3. Wastewater and Stormwater Collection
A sewerage system collects wastewater and can be in the form of blackwater
separated from greywater, or mixed with it (sewage). Gravity is used wherever
possible to convey the wastewater. It is not surprising therefore that natural
stormwater drainage is usually used, because this is how rainwater run-off is
conveyed in nature by gravity. The principle of using gravity as the driving
force for conveying wastewater in a sewerage system should be applied wherever
possible, because this will minimise the cost of pumping. Natural stormwater
drainage occurs in what is usually termed a catchment basin. In a catchment
basin, rainwater run-off flows to a common point of discharge, and in so doing,
forms streams and rivers. Crossing a catchment boundary may mean that the water
has to be unnecessarily pumped, requiring an energy source. A wastewater
sewerage system should therefore be within a stormwater catchment basin.
Sewerage systems can be classified into combined sewerage and separate
sewerage. Combined sewerage carries both stormwater and wastewater, while
separate sewerage carries stormwater or wastewater separately. Recent trends
have been for the development of separate sewerage systems. The main reason for
this is that stormwater is generally less polluted than wastewater, and that
treatment of combined wastewater and stormwater is difficult during heavy
rainfalls, resulting in untreated overflows (commonly termed combined sewer
overflow, CSO). In practice there is usually ingress of stormwater into
wastewater sewerage pipes, because of unsealed pipe joints, and unintentional or
illegal connections of rainwater run-off. Conversely there may be unintentional
or illegal wastewater connections to stormwater sewerage.
Wastewater sewerage systems can be classified into three major types:
- Conventional sewerage
- Simplified sewerage
- Settled sewerage
3.1 Conventional sewerage
Conventional sewerage is also termed deep sewerage because the sewerage pipes
are laid deep beneath the ground. Pumping is generally required at various
stages of the sewer pipe network, especially if the landscape is fairly flat.
The larger the population served by the sewerage system, and the longer the
planning horizon is to cope with future population increases, the larger the
diameter of the final pipes becomes. The costs of the pipes, inspection
manholes, pumps and pumping stations and their construction/installation are
therefore high. The costs of operation and maintenance are correspondingly high
because of very conservative design assumptions.
3.2 Simplified sewerage
Simplified sewerage is also known as shallow sewerage, reflecting the
shallower placement of the pipes in contrast to the conventional or deep
sewerage. The purpose of simplified sewerage is to reduce the cost of
construction and the corresponding cost of operation and maintenance. Simplified
sewerage design is based on hydraulic theory in the same manner as for
conventional sewerage but has less conservative design assumptions. Smaller
diameter pipes are used when water use per person is known to be less and the
minimum depth of cover of pipes can be as low as 0.2 m when there is only light
traffic. Manholes can be replaced by inspection cleanouts because of the shallow
pipes. The design planning horizon can be shorter because the population
projection may be uncertain. In a variation of the simplified sewerage, the pipe
layout passes through property lots (condominial) rather than on both sides of a
street (conventional). Figures 4 and 5 show the sewerage layouts in conventional
sewerage and in condominial sewerage. The cost of construction of simplified
sewerage can be 30 to 50 % less than conventional sewerage depending on local
Shallow sewerage is also conducive to local community participation because
sewer pipes have to cross property boundaries. The community has to agree to
this arrangement which extends after construction for maintenance (e.g.
unblocking of sewer pipes). The shallow pipe, and hence the shallow trenches,
also allow members of the community to participate by, for example, providing
labour for digging the trenches. This is in contrast to conventional sewerage
where specialised machinery is required for the deep trenches.
Simplified sewerage was originally developed in Brazil and is increasingly
being used in other parts of the world. The ‘International Source Book on
Environmentally Sound Technologies for Wastewater and Stormwater Management’
(hereafter referred to as the Source Book), published by IWA and IETC, provides
useful case studies.
3.3 Settled sewerage
Settled sewerage refers to sewerage for conveying wastewater that has been
settled, for example, in a septic tank. Settled sewerage originated to convey
the overflow from septic tanks where the soil cannot cope or absorb the
overflow. This usually occurs when the groundwater table is high, or where the
soil permeability is low, or where there are rock outcrops. It can also be used
when effluent from septic tanks pollutes groundwater and it is necessary to
convey the effluent off-site and treat it. Because there are no solids that can
potentially sediment in the sewerage pipes, there is no requirement for the
self-cleansing velocity. Smaller pipes and lower gradients can be used. The cost
of settled sewerage is between a third and a half of conventional sewerage.
Originally developed in South Australia to overcome problems with failing septic
tanks, it has been used quite widely worldwide to upgrade septic tank systems.
Where there is no existing septic tank, an interceptor box or tank can be
used. It functions like a septic tank and designed in the same way (Figure 6).
To reduce cost, the wastewater from a group of houses can be connected to one
interceptor tank. Just like in a septic tank, the accumulation of sludge has to
be removed regularly from an interceptor tank.
3.4 Stormwater collection
Stormwater flows through the landscape’s natural drainage system. Piped
stormwater collection was a development in European cities to overcome odour and
improve aesthetic appearance of wastewater disposed with stormwater. The
covering of ditches used for combined sewerage was an intermediate step in using
natural drainage to construct sewerage for combined wastewater and stormwater.
Piped sewerage also allows more land area for road and footpaths. With the
separate collection of wastewater there is an opportunity to return some
stormwater flow path to its more natural state to improve urban amenity value.