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Cost -v- Efficiency -v- Availability

When building an anaerobic digester for use in poor, rural communities in Kenya, the first concern to be addressed must be safety – obviously. We are producing a flammable gas and using feed stock that could contain all sorts of bacteria.

With that issue in mind, the next two issues are efficiency and cost.

In our first full-size digester, we used a 45 gallon oil drum 970mm high x 580mm dia), into which we put cow dung and water, leaving a gap at the top of only 25cm. Once sealed, the top of the drum quickly domed under pressure, but it has always managed to hold the pressure.

But we realised that we either needed an expanding digester tank or a gas storage facility.

The major expense when building an anaerobic digester is the container(s).

In Kisii, a single 45 gallon oil barrel, with two threaded outlets in one end (2″ and 5/8″) costs Ksh 2,000 (£17.25). (I can buy these in the UK for £6.20 or Ksh 720).

The ideal, which I have not been able to source in Kisii is a drum with a lid and clamp:

45 gallon drum with lid and clamp

Smaller plastic, cylindrical  water tanks cost a little less, but would not hold pressure, so two would have to be used to make an expanding tank, or a digester and storage tank.

When I started on this and similar projects, I imagined using scrap or discarded materials. Unfortunately (for us), there is very little useful scrap in Kenya. It is all used by the locals. Nothing is left to waste, and if someone does find something discarded, they will recuperate it for possible use at a later date.

That has dealt with cost, to some degree. What about efficiency and ease of use?

With a good anaerobic digester, all the user needs to do is turn on a gas tap and light the burner. But after about six months of use, the digester needs to be emptied and refilled. It then needs to be allowed to re-boot before any usable gas is produced. This can take up to a week, depending upon the weather in the area. In Equatorial Africa, this could be a couple of days.

Then there is the question of actually emptying the digester. The 2″ hole in the end of a 45 gallon barrel is not ideal, either for loading or emptying. Plastic water storage containers, with their large opening in the top would make this task a lot easier.

However, if the standard oil drum is to be used, I think that it should be horizontal with the 2″ hole as low as possible.  The drum would receive more direct sunlight and it will be easier to empty – probably.

I still believe that the best  and safest design is our Mk II comprising two drums (digester and storage) and a header tank to provide pressure, water scrubbing and a safety outlet should an excess of gas be produced, but this model would be expensive.

The cheapest, using only one drum, is not efficient.

The Mk II would suit commercial environments, hotels, safari camps, etc., as they can afford a little outlay to produce what is effectively a free fuel that can be used for cooking and water heating.

So, it is back to the drawing board and calculator when I return to Kenya.

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