Process of sewerage system Design
Introduction
Design of sewerage
system in a town is mainly based on using the gravity of the available terrain
and directing the sewage flow to the desired lowest point. While doing so
economy of pipe material, minimum excavation and lowering, laying labour is to
be achieved. However, main constrains are minimum pipe diameter, range of
slope, minimum velocity in the sewers. It is difficult to minimum velocity in
the sewers until the contributory population does not increase to certain
minimum number. Parameters of design of water distribution system and those of
sewerage system are quite different and sewerage system has fewer options
available.
Concepts behind stringent parameters
Basic criteria of
proposing sewerage system is to establish 135 LPCD of water supply at consumer
level. If the system attains 24x7 water supply then design peak factor is of
2.5 to 3 is attained. But still even before attaining the 24x7 water supply
sewerage system can be considered. Population forecast for the city is
applicable to both the designs. In both cases system designs are for the
ultimate stage. With the advance of the infrastructure the development is
ensured. Water supply network provides the asking rate for the development and
allows the population to settle. Sewerage system network follows the developed
areas. Habitations follow the road network, then it follows the water supply
network and sewerage system network comes finally. New areas often are asked to
develop their own network and connect to the existing sewerage network.
However, these future developments are already considered in the development
plan and overall sewerage system network also considers the future development.
The expected load is considered and the system is proposed in stages and
initially is limited to the existing development. In this case also population
at the ends is having so low density that they cannot contribute sewage flow to
the system which can generate the minimum flow. This minimum flow is to generate
velocity 0.6 m/s, in the minimum diameter of pipe at the end. Hence, even in
the existing stage there are two parts in the design. One is for the area
having development potential and other is already developed. The distinctive
mark is the sewage contribution to generate minimum sewage velocity in minimum
diameter of pipe. Area down below is
developed part. It is this area where the sewage flow can be directly connected
to the sewerage system and sewers are safe from choking possibility as minimum
velocity of 0.6 m/sec will be attained at least once in a day. This velocity is called as the self-cleansing
velocity.These sewers shall have minimum diameter of 150 mm dia. in smaller
cities and in other cities minimum diameter is 200 mm dia. Those who satisfy
these conditions are known as “Conventional Sewers.”
Sewers which do not satisfy these criteria are
“Non-Conventional Sewers.” It may be due to lack of contributory population to the
man-holes, pipes used are of lesser dia., velocity attained is less than
minimum self-cleansing velocity. These sewers are proposed in the developing
areas. They are likely to be updated as and when the contributory population is
expected to increase in a due course of development. Their design period is
different than the conventional sewers.As
the velocity criterion is not fulfilled these sewers cannot be connected to HH
systems directly. It shall be connected to the sewer systems through septic
tanks only. It is expected to carry liquid waste only.
Manual refers to
the concept of design period as below,
2.5 DESIGN PERIOD
The project components may be designed for
the periods mentioned in Table 2-1 overleaf.
|
Sr. No.
|
Component
|
Design
period Years
(from
base year)
|
|
1
|
Land Acquisition
|
30
years and more
|
|
2
|
Conventional sewer s (A)
|
30
|
|
3
|
Non-Conventional sewer (B)
|
15
|
|
4
|
Pumping mains
|
30
|
|
5
|
Pumping stations – Civil work
|
30
|
|
6
|
Pumping machinery
|
15
|
|
7
|
Sewage Treatment Plants
|
15
|
|
8
|
Effluent disposal
|
30
|
|
9
|
Effluent Utilization
|
15
or as the case may be
|
|
(A)
Typical
underground sewers with manholes laid in the roads.
(B)
All types such as
small bore, shallow sewers, pressure sewers, vacuum sewers
|
||
Concept of Project Area
For any city
the project area for sewerage system design shall be developable area for the
design period of 30 years. Population projection for the city as a whole shall
be done for thirty years to arrive at the total waste water generation of the
city after thirty years. It shall include both domestic and non-domestic waste.
It may not be
expected that the whole city drains at a single point. Hence, the natural
drains are traced and the area bound to drain to the particular natural drain
is decided. With suitable Pumping stations, location of STP shall be decided.
Choice is to weighed between a single STP and zonal STPs. For detailing
purpose, wards of the cities are to be studied. Development potential remaining
in the wards is to be identified and they are allowed to develop in a natural
way. After reaching certain population density people will prefer to move
outside and may be attracted to de-congested areas. Such a density is known as
the THRESHOULD DENSITY. Noting these observations, maximum densities in other
wards shall be decided. A notional sector planning is expected to be done in
the developable areas and the calculated population in the city after design
period shall be tallied with a justified distribution in the ward boundaries.
Knowing the areas of the wards and the population in the design year gross
density in the ward at design stage shall be known. It shall be modified to the
net density after reducing the areas where development is not feasible. All
these considerations are also affected by the drivers of the development for
the city.
Having marked
zones, known as drainage districts, ward areas are to be super imposed on the
same map and the densities are considered in that area to operate in the design
period. Knowing the present population density from the Google map as the
existing density, design of flows and pipes can be proposed from the density in
the design year.
These pipes
in the future period will be justified whenever the population settles down in
that area, but till then provisional connection pipe lines be proposed from the
software. However, flows can be cumulated downstream, and allowed to attain the
velocity criterion of 0.6 m/sec.
Manual also
refers to the similar process as below,
2.7
PROJECT AREA
The factors that influence the
determination of project area include natural topography, layout of buildings,
political boundaries, economic factors, CMP, etc. For larger drainage areas,
though it is desirable that the sewer capacities are designed for the total
project area, sometimes the political boundaries and legal restrictions prevent
construction of sewers beyond the limits of the local authority. However, when designing sewers for larger areas, there is
usually an economic advantage in providing adequate capacity initially for a
certain period of time and constructing additional sewers, when the pattern of
growth becomes established. The need to
finance projects within the available resources necessitates the design to be
restricted to political boundaries. The project area under consideration should
be marked on a key plan so that the area can be measured from the map.
Concepts for the layouts of sewerage
Sewerage
system is to be designed for the design year of 30 year ahead and laid to the
extent of the existing habitations. It will be distinguished into two
categories, conventional sewers and non-conventional sewers. In many fringe
areas, population density will be much less in the existing stage and also HH
can be well separated from each other. Some HH can be in clusters. Roads might
not have been developed fully. In such cases the main option is to adopt “on
site sanitation.” Habitations are allowed to settle and other developments are
also allowed to be defined. Water supply network shall be in place and LPCD
levels are defined to 135 LPCD at the consumer ends. Sewers can be laid only
after the roads are defined. This may cause the time difference. Conventional
sewer is laid as proposed. It has the contributory population defined in the
developable ward area. It can accommodate the sewage flow, if it reaches by non-conventional
system. Once the settlement of population occurs, Phase II Estimates can be
prepared for the conventional sewers or conversion of non-conventional sewers.
Criteria mentioned above are applicable to the Conventional sewers only, but
they become applicable when the conversion of non-conventional sewers is under
considerations. It is for this reason software design shall be for the full
area and model needs to be updated as and when the settled population
increases. Prospective urban local bodies may retain the non-conventional sewer
diameters to the design minimum diameters. ULB is expected to face distraction
from the prospective consumers as they have already drained their waste into
septic tanks and connecting to the drain sewers can cost them further. In such
cases present estimates shall provide for the connecting drains to main sewers,
even in the case of non-conventional sewers. ULB will become responsible for
the O&M of non-conventional sewers also. Moreover, ULB shall provide
facilities for the periodical cleaning of the septic tanks at reasonable
charges. All connected houses whether to conventional or non-conventional
sewers, are taxable at same rates.
Manual has
explained it as below,
2.9
LAYOUT AND ARRANGEMENT OF SEWERAGE
The
layout of collection systems shall resist the tendency to go in for underground
sewerage flat out even in habitations that are only sparsely developed. The options of either time-deferred underground sewerage or
incremental sewerage commensurate with the pace of development by such options
as small bore, shallow sewers, twin drains, etc., to start with and eventual
underground sewerage when habitations have been populated to a certain level
where the revenue will be able to sustain the
O&M.
The
layouts by small communities shall be mandated to include the small bore sewer
system / twin drain in both sides of roads, whereby the house side drain will
receive the septic tank effluent and the road side drain will receive the storm
water runoff. In metropolitan urban centers,
decentralized sewerage shall be confined to institutional boundaries only and
not culled out of habitations itself and zoning of sewerage with STPs fanning
out radially outwards is to be encouraged.
A flat out choice of underground
sewerage with sewers in middle of roads shall be discouraged and incremental sanitation as settled sewers, small-bore
sewers, twin drain for septic tank effluents and
sewers on shoulders of wide roads are to be evaluated.
2.11 Guidelines on
house sewer connections
a)
There is a compelling need to amend the bye-laws to
make it compulsory for the population to avail house service sewer connection
wherever public sewer is provided and if this is notforthcoming, the local
authority shall effect the house sewer connection and institute revenue
recovery proceedings.
b) Include house-service sewer connections as part of the
sewerage project itself
c)
Float Equated monthly installments (EMI) schemes for repayment of house service
sewerconnection costs.
Concepts in planning of sewer system
Total area covered in the city
limits can be far more as compared to the area occupied by the present
development. Still the Design of sewerage system shall be done for design
period of 30 years. Development plan is available and land use pattern is
decided for the design period. Residential area development in future is also
known. It is designer’s anticipation, that the sectorial development in future,
will be proposed to be shaped. It shall be based on the basis of development
plan reservations for particular type of development. Normally agricultural use
and the residential use are dependent on the demand for the residential use. If
the financial advantage is seen or anticipated in the conversion of land use
from the Agricultural to NA, then the rates of land use increase and the
residential settlements take place. This is a slow process and invites
programmed land monetization. These developments are in clusters, nearer to the
edge of present development. Infrastructure development also takes time.
Initially, the onsite sanitation is the main solution. Then the connecting area
drain to the main sewers becomes the ‘popular’ decision, limited to budget
provisions. Over a period the incremental sewerage system upholds in this type
of areas.
Manual
also refers to the similar process as below,
2.14.5 Planning of Sewer System
The design principles in Chapter 3 of this manual shall
be followed. In essence, it stipulates that the options of small bore sewers,
shallow sewers, twin drains and underground sewers all have to be relatively
evaluated to sub regions of the project site instead of blindly going in for
total underground sewer flat out. Incremental sewer shall also be considered
based on the phased development of the region instead of directly opting for
total underground sewer system.
3.4 TRIBUTARY AREA
The natural topography, layout of buildings, political
boundaries, economic factors etc., determine the tributary area. For larger
drainage areas, though it is desirable that the sewer capacities be designed
for the total tributary area, sometimes, political boundaries and legal
restrictions prevent the sewers to be constructed beyond the limits of the
local authority. However, in designing sewers for larger areas, there is
usually an economic advantage in providing adequate capacity initially for a
certain period of time and adding additional sewers, when the pattern of growth
becomes established. The need to finance projects within the available
resources necessitates the design to be restricted to political boundaries. The
tributary area for any section under consideration has to be marked on a key
plan and the area can be measured from the map.
Concept behind the coverage of sewerage
system
Sewerage
system aims at Benchmark level for coverage as 100%. It is presumed here that
even though the particular consumer has constructed a septic tank for immediate
disposal of waste water, he shall connect the outflow from septic tank to the
available sewerage system up to 30 m from the boundary of his compound. Hence,
the Govt. intends to extend the coverage of the sewerage system to 100%.
Consumers shall have to co-operate. Central Govt. has given the enactment done
for Goa State for users and consumers are bound to follow the Govt. intention.
On the similar lines Central Govt. wants that other states shall follow the enactment
Procedure, to make the consumers compulsory
2.21 MANDATORY REQUIREMENTS IN
SANITATION SECTOR
These shall be as follows
1.
Each state
government shall mandatorily pass a “Sewerage & Sanitation act” and notify
the rules thereunder. The reason for this is to empower the ULB’s to prevail on
the property owners/ occupiers to avail house service sewer connections once
the sewerage system is developed by the ULB, within 30 m of the premises irrespective of whatever be the mode of
existing sewage disposal system.In
case the owner/occupier fails to do so the ULB by virtue of its powers can
disconnect essential services like, water supply and electricity after the
expiry of the notice period. An example of such a provision can be seen under
rule 10(5) of the Goa Sewerage System and sanitation services management rules
2010 enacted under the Goa Sewerage system and services management act 2008 as
contained in Appendix C. 2-2 of Part C Management.
2.
Similarly, each
state government shall mandatorily formulate and notify appropriate act and rules for septage
management.
2.17.2 Reconstruction Planning of Sewers
h) While designing
the sewer system itself, trunk sewers shall be designed to be possible to be
used for such diversions by temporarily using the sewer as a pumping line under
low pressure. After all, these sewers are laid using long sewer pipes of 6 m
length, and the load carrying capacity needs a rating of at least about 4
kg/sq. cm and this is adequate for such low head pumped diversions. Temporary
pumping lines of low pressure can be laid above ground along property boundary
compound walls by using double-flanged DI pipes which are easy to lay and
dismantle.
PER CAPITA SEWAGE FLOW
The entire spent water of a community
should normally contribute to the total flow in a sanitary sewer. However, the
observed dry weather flow quantities usually are slightly less than the per
capita water consumption, since some water is lost in evaporation, seepage into
ground, leakage etc. In arid regions, mean sewage flows may be as little as 40%
of water consumption and in well developed areas; flows may be as high as 90%.
However, the conventional sewers shall be designed for a minimum sewage flow of
100 litres per capita per day or higher as the case may be. Non-conventional
sewers shall be designed as the case may be.
For some areas, it is safe to assume that
the future density of population for design as equal to the saturation density.
It is desirable that sewers serving a small area be designed accordingly on
saturation density.
For new communities, design flows can be
calculated based on the design population and projected water consumption for
domestic use, commercial use and industrial activity. In case a master plan
containing land use pattern and zoning regulation is available, the anticipated
population can be based on the ultimate densities as in Table 3.1.
From
the above Table it will be clear that the Peak factors for the sewage flowing
to the sewers is quite different from the water supply distribution Peak
factors.
|
Population
|
Population
|
|
Up to 50000
|
3.0
|
|
50000 to 200000
|
2.5
|
|
Above 200000
|
2.00
|
Same Table is rearranged as,
|
Population
|
Population in Lakhs
|
Population
|
|
Up to 50000
|
0.5
|
3.0
|
|
100000
|
1.00
|
2.5
|
|
150000
|
1.5
|
2.5
|
|
200000
|
2.0
|
2
|
Converting into graph,
For convenience the graph is plotted for the Population in lakhs
vs peak factor. It is found that the R2value is exactly unity. Hence
the generated equation of the graph is applicable to be expanded in both the
directions. Accordingly using the equation value of peak factors has been
calculated for lower values of population up to 5000 (0.05 L) population and
tabulated above.
Population mentioned in the Peak factor statement does
not refer to the city population. But it is referring to population in the
influence area of the concerned Storage.
Peak factor in water supply distribution
For Peak Factor concept, demand pattern for water requirement in
24 hours is used. Average hourly demand is calculated from the average daily
water supply in LPCD. In case of 135 LPCD water supply at consumer end average
hourly supply works out to 135/24 = 5.63 LPH per capita. With peak factor 3 in
morning hours Demand in this hour will work out 5.63*3 = 16.89 LPH per capita.
Application
of Peak factor in sewerage system
Use the same peak factor 3 for sewerage design. Peak factor
depends on
the density of population, topography of the site, hours of water
supply and hence, individual cases may be further analyzed if
required. Relevant figures for sewage
flow will be calculated at 80% of these figures. i.e. 16.89*0.8= 13.51 LPH.
This rate is supposed to be flat in the peak hour.
For
minimum dia. of 150 mm and with n=0.013, for velocity to attain 0.6 m/sec and
sewer to run 50% full, required flow is minimum as given below,
|
% full
|
Dia. in mm
|
Slope 1 in
|
Hz-Will. C-value
|
Dia. Power 0.63
|
Dia. Power 2.63
|
slope power 0.54
|
Velocity in m/sec
|
Discharge in
cum/day
|
Discharge in
cum/hr
|
Peak hour
Discharge in cum/hr
|
|
|
|
|
|
|
|
|
|
|
|
Peak = 3
|
|
0.5
|
150
|
170
|
140
|
23.49
|
528591
|
0.06
|
0.94
|
716.37
|
29.85
|
89.55
|
|
0.4
|
|
|
|
|
|
|
0.85
|
482.83
|
20.12
|
60.35
|
|
0.3
|
|
|
|
|
|
|
0.73
|
280.82
|
11.70
|
35.10
|
|
0.2
|
|
|
|
|
|
|
0.58
|
126.08
|
5.25
|
15.76
|
|
0.1
|
|
|
|
|
|
|
0.38
|
30.09
|
1.25
|
3.76
|
Manual refers to minimum dia. = 150 mm
Minimum slope = 1 in 170
Maximum c-value as applied with use of DWC pipe =140
Based on these conditions velocity attainable can be calculated.
In the initial stages the Pipe will not remain full.
Hence, change in velocity with same parameters but for change in
the depth of water are Tabulated.From table it is seen that contributory population shall be such
that it should create the flow of 11.70 cum/hr average and 35.10 cum/hr peak
flow to attain 0.3 d in the pipe and to give 0.6 m/sec velocity.
On the other hand it is assumed that sewer in the initial stages
is receiving sewage from both the sides. Construction of man-holes is normally
at every 30 m on the sewer line. It is expected that at every 10 m width there
is HH. On both sides there will be 6 HH in 30 m. Hence, the initial manhole
will receive sewage from 6 HH. Following table will show that with number of
different HH and 5 persons per HH Peak flows will be tabulated.
|
HH
|
Pop.
|
LPCD
|
Flow ave. per
hour in cum/hr
|
Flow Peak per
hour in Cum/hr
|
|
both sides
|
5 per HH
|
80%of 135
|
|
PF = 3
|
|
30
|
150
|
108
|
0.675
|
2.025
|
|
24
|
120
|
108
|
0.54
|
1.62
|
|
18
|
90
|
108
|
0.405
|
1.215
|
|
12
|
60
|
108
|
0.27
|
0.81
|
|
6
|
30
|
108
|
0.135
|
0.405
|
On one manhole length of 30 m and assuming about 30 HH are
contributing flow is not satisfactory to give 0.6 m/sec velocity.
|
HH
|
Pop.
|
LPCD
|
Flow ave. per
hour
|
Flow Peak per
hour
|
|
both sides
|
5 per HH
|
80%of 135
|
|
PF = 3
|
|
500
|
2500
|
108
|
11.25
|
33.75
|
|
30
|
150
|
108
|
0.68
|
2.03
|
|
24
|
120
|
108
|
0.54
|
1.62
|
|
18
|
90
|
108
|
0.41
|
1.22
|
|
12
|
60
|
108
|
0.27
|
0.81
|
|
6
|
30
|
108
|
0.14
|
0.41
|
For 0.6 m/sec velocity with 0.3 = d/D, 500 HH with 2500 population
can give required flow with the given constraints in 150 mm dia. pipe line.
The flow in sewers varies from hour
to hour and seasonally. However, for the purpose of hydraulic design estimated
peak flows are adopted. The peak factor or the ratio of maximum to average
flows depends upon contributory population as given in Table 3.2.
Table 3.2 Peak factor for Contributory
Population
|
Contributory Population
|
Peak Factor
|
|
Up
to 20000
|
3
|
|
20001
to 50000
|
2.5
|
|
50001
to 750000
|
2.25
|
|
More
than 750000
|
2
|
Source:
CPHEEO, 1993Part A: Engineering 3 - 4 CHAPTER 3: DESIGN AND CONSTRUCTION OF
SEWERS
The peak factor also depends upon the density of population, topography of the site, hours of water
supply and hence, individual cases may be further analyzed if
required. The minimum flow may vary from 1/3 to 1/2 of average flow.
Rearranging same
Table data,
|
Contributarypopulatin
|
Contributarypopulatin
in Lakhs
|
Peak factor
|
|
20000
|
0.2
|
3
|
|
50000
|
0.5
|
2.5
|
|
100000
|
1
|
2.25
|
|
150000
|
1.5
|
2.25
|
|
200000
|
2
|
2.25
|
|
250000
|
2.5
|
2.25
|
|
300000
|
3
|
2.25
|
|
350000
|
3.5
|
2.25
|
|
400000
|
4
|
2.25
|
|
450000
|
4.5
|
2.25
|
|
500000
|
5
|
2.25
|
|
550000
|
5.5
|
2.25
|
|
600000
|
6
|
2.25
|
|
650000
|
6.5
|
2.25
|
|
700000
|
7
|
2.25
|
|
750000
|
7.5
|
2.25
|
|
800000
|
8
|
2
|
This data is converted into graph as
below,
|
PF
|
3.33
|
3.31
|
3.29
|
3.27
|
3.24
|
3.14
|
3.05
|
2.96
|
|
POP. In Lakhs
|
0.01
|
0.02
|
0.03
|
0.04
|
0.05
|
0.1
|
0.15
|
0.2
|
In case of design of sewerage system
contributory population of 2500 is minimum required for 0.3=d/D.
However, manual refers to design of
sewers at 0.5 depth. It requires 81 cum of flow per hour. With PF =3.
|
HH
|
Pop.
|
LPCD
|
Flow ave. per
hour in cum/hr
|
Flow Peak per
hour in Cum/hr
|
|
both sides
|
5 per HH
|
80% of 135
|
|
PF = 3
|
|
1200
|
6000
|
108
|
27.00
|
81.00
|
Contributory population will decrease
if the PF is considered as 4.
|
HH
|
Pop.
|
LPCD
|
Flow ave. per
hour in cum/hr
|
Flow Peak per
hour in Cum/hr
|
|
both sides
|
5 per HH
|
80% of 135
|
|
PF = 4
|
|
900
|
4500
|
108
|
20.25
|
81.00
|
For each city PF is also dependent on
the other considerations such as topography, hours of water supply, density of
population etc.
Manual refers to guidelines for the Ha
density assumptions when such data is not available. It is given below,
When
a master plan containing land use pattern and zoning regulations is available
for the town, the anticipated population can be based on the ultimate densities
and permitted floor space index provided for in the master plan.
In
the absence of such information on population, the following densities are
suggested for adoption as in Table 3.1.
Table 3.1 Densities of Population vs.
Populated areas
|
Size
of town (Population)
|
Density
of Population per Ha
|
|
Up
to 5000
|
75-150
|
|
Above
5001 to 20000
|
150-250
|
|
Above
20001 to 50000
|
250-300
|
|
Above
50001 to 100000
|
300-350
|
|
Above
100001
|
350-1000
|
Source: CPHEEO, 1993
In
cities where Floor Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed
by the local authority this approach may be used for working out the population
density. The FSI or FAR is the ratio of total floor area (of all the floors) to
the plot area.
The
densities of population on this concept may be worked out as in the following
example for an area of one hectare (ha)
Roads 20%
Gardens 15%
Schools (including playgrounds) 5%
Markets 2%
Hospital and Dispensary 2%
Total 44%
Area
available for Residential Development = 100 - 44 = 56% or 0.56
Actual
total floor area = Area for residential development x FSI
Assuming
an FSI of 0.5 and floor area of 9 m2/person
Number
of persons or density per hectare = (0.56*10000*0.5) / 9 = 311
Densities as mentioned of 150 persons
per Ha are also not found at the fringe areas and HH are situated but with very
lean density. We have seen that the at least 5000 population is required to get
the adequate flow in the system at the starting points. Hence, the conventional
sewers are supposed to be on inner side of the development when such flow is
available. In time to come the density will increase and then no-conventional
sewers will be converted into the conventional sewers.
Present cities are not processed and
established for 24x7 supplies. Hence, Peak factor in this situation is
considered as 4 to avoid larger no. from becoming non-conventional sewers. It
has the reasoning based on the new areas are now equipped with toilet flushing
system and they use flushes in the peak hours hence Peak factor is improved
from 3 to 4 and even more. Flushes are capable of giving 9 lit/min flow. It
gives 9*60 = 540 lit per hour flow in the system. There can be a probability of
operating simultaneous flushes in the system giving rise to more peaks. Hence,
in the starting region up to increase of population of 5000 persons PF of 4 is
justified. Such assumptions will differ from case to case.
Water supply norms for various types of areas as per
standards.
Following Table gives the utilization
wise distribution of water supplied to various areas. For sewerage system to be
proposed minimum level of water supply is to be attained for the future
population up to 30 years.
In rural water supply and in semi-urban
areas sewer systems are not proposed. It is recommended that for proper sewer
system to function maximum velocity in a day shall be 0.6 m/sec. If this
self-cleansing velocity is not attained then the solids in the waste shall be
preferably separated and not allowed flow in the sewers.
|
Purpose
|
40 LPCD
|
70 LPCD
|
135 LPCD
|
150 LPCD
|
|
Drinking
|
3
|
3
|
3
|
3
|
|
Cooking
|
5
|
7
|
12
|
12
|
|
Bathing
|
15
|
25
|
30
|
30
|
|
Washing utensils and house
|
7
|
10
|
40
|
40
|
|
Ablution
|
10
|
25
|
40
|
40
|
|
Others
|
0
|
0
|
10
|
25
|
|
|
40
|
70
|
135
|
150
|
Considering, 135 LPCD, as the
acceptable minimum flow for sewerage system development. Break up of 135 LPCD is given below. It can
be seen that uses drinking, cooking, washing utensils and house can come at any
different timings of the day. But for peak to observe Bathing and ablution use
shall come in peak hour. Total consumption for these uses is 70 lit. It will
mean that 65 Lit (135-70) will be utilized in 24 hours. It calculates to 65/24
= 2.70 Lit/hr.
|
Purpose
|
Drinking
|
Cooking
|
Bathing
|
Washing utensils and house
|
Ablution
|
Others
|
|
Maximum waste
generation per hour as per table is 25.84 lit/hr. Average utilization of
supplied water is 4.5 lit/hr. Hence, the working peak factor works out to be
(25.84/4.5) = 5.74.
|
||||
|
135 LPCD
|
3
|
12
|
30
|
40
|
40
|
10
|
135
|
|
||||
|
Hours
|
Hour wise Dist.
|
Hour wise Dist.
|
Hour wise Dist.
|
Hour wise Dist.
|
Hour wise Dist.
|
Hour wise Dist.
|
Lit/hr
|
Waste/hr
|
Cum.lit/hr
|
Cum. Waste lit/hr
|
Cum.Ave. lit/hr
|
Hours
|
|
0
|
|
|
|
|
|
|
0
|
0
|
0
|
0
|
4.5
|
0
|
|
1
|
|
|
|
|
|
|
0
|
0
|
0
|
0
|
9
|
1
|
|
2
|
|
|
|
|
|
|
0
|
0
|
0
|
0
|
13.5
|
2
|
|
3
|
0.05
|
0.1
|
|
2
|
1
|
1
|
4.15
|
3.32
|
4.15
|
3.32
|
18
|
3
|
|
4
|
0.05
|
0.1
|
|
2
|
2
|
|
4.15
|
3.32
|
8.3
|
6.64
|
22.5
|
4
|
|
5
|
0.2
|
0.1
|
|
2
|
2
|
1
|
5.3
|
4.24
|
13.6
|
10.88
|
27
|
5
|
|
6
|
0.2
|
0.5
|
2
|
8
|
|
1
|
11.7
|
9.36
|
25.3
|
20.24
|
31.5
|
6
|
|
7
|
0.3
|
1
|
15
|
5
|
9
|
2
|
32.3
|
25.84
|
57.6
|
46.08
|
36
|
7
|
|
8
|
0.2
|
1
|
5
|
4
|
9
|
|
19.2
|
15.36
|
76.8
|
61.44
|
40.5
|
8
|
|
9
|
0.1
|
1
|
2
|
|
|
|
3.1
|
2.48
|
79.9
|
63.92
|
45
|
9
|
|
10
|
0.25
|
2
|
|
|
|
|
2.25
|
1.8
|
82.15
|
65.72
|
49.5
|
10
|
|
11
|
0.25
|
2
|
|
|
2
|
1
|
5.25
|
4.2
|
87.4
|
69.92
|
54
|
11
|
|
12
|
0.2
|
0.1
|
|
2
|
2
|
|
4.3
|
3.44
|
91.7
|
73.36
|
58.5
|
12
|
|
13
|
0.1
|
0.1
|
|
5
|
|
1
|
6.2
|
4.96
|
97.9
|
78.32
|
63
|
13
|
|
14
|
0.1
|
0.1
|
|
|
|
|
0.2
|
0.16
|
98.1
|
78.48
|
67.5
|
14
|
|
15
|
0.2
|
0.2
|
|
|
|
|
0.4
|
0.32
|
98.5
|
78.8
|
72
|
15
|
|
16
|
0.1
|
0.3
|
|
2
|
2
|
1
|
5.4
|
4.32
|
103.9
|
83.12
|
76.5
|
16
|
|
17
|
0.1
|
0.3
|
|
|
9
|
1
|
10.4
|
8.32
|
114.3
|
91.44
|
81
|
17
|
|
18
|
0.2
|
0.2
|
3
|
|
|
|
3.4
|
2.72
|
117.7
|
94.16
|
85.5
|
18
|
|
19
|
0.3
|
0.5
|
3
|
4
|
1
|
1
|
9.8
|
7.84
|
127.5
|
102
|
90
|
19
|
|
20
|
0.1
|
2
|
|
4
|
1
|
|
7.1
|
5.68
|
134.6
|
107.68
|
94.5
|
20
|
|
21
|
|
0.2
|
|
|
|
|
0.2
|
0.16
|
134.8
|
107.84
|
99
|
21
|
|
22
|
|
0.1
|
|
|
|
|
0.1
|
0.08
|
134.9
|
107.92
|
103.5
|
22
|
|
23
|
|
0.1
|
|
|
|
|
0.1
|
0.08
|
135
|
108
|
108
|
23
|
|
|
|
|
|
|
|
|
0
|
0
|
|
|
|
|
|
Lit.
|
3
|
12
|
30
|
40
|
40
|
10
|
135
|
108
|
|
|
|
|
Graphical representation of utilization
of water and generating the waste is shown below. It can be seen that the peak
occurs at 7 to 8 am. Main utilization is bathing and ablution. Use of flush is
expected with 135 LPCD water supplies. Compared to the average flow s per waste
generation of 80% of 135 LPCD flow = 108 LPCD, is 4.5 lit/hr. Maximum hourly
waste generation seems to 25.84 lit/hr. Hence, the worked out Peak factor is
5.74.
With the same data another graph is
generated to show the cumulative waste generated per hour to cumulative average
waste generated. It indicates that the once the peak is reached the waste
generated quantity is rated above average flow.
Approaching the velocity of 0.6 m/sec. with the present setting
how much population is minimum required is tried to be predicted. As per Table
it works out to 800. However, the peak factor is 5.75 instead of 3 as given in
the manual.
|
|
|
|
Population wise rated utilization
|
Velocity in 150 mm dia.sewer with slope 1 in 170, and designed
for half full
|
||||||
|
135 LPCD
|
135
|
|||||||||
|
Hours
|
Lit/hr
|
Waste/hr
|
30
|
60
|
90
|
200
|
400
|
600
|
800
|
in m/sec
|
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0.00
|
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0.00
|
|
2
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0.00
|
|
3
|
4.15
|
3.32
|
99.6
|
199.2
|
298.8
|
664
|
1328
|
1992
|
2656
|
0.08
|
|
4
|
4.15
|
3.32
|
99.6
|
199.2
|
298.8
|
664
|
1328
|
1992
|
2656
|
0.08
|
|
5
|
5.3
|
4.24
|
127.2
|
254.4
|
381.6
|
848
|
1696
|
2544
|
3392
|
0.11
|
|
6
|
11.7
|
9.36
|
280.8
|
561.6
|
842.4
|
1872
|
3744
|
5616
|
7488
|
0.24
|
|
7
|
32.3
|
25.84
|
775.2
|
1550.4
|
2325.6
|
5168
|
10336
|
15504
|
20672
|
0.65
|
|
8
|
19.2
|
15.36
|
460.8
|
921.6
|
1382.4
|
3072
|
6144
|
9216
|
12288
|
0.39
|
|
9
|
3.1
|
2.48
|
74.4
|
148.8
|
223.2
|
496
|
992
|
1488
|
1984
|
0.06
|
|
10
|
2.25
|
1.8
|
54
|
108
|
162
|
360
|
720
|
1080
|
1440
|
0.05
|
|
11
|
5.25
|
4.2
|
126
|
252
|
378
|
840
|
1680
|
2520
|
3360
|
0.11
|
|
12
|
4.3
|
3.44
|
103.2
|
206.4
|
309.6
|
688
|
1376
|
2064
|
2752
|
0.09
|
|
13
|
6.2
|
4.96
|
148.8
|
297.6
|
446.4
|
992
|
1984
|
2976
|
3968
|
0.12
|
|
14
|
0.2
|
0.16
|
4.8
|
9.6
|
14.4
|
32
|
64
|
96
|
128
|
0.00
|
|
15
|
0.4
|
0.32
|
9.6
|
19.2
|
28.8
|
64
|
128
|
192
|
256
|
0.01
|
|
16
|
5.4
|
4.32
|
129.6
|
259.2
|
388.8
|
864
|
1728
|
2592
|
3456
|
0.11
|
|
17
|
10.4
|
8.32
|
249.6
|
499.2
|
748.8
|
1664
|
3328
|
4992
|
6656
|
0.21
|
|
18
|
3.4
|
2.72
|
81.6
|
163.2
|
244.8
|
544
|
1088
|
1632
|
2176
|
0.07
|
|
19
|
9.8
|
7.84
|
235.2
|
470.4
|
705.6
|
1568
|
3136
|
4704
|
6272
|
0.20
|
|
20
|
7.1
|
5.68
|
170.4
|
340.8
|
511.2
|
1136
|
2272
|
3408
|
4544
|
0.14
|
|
21
|
0.2
|
0.16
|
4.8
|
9.6
|
14.4
|
32
|
64
|
96
|
128
|
0.00
|
|
22
|
0.1
|
0.08
|
2.4
|
4.8
|
7.2
|
16
|
32
|
48
|
64
|
0.00
|
|
23
|
0.1
|
0.08
|
2.4
|
4.8
|
7.2
|
16
|
32
|
48
|
64
|
0.00
|
|
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0.00
|
|
Lit.
|
135
|
108
|
3240
|
6480
|
9720
|
21600
|
43200
|
64800
|
86400
|
|
If as per manual Peak factor is
considered as 3 it does not give the cumulative population as shown above. As
per manual area serving the desired population cumulating to particular manhole
generating maximum velocity less than 0.6 m/sec., the pipe lines in this area
cannot be termed as sewers. These are non-conventional sewers. By definition
these non-conventional sewers shall not receive the sewage but it shall receive
only the liquid drain.
Actually peak flows per hour flattens
the velocity rating in that hour. Bath and toilet (in particular) waste has
much higher rate at which it can flow in the sewer. Concerned peak factors are
quite high. Manual has not referred to this possibility. It may be since the
each house draining may not have the flushing system fitted to their toilets.
So also Manual has also questioned about the quantity of waste water reaching
to the sewers. Bathroom and kitchen waste may run to the kitchen garden.
Toilets wastes have no option than to run through septic tank. Velocities are
dampened substantially. However, the quantity shall be same flowing with lesser
velocity. It may dampen the peak factors.
Peak factor will depend on the size of
city and other amenities available in that city. Density of population also
matters. Higher density will allow better peak factors. Development pattern of
the city also matters as the living standards are prominent features to run
more waste to drains/sewers.
Sewerage system is designed for the 30
years ahead. Apart from the increase in population development and living
standards are also upgraded over a period. Other developments are functional in
this period. Roads, education, Land value, area development, educational
facilities, markets, will also develop in their own ways, simultaneously with
the financial and infrastructural inputs. In time to come the waste generations
are expected to increase and more and more conventional sewers will be added to
the system.
Rationalization of peak factors is
thought of by adopting flows corresponding to prospective population of 5000 to
reach to the influenced manhole, with increased value of ‘4’
instead of ‘3’, to account for the moderation of concern of
development pattern, is proposed to be adopted. This population, however, can
change according to the size of city, present development and present living
standard.
As far as the living standard is
concerned, it is related to the per capita income. These figures are available
for each city. It can be understood from these figures that the income level at
Pune and remote cities in the State from Metros are having a substantial
difference. In case of Pune the per capita income is more than Rs.100000/-
while that at Chandrapur is hardly Rs. 16000/-. Both are having Average LPCD
levels above 135 LPCD. But quantity flowing to sewers may have a substantial
difference.
Conclusion
Design of sewerage system with the use of softwares will
give the quick results, but the scenarios which can be generated in waters
supply distribution system, cannot be generated. Sewerage system is not
proposed to be laid in advance before the population is settled in place.
Hence, only well settled population is to be provided with
the amenity. System is designed for the ultimate stage and the main sewers
satisfy the manual criteria. In other areas sewers are proposed but they are
non-conventional, (small bore, drains, twin pipes) and they need to be upgraded
time to time. Non-conventional sewers have less velocity than self-cleansing
velocity. Hence, septic tanks are necessary to remove solids. Peak factor in
the sewerage system is dependent on the contributory population to the sewer.
It shall be about 5000 population. In many cases other conditions, terrain,
hours of water supply prevail. In such cases it is better to apply modified PF
of more than 3 up to the desired contributory population is reached.
