The grit chamber removes inorganic grit such as sand, gravel, and other mineral matter that has a nominal diameter of 0.15 to 0.20mm or more. Grit chambers are sedimentation basins usually placed after the grit screen and before the primary sedimentation tank.
The grit chamber can remove grit components like coffee grounds, vegetable cuttings, ash clinkers, wood particles, tea residues. Although some of them are organic, they are non-biodegradable over time span involved in grit collection and disposal.
Generally, Grit chambers are designed to remove all the particles of specific gravity 2.65 or more, with a settling velocity of about 21mm/s for 0.20mm diameter and velocity of about 15mm/s for particle size above 0.15mm.
Some of the grit chambers are designed to remove particles of size
Note: Flow velocity of about 0.25 o 0.3m/sec is adopted for the design of grit chamber
Detention Time of Grit chamber is about 30 to 90seconds.
Grit chambers can be divided into the two general types those are
Now, we will take a look at these types in details
In horizontal flow type, the flow of sewage passes through the channel in a horizontal direction. Horizontal flow type grit chamber is most widely used grit chamber. The horizontal flow type grit chamber is designed to provide a straight line horizontal flow, and flow velocity remains constant even if there are variations in discharge.
That is why it is also called as Constant Velocity Grit Chamber.
Also Read: Design of Grit Chamber | Step-by-Step
Note: Only inorganic matter is allowed to settle in grit chamber.
In such constant velocity grit chambers, the chamber is enlarged in which the cross-section is increased, so as to reduce the velocity of sewage flow to such extent that the heavy inorganic materials settle down by gravity and the lighter organic materials remain in suspension and goes out with effluent to the further process.
Aerated Grit Chamber consist of spiral flow aeration tank, where the spiral velocity is controlled by the dimensions of the chamber and also quantity of air provided to the tank.
In the spiral flow tank waste water enters in a spiral flow direction and the shearing action of air bubbles is supported to strip the inert grit of much of organic material that comes close to the surface.
The efficiency of aerated grit chamber is depends on the spiral flow velocity and detention time. If the flow velocity is too high then grit will not settle and carried away, and if velocity is too low then organic matter may also settle down along with grit.
Detention time is taken about 3 minutes for design purpose at maximum flow. Length to width ratio may range from 2.5 to 2.7m with depth of 2 to 5m. Also width to depth ratio varying between 1:1 to 5:1.
To get best out of the grit chamber it is mandatory to carry out cleaning with regular intervals. The grit chambers can be cleaned periodically at about 3 weeks interval, by the following methods
Hand cleaning is carried out for smaller plants of capacity generally less than about 4.5 million litres per day. Hand cleaning is carried out by labours using shovels, and it is difficult for labours to maintain good sanitary conditions in the chambers so this method is not used for larger plants.
Mechanical and Hydraulic cleaning is carried out for larger grit chambers i.e. generally for plants with a capacity of more than 4.5 million litres per day. In hydraulic cleaning, grit is removed by the force of water-jet directed from a central point and removed through the pipes at bottom of the chamber.
Mechanical cleaning is carried out for larger plants. Mechanical cleaning is done by using machines. The mechanical machines used are scrapers, buckets, elevators. It is easy to maintain good sanitary conditions using mechanical cleaning.
After the cleaning of grit chamber, the settled and removed grit may contain some organic matter and can be washed before its disposal by using certain machines and washed water returned back to plant influent. Washed grit may still contain about 1 to 5% of putrescible organic matter.
The disposal of grit and silt can be easily be done by disposing either by burning or burring and filling the low lying areas by dumping.
Working principle of grit chamber is, grit chamber is a primary sedimentation tank designed to settle the heavy inorganic matter and pass off the small suspended particles. Scouring velocity is maintained in the grit chamber so that is could not be too high that heavy matter also gets passed off and not too low that suspended organic matter also settle.
Formula for scouring velocity given by the Shield’s Formula
V = 3 to 4.5 (g(Ss -1)d)1/2
The grit chambers can be cleaned periodically at about 3 weeks interval.
The purpose of providing grit chamber in the sewage treatment process is to remove the inorganic materials mentioned in the introduction.
Grit in sewage treatment causes a lot of problems like, erosion of pump impellers, it settles in pipes carrying sewage. So there is need to remove the grit.