Centralised –Decentralised
transportation system
M.K. PANDEY/P. Jenssen
2111
2005
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Centralised system
 Collection system 70 - 90 %
 Treatment
10 - 30 %
(Otis 1996, Mork et al. 2000)
Sewer lines
 The cost of conventional gravity
system is up to 4 times higher than the
cost of treatment and disposal
Wastewater
Wastewater
treatment
plant plant
treatment
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
House hold human waste and
wastewater
Urine
{
＋
}
Excreta
Feaces
＋
Flush
＋
Anal cleansing
Wastewater
Black water ＋ Greywater
Or Sewage
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
House hold human waste and
wastewater
Important constituents
•Organic matter
•Neutrients- Nitrogen,
Phosphorus, Potassium
•Pathognes
{
Urine
＋
}
Excreta
Faeces
＋
Flush
＋
Anal cleansing
Wastewater
Black water ＋ Greywater
Or Sewage
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Wastewater transportation
 Wastewater transported to treatment plant as
quickly as possible
 Self cleansing velocity should be maintained at
low flow
 Velocity should not be higher than the maximum
allowable velocity – to prevent wear and tear of
the pipes
 Formation of Hydrogen sulphide, airlock should be
prevented
 Should not be close to W/S lines
 Proper selection of type and shape of sewer
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Conventional gravity sewer
•Pollution due to combined sewer
overflow
•Large dia sewer
•Interference to other infrastructure
•Contamination of water distribution
system
•High chances of system failure
WW Treatment
Plant
G.L
Over flow
structure
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Pumping system
River
Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Types of conventional sewerage system
Combined sewer
•
•
•
•
•
•
•
Storm and sanitary sewage (wastewater) collected in one sewer
Suitable at places where rainfall is evenly distributed throughout
the year
Overflow structure required to divert the flow more than the
design flow
Large dia sewer required
Large volume of wastewater to be treated
Plumbing work reduced in houses
Separate sewer – Storm sewage and sanitary sewage conveyed in
separate sewer
Chances of clogging
Prone to formation of H2S
Partially separate system
Rainwater from houses and yards discharged into sanitatry sewers
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Investment Cost
 Collection system 70 - 90 %
 Treatment
10 - 30 %
(Otis 1996, Mork et al. 2000)
Sewer lines
 The cost of conventional gravity
system is up to 4 times higher than the
cost of treatment and disposal
Wastewater
treatment plant
Wastewater
treatment plant
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system
Collection in a septic tank
and transport the effluent
wastewater to nearby
treatment system
•Soak pit
•Constructed wetland
•Infiltration system
•Pond system
•Sand filter
Septic
tank (S.T)
Natural Treatment
Compost or
transport to faecal
sludge treatment
facilities.
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system
Low flush
or pour
flush
Collection and treatment of
blackwater and Greywater
separately
•Soak pit
•Constructed wetland
•Infiltration system
•Pond system
•Sand filter
Septic
tank (S.T)
Compost or
Transport to faecal
sludge treatment
facilities
Settling
tank and
greese
tap
Natural
Treatment
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Department of Plant and Environmental Sciences
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system
Low flush
or pour
flush
Urine
Faeces
Collection and treatment of
urine, faeces and greywater
separately
Settling
tank and
greese
tap
Natural
Treatment
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system- STEG
Septic tank effluent gravity (STEG)
50 mm
50 mm to
200 mm
100 mm
Can be laid in variable grade - because no solid to settle
Uniform slope with no high points to prevent airlock
H2S formation
Air release valve in high points
Clean out ports at junction
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system- STEP
Septic tank effluent pump (STEP) and pressure sewer with
grinder pumps
Sewer are under pressure – pressure generated by high head
turbine pump
Advantage in high groundwater and rocky soil and rolling terrain
- can follow the terrain
If grinder pumps used- septic tank not required
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Decentralized system- Vaccum sewer
Vacuum sewer
Vacuum applied to transport sewage
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Hydraulics of wastewater collection system
Velocity and headloss are two governing parameter
Hazen williams equation (1)
V  0.849  C  R 0.63  S 0.54
Where
V= Velocity
of flow, m/s
C = Hazen –williams coefficient, 150 may be used PVC pipe
R = Hydraulic Radius, (wetted area/wetted perimeter), m
 (e.g for pipe flowing full
R  D/ 4
D = inside dia of sewer, m
S = Slope of energy gradeline, m/m,
hf = head loss due to friction, m
L = Length of pipeline
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S
hf
L
NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Hydraulics of wastewater collection
system
Manning`s equation (2)
V 
R
Where
2
3
 S 0.5
n
V= Velocity of flow, m/s
n = Manning’s coefficient, 0,013 to 0.009 may be used for PVC pipe
R = Hydraulic Radius,
R  D/ 4
(flowing full)
D = inside dia of sewer, m
S = Slope of energy grade line, m/m,
hf = head loss due to friction, m
S
L = Length of pipeline
hf
L
Sewer line (gravity sewers) are designed as a open channel or
flowing just full
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Hydraulics of wastewater collection
system
 The velocity should be less than 1.5 m/s to avoid excessive
frictional loss.
 No minimum velocity required for STEG system – (but usually
kept at 1m/s)
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Information's required for design and
layout of STEG collection system
Site characteristics
–Topography of the area
–Depth of soil
–Depth of water table
–Depth of freezing zone
Equivalent dwelling unit (EDU)
–Residence with given number of residents e.g if 1 EDU is
defined as residence with 4 person then 8 person residence is
2EDU
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Information's required for design and
layout of collection system
Peak flow rates
–collection system based on peak flow rates
–1.3 to 1.9 lit/min/EDU (USA)
–0.8 to 1.2 lit/min/EDU (Norway)
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
STEPS for the design of Sewer
collection system
Prepare a longitudinal profile
Select a pipe size
Calculate the velocity using Hazen Williams equations
Calculate the pipe cross sectional area and determine the actual
capacity
Check for the surcharged condition
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NORWEGIAN UNIVERSITY OF LIFE SCIENCES
THANK YOU
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