Sugar Land Regional Sewerage System - Executive Summary
BRAZOS RIVER AUTHORITY |
SUGAR LAND REGIONAL SEWERAGE SYSTEM
The Sugar Land Regional Sewerage System is an innvoative and cost-effective regional sewerage system that serves the wastewater treatment needs of the City of Sugar Land, Fort Bend County Municipal Utility District No. 13, Nalco Chemcial Company and Imperial Sugar, all located near Sugar Land, Texas. The plant was opened in 1975 and has a system capacity of 3.7 million gallons per day actual average flow (3.9 mgd is the current six month average) ; 6.0 million gallons per day permitted average flow; and 17.39 million gallons per day permitted peak flow. Plant awards include the 1993 Operation and Maintenance Excellence Award by the Texas Water Commission for the "Best Wastewater Facility in the State of Texas for Category of Medium Secondary Treatment, 1993 EPA Region 6 Regional Administrator's Environmental Excellence Award for Excellent Wastewater Treatment Operations and Maintenance, 1992 Texas Water Commissio
n Operation and Maintenance Excellence Award for the Best Wastewater Facility in the State of Texas for the Category of Medium Secondary Treatment, and a Certificate of Appreciation from EPA Region 6 in recognition of the Authority's contributions to the Region's Beneficial Sewerage Sludge Use Program and the Authority's leadership in promoting the beneficial use of sludge.
Only chlorine and sulfur dioxide, used to disinfect the water, are stored above the allowable threshold quantity.
Chlorine is used in two independent processes at the treatment plant. These processes are disinfection of the treated effluent flow and the partial disinfection of the water stored in the two final clarifiers.
In the treatment plant effluent, chlorine is used to destroy disease-causing organisms. It is essential that all plant effluent have a chlorine residual of at least 1.0 mg/l after a contact time of 20 minutes. The chlorine is fed at a rate to equal the demand needed to destr
oy any pathogens present, or the demand plus the required minimum 1.0 mg/l residual required by the Systems discharge permit.
Chlorine is stored in six, one-ton cylinders with two cylinders connected to a header pipe. The header pipe in equipped with an automatic switch-over valve that allows chlorine to be fed from one cylinder until it is empty and then switches to the other connected cylinder. The remaining 4 cylinders are used to replace the emptied cylinders. Each cylinder connected to the common header has a separate cylinder mounted vacuum regulator that requires a constant vacuum source to operate. If the vacuum is reduced or lost the regulator will close shutting off the delivery of chlorine gas. The chlorinating equipment utilizes automatic controls that receive signals from the effluent flow metering equipment and pace the volume of gas delivered with the effluent flow volume.
Induction units provide the vacuum required to keep the regulators open and to pull the ch
lorine gas to the point of application. These induction units create a vacuum through a motor-driven open propeller pump unit located at least 24 inches below the minimum water surface. This pump also provides mixing of the chlorine gas with the treated wastewater. The chlorine feed rate is initially adjusted manually to obtain the desired residual concentration utilizing rotometers. Once adjusted, the rotometers are automatically paced or adjusted based on the signal received from the effluent flow metering equipment. From time to time, manual adjustments are necessary to compensate for variations in water quality. There are 4 chlorine contact basins, all constructed of concrete. Each basin can be placed into operation independently, based upon the facilities need to achieve the required 20-minute detention time.
In addition to application of chlorine at the contact chambers, a chlorine solution feed system is installed at the final clarifier structures. This feed system chlor
inates the effluent from these structures and controls algae growth within the effluent trough of the structures. Two, one-ton chlorine cylinders are located in this area. One cylinder is connected to the feed system and the other is for spare inventory. This chlorine feed system conveys the chlorine gas by vacuum created by passing water through an injector. The injector also mixes the gas with the vacuum source water creating a chlorine solution that is distributed to the final clarifier through a >" perforated plastic pipe.
The concentration of the chlorine residual measured at the effluent weirs of the chlorine contact chambers is used to determine the sulfur dioxide dosage at the effluent structure. The plant effluent flow must be dechlorinated to less than 0.1 mg/l before is can be discharge to the receiving stream.
The sulfur dioxide feed system operates the same as the chlorine feed system with the exception that sulfur dioxide is stored in two, one-to
n cylinders, each operating as its own group and two additional one-ton cylinders for backup inventory. As with the chlorine system, each group is connected to a common header that is mounted on anchored supports. With the exception of internal seals, within the regulator and rotometer all of the equipment used for sulfur dioxide feeding and induction are identical to the equipment used for chlorine induction.
Sulfonation, the process of sulfur dioxide reducing 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 following the chlorine contact chambers.
Chlorine and Sulfur Dioxide Leak Detection Equipment
Chlorine and sulfur dioxide gas feeders and rotometers are wall-mounted in a concrete block structure equipped with leak detection equipment. This detection equipment continuously monitors the atmosphere within th
e structure and adjacent to the storage cylinders. Should a leak occur an audible and visual alarm is activated warning staff of the leak. Additionally, this alarm is sent to the on-site SCADA system sounding another alarm and recording the event. Sufficient valving has been provided so the system can be shut down should a leak occur anywhere from the feed source or cylinder, to the point of use at the end of the chlorine contact chambers.
Accidental Release Prevention and Emergency Response Policies
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, smei-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 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 105 degrees F. No passive mitigation measures are in place. Therefore, the distance to the end point for chlorine vapor is 1.3 miles in an Urban 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 Waste Water 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. No passive or active mitigation measures are in place. Therefore, the distance to
the end point for chlorine vapor is 0.1 miles in an Urban environment. The very same conditions apply to Sulfur Dioxide. According to Exhibit 4-19 (Sulfur Dioxide Vapor), the release rate is 7 lbs/min, so the distance to the end point for sulfur dioxide vapor is also 0.1 miles in an Urban environment.
Accidental Release Prevention Program
The 1-ton Chlorine and Sulfur Dioxide cylinders are stored beneath a specially constructed dock/storage building. This building has a roof and metal siding extending to all but 4 feet from the floor to create a natural breezeway. The ends of the structure are open to allow loading and unloading of containers. In the case of an accidental liquid spill, drains are provided beneath the drums. The liquid will drain to our main lift station and be processed with incoming sewerage.
Both the chlorination and sulfonation systems are equipped with automatic switchover devices. When gas pressure is reduced in one drum indicating that it is empt
y, it automatically switches over to the full or standby drum that has previously been hooked up and properly tested for leaks. Overhead vacuum piping goes from the cylinders overhead into the Chlorine and Sulfur Dioxide Feed Rooms - here feed rates can be adjusted as necessary to meet permit requirements. Fresh air is drawn into each room, which is equipped with exhaust fans which pulls air out from the floor level.
A "Multipoint / Multigas Gas Detector" is installed to constantly monitor for both Chlorine and Sulfur Dioxide gas leaks. Four sensor modules are installed to monitor gas leaks at the following locations
1. Chlorine Sensor Module inside the Chlorine Feed Control Room.
2. Chlorine Sensor Module between the two one ton cylinders located on the scales inside the Chlorine Dock / Storage area.
3. Sulfur Dioxide Sensor Module inside the Sulfur Dioxide Feed Control Room.
4. Sulfur Dioxide Sensor Module between the two one ton cylinders located on the scales inside the
Sulfur Dioxide Dock / Storage area.
Alarms at a remote receiver are both visual and audible and the System's SCADA also receives the same alarm(s) which are manifest via an alarm printer, visual, and read-out modes.
The vacuum piping proceeds from the Chlorine and Sulfur Dioxide Feed Rooms underground with the Chlorine vacuum piping routed to the Influent Chlorine Contact Basin and Sulfur Dioxide vacuum piping routed to the Effluent Contact Basin. The vacuum piping is terminated at a Chemical Induction Unit that creates high vacuum as water passes through a venturi and the turbulence created by a recessed impeller in the Unit contributes to complete chemical mixing with the water. At any point along the vacuum lines (above or below ground) where the lines may be broken, it breaks the vacuum and a spring-loaded inlet safety valve on the vacuum regulator immediately closes and isolates the pressure gas supply.
In addition to application of chlorine and subsequent dechlorin
ation using Sulfur Dioxide at the Effluent Contact Basins, a chlorine solution feed system is installed at the final clarifier structures. This feed system chlorinates the effluent from these structures and controls algae growth within the effluent trough of the structures. Two, one-ton chlorine cylinders are located in this area within a cement containment structure - the structure is open with no roof or walls. The containment was constructed in order to minimize the surface area evaporation rate in the case of a liquid chlorine spill. The drum storage containment is located just off an access road so a "vehicle crash guard" has been installed to prevent potential damage to the cylinders in the event where a vehicle could veer off the access road.
One cylinder is connected to the feed system and the other is for spare inventory. This chlorine feed system conveys the chlorine gas by vacuum created by passing water through an injector. The injector also mixes the gas with the va
cuum source water creating a chlorine solution that is distributed to the final clarifier through a >" perforated plastic pipe. This system omits the Chlorine Feeder and rates are manually adjusted directly via the drum mounted vacuum regulator. If at any time the vacuum lines are broken, it breaks the vacuum and a spring-loaded inlet safety valve on the vacuum regulator immediately closes and isolates the pressure gas supply.
Every time a new cylinder is installed and tested for leaks using ammonia gas the exposed vacuum line is tested as well.
All areas where Chlorine or Sulfur Dioxide is stored are adequately posted with warning signs indicating the presence of these hazardous gases. All above ground piping that convey these gases are clearly labeled for easy tracing in the event of a chemical spill or leak.
Five-Year Accident History
The plant has not had any accidental releases of regulated chemicals in the last five years.
Emergency Response Program
as the capability to respond to and mitigate the release of on-site hazardous chemicals that present a moderate health and/or safety threat to SLRSS 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 Sugar Land's and the City of Houston's HAZMAT teams will respond by calling 911. Training and exercises are conducted periodically.
Planned Changes to Improve Safety
A leak detection system will be installed utilizing our existing "Multipoint / Multigas Detector" with funds appropriated in the FY2000 Budget for the Clarifier Weir Chlolrination System.