EQ (Equalization) Tank

Among the most effective waste management procedures is equalization of the wastewater. Equalization can be of two types: flow equalization and contaminant equalization (COD, TSS, pH, etc.). The use of flow equalization tanks following the API separators provides a relatively constant flow rate and contaminant loading to the biological treatment system. We are trying to reduce or control variability in wastewater characteristics.

With equalization, you can provide a more uniform effluent to downstream processes. This becomes particularly beneficial (and important) when biological treatment follows. And here again, we find in wastewater, as in life, that size really does matter, particularly so when it comes to EQ tanks. The equalization tank(s) must be large enough to absorb and dampen concentrated batch dumps or spills within the process.

Equalization tank

In complex chemical and petrochemical plants there are numerous waste streams that enter the wastewater treatment plant. Some of these streams are a readily biodegradable source of substrate (food) for the bacteria in the biological treatment unit. Other streams are inhibitory while still others might have a toxic impact to the bacteria. The EQ tank dampens or neutralizes the impact of inhibitory or toxic compounds in the wastewater. The graphic below shows several waste streams that enter the wastewater treatment plant in a refinery.


The goal of an EQ tank is very simple:

  • Equalize the flow rate,
  • Equalize or balance the organic concentration,
  • Dilute or dampen the impact from inhibitory compounds,
  • Neutralize the pH,
  • Even out temperature fluctuations, and
  • Minimize chemical usage (e.g., downstream pH adjustment).

Some aspects to the design and operation of an EQ tank are as follows:

  • The equalization tank should be completely mixed.
  • Can be operated at constant volume which results in variable outflow.
  • Can be operated at variable volume which results in constant outflow.
  • Highly variable waste streams favor variable volume/constant outflow operation.

Operator input for an EQ is quite basic and involves the following:

  • Sample points on the side of the EQ tank should be checked regularly to confirm the surface oil level in the tank.
  • Oil skimming should occur as needed.
  • Bottom solids should be removed from the tank periodically.

The Excel spreadsheet shown below shows the typical testing I do on the wastewater leaving the EQ tank. The EQ tank effluent is typically the inlet wastewater to the biological treatment system.

EQ tank laboratory analysis