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Septic Tank

Made of

Concrete Plastic Fiberglass Brick

Brief description

A localized underground wastewater treatment system for areas not connected to a central Sewage System. It uses natural processes (settling and anaerobic bacteria) to treat sewage before releasing it into a drain field.

Use / Function

  • Primary use: Treating household wastewater (from toilets, sinks, showers).
  • Secondary uses: Separating solids from liquids.
  • Scale: Single household to small community.

Operating principle

  1. Settling: Wastewater flows into the tank. Heavy solids (sludge) sink to the bottom. Lighter solids (grease, oil - scum) float to the top.
  2. Anaerobic Digestion: Bacteria in the tank break down the organic matter in the sludge and scum, reducing their volume.
  3. Effluent: The liquid layer in the middle (effluent) flows out of the tank into a drain field.
  4. Filtration: The drain field (perforated pipes in gravel) distributes the effluent into the soil, where aerobic bacteria further treat it before it reaches groundwater.

How to create it

1. The Tank

  • Must be watertight to prevent leakage and groundwater infiltration.
  • Inlet: Where sewage enters. Has a baffle to prevent disturbing the scum layer.
  • Outlet: Where effluent leaves. Has a baffle or filter to prevent solids from clogging the drain field.
  • Access Ports: Lids for inspection and pumping out sludge (every 3-5 years).

2. The Drain Field (Leach Field)

  • A network of Pipes with holes, laid in trenches filled with gravel.
  • The soil must be permeable (perc test required).

Materials needed

  • Tank: Concrete (precast or poured), Plastic (polyethylene), Fiberglass, or Brick (historical, sealed with mortar).
  • Pipes: Plastic (PVC) for connections and perforated drain lines.
  • Gravel/Stone: For the drain field bed.
  • Soil: The final filter.

Variants and improvements

  • Aerobic Treatment Unit (ATU): Uses an air pump to inject oxygen, promoting aerobic bacteria which digest waste faster and cleaner than anaerobic ones.
  • Mound System: An elevated drain field for areas with high water tables or shallow bedrock.
  • Sand Filter: Effluent passes through a bed of sand before entering the soil.

Limits and risks

  • Maintenance: Sludge builds up and MUST be pumped out regularly (3-5 years) or it will clog the drain field (expensive failure).
  • Chemicals: Bleach, harsh cleaners, and paints kill the beneficial bacteria, stopping the treatment process.
  • Groundwater Contamination: If the soil is too permeable or the water table is too high, untreated sewage can pollute drinking water wells.
  • Capacity: Cannot handle massive surges of water (e.g., stormwater) without flushing out solids.