Does my sewage system produce methane?

This question was put to me:

On  recent tour of your house with Ryde TAFE college, you told me the septic tank did not give off methane because of the anarobic bacteria. You asked me to ask the question on your blogg. Being an old fart, I do not know how to use these wiz bang electronic garbbage, so in search of knowledge, I am contacting you directly. My knowledge on septic tanks is limited to the anarobic bacteria break down the liquids; while airobic bacteria breaks down the crust which forms on top. Chemical are not used for fear of killing these little criters. If the bacteria are killed the whole tank must be emptied and the tank walls scrubbed before the tank is allowed to be refilled and the bacteria begins to form again. My understanding/ mis-understanding is while the bacteria is breaking down the solids/liquids they release methane gas.
One thing I was please to hear from you is “search for the truth and do not accept anything less”, those inspiring words has me at you door step. If the answer does help others please feel free to transfer my question to your blogg.


FL, the sewage system here works like this:

  • raw sewage goes into a tank at one end where it sits for about a day or two while more and more sewage is added
  • when that first tank gets full a pump transfers the top couple of hundred litres from the top of that tank to another two connected tanks in which air is continually blown into the sewage there by a 60 watt pump at 60 litres of air a minute
  • if those two connected tanks are full then, when the pump transfers sewage from the first tank, that transferred sewage displaces water from them to a final tank where the water is held until it’s pumped to flush the toilet, wash the clothes or hose the garden
  • all the tanks are ventilated with inflowing air naturally entering them and being drawn across the sewage to a vent pipe at one end which rises above the house roof

So air is pumped into about half of the stored sewage.  This is aerobic.  That is, it’s full of air.

About half of the sewage has only the air that is in the water or passes across it as it is drawn up to the vent pipe. This is slightly anerobic.

Some observations:

  • frogs live in the tanks and I hear them calling out.  As far as I can tell, they’re not saying, “Help, I’m full of it, get me out of here”.  Judging by the little ones I disturb when I inspect the tanks they’re happy in there
  • some slight odour is detectable when sewage is transferred from one tank to the other

As the system doesn’t smell I think its almost entirely aerobic; it’s easy to tell when something’s anerobic because there’s a smell.

So the process doesn’t produce a significant amount of methane but it does produce some.  My research suggests the amount is minor.

There are sewage systems in India and other countries where the gases are harvested to cook with and they have methane, but they are different to mine.

Great to have your question, thanks,


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  • Michael Mobbs

    Michael is a former Environmental Lawyer who is uniquely placed to consult in four main areas:

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