my first biogas digester

[Ivan]

I've finally made one! or started, at least.

Digester - 20litre plastic container that some waste veg oil came in. Semi-transparent so you can see what's going on. Sealing screw top lid, that I will conect some pipework through. Intended for use as a batch digester.

Substrate - grass clippings, a few weeks old so starting to rot (only because I cut the grass a few weeks ago, not for any particular reason). Approx 1litre of fresh cow poo. Smelly, still semi-liquid and not crusty (methanogens hopefully not started to die off yet).

Procedure
Add about 200ml glycerine waste to tub to give it a good wash out, to remove the oil residue, crusty chip residue etc. Dilute with water. Fit lid, shake, empty. Add grass clippings. Approx 1/3 fill the container. Then add cow poo: pour from ice-cream tub, using a large twig to push it into the container. Fill to 3/4 full using tap water (probably should use rain water, but tap water was more convenient). Give a good stir using a short length of old 15mm copper pipe, screw on lid to seal.

Today, it's exactly one week old and I had to unscrew the lid enough to relieve the pressure, as the container had started to inflate like a balloon. If you kick the sides of the tub, you can see bubbles rising to the surface. Haven't tested the gas yet - you need to relieve the first two volumes to avoid potenial for explosive mixture. Looking good so far, though! I can post a pic if anyone wants to see.

[Ted]

Interesting. I've got a whole bunch of 20 liter plastic containers I could use for this too as well as access to plentiful supplies of the prime ingredients. But is there any (easy) way to store the gas under pressure so that it can be used some time after it is produced and maybe also reduce the storage volume needed at the same time?

[Ivan]

I don't know what pressure the methanogens will tolerate. From my wine-making days, I remember that yeasts will happily work up to 100psi or so, but whether methanogens would, and whether they would alter their efficiency in doing so, I really don't know.

Compressing a flammable gas is a safety issue, and really you should only use certified pumps for flammable gases (=expensive). I remember with LPG, we used to use diaphragm pumps to good effect - the sort used for crop spraying, or general liquid pumping. They weren't designed for flammable liquids but never gave any problem.

Most biogas digesters use either a big balloon of some kind (mylar works well), or a floating gas holder (mini- gasometer), and store at only slightly over atmospheric. It's low BTU gas, so if you are storing from summer to use in winter, I suspect you'll need quite a large volume

[Twenty4Seven]

I have a steady supply of byproduct from my biodiesel plant which I understand is suitable for use in an anaerobic digester..... This could be the answer to the vexed question of what to do with it..... Not sure if the spent material would be any use as compost though if NaOH was used in the biodiesel process.... might have to switch to KOH.

Nick

[Ted]

It's low BTU gas, so if you are storing from summer to use in winter, I suspect you'll need quite a large volume

This was exactly what I was thinking of, produce over 12 months and consume over 6. Do you have any idea of figures for BTUs?

[northern installer]

Come on Ivan! lets see the pics?

[Ivan]

Here it is. If you look closely you can see some bubbles of gas against the plastic side of the container

[frotter]

Mmmmm...... bloated.

 

[Ivan]

I think that's a good description. Incidentally, it was working well for a few days, but ambient temperature has dropped over the last couple of days - really quite cold today, so output seems to have slowed. Need some more sunshine.

Ideally, an insulated oil drum with some solar heat input would work well. Or maybe even an SFA solar panel -mine in the back garden rarely drops below 40C, except when it gets very cold, although you'd need to do something to prevent overheating - it can boil water all day in sunny weather. Perhaps the SFH would be a better design, as we don't really want slurry going into the tubes.

[welshboy]

Hi Ivan,
Inspired by the concept (fuel price is the impetus) of a digester I have been trying to calculate the size of a digester required to run our aga on Biogas. The calculations go as follows.
18 therms week required (source Aga website).
100000 btus per therm.(source wiki)
 therefore  18 *100000 /7 /24 = approx 11000 btu hour.
 At 900 btu cubic foot (source Fry Article) that is about 12 cubic ft gas hour.
At 50/50 cow poo/grass that is 4 cubic ft per pound of material- so about 3 lb an hour or say 75 lb day.
 As an approximation this would be 5gallons cow poo, 12 gallons of grass (Source below)and 17 gallons of water.
34 gallons a day over a cycle of say 30 days = 1020 gallons of storage in digester.
assume digester is only half full (maximum  surface area to avoid crust developing).
then 2040 gallons capacity. 
2040/6.25 = 326 cubic ft.
Or about 9.5 cubic metres
Any need to factor in efficiency ?  Any comments gratefully received even negative ones-

PDF] Volume-to-Weight Conversions for Recyclable Materials
9k - Adobe PDF - View as html
800 pounds / cubic yard. 2,3. Hand stacked. 35 pounds / 12 inch stack. 2, ... Organics - Weight Conversions. Grass Clippings. 667 pounds / cubic yard. 4. Leaves ...
www.mass.gov/dep/recycle/approvals/dsconv.pdf

[KenB]

Ivan,

How about a modular system based on standard 1000 litre IBCs? 

The wide filling neckon an IBC would allow a solar water heating coil to be fitted easily.

The gasometer could be made from a couple of 1800 litre "Roto" bulk orange juice containers.

An inverted IBC, fitted with a 4" grey soil pipe as the filler tube could be "topped up" with additional waste as required, with the smaller drain-cock outlet being used for gas exit.

Biogas will be about 60% of natural gas calorific value because of the CO2 impurity  - so about 7kWh per m3 (methane is 11kWh/m3).

Using cow dung slurry as a starter,  grass cuttings and other shredded biomass will give a much more efficient digester.

Have a look at the work by Dr. Anand Karve of the ARTI institute -

http://www.arti-india.org/content/view/45/40/

Don't try to reinvent the wheel. 



Ken

[KenB]

Welshboy, - EDITED with some new information -I had a rethink about the scaling factor 

The 180,000 BTU per week Aga is 527kWh of natural gas per week. That's 75kWh per day roughly.

Natural gas is about 11kWh per m3, so you would need approximately 48m3 of methane production per week -probably about 55m3 to account for the lower calorific value of the biogas..

The ARTI gas generator produces 500g of gas per day, 1m3 of gas is 667g.(about 8.25kWh) from a 1000 litre vessel.

However this is the gas storage vessel, not the size of vessel needed to house the 15 litres of water plus 1.5 to 2kg of feedstock.

By this estimation you would need to scale the feed requirement of  ARTI system by a factor of 10 to supply your daily demand - this could be done using a 1000 litre IBC as the fermentation vessel with a separate IBC to act as the gas storage buffer.

With several of these modular IBCs  you would rotate them so that some are being refilled when exhausted whilst the other ones are gassing. With a 3 way rotation you might achieve this with 7 IBCs one for each day of the week.

The idea of an IBC mounted in a steel swivel frame so that it could be easily inverted is one suggestion. 

Half fill with the grass cuttings, foodwaste etc,  top up with warmed water to expel the air and invert.  As the gas is produced it bubbles to the top and displaces the water through a "piddle tube" or one way vent.  The gas outlet is via the drain cock.

The spent digestate is strained and then composted, the liquid is used as liquid fertiliser.




Ken

[mikethebee]

Ivan, you inspired me to suggest to my cousin that he tries the same thing, but how is yours doing? Not heard much recently :-/

[Ivan]

It's very weather dependant. It seems pretty obvious to me that it needs to be upper 20s temperature to work, or even low 30s. When it gets a good morning's direct sunshine it seems to bloat up, but in the weather we've been seeing recently, it doesn't seem to make much gas. I think a nice project would be a hot water cylinder as the digester (ready-insulated), heated by solar to the mesophilic range (50C - 60C, I think - most efficient production of gas, and fastest at this temperature), using the immersion port as the loading port and maybe a retro-fitted immersion port (you can buy them from BES, I think) at the bottom for effluent to flow out. The hot water ouput would be the gas output. You'd need to work out a method of stirring the mixture daily. Most biogas digesters do this with a mechanical paddle, but of course it's got to be sealed to prevent gas escaping, so not easy. The alternative, would be to recycle some of the gas, feeding it into a bubbler mounted in the bottom of the tank (via cold water inlet). Once a day, a decent volume of gas is recycled by a reasonably powerful pump. This gas would disturb and mix the digestage sufficiently to break up the detrimental crust which forms on the surface.

I haven't got time at the moment - go on, you know you want to build it!!

[mikethebee]

Wow, for a man of little time you have certainly been thinking 'design'. Sounds workable, but we wanted to reuse parts around the farm, rather than buy new.

Maybe the paddle could be turned mechanically by the gas pressure flowing through a 'turbine' of some sort. The temperature thing is a problem, solar needs sun too. Maybe compost heap heat (CHH) could be used. We'll give it a go anyway.

Have you tried using the gas yet?