Automated Chlorine Control Brings Precision to Water Reclamation Operation

About the author: Steve Kobler is the water disinfection technician for the Otay Water District, Otay, Calif.

Using reclaimed water for non-potable purposes as a means of conserving potable water supplies is the most prevalent method of water reuse in the United States today. One of the significant challenges for water reclamation facilities is to keep up with the demands for safe, compliant chlorine (Cl2) treatment. One utility that is effectively meeting this challenge is Southern California’s Otay Water District.

The Otay Water District, established in 1956, is a publicly-owned water and sewer services agency whose area encompasses 132 square miles of southeastern San Diego County. District facilities provide water and/or sewer service to approximately 118,000 people. A distribution water district with approximately 36,000 total connections, the Otay Water District receives treated water purchased from the San Diego County Water Authority with connections to surrounding water agencies for high flow and emergency situations.

In addition, Otay Water District owns and operates the Ralph W. Chapman Water Recycling Facility, producing up to 1.3 million gallons per day of high-quality reclaimed water. Through a dedicated pipeline system, the reclaimed water is transported to the eastern Chula Vista area where it is used to irrigate a golf course, public parks, roadway landscapes and various other approved uses.

Chlorine System Overview

Chlorination is the final stage of treatment at the facility before the effluent is discharged to a settling pond four miles away. Since this water is destined for reuse, this procedure must be dependable and accurate. Disinfection is performed through gas chlorination, utilizing a bank of V-2000 chlorinators manufactured by USFilter’s Wallace & Tiernan Products, Vineland, N.J. The chlorinators’ V-notch orifice consists of a precisely grooved plug sliding in a fitted ring, resulting in accurate gas flow control and repeatability.

The chlorinators provide for safe, all-vacuum operation. Gas from cylinders flows to the chlorinators operating under a vacuum produced by a flow of water from aspirator-type injectors. Chlorine is transmitted to the control module and then to the vacuum-regulating valve and then to the injectors. At the injectors, metered gas is dissolved in the water stream. From the injectors, the chlorine solution flows to diffusers in the chlorine mixing chamber where it is mixed with effluent from the plant’s final settling tanks.

Following mixing, the flow enters the distribution channel and is directed into the plant’s 8¢-wide, 50¢-long and 6¢-deep chlorine contact tank. The contact tank provides a detention time of 20 minutes at average design flows. Following chlorination, flows from the plant are pumped to the settling ponds through four miles of pipe that gives a total of 21/2 hours of detention time.

High Cl2 Residual Requirement

To meet the facility’s permit, reclaimed water must contain 10 mg/L chlorine residual when it reaches the district’s settling ponds. The district originally relied on flow pacing and periodic residual measurement to maintain chlorine dosage at the plant. However, this practice resulted in wide fluctuations in the residual level of water entering the settling ponds. To ensure the plant met its permit requirement, the chlorine dosage was increased substantially so that the water contained 10 mg/L chlorine even during the lowest swing in residual level. This resulted in a relatively high chlorine use rate of 550–600 lbs. per day.

The district realized that overfeeding chlorine was only a temporary solution. Consistently meeting the tough effluent residual standard required that it establish a reliable, long-term method for chlorination control. To this end, Otay District managers toured a treatment plant in nearby San Juan Capistrano that was conducting a demonstration of an automated, demand-based chlorine feed using a High Resolution Redox (HRR) controller, manufactured by USFilter’s Stranco Products, Bradley, Ill. Installing the unit brought chlorine fluctuations under control, eliminating the serious high and low conditions experienced at the San Juan Capistrano facility. Based on the results of the demonstration project, Otay Water District management decided to install the system at its reclamation facility.

The controller monitors the oxidant demand of flows exiting the plant’s chlorine contact tank and automatically feeds the required amount of chlorine. An HRR sensor located in the contact tank responds to the oxidation reduction potential (ORP) of the treated water, automatically modulating chlorine dosing. The controller is programmed with an operator-determined HRR setpoint that corresponds to the disinfection value required to meet Cl2 residual parameters for flows entering the district’s settling ponds.

The adoption of demand-based chlorine control took the highs and lows out of the chlorination process at the plant. This, in turn, eliminated chlorine overfeed. Prior to automation, the facility was consuming an average of 550 lbs. of chlorine a day. With automated chlorine control, chlorine usage has dropped to approximately 350 lbs. per day.

In addition to automating chlorine control, Otay Water District management found that the control system provides an early warning when the plant is going in and out of nitrification. HRR readings increase dramatically prior to the plant going into nitrification and then drop when it is going out of nitrification. The district’s water disinfection technician generally notices this increase four to five days before normal laboratory testing procedures at the plant can determine that the facility is going into nitrification.

Conclusion

Chlorination is a critical step in water reclamation, a water reuse method that effectively conserves and extends freshwater supplies. The development of cost-effective systems that provide performance improvement in the disinfection and chlorination process helps utilities meet the growing trend toward water recycling and reuse.

Photos courtesy of USFilter’s Chemfeed & Disinfection Group.