What is an equalization tank?
Equalization tank: Flow equalization basin
In short
An equalization tank (or basin) is a holding volume that stores wastewater during peak periods and releases it at a controlled rate, so the treatment processes downstream see a steady flow and load instead of the morning and evening surges a sewer delivers. It protects biological treatment from washout in storms and avoids sizing every process for the peak hour.
Plants also use them to run pumps and clarifiers at their design rates. In EPA unit-process data, flow equalization is recorded at US plants Klarifi tracks, and 712 plant-wide upgrade line items in city capital plans include new or expanded basins.
In-line versus off-line equalization
An in-line basin receives all the flow and discharges it at a constant rate, which also dampens concentration swings. An off-line (side-line) basin only fills when flow exceeds a set point and drains back when it drops, which needs less volume and mixing energy but does less to smooth loads. Most municipal basins are off-line and sized from a mass diagram of the daily flow pattern, typically 10 to 20 percent of average daily flow.
Mixing, aeration and odor
Stored sewage goes septic within hours, so basins are mixed and usually aerated with coarse-bubble diffusers or floating aerators to keep solids in suspension and stop hydrogen sulfide forming. Covers and odor control are common retrofits. Industrial pretreatment systems use equalization for the same reasons, plus pH neutralisation.
Equalization and wet-weather capacity
Where combined or leaky sewers bring storm flow to the plant, equalization storage is often the cheapest way to meet peak-flow permit conditions and avoid bypasses. That is why storage tanks and basins appear in so many sewer-overflow consent decrees and capital plans.