Decomposition of motor oil

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Apr 25, 2009
England, UK
Hi there. Does anybody know how long it takes motor oil to decompose in a landfill site? I know this is a strange question but it has come up in discussion in my classroom and I honestly have no idea. A 'ball park' figure would be fine as I expect it depends on the conditions.
interresting question for sure, I know from local DERM that oil seeps 10ft a year into the ground here, we have coral rock 6/12 inches under the top layer.
oil decomposing? is that possible? after all it takes a very long time and perfect conditions for the earth to manufacture crude oil.
in a landfill I would think oil would disperse instead of decompose
Motor oil is not biodegradable like vegetable oil. It won't decompose. After all, it's been in the Earth for millions of years before we pulled it out!
Landfills process their leachate. They all leak from above (rainwater) ..and are allowed to leak from their liners. Oil would not just lay in the landfill. It would probably be pretty clean due to the "ground filtering" when it ended up in their waste water treatment plant. Depending on the system, a skimmer should remove it from the waste water stream. Some even recirculate it. I think it's a method of allowing natural venting, from heating, to deal with the rainwater that invariably finds a way in. They're all vented and even power generators with the off gassing.

Some bacteria will attach to just about anything. That's how you remove some chemical pollution where removal is just too impractical. You take a sample, identify the bacteria ..and mutate it in a lab. You then introduce it into the site and either aerate it or turn/churn it ...eventually, you have no more waste ..other than the byproduct of the bacteria.

..but (short verson) ..I dunno. Good question.
I think the bigest problem is the contaminants in used oil, in addition to the additives present in virgin and used oil. I would not think that the base stocks are a big issue with regard to biodegradability.
Linear hydrocarbons are easiest for bacteria to digest, polycyclic aromatic ones the hardest, with branched (PAO) in the middle somewhere.
Hi, so if it was enhanced mineral oil for a car engine, roughly how long are we talking? One website I came across estimated 10 - 30 years for it to decompose. Does that seem reasonable?
I dunno. The better question is when does it no long presents an environmental hazard.

I don't know that answer either
That's a shame, as that was exactly my next question too - how long before it's no longer an environmental hazard?

Mind you, as I now understand it, contained in a modern landfill, any oil that comes out in the leachate is treated and only 'safe material' is released to the environment.
I've read where superfund cleanups were used to clean areas at a cost of millions of dollars but that microbes do as good as a job for free or very little cost.
Originally Posted By: Georgia
That's a shame, as that was exactly my next question too - how long before it's no longer an environmental hazard? Mind you, as I now understand it, contained in a modern landfill, any oil that comes out in the leachate is treated and only 'safe material' is released to the environment.

Landfills are routinely monitored for the content of their effluent. There are also monitoring wells at strategic points outside of the landfill to track any ground contamination. The outfit has had their waste treatment plan approved by the state's DER/DEP/whatever. They may detail specific features required from the facility, but don't provide the consulting for the actual design. Violations are handled with ever increasing fines. The fines are based on your environmental track record. Violations are called excursions.

It's all public record. I'm sure you could find out what the content of any landfill's effluent from your state's environmental agency.
It depends on the microbes present and the environment they are decomposing in. I know for a fact motor oil will decompose in a matter of months in an thermophylic compost pile. IT is carbon which is basically sugar to bacteria. If there is enough oxygen and nitrogen present the oil can be decomposed, In an anerobic environment it takes much longer.
Like myself, many years ago most people just dumped their oil in a jug into the trash. Shame on us. Same mindset as us that used to smoke. Pollute the land and our lungs.
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Thank you so much for all of your posts - they're really helpful.
About the zinc, phosphorous etc. - that's contained within the landfill anyway isn't it? So that wouldn't be going anywhere, would it?
If the motor oil ends up in a modern monitored landfill, it's not really that much of an environmental hazard then is it? From what I've been reading and finding out, unfortunately quite a lot still does end up in such landfills. Obviously it would be better to recycle it but if it does eventually decompose due to these microbes etc., then it is not an environmental hazard providing the landfill company / monitors etc. do their jobs properly and treat the leachate. Is that right? (Obviously I shall be recycling my motor oil!) Thanks for all replies.
Google "biodegradability OECD 301 B" or something similar and you will find your answer in detail, but in general, a group II base oil passes the biodegradability test, as well as a lot of the synthetics.
The generally accepted rule for claiming biodegradability in oils is what the OECD calls "ready biodegradability", and tests are frequently run to certify this. I supply oils that meet this to plants that export to Europe.

The problem lies in the additives and in the case of used oils, the contaminates. Many oils can be produced with environmentally friendly additives.
Here is an experiment that can be done using naturally occurring bacteria in soil to biodegrade oil:

Note that this is done with plenty of water, oxygen and nutrients (nitrogen and phosphorus). Under these conditions bacteria can biodegrade the oil quickly.

The problem with most landfills is that the conditions underneath the surface are anaerobic so things like oil breakdown very slowly. This is not a major problem as long as leachate can be collected and treated. But, even though modern landfills have clay and synthetic liners and elaborate leachate collection many still have some leakage to ground water.

You could try the same experiment above but put all the ingredients into a closed beaker and don't add any air. It would need a water trap vent to let gasses such as methane escape.
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