L-13Grit Chamber

Environmental Engineering-II

Preliminary Treatment

Introduction and Objective• Grit removal basins are the sedimentation

basins placed in front of the fine screen to remove the inorganic particles having specific gravity of 2.65 such as sand, gravel, grit, egg shells, glass pieces, gravel, grit, egg shells, glass pieces, metal fragments etc. and other non-putresciblematerials that may clog channels or damage pumps due to abrasion and to prevent their accumulation in sludge digesters.

Settling Categories• Type I - discrete settling in dilute suspensions,

grit chamber good example• Type II – flocculent materials in dilute

suspensions, primary settling basin good example• Type III (Zone or hindered settling) – high • Type III (Zone or hindered settling) – high

concentrations (>1000 mg/L), particles interfere with each other’s settling, secondary settling basin

• Type IV (Compression settling) – weight of particles cause more settling, sludge zone in clarifiers

Types of Grit Chambers• Grit chambers are of two types:

mechanically cleaned and manually cleaned.

• In mechanically cleaned grit chamber, • In mechanically cleaned grit chamber, scraper blades collect the grit settled on the floor of the grit chamber.

• The grit so collected is elevated to the ground level by several mechanisms such as bucket elevators, jet pump and air lift.

• The grit washing mechanisms are also of several designs most of which are agitation devices using either water or air to produce washing action.

• Manually cleaned grit chambers • Manually cleaned grit chambers should be cleaned at least once a week.

• The simplest method of cleaning is by means of shovel.

• There are two general types of grit chambers

1. Horizontal flow grit chamber and

2. Aerated grit chamber

Horizontal flow grit chamber• Velocity close to 0.3 m/s is maintained.

• Such velocity will carry most of the organic matter through the channel and will tend to re-suspend any that settle, but permit heavy grit to settle.to settle.

• Therefore velocity in the grit chamber shall be neither too low as to cause settling of lighter organic particles nor should be so high as not to cause the settlement of entire silt and grit present in the sewage.

• Length – 10 to 18 m

• Depth of liquid – 1 to 1.3 m

• Velocity is controlled by means of velocity control device.

Aerated Grit chamber

Vortex type grit chamber

Design criteria1. Settling Velocity:- is given by

Stoke’s Law for laminar flow:

vs = g(ρp – ρw)dp2/18µ

v = g(G – 1)d 2/18υvs = g(Gs– 1)dp2/18υ

Vs = settling velocity

dp= size of particle

Gs = Specific Gravity

ρp = density of particles

ρw = density of water

µ = dynamic viscosity

υ = µ/ ρw = kinematic viscosity

g = Gravitational accelerationg = Gravitational acceleration

2. Overflow rateSOR is given by

SOR = Q / As

Q = flow rate

A = Surface areaAs = Surface area

3. Detention period

• Detention period may vary from 45 to 90 seconds.

• Generally it is taken as 60 sec.• Generally it is taken as 60 sec.

4. Bottom scour and flow through velocity

5. Velocity Control Devices

6. Number of units

• For manually cleaned grit chambers at least two units shall be provided..

• For mechanically cleaned units one • For mechanically cleaned units one additional manually cleaned unit as by-pass shall be provided

7. Dimensions• Free board of 150 to 300 mm shall be

provided.

• Length can be calculated as

L = vh x t• Depth can be calculated as • Depth can be calculated as

D = vs x t• Bottom slopes shall be provided.

• Breadth depends upon velocity control device adopted.

8. Head loss

• Head loss – 0.06 to 0.6 m

• Depends upon velocity control device adoptedadopted

Disposal of Grit• The moisture content of grit varies

from 14 to 34 percent.

• Grit is sometimes washed after collection to reduce the organic collection to reduce the organic concentration which may be as much as 50 percent of the total solids.

• Filling Low lying area.

• Incineration.

• Well-washed grit has been used on sludge drying beds, as a cover for sludge drying beds, as a cover for screenings, and as a surfacing material for walks and roadways.

Partial flume

Aerated Grit chamber

Aerated grit chamber

Drained grit chamber with sediments

Objective Questions1. In Grit chamber ______________ settling is

observed.

2. For grit chambers detention period of __ sec is usually adopted.

3. Grit chamber removes particles having Sp. Gr. In 3. Grit chamber removes particles having Sp. Gr. In the range of ____ to _____.

4. Pick out odd one

a. Silt, Broken glass, card board, egg shells, metal fragments

b. Parshall flume, sutro weir, V notch, Proportional flow weir

Theory Questions

Q1. Explain necessity and working of grit chamber with neat sketch.

Q2. Write a detailed note on ‘ design of grit chamber’.grit chamber’.