3. Design of wastewater collection system.pdf

Structural design

1. Sanitary engineering
CET 06216
Chapter 3
Design of Wastewater
Collection Systems
Eng. AbdulbastIdrisa
MISUNGWI CDTTI

2. 3.1 DEFINITION OF KEY TERMS
Sewerageis the infrastructure or system that
conveys sewage or surface runoff (storm water,)
using sewers.
Dry weather flow(DWF) is the normal flow in a
sewer during the dry weather.
Wet -weather flow(WWF) is the flow in a sewer
during the wet weather.

3. Cont
Manholerefers to as a small covered opening in a
paved area allowing access beneath, especially one
leading to a sewer.
Concentration timeis defined as time needed for
sewage to flow from the most remote point in a
sewerage as the time to the treatment system.
Return periodalso known as a recurrence interval or
repeat interval, is an average time or an estimated
average time between events such as sewage to occur.

4. 3.2 WASTE WATER DESIGN
DESIGN CRITERIA
Population projection
Flow estimate

5. 3.2.1 Population and Projections
Accurate population projections are essential for
estimating Wastewater flows and Biological loads
This ensures proper sizing of treatment plants and
associated infrastructure.
Existing plants: Use actual flow and load data.
New developments: Use population estimates and
assumptions (e.g., housing plans, bedroom counts)
Projections should ideally cover a 10-year design period,
though phased/staged developments are common due to
economic or planning constraints

6. Cont
Forecasting population is relatively simple if land
use plans are available and adhered to.
The most common approach is normally using the
past trends of population growth on the basis of
the census data.
Then forecasting future population by assuming
validity of some mathematical models (i.e. simple
compound arithmetic expression given below).

7. Cont
Where:
P= future population
P
o= present population
r= rate of annual population growth
n= number of future years

8. Factors Affecting Population per
Household
a)Size (more bedrooms →more occupants)
b)Area type (holiday vs. permanent residences)
c)Income level (lower-income areas often have
more residents per unit)

9. 3.2.2 Flow Estimation
Why Flow Estimation Matters
Flow rate and sewage strength directly affect
i.Sizing of treatment units
ii.Hydraulic design
iii.Operational efficiency

10. Estimation Methods
1. Fully-seweredhouseholds (with internal
plumbing):

11. Cont. ….
2. Dwellings without full in-house
water supply:

12. 3.1 Flow Variation
In wastewater design, flow variation is a crucial consideration
since actual flows fluctuate throughout the day, seasonally, and due
to weather conditions.
Understanding and accounting for these variations helps ensure
treatment units are neither under designed nor oversized

13. .
Flow Type What It IncludesUse In Design
Average Dry
Weather Flow
(ADWF)
Domestic
wastewater only
Process unit sizing
(Peak Dry Weather
Flow )PDWF
ADWF X Peak
factor
Daily peak
handling (pumps,
pipes)
Peak Wet Weather
Flow (PWWF )
PDWF +
infiltration/stormwa
ter
Extreme conditions
(hydraulics,
overflows)

14. Factors Influencing PWWF
Factor Effect on Flow
Rainfall intensity
Higher intensity increases runoff ingress into the
system
Natural groundwater
table
High tables →continuous infiltration
Pipe watertightness
Poor quality →cracks or leaks let
groundwater/stormwaterin
Inspection/maintenance
frequency
Infrequent inspections →unnoticed damage,
more inflow
Soil/geology
Sandy/unstable soils →higher risk of pipe shifts
and cracks
Topography
Flat, low-lying areas more prone to flooding
and infiltration

15. Cont
The other two common alternative approaches
used to determine design flow are :-
I.Estimating peak/average sewage flow ratioas a
function of community size (see Table 3.1) to
determine design discharge (Q
d) based on
average discharge values within various
reaches of the sewer network.

16. Cont
ii.Using factor of 1.2 and distributing total
daily sewage over only a limited.
•Number of hours (i.e. 10-16 hours
within a day ) to determine design flow
discharge (Qd).
•In industrial areas wastewater
production is constant during working
hours.

17. Cont
Table 3.1 Peak/average flow ratios in relation to
population (Source: Okun and Poghis, 1975)
Population Peak/average flow ratio
1,000 5.0
2,000 4.5
5,000 4.0
10,000 3.5
20,000 3.0
40,000 2.5
100,000 2.2
500,000 1.6

18. Cont
Peak factor of 2 is taken for larger populations
(≥500,000) or about 3.5 for smaller
populations (≤ 20,000), generally peak factor
of 3 is used.

19. Cont
Sewers are normally designed to flow 0.8 of
their full capacities to allow for some margin
to take into account possible sudden
excessive flows and foul gases.
Design period may be taken as 30 years
and the increase in capacity may be taken
not to exceed 25%.

20. Cont
Seepage through sewer joints should be
added to determine Q
dfor sewer design
purposes.
The seepages depend on:
i.Joint types
ii.Sewerage
iii.Workmanship
iv.Weather (rainfall amount)

21. Cont
The common four methods for estimating
seepage are:-
1.Rate per unit sewer length per day
Ranging from 10 to 200 m
3
/km-day.
2.Rate per unit area sewered
Ranging from 5 –20 m
3
/ha –d plus 4 m
3
/d -
manhole.

22. Cont
1.Rate per pipe unit length and unit diameter
per day
Varying from 50 to 5000 l/day.km.mm pipe
diameter or m
3
/mm.km.d
2.Infiltration rate as a percentage of the total
sewage generation per day
About 20 to 60 %

23. 3.5SEWERS SYSTEM DESIGN
Sewers design process should be observe the
following important items:
A.Manhole locations
Sewers should have manholes at the following
locations:-
i.At each change of direction and slope
(horizontal and vertical)
ii.After every 50 m of sewer length for control
during maintenance

24. Cont
i.Whenever the pipe diameter changes
ii.Whenever several pipes come together to
join a single drain main
iii.At the boundary of property before the
drain joins a public sewer

25. Cont
Fig.3.1shows details of sewer drains and manhole constructions.

26. Cont
B.Sewer joints
Sewers should be joined to main drain at an
oblique angle in the direction of flow.
Secondary sewers should be joined to main
sewers (with bigger diameters) such that they
are in line with tangents at the top sides of the
main sewers to avoid back flows in the
secondary sewers.

27. Cont
C.Sewer flow velocities
Sewer velocities should be such that ;-
•V
min= 0.6 to 0.9 m/sec. in the case of
separate sewers and 0.75 m/sec. in the case
of combined sewers to avoid silting
(sedimentation).
•V
max= 2.5 m/sec. to avoid abrasion of
sewers.

28. Cont
D.Sewer slopes
Sewer gradients are determined by
considering the following:-
i.Slope of the ground
ii.Minimum and maximum flow velocities
permissible
iii.Diameter of sewers and the sewage
discharge

29. Cont
E.Sewer depths
Minimum depth from ground surface to the top
of main minimum diameter (300 mm) should be
1.0 m.
Maximum depth to invert level of any pipe
should be 5.0m.

30. Cont
The deeper the sewer in ground the higher the
operation and maintenance cost and the
higher the extent of ground water infiltration
into the sewer with time (age).

31. Cont
F.Sewers ventilation
Prevents the accumulation of explosive,
corrosive or poisonous gases and vapours (e.g.
methane –CH
4, hydrogen sulphide –H
2S,
petrol vapour, etc.).
Prevents high concentration of unpleasant
odours causing nuisance.

32. Cont
Relieves air pressures above or below
atmospheric pressure caused by sudden rise or
fall of sewage.
This is done to permit free flow of sewage.

33. Cont
Sewer ventilation can be provided by:
i.Installing ventilation columns(Fig. 3.2)
at upper end of every branch sewer and
also at every change in size of sewers.
Usual spacing is about 300 m and their
sizes are determined by sewer
diameters.

34. Cont
Fig.3.2Ventilation column

35. Cont
i.Using perforated manholes.
These are characterized by bad odour
nuisance and admit surface runoff
water in large quantities into sewers.
ii.Providing un-obstructed outletsin the
case of storm water drains or sewers
which can act as partial ventilators.

36. Cont
iv.Using forced draughtscaused by exhaust
fans to expel out foul gases from the
sewers.
v.Using house vents and soil pipeswhich
have added advantages of ventilating
house drains and public sewers.

37. 3.6MATERIALS AND FORMS OF SEWERS
The materials used for pipe sewers should
be:
i.Have adequate strength against earth
pressures and other loads.
ii.Resistant to corrosion.
iii.Impervious.
iv.Had to resist erosion due to abrasive sewage
materials such as grit.
v.Economical.

38. Cont
The materials employed for pipe sewers are:
I.Vitrified clay or salt glazed stone ware
II.Cast iron
III.Steel
IV.Concrete
V.Asbestos cement
VI.Plastics

39. Cont
Sewer section
The cross-section of sewers used (which is either
circular or non-circular) depends upon:-
i.Efficiency of flow with reference to hydraulic mean
depth and roughness of material.
ii.Structural stability
iii.Resistance to internal and external pressures
iv.Resistance to corrosion
v.Ease of operation and maintenance
vi.Cost

40. 3.7SEWAGE FLOW AND SEWER SECTION DESIGN
The flow of sewage in sewers takes place
under two conditions:-
i.Open channel flow (partially flow)
When the hydraulic grade line lies on the surface
of the flowing liquid and is exposed to the
atmosphere.
ii.Closed channel flow (full flow)
When sewage flows in a conduit at a pressure
above or below that of the atmosphere.

41. Cont
Based on these two conditions of flow, the
hydraulic formulae governing sewage flow
generally used include:-
(Hazen-William) (1)
(Mannings –Strickler) (2)

42. Cont
Where:
v = velocity of flow
C = roughness coefficient
R = hydraulic radius (= D/4 for Darcy –Weisbach
equation)
S = channel or pipe gradient
f = non-dimensional coefficient
(Darcy –Weisbach) (3)

43. Cont
Table 3.3:Typical roughness coefficient (C) values recommended by various authors
for different sewer pipe materials.
Pipe material Smooth (new) Rough (old) Typical
Wood 90 80 85
Vitrified clay - - 100
Cast iron 90 70 80
Concrete(plain or reinforced) 100 60(50) 70
Asbestos cement 140 100 100
Plastic 140 100 120

44. 3.8 DESIGN OF STORM RUNOFF WATER DRAINAGE SYSTEM
Runoff discharges for designing rainfall runoff drains
Volume of rainfall storm runoff is a function of catchment or watershed
and rainfall characteristics:
i.Vegetation
ii.Soil infiltration capacity
iii.Topography (slope steepness and slope length)
iv.Catchment size and shape
The rainfall characteristics affecting runoff volume generation are,
intensity and duration
Many empirical methods are available for estimating runoff discharges
while considering the catchment characteristics variably i.e. The rational
method.

45. Cont
The Rational method
For estimating runoff discharges is:-
Where:
Q
P = rainfall storm runoff discharge (M
3
/s)
C = rainfall storm runoff coefficient (see Table 3.6)
i = average rainfall intensity (mm/h) with duration equal
to time of concentration (gathering time) of the
catchment under consideration
A = catchment area (ha)

46. Cont
Table 3.6: Runoff coefficients for different catchment characteristics.
Surface Coefficients, C Surfaces Coefficients, C
Roofs/grazed surfaces 0.90 –0.95 Town 0.50 –0.70
Asphalt concrete 0.85 Suburbs 0.30 –0.50
Paved streets 0.50 Play grounds 0.20 –0.30
Gravel roads 0.20 Unimproved areas 0.10 –0.20
Gardens 0.10 Gardens 0.05 –0.10
Town centre 0.70 –0.90

47. Cont
Judgment should be used in selecting rainfall storm runoff
coefficients, C, when using the Rational method to determine runoff
discharges.
Average coefficients are normally determined for catchments with
varying characteristics.
For example, if 30% of a catchment is a garden, 15% is play
grounds and 55% % is unimproved area, the average coefficient
is:
0.1 ×30/100 + 0.25 ×15/100 + 0.15 ×55/100 = 0.15.

48. Cont
4.Design of storm runoff drains (sewers)
Storm intensities increase with return periods.
The longer the return period for a design storm selected the
larger will be the structure.
Thus, choosing a design storm of a given return period for the
design of storm runoff drains is an economic problem, i.e. the
cost of construction and cost of damage that can result from
floods and other inconveniences.

49. Cont
The following guidelines can be used to determine design storm return
periods for specific runoff drains:
Tr ≤ 1 yr for villages in the tropics.
Tr = 1 –5 yrs in small towns
Tr = 10 yrs in large towns and central business areas and soil conservation works.
Tr = 20 –50 yrs in commercial city centres.
Once the runoff discharges are obtained the storm water drainage sewers
are designed in the same way as the waste water sewers.
Open channel drains are designed using Manning -Strickler equation based
on flow velocity and design discharge.

50. Cont
Where:
v = flow velocity (m/sec)
C = roughness coefficient (m
1/3
/s)
R = hydraulic radius (m)
S = channel longitudinal slope (m/m)
A = cross section area of flow (m
2
)
Q = discharge (m
3
/sec)

51. Cont
Maximum and minimum velocities and hydraulic roughness values for
different channel conditions are given in Tables 3.7 and 3.8
respectively.
Table 3.7 Maximum and minimum design velocities for channel drains.
Channel lining condition Minimum velocity
(m/sec)
Maximum velocity
(m/sec)
Soil without vegetation - 0.40
Grass lined ditches 0.75 1.50
Concrete channel 0.75 2.0 –4.00*

52. Cont
Table 3.8 Hydraulic roughness values for different channel lining
materials
In deriving the runoff discharges, first determining
concentration time for the runoff contributing areas
(catchments).
Lining materials Hydraulic roughness (C-m
1/3
/s)
Earth bottom and rubble sides 24 –45
Grass lined ditches 25 –35
Concrete channels 50 –70
Irregular rock cuts 20 –30

53. SAMPLE QUESTIONS
1.Define the term ‘‘sewerage’’?
2.Distinguish between sewage and
sewerage.
3.Briefly explain different types of
sewerage system.
4.Distinguish between separate and
combined sewerage systems.

54. Cont
9.What are the advantages and disadvantages
of separate sewerage system?
10.What are the advantages and disadvantages
of combined sewerage system?
11.List down common data required for
sewerage system design.
12.What factors contribute to sewage
discharge?

55. Cont
9.What is meant by dry-weather flow(DWF)?
10.Dry-weather depends upon various factors,
mention and explain in detail at least four (4)
factors.
11.How sewage design flow maybe determined,
explain?
12.What factors affect seepage through the sewer
joints?

56. Cont
9.State, how seepage through the sewer
joints maybe determined?
10.Sewers design process should observe
various important items. List them and
state the criteria or specify the range to be
considered in each item during the design
process.

57. Cont
15.The materials to be used for pipe sewers
should meet various conditions and
specification, mention at least five (5)
conditions.
16.Mention important materials that are mostly
used in sewer pipe.
17.The cross-section of sewers used (which is
either circular or non-circular) depends upon
various factors, mention those factors.

58. Cont
15.Why circular sewer section is mostly
preferred in sewer design.
16.Differentiate between partially and full
flow as far as sewage flow is concerned.
17.What is meant by wet-weather flow
(WWF)?
18.Distinguish between DWF and WWF?

59. END
THANK YOU

60. What’s next
Chapter 4
DESIGN OF WASTEWATER TREATMENT
SYSTEMS