Transcript SWAT - UNESCO-IHE

Course 2 Unit 8:
Alternative Sewer systems
Lecturer: Dr. Eddy Akinyemi
[email protected]
This presentation was first put together by Eddy Akinyemi, and then
edited and altered by Elisabeth von Münch
1


Dr Akinyemi has over 25 years experience in scientific
research, education and consulting internationally in
civil engineering in general and drainage and sewerage
and transport engineering in particular. Dr Akinyemi
worked as a university academic for 14 years in
Canada, United States, Netherlands and Nigeria. His
last position as a university academic was as a
Professor of Civil Engineering in 1991.
In the past 12 years, he worked at the UNESCO-IHE
Institute for Water Education, Delft, The Netherlands as
a trainer, researcher and consultant in the provision of
sound engineering solutions to drainage, sewerage,
mobility and environmental problems in African, Asian,
Latin American and Eastern European cities. Key
research topics include integrated infrastructure
design, road drainage and operationalisation of the
concept of sustainable development and environmental
capacity.
2
This unit deals with which part of the
sanitation system?
Crop grown with ecosan products as fertiliser (closing the loop)
Part A
Household
toilet
Household
toilets, but can
also include
showers, bath
tubs, sinks
Part B
Part C
Part D
Treatment
& storage
Collection &
transport
Urine, faeces,
greywater
transport
(road-based
vehicles in
combination
with pipes)
Part E
Re-use in
Agriculture
Transport
Treatment
for faeces
and
greywater,
storage for
urine
Transport of
sanitised urine and
faeces by truck;
treated greywater
transport by pipes
Sale of fertiliser
(sanitised human
excreta); irrigation
with treated
greywater
 Alternative sewer systems are an important option to transport
greywater or blackwater (in the case of vacuum sewers)
3
Introduction


Course 2 Unit 8
Wastewater can be transported in gravity, vacuum, or
pressure-sewer systems that carry wastewater from
homes and other buildings to a wastewater treatment
facility
Note: this unit only deals with transport of wastewater
by pipes. At the end of the pipe, you still need a
treatment step!
4
CATEGORIES OF SEWER SYSTEMS
1.
2.
3.
4.
5.
Conventional gravity sewerage
Simplified sewerage
Small-bore gravity sewerage
Vacuum sewerage
Pressure sewerage: Septic tank effluent pumping
(STEP)
These systems are explained in the following slides.
5
Overview of sewer systems and their possible ecosan applications
Type of sewer
system
Application in conventional
sanitation
Application for ecosan
1. Conventional
gravity
sewerage
Common solution world-wide for
those who can afford it
Rarely used in ecosan
applications (very high cost;
complex infrastructure)
2. Simplified
sewerage
Used for mixed domestic
wastewater, e.g. in Brazil
Can be used for ecosan
projects but difficult to
enable safe reuse
3. Small-bore
gravity
sewerage
Has been used as a low-cost
solution to transport domestic
wastewater (after septic tanks)
Suitable system to transport
greywater after pre-settling
to remove solids
4. Vacuum
sewerage
Used for specialised applications,
e.g. on cruise ships
Suitable system for
blackwater obtained from
vacuum toilets (but not lowcost)
5. Pressure
sewerage
(STEP)
Used to upgrade on-site systems
where septic tanks are used
(industrialised countries, e.g.
USA, Australia)
Not commonly used; no
major advantages over
small-bore gravity sewers
but more expensive
6
What are the key differences for the
designs of the different sewer systems


Characteristics of the wastewater conveyed (settled or
non-settled)
Criteria and standards for the design of the sewers,
e.g.:




Minimum velocity in pipe
Minimum slopes of pipes
Minimum pipe diameter
Design peak flow factor
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Course 2 Unit 8
1. CONVENTIONAL GRAVITY
SEWERAGE
8
Conventional Gravity Sewerage:
Basics

Sewer systems are constructed in three different
"tiers" of pipe size and function:



The smallest pipes are known as "collectors." Wastewater
from homes, businesses, schools etc. enters the system
via these collectors.
From the collector pipes, wastewater flows into a system
of larger pipes called "trunk lines"
The trunk lines connect to a massive system of pipes and
pump stations that carry wastewater directly to the
treatment plant.
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Conventional Gravity Sewer: Basics

A city's (conventional) wastewater sewer system
basically consists of a network of sewers (hydraulic
conveyance structures), manholes, service
connections and pump stations to collect and convey
wastewater to a treatment plant or other authorized
point of discharge.
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11
Layout of a typical conventional Gravity Sewer
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Course 2 Unit 8
Conventional Gravity Sewer – Design
details (1)


It requires strict alignments in both the horizontal and
vertical plane. Manholes are installed whenever
direction and grade change, and to allow for cleaning.
The minimum recommended slope is 2‰ (2 per
thousand).
Because sewers must carry both liquid and solids,
maintenance costs of sewer systems often exceed the
cost of wastewater treatment. Sewers generate
odours and corrosive gases which form acids that
slowly dissolve the piping and manholes. Eventually
the sewers collapse creating the necessity for
expensive repairs and/or replacement.
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Conventional Gravity Sewer- Design
details (2)


The manholes are a source of inflow and infiltration,
increasing the volume of wastewater to be carried, as
well as the size of pipes and lift/pumping stations, and,
ultimately, increasing costs.
Duncan Mara: “Design codes stipulate minimum pipe
diameters, gradients and depths that may be
unnecessarily conservative and expensive”
14
Conventional Gravity Sewer- Design
details (Netherlands)




A minimum diameter in the Netherlands is 25 cm, minimum
slope of 4‰ is considered.
We need cleaning once every 6 or 7 years. Our sewers are
not self-cleansing, result: sedimentation, sewer gases
(hydrogen sulfide). Concrete sewers have a reduced life.
Other countries design for self-cleansing: a minimum slope of
4 ‰ for minimum diameter sewer. Ventilation of sewers is
possible. You will see in Germany, Austria etc the ventilation
holes in manhole covers.
For larger diameter sewers the minimum slope is inversely
proportional to diameter
Source: Bert van Duijl (retired lecturer at UNESCO-IHE)
([email protected])
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Conventional Gravity Sewer Advantages

Properly designed and constructed conventional
gravity sewers provide the following advantages:


Can handle grit and solids in sanitary sewage.
Can maintain a minimum velocity (at design flow),
reducing the production of hydrogen sulfide and methane.
This in turn reduces odors, blockages, pipe corrosion, and
the potential for explosion
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Course 2 Unit 8
Conventional Gravity SewerDisadvantages


The slope requirements to maintain gravity flow can
require deep excavations in hilly or flat terrain, driving
up construction costs.
Sewage pumping or lift stations may be necessary as a
result of the slope requirements for conventional
gravity sewers, which result in a system terminus (i.e.,
low spot) at the tail of the sewer, where sewage
collects and must be pumped or lifted to a collection
system.

Pumping and lift stations substantially increase the cost
of the collection system.
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Conventional Gravity SewerAssessment

Requires a reliable multiple-tap in-house water supply



requires water supply and consumption of at least approx
100 L/cap/d for problem-free operations
Not an option for low-income urban communities
Unaffordable and inappropriate for low-income
communities

Thus, unsustainable in environmental, social and financial
terms
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Course 2 Unit 8
2. Simplified sewerage
(Also called: Condominium sewerage, communitybased sewerage)
Some information was taken from this paper:
Paterson, C. , Duncan M. and Curtis, T. (2007) Pro-poor sanitation
technologies, Geoforum 38, p. 901-907 (also provided under Extra
Materials
19
The “guru” on simplified sewerage:
Duncan Mara


Duncan Mara is a professor in Civil Engineering at the
University of Leeds, UK and the guru on simplified sewerage
– since many decades!
He keeps a website where he makes available many
presentations, reports, publications on this topic:
http://www.personal.leeds.ac.uk/~cen6ddm/


Duncan Mara is known to be an “ecosan sceptic” – you can
also read about that on his website
I think the principles of simplified sewerage (and small-bore
sewerage) could well be applied to greywater transport, and
therefore I see it as one of the technology options within the
ecosan concept
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Simplified Sewerage – what is it?



Consists of shallowly-buried plastic pipes, low-cost
cleanouts instead of frequent/costly manholes, and a
minimum number (if any) of lift stations
Management requirements are equal or lower than
conventional gravity sewers (depends on number of lift
stations)
This type of sewerage is widely known in Latin
America but little known in Africa and Asia

Most widely used in Brazil (field tested in early 1980s)
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Simplified sewerage: overview




Arno Rosemarin on Ecosanres Discussion Forum 8 Nov.
2007:
“The work of our good friend Duncan Mara at Leeds
Univ may be of interest at least when it comes to what
piping methods can be used in dense slums. He refers
to decentralised simplified or condominium sewerage.
Sometimes it is called "slum networking".
In any case a ca 100 mm-diameter piping is laid down
underground often beside the common open trench or
drain one always sees in these communities.
Brazil is the country that practises this the most to
move black and greywater out of slums. Drainage of
stormwater is just as important and needs to be dealt
separately as well. and then there is solid waste....”
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Course 2 Unit 8
Simplified Sewerage – design details (1)





Receives wastewater directly from each site (no presettling)
Designed less conservatively than for conventional
sewerage systems to reduce costs, but still
accounting for transport of grit and solids
Conveys the wastewater to a centralized treatment
plant as rapidly as possible
Collects all household wastewaters in small-diameter
pipes laid at fairly flat rates
Sewers are often laid inside the housing block, or in
the front garden or under the pavement (sidewalk),
rather than in the centre of the road as with
conventional sewerage
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Simplified Sewerage – design details (2)



Conservative design codes (of conventional sewerage)
are relaxed in order to reduce the pipe diameters,
gradients and depths, while still maintaining sound
physical design parameters
In high-densities areas, the resulting sewage flows are
high even if household water consumption is relatively
low
Even in the highest part of the network where the flow
is intermittent, solids are gradually moved along the
pipes each time there is a flush of flow
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Simplified Sewerage – design details (3)



Vitrified clay or PVC pipes can be used, with simple
joints and minimal leakage
Simple pipe junctions and cleanout and inspection
units are used in place of manholes
Pipes are often laid inside a housing block, in the front
garden, or under the pavement, rather than in the
centre of the roads as with conventional sewerage

Cost savings in excavation, backfill materials and pipe
quality
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Simplified Sewerage: Alternative options for routing
branches
Location preferable in backyard (house connection short) and a clean-out for each
house. Each family is equipped with rodding sticks.
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Simplified sewerage: the need for
community consultation



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Community participation is essential at all stages
Households are responsible for unblocking length of
sewer laid in their own plots
Negotiations may be lengthy, but neglecting the
opinions of the community has proved a false economy
The system can be introduced gradually to a
community, block by block
Issue of cost recovery:

Urban poor are willing to pay for services including
sanitation, provided the services are worth paying for
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Simplified sewage: advantages and
disadvantages

The advantages and disadvantages are very similar to
those listed for small-bore sewerage (see Part 3
below)
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Course 2 Unit 8
3. SMALL-BORE GRAVITY
SEWERAGE
Note: Once again, there are many different names in use for the same thing:
Small-bore gravity sewer
Small-diameter gravity sewer
Settled sewerage
Solids-free sewerage
All refer to the same thing!
29
Small-bore gravity sewerage: What is it?



It uses gravity (preceded by a septic tank) to transport
sewage. The settling that first occurs in the septic
tank eliminates much of the solid matter from the
wastewater.
Requires less hydraulic gradient and velocity to
transport the wastewater through the lines than is
necessary with conventional sewer.
 It has many similarities to the simplified sewerage
with the main difference being that the wastewater is
first settled in a septic tank at household level
30
Small-bore gravity sewer: Design details

It consists of house connections, an interceptor tank
(septic tank), sewers, cleanouts/ manholes, vents, a
sewage treatment plant




The system may include some pump stations (where
gravity flow is not possible) – but not as part of the
household equipment
No pumps are installed in the house connections.
It is designed to receive only the liquid fraction of
household wastewater. The solid component of the
wastewater is kept in an interceptor tank (septic
tank).
[Some people use this definition: “interceptors” have a
solid retention of 3 months and septic tanks of 6
months to 2 years]
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Course 2 Unit 8
Small-bore gravity sewerage
32
Small-bore gravity sewerage:
Design details (1)



The pipeline follows the profile of the ground, buried in
a shallow trench.
The collection force main, buried about 0.75 m deep,
can gradually increase in size, from 50 mm at the
beginning to 150 mm at its downstream end. Mainline
is generally buried approximately 1.2 m in depth
following the contours of the land/street
Pipelines should be kept at depths which provide at
least the following:



300 mm cover on plots
1 m cover on public land such as in road reserves
1.2 m cover when crossing roads.
33
Small-bore gravity sewerage:
Design details (2)



Pumping stations may be needed in sewer mains to
reduce excavation. Flow velocity is maintained at 0.6
m/s to keep solids from settling and to minimize
hydraulic losses.
To reduce the chances of a blockage in the main
sewer, at least the first two meters of the connecting
pipe from the interceptor tank to the plot boundary
should have a diameter slightly smaller (50mm
diameter) than the sewer main.
Mains should consist of plastic pipe with a minimum
diameter of 100 mm as this is economical, smooth and
resistant to corrosion
34
Small-bore gravity sewerage:
Design details (3)


Provide pipes with watertight joints.
It requires separate design calculations or analysis for
each sewer section in which:



the type of flow does not vary, and
the slope of the pipeline is reasonably uniform
Can use Manning equation with “n” equal to 0.013
35
Small-bore gravity sewerage:
Design details (4)



Course 2 Unit 8
Hydraulic design should be checked to ensure that it can be
flushed between successive cleanouts at 0.5 m/s without
backing up into adjacent septic tanks.
Small bore sewers with interceptors are applied in Australia
for example. In Australia the velocity is 0.45 m/s when the
pipe is running full or half-full, but at pumping stations higher
velocities my be used, determined by economical
considerations.
Clean-outs should be located as follows:




at the upstream ends of the system
at the intersection of sewer lines
major changes of direction
at high points, and at intervals of 150 to 200 m in long flat
sections
36
Small-bore gravity sewerage:
Design details (4)

It is not necessarily laid to uniform grades and may
have low points or dips which may remain full under
static conditions.





The horizontal alignment can also curve to avoid objects
It is designed on hydraulic considerations only as it is
not intended to carry solids
It should be low enough to receive flows from the
majority of service connections by gravity.
It can be installed with sections below the hydraulic
grade line, thus flow may alternate between open
channel and pressure flow.
In principle, it is a gravity flow system. There is no
sludge in the system so the pipe may go up and down
and certain pipe sections may function as pressure
pipes.
37
Small-bore gravity sewerage:
Design details (5)


System mains are usually run down the side of streets
rather than down the middle and below the pavement
like those of most conventional sewers. Collector
mains may be located on both sides of the street to
minimize pavement crossings, or, less commonly, they
may run along back lots to be closer to preexisting
septic tanks.
Mains are simple to install. The mains can be laid at
varying grades and can be easily routed around
obstacles discovered during construction, such as
large boulders.
38
Small-bore gravity sewer:
Operational requirements




Ensure that no solids enter the system, and that the
interceptor tank functions properly
Regularly remove sludge in the interceptor tank and
blockages in the sewage pipes
Regularly flush the system
Check and maintain pipeline system components
39
Course 2 Unit 8
Small-bore gravity sewerage Features

It can be adapted to a variety of terrains




Lines are laid at a relatively constant, shallow depth,
following the natural contour of the land.
The up and down flow patterns are possible as long as the
beginning of the system is higher overall than its final
destination—the outlet to the treatment facility.
When it is necessary for the flow to be directed upwards,
effluent pumps can be utilized to move the wastewater to
higher elevations.
It is most appropriate for areas that already have
septic tanks, but where the soil cannot (or can no
longer) absorb the effluent, or where the population is
too dense and there is no room for soakaways.
40
Small-bore gravity sewer:
Advantages (1)

Sewage flow rates do not have to be self-cleansing
rates (since solids do not need to be transported)



The system can be used by people using very little water
because the sewers do not need to be flushed
There are fewer treatment requirements of the
(settled) sewage, because the solids are kept in
interceptor tanks
Labour content in construction and hence the benefit
to the community is much larger than a gravity sewer
contract because the shallower trenches can be hand
dug
41
Small-bore gravity sewer:
Advantages (2)




It is well suited for communities where the houses are
far apart, or where most houses are served by an
existing septic tank
Manholes are not required; instead, clean out ports are
used to service collector pipes
The pipes used can be made of light weight plastic and
can be buried at a relatively shallow depth.
If a community has simple slopes all going in the same
direction, then this system may be the best option. But
if the treatment plant is uphill, or if the town has
undulating topography, then pressure sewers may be a
better option.
42
Small-bore gravity sewer:
Drawbacks

The system cannot tolerate gross solids so direct
connections to the system cannot be tolerated. It is
important to remember this fact so that connections
made in the future do not connect directly


Bert van Duijl: “In some countries small bore sewers with
interceptors or septic tanks were a complete failure. The
reason is that septic tanks and interceptors are
considered as house installations. House owners do not
maintain the tanks until there is a problem. Problems do
not occur at the tanks but in the small diameter public
sewers.”
Corrosion and odours are major problems because of
the septic nature of the effluent
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Course 2 Unit 8
4. VACUUM SEWERAGE
Technology leader for vacuum sewer systems:
Roediger Vacuum and Haustechnik - “World leading systems for
the collection of wastewater using vacuum” http://www.roevac.de/page/en
44
Vacuum sewerage - What is it?



Wastewater from one or more homes flows by gravity
to a holding tank known as the valve pit. When the
wastewater level reaches a certain level, sensors
within the holding tank open a vacuum valve that
allows the contents of the tank to be sucked into the
network of collection piping
There are no manholes; instead, access can be
obtained at each valve pit.
The vacuum or draw within the system is created at a
vacuum station. Vacuum stations are small buildings
that house a large storage tank and a system of
vacuum pumps.
45
Vacuum Sewerage - What is it?
46
47
Vacuum Sewerage - How does it work
Course 2 Unit 8
48
Vacuum sewerage – Design details (1)


Wastewater from the houses enters a vacuum pot
under gravity. At a pre-determined level the valve in
the pot opens and the wastewater is “sucked” into the
pipeline system. A volume of air is "sucked" into the
pipeline system with the slug of wastewater.
The wastewater slug soon disintegrates and flows to a
low point in the sewerage system, where it reforms.
Subsequent flows of air push the wastewater slug
through the system to the vacuum/pumping station.
49
Vacuum sewerage – Design details (2)


It consists of a holding tank/sump/valve pit, typically
located along the property boundary receiving sanitary
sewage via conventional plumbing under gravity. When
a set volume of wastewater has accumulated in this
sump, a pressure sensor signals the vacuum valve to
open for a set period allowing the wastewater and air
(at atmospheric pressure) to enter the system (which
operated below atmospheric pressure).
Used to minimize the construction depth, trunk
sections of the pipe system are laid with a small
gradient, with lift stations.
50
Vacuum sewerage – Design details (3)

Slopes

Vacuum mains are slightly sloped towards the vacuum
station. Velocity in a vacuum main is independent of both
slope and pipe diameter, unlike gravity sewers that
require a minimum slope for a given pipe diameter to
obtain the 0.6 m/s scouring velocity. The pressure
differential results in velocities of 4.5 to 5.4 m/s.
51
Course 2 Unit 8
Vacuum sewers - Applications

Compared to conventional methods of sewerage, vacuum
technology can provide major advantages in the
following circumstances:










The topography is flat
Groundwater table is high
Sewer system is located near a lake, river, coastline or floodplain
Ground has an adverse gradient
Wastewater flows are highly variable e.g. holiday establishments
or local recreational facilities
Difficult ground conditions e.g. rock, running sands, peat, swamps
etc.
Refurbishment of sewer systems
Rural area where houses and buildings are not close to each
other
Crossing rivers, streams, railway lines, major road etc
Groundwater protection areas
Source: http://www.roevac.de/page/en/page_ID/45?PHPSESSID=676aca471db0af8e6190e9f9bda5bda1
52
Vacuum Sewer - Summary (slide 1 of 2)
53
Vacuum Sewer – Summary (slide 2 of 2)
54
Course 2 Unit 8
5. PRESSURE SEWERAGE: Septic Tank
Effluent Pump (STEP) System
Note: Not a suitable system if very low costs are important
55
Pressure Sewerage - What is it?





It is a “closed system.” i.e. the system is constructed
as a continuous line of pipe
It employs cleanouts instead of manholes as access
points for cleaning and monitoring the lines
It uses positive pressure to propel wastewater through
the lines
It employs small-diameter (37-200 mm), light-weight
polyvinyl chloride (PVC) or polyethylene (PE) pipes
Pressure is created from the wastewater being
pumped into the watertight lines at several
connections. In addition, wastewater is literally
pushed through the pressurized lines, eliminating the
need for gravity.
56
Pressure Sewerage: What is it?


The sizes of mains are calculated based on the peak
hydraulic flow rate and the hydraulic velocity needed
to transport the wastewater through the entire
system. The mains typically range from 100 mm to 400
mm in diameter.
Necessary pipe diameter is calculated by using the
Hazen-Williams formula, Manning’s equation, or the
Darcy-Weisbach equation
57
Pressure Sewerage - Advantages


It can be installed at shallow depths, to follow the
natural contour of the land and can be easily
reconfigured to accommodate unforeseen obstacles
and tight spaces.
The watertight piping and the pressure maintained in
the lines eliminates inflow and infiltration from
groundwater and stormwater into the system.
58
Basic features of STEP
Course 2 Unit 8
59
60
Septic Tank Effluent Pumping (STEP)
- Advantages and disadvantages
61
Relevant website for alternative sewer
systems (in particular simplified
sewerage)


Material from Professor Duncan Mara (Leeds university,
UK). A fountain of knowledge with design details for
these alternative sewer systems.
http://www.personal.leeds.ac.uk/%7Ecen6ddm/MProdInd
ex.html
http://www.personal.leeds.ac.uk/%7Ecen6ddm/simpsew.
html --> video clip of a condominium sewer system in
South Africa (without many technical details,
unfortunately)
62