Balance and equalisation tanks

Function

Buffer changes in wastewater flow or load

Possible contents

Mixed wastewater and accumulated solids

Check equipment

Mixers, pumps, level devices and connections

Critical information

Process role and permitted interruption

How balance and equalisation tanks work

A balance or equalisation tank reduces variation before wastewater reaches downstream treatment. Depending on the design, it may smooth flow, concentration or both. Taking it out of service can therefore pass more variation to the next treatment stage.

The name of the tank does not establish its contents. The process and actual influent determine whether it holds sewage, industrial effluent or a mixture, and those characteristics can change over time.

Deposits and mixing equipment

Solids may settle according to tank shape, wastewater characteristics and operation. Floating material and settled sludge can need different removal methods, and deposits may obscure mixers or level devices.

Rated capacity says little about the amount of settled material. Inspection, level data and operating history give a better basis for estimating what may need to be removed.

Cleaning and treatment continuity

Continuing inflow and the effect on downstream treatment set the limits for taking an equalisation tank out of service. EPA guidance notes that equalisation smooths downstream flow and loading, so removing that stage temporarily can alter the conditions seen by the next process.

Any removed material must be assessed from its actual source and composition; industrial inputs can change waste characteristics. Work involving confined-space entry or moving or electrical equipment needs competent people and controls suited to the installation.

Return to service and inspection

Cleaning records should say what deposits were removed and which mixers, pumps, level devices or other components became accessible for inspection. Areas that could not be assessed should also be noted; repair and testing remain separate from cleaning.