For those unaware, a biogas digester is a low energy device which is fed manure, and produces biogas and compost. The biogas is a mix of mostly CH4 and CO2, with traces of N2, H2, H2S and O2. The compost has about the same proportions of nutrients as the original manure, but in a form that's more absorbable by plants, and furthermore is nearly sterile -- almost all the pathogens that were in the manure are destroyed.

Using manure alone produces relatively low gas yields -- the main benefit of using the digester in such a situation is the improved compost.

Adding fats and grease to the digester can vastly boost production of biogas... adding 20% fats can boost gas production by a factor of 5. However, for a digester that's built close to livestock, it might not be economical to buy and transport fats and grease for this purpose.

The solution: Build, near the livestock, a small algae farm -- use a species of algae which produces lots of oil. Then, without separating the oil from the algae, move the algae into the biodigester. The algae get digested, oil and all, and biogas is released.

Most large biodigesters feed their gas directly to diesel generators, where are generally nearby. If this is the case, then the exhaust from the diesel engines can be fed back to the algae, providing a source of sterile CO2.

[edit] And if we're lucky, there will be enough waste heat from the generator to sterilize a portion of the biodigester effluent, allowing it to be used as fertilizer for the algae.

[edit2] Alternatively, if the biogas from the digester is upgraded to biomethane (and sold to the local utility company) instead of being used to generate electricity directly, then the upgrader will produce a stream of pure, sterile, CO2; naturally, this can be fed to the algae.
-- goldbb, May 24 2010

I misread this as a biogas disaster. Sorry.
-- pocmloc, May 24 2010


From the title, I thought bean was at it again. But then I saw coherent writing, and so did a double- take.
-- RayfordSteele, May 24 2010


How would beanangel have written this idea? :)
-- goldbb, May 25 2010


I am surprised that fats/oils augment biodigesters. It would seem to me that oils would float on top of the liquid phase mix.

I am surprised that biodigesters output O2. I thought they were strictly anaerobic.

I think not infrequently about biodigesters and their semiferal cousin, the landfill. Landfill gas is captured and burned on site as described but it does not seem to me that landfill construction is optimized for production of landfill gas.
-- bungston, May 26 2010


8/7, oops, sorry, didn't mean to delete your anno. And I had the funniest riposte, too.

bungston, Biodigesters output trace amounts of O2, somewhere in the zero to two percent range, according to Wikipedia.

Use of grease and fat as feedstock for biodigesters is not new.

I will admit that I couldn't find anything on oils as biodigester feed stock. I assume this is because if one has a cheap source of plant oil, it's usually more cost effective to extract, then refine that oil, and then burn or sell it. However, extracting and refining oil from algae is costly -- according to Wikipedia, about $2 per gallon, then another $1 per gallon to refine.

But if the algae can simply be digested, without extracting or refining the oil, the cost per unit of electricity produced might be much cheaper.
-- goldbb, May 26 2010


"Anaerobic" refers to the fact that the reaction does not require or consume oxygen; it has nothing to say on the issue of what is produced.
-- BunsenHoneydew, May 31 2010


When I read H2S as a byproduct I trhought, that can't be right. When I worked in the oil patch they told us that two parts per million and you'd be pushing up daisies. Looks like they lied.
Quite a difference between 2 parts and 800 parts per million.
-- 2 fries shy of a happy meal, May 31 2010


2fries, the presence of H2S means that you absolutely would not want to breathe an atmosphere of biogas :) Of course, since it's O2 content is so low, you'd probably asphyxiate before the H2S killed you.

On the other hand, biogas diluted with air wouldn't asphyxiate right away, in which case H2S poisoning might be your doom... unless a spark produced an explosion, killing you much faster.
-- goldbb, Jun 01 2010


I had an idea to make methane from algae on time. I wrote it down and thought the oil would come out the other side or would be broken down into methane also. But centrifuge would have to separate digested algae waste from the oil
-- travbm, Oct 29 2015



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