Comminutors and grit chambers are devices used in primary wastewater treatment to reduce the size of solid materials and remove heavier inorganic particles like grit. Comminutors shred large solids using a revolving drum with slots, while grit chambers use lower flow velocities to settle out particles with specific gravities above organic materials. Grit chambers can be horizontal flow or aerated, and are cleaned periodically. Detritus tanks and skimming tanks provide additional removal of finer solids and floating matter before sedimentation. Equalization tanks help stabilize the flow rate and characteristics entering secondary treatment.

2. COMMINUTORS
• A device used to reduce the size of the solid materials in wastewater
by shredding (comminution).
• The shredding action is like many scissors cutting to shreds all the
large solids in the wastewater.
• The rotating-screen-type comminutor consists of a motor-driven,
revolving, vertical drum-screen with 6mm – 10mm slots almost
completely submerged in the wastewater flow.
• Material that is too large to pass through the slots is cut into pieces
by the cutting members acting like shears.
• The sheared particles then pass through the slots of the drum out and
of a bottom opening through an inverted siphon

3. COMMINUTOR

4. GRIT CHAMBERS
• Grit chamber is a long narrow or circular tank in the primary sewage
treatment plant
• Designed to reduce the velocity of the flow of sewage to eliminate
the girt materials (specific gravity 2.4-2.65) such as sand, ash and
clinkers, eggshells, bone chips and many inert materials inorganic in
nature.
• Grit settles more rapidly than organic materials
• They are provided to protect moving mechanical equipments from
abrasion
• They reduce formation of heavy deposits in pipe lines, channels and
conduits
• They also reduce the frequency of digester cleaning

6. TYPES
1. Horizontal flow grit chambers
2. Aerated grit chambers
1. Horizontal flow-
• They are 10 to 18 m long narrow open channel with a depth of liquid 1 to
1.3 m
• are designed to maintain a velocity 0.3m/s
• Such a velocity will carry most organic particles through the chamber and
permit the heavier grit to settle out
• The flow velocity should neither be low as to cause settling of lighter
organic matter, nor should be so high as not to cause the settlement of entire
silt and grit
• To maintain a constant velocity a control section is used such as proportional
flow weirs, parshall flumes etc

7. CLEANING OF GRIT CHAMBER
• Mechanically cleaned
Mechanically cleaned grit chambers come with mechanical equipment that
accumulates and washes grit chambers, serving on either intermittent or
continuous basis.
• Manually Cleaned
Manually run grit chambers are equipped with adequate capacity to hold the
grits between the time of cleaning.
 Disposal of grit is by dumping or burying or by sanitary land fill

8. 2. Aerated grit chambers
• A special form of grit chamber consisting of a standard spiral flow aeration
tank with air diffusion tubes placed on one side of the tank 0.6 to 1m from
bottom
• Aerated type control the separation of organic and inorganic solids by
producing a rolling flow pattern
• The heavier particles drop down to the floor whereas lighter particles are
carried with the roll of spiral motion and eventually out of the tank

9. DETRITUS TANK
• A grit removal unit which also removes silt as well as some organic
matter
• Because flow through velocity is less and detention time is more
• Idea is to remove finer particles than those removed by a grit
chamber
• It is continuous flow settling tank of rectangular or square shape
• The sides are vertical and tapered towards the bottom to form a
trough for collection of detritus
• The overall depth of detritus tanks varies from 2.5m to 3.5 m
• Detention time – 3 to 4 mts
• Velocity of flow- 0.2 to 0.3 m/s
• The light organic matter can be washed out by passing compressed
air, by washing etc

10. SKIMMING TANK
• Installed just ahead of sedimentation tank
• To remove the floating matter like oil, fat, grease , soap, waxes, fatty
acids, fruit skins etc.,
• Major sources are kitchen, motor garrages, oil refineries, soap
factories etc
• If these are not removed they seriously affect the working of various
treatment units
• It’s a chamber so arranged that the floating matter like oil, fat, grease
etc., rise and remain on the surface of the waste water until
removed, while the liquid flows out continuously under partitions or
baffles.
• Most of them are circular or rectangular, with a detention time of 3 to
5 mts
• The chamber is a long trough shaped structure divided up into two or
three lateral compartments by vertical baffle walls having slots for a
short distance below the sewage surface and permitting oil and
grease to escape into stilling compartments.

12. • For efficient working air diffusers are provided at the bottom
• The rising air bubbles congeal the greasy and oily material and
push it to the side compartment
• The floating matters can be removed either manually or with
mechanical equipment
• The skimmings obtained should be disposed off by burial or by
burning or by digestion
• It may also be converted into soap lubricants, candles and other non
edible products
• The digestion of skimmings produces of gases of high fuel value

13. EQUILIZATION TANK
• When the characteristics of the waste water varies during the day and
also when the discharge is either not uniform or else is discontinuous,
equilization is necessary
• The process consists of holding the ww for some predetermined time,
in continuously mixed holding tanks so as to get ww of uniform
character and at uniform rate
Effects of flow equalization
 10-20% of BOD entering is stabilized in the equalization basin
 23-47% of SS is further removed in the primary clarifier
 reduce shock load on biological process