Deals with stoke's law, discrete particle settling, grit channels, proportional weirs, grit chambers, aerated grit chambers, vortex degritters and cyclone degritters.

1. Stoke’s Law and Grit
Separators

2. Stoke’s Law and terminal settling
velocity of particle
Forces acting on a suspended particle are
• Gravity force
• Buoyant force
• Drag force
– Increases with increasing speed – zero for zero speed
gpparticle ∨ρ
gpfluid ∨ρ
2
2
pfluidpd vAC ρ
pp dvπµ3 For laminar
flow conditions
Vp is volume of the partcicle
2
3
2
4
23
4






=






=
p
p
p
p
d
A
d
V
π
π

3. Net force of the particle (ma) makes the particle to accelerate
When drag force becomes equal to the net of gravity force
and buoyant force, net force and acceleration of the
particle becomes zero
Particle then settles at constant velocity (terminal settling
velocity)
Stoke’s Law and terminal settling
velocity of particle
p
fluid
fluidparticle
d
p d
C
g
v







 −
=
ρ
ρρ
3
4
34.0
324
++=
RR
d
NN
C
ν
pp
R
dv
N =
Where
Where
ν
ρ
ρρ
18
2
p
w
wp
p
dg
v





 −
=
For laminar flow
p
w
wp
p dgv 




 −
=
ρ
ρρ
33.3
For turbulent flow
ν is 1.003 x 10-6

4. Discrete Particle Settling
Settling tanks are designed for a selected design terminal
settling velocity (vt)
A particle is considered removed if it touches the bottom of
the tank
For 100% settling removal, particles with vt terminal settling
velocity has to be ≥ surface loading or overflow rate
In a settling basin design settling velocity, detention time and
depth are related
Actual design takes into account the effect of inlet and outlet
turbulence, short circuiting, sludge storage, and velocity
gradients due to operation of sludge removal equipment
A
Q
vt =
timeDetention
depthTank
vt =
Q is flow rate
A is surface area
is hidraulic retention timeԎ
A
QH
v
WH
Q
L
H
v
WH
Q
v
WHvQ
L
Hv
v
v
L
v
H
t
t
h
h
h
t
ht
==
=
=
=
=
=
τ

5. A
Q
vt =
Indicates grit removal efficiency is independent of depth and
detention time of the channel/chamber
Depth can be reduced – scouring problem is a limitation –
horizontal flow velocity should be <0.4 m/sec.
Increase of depth or width of channel reduces horizontal
flow velocity and results in settling of organics





 −
=
ρ
ρρp
s
f
Kgd
V
8
‘k’ depends on the material being scoured
(0.04 for unigranular particles and 0.06 for
sticky interlocked matter)
‘ρp and ρ’ are densities of particles and liquid
‘d’ is particle diameter
f is Darcy-Weisbach friction factor
(influenced by surface roughness and
reynolds number, etc. – typical values 0.02-
0.03
Camp-Shields equation
for scour velocity
Discrete Particle Settling

6. Only a fraction of the particles with the terminal settling
velocity < design settling velocity are removed in the
settling tank
Discrete Particle Settling
td
tp
v
v
removedFraction =
( )
( )
∑
∑
∫
=
=
=+= n
i
i
n
i
i
td
tpi
td
tp
td
n
n
v
v
dX
v
v
XremovedfractionTotal
1
1
vtp is terminal settling velocity
vtd is design terminal settling velocity
Xtd is fraction with terminal settling velocity >vtd
dX is the fraction of grit with vtp
ni is the fraction of grit falling in the ith
category

8. Grit removal facilities
Sand, gravel, cinders, and other heavy solid particles having
high specific gravity and settling velocities > organic solids
Located after bar screens, ahead of wastewater sump and
pumps – provided to
– Protect moving mechanical equipment from abrasion and
abnormal wear (pumps, centrifuges, valves, bearings, etc.
– Reduce formation of heavy deposits in tanks, basins,
pipelines, channels, conduits, heat exchnagers, etc.
– Reduce frequency of cleaning of digesters
Designed for the removal of grit particles of size 0.15 (100
mesh) or 0.21 mm (65 mesh) and specific gravity >2.65

9. Types of grit chambers
• Horizontal flow type
– Long grit channels with influent distribution gate and weir at
effluent end – mostly manual grit removal – preferred for low
flows (<1 MLD)
– Horozontal grit chambers (square shaped - facilitates proper
functioning of raking mechanism – mechanical grit removal -
rectangular chambers!
• Aerated type – spiral flow aeration tank is used –
preferred for larger flows (>2 MLD)
• Vortex type
– Cylindrical tank with tangential entry of flow creating vortex
flow pattern
– Centrifugal and gravitational forces cause the grit to
separate
– Grit cyclones and hydroclones!
– Preferred for moderate flows (1 – 2 MLD)

10. Horizontal flow grit channels
• Not much favoured
• Representative design data
– Detention time: 45-90 sec. (60 typical)
– Horizontal flow velocity: 0.25 – 0.4 m/sec. (0.3)
– Head loss: 30-40% of channel depth (36%)
– Added length for inlet and outlet turbulence allowance: 25-
50% of actual length (30%)
• At the design horizontal flow velocity heavier grit particles
settle while the organic particles tend to get resuspended
• Oil separation is often integrated
• Has isolating gate valves at the inlet and flow control
section at the outlet
– Parabolic channel cross section as substitute to flow control
section (meant to regulate horizontal flow velocity in the face
of flow variations
• Provisions for draining out the wastewater and manually
removing the accumulated grit

12. Grit channels

13. Square (or rectangular) horizontal flow grit
chambers
– Square tanks with grouted corners
– Influent is distributed across the section of the tank by a
series of vanes (adjustable deflector plates) and gates
– Wastewater flows in straight lines across the tank and
overflow a weir into the outlet
– Mostly designed to remove 0.15 mm grit with 95% efficiency
– Rotating, center driven rake mechanism mounted on a
bridge (spanning the basin) rakes grit into a side sump/
collection hopper
– Rake arms have outward raking blades
– From sump reciprocating rake mechanism resuspends
organic particles, and concentrates and moves up grit on the
incline
– Concentrated grit is washed in a classifier (submerged
reciprocating rake or an inclined screw conveyor)
Horizontal flow grit chambers

14. Grit separator-classifier
• Reciprocating rake mechanism set in an inclined concrete
channel along the collection basin
– All drive components and bearings are located above the
liquid level and protected from corrosion
– Putrescible organics are liberated and washed and a recyle
pump returns the organic matter into the degritted sewage
• Grit screw classifer: a tubular sheet shaft and helical flight
assembly set in a semicircular inclined concrete
trough/channel
• Cyclone separator with a reciprocating rake classifer: grit
from the collection basin is pumped through the cyclone
separator for solids concentrating and delivering into the
hopper of the classifier for washing and discharge

17. Aerated grit chambers
• Rectangular tank with aeration by coarse bubble diffusers
along one side creats spiral flow perpendicular to the flow
– Velocity of roll governs size & SG of the particles setting
– Strategically positioned longitudinal circulation baffle directs
the rotational flow and a vertical baffle at the far end
prevents flow short-circuiting
– Wastewater is introduced in the direction of the roll
– Wastewater makes 2-3 turns at peak flow
• Grit hopper: Located along one side of the tank, 0.9 m
deep, has steep sloping sides
• Design information
– Designed for 0.21 mm grit particles removal
– HRT is 2 to 5 minutes at peak hourly flow
– Air diffusers: Located 0.45-0.6 m above the normal plane
– Width to depth ratio: 1:1 to 5:1 (1.5:1)
– Length to width ratio: 3:1 to 5:1 (4:1)
– Air supply: 0.2 to 0.5 m3
/m.min.
• Expansion by air should be considered in head loss
estimation

19. 2000
2500
800 m
Slope 1 in 10
1200
300600m
400
300
300250250
1300
Aerated grit chambers
550
1750
250
2501500m
Slope 1 in10
1750
250
500
Tubular diffuser
Overflow weir
Grit trench
Inlet
250
Drain for degritting
Drain for draining
out the sewage

20. Grit and its removal and disposal
Removal of the accumulated grit
• Removal can be simultaneous to operation
– Grab buckets traveling on monorails
– Chain and bucket conveyors running full length of grit
hopper
– Screw conveyors, tubular conveyors, jet pumps and airlifts
– Traveling bridge grit collectors
• Removal can be after taking off the grit chamber and
involve
– Emptying of the grit chamber
– Manual removal of the accumulated grit
Grit characteristics and disposal
– Predominantly inert and relatively dry
– Volatile content: 1 to 56%
– Moisture content: 13 to 65%
– 3

21. • If no grit chambers provided and if grit accumulates in the
primary sludge then for removal
– diluted primary sludge is passed through cyclone degritter
• Grit separators and grit washers are used to remove
organics
– Water sprays in both cases help in grit washing
– Inclined submerged rake - necessary agitation is provided
for organics removal while the grit is raised on incline to
discharge point
– Inclined screw – lifts the grit up the ramp
• Grit disposal
– Transport to a landfill and stabilize with lime prior to
landfilling (!)
– Incinerate the grit with other solids
– Pneumatic conveyors for conveying grit over short distances
Grit and its removal and disposal

22. Vortex type grit chambers and Cyclone
grit separators
• Type-1
– Wastewater enters and exits tangentially
– Rotating turbine maintains constant flow velocity and
promotes separation of organics from grit
– HRT: 20-30 (30) seconds for average flow
• Type-2
– Vortex is generated by the flow entering tangentially at the
top of the unit
– Effluent exits the center of the top of the unit
– Sized to handle peak flow rates upto 0.3 m3/sec. per unit
• Cyclone grit separators
– Usually used in inclined position and deliver grit to classifer
– Used mostly for the separation/classification of grit grit
collection basins or from primary sludge

24. Grit Cyclones
Standard cyclone
• Has proper geometrical relationship between cyclone
diameter, inlet area, vortex finder, apex orifice and length
• Inlet:
– Two types of designs: involute feed type and tangential feed type
– Area of the inlet nozzle at the point of entry is 0.05D2
– Rectangular inlet with length parallel to cyclone axis
• Vortex finder:
– Controls both the separation and the flow leaving the cyclone
– Extends below the feed entrance
– Size is 0.35 times the diameter
• Cylindrical and conical sections:
– Cylindrical section length = dia. & dia. = cyclone dia.
– Conical section has included angle of 10-20°
• Apex orifice:
– Cone terminates in apex orifice (size is 10-35% of cyclone dia.)

26. Classification/separation of grit
• Definition of classification/separation
– Historical definition: the particle size of which 1% to 3%
reports to the cyclone overflow
– Currently used definition: the particle size of which 50%
reports to the overflow (called as D50C)
• Actual recovery:
– does not decrease below a certain level (that certain amount
of grit is always recovered to the underflow and bypasses
classification)
– This minimum recovery level is equal to the liquid recovered
– Certain % of all size fractions reports directly to the
underflow as bypassed solids in equal proportion to the
liquid split
• Corrected recovery:
– Produced from adjusting each size fraction of the actual
recovery curve by an amount equal to the liquid recovery

27. Classification/separation of grit
• Reduced recovery:
– D50C point changes from one application to another and the
recovery curves shift along the horizontal axis
– Reduced recovery curve is a corrected recovery curve
obtained through division of the particle size of each size
fraction by the D50C value
– Reduced recovery curve remains constant over a wide range
of cyclone diameters and operating conditions provided the
girt has a single specific gravity and has a typical or normal
size distribution
– The following mathematical relationship can be used for
calculating the reduced recovery
• Reduced recovery, along with the bypassed solids, can be
used to predict size distribution of grit in the underflow
2expexp
1exp
44
4
−+
−
= X
X
rR Rr is reduced recovery
X is ratio of particle dia. to D50C

28. Partical dia. Vs. Particle recovery

29. Cyclone design
• Feed enters the cyclone under pressure and comes out,
preferably at atmos. pressure, as overflow (through vortex
finder) and underflow (through apex orifice)
• Standard cyclones performance is indicated for water at
20°C, with 1% (by volume) grit (of 2.65 SG) when
operated at 69 kPa (10 psi) pressure difference
• D50C (particle size that is removed in the underflow with
50% efficiency) is used for indicating the performance
– D50C and cyclone diameter are related by
( )
( ) ( ) 3215050
66.0
50 84.2
CCCbaseDnapplicatioD
DbaseD
CC
C
=
=
D is diameter of cyclone in cm
C1, C2 and C3 are corrections
D50C (base) is size achieved by a
standard cyclone when operated under
standard conditions
D50C (application) is size achieved
under actual operating conditions by
standard cyclone

31. Cyclone design
• C1 correction
– It is for the influence of solids level in the feed (increasing
solids level increases the C1 value) – it is given by
• C2 correction
– It is for the influence of pressure drop across the cyclone
(increasing drop decreases C2 value) – recommended
pressure drop is 40-70 kPa – it is given by
• C3 correction
– It is for the influence of specific gravity of the solids/grit
(increasing specific gravity decreases C3) – it is given by
43.1
1
53
53
−





 −
=
V
C V is % solids by volume in the feed
( ) 28.0
2 27.3
−
∆= PC ΔP is pressure drop in kPa
5.0
3
65.1






−
=
ls SGSG
C GSs is specific gravity the solids/grit
SGl is specific gravity of the liquid

33. Cyclone design
• Vortex finder: typical diameters are 25-45% of the cyclone
diameter – increasing diameters coarsen the separation
• Feed inlet area: influence on separation is not as
significant as that of vortex finder
• Apex size: influence on separation very less unless the
diameter is too small and physically contraining underflow
and forcing the material through overflow
• Cyclone retention time: increasing retention time increases
the separation – retention time can be altered by
– Changing the length of the cylindrical section
– Changing the conical section inclusive angle
• Flow rate through the cyclone: increasing flow rates
increase the pressure drop and affect the separation
– Larger votex finders or larger inlet areas increase flow rates

35. Other aspects on cyclones
• Overflow discharge at positive pressure forces some of the
overflow through the underflow and increase bypass solids
– discharge at a point lower than the feed entrance can establish
siphon and disturb solids separation (a vent on overflow piping
can avoid the problem)
• Discharge of underflow at a negative pressure can result in
some of the overflow to flow as underflow
– Discharge against a positive pressure reduces underflow discharge
– to ensure solids discharge larger apex orifices are needed
• Apex orifice operating at atmos. pressure should produce
cone shaped discharge with 20-30 angle with hollow
centre – if hollow centre not produced then larger apex
needed
• Flow velocity should be such that particle settling should
not occur in piping and erosion/wear should be minimum
• Material of construction can be metal housing with
replaceable liners (natural gum rubber, Nihard, Neoprene
and nitrite, Urethane, etc.) – ceramic lining can be used at