Grit – You Don’t Know What You’re Missing!

Barter, P.1 and Sherony, M.2
1Hydro International UK, 2 Hydro International US Wastewater


Grit removal is a long established practice within the UK wastewater industry with a number of
technologies having evolved to address the challenges of grit removal at wastewater treatment
plants. The question however is that do these systems really work? Are grit removal systems being
designed optimally? Aeration lanes suffer from a build-up of grit and often require draining down
and cleaning. Anaerobic digesters also have to be routinely taken out of service to have deposited
grit removed. These operations are expensive and time consuming, as well as having a major impact
on the efficiency and productivity of the wastewater treatment plant. Is the assumption of a 2.65 specific gravity, 200 micron particle size typically used in design adequate?

This paper examines what we are missing when we consider grit removal and how that may be
avoided. Drawing on experiences with reference to particles size analysis work carried out in the US
and UK, it outlines what can and should be done to prevent grit from entering the wastewater
treatment works and causing the downstream problems observed.

Grit removal, particle size analysis, specific gravity, anaerobic digester cleaning


Grit removal as part of the wastewater treatment process is a long established practice within the
UK water industry; grit causes problems with down-stream plant such as pumps and valves by
causing undue wear and tear, it blocks pipes and channels and reduces the capacity of aeration tanks
and anaerobic digesters.

The removal of grit is based on the ability of the process to remove an identified particle from the
water by establishing a steady, stable and predictable flow regime, which allows the particle to settle
and be removed from the bulk flow without being re-entrained.

In many cases the suggested ideal target grit particle is a spherical, homogenous, 200 micron sized
particle with a specific gravity of 2.65, but the question really is “how accurate are these
assumptions and is this the grit particle to target”? What are the consequences and effects if these
parameters are wrong?

Other questions to be considered are what effect does varying flow have on the process? How
should the consequences of this flow variance be handled?
This paper examines these questions from a fundamental perspective including assessing properties,
characteristics and their effect on the sizing consideration of the grit removal system.

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