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Course 2 Unit 8: Alternative Sewer systems

Lecturer: Dr. Eddy Akinyemie.akinyemi@unesco-ihe.org

This presentation was first put together by Eddy Akinyemi, and then edited and altered by Elisabeth von Münch

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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.

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This unit deals with which part of the sanitation system?

Part E

House-hold toilet

Part A Part B Part C Treatment & storage

Part D

Re-use in Agriculture

Collection & transport

Transport

Household toilets, but can also include showers, bath tubs, sinks

Urine, faeces, greywater transport (road-based vehicles in combination with pipes)

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

Crop grown with ecosan products as fertiliser (closing the loop)

Alternative sewer systems are an important option to transport greywater or blackwater (in the case of vacuum sewers)

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Introduction

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!

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CATEGORIES OF SEWER SYSTEMS

1. Conventional gravity sewerage2. Simplified sewerage 3. Small-bore gravity sewerage 4. Vacuum sewerage 5. Pressure sewerage: Septic tank effluent pumping

(STEP)

These systems are explained in the following slides.

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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 low-cost)

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

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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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1. CONVENTIONAL GRAVITY SEWERAGE

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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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Layout of a typical conventional Gravity Sewer

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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”

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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) (bertvanduijl@planet.nl)

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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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Conventional Gravity Sewer- Disadvantages

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 Sewer- Assessment

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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2. Simplified sewerage

(Also called: Condominium sewerage, community-based 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

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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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Simplified Sewerage – design details (1)

Receives wastewater directly from each site (no pre-settling)

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

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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3. SMALL-BORE GRAVITY SEWERAGE

Note: Once again, there are many different names in use for the same thing:

Small-bore gravity sewerSmall-diameter gravity sewerSettled sewerageSolids-free sewerage

All refer to the same thing!

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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

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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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Small-bore gravity sewerageCourse 2 Unit 8

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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.

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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

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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

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Small-bore gravity sewerage: Design details (4)

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

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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.

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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.

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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

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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.

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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

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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.

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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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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

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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.

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Vacuum Sewerage - What is it?

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Vacuum Sewerage - How does it workCourse 2 Unit 8

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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.

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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.

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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.

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Vacuum sewers - Applications

Compared to conventional methods of sewerage, vacuum technology can provide major advantages in thefollowing 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

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Vacuum Sewer - Summary (slide 1 of 2)

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Vacuum Sewer – Summary (slide 2 of 2)

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5. PRESSURE SEWERAGE: Septic Tank Effluent Pump (STEP) System

Note: Not a suitable system if very low costs are important

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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.

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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

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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.

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Basic features of STEPCourse 2 Unit 8

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Septic Tank Effluent Pumping (STEP)- Advantages and disadvantages

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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/MProdIndex.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)