It is discussed about preliminary and primary waste water treatment

1. Wastewater Treatment
Chapter Three
Preliminary and Primary Wastewater Treatment Methods
Civil Engineering (Weekend)
By Manamno B. (MSc.)
Email: manamnobeza@yahoo.com

2. Content of the Chapter
3.1 Preliminary Treatment Methods
3.1.1. Screening
3.1.2 Grift removal of basins
3.1.3 Flow equalization
3.1.4 Tanks for removing oils and greases
3.2 Primary Treatment
3.2.1 Sedimentation
3.2.2 Sedimentation aided with coagulation (Type II – flocculent
settling)
3.2.3 Sedimentation basin design and other alternatives

3. 3.1 Preliminary Treatment Methods
• Separating the floating materials (like dead animals, tree branches,
papers, pieces of rags, wood, etc.), and also the heavy settleable
inorganic solids.
• It also helps in removing the oils and greases, etc. from the sewage.
• This treatment reduces the BOD of the wastewater, by about 15 to
30%.
• The processes used are:
- Screening for removing floating papers, rags, clothes, etc
- communitors
- Grit chambers or Detritus tanks for removing grit and sand; and

4. Cont....
1. Screening
 It is the process of passing the sewage through d/t types of screens to trap
and remove the floating matter present in sewage.
What if floating materials not removed?
 will choke the pipes, or adversely affect the working of the sewage pumps.

5. Cont....

6. Screens are sometimes classified as fixed or movable, depending upon whether the screens are stationary or
capable of motion.
Considerations for selection of a Screen
• Flow – average and variation
• Type of effluent – Storm, Industrial, Municipal
• Sewer type – long sewers result in heavy flushes of rags
• Degree of screenings removal required – screen aperture
• Type of cleaning required – Manual, Automatic.
• Availability of power and wash water
• Health and Safety – screenings contain pathogens mand attract insects
• Handling and transport requirements
• Disposal options
Cont....

7. 2. Comminutors
• Comminutor is a mechanism consisting of a drum, which rotates on a vertical axis,
and through which all the wastewater must pass from the outside inwards.
• Comminutors or Shredders are the patented devices, which break the larger sewage
solids to about 6mm in size, when the sewage is screened through them.
• Comminutors are of recent origin, and eliminate the problem of disposal of
screenings, by reducing the solids to a size which can be processed
Cont....

8.  Disposal of Screenings
The material separated by screens is called the screenings. It contains 85 to
90% of moisture and other floating matter.
To avoid such possibilities, the screenings are disposed of either by burning,
or by burial, or by dumping
The process of burning is called Incineration
The screenings may also be disposed of by burial. The process is technically
called composting.
Another method of disposing of the screenings is by dumping them in low
lying areas (away from the residential areas) or in large bodies of water, such as
sea.
Digestion of screenings along with the sewage sludge in a sludge digestion
tank has also been tried, but not found successful.
Cont....

9. 3. Grit Removal Basins
Grit removal basins, such as Grit chambers or Grit channels or Detritus tanks
are the sedimentation basins placed in front of the wastewater treatment plant
The grit chamber removes the inorganic grit, such as sand, gravel, and other
mineral matter that has a nominal diameter of 0.15 to 0.20 mm or larger.
Grit chambers are, in fact, nothing but like sedimentation tanks, designed to
separate the intended heavier inorganic materials by the process of
sedimentation due to gravitational forces, and to pass forward the lighter
organic material
Cont....

10. Tanks for Removing Oils and Grease
1. Skimming Tanks
Skimming tanks are sometimes employed for removing oils and
grease from the sewage, and placed before the sedimentation-
tanks.
They are, therefore, used where sewage contains too much of
grease or oils, which include fats, waxes, soaps, fatty acids, etc
2. Vaccuators
Grease can also be removed from the sewage by vacuum floatation
method, by subjecting the aerated sewage to a vacuum pressure of
about 0-25cm of mercury for 10 to 15 minutes in a vaccuator.
This causes the air bubbles to expand and move upward through the
sewage to the surface. The rising bubbles lift the grease and the lighter
waste solids to the surface, where they are removed through skimming
troughs.

11. Primary Wastewater Treatment
Primary treatment consists in removing large suspended organic solids. This is usually
accomplished by sedimentation in settling basins.
Sedimentation:
• is the physical separation of suspended material from water or
wastewater by the action of gravity.
• Sedimentation tanks are tank designed to remove this organic matter
from the sewage effluent coming out from the grit chambers.

12.  Type of Settling
• Depending on the particles concentration and the interaction between
particles, 4 types of settling can occur
1. Discrete particle settling (Type I): The particles settle without interaction
and occur under low solids concentration
2. Flocculent settling (Type II): Particles initially settle independently, but
flocculate in the depth of the clarification unit
• The velocity of settling particles is usually increasing as the particles
aggregates
Cont....

13. 3. Hindered/zone settling (Type III) : Inter-particle forces are
sufficient to hinder the settling of neighboring particles. The
particles tend to remain in fixed positions with respect to each
others.
4. Compression settling (Type IV): occurs when the particle
concentration is so high that so that particles at one level are
mechanically influenced by particles on lower levels.
• The settling velocity then drastically reduces.
Cont....

14. Cont....
Hindered

15. Settling of Discrete Particles (Type I Sedimentation or Settling)
• Fundamental principle underlying the process of sedimentation is that the
organic matter present in sewage is having specific gravity greater than that of
water (i.e. 1.0).
• In flowing sewage, they are kept in suspension, because of the turbulence in
water.
• The design of sedimentation basins is thus, totally dependent upon:
• the settling velocity of the sewage solids.

16. • The settlement of a particle in water brought to rest is opposed by the
following factors:
a. The velocity of flow which carries the particle horizontally.
b. The viscosity of fluid in which the particle is travelling
c. The size, shape and specific gravity of the particle.
• The greater is the specific gravity, more readily the particle will settle.
• The size and shape of the particle also affect the settling rate.
• the weight and volume of the spherically shaped particle, varies with the
cube of its diameter (volume being equal to
πd3
6
, where d is the diameter)
or its size
• Its area varies with the square of the diameter (area being equal to
πd2
4
).
• Hence, very small sized particles will settle very slowly
Cont....

17. Cont....

18. Cont....

19. Derivation of Stokes's Law: 3 forces on settling particle
Drag force, particle weight, buoyancy force
1. The drag force: is given by Newton's law, as
Drag force(𝐹𝐷) = CD Aρw
v2
2
Where, CD = Coefficient of drag
A = Area of particle
w = Density of water
v = velocity of fall
Cont....

20. 2. The weight of the particle (W) = 𝑚𝑔 = 𝑉
𝑝pg
3. Force of buoyance (𝐹𝑏) = 𝑉
𝑝wg
Where: Vp= volume of particle
w= density of water
p= density of particles
upward and downward force will become equal when v (in drag force
equation) becomes equal to vs.
CD Aρw
vs
2
2
+ 𝑉
𝑝wg= 𝑉
𝑝pg
CD Aρw
vs
2
2
= 𝑉
𝑝pg- 𝑉
𝑝wg
Cont....

21. CD A ∗ ρw ∗
vs
2
2
= vpg(p − w
)
vs
2
=
2 vpg(p − w
)
CD A ∗ ρw
For spherical particles
vp =
4
3
πr3and A = πr2
vs
2
=
4𝑑g(p−w
)
3CDρw
, d = particle diameter
𝐯𝐬
𝟐 =
𝟒𝒅𝐠(𝐆 − 𝟏)
𝟑𝐂𝐃
General equation to calculate settling velocity of particles
Cont....

22. The coefficient of drag (CD) is a function of Reynolds number, Re
CD =
24
Re
(laminar flow or streamline flow)
Where, Re is the particle Reynolds number =
𝑉𝑠𝑑

Therefore, the above equation then becomes,
vs
2 =
4
3
∗ g(G − 1)d
24
𝑅𝑒
vs
2 =
4
3
∗ g(G − 1)d ∗
𝑅𝑒
24
vs
2 =
g
18
∗ G − 1 d ∗
vsd

𝑤ℎ𝑒𝑟𝑒,  = 𝑘𝑖𝑛𝑒𝑚𝑎𝑡𝑖𝑐 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑(m2/s)
Cont....

23. vs =
g
18
∗ G − 1 ∗
𝑑2

The above Stoke's equation is valid for particles of size less than 0.1mm; in which case, the
viscous force predominates over the inertial force, leading to what is known as streamline
settling
a) For transition settling (d between 0.1mm and 1.0mm)
Here 1 < 𝑅𝑒
< 10
3
CD =
24
Re
+
3
Re
+ 0.34
b) For turbulent settling (d > 1.0mm)
Here 𝑅𝑒 >
10
3
, CD = 0.34 to 0.4
For turbulent settling, the vs reduces to:
vs = 1.8 gd ∗ (G − 1)
c) For streamline settling (d < 0.1mm)
Here Re < 1; and
CD=Re/24
Cont....

24. Sedimentation Tanks
• are designed for effecting settlement of particles by reducing the flow velocity or by
detaining the sewage in them.
• They are generally made of reinforced concrete and may be rectangular or circular in
plan.
• Long narrow rectangular tanks with horizontal flow are generally preferred to the
circular tanks with radial or spiral flow
Cont....

25. Fig. of rectangular sedimentation tank
Settling zone
Sludge zone
Inlet
zone
Outlet
zone
Inflow
Outflow
Sludge
Sedimentation basin has four district zones: inlet, settling, outlet
and sludge zone
Scraper blade
Cont....

26. o Sedimentation tanks may function either intermittently or continuously.
o The Intermittent settling tanks are simple settling tanks which store
sewage for a certain period and keep it in complete rest.
o In a continuous flow the flow velocity is only reduced, and the sewage is
not brought to complete rest, as is done in an intermittent type.
Cont....

27. Design of a Continuous Flow Sedimentation Tank
Depends on the following assumptions:
1. Particles settles in settling zone of the tank
2. The flow is horizontal and steady and the velocity is uniform in all parts
of settling zone for time equal to 𝐭𝐝.
3. The concentration of suspended solid of each size is the same at all
points of the vertical cross-section at the inlet end.
4. A particle is removed when it reach the bottom of settling zone.
Cont....

28. Fig. of vertical cross-section of the tank
Cont....

29. • Let L and H be the length and depth, respectively of the settling zone. Let B
be the width of the tank and Q be the discharge rate. Then, the horizontal
velocity is:
Vh =
discahrge
cross_scetional area
=
Q
BH
• And the time of horizontal flow is:
𝑡𝑑 =
𝐿
𝑉ℎ
=
𝐿𝐵𝐻
𝑄
=
𝑉
𝑄
................................a
• The time for falling through distance H will be:
𝑡𝑑 =
𝐻
𝑉
𝑠
… … … … … … . 𝑏
Equating equation a and b:
𝐻
𝑉
𝑠
=
𝐿𝐵𝐻
𝑄
Cont....

30. 𝑉
𝑠 =
𝑄
𝐿𝐵
=
𝑄
𝐴𝑠
… … … … … 𝑐, 𝐴𝑠 is surface/ plane area
Equation c defines the overflow rate or overflow velocity
And suggests all particle with 𝑉
𝑠 >
𝑄
𝐴𝑠
will reach the bottom before the end outlet
of the tank.
• If a smaller particle having 𝑉
𝑠′ <
𝑄
𝐴𝑠
enters the tank at point C it will settle
through only at height h. then,
𝑡𝑑 =
ℎ
𝑉
𝑠′
=
𝐿𝐵𝐻
𝑄
ℎ = 𝑉
𝑠
′
𝐿𝐵𝐻
𝑄
=
𝑉
𝑠
′
𝑉
𝑠
𝐻
Therefore, these particles will not settle if they enter the basin above point C.
Cont....

31. parameter value
range typical
Detention time, hr 1.5-2.5 2
Overflow rate,m3/m2/d
Average flow
Peak flow
32-48
80-120 100
Weir loading, m3/m/d 125-500 250
Dimensions, m
Rectangular
Depth
Length
Width
Sludge scrapper speed, m/min
Circular
Depth
Diameter
Bottom slope, mm/m
Sludge scrapper speed, m/min
3-5
15-90
3-24
0.6-1.4
3-5
3.6-60
60-160
0.02-0.05
3.6
25-40
6-10
1
4.5
12-45
80
0.03
Table. Design criteria for primary sedimentation tank
Cont....

32. Sedimentation Aided with Coagulation
What is Coagulation?
 Coagulation is the destabilization of colloids (very fine SS) by addition of chemicals
that neutralize the negative charges
 Colloids have a net negative surface charge
 The chemicals are known as
coagulants, usually higher valence
cationic salts (Al3+, Fe3+ etc.)
• Coagulation is essentially a
chemical processes
--
-
-
-
-
- --
-
-
-
-
-
--
-
-
-
-
- --
-
-
-
-
-
32
Cont....

33.  Then, the destabilized particles agglomerate into a large size particles
known as flocs which can be effectively removed by sedimentation or
flotation.
 The process of forming flocs is called flocculation.
Fig. of sedimentation by coagulation
33
Cont....

34. Why coagulation and flocculation?
G
r
a
v
I
t
y
s
e
t
t
l
I
n
g
Colloids – so small: gravity settling not possible 34
Cont....

35. S. No.
Name of
coagulant
B.O.D.
removed
as %age of
total
present
SS removed
as %age
of total
Present
Dosage
required
in ppm
pH value
required
for proper
functioning
Remarks
l.
Ferric
chloride
80 - 90 90 - 95 25 - 35 5.5 to 7.0
This coagulant is widely used for sewage
treatment, wherever, coagulation is
adopted.
2.
Ferric
sulphate
with lime
60 80 35 - 40 8.0 to 8.5
Ferric sulphate has been found to be
more effective than chlorinated
copperas, if used in conjunction with
lime. Hence ferric chloride and ferric
sulphate are mainly used, as coagulants
in sewage.
3. Alum 60 80 40 - 90 6 to 8.5
It is generally not used in sewage
although used for treating water
supplies on a large scale.
4.
Chlorinated
copperas
70 - 80 80 - 90 35 - 80
5.5 to 7.0
and
9.0 to 9.5
This coagulant is effective for producing
sludge for activated sludge process.
35
Properties of the important coagulants used in sewage treatment
Cont....

36. Example
1. Find the terminal settling velocity of a spherical discrete particle with diameter 0.5mm
and specific gravity of 2.65 settling through water at 200C. (𝜌𝑤 = 1000kg/m3 and 𝜇 =
1.002 × 10−3
𝑁𝑠/𝑚2
Solution
i. Assume laminar flow
𝜐 =
𝜇
𝜌
=
1.002×10−3
1000
= 1.002 × 10−6m2/s
𝑉
𝑠 =
9.81
18
2.65 − 1 ×
0.00052
1.002 × 10−6
= 0.22𝑚/𝑠
ii. Check Reynolds number𝑅𝑒 =
𝑉𝑠𝑑

=
0.22×0.0005
1.002×10−6 = 112 ⟹ 𝑡ℎ𝑒 𝑓𝑙𝑜𝑤 𝑖𝑠 𝑡𝑟𝑎𝑛𝑖𝑡𝑖𝑜𝑛𝑎𝑙

37. iii. For transitional flow, estimate the value of CD
CD =
24
Re
+
3
Re
+ 0.34 =
24
112
+
3
112
+ 0.34 = 0.84
iV. Using the general equation evaluate Vs
𝐯𝐬
𝟐 =
𝟒𝒅𝐠(𝐆 − 𝟏)
𝟑𝐂𝐃
=
𝟒 × 𝟎. 𝟎𝟎𝟎𝟓 × 𝟗. 𝟖𝟏(𝟐. 𝟔𝟓 − 𝟏)
𝟑 × 𝟎. 𝟖𝟒
= 𝟎. 𝟏𝟏𝒎/𝒔
V. With 𝑉
𝑠 =
0.11𝑚
𝑠
, 𝑟𝑒𝑝𝑒𝑎𝑡 𝑡ℎ𝑒 𝑎𝑏𝑜𝑣𝑒 𝑠𝑡𝑒𝑝𝑠
Cont....

38. 2. Design a suitable rectangular sedimentation tank (provided with mechanical cleaning
equipment) for treating the sewage from a city, provided with an assured public water
supply system, with a maximum daily demand of 12 million liters per day. Assume suitable
values of detention period and velocity of flow in the tank and 80% of water supplied to the
city becomes sewage.
Solution
• we have the quantity of sewage required to be treated per day (i.e. maximum daily). = 0.8
*12 million liters = 9.6*106 liters/day
• Now assuming the detention period in the sewage sedimentation tank as 2 hours (it is
taken between 1 to 2hr)
• Capacity of the tank required
𝑉 = 𝑄 × 𝑡 =
9.6∗106 liters/day
24
× 2 = 800𝑚3
• Assuming that the flow velocity through the tank is maintained at 0.3m/minute
• The length of the tank required = 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑓𝑙𝑜𝑤 × 𝐷𝑒𝑡𝑒𝑛𝑡𝑖𝑜𝑛 𝑝𝑒𝑟𝑖𝑜𝑑 = 0.3 ×
2 × 60 = 36𝑚
Cont....

39. • Cross-sectional area of the tank required=
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘
𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘
=
800
36
= 22𝑚2
• Assuming the water depth in the tank (i.e. effective depth of tank as 3m ( Usual values of
effective depth (i.e. depth excluding the bottom sludge zone) range between 2.4 and 3.6m
(generally not exceeding 3m)).
• The width of the tank required =
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎
𝐷𝑒𝑝𝑡ℎ
=
22.2
3
= 7.4𝑚
• Since the tank is provided with mechanical cleaning arrangement, no extra space at
bottom is required for sludge zone
• Assuming a free-board of 0.5m
• The overall depth of the tank = 3 + 0.5 = 3.5𝑚
• Hence, a rectangular sedimentation tank with an overall size of 36m * 7.4m * 3.5m can be
used. Note: This satisfies the requirements like: length not more than 4 to 5 times the
width; and the width not more than 7.5m or so; the depth between 2.4 to 3.6m, etc
Cont....