Technology brief

Wastewater
treatment
Introduction
Wastewater treatment is essential to prevent pathogens fromentering the environment and causing disease. While traditional
wastewater treatment often addresses sewage as waste, there
are also opportunities for it to be viewed as a valuable resource.
For example, biogas created from sludge can provide cheap,
clean energy, and composted faeces can make a highly effective
fertiliser. This technology brief gives an overview of thesemethods to encourage a holistic view of wastewater treatment.
Treatment can be roughly
divided into centralised
and decentralised systems.
Centralised systems gather
wastewater from a large
population and treat it
simultaneously. Decentralised
systems serve smaller
populations, ranging in size
from a household to a small
community. This technology
brief focuses on decentralised

Collection
and storage

1 | Wastewater treatment

Removaland transportsystems, which have often beenfound to be more affordable
and appropriate for low-income
urban communities.
There is a chain of steps in thejourney of wastewater fromcollection to disposal or use,
and it is important to consider
them all, as different treatment
processes can occur at different
steps in the chain.

Treatment

Disposal or use

Collection and storage
This technology brief discusses thecollection and storage of wastewater.
For information on the collection andstorage of faeces and sludge (for example,
in pit latrines, dehydration vaults andcomposting toilets) please refer to thetechnology briefs Household latrines and
Urban pit waste management.

sludge and scum within the tank and let
the water pass out for further treatment.
The final products of the septic tank are
therefore water (requiring further treatment
before it becomes safe), sludge and scum.
The septic tank will need to be periodicallyde-sludged to remove the latter (about once
every four years).

Septic tank

Septic tanks are generally installed
underground, and are therefore good forareas with limited land available. They areused in conjunction with flush toilets andrequire a regular supply of water, and are
therefore not suitable for areas wherewater is in short supply.

A septic tank is a water-tight chamber forthe primary treatment of wastewater andgreywater. It can function as a holdingtank – in which case the contents must be
periodically removed – or drain into any
type of sewer system or treatment works.
In rural areas, septic tanks can also drain
into leach fields.
Septic tanks are generally made fromconcrete, fibreglass, PVC or plastic, andcontain at least two chambers. Within theseptic tank, solids settle to the bottom andscum rises to the top. Baffles keep the

Septic tanks do not generally smell, andare easy to use. However, when installinga septic tank, it is vital to think about
what should be done with the effluent.
Committing to proper maintenance is alsoessential, as an overflowing or leaking
septic tank in a densely populated area
can pose a serious health hazard.

Suction tanker forperiodic sludge removal

Water cistern
Valve

Latrine superstructure

Connecting drain

Access cover

Inlet baﬄe
U bendDrainage trench for eﬄuent disposal

Scum

EﬄuentFig 1: Septic tank

2 | Wastewater treatment

SludgeEﬄuentOutlet baﬄe

Anaerobic baffled reactor
An anaerobic baffled reactor (ABR) is
similar to a septic tank, except thatafter the settling tank the wastewater isforced to flow slowly through a series ofbaffles. The increased contact time withbacteria in the sludge means that very
high reductions in pathogens are possible.
Like a septic tank, the final products are
effluent and sludge. WaterAid uses ABRs asa means of primary wastewater treatment,in combination with constructed wetlands.
Like septic tanks, ABRs require a regularsupply of water or wastewater. It can takeseveral months for an ABR to reach full
treatment capacity, as the bacteria taketime to colonise the tank. This process
can be sped up by ‘seeding’ the tank withbacteria from another ABR.
A note on greywater
Wherever possible, greywater (fromwashing clothes or other domestic uses)
should be collected and treated in acontrolled way. The amount of greywaterproduced by a household will vary greatly
by location, income and cultural practices.
Any standing pools of domestic water cancreate a potential breeding ground formosquitoes, and should be discouraged.
A simple soak pit will provide a decent
method of dealing with greywater;
however, it is important to consider thesoil type, water table depth, quantityof greywater and the space available.
Where kitchen gardens exist, greywater
can be safely used for irrigation if diluted
with an adequate amount of clean water.

3 | Wastewater treatment

Removal and transport

Kitchen and bathroom blocks

This technology brief focuses on the transportof wastewater. For information on transfer
stations and manual and motorised emptyingof pit latrines please refer to the technology
brief Urban pit waste management.

Simplified sewerage
Simplified sewerage is a relatively
cheap sewer design that has been used
extensively in Brazil. Cost savings are
made over conventional sewer design bylaying small diameter sewers in shallow
trenches under pavements or in yards,
where they are protected from heavy
traffic loading. This also allows savings to
be made by shortening the connection
distance from the house to the sewer line.
Simplified sewers can connect households
to a main sewer line or a decentralised
treatment works.
This method achieved success in the
Orangi Pilot Project, carried out inKarachi, Pakistan; however, it requires a
great deal of community participation tobe effective. People must understand theimportance of not flushing anything that
could block the small-diameter sewers,
and excellent design and engineering are
essential. It also requires a regular supplyof water for flushing, so might not beappropriate in water-stressed areas. Most
critically, simplified sewers can only work
if the topography of the area lends itself
to gravity-driven pipes.

4 | Wastewater treatment

Households

Collection sewer

Fig 2: Simplified sewerage

Solids-free sewerage
A solids-free sewer is similar to a simplified
sewer, except that they do not convey solidmaterial. Household wastewater drains into
a septic tank, settling tank or other primary
treatment method, before entering thesewers. This means that the sewers are very
unlikely to clog, and can be laid at very flator even negative gradients (provided thatthe final downstream end is lower than the
upstream end).
In addition to the criteria described forsimplified sewers, this method also requires
frequent maintenance, as the septic tanks
or settling tanks require periodic emptying.
However, the gradient flexibility means that
topography is less of a critical factor.

Treatment
Constructed wetlands

Waste stabilisation ponds

Constructed wetlands, also known asreed beds, use wetland plants (or, more
specifically, micro-organisms within
the roots) to remove pollutants fromwastewater. There are various constructedwetland designs, lying above or below theground surface, but all require a large areaof land. They also require some form of
preliminary and primary treatment to takeplace (as they cannot deal with large solids
or sludge). They can range in size from ahousehold treatment option to a large,
centralised facility.

Waste stabilisation ponds (WSPs) arelarge pools where wastewater settles anddecomposes because of naturally occurring
bacteria. A carefully designed series of atleast three linked ponds provides the besttreatment. It is not necessary to screen thewastewater before it enters the pond. WSPs
work in most areas, but are most efficient inwarm, sunny climates. They are especiallyappropriate for communities that haveample, inexpensive space away from homes
and public spaces. It is important that theponds have an impermeable liner and areprotected by a fence to stop people bathingin or using the water. The ponds require
desludging every 10-20 years and need
to be expertly designed for the maximum
removal of pathogens.

Operational costs for constructed wetlands are
low and, although careful design is necessary,
there is no need for skilled supervision.

1 Anaerobic

2 FacultativeInlet

2m - 5m

1 Anaerobic

3 Aerobic maturation

OutletLiner
Sludge

Inlet
02

02

02

02

1m - 2.5m

2 Facultative

OutletSludge

Inlet
02

02

02

02

3 Aerobic maturation

0.5m - 1.5m

Fig 3: Cross-section of waste stabilisation ponds

5 | Wastewater treatment

OutletAnaerobic biogas reactor
An anaerobic biogas reactor produces gas
from the fermentation of sludge, which canbe collected and used to fuel engines forelectricity, or burned for light and cooking.
They can be built for a single household
or for a whole community, and require
expert design and careful monitoring for
optimum performance.
Biogas reactors usually have a domed
shape. Wastewater flows in and sludge
settles to the bottom. Sludge fermentation
creates gas, which collects in the dome at

the top of the reactor. The dome can be
fixed or floating, rising and falling with the
production of gas. Biogas reactors produce
sludge that is not safe to handle, and must
therefore be disposed of carefully.
While biogas reactors are commonly used
to treat wastewater from communities, they
can also be successfully used on a smallerscale at the household level. For maximum
efficiency and gas production, families can
supplement their household wastewaterwith animal manure.

Biogas outlet

Expansionchamber

Seal

Inlet

BiogasOutletSlurryFig 4: Anaerobic biogas reactor

6 | Wastewater treatment

Case study

Cookingon biogas

Eva Eugene Millinga,
42, cooks over her
biogas stove in herhouse in Dar es
Salaam, Tanzania.
WaterAid/ Eliza Deacon
In Dar es Salaam, Tanzania, mostresidents use pit toilets, which are difficultand expensive to empty once full. Mattius
Millinga, who previously had a businessremoving solid waste, has also ventured
into a new business emptying pits,
using a low-cost pump called a Gulper,
motorised tricycles to transport theextracted sludge, and a small treatment
plant near his home.

7 | Wastewater treatment

The decentralised wastewater treatment
system (DEWATS) includes a sludge
discharge station, which receives amaximum of 40m3 of sludge per day,and a baffled anaerobic reactor thatdigests sludge and produces biogas,
contained by a dome. The biogas is usedin the Millingas’ home, while the sludge
is dried on drying beds and used as soil
conditioner, and the effluent goes onto
planted beds for final treatment.

Ecological sanitation
Ecological sanitation (ecosan) latrine
systems are designed to biodegrade
waste into a humus-like soil which is safe
to handle, rich in nutrients and can beused to increase agricultural production.
In this system, urine and faeces are seen
as resources rather than waste.
Urine-diverting composting toilets separate
urine and faeces using a specially designed
slab. The faeces are safely left to compostin vaults before being removed and applied
to land as fertiliser. Urine can be diluted
with water and applied to land after only
a few days, as it is sterile. This system canbe highly beneficial in agricultural areas
where artificial fertiliser is expensive or
hard to obtain, and the improvements inagricultural production can be impressive.
The ecosan system also does not requirewater, which makes it suitable forwater-stressed areas; however, it does
require adequate amounts of soil and ash,and a large handful should be thrown into
the faeces vault after each use (to preventthe latrine from smelling).
Ecosan can be extremely beneficial, but it isvery important that the system is correctly
managed. Latrines can be constructed
and maintained by families without
much outside help, but require ongoingcommitment for proper care and use. It istherefore essential to provide education on
correct use and ensure there is a demand
for the end product. Fertiliser use is usuallyseasonal, and thought should be given as
to where urine and compost can be stored
during times of low requirement.
It is very important that the dehydrating
vaults are kept dry; ecosan latrines are
therefore not appropriate for the disposal of
greywater or those who use water for anal
cleansing. The vault system also requires
slightly more space than a standard pit
latrine. Some communities are unwillingto accept the reuse of faeces and carefulsensitisation may be necessary.

8 | Wastewater treatment

Disposal or use
Some of the treatment processes listed
above do not provide a complete solution,with effluent or sludge still needing to bedisposed of. Some processes for dealing
with these end products are listed below.
They also give an indication of the valuethat is to be found in human waste, andmany sanitation systems can be easily
adapted to give these end results.

DryingIf sludge can be dried on site after
treatment it can then be transported off site
more cheaply, or used as fuel or compost.
There are relatively few tried and tested
methods for drying faecal sludge.
Solar drying
Simple and cheap solar driers havebeen trialled in Malawi. These use aglass panel above a black absorberplate to capture solar energy, a bit like agreenhouse. The sludge is spread over the
perforated absorber plate and is heated totemperatures of up to 57°C.
Compost
Biodegraded sludge can produce valuable
compost, which can be used to increase
agricultural production. Organic matter,such as dried leaves, should be added
for optimum results. The more liquid thesludge, the more organic matter should
be added.
Composting can be done either byleaving the sludge outside for a periodof a few months, or leaving it in a sealedcontainer. Care must be taken to ensurethat the sludge has been left for a longenough period that all harmful pathogens
are destroyed. The time for this to happen
will depend on the local climate andtreatment of the sludge.

9 | Wastewater treatment

Fuel
It is possible to harness energy from sludge,
as detailed in the two methods below.
BiogasBiogas is produced by fermenting sludge,
which is collected in anaerobic biogas
reactors. Gas from fermenting sludge is aclean fuel that burns efficiently. Fuel is often
a major part of a household’s expenditure,
and a clean, cheap supply can be good forboth the household budget and health.
Cakes
Once dried, sludge can be compacted
into ‘cakes’ or ‘briquettes’ and used as afuel source.
Disposal to sewers
In some areas, it may be possible toobtain permission to dispose of final
effluent into sewers. This provides agood solution for many small operators;
however, caution should be taken when
disposing of concentrated sludge intosewers because a sudden force can cause
problems at the wastewater treatment
centres downstream.

References

Useful resources

Tilley E, Ulrich L, Lüthi C, Reymond P
and Zurbrügg C (2014) Compendium ofsanitation systems and technologies (2ndrevised edition). Swiss Federal Institute of
Aquatic Science and Technology (Eawag),
Dübendorf, Switzerland. Available at:
http://wsscc.org/resources-feed/
compendium-of-sanitation-systems-andtechnologies-2nd-edition

Akvopedia Sanitation Portal. Available at:
http://akvopedia.org/wiki/Sanitation_Portal

Morel A and Diener S (2006)
Greywater management in low andmiddle-income countries. Swiss Federal
Institute of Aquatic Science and Technology(Eawag). Dübendorf, Switzerland.
Available at:https://www.sswm.info/sites/default/
files/reference_attachments/MOREL%20
and%20DIENER%202006%20Greywater%20
Management.pdf
Water, Engineering and Development
Centre (WEDC) (1991) The worth of water:technical briefs on health, water andsanitation. IT Publications, London, UK.
Available to purchase at: http://store.lboro.
ac.uk/product-catalogue/wedc/wedcbookshop/the-worth-of-water-technicalbriefs-on-health-water-and-sanitation

10 | Wastewater treatment

The School of Civil Engineering at Leeds
University has a large collection of information
on waste stabilisation ponds. Available atProfessor Duncan Mara’s webpages:
http://www.personal.leeds.ac.uk/~cen6ddm/
WSP.html
Andersson K, Rosemarin A, Lamizana
B et al (2016) Sanitation, wastewater
management and sustainability: fromwaste disposal to resource recovery.
United Nations Environment Programme
and Stockholm Environment Institute,
Nairobi, Kenya and Stockholm, Sweden.
Available at: https://wedocs.unep.org/
bitstream/handle/20.500.11822/8465/Sanitation,_wastewater_management_
and_sustainability_From_waste_disposal_
to_resource_recovery-2016sanitation_
wastewater_management_and_sustainabilit.
pdf?sequence=2&isAllowed=y
The World Health Organization fact sheetson environmental sanitation. Available at:
http://www.who.int/water_sanitation_
health/emergencies/envsanfactsheets/en/
index.html

Part of a series of WaterAid
technology briefs available atwww.wateraid.org/uk/technology

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