Equalization basins are used to smooth out wide variations in flow so that a constant or nearly constant flow rate can be achieved. Flow equalization improves the performance of the downstream processes and can reduce the size, number and cost of downstream treatment facilities.
They can either be in-line units, where all the flow passes through the basin or off-line units, where only the flow above some predetermined rate is diverted into the basin. The off-line basin is commonly used to capture the first flush from the combined sewers, while the in-line basin may be used if equalization of the plant loadings is also desired.
The in-line basins are normally used to even out hydraulic loads and daily flows since the flows differ from day to night in homes, industry and business. The in-line equalization basins also allow the operator to determine the appropriate amount of flow or hydraulic load to consistently feed to his plant to optimize the treatment process.
The off-line basins are normally used during and after rain events to catch what would normally cause a hydraulic upset at the treatment plant, such as a solids’ washout when all the solids end up in the stream. If the equalization basin is the right size and design, it should hold that first flush of rain from the inflow and the subsequent infiltration into the collection system and allow the operator to make the appropriate adjustments to the treatment operation.
Another use for the off-line equalization is as an organic or toxic dilution holding area. If a heavy load of organic or toxic material comes to the facility from an industry or a spill, it can be routed to the equalization basin and raw influent is then used to dilute it down to a concentration that the treatment scheme can accommodate.
Primary considerations in the operation include pump control systems, mixing and aeration requirements. The mixing equipment has to be adequate to prevent deposition of solids in the basin and aeration is needed to prevent the wastewater from becoming septic and odorous.  “Ten States’ Standards” recommends a minimum of 1.25 CFM per 1,000 gallons of storage capacity.
As mentioned, the pump control equipment is important to the optimization of the treatment process. If there are float switches, such as a lift station or wet well, they must be kept clean and free of grease and debris. The operational staff must know the pump flow rate for the pump used to feed the flow from the equalization basin to the influent. If it is only a portion of the flow, it must be added to the influent number to determine the correct influent flow. This correct flow rate is used in calculating food to microorganism ratio, organic loading and sludge age, just to name a few.
The requirements for aeration and mixing are normally met using the same piece of equipment, an aerator of some sort, depending on what style of equalization basin you have and what you want to accomplish. If your equalization basin is shallow with a large surface area, the best choice might be surface aerators. These aerators pump water from a depth of approximately 4 feet and throw it 4 to 6 feet up through the atmosphere to collect oxygen. At the same time, the pumping action keeps the basin mixed. The area and depth of your basin would determine the number and horsepower of this type of aerator. These types of aeration units may be used in deeper basins with the addition of a draw tube attached to the bottom of the pump.
If your basin is deep and has a smaller surface area, you may want to use the blower of a compressor-style unit. This apparatus consists of an air compressor or blower with air lines attached to bubble diffusers. The diffusers may or may not be attached to the bottom of the basin to add oxygen and facilitate mixing.
The number of aeration units of any type is also determined by what you wish to accomplish. If you want to keep the basin mixed well enough that the solids won’t settle to the bottom and go septic, then a limited amount of mixing and aeration will be needed. You would need to keep a dissolved oxygen level of .5 mg/l throughout the water column to maintain a thorough mixing.
If you wanted to use the equalization basin as an aeration basin, then more aeration would be needed as stated above:  “Ten States’ Standards” recommends a minimum of 1.25 CFM per 1,000 gallons of storage capacity. This should provide approximately 1-1.5 mg/l of DO throughout the column. This level of DO provides enough oxygen to the bacteria to significantly reduce the BOD, while maintaining the mixed character of the wastewater.
The lack of any aeration in an equalization basin causes it to act as a primary clarifier. As the water stills in the basin, the lighter materials (oil, grease, plastic) float to the surface and the heavier organic and inorganic material (coffee grounds, eggshells, pasta, sand, grit) settle to the bottom and begin to degrade, go septic and stink.
In addition to the problems caused from no aeration, equalization basins are not normally designed with scum removal. Sludge removal in equalization basins is extremely difficult because the liner in the equalization basins is often ripped by the equipment used to remove the accumulated sludge.
Equalization basins, either in-line or off-line can be used in different modes with different amounts of aeration to accomplish a variety of things. If your plan is to use them to fix your inflow and infiltration problem, they are not the right choice. They can be, at best, a stop-gap measure until you can correct the problems in the collection system.