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.
