service in two banks of four cylinder 
s each.  Each one-ton cylinder contains 2,000 pounds of SO2.   
 
Each header is provided with a separate vacuum regulator to reduce the gas from a varying supply pressure to a constantly regulated vacuum.  The vacuum regulators are equipped with gas pressure gauges and vacuum-sealing o-rings, to seal off the supply of gas in the event that the supply is exhausted and shut off.  When the exhausted supply is replaced, the fresh cylinder bank becomes the standby supply. 
 
Vacuum is applied to the De-chlorination System by Chlor-a-Vac induction units.  These units are suspended and are submerged to a depth of at least two feet below the minimum water surface.  Induction units create a vacuum with motor-driven open propellers, which also serve to mix the sulfur dioxide gas with the wastewater. 
 
Accidental Release Prevention and Emergency Response Policies 
 
*  Chlorine 
The 1-ton cylinders are stored in a block building with one personnel door on one side and one whole end of the building has s 
liding doors for handling the cylinders with overhead beam trolleys and hoists.  The storage\feed capacity is 12 cylinders, 6 on each side.  Each side is connected to the main header via whip lines.  Each bank is controlled separately by pressure valving and switchover valves to the rotometers and gas feeders.  The chlorine system changes from a pressure to a vacuum system at this point.   Normally one bank of cylinders is feeding and the other bank is either empty or ready to be exchanged with full cylinders or is full and on standby.  The chlorine control room is adjacent to the storage area and has an intervening block/glass wall between the two areas. There is adequate valving to isolate and stop any leaking section of pipe from the individual cylinders to the actual induction unit. 
The control room has a gas detection system with sensors in the control room and the storage room.  The gas detection system sets off an audible and visual alarm in the event of a leak.  Both the storag 
e and control area has ventilation systems.   There is also an open storage/staging area just outside the sliding doors of the storage/feed area. This area will hold 12 cylinders and is where cylinders are unloaded from the delivery truck or empty cylinders loaded on the same truck. 
 
*  Sulfur Dioxide 
The one-ton cylinders are in a storage/feed area enclosed by a metal building with block walls.  The area has room for 16 cylinders, 8 in each bank, but only 4 in each bank are manifold together.  Each bank is separated using pressure valving and switchover valves going into the rotometers and gas induction system.  The system changes from a pressure to a vacuum system here.  There is a dividing block/glass wall between the storage/feed area and the control room area.  There is one walk door for access to each area and a walk door from the control room to the storage/feed area.   There are ventilation systems in both areas and the storage/feed area has one wall consisting of sliding door  
panels for loading and unloading cylinders using overhead trolleys and hoists.  There is a gas detection system in place that has an audible and visual alarm to warn of a gas leak in either the storage/feed areas or the control room.  There is an additional outside storage/staging area adjacent to the large access doors with space for 8 additional cylinders.  This area is used for offloading full and empty cylinders from the delivery trucks. 
 
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 200 lbs./min. 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 104 degrees F.  The cylinders are stored in an enclosure as a passive mitigation measure.  Therefore, the distance to the end point for chlorine vapor is 2.2 miles in a Rural environment. 
 
Alternative Release Scenario 
 
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" valve/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.  The cylinders are located inside an enclosure open to the outside air providing a reduction in the release rate of 55%. Therefore, the distance to the end point for chlorine vapor is 0.2 miles in a Rural environment.  The very same conditions apply to Sulfur Dioxide.  According to Exhibit 4-19 (Sulfur Dioxide Vapor), the release rate is 3.9 lbs./min, so the distance to the end point for sulfur dioxide vapor is also 0.1 miles in a Rural environment. 
 
Accidental Release Prev 
ention Program 
 
Authority policy requires a written emergency action plan, annual exercises and reviews of the plan, emergency response training for facility personnel and coordination with local responders.  In addition, the Authority's Safety Officer and risk management consultants from our insurance company conduct semi-annual safety inspections. 
 
Five-Year Accident History 
 
The plant has not had any accidental releases of regulated chemicals in the last five years. 
 
Emergency Response Program 
 
WMRSS 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 WMRSS 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 Waco's and McLennan County's HAZMAT teams will respond by calling 911.  Training and exercises are conducted pe 
riodically. 
 
Planned Changes to Improve Safety 
 
*  Chlorine 
Currently the Chlorine alarm system is not hooked up to the SCADA system.  As soon as monies and equipment become available the alarms will be hooked up to the SCADA system allowing the alarms to show on the monitors in the operators room.  This will allow a constant monitoring of the system using the computers tied to the SCADA system.  We will also have the capability to monitor feed rates and chlorine usage more closely.  We also plan on relocating the SCBAs.  Currently there is one attached to the outside wall of the chlorine control room.  This location is too close to a chlorine leak if one occurs at the storage/feed room or the control room.  The SCBAs will be relocated a minimum 100 feet up hill from the chlorine rooms. 
 
*  Sulfur Dioxide 
The alarm system for the Sulfur Dioxide storage/feed and control room is not hooked up to the SCADA system.  This alarm system will eventually be hooked up to the SCADA system and wil 
l allow continuous monitoring through the SCADA computers.   The alarms will become a part of the SCADA monitoring system in the operator's room.  This will allow closer attention to the Sulfur Dioxide control/feed system making it possible to monitor feed rates and usage.  The SCBAs will also be relocated away from the SO2 feed/storage area.  Currently they are attached to the outside wall of the SO2 building.  This happens to be down hill from the chlorine storage/feed area also.  These SCBAs will be relocated along with the one from the chlorine building a minimum of 100 feet uphill from the entire Chlorine/Sulfur Dioxide storage/feed area.
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