Grit Removal...

Wastewater contains a certain amount of inorganic, relatively heavy material. This includes sand, bits of hard plastic, glass, pebbles, etc. These materials are abrasive and can prematurely wear pumps and other equipment. There are several common methods of removing grit. This facility uses a centrifugal grit seperator. The wastewater enters a small, circular tank at a tangent. This produces a circular flow pattern. Centrifugal force causes the heavier matter to move to the center bottom of the chamber. There they through an opening into a temporary holding tank. The grit is periodically pumped out, dewatered, disposed of at the landfill.


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Grit : sand, gravel, corn kernels, bone chips, coffee grounds.
Purpose of removal : (1) prevent excessive abrasion of mechanical equipment ; (2) prevent deposition of grit in pipes and channels (that eventually would lead to clogging) ; (3) reduce accumulation of grit in downstream units such as settling basins and digesters.
Aerated grit chamber : diffused air keeps organic solids in suspension as grit settles.



Square, clarifier-type : grit washing necessary to remove heavier organics.
Horizontal flow : was used in past, but rarely used today.
Purpose of "pre-aeration" in grit chambers : (1) freshen wastewater ; (2) remove gases ; (3) improve settling downstream.

"Centrifugal Grit Chamber"...

Characteristics of Grit...

- Sand, gravel, cinders, eggshells, bone chips, seeds, coffee grounds and other heavy materials
- Predominantly inert, composition variable
- Moisture content 13 - 65%, volatile organic content - 1 - 56%
- Specific gravity - clean grit particles - 2.65, for material with substantial organic material attached to inerts - approx. 1.3
- Bulk density in the range of 1600 kg/m3
- Most grits are retained on a No. 100 mesh sieve (0.15 mm or larger)
- Typical settling velocity for 100 mesh grit is 1.3 cm/s or 2.6 ft/min

Purpose of Grit Removal...

- To protect mechanical equipment from abrasion and abnormal wear
- To reduce conduit clogging caused by deposition of grit particles in pipes and channels
- To prevent loading the treatment plant with inert matter that might interfere with the operation of treatment units such as siltation of anaerobic digester and aeration tanks

Type of Grit Removal...

- Horizontal flow (square or rectangular configuration)
- Aerated (rectangular)
- Vortex-type

Horizontal Flow Type...

- Open channels with sufficient detention time to allow particles to settle and to maintain constant velocity to scour organics
- The velocity of flow controlled by ; (i) dimensions of unit, (ii) special influent distribution gates, (iii) special weir sections at the effluent end (proportional weir)
- Designed to remove grit > 0.21 mm dia. to as low as 0.15 mm dia.
- Grit removal is accomplished by a conveyor with a scraper, buckets or plows
- May require grit washing equipment to remove organics

Typical Design Information for Horizontal Grit Removal...

Parameters Range Typical
Detention Time (s) 45 - 90 60
Horizontal Velocity (ft/s) 0.8 - 1.3 1.0 - 1.25
Headloss in a Control Section
as Percent of Depth in Channel		 30 - 40% 36%
Allowance for Inlet and Outlet Turbulence 2 Dm - max - 0.5 L (At least 50% increase in theoretical length is recommended ;
Dm - max : depth, L : length)
Grit Quantities 0.004 - 0.037 m3/1,000 m3 -

Aerated Grit Chambers...

- Popularity of aerated grit chamber
- Less wear on grit-handling equipment
- In many cases, no need for separate grit washing equipment
- Normally designed to remove particles 65 mesh (0.2 mm) or larger
- Velocity of roll or agitation governs the size of particles of a given specific gravity to be removed
- Quantity of air is adjusted to provide the roll and washing of the grit to remove organic matter
- Grit removed by using grab buckets on monorails centered over the grit collection and storage trough or by a flushing through a drain


Typical Design Information for Aerated Grit Chambers...

Parameters Range Typical
Detention Time (Mins.) at Max. Flow 2 - 5 3
Depth (ft) 7 - 16 -
Length (ft) 25 - 65 -
Width (ft) 8 - 23 -
Width - Depth Ratio 1:1 - 5:1 1.5 : 1
Length - Width Ratio 3:1 - 5:1 4:1
Air Supply (ft3/min.ft of length) 2 - 5 -
Grit Quantities (ft3/mgal) 0.5 - 27 2.0

Vortex - Type Grit Chambers...

- Vortex created
- Grit move to the outside of the unit and collected
- Air scour to remove organics
- Grit removed by air lift pump or by bottom drain
- Typically designed to remove 95% of 100 micron (150 mesh) grit
- Head required to achieve this removal is a few meters, for example, 95% removal of 25 micron - head requirement - 5 - 7 m
- Typical units can handle peak flows up to 0.31 m3/s (7 mgd)

Typical Design Information for Vortex - Type Grit Chambers...

Parameters Range Typical
Detention Time (s) - 30
Diameter (ft) 4 - 24 -
Height (ft) 9 - 16 -
Removal Rates (%) 50 mesh (0.3 mm) 95+
70 mesh (0.24 mm) 85+
100 mesh (0.15 mm) 65+ 		-

Design Example for Horizontal Aerated Grit Chamber...

Design an aerated grit chamber with an average flow of 11.4 mgd (0.5 m3/s) with a peaking factor of 2.75.

1. Peak hourly flow rate for design = 11.4 x 2.75 = 31.35 mgd

2. Grit chamber volume (Assume 2.0 minutes for peak hourly flow and assume two chambers in use all the time).


3. Dimensions.

Use width to depth ratio of 1.1:1 and a depth of 8 ft

- Width = 1.1 x 8 = 8.8 ft

- Length = 2,910 ft2/(8 x 8.8) = 41.3 ft (Use 41 ft)

4. Check horizontal velocity.


(On the low side, but acceptable)

5. Air supply.

- Use 5 ft3/m. ft of length

- Air required = 41 ft x 5 ft3/m. ft of length = 205 ft3/min

6. Average grit removal.

- Assume 7 ft3/mgd

- Volume of grit = 11.4 mgd x 7 ft3/mgd = 79.8 ft3/day