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A grit chamber is the second step procedure that is used in the primary treatment of wastewater.
The grit chamber removes sand and other weighty matters which are inorganic such as metal fragments, rags, etc. If not removed in primary treatments, grit in the primary settling tank can induce abnormal abrasive wear and tear on mechanical equipment and sludge pumps, choke by deposition, and collection in sludge holding tanks and digesters. Therefore grit removal is essential to save the moving mechanical equipment and pump parts from abrasion.
Grit removal relies upon the differences in specific gravity between organic and inorganic solids to affect their separation. The specific gravity of the grit is usually 2.4 to 2.65.
1. Types of Grit Chambers
These chambers are divided into 2 types that are as follows:
1. Horizontal flow grit chambers
2. Aerated grit chamber
a. Horizontal flow grit chambers
These chambers are a lean open channel, which is about 10-18 meters long and 1 to 1.3 m in depth. Velocity in the grit chamber should be held in such a way that the velocity will carry most of the organic particles through the chamber and will re-suspend any that are designed.
A control Aerated settle but will allow the settling of weightier grit materials particles. It is to keep a velocity of 0.3 m/s.
A control department is utilized, in order to keep a fairly consistent velocity of flow.
b. Aerated Grit chamber
An aerated grit chamber is a unique form of grit chamber having a standard spiral flow aeration tank equipped with air diffusion tubes set at one end of the tank at about 0.6 to 1m from the bottom.
The weightier grit particles with their higher settling velocities settle down to the floor, whereas more lightweight organic particles will stay in suspension and carry with the roll of spiral motion because of the diffused air and finally move out of the tank.
It is further classified into two types, based on the cleaning mechanism.
a. Mechanically cleaned
b. Manually cleaned
a. Mechanically cleaned
Mechanically cleaned grit chambers are equipped with mechanical equipment for the accumulation and washing of grit chambers, which are served either on a continuous or intermittent basis.
b. Manually Cleaned
Manually run grit chambers should have adequate capacity for hold of grits between the time of cleaning.
The easiest method of cleaning is with the help of a shovel.
2. Uses
The uses of the grit chamber are as follows:
✔ It is used to prevent equipment from clogging.
✔ It is used to slow down the flow to settle heavy solids.
✔ It is used to save the waste treatment cost.
✔ It is used to control grit collection in sludge digesters.
3. Composition of Grit
The composition of grit varied depending on the following conditions-
✔ Types of street surfaces encountered
✔ Relative areas served
✔ Climatic conditions
✔ Types of inlets and catch basins
✔ Amount of stormwater diverted from combined sewers at overflow points Sewer grades
✔ Construction and condition of the sewer system
✔ Ground and groundwater characteristics
✔ Industrial wastes and
✔ Social habits.
4. Working Principle of Grit Chamber
The working principle of the grit chamber is as follows:
It operates as a sedimentation tank which is prepared to distinguish the planned weightier inorganic materials (specific gravity of about 2.65) and to move ahead of the more lightweight organic materials.
Differential sedimentation and differential scouring velocity are held in the chamber so that the flow velocity should neither be less as to cause the settling of more lightweight organic matter nor should it be more as not to cause the settlement of the silt and grit attending in the sewage.
The critical velocity of flow ‘vc‘ beyond which particles of a particular size and density once settled should always be smaller than the scouring velocity of grit particles. The critical velocity of scouring is provided by Schield’s formula-
V = 3 to 4.5 (g(Ss – 1)d)1/2
The horizontal velocity of flow of 15 to 30 cm/sec is utilized at peak flows. This identical velocity is to be held at all change of flow to make sure that only organic solids and not the grit is scoured from the bottom.
5. Advantages of Grit Chamber
The advantages of this chamber are as follows:
✔ To save running mechanical equipment from abrasion and abnormal wear.
✔ To decrease maintenance cost in the frequency of digester cleaning caused by an extreme collection of grit.
✔ To control weighty deposits in pipelines and channels.
✔ It also saves the cost of waste treatment by stopping solid materials in it.
6. Disdvantages of Grit Chamber
The disadvantages of the grit chamber are as follows:
✔ They are more probable to release toxic odors and toxic organic matter.
✔ Aeration system control and maintenance will affect different human resources.
✔ Compared to other girt removal technologies, they need more energy resources.
✔ Initial construction cost is high.
✔ Regular maintenance is needed.
7. FAQ
Here are the most asked question related to this topic.
a. How does a grit chamber work?
It works as a sedimentation tank which is prepared to distinguish the planned weightier inorganic materials (specific gravity of about 2.65) and to move ahead of the more lightweight organic materials.
b. What is a grit chamber?
It is the second step procedure that is used in the primary treatment of wastewater.
b. What is an aerated grit chamber?
An aerated chamber is a unique form of grit chamber having a standard spiral flow aeration tank equipped with air diffusion tubes set at one end of the tank at about 0.6 to 1m from the bottom.
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