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Septic Tanks

Dorothee Spuhler, seecon gmbh

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Septic Tanks

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Copyright

Included in the SSWM Toolbox are materials from various organisations and sources. Those materials are open source. Following the open-source concept for capacity building and non-profit use, copying and adapting is allowed provided proper acknowledgement of the source is made (see below). The publication of these materials in the SSWM Toolbox does not alter any existing copyrights. Material published in the SSWM Toolbox for the first time follows the same open-source concept, with all rights remaining with the original authors or producing organisations.

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Disclaimer

The contents of the SSWM Toolbox reflect the opinions of the respective authors and not necessarily the official opinion of the funding or supporting partner organisations.

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 respect to the results that may be obtained from the use of the information provided.

 

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Copyright & Disclaimer

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

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Septic Tanks

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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.

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1. Concept

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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.

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1. Concept

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What is a septic tank? (2/2)

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1. Concept

Source: adapted from TILLEY et al. (2008).

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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�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

Source: SANIMAS (2005)

Septic Tanks

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Examples

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Newly constructed septic tank

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1. Concept

Source: http://mtaquiferrescue.com/images/completed_septic_tank.bmp [Accessed: 27.05.2010]

Septic tank at community level

Source: SANIMAS (2005)

Septic Tanks

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Examples

Septic tank followed by a soak pit for the infiltration of the effluent

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1. Concept

Source: SANIMAS (2005)

LOST !!

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Examples

Septic tank combined with leach field

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1. Concept

Source:http://www.jetrod.biz/images/septic%20tank%20&%20soakawa [Accessed: 26-.05.2010]

Source: US EPA (n.y.)

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Examples

Septic tank connected to a simplified sewer system (solids-free sewer)

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1. Concept

Source: US EPA (2004)

Source: http://www.orenco.com/images/ccs_prostep.jpg [Accessed: 27.05.2010]

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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.

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2. How it can optimize SSWM

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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

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Aqua privy – a variation of the septic tank

    • 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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3. Design Principals

Source: WAaF (2002

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    • 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)

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    • 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

Health aspects

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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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    • 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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5. Operation and Maintenance (O&M)

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    • 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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6. Applicability

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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

Disadvantages:

  • High cost compared to dry or composting toilet systems
  • Water reliant
  • Low reduction in pathogens
  • De-sludging required: Manual de-sludging 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_Dewats-Handbook.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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“Linking up Sustainable Sanitation, Water Management & Agriculture”

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Septic Tanks