GRIT CHAMBERS AND COMMINUTORS
Preliminary treatment of wastewater is the removal of debris and coarse biodegradables from wastewater with or without stabilizing wastewater by chemical addition.
The wastewater contains rags, grit, sticks, other large debris, and foreign objects. They make the operation process more complex and problematic.
Therefore, the method of removal of those objects before primary or subsequent treatment processes is known as the preliminary or pretreatment of wastewater.
Preliminary or pretreatment includes the following methods of treatment.
a) Screening and Comminution
b) Grit removal
c) Pre-aeration
d) Equalization
e) pH control
f) Flotation
Comminutors
Comminution is the process of shredding and screening the debris in wastewater to comply with the sludge handling facilities. As an alternative to racks or screens, a comminutor or shredder cuts and grinds up the coarse solids in the wastewater to about 6-10 mm so that the solids will not harm subsequent treatment equipment. The chopped or ground solids are then removed in primary sedimentation basins.
A comminutor consists of a fixed screen and a moving cutter. Comminution can eliminate the messy and offensive screenings for solids handling and disposal. Comminutors are installed directly in the wastewater flow channel and are equipped with a bypass so that the unit can be isolated for service maintenance.
Grit chamber
Grit is an inert matter in wastewater that will not decompose during the wastewater treatment process. Generally, those have a specific gravity of around 2.65.
It may be a piece of rock, sand particle, metal, bone, cigarette filter tips, metal fragments, coffee grounds, seeds, large food wastes (organic particles), or even a piece of eggshell.
Grit originates from domestic wastes, stormwater runoff, industrial wastes, and groundwater seepage.
These substances can promote excessive wear of mechanical equipment and sludge pumps, and even clog pipes by deposition.
Grit chambers should be provided for all wastewater treatment plants, and are used on systems required for plants receiving sewage from combined sewers or sewer systems receiving a substantial amount of ground garbage or grit.
Grit chambers are usually installed ahead of pumps and comminuting devices.
Grit chambers can be cleaned manually, mechanically, or using aerated or vortex-type de-gritting units. The chambers can be square, rectangular, or circular.
Grit chambers are commonly constructed as fairly shallow longitudinal channels to catch high specific gravity grit (1.65). The units are designed to maintain a velocity close to 0.3 m/s and to provide sufficient time for the grit particle to settle to the bottom of the chamber.
Grit Removal
Grit occupies a large space in the wastewater treatment process if they are not removed. It abrades the machinery and surfaces in the system. Therefore grit removal is very important in wastewater treatment. Grit removal increases the effectiveness of the other treatment processes and reduces the maintenance cost of high-speed centrifuges.
Grit removal is quite economical compared to the other unit treatment processes.
There are several types of grit chambers. Some of them are; Horizontal flow grit chambers, Detritus chambers, and Aerated grit chambers.
Horizontal Flow Grit Chamber
The horizontal flow grit chamber is a narrow rectangular chamber that ensures a horizontal channel flow of wastewater.
The velocity of flow in this chamber is designed in such a way that the suspended organic matter flows forward with the water and the grit (heavier than the suspended organic matter) to settle in the tank. Generally, the velocity of wastewater flow in the grit chamber is 0.3 m/s.
Detritus Chamber
Detritus grit chamber is a chamber that is designed to have a lower velocity of flow. This may allow the organic matter to settle with grit. This grit-organic matter mixture is called “detritus”.
The organic matter is separated either by gentle aeration or washing detritus to re-suspend. There are several methods to accomplish the separation.
Aerated Grit Chamber
In the aerated grit chamber, air is diffused to separate the grit from the other matter. Circulation of air promotes the accumulation of grit in the bottom of the tank. Then the grit can be removed.
The aeration in the preliminary treatment process improves the primary sedimentation more effectively and removes odor. This is also known as pre-aeration.
Example – Design of Grit Chamber
The designed hourly average flow of a municipal wastewater plant is 0.438 m3/sec. Design an aerated grit chamber where the detention time of the peak flow rate is 4.0 min. Generally, the detention time is between 3 – 5 min.
Solution
Step 1
Determine the peak hourly flow Q assuming a peak factor of 3.0.
Q = 0.438 m3/sec x 3 = 1.314 m3/sec
Step 2
Calculate the volume of the grit chamber. Here, it is assumed two grit chambers will be constructed.
Therefore, for each unit;
Volume = 1.314 m3/sec x 4 min x 60 sec/min / 2 = 157.68 m3
Step 3
Determine the size of a rectangular chamber, assuming the width of the chamber = 3 m and depth-to-width ratio = 1.5:1. The depth-to-width ratio usually varies between 1.5:1 and 2.0:1.
Depth = 3 m x 1.5 = 4.5 m
Length = volume / [depth x width] = 157.68 m3 / [4.5 m x 3 m] = 11.68 m
Each of the two chambers having the size of 11.68 m long x 3.0 m wide x 4.5 m deep are to be constructed.
Step 4
Calculation of average volume of grit produced per day
Assume 01 m3 of inflow of wastewater produces 52.4 ml of grit.
Volume of grit = 52.4 ml x 0.438 m3/sec x 86,400 sec / day = 1,982,938 ml / day ≈ 2 m3 / day.