Grit chamber_ An Overview
The Grit chamber is second unit operation in the primary treatment of wastewater. Its purpose is to remove suspended inorganic particles from the wastewater, such as sand and gritty materials. Except for some industrial wastewaters that may contain grit, this is normally confined to municipal wastewater and is not necessary for industrial effluent treatment plants.
The grit chamber is used to remove grit, which is made up of heavy particles such as sand, gravel, cinder, or other materials with a specific gravity substantially greater than the organic contents in wastewater. Grit chambers are used to protect moving mechanical equipment from abrasion and excessive wear, to prevent grit buildup in pipes, channels, and conduits, and to limit the frequency with which the digester needs to be cleaned.
How to calculate Horizontal flow velocity in Grit chambers?
The settling of grit particles in the chamber is referred to as Type – I settling since it is expected that the particles settle as distinct entities.
Inlet zone, outflow zone, settling zone, and sludge zone are the four compartments of the grit chamber.
Components of grit chamber
Zone – I: Inlet zone: This zone evenly distributes the entering wastewater over the grit chambers cross area.
Zone - II: Outlet Zone: After the grit is removed, this zone gathers the wastewater.
Zone – III: Grit Material Settling Zone: Grit material settles in this zone.
Zone IV: Sludge zone: This is a zone where settled grit collects.
L – The settling zone's length
H – The settling zone's depth
v — Horizontal wastewater velocity
Vo - Settling velocity of the smallest particle in the grit chamber to be eliminated.
For “Vs” being the settling velocity of any particle, the particles will be completely removed and these particles will be partially eliminated for “Vs <Vo”
Where,
“Vo” denotes the settling velocity of the smallest particle to be eliminated. The smallest particle intended to be eliminated in the grit chamber is 0.2 mm in size, while in practice, the smallest particle is sometimes believed to be 0.15 mm in size.
“Vo” denotes the final velocitywith which this tiniest particle will settle. This velocity is commonly referred to as the surface overflow rate or surface settling velocity, and can be represented as flow or discharge per unit surface area of the tank.
Now, with a settling velocity of “Vs ³Vo”, for 100% particle removal, we have:
Detention time = L/v = H/Vo or L/H = v/Vo
The magnitude of ‘v’ should not exceed critical horizontal velocity Vc.
To prevent scouring of already deposited particles, the above equation becomes,
L / H = Vc / Vo
Critical velocity, Vc, is given by the following equation:
Where, b = constant
=0.04 for uni-granular sand
= 0.06 for non-uniform sticky material
f = Darcy –Weisbach friction factor
= 0.03 for gritty matter
g = Gravitational acceleration
S = Specific gravity of particles to be removed (2.65 for sand), and
D = Diameter of the particle, m
The grit chambers are designed to remove the tiniest particles, measuring 0.2 mm in diameter and having a specific gravity of 2.65.
Conclusion
Horizontal flow grit channel is an enlarged channel or a long basin in which the cross-section is expanded to slow sewage flow velocity to the point where the heavier inorganic particles settle down by gravity while the lighter organic particles remain in suspension and are discharged with the grit basin's effluent.
The flow velocity should not be too high to prevent the complete silt and grit included in sewage from settling, nor should it be too low to facilitate the settling of lighter organic materials. The flow velocity should also be high enough to scour out and reintroduce the settled organic elements back into the flow.
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