Cooling Water Intake Decisions Are Heating Up
Careful choices for 316(b) compliance can save millions of dollars
The objective of the 316(b) rule is to reduce the impingement or entrainment of aquatic organisms in cooling water intakes—keeping fish, crustaceans, mammals, sea turtles, larvae, eggs and other organisms from getting drawn into the cooling water system or trapped against the screen. In short, compliance is built around creating an intake gentle enough to allow fish and other aquatic life to float on by, escape the inflow current, or be effectively excluded from or removed from the intake systems.
According to EPA, there are 1,065 facilities required to comply with 316(b) because they utilize at least 2 million gallons per day (GPD) of cooling water. Compliance with the rule will be required at each affected plant when its NPDES permit is up for renewal. About 40 percent of those facilities—about half of which are power plants and half are manufacturing facilities—already comply.
Balancing Ecology and Economy
As EPA often does, the agency calls on facility owners to choose among best available technologies. In its rule, released May 2014, EPA listed seven permissible approaches to compliance, including:
- Operate a closed-cycle recirculation system.
- Operate a cooling water intake structure with a maximum through-screen design intake velocity of 0.5 feet per second (fps).
- Operate a cooling water intake structure with a maximum through-screen intake velocity of 0.5 fps.
- Operate an offshore velocity cap.
- Operate a “modified traveling screen” that meets the rule’s specifications and is defined as “best technology available for impingement reduction.”
- Operate any combination of technologies, management practices and operational measures that the director determines is best technology available for impingement reduction.
- Achieve the specified impingement mortality performance standard, which includes keeping mortality below 24 percent.
That is a very broad set of options, notes David Anderson, North American Business Development Director-Water Intakes, for Bilfinger Water Technologies.
Compare the highest-cost options—new closed-cycle recirculation systems, estimated to cost as much as $400 million in some facilities, or ongoing monitoring programs to document compliance under option 7—with screened intakes that can cost from $500,000 to $5 million, Anderson suggests. Option 4, a velocity cap, can cost more than $50 million to retrofit to an existing plant. That vast cost range highlights the possibilities of balancing ecology and economy, he says.
A careful reading of the rule also highlights areas of vagueness in the rule’s language as well as areas of extreme clarity, Anderson notes. For instance, options 6 and 7 are quite unspecific, leaving room for engineers and regulators to develop site-specific solutions. Of course, that’s costly, but it creates opportunities to apply a variety of approaches in a solution that all parties can agree addresses the environmental objective as well as local challenges.
On the other hand, options 2, 3 and 5 are extremely clear, and point to very specific solutions. In 2 and 3, certain passive intake screens with flow modifiers—such as Johnson Screens Vee-Wire passive wedge wire cylinder intake screens—meet the specifications perfectly. In option 5, Geiger modified traveling screens are mentioned by name in EPA’s rule as an example of the acceptable technology.
Cost Efficiency at Work
The Geiger modified traveling screen design is extremely efficient in space and materials. The Geiger MultiDisc is a direct replacement for old-style screens, requiring no structural modifications. The system gently removes and transports fish and other organisms from the intake flow—the modification referred to in the term “modified traveling screens”—and features an on-demand screen washing system. The MultiDisc can be designed with two or infinitely variable speeds to suit the site and operation.
The Geiger MultiDisc is the only through-flow style modified traveling screen that eliminates debris carryover, which at many sites has been an expensive root cause of lost generation, notes Anderson. Optimizing the flow and cleaning rates reduces energy demand and still allows the Geiger screens to process up to 100,000 m3 of water per hour.
The Johnson Screens Vee-Wire passive cylindrical intake screens are a study in efficiency. Meticulous design and construction yields durable, dependable intakes with minimal operating and maintenance costs. The Vee-Wire design yields a plug-free screen with a smooth surface, minimizing injury to passing fish and other organisms. With slot widths ranging from 0.5 mm (0.020 inch) to 9.5 mm (3/8 inch), the Vee-Wire screen is a highly effective debris filter while ensuring a minimal pressure drop to the intake—typically under ¾ foot of water head loss. The system’s Hydroburst Air Backwash System is simple and highly effective for cleaning the screen periodically.
What makes the Johnson Screens system so effective—and such an ideal fit with the new 316(b) rule—is their sophisticated, open pipe double flow modifier system. Though velocities are high in the flow modifier pipes at the center of the system—internally—they are kept low, constant and even across the screen surface. Maintaining a steady intake flow of less than 0.5 fps at the screen minimizes entrainment and impingement, and reduces the intake’s “zone of influence” to a distance equivalent to roughly half of the screen’s diameter, resulting in low impact on fish and other organisms in the surrounding water.
This isometric diagram highlights the double flow modifiers that enhance the efficiency of this intake while maintaining a low, constant and even flow along the screen surface.
Backed By Engineering
Besides over 40 years of intake design experience, Johnson Screens’ passive intake screens are supported by an engineering team equipped with powerful computational fluid dynamics (CFD) and finite element analysis (FEA) modeling tools, which have been proven for exceptional accuracy in predicting the performance of intake systems before they are even built. In fact, physical CFD testing just ½ inch from the screen surface of its intakes validated the strong correlation between models and reality, and allowed Johnson Screens to establish the reliability of its models across a wide range of cylinder diameters and configurations.
This computational fluid dynamics (CFD) model illustrates the high flow rate inside the screen and the low, steady flow around the screen, minimizing entrapment and impingement of aquatic organisms.
The result is the ability to tailor design and construction to the very specific demands of any site. Using the COMSOL Multiphysics modeling system, Bilfinger’s engineers factor in depth, debris, turbulence, wildlife and wave action, then model CAD designs against the fluid dynamics of wave loading, system stress and self-cleaning demands. Integrating modeling into the design process allows the company to make the most efficient use of resources to create intake screens ideally suited to the site and demonstrated to comply with the 316(b) rule.
An installation at Wisconsin Electric Power’s Oak Creek Plant outside Milwaukee provides an excellent example of the process. The plant draws 1,560,000 gpm (2.2 billion GPD) of cooling water from intakes located three miles offshore and 50 feet deep in Lake Michigan. Using computational fluid dynamics modeling to illustrate flows around and through the passive cylindrical screens and their flow modifiers, Johnson Screens and Wisconsin Electric Power were able to demonstrate the system’s compliance with the 316(b) rule.
“By modeling the dynamics of the lake at the position and depth of the intake screens, Bilfinger was able to construct the cylinders without over-sizing the structural supports, saving money for the utility and optimizing the design for the site,” notes Mark Watson, Johnson Screens Intake Screen Product Line Manager for Bilfinger Water Technologies. “In addition to the 316(b) issues, we had to demonstrate that the screens could withstand the forces of a 100-year storm event.”
Extensive modeling allowed Johnson Screens to refine the design and optimize the structural plans for this intake screen—one of 24 deployed three miles offshore in Lake Michigan for a Wisconsin Electric Power project that complies with new EPA 316(b) rules.
In addition, Johnson Screens used its copper-rich Z-Alloy for its Vee-Wire, creating a corrosion-resistant surface that repels the zebra and quagga mussels that have invaded the Great Lakes and are a costly threat to intake systems, Watson adds. In all, the Wisconsin Electric Power installation can serve as an excellent model for so many facilities that must now come into compliance with 316(b), creating an economically viable approach to the ecological challenges at the cooling water intake.
For more information on Bilfinger Water Technologies, including Johnson Screens’ passive wedge wire cylindrical screens and Geiger modified traveling band screens, visit http://www.wateronline.com/ecommcenter/bilfinger.
Jim Lauria is Senior Vice Director of Bilfinger Water Technologies’ Water Treatment Business Unit. Jim has over 20 years of global experience as a senior executive in the water industry, and holds a Bachelor of Chemical Engineering from Manhattan College. You can reach him at firstname.lastname@example.org or (805) 338-9352.
Image credit: "DSC_0362_pp," WalterPro4755 © 2014, used under an Attribution-ShareAlike 2.0 Generic license: https://creativecommons.org/licenses/by-sa/2.0/