1998 RMWEA Annual Conference
Dar Barrie, Littleton / Englewood WWTP
Greg Farmer, Littleton / Englewood WWTP
Kirk Petrik, Brown and Caldwell
James Widner, Brown and Caldwell
In 1977 the Littleton / Englewood wastewater treatment plant was placed on-line with conventional gas chlorination and sulfur dioxide to meet dechlorination requirements. In the phase 1-A plant expansion in 1991 a liquid chemical disinfection system was installed as an inexpensive alternative that would alleviate the expenses of meeting the Uniform Fire Code regulations for toxic gases.
This renovation converted the existing gas process area to a liquid chemical system using sodium hypochlorite for chlorination and sodium bisulfite for dechlorination. The new design was intended to last five years and would allow time for continued development of alternate disinfection processes such as UV.
During the design, engineers and plant staff met to develop an operating criteria, and options to be incorporated into the system. This evolved into a very complex piping system with all pumps being able to draw off any tank. Control of this system followed to accommodate these options. In the early 1990’s little was known about corrosion and off-gassing that occurs with pumping sodium hypochlorite. After one year of operation substantial amounts of leaks and other problems occurred. The 1991 system accomplished the design intent for the temporary five-year period. The total cost for the 1991 system installation was $466,000; inexpensive in relation to the other options available at the time.
Operations and Maintenance Action Team:
In 1997, plant staff reviewed the disinfection system due to the failure of the sodium hypochlorite storage tanks, which were now six years old. The system had met its intended life span and the staff had learned a lot about operation of a liquid system.
The plant operations and maintenance divisions formed an action team to identify and correct problems in the operation of the disinfection system. Team members were assigned to research alternatives to correct the problem areas.
To gather information for the system renovation we utilized operator process experience, maintenance repair experience, and the expertise of the chemical supplier, storage tank manufacturers, pump distributors, and the Internet. We found more information was available than we needed. Everything from exploding ball valves to, threaded joint sealing procedures. We then held a brainstorming session to validate the information as it applied to our process situation.
Chemical Supplier Expertise:
As part of the research project we visited the sodium hypochlorite manufacturing facility to see how they operate in a production setting. We gained a great amount of information based on their experiences in system set up and operation. What they put a storage system through in one day of production is equal to one year of process operation at the plant. The trial and error experience of this resource was invaluable to our process system design and operation. This information became the cornerstone of our concept.
This is a summary of validated design information. Combined with five years of liquid system operation and actual trial and error experience in installing and operating our new system.
Keeping all this information in mind the plant philosophy of KISS was developed and the renovation plan was put into action. Keep- It- Simple, "Stupid!"
Tank Selection / Installation:
Our selection was an extreme service, natural colored cross-linked polyethylene, top entry tank, with a single molded outlet flange at the bottom. The tank was set on an optional tank cushion support. The tank is full drain capable with all penetrations for fill, overflow, and vent, field installed with standard PVC bulkhead connectors on the boss areas at the top of the tank. The tank material may be worked with standard woodworking tools. A four inch flanged level measurement device was directly installed over the flange pattern cut, drilled and tapped into the top of the tank.
System Piping and Hardware:
Our system piping used all duo-block valves. The flange at the bottom of the tank received a 3" valve. This is directly connected to a flex isolation device fabricated from 2" PVC barb fittings and 2" PVC hose, connected to a tee where the tank drain and transfer pump connection is made through2" PVC cam-lock connection. The remainder of the system is 1" PVC supply and discharge. Just prior to the carrier water inlet a check valve was installed. The total run of pipe intake and discharge is 15’. Threaded connections were held to a minimum, no matter what you do they will leak.
To pump the sodium hypochlorite we chose a peristaltic pump that accepts a 4>20 Ma control signal and is powered by 120 Volt AC power. The pump is a self-contained unit with the control system that is factory sealed making it wash-down compatible. Should the control system fail, we install a backup unit and send it back to the manufacturer for repair.
Maintenance of the pump consists of periodically replacing the pump tube. To avoid using a pump tube until it fails, a pump revolution totalizer is utilized to signal when a tube needs replaced. Because of its flexibility and reliability we have used this pump in most chemical injection systems within the plant.
Transfer / Emergency Response Pump:
This pump is very important to the overall operation of the system. It is a magnetic drive, 120V portable polyethylene centrifugal pump. Each process tank is equipped with a PVC cam-lock fitting that enables us to hook up the pump to any system and transfer from tank to tank in any number of configurations. We can also hook up to the containment area sump and transfer chemicals from point to point with limited entry into the building.
Old system vs. New
- The old system utilized a manifold piping system to distribute chemical to the pumps. This involved numerous connections (valves, fittings, checks, and unions) each connection is a potential leak point.
- In the new system we move the pumps to the tank or move the chemical from tank to tank. THIS IS THE ONE TANK, ONE PUMP, ONE SYSTEM philosophy that makes the system simple.
- The transfer pump can be used as emergency response, area clean up, chemical transfer, it is polypropylene with a magnetic drive. There are endless combinations of connections to achieve a desired result leaving the choice to the Best Professional Judgement of the staff on hand.
- The KISS concept was carried over to the sodium bisulfite system to achieve the same reliability standards set by the sodium hypochlorite renovation. The concept has been moved throughout the plant chemical applications.
- The 120V operation of the peristaltic pump adapts itself to many situations. When doing the renovations we set up temporary operating systems outside. This system can be operated off of a 120V engine generator in an emergency using a Hach kit to monitor system performance. At one point we simply plugged in the pump and fed sodium bisulfite from a five-gallon bucket during a signal-connecting phase after the renovation project.
This system concept is extremely versatile and the simple nature of the design lends itself to a high degree of reliability. Operations personnel handle routine maintenance and process system applications. Should a system fail we simply turn on another system. If needed, pumps can easily be moved from location to location by unplugging the 120 V standard power cord and the 4>20 Ma signal and moving the pump. This is an added benefit to the off hour and weekend operation of the plant, in that very little special skills are required to troubleshoot and maintain a running system. The KISS system is driven by ease of operation and component flexibility. This hardware system allows the plant operations staff to reliably and accurately utilize the ORP application system.
Chemical Dosage Control-
The liquid chemical system that was installed in 1991 was furnished with a wet chemistry chlorine residual analyzer. Sample streams were pumped to the analyzer, metered, and combined together before testing. This system required continual operator attention to refill reagents, keep sample volumes accurate and to keep calibrated. Due to lack of dependability the system was never put on-line.
The operators then relied on using a Hach DPD test kit to set the sodium hypochlorite dosage. We soon found that we were getting interference in the test. Samples that were known to contain no chlorine were showing total chlorine residuals of up to 0.2 mg/l. This made it difficult to set the proper dosage and obtain consistent fecal coliform counts. Sodium bisulfite was overdosed to assure compliance with a 0.0003 mg/l final effluent chlorine residual limitation.
Automatic Chemical Dosage Control
In 1996 a Strantrol 900 oxidation reduction potential (ORP) system was installed.
This system provides dosage control for both the chlorination and dechlorination process.
ORP measures the transfer of electrons. Chlorine forms that are toxic to microorganisms are missing one or more electrons. To satisfy their need for electrons, they steal them from organics or other donors through an electrical attraction. This electrical potential is the ORP and is measurable in millivolts. The higher the ORP the greater the disinfecting action.
An ORP probe with a platinum tipped electrode is submersed into the chlorine contact tank distribution channel. The probe monitors the ORP value and compares it to an operator defined ORP setpoint. Adjustments to the dosage (peristaltic pump speed) are automatically made to maintain the setpoint. The proper setpoint is determined by the ORP that provides the desired fecal coliform count.
Likewise, another probe is located downstream of the sodium bisulfite injection point and monitors and adjusts the sodium bisulfite dosage to maintain the setpoint..
Once programmed correctly, the Strantrol ORP system requires little maintenance. An auto wash system pumps a weak acid solution across the electrode once per day. The time of day and duration of the cleaning are user defined. The system stays in operation during the cleaning by maintaining the average dosage of the 30 minutes prior to the cleaning cycle. Once out of the cleaning cycle, real time data is used for control.
In addition to the daily autowash cycle, plant operators manually clean the electrode twice per week. This involves removing the probe from the liquid stream and gently scrubbing it with a toothbrush. This procedure also allows the operator to visually check the condition of the probe.
Alternate Dosage Control Methods
In addition to automatic ORP dosage control, two other methods for controlling chemical dosage are available.
The ORP system combined with the simplified chemical storage and conveyance system at the Littleton / Englewood WWTP has made the chlorination and dechlorination process dependable and easy to operate.