Anaerobic digestion

by John Cossham 11th April 2011, 7:35 pm

Mr Sfstk8td said

OK, so I got turned onto another question from the thread on SA changing
rules on compost and digestates from food waste and household green
wastes. At first I thought that by digestates they'd meant poo, but
when I read the article in detail, it appears they mean anaerobic
digestates. Well, that makes sense.

So, the question is, what
are the benefits of an anaerobic digestate? I know that fermented
greenstuffs are used for cattle fodder, and this is a type of anaerobic
digestate, but I'm sure what this article is talking about is, at least
functionally, different. The silage doesn't completely break down the
plant mass, just starts the rumination of some of the complex
carbohydrates a bit early, so to speak. The churned, turned and
temperature/humidity controlled industrial anaerobic digestates is a
different kettle of fish though.

What advantages does anaerobic
digestate present for the smallholder? Is it worthwhile to seek a
source for ammendments of this type? Does it make sense, or is it even
possible, to make anaerobic digestates on a small scale? Oh dear, I
believe I've just opened up a new niche for myself.

The thing about AD is that the process extracts usable energy from the materials. Although I'm a fan of composting (and aerobic process) it does only result in a)humus to add to your soil, with some slow-release nutrients, and b)reduced landfill.

But with AD, you get a load of gas, a mix of methane (CH4) and some CO2, H2O and some others, smelly stuff like hydrogen sulphide and mercaptans. This can be cleaned up, leaving the methane, which is able to be burnt to generate heat (and boil water to spin turbines to make electricity, or just boil your kettle at home, or central heating etc) or to go through a fuel cell to make heat and electricity.

And you're left with the digestate, which can be composted aerobically and forms a compost, I'm told, much like any other.

So, what did you want to know?

by mr_sfstk8d 11th April 2011, 8:21 pm

So the AD itself then must be composted for garden use, not applied directly?

There are a few very large corn product distilleries in my area, mostly ethanol for fuel, some potable, i.e. vodka. I was wondering if this steaming pile of mash was suitable to use directly or would take some further post processing before going into the veg patch.

by John Cossham 11th April 2011, 10:12 pm

I think that the products of AD should be aerobically composted with other materials first, before use on the garden. However, I have no experience of using AD digestate. I did once get about 200kg spent hops from a brewery, these composted well mixed with the usual diet of shredded twigs, food waste, greengrocers 'resources' etc which I get several times a week.

If you're really interested, I could post the question on the US Composting Council enquiries list that I'm a member of (despite my being in the UK) to ask if raw digestate can be used. PM me if you want me to do this; we can post any answer here, in public.
John

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