Getting Off the Roller Coaster
An ineffective system for controlling chlorination and dechlorination, combined with the necessity to maintain stringent disinfection and residual chlorine limits, brought about excessive chemical overfeed, permit violations and fines at the municipal wastewater treatment plant in Stuttgart, Arkansas. By adopting a new approach for controlling the chlorination/dechlorination process, the plant is now meeting these strict requirements and excessive chemical overfeed has been eliminated, reducing chemical costs by more than $18,000 per year. The plant also installed new gas chlorinators and sulfonators with sonic regulation, further providing for safe, reliable gas disinfection and dechlorination.
The Stuttgart plant is a 4.5 million gallons per day (MGD) trickling filter plant that serves a population of 10,000 as well as two major industries (a soybean processing plant and two rice mills) that contribute combined loadings equivalent to another 10,000 population. The facility was completely overhauled in 2001, including the addition of sand filtration for tertiary treatment.
Disinfection is the final stage of treatment before effluent is discharged into Kings Bayou. Chlorine is used to disinfect the wastewater to meet a stringent effluent limit of less than 200 total coliform per 100 milliliter (ml). Following disinfection, the water is dechlorinated to meet the plant's chlorine residual discharge limit of 0.1 milligrams per liter (mg/L).
Chronic Cl2/SO2 Underfeed and Overfeed
Chlorination had been carried out by flowpacing chlorine (Cl2) feed and periodically adjusting dosage rates based on manual residual analysis. Dechlorination was accomplished by feeding a corresponding dose of sulfur dioxide (SO2) following disinfection.
Basing chlorine feed rates on a combination of flow and residual measurement resulted in chronic Cl2/ SO2 overfeed and underfeed. This was happening primarily because residual measurement does not always predict coliform numbers as a basis for dose in a continuous flow system. Because the chlorine profile and the chlorine demand are constantly changing, the kill rate of bacteria (total fecal coliform most probable number/100 mL) was not dependable. Flowpacing and manual dosage adjustments based on periodic residual testing produced erratic chlorination even though high Cl2 rates were being deliberately maintained.
It is critical for operators of municipal wastewater treatment plants to maintain proper chlorine disinfection without exceeding discharge limits for chlorine residual.
Dechlorination control was also a problem. When water reached the dechlor chamber, SO2 feed rates were sometimes too low to achieve proper dechlorination. Despite operators continuously adjusting SO2 feed rates ever higher in and effort to compensate, undertreatment "blips," occurred where chlorinated water exceeding 0.1 mgL was discharged from the plant.
These problems resulted in chronic disinfection and chlorine violations, fines and an administrative order from the Arkansas Department of Environmental Quality (DEQ). The facility constantly swung back and forth between the two extremes. Once operators would finally get fecal coliform levels below permitted levels, then the plant would then go out of compliance for excessive residual chlorine.
In addition to fines and excessive high chemical costs, the high degree of operator reaction involved with trying to control chlorination and dechlorination often consumed a significant amount of hours of labor that could have been better utilized performing other duties at the plant. If residual testing showed the chlorine residual was too low prior to dechlorination, for example, the operator would have to make an arbitrary decision on how much to increase the feed rate. Often, the operator would increase the rate on the high side, "just to be safe," and would then check residual an extra time because there was no way of predicting how the higher feed would affect the residual level. If subsequent texts indicated the residual level was still not right, the procedure would then have to be repeated until the proper level (as well as the associated SO2 feed rate to remove the chlorine prior to discharge) was reached, which proved to be only temporary.
Taking a New Approach
Plant management determined a new strategy for chlorine and dechlorination control was required to maintain compliance, reduce chemical usage, and reduce manpower requirements at the facility. Following a comprehensive review, the plant adopted an approach that monitors the oxidation reduction potential (ORP) in the waste stream following chlorination and sulfonation and automatically matches chemical feed rates to the changing demand.
When microorganisms are destroyed though an oxidation process, such as chlorination, an electromotive force is generated that is measurable in millivoltage. This millivoltage is called ORP, or Redox potential. The strength of this force is directly proportional to the oxidative strength of the chlorine solution. The higher the disinfecting ability of the chlorine present, the higher the voltage. The higher concentration of organics, the lower the voltage. Because of this direct correlation between ORP and disinfection, the current ORP at any given time is an accurate measurement of the current chlorine demand in the system.
A High Resolution Redox (HRR) control system from USFilter's ChemFeed & Disinfection Group was installed to monitor the chlorine demand in the system (by real-time measurement of ORP) and feed the rate of chlorine and SO2 required to meet setpoint parameters. The system is a three-channel controller. One HRR sensor, located at the beginning of the chlorine contact chamber, monitors chlorine demand for disinfection control. The second sensor is located in the dechlorination tank, which monitors reductant demand for SO2 control. The controller converts the ORP signals to 4-20mA signals that drives the plant's chlorinators and sulfonators, modulating dosing to meet the ever-changing oxidant and reductant demands in the system. The third sensor is located further downstream in the chlorine contact tank and monitors chlorine demand following retention -- to verify that at least 1.0 parts per million (ppm) of free chlorine (based on HRR millivolt readings) is present in the water just prior to dechlorination.
The facility constantly swung back and forth between the two extremes. Once operators would finally get fecal coliform levels below permitted levels, then the plant would then go out of compliance for excessive residual chlorine.
The controller also compensates for changes in lag time between the chemical injection point and the sensor location. If flow rates increase (shortening lag time), or decrease (lengthening lag time), the controller continues to respond accurately, accounting for changes in flow rate to maintain accurate control. Each channel of the controller is programmed with an operator-determined HRR setpoint that corresponds to the disinfection or dechlorination value required to meet the plant's discharge limits. The controller automatically goes into alarm mode if HRR readings fall out of the setpoint range.
Once becoming fully on-line in 2001, the HRR controller has provided continuous, accurate chlorination and dechlorination control. Through automated chlor/dechlor control, the plant has consistently remained in compliance with both disinfection and chlorine residual. Chemical savings is approximately $18,000 per year due to the significant decrease in overtreatment. Chlorine usage has been cut in half, and SO2 usage has been reduced by more than 25 percent. Sulfur dioxide setpoints could, in fact, be lowered in the future to provide further chemical savings. However, due to the plant's recent compliance history, operators remain justifiably conservative when it comes to deciding how much to decrease SO2 feed rates.
Because of this direct correlation between ORP and disinfection, the current ORP at any given time is an accurate measurement of the current chlorine demand in the system.
Adopting automated chlor/dechlor control has also benefited the plant by providing ready access to essential performance data. The datalogging and graphing features of the controller support continuous improvement by allowing operators to track performance over designated intervals and take "snapshots" of realtime conditions.
Improvements made to the plant's chlorination and dechlorination operations have not stopped with the adoption of demand-based control. The plant recently installed new S10K gas chlorinators and sulfonators from USFilter's ChemFeed & Disinfection Group. With the all vacuum operated, sonically regulated units, direct cylinder mounting puts the vacuum-regulating valve right at the source, reducing gas pressure to vacuum immediately. The unit's rotameters are mounted remotely, next to signal conditioning units (SCUs) that allow for automatic proportional control in the even the plant's HRR control system ever shuts down for any reason. A built-in automatic switchover system on the chlorinators and sulfonators eliminates the need for external switching devices. The non-isolating feature allows cylinders to be emptied thoroughly, for complete gas consumption.
Meeting the Challenge
Solving chronic disinfection and dechlorination problems at the Stuttgart plant was a substantial challenge for management, requiring leading technologies to improve process control. It is critical for operators of municipal wastewater treatment plants to maintain proper chlorine disinfection without exceeding discharge limits for chlorine residual. Attempting to accomplish this through flowpacing, however, proved unsuccessful. But by changing Cl2 and SO2 control strategies to an automated system that modulates chemical feed rates to match the ever-changing demands in the system, strict limits for total fecal coliform and residual chlorine are now being met.
This article originally appeared in the June 2003 issue of Environmental Protection, Vol. 14, No. 5.
This article originally appeared in the 06/01/2003 issue of Environmental Protection.