Challenge

The Zanesville, Ohio, Wastewater Treatment Plant, with a peak flow of 20 MGD, serves a population of nearly 58,000. The trickling filter facility is required to perform chlorination from May 1 through  October 31 and utilizes V-10K chlorinators and sulfonators to feed gaseous chlorine and sulfur dioxide. Originally, the chlor/dechlor process was controlled based on flowpacing and residual tests performed by operators every two hours. Under the flowpacing/residual measurement approach, the plant had great difficulty maintaining consistent feed and control. “We were constantly overfeeding and underfeeding chemicals in an attempt to maintain consistent kill and stay within our residual limits,” says Chief Operator Scott DiFolco. “Although our fecal limit is 2,000 mpn/100 mL over a 7-day average, there were days when our fecal count was TNTC (too numerous to count). Plus, we had an equally tough time meeting our residual limit of .038 ppm.”

The plant is gearing up for a major upgrade and the plan had included switching to an ORP (oxidation reduction potential) system for controlling the chlor/dechlor process. “Since plans called for a new ORP system anyway, we decided to jump the gun a little and make the change to ORP right away,” says DiFolco.

Solution

The plant installed a Strantrol^® 890 automated, demand-based chlorination and dechlorination control system from USFilter’s ChemFeed & Disinfection Group in June 2002.

• Utilizing High Resolution Redox® (HRR) technology, the Strantrol controller provides automatic, demand-based control of chlorination and dechlorination, matching feed to demand and treating the system with only the chemical dosages required to maintain consistent compliance.
• One of the controller’s probes, located approximately 10 minutes downstream from chlorine injection, monitors for chlorine activity and demand. A second probe, monitoring dechlorination, is located just before effluent discharge.
  

Results

The Strantrol controller is programmed with a predetermined HRR setpoint that corresponds to the disinfection or dechlorination value required to meet the plant’s discharge requirements.

• The system monitors both the oxidant and reductant demand in the water and automatically modulates the amount of chlorine and sulfur dioxide required to meet chlorine residual and fecal coliform limits in the plant effluent. “The system feeds what it needs, so we no longer overfeed or underfeed chemicals,” says DiFolco.
• The controller 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 while maintaining accurate control.
• “Having the Strantrol 890 is like having an operator 24/7, making adjustments as needed,” says DiFolco. “As far as I’m concerned, it’s an excellent piece of equipment and money well-spent.”