Science & Technology in Action

11th Edition

Septic Tanks


Construction Studies
This lesson outlines the structure and operation of septic tanks. It describes primary, secondary and tertiary treatment processes. Septic tanks are so unobtrusive that they are easily forgotten and this can lead to major problems. All septic tanks must now be registered with the local authority. The EPA supervises the inspection of the tanks by the local authority.
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Lesson excerpt

Sewage treatment process
The treatment of sewage can be split into three levels of increasing purification.

  1. Primary treatment involves the removal of solid material (or sludge) by screening and sedimentation, which can then be rendered biologically safe and stabilised by biodigestion (essentially composting). 
  2. Secondary Treatment: involves biological oxidation by aerobic microorganisms. These break down the organic materials in the sewage into carbon dioxide and water. They also destroy most of the pathogenic organisms present allowing the resulting liquid to be discharged into water courses in accordance with licenced limits. Since these are aerobic bacteria there is no unpleasant smell. Nutrients are not significantly reduced during the process.
  3. Tertiary treatment is the removal of nitrates and phosphates (NO3-, PO43-) by precipitation or ion exchange which prevents eutrophication. In the most modern plants this treatment can also involve exposure to ultra-violet radiation to ensure complete biological safety by killing the pathogens such as E coli which have survived secondary treatment. Wastewater sludge is a by-product of the sewage treatment process. In modern sludge treatment systems about half of this solid is converted to methane by anaerobic bacterial action while the rest is dried and pelleted before being spread on land. The methane can be collected and used as a fuel.

Due to the very spread-out nature of rural housing a centralised sewage treatment system is not a viable option, thus each house needs an individual sewage and waste water treatment system. The commonest system in use today is a septic tank. 

True or False?

  1. Anaerobic bacteria produce foul smells and poisons. true
  2. Septic tanks are the most efficient way of disposing of sewage. false
  3. A septic tank should be checked more than once a year. true
  4. Rainwater should pass through the septic tank. false
  5. Anaerobic bacteria break down sludge in the first chamber. false
  6. It is safe to dispose of fats and oils using the kitchen sink. false
  7. Drain cleaner and bleach reduce the efficiency of septic tank true
  8. Only new septic tanks must be registered and inspected. false
  9. Lush green grass on a percolation area indicates that the system is working efficiently. false
  10. Eutrophication is a serious problem in some areas. true
  11. There are more than 400,000 septic tanks in use in Ireland. true
  12. It is safe to place a well near a percolation area. false