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5 June 2013

Organics anaerobic digester = septic tank?

Is an organics anaerobic digester equal to a septic tank? Close, but no cigar – critical feedstock and system parameters to consider when selecting a system

Example anaerobic digestion system

Example anaerobic digestion system

Recently someone asked me if a septic tank was an anaerobic digester. I responded yes, because the decomposition process in a septic tank is anaerobic. He then asked, “If that’s the case, why couldn’t we just take our organic waste and put it in a septic tank?”

I proceeded to explain that the quantity and quality of the valuable byproducts captured in a digester is a direct result of microorganism activity, which in turn is determined by the anaerobic digester’s feedstock and system parameters. These parameters are important because anaerobic digestion occurs in three phases, each with different operating microorganisms, with different optimal environmental conditions.

Instead of why couldn’t we use a septic tank, the real question becomes why would we? Let’s examine some of the feedstock and system parameters that contribute to the complexity of an organics anaerobic digester and should be considered when selecting a system.

Moisture Content

  • Wet (pumpable) – requires addition of water
  • Dry (stackable) – may be cheaper due to fewer moving/technical parts
  • Low solids (<20%) – more contact between microorganisms and their food
  • High solids (>20%) – more compact system (less water = less volume), higher energy production (more organic material to turn to biogas)

Retention Time

  • Time it takes for AD process to be completed (how long substrate is in system)
  • Dependent on amount and type of substrate and system
  • Typically 15 to 40 days for high solids
  • Mixing (with impeller, pumps, pressurized gas) decreases retention time


  • Psychrophilic (<25°C)
    • Low biogas yield and long retention time
  • Mesophilic (30-40°C)
    • More stable – microorganisms are more tolerant to environment condition changes
  • Thermophilic (50-55°C)
    • Higher biogas yield and shorter retention time
    • Sterilization of substrate
    • Sensitive to temperature and pH changes

Loading Rate

  • Batch
    • Substrate is added at beginning of process and is sealed for duration
    • Lower equipment and design cost because it is simpler
    • Gas generation curve used to determine when process is complete
  • Continuous
    • Substrate is constantly added
    • Constant biogas production
    • Less risk of odor
    • Greater risk of overloading substrate (different phases in direct competition with one another, pH issues)


  • Single stage
    • All AD reaction stages occur in one vessel
    • Less space required for system
    • Less control of the reaction (different phases in direct competition with one another, pH issues)
    • Higher risk of odor (pH decreases – substrate becomes more acidic – methanogens die and more H2S forms, causing odor)
  • Multiple stages
    • Different stages occur in different vessels
    • Maximum control over microbial communities
    • Usually dual stage – methanogenesis separated from other processes
    • First tank can act as buffer for loading rate

So, now I hope I have convinced you that a basic septic tank just has to yield to a more complex anaerobic digester for organic waste.

Categories: Organics
Posted By Laura Hammerer at 11:00 AM  |  11 Comments on Organics anaerobic digester = septic tank?

11 Responses to “Organics anaerobic digester = septic tank?”

  1. Dumindhi says:

    I’d like to know what are the kind of parameters and machinery needed to handle the mixing of organic waste and sewage sludge to make bio gas and fertilizers?

  2. Robert Head says:

    How much recoverable methane gas can be produced in a typical 750 gallon residentail septic tank in southern USA: over a 1 day, 7 day, and 30 day period?

  3. Bill_Corr says:

    Keep on pursuing this idea of a household anaerobic digester. They do it in India:

    I’m exploring what needs to be done to make ADs a reality not in India, but in California.

  4. J Price says:

    septic systems can work if you separate washing machine dishwashers showers etc. into a grey water. This helps keep the solids ratio high enough as detergents antibacterial soap chlorinated water will slow down or stop the digestion. Generally showers and laundry water is not a biohazard initial mixed with waste

    • Laura Hammerer says:

      Agreed – this is something that has to be considered if you expect to collect methane from a household anaerobic digester.

  5. ZEESHAN says:

    dear sir,
    asslam o alikum!
    i am designing three stage biodigester to produce methane, would u like to guide me please about its feasible design with reference to mesophilic, thermophilic and hyperphilic stages.

  6. […] Organics anaerobic digester = septic tank? Critical parameters to consider when selecting a system […]

  7. Kimanzi mutwai says:

    How do I get digester bacteria?

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