Transcript Slide 1

Septic Tanks
Dorothee Spuhler, seecon gmbh
Septic Tanks
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The contents of the SSWM Toolbox reflect the opinions of the respective authors and not necessarily the official opinion of the funding or
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Depending on the initial situations and respective local circumstances, there is no guarantee that single measures described in the toolbox
will make the local water and sanitation system more sustainable. The main aim of the SSWM Toolbox is to be a reference tool to provide
ideas for improving the local water and sanitation situation in a sustainable manner. Results depend largely on the respective situation
and the implementation and combination of the measures described. An in-depth analysis of respective advantages and disadvantages and
the suitability of the measure is necessary in every single case. We do not assume any responsibility for and make no warranty with
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Septic Tanks
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Contents
1. Concept
2. How can it optimise SSWM
3. Design principals
4. Treatment efficiency
5. Operation and maintenance
6. Applicability
7. Pros’ and Cons’
8. References
Septic Tanks
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1. Concept
Background
Septic tanks are the one of the most simple on-site water based
sanitation systems and are used all over the globe.
It is a simple tank which pre-settles the solids contained in the
wastewater flow while the liquid flows through.
Anaerobic digestion degrades the settled sludge, but the remaining
sludge, much like raw faecal sludge from pit latrines requires regular
emptying and treatment. The emptying and correct discharge or
treatment and reuse of the vaults can be expensive.
Consequently the clandestine discharge of emptied faecal sludge from
septic tank consists in a major health risk and environmental problem
in low-income areas.
Septic Tanks
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1. Concept
What is a septic tank? (1/2)
A septic tanks is an underground watertight chamber (generally rectangular,
sometimes round) made out of brick work, concrete, fiberglass, PVC or plastic.
They are used as primary wastewater treatment unit for the on-site
treatment of blackwater from cistern or pour-flush toilets, greywater or
biodegradable industrial wastewater.
Wastewaters are flushed trough a pipe into the tank. Solids (e.g. scoops,
kitchen waste etc.) settle to bottom of the tank where the organic fraction of
the solids are reduced via anaerobic digestion.
A baffle retains the settled solids. The only moderate treated liquid
(supernatant) flows out of the tank on the opposite site of the inlet and is
either infiltrated into the soil (in a soak pit, leach field or mound) or
transported via a simplified sewer system to a (semi-)centralized treatment
facility.
One every few years, the accumulated sludge in the bottom of the chamber
needs to be dug and disposed correctly.
Septic Tanks
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1. Concept
What is a septic tank? (2/2)
Source: adapted from TILLEY et al. (2008).
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1. Concept
How does it work?
• The solids settle to the ground while scum
(oil and fat) floats to the top and liquid
flows through (Physical Treatment).
• Anaerobic microorganism living in the bottom
Source: SANIMAS (2005)
sludge start to degrade the organicfraction of the
wastes, transforming it into mehtane (CH4), carbon dioxide (CO2),
hydrogen (H), nitrgoen (N) and hydro sulphide (H2S) = Biogas.
• This process called anaerobic digestion (Biological Treatment).
• Biogas has similar properties to natural gas and can used for heating
or cooking.
• Ventilation is necessary if biogas is is not recovered.
• The settling capacity of the solids depends on the wastewater flow.
A rather slow flow will settle better. But a turbulent flow will increase
the contact of the new sludge with the microorganism responsible for
degradation.
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1. Concept
Examples
Source: SANIMAS (2005)
Newly constructed septic tank
Septic tank at community level
Source: http://mtaquiferrescue.com/images/completed_septic_tank.bmp [Accessed: 27.05.2010]
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1. Concept
Examples
Septic tank followed by a soak pit for the infiltration of the effluent
LOST !!
Source: SANIMAS (2005)
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1. Concept
Examples
Septic tank combined with leach field
Source: US EPA (n.y.)
Source:http://www.jetrod.biz/images/septic%20tank%20&%20soakawa [Accessed: 26.05.2010]
Septic Tanks
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1. Concept
Examples
Septic tank connected to a simplified sewer system (solids-free sewer)
Source: http://www.orenco.com/images/ccs_prostep.jpg [Accessed: 27.05.2010]
Septic Tanks
Source: US EPA (2004)
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2. How it can optimize SSWM
Septic tanks are better than nothing: the solid fraction, and thus a large
fraction of the pollution are removed, even thought the effluent is not
safe and bears many pathogens. Optionally, even biogas can be recovered
for cooking and heating.
When effluents don’t contain solids anymore, simplified sewers, which are
much cheaper and more easy to install and maintain than conventional
sewers, can be installed.
The liquid flowing out of the septic tank can also be used for fertigation if
health precautions are taken or groundwater recharge resulting in on-site
recycling of water and nutrients.
Also the sludge collected in the septic tank needs to be emptied regularly,
it may be further treated (e.g. composting) and used as a fertiliser, further
increasing soil quality and food production.
Septic Tanks
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3. Design Principals
Septic tank
• Minimal two compartments: one settling chamber, and the following to calm
the turbulent liquid.
• Made out of concrete or bricks, pre-fabricated concrete rings, PVC or
fibreglass septic tanks
• A baffle between the chambers prevents scum and solids from escaping from
the settling chamber.
• A T-shaped outlet pipe (30 cm below water level), will further reduce the
scum and solids that are discharged. (SASSE 1998)
• If only two chambers, the first one should be 2/3 of the total length. (TILLEY et al. 2008)
• Chamber depth of 1.5 to 2.5; sometimes the first chamber is made deeper as
the others (for higher sludge retention volume).
• Hydraulic Retention Times (HRT) of about 24 to 48 hours. (MOREL & DIENER 2006; TILLEY et al. 2008)
• Sludge Retention Time (SRT) of several years.
• Sludge production depends on number of users and portion of settable solids,
average annual temperature, desludging intervals etc.
• 80 to 100 L of volume should be provided per domestic user. (SASSE 1998)
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3. Design Principals
Aqua privy – a variation of the septic tank
Source: WAaF (2002
• Simplified septic tank
• Only one chamber (settling
chamber)
• Biogas is vented trough a pipe
• Supernatant is used to protect
as a seal for the sludge to
prevent smell, flies etc. (WHO 1992)
• Overflowing liquid is infiltrated
into the ground by a soak pit
• Accumulated solids need to be
removed frequently and require
further treatment
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4. Treatment Efficiency
• Biological Oxygen Demand (BOD): 30 to 50 % (UNEP 2004)
• Total Suspended Solids (TSS): 40 to 60 % (UNEP 2004)
• E. coli: 1 logarithmic (log) unit (TILLEY et al. 2008)
Health aspects
• Effluent from septic tanks can contain pathogens. (WHO 1992)
• Since there is no way of differentiating between freshly added excreta and
excreta in the tank, the septic tank sludge must be considered as
microbiologically contaminated. (WHO 2006, Vol. III)
• Many of the problems with septic tank systems arise because no adequate
consideration is given to the disposal of the faecal sludge and tank effluent.
• Direct reuse of effluent is not to be considered.
• Underground infiltration of effluent is recommended.
• Sludge should be stored minimal four weeks before reuse. (WHO 2006, Vol. III)
• Aerobic composting of sludge before use to reduce health risk.
• Underground construction prevents direct contact with human.
• Care should be taken during operation and maintenance interventions (prevent
contact with sludge and effluent).
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4. Treatment Efficiency
Corrrectly treated,
faecal sludge from
septic tank can be
reused in agriculture
Source: WHO (2006), Vol. IV
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5. Operation and Maintenance (O&M)
• Start-up: Seeding (inoculation) is required (take sludge from
another septic tank…).
• No harsh chemicals should be introduced into the tank as this could
kill the microorganisms.
• De-sludging is needed when 1/2 to 2/3 of depth (between the
water level and the bottom of the tank) are occupied by sludge and
scum (WHO 1992)
• Desludging is required every 1 to 5 years.
• Desludgeing can be done manual (with a gulper) or mechanised.
Manual desludgeing bear more health risks.
• Some sludge must be left in the tank to maintain the
microorganism responsible for anaerobic digestion.
• Emptied sludge must be treated by composting, further anaerobic
digestion, waste stabilisation ponds, constructed wetlands or drying
beds.
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6. Applicability
• Wastewaters with high percentage of settable solids and organics
(typically for water from toilets and greywater, but can also be
used for biodegradable industrial waters).
• Adapted for household, community or institutional level.
• If effluents need to be infiltrated on-site, not adapted for areas
with dense settlements, rocky grounds, high groundwater table or
areas prone to frequent flooding.
• Sludge needs to be collected and treated.
• Every climate, for effective anaerobic digestion, moderate
temperatures are required.
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7. Pros’ and Cons’
Advantages:
• Can be built and repaired with
locally available materials
• No real problems with flies or
odours if used correctly
• Long service life
• Little space required due to
underground construction
• Low investment costs, low
operation and maintenance costs
depending on the availability of
water and the requirement for
emptying
• No energy required
Septic Tanks
Disadvantages:
• High cost compared to dry or
composting toilet systems
• Water reliant
• Low reduction in pathogens
• De-sludging required: Manual desludging is hazardous to health
and mechanical de-sludging
(vacuum trucks) requires the
infrastructure and may be rather
costly
• Only suitable for low-density
housing in areas with low water
table and not prone to flooding
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8. References
MOREL, A., DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of different treatment systems for
households or neighbourhoods. (=SANDEC Report No. 14/06). Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG),
Department of Water and Sanitation in Developing Countries (SANDEC) Available at:
http://www.eawag.ch/organisation/abteilungen/sandec/schwerpunkte/ewm/projects/project_greywater [Accessed: 19.05.2010]
SANIMAS (2005): Informed Choice Catalogue. PPT-Presentation. BORDA and USAID
SASSE, L. (1998): DEWATS Decentralised Wastewater Treatment in Developing Countries. Bremen: Bremen Overseas Research and
Development Association (BORDA) Available at: http://www.borda-net.org/modules/wfdownloads/uploads/062%20BORDA_DewatsHandbook.pdf [Accessed: 27.05.2010]
TILLEY, E., LUETHI, C., MOREL, A., ZURBRUEGG, C., SCHERTENLEIB, R. (2008): Compendium of Sanitation Systems and Technologies.
Duebendorf and Geneva: Swiss Federal Institute of Aquatic Science (EAWAG) & Water Supply and Sanitation Collaborative Council
(WSSCC) Available at: http://www.eawag.ch/organisation/abteilungen/sandec/publikationen/compendium_e/index_EN Accessed:
09.04.2010]
UNEP (2004): Improving municipal wastewater management in coastal cities. Training Manual Version 1. United Nations Enivrionmental
Programme (UNEP), Global Programme of Action (GPA). Available at:
http://www.unep.org/publications/search/pub_details_s.asp?ID=3632 [Accessed: 27.05.2010]
U.S. EPA (2004): Background and Technology. PDF Presentation. United States Environmental Protection Agency, Office of Water Office
of Research and Development. Available at: http://www.epa.gov/owm/septic/pubs/septic_technologies.pdf [Accessed: 22.03.2010]
U.S. EPA (n.y.): Planning for Septic Systems - Use of Online Decentralized Systems in Developing Areas. United States Environmental
Protection Agency, Office of Water Office of Research and Development . Available at:
http://www.epa.gov/owm/septic/pubs/planning_for_septic_systems.pdf [Accessed: 27.05.2010]
WAaF (2002) Sanitation Technology Options. Pretoria: Department of Water Affairs and Forestry (WAaF) Available at:
http://www.bvsde.paho.org/bvsacd/cd66/Technical.pdf [Accessed: 27.05.2010]
WHO (1992): A Guide to the development of on-site sanitation. Geneva: World Health Organisation (WHO). Available at:
http://whqlibdoc.who.int/publications/1992/9241544430_eng.pdf [Accessed: 27.05.2010]
WHO (2006): Guidelines for the safe use of wastewater excreta and greywater. Volume III. Wastewater and Excreta Use in Aquaculture.
Geneva: World Health Organization (WHO).
WHO (2006): Guidelines for the safe use of wastewater excreta and greywater. Volume IV. Excreat and Greywater Use in Agriculture.
Geneva: World Health Organization (WHO)
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