Grit chamber: purpose, construction, design

A grit chamber is a narrow, long channel or tank placed after the screens (and before the primary sedimentation tank) in which the velocity of sewage is controlled so that heavy inorganic particles (sand, gravel, cinders, eggshell fragments of size about 0.15 mm and above, specific gravity about 2.65) settle, while the lighter organic matter stays in suspension and passes on.

Why a grit chamber is needed

• Grit is abrasive and wears pumps, pipes and sludge-handling equipment.
• It settles in channels, tanks and digesters, reducing their capacity and causing clogging.
• If grit mixes with organic sludge in the primary tank, the sludge becomes heavy and difficult to digest and dewater.
• Removing grit separately gives clean, easily disposable inert material.

Purpose

To remove grit without removing the organic solids, and so protect the downstream mechanical equipment and save tank capacity.

Construction (horizontal-flow type)

 Plan                      Section
 ----------------------    ~~~~~~~~~~~~~~~~~~~~~~~ water level
 inlet -> |           |    -> v = 0.2-0.3 m/s ->
 zone  -> |   channel |  ->  \_ grit settles   _/
 -----------------------      grit storage hopper
          proportional weir / Parshall flume at outlet

• A rectangular channel, normally two or more units in parallel so that one can be cleaned while the other works; inlet and outlet have tapered transitions.
• The outlet has a proportional (Sutro) weir or a Parshall flume to keep the horizontal velocity almost constant at about 0.3 m/s even when the flow varies.
• A hopper at the bottom (or extra depth of 0.2-0.3 m) stores the grit; it is removed by hand-scraping in small plants or by mechanical scrapers, screw conveyors or air-lift in large plants.
• Aerated and vortex-type grit chambers are other variants.

Design criteria

1. Flow-through velocity: about 0.2-0.3 m/s. It must be below the scour velocity of grit, $v_c=\sqrt{\dfrac{8k}{f}(G-1)g\,d}$ ($k=0.04$-$0.06$, $f=0.02$-$0.03$), but high enough to carry organic matter ($v \ge$ about 0.15 m/s).
2. Detention time: 30-60 seconds (usually about 1 minute) at maximum flow.
3. Settling velocity for 0.2 mm grit of specific gravity 2.65 is about 0.02-0.04 m/s (Hazen: $v_s=418(G-1)d^2\dfrac{3T+70}{100}$ mm/s, $d$ in mm).
4. Length $L=\dfrac{v\,H}{v_s}$, increased by 20-50% for inlet and outlet turbulence.
5. Cross-section $A=Q/v$, with depth $H$ chosen (0.6-1.5 m) so that width $B=A/H$ is practical; allow 0.3 m free board and 0.2-0.3 m grit storage depth.
6. Surface loading (overflow rate) about 800-1200 m³/m²/day.
7. Grit removed is about 0.03-0.1 m³ per 1000 m³ of sewage.

Example: $Q=0.2$ m³/s, $v=0.25$ m/s gives $A=0.8$ m²; with $H=0.8$ m, $B=1.0$ m; if $v_s=0.025$ m/s then $L=0.25\times 0.8/0.025 = 8$ m, plus 25% = 10 m.