Flow equalization is critical where significant variations in flow rates and organic mass loadings are expected. Flow equalization is also important if a plant is expected to receive a significant amount of septage or is taking in a significant amount of industrial wastes. Flow equalization is strongly recommended when a plant needs to achieve nitrification and denitrification. It is important to note, however, that the size of the influent equalization basin must be carefully considered because an oversized basin can cause negative downstream-treatment-process impacts. A plant utilizing an influent equalization basin will be able to have a true batch reaction.
Influent-flow equalization benefits the SBR process in the following ways:
- Allows for a smaller SBR-basin size because it allows for storage until the processcycle is complete.
- Allows for one basin to be taken off line for maintenance or for seasonal variations. Routine maintenance is necessary for all tanks. For plants that have seasonal variations, taking one basin off line is cost-effective due to a reduced need for electricity, staff hours, and tank maintenance.
- Allows for scum and grease removal at a single point before it enters the SBR tank. Entrainment by mixing should not be the sole means of scum control. A mechanism or process for removing scum, grease, and floatables should be provided in the equalization tank.
- Allows plants that must denitrify to ensure that an adequate amount of carbon is available in the denitrification fill phase.
- Allows for an equal flow volume into the basin, keeping the food to microorganism ratio (F/M) fairly stable.
With the use of influent-flow equalization and bar or mechanical screens, the wastewater stream entering the SBR is free of grease, scum, rags, sticks, floatables, and other debris, making it easier to treat.
As stated previously, each SBR design is unique and in some situations influent-flow equalization basins may not be required to obtain optimum treatment. Examples of where influent-flow equalization is not needed include (but are not limited to) plants designed with three or more SBR basins and plants that do not need to nitrify and denitrify.
If a plant is operating with a two-basin system without influent-flow equalization, then it should have an adequate supply of essential spare parts onsite. This will allow broken components to be returned quickly to service without the need to wait for back-ordered parts.
The influent-equalization basin should have a form of agitation or mixing to keep the solids in suspension. A mechanical-mixing unit can be used for this purpose. Maintenance on this basin should be minimal as the solids are in suspension due to the agitation; however, a means to bypass the equalization basin and to dewater the basin should be provided. Pumps that direct influent to the SBRs should be in duplicate. Influent-flow equalization should be designed to hold peak flows long enough to allow the active treatment cycle to be completed.