Sewage Treatment Grit chamber

Ashish KumarC07202

Civil(6th Semester)

Classification of Sewage Treatment processes:

◦ Preliminary Treatment:-includes Screening for removing floating papers , rags , clothes etc. ; Grit Chambers or Detritus Tank for removing grit and sand ; and Skimming tanks for removing oils and greases.

◦ Primary Treatment:- includes sedimentation tanks which are designed to remove a part of he organic matter from the sewage effluent coming out from the grit chambers.

◦ Secondary Treatment:- further treatment of effluent coming from sedimentation tank.filters,aeration tanks,oxidation ponds and aerated lagoons,anaerobic lagoons,septic tanks,inhoff tanks etc. are used for biological decomposition of organic matter.

◦Final or Advanced Treatment:chemical processes carried out for removing iron,phosphate etc..

Grit: Municipal wastewater is a combination of

Inorganic solids such as pebbles,sand,silt,egg shells, glass,metal fragments

Organic solids such as bone chips,seeds,coffee and tea grounds,vegetable cuttings,ash clinker,wood pieces etc

These are called Grits.

Grit Chambers◦ These are placed usually after the fine screens and before the

primary sedimentation tank◦ Grit chamber removes the inorganic grit of nominal diameter

of 0.15-.20 mm or more.◦ In general, grit channels are desingned to remove all particles

of higher specific gravity of 2.65 or so, with a nominal dia of .20 mm or more, having settling velocity of 21mm/s .

Principle of Grit RemovalArea of channel is increased so as to reduce the

velocity so that particles get the required time to settle down.

Settling Velocity : The settling velocity of a spherical particle is expressed by stoke’s law which takes into account the velocity of flow,viscosity of water, size , shape and specific gravity of the particle. ( Drag force=Effective Weight of particle)

Grit channels can be divided into two types:

Horizontal flow type( non aerated): the flow passes through the channel in a horizontal direction. The chamber(or channel) is designed to give a horizontal straight line flow velocity, which is kept constant over varying discharge.

Aerated type : consists of a spiral flow aeration tank , where the spiral velocity is controlled by the dimensions/shape of the chamber and the quantity of the air supplied to the unit.

Constant Velocity Horizontal flow Grit channels

Such a grit channel is an enlarged channel or a long basin in which the cross-section is increased, so as to reduce the flow velocity of sewage to such an extent that heavy inorganic materials do settle down by gravity.

provision of chambers: 1]multiple units: Additional chambers are used for taking increased discharge at peak hours.Generally 2-3 separate chambers are provided. Also helps in manual cleaning.

2]Single unit: Velocity control sections are used.

Proportional flow weir or a sutro weir It is provided at the effluent end of the rectangular

channel, which helps in varying the flow area of the section in direct proportion to the flow, and thus, helps to maintain a constant velocity in the channel.

A channel with a varying cross-section geometry is an alternative to providing a modified wier.

Design Q- A rectangular grit chamber is designed to remove particles with a dia of 0.2 mm,

specific gravity 2.65. Settling velocity for these particles has been found to range from 0.016-0.022 m/sec, depending upon their shape factor. A flow through velocity of 0.3 m/sec will be maintained by proportioning weir. Determine the channel dimensions for a maximum wastewater flow of 10,000 cum/day.

Solution: Horizontal velocity flow=V(h)= 0.3m/sec Discharge=Q=10,000 cum/day=0.116 cumecs Area of cross section = Q/V(h)= 0.385 meter square Assuming a water depth(H) of 1 m above the crest of the weir , which is kept at 0.3 m above

the tank bottom, we have the width(B) of the basin as 1xB=0.385 => B= 0.385 m ; say 0.4 m Overall depth of grit chamber(D)= Water depth above the crest of weir+0.3m+free board of

0.45 m = 1.75 m

Settling Velocity is b/w 0.016 to 0.022 m/sec, and hence let it be 0.020 m/sec. Now, settling velocity=V(s)=0.020 m/sec => Detention time= Water depth in the basin/settling velocity = 1/.02=50 seconds Length of Tank= Horizontal flow x detention time=0.3x50=15 m Length of the tank is increased by 10-30% to account for non idealities in the flow and settling of

particles.

Hence, use a rectangular tank, with dimensions: Length=20 m Width(B)=0.4m Depth(D)=1.75mDesign of proportional weir is to be done using the equation x= 2.B.V(h)/C(d).sqr root of 2g. Pi.sqr root of y where B=width of channel ….C(d) is coefficient of discharge=0.62….x and y are co-ordinates on

weir profile as shown in fig. take y=a=recommended value=0.025m for domestic wastewater putting a in equation we get b= 1.467. B. V(h)