When Baffles Fall Flat
It’s clear that strategic use of baffles in a clearwell tank enhances efficiency and effectiveness during the disinfection stage of potable water treatment. Inefficient flow patterns cause the formation of dead zones or recirculation patterns that reduce disinfection effectiveness and promote the formation of byproducts, both of which present a danger to the health of the consumer. However, the challenge of designing and engineering an ideal mixing pattern while minimizing hydraulic energy loss in a contact tank is substantial. Decades of research, improved modeling, and innovative designs are all constantly in development to create better solutions.
Why Efficient Mixing is So Important
When disinfectant is mixed with treated water during the clearwell stage, it activates the removal process for bacteria, parasites, viruses, and other pathogens. As the microorganisms are exposed to chlorine or chloramine, the disinfectant is consumed. Careful dosing is designed to ensure that there is sufficient chemical to destroy all microorganisms plus just enough residual to maintain protection from further exposure in the distribution system. There are both minimum and maximum levels to meet, so it’s critical that mixing processes are as effective as possible.
In treatment tanks with inefficient flow patterns, some particles can flow through the tank in a single hour, short-circuiting the disinfection contact time, while others might remain for two days, eddying endlessly in small recirculation zones. In these cases, even when operators combine different mixing strategies, such as combining air or mechanical mixing with baffles, it’s difficult to get reliably similar levels in samples taken from across the entire volume, and that presents a problem in complying with regulatory standards.
Conventional Baffle Configurations
Traditionally, solid baffles constructed from metal or concrete were built into contact tanks to slow down the passage of water from entry to exit by creating a serpentine pathway for the water to follow. This long, slow path was intended to increase disinfectant contact time for the removal of pathogens and viruses from the water. However, this basic classical design has been found to promote the formation of recirculating dead zones and short-circuiting in the chambers of the mixing tank. The typically strict geometric configuration features many sharp turns along the course where flow velocity rapidly decreases or disappears entirely, forming a dead zone, and other areas where stable recirculation patterns are established. These disrupted zones, where the fluid velocity differs significantly, encourage short-circuiting since high energy incoming water will inevitably avoid mixing with zones of lower energy and will flow instead along a direct path to the exit
The solid baffles themselves also interact with the turbulence generated in the water as it follows 180-degree flow inversions along the path. This interaction of turbulent flow with solid baffle structures drains energy through friction, thereby reducing the liquid’s overall kinetic energy. Unfortunately, lower kinetic energy, in turn, further reduces mixing efficiency, which then requires higher doses of disinfectants as well as more supplemental energy simply to keep the water moving through the step. These compounding issues, combined with higher treatment costs and increased regulatory monitoring have led scientists to investigate and design more efficient mixing tank systems.
Optimizing Flow Patterns in Clearwell Tanks
Since a simple solid baffle configuration that forces a basic serpentine route through the holding tank generates so many efficiency issues, a lot of research has been devoted to discovering the many physical factors that can promote - or hinder - mixing in these systems. Some of those factors include:
- Configuration & orientation of internal baffles
- Baffle spacing/channel width
- Creating narrow openings at top and bottom of baffles
- Use of porous or perforated baffles
- Varying shapes, sizes, and arrangements of perforations
- Use of turning vanes near baffle corners
- Direct mixing strategies
- Inlet configuration
- Dosing strategies and locations
These decades of research into optimizing flow patterns for mixing and contact time, while minimizing energy requirements during disinfection, have produced many design possibilities. Some of the most successful, recent designs include slot baffle, perforated baffle, and other porous baffle designs.
It’s well established that while traditional solid baffle walls may increase the average residence time in a clearwell tank, mixing efficiency is very low and the actual residence time of any volume of water can vary from a few minutes to a few days. Indeed, changing location and even orientation of solid baffles adds very little to overall efficiency in these cases. Recent studies, however, suggest that the presence of slots or openings in the baffles can actually improve mixing efficiency by reducing short circuits and increasing fluid contact mixing throughout.
The specific mechanisms through which perforated baffles aid in mixing are demonstrated when water flows energetically through baffle perforations and penetrates recirculation zones in adjacent chambers. This not only disrupts the primary short circuit flow but turns dead zones into areas of active mixing, while turbulence created around the baffle perforations create even more new mixing zones. This effect continues downstream with each succeeding set of perforations continuing to disrupt regulated flow in the following sections and generating new mixing zones.
Using perforated baffles to both increase mixing efficiency, and maintain hydraulic performance, requires the installation of several baffles throughout the flow path length. Thus, the regulated flow is continually disrupted and mixing is optimized. The most critical design parameters are the size and shape of the holes, the arrangement of the holes within the baffles, and the perforation density.
Developing a clearwell tank and baffle design for optimal hydraulic and mixing efficiency is not a simple, straightforward task. A straightforward strategy of maximizing the distance traveled from inlet to outlet is clearly inadequate, so current efforts tend to focus on promoting turbulent mixing while still mitigating effects which lower hydraulic efficiency. This kind of optimization is impossible to do real-time in a full-size structure, as the need to make repeated complex changes to baffle configuration, inlet design, and other mechanisms would completely disable operations. Even testing in laboratory size tanks is prohibitively expensive when more than a handful of designs are being tested.
Today, the use of 3D models derived using computational fluid dynamics provide an opportunity to test a wide range of materials and baffle configurations at a relatively low cost. These models not only to calculate overall mixing and hydraulic efficiency but can also detect and minimize areas of internal recirculation and short circuiting, as well as predict formation of DBPs. Water treatment facilities that are taking advantage of these tools can quickly identify and implement design changes that improve effectiveness of the clearwell stage across all measures.
