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The Biology and Operation of a Septic System

Uploaded by picco20 on Feb 09, 2002

Introduction to Septic Systems

The septic system is a grouping of components working together to decompose household waste water. The septic tank holds the waste and breaks the organic compounds into a solid, liquid or gas. The solid waste (called sludge) settles to the bottom of the tank and must periodically be removed. The liquid waste is flushed to the distribution box and is then allowed to drain into the ground through a series of perforated pipes (often called lines or a leach field). Gases follow the same path as liquids and eventually rise through the soil and enter the atmosphere. Tanks can be arranged in series for additional waste treatment. Many older systems have no distribution mechanism.

The Septic Tank Layers or Horizons

Wastewater flows through a pipe into the septic tank. Baffles or "tees" at the inlet and outlet of the tank slows the flow of water and prevents sewage from flowing directly through the tank. Bacterial action within a septic tank helps to break down the solids in the wastewater. The tank must be large enough, and the rate of flow small enough, to ensure sufficient "residence time" of wastewater within the tank. The solids which cannot be broken down sink to the bottom of the tank and accumulate as sludge.

Grease, foam, and lighter particles float to the surface and form a layer of scum. The exit baffle holds back sludge and scum while allowing a partially digested wastewater to flow out of the tank.

This picture depicts the three layers within the septic tank. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.

Organic Substances

  • The concentration of natural and synthetic compounds in effluent are generally expressed in terms of:
  • Biological Oxygen Demand (BOD), the measure of how much oxygen is required to finish digesting the organic material left in the effluent.
  • Chemical Oxygen Demand (COD), the measure of how much oxygen is required to finish the decomposion of chemicals.
  • Total Suspended Solid Content (TST)
  • Total phosphorus and nitrogen - measures the nutrients remaining in the water

    A properly designed and maintained septic tank removes most of the organic
    substances from raw wastewater. Additional removal...

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    Uploaded by:   picco20

    Date:   02/09/2002

    Category:   Biology

    Length:   10 pages (2,361 words)

    Views:   2643

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