A Change for the Better

A new disinfection systems is ensuring safe, all-vacuum liquid chemical feed at Middletown, Ohio's wastewater treatment plant

• By Steve Thompson
• Jan 01, 2005

When the staff at Middletown, Ohio's wastewater treatment plant decided two years ago to switch from gaseous chlorine to sodium hypochlorite for disinfection, they also wanted to find an alternative to using a pressurized liquid chemical delivery system. Today, an all-vacuum system utilizing high-speed chemical dispersion is providing the necessary versatility while bringing reliability, efficiency, and greater safety to disinfection operations.

Background
Middletown's 26-million-gallons-per-day (mgd) activated sludge plant serves a population of 55,000 as well as the local paper industry. The plant uses physical separation and biological treatment to provide primary and secondary level treatment before discharge to the Great Miami River. Chlorine disinfection and dechlorination are the final treatment steps from May through October. The plant's chlorine contact chamber is designed to provide a minimum of 17 minutes detention time at peak hydraulic loadings of 48 mgd.

Ultimately, the new chemical feed system utilized vacuum induction to provide precise dosing of sodium hypochlorite and high-speed chemical dispersion.

When the plant decided to switch disinfection operations, a number of disinfection alternatives to gaseous chlorine were considered, including ultraviolet light (UV), which has become more widely used in the area. UV was eventually ruled out, however, because the plant's influent can sometimes be characterized by a color due to industrial flows from the local paper industry. There was concern that the amber color would absorb the ultraviolet light and reduce its effectiveness.

After deciding to switch from gaseous chlorine (Cl₂) to 15 percent sodium hypochlorite (NaOCl) as the plant's disinfection agent, the plant staff's biggest concern was the safety impact of personnel handling sodium hypochlorite under a pressurized system. Plant staff wanted to avoid the potential for problems associated with leaks and off-gassing in a pressurized system.

Ultimately, the new chemical feed system, selected by the plant's consulting engineer, Finkbeiner, Pettis, & Strout Inc., used vacuum induction to provide precise dosing of sodium hypochlorite and high-speed chemical dispersion. This solution was based on leading technologies and a clear view of process control and system integration to bring about a fundamental change in chlorine delivery at the plant while providing a greater level of safety.

The plant's existing gaseous chlorine delivery system was replaced with two LVN-2000 Liquid Chemical Feed systems from USFilter's Wallace & Tiernan Products, equipped with a Water Champ® Chemical Induction Unit (CIU) to draw and meter sodium hypochlorite from supply tanks to the point of application. The covered, outdoor area that had been used for storage of one-ton chlorine cylinders was enclosed and three 2,500-gallon sodium hypochlorite storage tanks and a 500-gallon day tank were installed along with pneumatic transfer pumps. The plant's gas chlorinator system was removed and replaced with two wall-mounted LVN-2000 vacuum chlorinators, and the existing gas diffuser system was removed and replaced with the Water Champ CIU.

High-Energy Induction and Dispersion
Due to the older (1955) design of the plant's chlorine contact tanks, the staff was concerned with obtaining adequate chemical induction and dispersion. With the former gaseous chlorination system, operator testing would often indicate unequal distribution of chemicals within portions of the contact area at certain flow levels. Maintaining proper disinfection often required maintaining a high average chlorine residual in the contact tanks in order to compensate for areas of poor chemical dispersion. A corresponding amount of sulfur dioxide was then required to eliminate chlorine residual in the final effluent.

The plant staff's biggest concern was the safety impact of personnel handling sodium hypochlorite under a pressurized system. Plant staff wanted to avoid the potential for problems associated with leaks and off-gassing in a pressurized system.

When selecting the new system, it was determined that increasing and optimizing mixing energy would help ensure thorough chemical induction and dispersion throughout the contact area. This is accomplished with the Water Champ CIU, supplied by USFilter's Stranco Products, which serves as a vacuum source, eliminating the need for conventional inductors. The Water Champ design includes a 7.5-horsepower (hp) chemical process duty motor mounted to the unit with a vacuum body and airfoil design propeller at the other end. Chemical solution is injected into the body of the unit and is dispersed while being simultaneously mixed by the propeller. The airfoil design of the titanium propeller rotates at 3,450 revolutions per minute (rpm), enabling the unit to achieve high-energy transfer.

The unit has been installed in a wet well located at the entrance to the plant's first contact chamber. Because chlorination is a seasonal procedure, the unit is easily removed for annual maintenance using a stainless steel guide rail and hoist.

Positioned horizontally in the center of the flow regime, the CIU provides for countercurrent mixing at a very high velocity gradient across the entire width of the well. The submerged mixer generates high zone diffusion and a high turbulence region extending across the entire channel as sodium hypochlorite is simultaneously dispersed. Flows passing the mixing area make contact with turbulence and distributed chemical, providing for a high rapid mixing rate.

Precise Chemical Feed Control
The new LVN-2000 chlorinators each have a maximum flowrate of 7.7 gallons per minute (gpm) and are equipped with multifunctional touchpad controls, facilitating flow monitoring and adjustment. Staff liked the accuracy and reliability of the V-Notch variable control valve that had been an integral component of the plant's former V-2000 gas chlorinators. One of the factors in the selection of the LVN-2000 liquid chemical feed system was that, with this system, the manufacturer has configured the field-proven V-Notch control valve for liquid metering. The V-Notch orifice consists of a precisely grooved plug sliding in a fitted ring. Any position of the plug in the ring results in a specific orifice size and corresponding feed rate, resulting in the accurate flow control necessary for this application.

In addition to the V-Notch variable orifice and positioner, the wall-mounted chlorinators consist of a flowmeter, vacuum regulator, and dedicated control unit. The vacuum generated by the CIU submersed in the wet well leading to the plant's chlorine contact tanks draws chemical through the liquid chemical feed system from the day tank. A vacuum regulating valve maintains a constant vacuum to the V-Notch orifice regardless of fluctuations in the vacuum supply, providing a smooth flow of solution with no pulsing output.

The facility is planning to connect the feed systems to the plant's existing computer feedback networks, enabling operators to adjust chlorine feed rates remotely. In addition, the plant is currently looking into adding an oxygen reduced potential (ORP) controller to provide automatic chlorine control based on the real-time chlorine demand in the system.

Performance
Since its startup in May 2002, the new liquid chemical feed system at Middletown's wastewater treatment plant has operated precisely as planned. The new system has helped to bring added safety and flexibility while optimizing performance in disinfection operations. Plant staff especially like the aspect that the new feed system is vacuum induced. If there are ever any leaks in piping, the system will draw air rather than spill sodium hypochlorite.

Due to the older (1955) design of the plant's chlorine contact tanks, the staff was concerned with obtaining adequate chemical induction and dispersion.

By placing properly engineered, high-mixing energy directly into flows entering the contact tank, the plant has significantly improved chemical efficiency and thereby reduced chemical usage accordingly. Because the new submersible mixing system provides a far more homogenous solution of chlorine residual within the entire contact area than the previous gas system, plant operators have been able to reduce the average chlorine residual in the treated effluent 14 pounds Cl₂ as NaOCl per million gallons (MG) versus 17.3 pounds Cl as Cl₂ per MG. In addition, the plant is using 30 percent less sodium hypochlorite than the consulting engineer's original projections.

Precise chemical feed control and more efficient chemical mixing has allowed the plant to switch to the more expensive chemical and still provide cost-effective chemical feed.

This article originally appeared in the January/February 2005 issue Water and Wastewater Products, Vol. 5, No. 1.

This article originally appeared in the 01/01/2005 issue of Environmental Protection.