My compact biogas dragon!

At the beginning of the year 2016, I decided to try and make a Biogas system to convert my household food waste and manure from my small scale rabbitry  to biogas.

I searched the internet for ideas and came across the Solar CITIES IBC system developed by T.H. Culhane.

The challenge was that I did not have enough space to fit the IBC tanks in my small backyard. Secondly the IBC tanks here in Nairobi ,though readily available, were a little too pricey for what I considered as "my experiment".

I then decided to try the same system using 200 litre HDPE plastic containers that were easily available where I live in the outskirts of Nairobi Kenya. The same had been done by Shaun in Australia and documented in his blog here.

My goal was to achieve the plan below:

It seemed easy enough at the start: just a 2inch PVC pipe, some fittings and a 200 ltr blue HDPE plastic drum. What I was not aware of was that sealing and glueing PVC to HDPE plastic was not as easy at it may appear. Firstly, I did not have access to uniseals as documented by Culhane for the Solar CITIES IBC system. I had thought super glue and sand will do the trick. However, I came to learn that PVC does not chemically bond to HDPE plastic! The holes where the PVC pipe enters the drum, needed to have been perfectly smooth and very tight. Suffice it to say that mine were not so perfect. What followed was several weeks of leakages and frustration!

To correct the imperfectly cut hole I tried to weld the HPDE with a soldering iron. It reduced the imperfection but did  not fully eliminate the minor leakages. I tried some silicon sealant, but somehow the slurry would sip out, albeit ever so slightly. Utter frustration!

What finally worked was a special glue from india that I purchased on ebay called M-Seal. It is a play-doh like 2-part epoxy that  contains two small packets of compounds that react when kneaded together.  It hardens to a very solid waterproof material.

Lesson Number 1: Clean and airtight or... just get a plumber :
Fixing leakages once the drum is full of manure is a  messy and difficult affair.  A biogas digester must be 100% airtight. No short-cuts here. Some plumbing skills are needed to pull this off. To avoid messy leakages and a subsequent epic fail to this project, do the following:

• Before you start the digester construction, clean the drum to be used thoroughly, especially if it had industrial chemicals. Soak with detergent and water for a day or so then rinse and leave it to dry for a couple of days.
• Pipe insert holes should be  clean, dry, cut perfectly smooth and very, very tight fitting. In short, all the pipe fittings to the digester should be done meticulously well. Quality plumbing work is a must here.
• Before putting the manure/water slurry mixture, test for leakages. Do so by first closing the PVC inlet and outlet  pipes tight. Sprinkle soapy water onto all the pipe joints and areas where the pipes enter the drum. Now,  blow hard into the gas outlet valve. If there are any leakages, they will create bubbles. Fix them. If you find blowing too difficult, you can improvise with a foot/floor pump used for bicycles or whatever works for you.  Repeat the checks,  until it's 100% gas/water tight . There is no short-cut to this.
• Position the drum where you finally want it to stay permanently before filling it. It is very heavy once full! 

Anyway, back to my story:  

Finally, the leakages stopped and I painted my digester black and properly attached a floating, gas storage tank.

The little grey pipe suspended in the air  is as a filter for Hydrogen Sulphide(H2S) (a poisonous foul smelling gas that occurs in trace amounts in Biogas). It is purely optional to add this filter to a biogas digestor.  Its is just a 2 inch PVC pipe, about 1 m long, filled with compacted regular dish washing steel-wool obtained from supermarket store. The idea is that as the Biogas passes though it, the traces of  Hydrogen Sulphide in it reacts with iron oxide in the steel wool to generate a salt compound and water. Ideally, the steel wool, should be damped with a little water and left to rust a little.

There is a long story before the perfect scenario above came to be. Well, at first before I noticed any leakages, I had added about 20 litres of horse manure (which I obtained from a stable near our church) mixed at a ratio of 1:1 with water and topped the digestor with water. I waited for a couple of weeks. Nothing! No gas! Sometimes on a hot day I could feel some bubbling activity inside the digestor. That's when I noticed the minor leakages and  started to fix them. A few weeks went by, a little leak here, a little leak there, leakage fixed. Still, no gas!

I decided to try cow dung instead. I got some fresh dung, about 30Kgs, from a near by Maasai manyatta (homestead), mixed it with water at a ratio of 1:1 and put it in. It was more than 60 litres, I think. It pushed out an equal amount of the horse dung slurry that was already inside. Which I drained into the sewer system. Then I waited for a week. Nothing.  Ten days, nothing. I noticed a few more leakages which I proceeded to fix. After I had completely eliminated all the leaks, still no gas! A month went by and still no gas. I read all I could find on the internet about the system.

Following my "further internet research", I decided to get some 10 ltrs of affluent from a working Biogas plant upcountry to "kick start" the system. As a result, there was a brief but aggressive bubbling activity in the digester for a couple of days and then it stopped. Unfortunately, I had not sealed my storage tank properly and the little gas it produced leaked away. I sealed that too. But then, the system just went dead. No bubbles, just .. nothing.

I finally gave up on it and just left it alone.  After all, what else could I do with a sealed  200 litre drum full of  soupy cow/horse manure?

About 3 months or so went by. I had completely left the unit unattended, having given up on it. Then one day ... I noticed the floating drum had risen slightly.  I decided to test the gas. Voila! it was flammable!!!

It was sheer joy and excitement for me! Gas at last!

Lesson Number 2: Patience
Biogas generation is a natural but complex biological process. Once the bacteria is introduced via the manure slurry into the airtight, oxygen free container at room temperature, it requires no other input from anyone until their food runs out. Period. The setup has to be just right for the anaerobic bacterial  microbes called methanogens and acidogens  to multiply and balance out before they can start producing flammable gas. It's a delicate equilibrium. Only time will allow it to be achieved. So, in summary:

• The drum must be clean and free of industrial chemicals.
• Let the bacteria do their thing!  Avoid the urge to add PH correcting chemicals. In short, NO CHEMICALS PLEASE!
• In theory, it should generate flammable bio-gas after 1 or 2 weeks. However, depending on the environment in and out of the drum (i.e. manure source, temperature, leakages e.t.c.), it could be a month or more. Mine took more than three months! In retrospect, I now understand that the delay in biogas  production in my system must have been a result of some bacterial and chemical imbalance inside the digester that could only be cured by time.  What is called a well buffered state.
• Do not add Kitchen food waste until the first flammable gas is produced. The mixture may just get too acidic and take even longer to settle. Even when it has finally produced flammable gas, add only a small amount(of mashed kitchen food waste) at a time. Just 1/40 of the volume of the digester. I only do this once in three days. 
• I personally found that horse manure is harder to work with as a starter. It's too course, and can clog the inlet pipe easily. Cow manure is way smoother and better because cows chew cud. 
• Finally, be patient. Very patient!

My Biogas storage tank(floating blue drum) full of gas!
Notice that  I used bangee cords to stop the floating drum from tipping over.

My full storage tank is about 180 Ltrs and can run for about 45 minutes on a single burner. When the the digester is under warm sunny conditions it can generate enough biogas to fill the tank in about 2 days or less. In cold conditions it can take 4 days or more to fill it.

The Biogas produced is not compressed and hence it cannot use a regular LPG burner as it is. The high pressure regular or restriction found on LPG gas stoves has to be modified slightly for it to work with Biogas. I purchased a cheap cast-iron single gas burner from the super market store. Using a small steel nail of 2mm diameter, I widened the gas outlet hole. Suffice it to say that once modified for bio-gas it cannot be used any longer for LPG!

Clean blue flame! This modified burner can no longer be used for LPG because the outlet hole has been increased.

I also reduced the size of the burner air inlets by covering them slightly to stabilized the Biogas flame.

It's enough to cook some tea or a small meal of pasta.

The foil I placed under the burner just helps to concentrate the heat onto the cooking port.

Lesson number 3: Bigger is better
The digester produces gas fairly continuously. When I don't use my Biogas it accumulates. It is advantageous to have a large storage container. Otherwise the gas will soon bubble out from under the floating drum once it reaches it's maximum capacity.

• Get the biggest practical floating storage drum you can work with.
• 200 lt of biogas is about 30-45 minutes on a single burner. 500 ltrs will give you about 1 and a half hours worth of cooking. 1000 litres of biogas is well over 2hrs of continuous cooking! Make your choice!

The final working setup.

Update -27th July 2016

My attempt to increase the gas storage: the tee section feeds into an old lorry inner tube.

I added an old lorry inner tube and placed it in a protective polythene bag. There is a weight pressing against it on the opposite side(not shown). I hope it works!

Update -5th Oct 2016

A few weeks ago, while feeding my digester with kitchen waste, I added some spoilt sour milk and  it stopped producing gas by the next day! I think it became acidic.

I tried to add a couple of spoons of  baking soda and it seemed to have solved the problem but the gas production became very slow. I decided to live it alone without adding any new feedstock to let it resolve itself with time.

Sure enough, it gradually started producing a reasonable amount of gas again after a few weeks. By  yesterday it looked like the good old digester again!

Lesson learnt: avoid adding acidic food into the digester.