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Grit chamber is a long narrow or circular tank in the primary sewage treatment plant that is designed to reduce the velocity of the flow of sewage to eliminate the girt materials such as sand, ash and clinkers, eggshells, bone chips and many inert materials inorganic in nature.
Composition of Grit
Grit is non-putrescible and possesses a higher hydraulic subsidence value than organic solids. The specific gravity of the grit is usually 2.4 to 2.65. The composition of grit varied depending upon 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.
Types of Grit Chambers
Major classification of grit chambers is made into 3 types-
- Velocity controlled V-shaped long grit channels
- Square shaped chambers with entry and exit on opposite sides and mild hopper
- Vortex type cone and the centrifugal action plummets the grit to the bottom
Further depending upon the several factors such as the quantity and quality of grit to be handled, head loss requirements, space requirements,
topography and economic considerations with respect to both capital and operating costs are divided into-
- Mechanically Cleaned
- Manually Cleaned.
In very small plants, mechanization may be uneconomical. For Sewage treatment plant flows of more than 10 MLD, mechanized grit removal units are preferred.
1. Vortex Type Grit Chamber
In this type of grit chamber, the sewage is fed in the tangential direction to induce a vortex type of flow, which will funnel the grit towards the center, and hence be drawn down at the bottom chamber. An agitator is used to keep the grit in suspension.
Due to the vortex type of flow, the grit which has a heavier specific gravity of 2.4 to 2.65 settles down which is later drained onto a grit filter bed by gravity or pumped to the beds. This unit has its advantages in situations where sewage flow rates and durations vary widely.
2. Aerated Grit Chamber
It is a special form of grit chambers having a spiral-flow aeration tank installed with air-diffusion tubes placed on one side of the tank. As the sewage enters into the grit chamber the shape of the chamber and the air makes the sewage flow in a helical pattern.
Due to the helical flow pattern, the heavier grit particles settle down while the lighter organic particles are carried with a roll of the spiral motion and eventually out of the tank.
Clearing of the Grit Manually
Manual clearing of the grit is to be avoided except in the case of very small STPs of less than 1 MLD where velocity controlled channels can be cleared by the operator using a shovel and walking on a platform along the length.
Disposal of Grit
Grit is usually washed after removing it from the chamber, unless it may contain a considerable amount of organic matter which causes an unpleasant odor. They are mainly disposed of by dumping or by a sanitary landfill. The method of disposal is selected on the basis of physical and chemical characteristics of grit, quality, availability of land for disposal, etc.
Working Principle of Grit Chamber
The grit chambers work as a sedimentation tank which is designed to separate the intended heavier inorganic materials (specific gravity about 2.65) and to pass forward the lighter organic materials. Differential sedimentation and differential scouring velocity are maintained in the chamber so that the flow velocity should neither be too low as to cause the settling of lighter organic matter nor should it be too high as not to cause the settlement of the silt and grit present in the sewage.
The critical velocity of flow 'vc' beyond which particles of a certain size and density once settled should always be less than the scouring velocity of grit particles. The critical velocity of scour is given 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 used at peak flows. This same velocity is to be maintained at all fluctuation of flow to ensure that only organic solids and not the grit is scoured from the bottom.
Advantages of Grit Chamber
- To protect moving mechanical equipment from abrasion and abnormal wear.
- To reduce maintenance cost in the frequency of digester cleaning caused by excessive accumulation of grit.
- To prevent heavy deposits in pipelines and channels.
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