physical unit operations

Physical Unit Operations

Grit Chamber Unit-III

Grit Removal in Grit Chamber

• Sand, ash, Cinder, Bone Chip, egg shells, etc., of size less than 0.2 mm are included in grit. It is not putrescible and possesses higher subsidence value than the organic matter. It is therefore possible to remove grit from the waste water easily by reducing the wastewater velocity in long channel called as grit channel. The velocity is reduced to about 0.3 m/sec. The settled grit is washed before its disposal.


• Grit Chamber is provided for the purpose of removal of silt and sand particles mainly so that the same will not cause, the wear and tear of vanes of pumps, clogging of pipes, as well as, valve operation difficult. Removal of grit also reduces accumulation of inert material in subsequent treatment units. Cementing effects are also prevented in settling tanks and digester by removal of grit.


• Grit Removal Unit may be a grit Channel, Grit Chamber or a Grit Basin. The Word grit chamber shall be used in subsequent discussion.

• There are two types of grit chambers • (i) Horizontal flow Grit Chambers• (ii) Aerated Grit Chamber


• Horizontal flow grit chamber are designed to maintain a velocity of around 0.3 m/sec. Such a velocity falls then the organic particles also settle down and if velocity becomes high grit particles will not settle. So to cause only settlement of grit, it is necessary to maintain constant velocity of around 0.3 m/sec. The Waste water flow varies and therefore it is required to maintain the constant velocity by providing proportional flow weirs, partial flumes and palmer- Bowlup flumes.

Horizontal Flow Grit Chamber


• Aerated Grit Chambers are Used for Selective Removal of Grit in medium and large sized wastewater treatment plants.

Aerated Grit Chambers

Grit Collection & Removal

• Mechanical Grit Collection in velocity controlled horizontal flow grit chambers and aerated grit chambers is achieved by the conventional equipment with scrapers screws buckets plows or some combinations of these. In some cases steep bottom slope is provided which will collect the grit at Central Point of Removal.


• Grit Removal is achieved by air pumps for small aerated grit chambers. Grit can also be removed by tubular conveyors, buckets type collectors, elevators screws conveyors, grit pumps and clam shell buckets.

Clam shell buckets

Quantity of Grit

• Grit Quantity varies greatly. It depends upon the following factors:

• Type of Sewerage System i.e. Separate or Combined Sewerage System

• Climate Condition• Soil Type• Sewer Grades• Type of Industrial Wastewaters• Relative Use of Garbage and grinders• The grit quantity may range from 5 to 200 m3/ Million Cubic

Meter of Wastewater. The typical Value can be Considered as 30 m3/ million Cubic meter of wastewater.

Quantity of Grit

Grit Disposal

• Various methods are Used for Grit Disposal • They are • Sanitary Landfill: In low lying areas or large natural pits

the grit is disposed. Such a method of disposal is preferred when site of disposal is far away from city or town.

• Land Spreading • Incineration with Sludge: Incineration is burning at very

high temperature in excess Oxygen. City solid waste can be incinerated. Sludge and grit can be taken to the incinerator for burning. Grit can also be buried when its quantity is small.

Sanitary Landfill

Land Spreading

Incineration with Sludge

Design of Grit ChambersFollowing Information should be collected for designing of the grit chambers• Wastewater Characteristics and Size of Grit particles to be

removed. • Design average, peak and lowest flow.• Information about existing plant if it is to be expanded.• Type of Grit Chamber to be provided i.e. Horizontal flow,

aerated etc. • Influent pipe data and static head force main and hydraulic

grade line in case grit removal preceded pumping station.• Head loss constraints for Grit Removal Efficiency.• Treatment plant design criteria by Bureau of Indian Standards.

Design Criteria for Horizontal Flow Grit Chambers

• Detention Time = 40 to 60 Sec• Horizontal flow velocity = 15 to 30 cm/sec

approximately • = 4 • Where g= Acceleration due to Gravity= 9.81

m/sec2 • Ss= Sp Gravity of Grit= 2.65• D= dia of Grit in m.

Design Criteria for Horizontal Flow Grit Chambers

• (iii) Surface Overflow Rate (SOR) • = 500-1500 m3/m2/day • Length to Breadth Ratio = 6 to 15 • Length to depth Ratio = 10 to 30 • Depth= (1.5 to 2) + free board• Free Board = 0.75 to 1.0 m

Design Criteria for Aerated Grit Chamber

No. Design Parameter Range of Variation Remarks1 Dimension

DepthLengthWidthWidth/ Depth Ratio

2-5 m7.5 to 202.5 to 7.0 1:1 to 5:1

Width of Basin is limited to prevent turbulence in the bank.

2 Transverse Velocity at Surface

0.6 to 8 m/sec

3 Detention Time at Peak flow, minutes

2 to 5 If Grit Chamber is Used to remove Grit less than 0.2 mm a longer detention time may be provided.

4 Air Supply L/S m 4.6 to 12.4 Litres/Sec/m length of the Tank.

Design Example• Design a Suitable Grit Chamber to Cater a town of 2

Lakh population with 150 l/capita/day Sewage Contribution

• Assume Peak Factor = 1.5 • Maximum Sewage Contribution per day • = Average Contribution x Peak factor • = 150 x 200000 x 1.5 litres• = 150 x 200000 x 1.5 m 3 1000 = 45000 m3

Design Example• 45000 m3 sewage produced daily i.e. in • 24 x 60 x 60= 86400 Seconds• Therefore Maximum Sewage Discharge • = 45000 m3/ Sec 86400 = 0.52 m3/Sec Assume horizontal velocity 0.3 m/sec and detention time 50 secTherefore Length of the tank Required= Velocity x Detention Time = 0.3 x 50 = 15 m

Design Example• Now Settling Velocity of Grit Particles are governed by Hazen’s

Modified Equation as Given Below:• Vs= 60.6 (Ss-1) d (3t + 70) 100Where, t= temp of Waste water 0 C d= dia of particle in cm Vs= Settling Velocity in cm/sec Ss= Sp Gravity of Grit, 2.65If Ss= 2.65 above equation becomes Vs= d (3t + 70)Taking t= 27 0C Vs= 3 m/sec

Design Example

• We want to remove 0.2 mm particle so Settling Velocity = 3 cm/sec

• Therefore Depth of Tank= 3.0 cm/sec x 50 sec.3

• = 150 cm• = 1.5 m• Taking Length to Width Ratio as 10 : 1 • Width of the tank = 1.5m

Design Example

• Now Check for SOR Surface Overflow Rate• Width= 1.5 m• Length=15 m• Plan Area= 1.5 x 15 = 22.5 m2 • Max Sewage flow = 45000 m3/day • Max SOR = 45000 m3/day 22.5 = 2000 m3/m2/day Which is higher than permitted as per Criteria

Design Example

So, take length: Width ratio as 6:1 Therefore Width= 2.5 m Therefore Plan Area= 2.5 x 15 = 37.5 m2

Therefore Max SOR= 45000 m3/ day 37.5 m2 = 1200 m3/m2/day Which is less than 1500 therefore O.K.

Grit Chamber

References

• Sewage Disposal & Air Pollution Engineering: S.K. Garg

• Environmental Engineering : By Prof B.R.Shah

Prof A M Malek

• Internet Websites

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