2 answers 4 2 ideas?

[Bob]

Hi Qcks,

Very interesting questions & ideas.

Often times people describe a "scum" that accumulates within an anaerobic digester. What is the composition of this scum? Is it organic waste or something else? if it is an organic, is fiberous material or is closer to a lipid? If it's a lipid, have there been any studies conducted on it regarding it's collection and transformation into a biodiesel fuel?

The composition of the scum will depend on the feedstock, and can include a combination of fibrous material and/or lipids -- essentially whatever is less dense than the liquid layer that will float to the top, or be transported there by the rising bubbles. Over time, unless the digester is continuously mixed or wetted, this floating layer dries out and hardens into an impenetrable mass.

I suppose you could look at removing the scum and processing it into biodiesel (assuming it's content was suitable), but my own preference would be to try to prevent formation of the layer in the first place, by mixing it.

Most of the material i've found online seems to indicate that the Archea are more delicate, and produce a lower nitrate containing solid, thus making them less suitable for methane production and less profitable overall. Specifically, i'm interested in finding out why sediments from archea systems are lower in nitrates then their prokaryotic counter parts, and if this effects there tolerance of nitrogen.

Interesting question. I wonder which came first, the nitrogenous sediment, or the organisms that feed on it.

[Bob]

i honestly can't tell if your being facetious or not

No, not being facetious, but probably displaying my ignorance.

...but continued research on the web seems to indicate that the methanogens seem to need nitrates.

Yes, of course -- which is implied in the need for proper C/N balance. I don't know the details of specific requirements for specific organisms, but a 30:1 ratio has long been established as a general rule for the aggregate.

So much so, in fact, that some seem to have the ability to fix atmospheric nitrogen.

Interesting. I had not heard that. How would obligate anaerobes get it from the atmosphere? From facultative intermediates?

While i doubt the archea within a methane generator will ever be in short supply of nitrate

Usually true, but of course depending entirely on feedstock. Back to the 30:1 C/N ratio. Any feedstock with manure (i.e. not just leaves, straw, sawdust, paper, etc.) would probably provide the required amount of N.

it implies that they're better suited for use in systems in which there is no urine seperation.

In my experience, the urine content tends to lower the ratio below optimum. There is still plenty of N without it. But there again, it will always depend on what you are trying to digest.

[Bob]

Further explanation of my earlier question (that you thought might have been facetious).

I wonder which came first, the nitrogenous sediment, or the organisms that feed on it.

In the natural world, when one wants to know something about the organisms that live in any particular environment, usually the first, best (maybe only) answer, until detailed analysis is done, is "Those that feed on X." "X" being the nutrients and organic material found there.

In other words, if a nitrogen-rich environment is found, one would expect to find organisms there that feed on it. Same thing with any other nutrient or substrate.