ORP System Helps Plant Track C12 SO2 Dosages

The Joppatowne Wastewater Treatment Plant in Harford County, Md., replaced the flow pacing method for establishing chlorine (Cl2) and sulfur dioxide (S02) dosages with an automatic chemical feed system based on oxidation reduction potential (ORP), the measure of actual oxidant demand.

The Joppatowne Wastewater Treatment Plant in Harford County, Md., replaced the flow pacing method for establishing chlorine (Cl2) and sulfur dioxide (S02) dosages with an automatic chemical feed system based on oxidation reduction potential (ORP), the measure of actual oxidant demand.

The facility has reduced chemical use for chlorination and dechlorinaton by 25 percent while consistently meeting the 200 mpn (most probable number) fecal count disinfection requirement and Cl2 residual discharge limits of <0.1 mg/L. Operators have also been able to use ORP readings in the chlorine contact chamber to improve the operation of their biological nutrient removal (BNR) process.

An Upgrade to a BNR Facility

The Joppatowne plant, which serves a residential population of 40,000, was upgraded to a BNR system in 1995. It has a peak capacity of 3.5 mgd and receives an average daily flow of approximately 1.0 mgd. The influent first enters an anoxic tank, then moves into an aeration basin. Internal recycle pumps return mixed liquor from the end of the aeration basin to the anoxic zone. Beyond the aeration basin, water flows into a secondary clarifier, then to the contact tanks for disinfection and dechlorination.

An Upgrade to a BNR Facility

The chlor/dechlor contact chambers are designed for a 30 minute contact time at 3.5 mgd. At the current flow of 1.0 mgd, total contact time is slightly more than an hour.

An Upgrade to a BNR Facility

The Joppatowne plant discharges into the Little Gunpowder River, which flows into Chesapeake Bay. Per state and federal regulations, the effluent fecal coliform count cannot exceed 200 mpn, nor can the discharge carry a detectable chlorine residual. The plant has a 67,000 gallon chlorine contact tank and a 20,000 gallon dechlorination tank which usually operate in series, but can be isolated for cleaning or maintenance.

An Upgrade to a BNR Facility

The districts High Resolution Redox (HRR) controller, manufactured by U.S. Filter/Stranco, monitors and controls chlorination, then tracks dechlorination via sensors in each contact tank. The automated chlor/dechlor controller is tied to an A-B selector switch so that each tank can be run independently if necessary. Operators can discharge straight from the first tank if the second is down for maintenance, or vice-versa. The goal is consistent disinfection followed by total dechlorination.

Flow Pacing Results
in Imprecise Chlor/Dechlor

Prior to installing automated HRR control for chlor/dechlor, operators relied on a flow pacing system. They verified disinfection by taking fecal coliform counts and checking chlorine residuals. However, chemical additions based on flow range did not always keep up with the actual chlor/dechlor demand.

Flow Pacing Results
in Imprecise Chlor/Dechlor

The flow pacing system consisted of a flow meter which regulated a 100-pound chlorinator and a 50-pound sulfonator. Regardless of the amount of C12 remaining in the water when it reached the dechlorination point, the sulfonator injected SO2 based on flow rate. Therefore, operators routinely overtreated with SO2 to avoid chlorine violations and the resulting fines, which can exceed $10,000 per violation. There was no practical way to continuously measure the level of oxidant present in the water.

Flow Pacing Results
in Imprecise Chlor/Dechlor

With the flow pacing method, operators were testing the water three to four times per eight-hour shift. They had to compensate for diurnal changes, peak and low flows, variations between weekday and weekend usage, and the influx of summer visitors each year. Due to the effects of these variables on C12 and SO2 performance, when the facility was upgraded for BNR in 1995, design engineers recommended and plant supervisors approved the installation of the automated controller to ensure precise control over the chlor/dechlor processes.

Demand-Based HRR Control

High resolution redox provides a direct measure of the rate of oxidative disinfection. A transfer of electrons is the initiating event in any oxidative disinfection reaction, such as chlorination. Chlorine forms which are toxic to microorganisms are missing one or more electrons and they satisfy their need for electrons by stealing them from organics or other donors. This potential for electron transfer, measurable in millivolts, is the HRR value.

Demand-Based HRR Control

The more abundant the toxic forms of C12, the higher the HRR readings. The more abundant the donors (including microorganisms and reductants such as SO2), the lower the HRR reading. The sensors located in contact Chambers A and B detect and relay these voltage readings to an automated controller. The controller converts the sensor signal into output which drives the chemical feed system, modulating C12 and SO2 feed rates according to actual demand.

Customizing the HRR System

After installing the automated controller at the Joppatowne facility, operators and system technicians monitored the chlor/dechlor readings for one week, comparing them to the results of manual tests and creating a datafile in order to determine the optimal program parameters for the specific plant. The seven-day period for data collection ensured that the system operated accurately under all typical flow patterns, including a weekend. By operating the chlorinator and sulfonator manually through the controller, they were able to correlate dosages with variations in demand and flow, then compare their findings with operations data recorded in the supervisor’s logbook.

Customizing the HRR System

The most important parameter is the fecal coliform count. It determines the HRR baseline for disinfection, providing a safe minimum dosage while responding instantly to changes in demand. The dechlor sensor at the end of contact Chamber B supplies equivalent information regarding the effectiveness of the SO2, corresponding directly to actual reduction potential in the water.

Correlating HRR
with the BNR Process

The automated controller also indicates how well the BNR process is performing. When the plant achieves total nitrification, the HRR reading at the chlorine contact chamber increases sharply though the chlorine feed rate remains stable, indicating that only free chlorine is present. No ammonia is escaping to combine with chlorine to form chloramines.

Correlating HRR
with the BNR Process

Chloramines yield a lower HRR reading. By correlating these readings with BNR process parameters, operators have established an optimal level of 2500 mg/l for mixed liquors, with ammonia levels falling between 0.0 and 0.5 mg/l. Plant records for the last two years show that the Joppatowne plant completely nitrifies most of the year, keeping it consistently “above average.”

Data Collection Essential
for Smooth Startup

The data collection interval is essential for optimizing the chlor/dechlor process and obtaining maximum precision in chemical dosages. The testing period helps personnel determine how to best configure components such as sensors, mixers, and pumps. It also provides an opportunity for troubleshooting so that operations are not disrupted later.

Data Collection Essential
for Smooth Startup

For example, the plant operators at Joppatowne noticed small gaps in data as they downloaded information from the controller. Asterisks appeared in the data file instead of numbers because the signal wires were not properly grounded. The wires were picking up stray electrical “noise” from motor starts and other equipment. Once the wires were correctly grounded, the problem was eliminated.

Data Collection Essential
for Smooth Startup

Inadequate mixing in the dechlorination phase was fixed when operators redirected piping and changed the sensor’s location, improving SO2 dispersion and shortening the lag time between chemical injection and the sensor to less than a minute. The sensor now hangs in a basin downstream of a set of baffles, ensuring accurate readings on thoroughly mixed effluent.

Data Collection Essential
for Smooth Startup

A weekly cleaning of the sensors is part of routine maintenance at the Joppatowne plant. Operators use a brush and a 10 percent hydrochloric acid solution to remove biological growth which can affect sensor accuracy by acting as a reductant. The cleaning operation requires less than five minutes. When properly maintained, the sensors should perform continuously for a minimum of two years.

Savings in Chemical
and Labor Costs

Since the July 1995 installation of the automated controller, the plant has maintained an average of 2.0 mpn fecal coliform and has had no chlorine violations, yet chemical usage is down 25 percent, a savings of approximately $5,000 annually. Because operators are no longer required to take test samples every two hours, their time can be allocated to other duties.

Savings in Chemical
and Labor Costs

The Joppatowne staff was recently assigned responsibility for another plant requiring several hours of attention each day, in addition to maintaining 11 main pumping stations and 72 homes on an innovative sewer system. The automated chlor/dechlor control has saved approximately eight hours of labor per day, allowing the Joppatowne facility to run with a minimal staff and distribute labor to other areas as needed.

Savings in Chemical
and Labor Costs

Operators propose using the third channel on the automated controller to monitor the headworks and alert them to unusual influent conditions. For example, the nearby marina occasionally uses a bactericide containing quaternary ammonium chloride for cleaning. The bactericide is toxic to nitrifiers at the plant. Having a sensor at the headworks would allow operators to prepare for its effects by using a bypass or equalizing tank, and minimize disruption to operations.

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