Removal of chlorine from treated effluent is a fundamental prerequisite of the wastewater treatment process, mandated by the U.S. EPA and Texas Commission on Environmental Quality.
Sulfur dioxide is one of several means available to meet this requirement. The gas is injected into the water by a sulfonator combined with an injector or inductor, which creates a vacuum that pulls the chemical into the treated water to eliminate the chlorine residual.
In the mid-1990s, the River Road Wastewater Treatment Facility in Wichita Falls, Texas, experienced several incidents in which chlorinated water was released into its receiving stream, the Big Wichita River, in violation of the plant’s permit.
These violations generally resulted from equipment failures, such as the analyzing instruments measuring the chemical residual in the treated water or the gas feed systems. In addition, north central Texas is prone to electrical storms during the spring, and power outages are common. These events would produce the same undesirable results as equipment malfunctions.
To prevent releases of chlorinated effluent even in adverse conditions, the staff at the Wichita Falls plant devised a simple mechanism using an existing programmable logic controller (PLC) and a hand-fabricated trap door.
Matter of time
The River Road plant is an activated sludge facility with 20-mgd design flow, 10-mgd average flow, and 13-mgd peak flow. Sulfur dioxide (SO2) gas is the plant’s primary dechlorination agent. Granular sodium metabisulphite is used as a substitute when the main dechlorination system is off-line for maintenance or equipment failures. During the latter, the staff found it impossible to prevent all water containing chlorine (Cl2) residual from escaping to the receiving stream because of the time lapse between discovering the problem and their intervention.
In line with this need, maintenance and electronics personnel began working on a fail-safe device that immediately dispensed sodium metabisulphite into the treatment channel once specific failure criteria were met.
After days of brainstorming, the parties agreed on an electro-mechanical assembly with a trap door that opened via a motorized latching mechanism. The trap door would span the walls of the treatment channel directly above the dechlorination process. Maintenance personnel set about designing and fabricating the trap door “box” from off-the-shelf parts.
Choosing criteria
The next step was determining how to activate the trap door remotely and on what conditions. The plant already had a PLC that contained vacant input/output points. In addition, the analog input signal for the SO2 residual was present through the PLC and monitored on the SCADA system.
Therefore, the team created an alarm tag in SCADA to reference that input signal, and set it to activate at a predetermined value. Once in alarm, a relay output in the PLC sent 120v AC current to the solenoid in the trap door. The solenoid retracted a small rod beneath the edge of the door, allowing it to swing down and release a container of granules into the water.
Because a loss of power would shut down the gas feed systems and fail-safe device, the staff also devised a method to activate the latter. The PLC was powered through an uninterruptible power supply (UPS) that energizes vital equipment during outages.
The staff wired a power monitor to the circuit breaker of the lead sulfonator, which opened a set of contacts if it sensed a voltage loss. This was then connected to a point on a PLC input module for monitoring purposes. Upon opening, the PLC generated an alarm and energized the same relay output used for a low SO2 residual condition. Now the solenoid activated for either a low residual or a power loss.
Refining logic
The final step was expanding and modifying the PLC’s ladder logic to energize relays and reset itself after alarm conditions had cleared.
If the low residual or power loss criteria were met, a one-shot timer energized for six seconds, allowing the solenoid to retract the trap door arm. Simultaneously, an alarm would sound over the plant’s public address system, alerting personnel to the problem.
The staff tested the fail-safe system with successful results, except for one significant flaw: Once dropped into the channel, the sodium metabisulphite dissipated too quickly to be effective for more than a few minutes.
To correct this, the team placed the chemical in a sack attached by a stainless steel cable to the trap door framework. When the door opened, the cable suspended the bag just below the water’s surface. After trying various containers, including pillowcases, the team chose double burlap bags to slow the liquefaction process.
The period for dissolution increased to more than 20 minutes, depending on the quantity of chemical placed in the bag. This meant response time by maintenance personnel was no longer a critical issue.
Strong track record
The fail-safe system has operated for more than 10 years with positive results. Permit violations for chlorine release are nonexistent at the River Road facility. About seven years ago, the staff installed a similar device at the city’s Northside Wastewater Treatment Plant, a 0.5-mgd (average) facility without the benefit of a PLC. The device, built entirely by the team from off-the-shelf parts, has proven equally successful.
Material costs have increased dramatically since construction and would vary depending on size requirements and vendor pricing. It is difficult to quantify actual savings from the devices, but not having to report chlorine release violations to the EPA or TCEQ is invaluable.
About the author
Frank Hill is an electronic instrument technician II at the River Road Wastewater Treatment Plant in Wichita Falls, Texas.
