Importance of Flow Equalization
Many municipalities are facing the challenge of having to equalize large volumes of stormwater sometimes mixed with raw sewage in order to continuously meet their permit limitations. The equalized flow is stored during a storm event and then slowly diverted to the treatment facility when influent flows return to normal levels.
In the past few years, the need for influent storage has been driven by both the increased popularity of the membrane bioreactor (MBR) process and the build-out of new stormwater treatment facilities. In the case of MBR plants, the need to equalize flow is directly related to maintaining a flux rate that the membranes can accommodate, ensuring the plant does not get bottlenecked.
With stormwater treatment facilities, the need is based on ensuring that the solids removal processes are operating at design flow rates and are not hydraulically overloaded, which negatively impacts performance. The equalization basins for these facilities are often large tanks with diameters ranging from 140 to 200 feet and liquid depths ranging from 20 to 50 feet.
Ideal for these types of applications are jet aerators and jet mixers from KLa Systems Inc. These aerators and mixers provide flexibility of operation and configuration in large-diameter tanks; consist of corrosion- and abrasion-resistant, in-basin components; use simple external pumping systems; and have a more than 20-year design life.
Jet Aerator or Jet Mixer?
The choice of using a jet aerator/mixer is typically made by the consulting engineer after carefully analyzing the source of water to be equalized. If the equalization tank has significant amounts of sewage mixed with the stormwater, an aeration system should be chosen, as the oxygen transfer component from the addition of air ensures that there will be no odors emanating from the facility.
If it is only stormwater, then in some cases hydraulic mixing with no air addition should be chosen, making blending and off-bottom suspension of total suspended solids (TSS) the main design parameters. These tanks are typically covered for odor abatement.
KLa Technology and Experience
The jet aerator/mixer systems are available in a variety of materials of construction. Typically, the piping and jets are fabricated of high-grade vinylester resin fiberglass reinforced pipe (FRP) with a stainless steel support system. Likewise, KLa jet nozzles are offered in all FRP with proprietary abrasion liners, a combination of FRP with polypropylene, or Kynar polyvinylidene fluoride (PVDF). An all-polypropylene jet is also available.
In all KLa installations, municipalities have been able to successfully mix and equalize large volumes of stormwater and sewage and maintain aerobic conditions when warranted, as well as accomplish off-bottom suspension of TSS. The KLa Systems jet mixer systems designs have been confirmed by computational fluid dynamics (CFD) modeling on four different occasions.
For more information, visit www.klasystems.com.
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