What is a Baffle?
In general terms, a baffle is a barrier that slows and directs the flow of a liquid through a space. The goal may be to reduce erosive effects of water traveling at fast speeds; to allow sediment or other particles to settle to the bottom instead of being carried downstream; or to direct moving water away from a particular feature or location. Baffles are also used to increase retention time in oxidation ponds, clarifiers, water cooling, reservoirs, and wastewater treatment plants.
In disinfection tanks, or clearwells, baffle curtains promote thorough mixing, eliminate shortcuts and force water to travel a predetermined route through the tank, eliminating dead zones. With a longer distance to traverse, the time that water spends within the tank (also known as contact time) is more uniform and predictable. Ultimately, this function aids in complete disinfection, which helps water treatment operators reach strict compliance standards for potable water in a cost-effective manner.
Briefly, the use of baffles in a clearwell tank have the following advantages:
- Directs water flow through a specific predetermined path
- Minimizes short-circuiting, where water travels quickly and directly from inlet to exit.
- Increases contact time with disinfectants, which increases their effectiveness.
- Improves passive mixing so that disinfectants are efficiently distributed throughout the water volume.
- Eliminates static (or dead) zones.
- Improves overall efficiency of the disinfection process, reducing the amount of chemicals required.
- Decreases the formation of DBP (disinfection byproducts).
- Increases likelihood of meeting compliance standards
Water Flow and Chlorine Contact Time
Flowing water naturally tends to travel in a straight line between points. In the case of a storage tank, this means that once it’s reached a minimum volume, incoming water will flow quickly to the outlet, bypassing the vast majority of tank space. Water in those neglected sections (both square and circular tanks) lose kinetic energy and become static. Water in these dead zones may remain trapped in a tank for weeks at a time while new incoming water flows directly to the exit, spending insufficient time in the contact zone.
Baffle curtains are typically configured to hang vertically in a clearwell tank, directing water along a prescribed, winding path. The effect is similar to switchbacks on very steep roads. The speed of traffic is forcefully reduced as the road snakes back and forth, and inevitably, individual vehicles spend substantially more time traveling on the road than if they simply crashed over the precipice and tumbled straight down into a ravine.
Mixing
Inadequate mixing during the disinfection step diminishes the likelihood that treated water will meet SDWA standards, since not all the water has necessarily encountered the disinfectant for an adequate amount of time. When the standard for detection is set at zero, there is no room for error. This zero-tolerance standard exists for microorganisms like cryptosporidium, giardia, legionella, total coliforms, and viruses.
These standards mean that even if 1000 gallons of water are completely free of microorganisms, while only a single gallon of water was poorly mixed, it’s still likely to harbor a small amount of microorganisms. No matter how small the detection rate is, it presents a credible health risk to the public, putting the entire disinfection process in violation of EPA standards.
Efficiency
Effective use of baffles to direct water flow is intended to increase the overall efficiency of the disinfection step. When water is thoroughly mixed with disinfectant and spends sufficient time in the contact tank, disinfection is thorough and effective. This reduces overall costs and ensures that stringent water quality standards are consistently satisfied. However, while some designs might readily achieve an ideal mixing efficiency in a disinfection tank, it might simultaneously lower the hydraulic efficiency as energy is dispersed through friction and excessive turbulence. This loss of energy would ultimately require auxiliary pumps to move the fluid through and beyond the contact tank. The ultimate goal, then, is to find an ideal balance between optimal mixing efficiency and optimal hydraulic efficiency.
Inefficient mixing can also introduce a third problem into the disinfection step. When disinfectant is insufficiently mixed into the water being treated, operators may find it necessary to increase the amount of chemicals applied to ensure that water, whether poorly mixed or traveling too rapidly through the tank, still receives an adequate dosage. This process of increasing the chemical dosage is called super chlorination and, while it substantially reduces the minimum contact time for effective disinfection, it also results in a final product that has a residual (free) chlorine concentration that’s 10 times the ideal level. Water with excessive levels of free chlorine can have objectionable odors, damage pipes and other materials exposed to the chemical, and present health risks to consumers. Further, high levels of disinfectant in the clearwell stage boosts the formation of DBP, another harmful contaminant closely regulated by the EPA’s SDWA.
When super chlorination is in use, subsequent dichlorination is typically accomplished with an additional filtration step using activated-carbon. If excessive levels of DBP are detected, even more treatment may be necessary. Overall, a clearwell that isn’t optimized for both mixing and hydraulic efficiency will inevitably increase treatment costs through excess chemical usage, additional filtration and treatment steps, and additional energy consumption to power mechanical mixers and pump water through the system.
