Biogas digester

[jason]

hi all
we are planning on building a biogas digester for our domestic property in devon we have 3.5 acers of grass and a futher access to another 7 dose this seem like a possible propposal these are wide spread accross india and china. so please dose any one have any ideas or infomation or maybe even plans on such a unit.
hope that this is not a pipe dream.
many thanks
jason

[martin]

I'm interested too! - John Seymour included full plans in his excellent "Self-sufficiency" book - originally written  in 1973 - there's a new edition of the book out now!
http://bookshop.blackwell.co.uk/jsp/id/0751364428/New_Complete_Self_sufficiency_The_Classic_Guide_for_Realists_and.html;jsessionid=3825F312E333F273FC5BEE06BBD79039.bobcatt2

[mick]

Jason

Not a pipe dream - I have seen biogas plants in India - few buckets of cow dung per week supplying gas to cook for a whole NGO (about 40 meals a day). If I had your amount of space I would probably devise one linked to the loo - get gas + fertilizer + solve sewage prob!

My starting point would have been Intermediate Technology's bookshop in London but it appears to have changed - now Practical Action focussing on development (see  http://practicalaction.org/?id=biogas ). They will still have something and will give you links.

Also see http://practicalaction.org/docs/technical_information_service/biogas_liquid_fuels.pdf - has useful refs in it.

'Half-developed' countries like South Africa have well developed biogas plants especially in agriculture.

Should be fun and not costly to build - don't know about byelaws re: gas production though

Regards,

Mick.

[jason]

 I knew it excellent site just ask the question and excellent responses thanks guys now time to do the home work
 will keep you posted
 thanks jason

[mick]

Jason,

I have continued to research through Google - I am inspired -  can produce electricity via gas engine as well as use as gas for water heating / central heating, cooking, etc - I imagine neighbourhood schemes that deal with all organic waste as well a sewage, returning gas / electricity to householders, etc, etc. See  http://www.oaktech-environmental.com/arrowbio_process.htm  for an up and running system that every local authority should be installing now!

Phew!

Found a lot more - I put in 'biogas human' !

Regards,

Mick.

[Ivan]

This is, in my opinnion, the one of the most definitive works on biodigesters. It is very old - early 70s I think. This is the text of a long out-of-print book written by L. John Fry, a South African Pig farmer who pioneered biogas in the early 70s. He wrote a series of books on biogas, this is one of them.

http://journeytoforever.org/biofuel_library/MethaneDigesters/MDToC.html


If you want a bit of historical info, you can find it here:

http://journeytoforever.org/biofuel_library/MENintvus/fryintvu.html


NOTE: As with all great inventors, he was BRITISH!!



Ivan

[jason]

OK my idea is to run may be a 5kw petrol generator  (watercooled) on biogas to run my navitron 9kw gshp along side 1kw of pv and a 1kw wind turbine and sell any excess to the grid and use the heat from the generator to preheat the water for the heat pump. most of the system is in place and just the biogas system to crack so thanks all for your help and direction.
jason 

[KenB]

Jason & List,

There have been some tremendous advancements in biogas reactors recently when you use starchy materials rather than cow dung.  Gas production is 20 to 40 times better.

Cow dung is almost free from any calorific value - the cow's efficient digestive system takes care of that!

Have a look at the reports bt Prof. A.D. Karve of the ARTI institute in India - well worth the effort.

http://www.ashdenawards.org/projects_2006_photos

http://www.ashdenawards.org/media_summary06_india_arti   for details

If you have a local baker nearby, you could offer to take the waste flour sweepings off his hands.  1 kg of waste flour will produce as much gas as 40 kg of cow dung - I know which I'd rather handle.   Other feedstocks include foodstuffs that would normally go onto a compost heap.


Ken

[Ivan]

The other option is to cut-out-the-middleman ie the cow.

There must be plenty of opportunity to run these on grass clippings at least during summer months in the UK.


Ivan

[jncg]

Ivan has hit upon the issue with small scale anaerobic digestion, you need a reliable and consistent supply of organic material and there are massive industries set up to cater for the care and harvesting of grass on any scale in this country, if you're lucky you'll have some growing outside your house.  Taking Ken's example, by using raw (not previously digested!) grass clippings as a base feed material you would only need 5kg of grass to give as much biogas as 40kg of cow slurry though massive variables such as water content and the cow's diet affect this slightly.  It just shows how efficient the cow is too.

Another thing to bear in mind is that you need a carbon rich, slower digesting 'carrier' such as grass or cow slurry.  If you tried digesting just flour, it could ultimately get very messy and overall efficiency wouldn't be great either.  Consider yourself a digester: if you eat buckets salad all day for a week and then ate just bread for the weekend, how would you feel on Monday morning?   

[dandoc]

The part which puzzles me on doing this is, having built your inner tube digestion unit, what do you
actually do with the gas?  Assuming you've gas units in your house - cooking, heating or whatever,
can you connect these to this unit easily and do they need rejetting?

Anyone better in the know?

cheers
Danny

[Ivan]

The gas has a lower calorific value, different gas/air burn ratio and a much slower flame speed - so you will need to rejet things to get it to work. In fact, it will burn from non-rejetted appliances, but from what I remember, it will have a habit of blowing out. There are some tips in L.John Fry's books for rejetting, and I am sure it is covered in the other texts mentioned.


You can use the gas for: cooking, heating, lighting (gas mantles) or running an engine/Combined Heat and Power unit. (It's use for lighting tends to only be in developing countries which do not have a source of electricity)

Ivan

[dandoc]

The gas has a lower calorific value, different gas/air burn ratio and a much slower flame speed - so you will need to rejet things to get it to work. In fact, it will burn from non-rejetted appliances, but from what I remember, it will have a habit of blowing out. There are some tips in L.John Fry's books for rejetting, and I am sure it is covered in the other texts mentioned.


You can use the gas for: cooking, heating, lighting (gas mantles) or running an engine/Combined Heat and Power unit. (It's use for lighting tends to only be in developing countries which do not have a source of electricity)

Ivan

Thanks Ivan - that's interesting.  We have a septic tank which takes waste from our house and next door - five people in all, all adults.  i had wondered whether there was any scope at all for using that and organic waste from such as grass clippings, but it looks like the amount of gas this would give would be pretty low.  Given that the only thing we use gas for, is for cooking, it sounds quite a bit of effort for relatively little gain unless the grass clippings make any difference.

i'd be interested if anyone has comments on these observations :

I have a book which I think is Fry's which suggests that you can get 1cu ft of gas from the organic waste which a human creates per day.  Even assuming that's all methane, that's only around 1000 BTU as i understand it.  Given that 1KW is around 3400 BTU/hour, we'd get the equivalent of around 1.5KWh worth of gas.  Unless I'm missing something, that's barely getting started, barely useful.  We have a plot which is about half an acre - we could probably run to a few chickens, but anything bigger than that such as pigs/cows is unlikely (Fry suggests these can give 9 and 8 cu ft. of gas from each per day).

Seems a shame if this is so pointless - we have easy access to the slurry from the septic tank after all

Danny

[Ivan]

Yes, the yield from a few humans is pretty low....enough to cook a daily meal, in India. However, if you add the grass-clippings (3.5acres is a lot of grass!), foodwaste and a few animals I think you would be talking quite significant amounts.

However, your septic tank has been designed to process the waste from a few humans - if you add all the grass clippings, it would no doubt overload it.

Has anyone seen any designs for modifying existing septic tanks to biogas?

Ivan

[jncg]

I must admit that I don't know the ratio of water to solids entering a normal domestic septic tank though that's where you'd have the most trouble to begin with.  Anaerobic digestion for reasonable gas yeilds requires the process to be accellerated by bringing the material, slurry, up to broadly body temperature.  When you bear in mind that only the organic solids and dissolved sugars in the slurry are making the gas in the first place, the water is just there to carry it all.  You will end up with the perennial AD problem of heating excessive quantities of water to extract the energy from the solids.

To give an example, on a dairy farm a continuous feed digester (as opposed to a batch process digester) will cope with all the animal waste and dairy wash water (including diluted chemicals) but if you have a shed roof or yard draining into the slurry store, it can easily tip the balance from making more gas than the process needs to sustain itself to the opposite:  There is insufficient organic material to make the gas required to heat the incoming slurry... So the digester temperature falls away and you risk needing to use alternative heat sources to maintain digestion, let alone provide a gas surplus.  I should point out that this example assumes the gas is used primarily to sustain the digester at temperature with the excess going to alternative uses. 

You can digest materials with lower water volumes but it's best to imagine an anaerobic digester as a stomach - as such a slurry is the best bet.  Best to be looking for about 10% total solids by weight for decent efficiency.

I suspect you could use a heavily modified septic tank in a purpose built, dedicated digester application.  You also have to bear in mind though that if you use human sewage in a digester and suppliment it with other organic material you'll still have the digested material (digestate) to dispose of.  This would be much easier if it didn't have the sewage content.  The only way to make this safe for use as a fertiliser on the ground would be to pasteurise it - massively energy intensive.  As Ivan says, if the digestate were to go from a digester to a septic tank you'd quickly overload it.

On another note, regarding the use of biogas in open burners. This isn't wise indoors as there is often a (ppm) sulphurous component to the gas.  This leaves a bit of a taste at the back of the throat and is a bit eye-watery if the exhaust from a burned accumulates in a confined area.  Like a typical UK kitchen...
 A storage type closed appliance is ideal to overcome this as it keeps the smell out and allows for fluctuations in gas production.  Rayburn, AGA, Esse etc are ideal.