:::: MENU ::::
Biogas Digester Design Project

At Nyasanda Community High School, in Ugunja, Kenya
2007-02-02 – 2007-03-15



By Karolina Hagegård
Master student of Biotechnology
At Chalmers University of Technology, in Gothenburg, Sweden

1         Abstract

This project resulted in a roughly 400 l low-cost plug flow biogas digester, made all in plastic, from polyethylene water drums, PVC sewage equipment and some other details like PET bottles. There was no skilled labour involved in the construction, and the materials were all locally available. The bearings leaked, but if that problem could be overcome, it seems this type of digester could be very feasible for rural African families to build and use, to produce cooking fuel and organic fertilizer for their household needs. The total cost for it, including gas container and insulation in the form of a little clay-straw house (but no stove), would be a little over 20,000 KES, or a little over 220 Euro.

Other results that came out of the project were a very serious Biogas Club in the work site school (who had  gained a lot of knowledge about biogas and digesters), interesting contacts between the school and e.g. the Anyiko Women’s Groups, biogas digester constructor Tobias Omoro, local development worker Benard Ochieng and the neighbouring technical school Nyati, and there was a lot of biogas interest spread all around as the writer of this report promoted the idea and handed out biogas instruction kits to anyone who was interested.

This is the report about a six weeks project at a secondary school in western Kenya, to design and construct a low-cost family size biogas digester that could later be constructed and used by the rural population in the neighbourhood, and in similar areas.

The report was written on my own initiative, and is mainly for documentation of what was done and what was learned, since this project will later serve as a part basis for my master thesis, Small-Scale Biogas by Lake Victoria. I have kept it rather informal, but still written it as a proper academic report, so that it should be able to serve as help for others wanting to go on with the work that we did. The chapters Results and Project Budget are mainly for those who wish to try and make our designed biogas plant work in reality, and then especially the subchapter Discussion about the design. The chapter How the work was carried out is about the project itself, the story of how it all happened. It is meant to be useful for anyone who wishes to do similar project work in similar conditions to these, and is also quite nice reading!

4         Background

The project described in this report has many roots, and there are many people that have been working, and will be working, in more or less strong connection with it for a long time. The key driving force behind it, though, can be said to be the fact that the area in which it takes place – rural Western Kenya – has a crucial need to find an alternative to firewood as cooking fuel. The importance of stopping the grave deforestation in the area has been pointed out by for example Nobel Peace Prize Winner Wangari Maathai. But the biogas technology also has other advantages than just serving as fuel for stoves, lamps or cars: It can also hygienize waste such as butchery waste or toilet sewage, and the residue serves as an excellent organic fertilizer, more nutritious than synthetic fertilizer. With these features, biogas has an interesting potential of becoming the key ingredient in a full shift towards a flourishing sustainable society.

Having realized these benefits, Swedish biogas expert Björn Martén has, together with his Kenyan contacts, for many years tried to bring the biogas technology into the Kenyan countryside, and make it a part of development projects that are already growing in so many places of this buoyant country. A pilot biogas plant built in Björn’s secondary school in Sweden was placed at Nyasanda Community High School (from now on called Nyasanda High), the project site of this report, in agreement with Ugunja Community Resource Centre (UCRC). The school has at times had it running, performing some experiments with it, and cooking tea with the gas, but now it was broken and leaking and had not been in use for a long time.

In this particular project, I wanted to build a biogas digester on the spot in Kenya, to make sure that it could be done easily, with locally available materials and at a low total cost. I had been involved in Martén's biogas work in Kenya once before, experimenting with insulation of the digesters, and working part of my time at Nyasanda High and the other part in the village Orongo, near Kisumu. This time I didn't know where to set up my work-site before I left Sweden. As it was very difficult to communicate with my contact people in Kenya from Sweden, I had planned to travel around to my different site options and see if I could set up a plan with some suitable co-operators once I was there.

When I came to the first of my options however, Nyasanda High, it so happened that I found there one of the students that I had worked with before, Kennedy Oduor. He was the one that I had left "in charge" of the continuance of the project the last time I left the school, and so we sat down to have a talk about what had happened. It was in that talk that I decided to set up my working site here once more.

This choice did not come without its problems; it had proved to be very difficult to communicate with the school from Sweden (in fact since last time I was there I hadn't heard a word about the progress of the project) and also the school had a history of seeing its development projects die a very quiet death as soon as the project coordinator had left the place (in fact that was exactly what had happened after I left). But Kennedy convinced me that there was a genuine will from the students to see the project go on, and as he was eager to work with me once again and make it happen, I felt that we had better prospects for the project to continue here then we could have in most places. We also had the declared desire from the principal to see the project start up again, which of course is a very vital thing to have.

And so we set up the work.

5         Project Objectives

The main task that I wanted to fulfil, was to design and construct a plug flow biogas digester of household size that could be used by households in rural Africa. The digester needed to be cheap and simple to construct (expensive and complicated ones have already been made in great numbers), made from only locally available materials and techniques. The goal was that any normal household in rural Kenya should be able to build one themselves, with only the supervision of someone who has built one before. That way the technology would be able to spread by itself, household to household, once the first digester has been put in use. Later, I will myself offer my supervision of this to anybody who wants to build one, and who is prepared to pay for the materials, but first I needed to come up with an efficient design.

On top of that, I also wanted to share the experience of designing and building this digester with some local people, who could, if the experiment turned out successfully, benefit from the knowledge gained and maybe put it in immediate use, building a digester of their own and using it.

After the decision to work at Nyasanda High had been made, it was also felt that it would be a very good idea if we could leave something which was actually working by the time I left, so that the school could once again serve as an ambassador for developing and spreading the knowledge about biogas, to its own students and to others. "Functional" would therefore be a good point, even if the design we had come up with had some flaws in other respects (for example was too expensive, or contained locally unavailable parts).

6         Results

As described in the previous chapter, there was call for several types of results. The first one, to come up with a suitable design of a biogas digester, was at least partly met. Some steps forwards were made, although in the end, the digester that we constructed leaked  from the bearings of the stirrer axis. Therefore, it must be said that the design we came up with is not yet perfect. It was, however, cheap (around 20,000 Kshs), demanded no skilled labour and was made in only locally available materials! Thus, if the leakage problem could be overcome, this type of digester should be feasible to build and use for a large number of rural families in the area.

The second objective, to share the knowledge with local people, worked better. As described in the chapter How the work was carried out , the Biogas Club of the school was very active in the design and construction work, and I believe they had a good understanding of the whole idea of this project when our time was over. To make this official, by the end of the project, certificates for "Participation in Biogas Digester Design Project" were handed out by me, and then handed out again later by the school as an official school certificate. I also left them my previous work, the instruction kit "Biogas – How to make it work" (Karolina Hagegård 2005), and they will be given this report once it is finished. That way they will have roughly the same knowledge I have about this project, and good conditions to take the work further, on their own.

I also managed to have a biogas lecture to some women's groups in Anyiko who were very enthusiastic, and "Biogas – How to make it work" were handed out to many interested people all around.

About leaving something functional behind in the school after the project time, well, the digester we constructed leaked and was therefore not functional. There was also an old biogas digester in the school, which was also not working. There was, on the other hand, a very serious biogas club left after the project time, that could very well make good things happen in the future. More about this in the chapter The Future.

6.1       The Constructed Digester

The end product of the project was a plug flow biogas digester made from two 210 l polyethylene superdrums, connected at the openings by a sawed open jerrican.

100_0670 100_0809

To combine the two superdrums into one digester, the top and bottom of a jerrican were sawed off, and the remaining part was fitted in the openings of the superdrums. It was a few millimetres too wide, so the superdrum openings were widened slightly by the use of a hacksaw. They were made only so big so that two people could just about squeeze the jerrican in, with some coaxing, and a bit of doubt along the way. Later the joint was sealed with silicon paste.

Through the whole digester ran a stirrer. It was made from a 1½" PVC sewage pipe, with plastic bottles attached to it as stirrer blades. The bottles – which need to be Dasani water bottles or an equally strong type – were attached by burning a hole in the pipe with an iron pipe that had been heated in the fire. After the iron pipe had gone through, it was moved gently around in circles to make the hole slightly larger and the edges of the holes soft. Then the pipe was removed and the bottle neck pushed through. The bottle was held for a few seconds, until the PVC edges had gone hard again. The bottles needed to be attached already inside the superdrums, because once attached to the pipe, they could no longer go through the opening. For the same reason, also the jerrican needed to be threaded onto the PVC pipe before attaching the bottles of the second drum. The ends of the sewage pipe were plugged by a stopper made from a curry powder can lid that was hammered into the opening and then sealed with silicon paste.

Figure 1: Experimenting with attaching bottles as ”stirrer blades” in a test pipe. Later, the strings were not used, as the attatchment turned out to be strong enough anyway.

The same sewage pipe (they could be bought in one piece of specific length) was also used for inlet and outlet, after it had been bent by careful and repeated heating over the fire until it went soft and flexible. The inlet opening needs to be the highest point of the construction, and the outlet opening on the same height as the slurry level inside the digester, or higher, if there is extra pressure (ΔP) inside the digester. E.g. 10 cm water column of extra pressure means the outlet opening should be 10 cm higher than the slurry level inside the digester.

Figure 2: Principle sketch of the constructed digester, showing slurry levels with a back pressure of ΔP on the gas side. This digester leans slightly towards the inlet, which can be favourable. It should not lean towards the outlet.

Both the in-/outlet pipes and the stirrer pipe were attached in the end sides of the digester by PVC pipe adapters. There were two correct size adapters for the in- and outlet, and two larger ones as bearings for the stirrer, so that the stirrer pipe could go fully through the narrow piece of the adapter. The larger adapters were, however, not quite large enough, so the narrow part of them were widened by the use of first round files, and then when we were more pressed for time, by heating the inside with an iron pipe until it melted, and then scraping the melted bits out before they cooled down (holding our breath to the toxic chloro-organic smoke). The resulting rough surface was smoothed by a round file. This contact surface between the insides of the adapters and the stirrer pipe – a surface the size of a roughly 3 cm broad band around the pipe – smeared with grease, was all that would hold the slurry back from leaking out by the stirrer bearings, but as was later shown, the bearings did leak.

The adapters were fixed in the bottoms of the superdrums by burning holes of size just smaller than the adapters into the superdrum, and then screwing the adapter in while the edges of the hole were soft. Afterwards we wanted to attach a plastic nut from a sink bottle trap on the inside, but we learned that a thick ring of melted polyethylene covered the adapter on the inside, so far that the nut could not be fitted on the threads. So we had to leave them as they were, only sealing the joint on the outside with PVC pipe cement. The holes were burned using some iron junk and a curry powder can, respectively.

Figure 3: Experimenting on a jerrican

The gas would be taken out at the top of the digester by a regular PVC hose-pipe. To attach the hose-pipe we made some home-made "pipe nipples" from bottlenecks. The small flange just below the threads of the bottleneck, above the larger flange, was removed by knives, saws and files, and the top of the cap was taken off, leaving the threaded part to be used as a nut. A hole was burned in the superdrum and then the bottleneck was screwed/pushed through from the inside, all the way to the remaining flange, while the edges were soft. The cap nut was immediately attached from the outside. The hose-pipe just fitted inside the bottleneck, and so to make it stay, and to make the joint tight, we agreed to tie soft plastics and strings around it, like our friend Tobias from the study visit had done. But first we also fixed the hose-pipe inside the bottleneck with the PVC pipe cement.

Figure 4: Experiment bottleneck pipe-nipple attatchment

From the digester, the gas would be continuously led into a gas container, made from two differently sized superdrums, one up-side-down inside the other. The larger drum would be filled with water. The gas would go into the gas container by a bottleneck pipe-nipple in the up-turned bottom of the smaller drum. This drum would maybe need to be held down by some stones, tied by strings attached to the handles (which would be next to the down-turned opening). The hosepipe leading to the gas container was cut at one point, and the joint sealed by a short piece of a hard plastic pipe. The idea was to let the hosepipe go on the outside of the hard pipe, and then tighten with a clip. Then, when one wanted to use the gas, one would block the hosepipe by the digester, by folding it a few times and tying strings around it, and then take the gas container hosepipe from the little bit of hard pipe, squeezing it to prevent too much gas from leaking out, and attaching it to the stove. While cooking, the gas would build up inside the digester, making the slurry overflow, and then sink back when the gas container was reconnected. The stones should be of enough weight to produce enough working pressure for the stove, by pulling the gas drum down while the water rises on the sides.

Figure 5: Gas container

Another idea would be to have two hose-pipes into the gas container, one for inflow and one for outflow of gas. Then the stove could just be connected and disconnected to the outflow, and the pressure would remain constant in the whole system at all times.

There was no time to construct this gas container within the project time, but the students were instructed to build it when the project coordinator had left.

6.2       Discussion About the Design

6.2.1        Techniques used

The techniques were incredibly simple to use. Especially attaching bottles to the stirrer axis was very easy, and yielded a perfect result every time, once the trick was learned. (The trick was, like in most of the other burning techniques here used, to use really lots of heat and to do the whole procedure in one go. With the superdrum this latter condition was not always possible, and in those cases it was best to have two sets of heated items, so that one could be heated in the fire while the other was being used.) The attachment of the bottles to the stirrer was also very strong and stabile, while the adapters in the superdrum were a bit more tending to be screwed out again. That was why we bought the PVC pipe cement to fix them with. Perhaps it would have been better to follow our first plan and secure them from the inside with a plastic nut. This would demand that the nut be screwed on while the melted plastic on the inside is still soft, or that the melted plastic be removed.

6.2.2        Digester

Björn Martén had estimated that the size of a family digester should be around half a cubic metre (500 l). This one was not quite that, but almost. The dimensions should be such that the length-to-width relationship be more that 2:1, and this one indeed was. The joint between the superdrums, however, was perhaps not great. It had the advantage that it was simple, flexible, would not wear out quickly (although it needed a lot of silicon paste in order to stop leaking) and could easily be taken apart to clean or make adjustments on the inside. But, on the other hand, it had the problem that all components of the slurry must be able to lift from the bottom or sink from the surface in order to pass to the second drum. This may not be a huge problem if used only with plant material (which doesn't hold any sand like cow-dung does, although it will, on the other hand, tend to float) and the stirrer is always turned while, or just before, feeding the digester. Besides, all the components of the slurry would in any case have to be able to get to the location of the outlet in order to finally escape the digester. In order to ease both those passages, it is probably better if the DS (Dry Substance) content of the slurry is rather high, so that the slurry cannot separate into a floating “scum” layer, a bottom “sludge” layer and a watery middle layer, but be rather evenly mixed when stirred by the stirrer.

Perhaps it would have been better to join the two drums with the help of a superdrum fundi. He or she could either make a permanent seal with melted plastics around the jerrican, where we now had silicon paste, or we could go with another plan that I had, but that we did not try, which was this: The superdrums have bands, about three cm broad, where they are slightly wider (a design thing). We could take off the tops of both superdrums, one a bit over the top band and the other one just at the top of the top band, so that the opening of this last superdrum would be slightly larger. Then maybe that one could be slipped outside the other one, creating an overlap of about three cm. Then the joint could be sealed with melted plastic, as mentioned before. This would of course make the total volume smaller, but there would be no obstacles for the material to pass inside, which could make the active volume larger. Also, if this works, a third drum could be added after the second, etc! Another problem though, with both those ideas, is that there would be no easy way to open the digester to do some work inside it, or to clean it all out, once the joint would be sealed.

A way to solve this last problem of not being able to open the digester, could be to make a hatch in the side of one (or more) of the drums. A rectangular opening of suitable size could be made in the side of the drum, and maybe the envelope area of a jerrican could be used to cover it, attached by screws. The screws would rust with time but could be replaced after some period without too high a cost. The joint would need to be sealed by silicon paste, preferably in an overlap between opening and cover. The hatch should be located fully underneath the slurry level while the digester is in use, so as to avoid biogas leakage out, and oxygen leakage in. After having been opened and closed many times the holes for the screws could go larger, but then one could change to larger screws, thus prolonging the life length of the digester for yet another while. Also, the jerrican cover could be replaced after some time, since it is not so expensive. This hatch type has not been tested in this project using the mentioned materials, but I tested this construction of a hatch in Sweden before the project, and it worked well. However, in that case I was using a slightly harder type of plastic drum than the superdrums, and the cover was of the same material as the drum (in fact it was cut out from an identical drum that had been discarded).

6.2.3        Bearings and stirrer

Something obviously needs to be done about the bearings for the stirrer. I discussed the issue with a friend who then pointed out that for one thing, we don’t need to have bearings at both ends. Only one end of the stirrer pipe needs to stick out so that it can be turned from the outside. The other end just needs something to hold it in position, which is an easier task then to allow it to go through and turn. Also, the pipe adapter which was used for bearings looks like this:

Figure 6: Cross-section of PVC pipe adapter, not quite to scale

In the project we used the side which is to the left in the picture (and which is threaded on the outside, though it doesn’t show here) to be pointing inwards to the digester, and the larger part to be on the outside. However, since the larger part has a funnel-shaped interior, if you instead turn that end inwards, then a conical collar of some kind could be made for the stirrer pipe:

Figure 7: Conical collar for the stirrer pipe

And when the cone is pushed into the funnel, a rather good seal against leakage would be created. Maybe one could even actively pull the stirrer outwards and fasten it there somehow whenever the stirrer is not used. Then the digester could be allowed to leak slightly, just around feeding and stirring time, but be tight during the largest part of the day.

There is also a slight possibility that the type of bearings that we did use could still work, if we had only done the job better. We were running out of time, and so when we had finally made the adapters large enough for the pipe to go through them, there was a crude and quite deep unevenness along the insides that was well visible with the eye. But the PVC pipe that we used as stirrer was slightly flexible, and thus could maybe tolerate some smooth unevenness and still make the joint tight. Then the pipe should be pushed through with force, and the grease should be water proof, and of a type that would make the plastic surfaces really slippery against one another (the type that we used wasn’t great).

In the digester that I finished in Sweden and which is a later version of the one that was present in the school before I came, the stirrer axis and the bearings were both made of steal. They were just two pipes of different dimensions, one just small enough to go inside the other one, and to make the joint tight, I used an o-ring. It was of the size that the stirrer axis could just be pushed through it with a little bit of force. A track was made on the inside of the bearing where the o-ring was fitted to stay in place, and then the axis was pushed through. This bearing kept tight despite rather high water pressure. Probably it could work also for our plastic version! The track could maybe even be made with a simpler tool than the lathe that was used in my case.
Figure 8: O-ring to tighten bearings with

The stirrer in itself was a great construction, although I now think it would be better to keep the caps on the bottles, so that they will not get filled up with slurry which is now the case (since the attachments are not tight and so the axis will be filled with slurry). It is not a great problem if the bottles do get filled with slurry, but it doesn’t help either, and it probably makes the stirrer heavier to turn.

6.2.4        Bottleneck pipe-nipples

Another perhaps week spot in the design was the many bottleneck pipe-nipples. Those offer many places for biogas to leak out and oxygen to leak in. Altogether there were three pipe-nipples in the digester, and at least one for the gas container. If the design were to be changed so as to exclude the jerrican joint as previously described, the pipe-nipples in the digester could be reduced to only one. In any case, one should really make an effort to make the pipe-nipples gastight, using any material available to seal them with.

6.2.5        In- and outlet pipes

The in- and outlet were constructed from the same sewage pipe as the stirrer axis. This was done in order to keep the cost down by having to buy only one pipe instead of two. However, it may very well so be that they are too narrow for the material of the slurry to pass well, especially since they were attached to the digester by an adapter which, in its most narrow part, was only around 3 cm wide. Another way to attach the same pipe to the digester would be to use the larger kind of adapter which was used for bearings, and even in this case let the pipe go through the narrow part, rather than stay in the wider part. Of course, we all know that that didn’t work very well for the bearings, but in this case the pipe would not need to be able to turn, and so the joint could be sealed with cement and silicon. Or else, a larger pipe could be bought, and in fact they were only 300/= a piece after all. But perhaps it should be mentioned that the pipe adapters of the larger size used were not as readily available as the smaller one, and in fact one of them even had to be bought in Maseno.

6.2.6        Gas container

The gas container made of two superdrums was of incredibly simple design and should be able to work well. It should be kept away from the sunshine in order to last longer and for the gas not to expand and maybe escape. The water would probably remove some of the carbon(IV)oxide from the gas which would make the heat value of the remaining gas higher. The water could maybe go slightly acidic from this, but since the resolved carbon(IV)oxide can continuously escape to the air along the sides, it shouldn’t reach any higher concentrations. This construction seems to be more durable and much more available than a decent bag type of container. For the pressure to be able to rise high enough to cook with, though, there needs to be enough space for the water to rise along the sides as the gas drum is pulled down by the stones. If that is not the case, an extra step is needed between the gas container and the stove. For example, one could use refuse bags and fill them from the gas container just when one wanted to cook. Then attach the refuse bag to the stove and put something heavy on the bag. This would be better than collecting the gas in refuse bags from the beginning, since the refuse bags will certainly leak gas, but in this case they would be used only for such a short time that it might not be noticeable. However, due to the low durability of the refuse bags, it is still not a great idea. If the bag is used in a room with an open wood-fire, there also seems to be a danger of some burning object falling on it, making a hole in it and causing an explosion. With the drum construction that risk seems very low. So probably the best thing would be to go with the drum version, and use a stove with a low enough working pressure.

As for having one or two pipes for the gas container, our first plan was to have two. The one-pipe design was first invented mainly because the pipe-nipples that could be bought were so expensive, but that problem was later overcome by the bottleneck pipe-nipple design. However, we stayed with the one-pipe idea since it was felt that the bottleneck pipe-nipples weren’t very gas tight, but on the other hand, it seems the two-pipe design has a lot of advantages, especially for the convenience of cooking, with maybe just a tap to open and close.

6.2.7        On the whole

On the whole, it seems we have created an ALMOST functional biogas plant, with only locally available materials, no skilled labour involved and at the cost of only around 20,000 Kshs! It seems clear that if only that bearing problem could be satisfactorily solved, at a low cost, it would be very feasible for a large number of families and other groups of people in rural Kenya to set up and use their own biogas digesters, roughly of the design that we have drawn up here.

7         Project Budget

This is the entire budget (as far as I could work out) that was spent for the project, within the project time. This is not necessarily what it costs to build a digester like this when you already know what to do, since some of this was purchased for experimentation. The real total cost might very well be higher though, as our digester leaked. But if one manages to find a cheap way to construct a non-leaking bearing, one should be able to get away a few thousand cheaper than this!

Item no.

Materials for digester
Superdrums x21)
2 ∙ 1 450 =
9, 35
Jerrican x2
100 + 100 =
1½" PVC pipe x1

Adapter 1½"

Adapter 1½"

Adapter 1” x2
Approx. 2 ∙ 100 =
Hose-pipe x1 role

Hose-pipe joint

Water bottles
7 ∙ 5 =
Bottles of Dasani water x7
7 ∙ 55 =
Pipe clips
7 ∙ 25 =
Bottle trap

Thread seal tape

Grease, 2 different kinds
150 + 100 =
Menile string

PVC pipe cement

Silicon paste




100 + 150 =


Screws x10

2 ∙ 50 =
Curry powder

Tape measure

Sand paper

New saw-blade

Round files x2
2 ∙ 100 =
More sandpaper

Junk iron pump ("nini")


More curry powder




Gas container
Superdrums x2 for gas container
1450 + 1050 =
Refuse bags 750x90mm




Insulation house (Future)
Polythene sheet, 15 m yellow

Wooden frames for straw house x5
5 ∙ 480 =

Funds for insulation house2)




36, 37, 38
Printing and photocopying of instructions and certificates
330 + 1200 + 580 =



Total sum


Sum without italic items

Table 1: Project Budget
The item numbers refer to a receipt with the same number.
Items that lack receipts are grey in the table.
Items that we used only for experimentation or non-construction project things or that in the end we turned out not to find very useful at all, are written in italics.
1) There is no receipt for these drums which were bought on the market, but they were the same price as the large one on item no 24
2) See Appendix A: Budget for funds left behind

8         How the work was carried out

The school set up a biogas club (as they had done before) to work with me for six weeks. The members were recruited by my previous student acquaintances Kennedy Oduor and Nancy Awuor finding some more students that they believed would work well in the club and that were interested. This of course had the disadvantage that some possibly interested students never had the chance to join, but, on the other hand, had the advantage that the whole club knew one-another and could work efficiently from the start. I wanted, this time, to have a small but very serious group, that could participate as actively as possible, and that could later recruit more members and teach them what they had learned. My desires were met.

The club then picked their chairperson, which turned out to be Kennedy, and it was said that the club would function independently, so that even if I would not be there one day, the work would not have to be halted. This would also be a strength after I had gone.

The time we had available for project work was during games hour 4–5 pm Mon–Fri and Saturdays after 12.30, when the school very generously offered us free lunch, so that the club could eat without wasting any time, and then work until we dropped.

We started with a thorough briefing, in which I told them about what kind of digester I wanted to make, and a short theory of biogas, its practice and its advantages and problems. We also talked about what we could expect from those six weeks and the time after that. Then followed three weeks of design talk. With the funds from ForumSyd, I bought two superdrums and some tools and accessories, and we looked at them and tried to combine them, talking back and forth about the advantages and disadvantages of different solutions. After some time of this, when I felt the students had enough background but hadn't yet quite caught up with the thinking in depth of the task, I gave them the assignment to, together but in my absence, put up a detailed building plan for the whole plant. Then we discussed their plan well.

After three weeks, when we felt we had had enough talking, and then had been delayed for some days for various reasons, the actual construction work started. We had some practice items – a piece of left-over pipe and a broken jerrican – on which we tried out the techniques before we used them on the more expensive things like the superdrums. The goal was for everyone to try all the techniques, at least on a practice item. Since the students did not have much time though, I ended up doing a lot of the actual construction myself, but three and a half weeks later the digester (if not the gas container) was ready, and we tested it for leaks.

It leaked.

We concluded the project with a party, summarising what we had done and learned and what could be done in the future. It was generally felt that the club should go on, and Mr. Owenga, the responsible teacher for the biogas project of the school, gave us his word on behalf of the school staff that they would be with us in helping the project.

One Saturday during the project time, four of the students were given a leave from school and went with me for a field trip to a biogas digester constructor in Maseno, Tobias Odhiambo Omoro. Tobias had picked up his biogas knowledge in primary school, and had then experimented on his own to make it work. He constructed and sold digesters for lighting and light cooking, and he also used a tiny little biogas digester in one of his other businesses which was to mend radio machines. The trip was very inspiring, and we took some pictures and told the rest of the club all about it when we returned. Some time later, Tobias also came to look at our work and gave some inputs.

8.1       Strengths and challenges in the work

There were eight students in the club, half of which were in form four (the final year), and the rest from different forms. This felt like an ideal size of the group, and all of the students were quite or very active. That is, they didn't say much at first, but after the idea had won their liking, and especially after they had made their own building plan, they made plenty of inputs and they really challenged my ideas, forcing me to defend them profoundly. This was terrific, as it brought to the surface many things that I had failed to explain earlier or assumed that they already knew, and so it gave all in the club a much deeper knowledge about the topic than they would have had otherwise. And it made the design better too!

The items were kept in class-rooms, easily accessible to the students, and we had great access to the school kitchen and its fires, without which access this work could never have been carried out.

We managed to make some contacts with other people in the field, for example Tobias, the biogas constructor, and Benard Omondi Ochieng at UCRC who introduced us to the Nile Transboundary Environmental Action Project. There was also some contact with the nearby school Nyati, Nyasanda Technical Institute. These contacts might prove important especially for the future.

On the very first day of construction, after weeks of theory, a new student suddenly joined the club, through some unclear communication. He gladly got to work on one of the terribly expensive superdrums with a wood saw. A tool with the subtleness of a sledge hammer, for a job with an almost surgical demand for precision. He didn't make it. Therefore, the top of that superdrum was as good as ruined. I think this was a good learning experience for all of us: When you have spent three weeks emphasizing how important carefulness is, and especially when it comes to the more expensive stuff, do not let someone from the outside slide in and make the critical move, just because you are too polite to say no! We also learned that it is very good to have a backup plan. In this case no harm was done, as we had saved the superdrum we wanted to use as gas container, and that we wanted to take the top off of. Now we used that one for the digester, and decided to take the top off the ruined one.

But the only real challenge in the work was the time. Our project time was during games hour at 4–5 pm Mon–Fri and from lunch on Saturdays. However, the students were never gathered any earlier than a ten past four at any time, and more often around twenty past. Sometimes they were held in class, sometimes they were cleaning classrooms and sometimes they were just late. I talked to the teachers, and they promised to let the biogas students go, even if the lesson finished a bit late, and never to call for my students during club hour, but they kept forgetting, and the students were not comfortable in leaving class earlier. They also felt obliged to take part in cleaning the class-rooms when it was their turn to do so, or else they would leave other students with an extra work load. This meant, however, that from the time I had expected us to have for the project, almost one third was lost, just because of students not showing up on time. There was also some misunderstandings between school staff and the kitchen  personnel which had the effect that there was no lunch on some Saturdays, although we had been promised there would be, and this set us back seriously, since Saturday was our only long working day. On top of this, my project was not of course the only thing to take place during games hour. One day of the week some students had a Christian meeting, and towards the end of my stay some of them, including the chairperson, started practicing volley-ball for some matches which were coming up soon. Also, during our fourth week, the form four students had exams.

The fact that half of the students were in the final year and therefore will be gone soon might not be such a great disadvantage, as one of the expressed tasks for the future was to recruit new members. Also, some of those students had worked with me before, which was an advantage.

On the whole, I think the work went very well. Despite the problems that we had, we managed to do a great deal of design and construction work, and the fact that the students were so active and that we all learned so much, really made it all worthwhile. I felt that I left a very serious and ambitious club behind me when I left, if even a club with a very difficult task, to make either of two leaking digesters function. But at least as for the period within the project time, we must say we did a good job!

9         The Future

As has been said before in this report, this project rendered a very serious biogas club at Nyasanda High, who were very eager to keep on working with biogas also in the future. It would be a very good thing if they could be able to do so, although this project did not leave any functional biogas digester behind! However, as this report is nearly finished I have recently heard that a team of experts from Orongo village, where my supervisor Björn Martén has also worked, is going to come to Nyasanda High and build a new biogas digester for the school like the one they have built in Orongo. It would be excellent if the Biogas Club could keep this digester running, learning how to operate it for a maximum production of gas. Then the school could be a good ambassador for the biogas technology, and the club could demonstrate the digester whenever the school wanted to show it to interested visitors. The excellent fertilizer that the digester produces should be used to fertilize the school crops. John Ombwayo, one of the Orongo experts, can be reached on 0734 38 85 80 or jokagie(at)yahoo.com.

But there are also the plans that we had before I left, for example the initiative that Benard Omondi from the UCRC told us about, the Nile Transboundary Environmental Action project, where a group of people could register their work as a development project, and if they handled the task well according to the criteria, there would be a reward in the end. The group will get plus points if they manage to integrate the community well in the project, and so we said that there should be cooperation with the Anyiko Women’s Groups who were so interested in the biogas technology. They can be contacted through my friend Michael Nyadenge, 0727 15 00 55, nyadenge01(at)yahoo.com. Benard can be reached on 0722 24 56 36, benmosh2001(at)yahoo.com.

Also, the neighbouring school Nyasanda Technical Institute (Nyati) were very interested in having a biogas digester of their own, and so it was said that the Biogas Club at Nyasanda High should later share their knowledge with Nyati.

To make any kind of use of the digester that we constructed, rather than just waste the materials, the club should either try to fix the bearings so that they no longer leak (with the help of the chapter Discussion About the Design of this report or by other means) or the bearings should simply be sealed with PVC cement, silicon paste and perhaps other things, so that the leakage would stop, even though the stirrer could no longer be used. Then the digester should be run with only cowdung. If only cowdung is used, a stirrer is not an absolute need.

The gas container described in this report was not constructed during the project time. This should be done, so that we can find out if it works or not. It is, however, not necessary to do so before there is a functional digester in the school, as it needs something to be tested with.

Another thing, which is also not necessary to be done before there is a functional digester, is the insulation house that we had planned to make around the digester to make the temperature inside it stabile. A lot of funds were left behind after the project for the club to be able to construct this little house in the next dry season, but it was said that if there was no functional digester, they might as well not build any insulation house and instead save the funds. In that case they should, however, use that part of the funds that was dedicated to Maintenance/Other (see Appendix A: Budget for funds left behind) to make a digester functional.

In order to do all this, the club must make sure to recruit new members, introduce them to the biogas technology and inspire them to carry on with the work.

Now, there are a few things to work with at least! As for my own part, I will keep my promise and come back to Nyasanda High sometime in the year 2008 or 2009, to follow up what has happened and, who knows, maybe work there again if there is scope for a good project. In the meantime, I hope that the Biogas Club will keep their promise to me to be very serious about their task, so that their ambition that they shared with me at our closure will be met, that the next time I come back, I will not find an empty work site, but a biogas project in full activity!

Let us all keep on working for the better of this world, for as long as we shall live!

10    Acknowledgements

For helping in this project, I would humbly like to thank Mr. Norbert K’Onyango, principal of Nyasanda Community High School who let me set up my work at his school and also made all the arrangements needed to make it possible (especially the Saturday lunches were invaluable). Also all the members of the Biogas Club – Samuel Otieno Apodo, Polycup Otieno Mwalawa, Nancy Oyugi Awuor, Ruphine Adhiambo Oguga, Victor Oluoch Onyango, Wycklife Adeny Otieno, Felix Oduor Onyango and chairperson Kennedy Odhiambo Oduor – and the teacher responsible of the club, Mr. Henry Owenga, as well as all the teachers at school for their kind interest and support. A very special thanks must go to the kitchen staff for their never-ending patience with our presence in their cooking fires, their generosity with firewood and drinking water and of course their good-humoured spirits that could brighten up the most depressing day! A great thank you also to my dear friend and mentor Mr. Aggrey Omondi at the Ugunja Community Resource Centre for cooperation, criticism and morale (and of course for managing the Funds Left Behind for the club). Thank you my other friend and mentor as well as supervisor Mr. Björn Martén, who started this biogas project and whose honest, ambitious spirit burns like a fire in the dark. Thanks to the hardware shop Abisa Enterprises for excellent service, and to the school gardener Isack Onyango who, among other things, lead us to his friend, the biogas expert Tobias Odhiambo Omoro, who also receives a special thanks for his great inspiration and interest in our project. Thank you Benard Omondi Ochieng, for friendship, criticism and valuable discussions. All my loving gratitude to Homeground Resort, for really being a resort for me where I could unwind and feel at home when the working day was over, and especially to Fred who bought medicine for me when I was sick and openheartedly helped me with anything that I ever needed. Thank you Tony Olalo at the bar/petrol station for throwing us a party at a “friendship price” (although it was a bit late in the delivery…), and in that, of course we have to thank (again) the owner of Homeground for letting us have our party there, even though I brought in someone else to cook. There were many other people that helped make my stay enjoyable and the project flow more easily, and although I cannot mention all of you, my thanks go also to you.

And of course, last but not least, my most humble thanks to the managers of the Jan Danielsson memory fund, the Green Cross of Sweden, who granted me with a 5000 SEK scholarship to perform this project, and to ForumSyd of Sweden who paid our expenses for the materials that we used.

11    Appendix A: Budget for funds left behind

This is the budget for the funds that were left behind after the project time, managed by Mr. Aggrey Omondi of the UCRC:

Budget for Biogas Club Funds 14/03/2007

Actual spent
Balance (Budget-Spent)
Roof to protect insulation house:

4 poles

12 fitos

2 kg 1 inch nails

1 kg 3½ inch nails

1 kg 4 inch nails




straw (from Anyiko women’s groups)





Trips (5 pers to Maseno + lunch à 40/=)

Maintenance/Other (broken parts, grease, tools,…)



All the members of the club, as well as the teacher Mr. Henry Owenga were given a copy of the above table.

12    Appendix B: Alternative design suggestions by Tobias Omoro

After the study visit to Tobias Omoro, he came to visit us at the school, and he gave us some alternative design ideas that he thought we should consider. We told him that our construction plans had already gone so far that it was difficult to change them now, but at least we talked through his sketches afterwards to point out significant features about them, and to motivate why we decided to stick with our own design. A lot of valuable points were made clear through these discussions, which is why I have decided to include them in the report. Here they are:
Sketch 1: Basically our Nyasanda High design

This is basically our design, that Tobias drew from memory after we had left. The improvement he suggested was mostly that he thought that instead of a hose-pipe between the drums for the gas to pass, we should use the black plastic tubes that electric wires are usually wrapped in. That was the material he usually used, and it could be welded directly onto the drums, he said, and also turned into a T-joint or any shape really. This was probably a good idea, and I even got us a discarded piece of such a plastic tube, but there was never any time for us to try it.

He also had some idea that the jerrican should have openings on two different levels from the ground, because it would create a water lock between the drums, in case you didn’t want to mix the two gases, as the gas from the first drum would, in his mind, be “weak”. But our opinion was that all gas needs to get out anyway so you might as well use it, and we didn’t want to make it more difficult than it already was for the slurry to pass the jerrican, so we cut it completely open. Also, we didn’t want the gas to displace too much slurry, since the active digestion volume would go smaller.

A weak spot that we pointed out in this sketch, compared to our design, was that his inlet pipe goes through the roof of the digester, whereas ours goes in through the side, near the bottom. That creates one more point in his version where there is risk of gas leakage.

Sketch 2: Two continuous tanks in a series

This could be called two serial tank reactors. Our biogas plant is a tube reactor, or plug flow reactor. What characterizes a tank reactor is that its contents are (in the ideal case) evenly mixed in the whole tank. That means that the outflow has the same composition as any volume element within the tank, whereas the inflow is of a rather different composition. In a tube reactor, on the other hand, the composition of the contents changes gradually all the way from the inlet to the outlet. For this to be possible, the reactor’s length must be much greater than its width and height, or else the contents will intermix through diffusion.

Except in some very special cases, the tube is more efficient than the tank. This is partly because the reactions can go quicker in the beginning of the tube, when the concentration of the substrates is high – in the tank they immediately get diluted – and partly because in the tube, they can really go all the way. In the tank, a percentage of the feed material is always washed out through the outlet without time to first react. In the biogas case it can also be said that there are different bacteria active at different stages of the tube, so that they can really specialize on the conditions at that stage. This is not possible in the tank, where there aren’t any different stages, and so this may be an extra benefit with the tube in the biogas case. A definite benefit with the tube in the biogas case is that it makes really sure that no pathogens from the feed can go quickly through to the outlet, and thus the hygienization gets more effective.

Smaller tanks in series are, however, more efficient than a single large tank, and as a rule of thumb, engineers say that 10 tanks in a series is a tube!

Sketch 3: Single continuous tank

A tank reactor can be of a batch type or of a continuous type. The ones that Tobias usually constructed and sold were of a batch type, which means that you fill it once, then you wait for all the procedures to take place and then you harvest your products and empty the reactor. You will have no problems with blocked pipes, and if you time it well, you will have the fertilizer ready right when you need it. The above sketch is, however, a continuous tank, which means that you feed it and let it overflow every day. This is better if you want to use the biogas for cooking, and thus need it all the time.

The above design is a rather nice one, I think. The way the inside of the inlet is placed means that it will probably stir the content a bit at feeding, the springs for the floating dome might help keep the pressure quite even, also with an uneven usage of the gas, and there is only one crucial place where the gas could leak, and that is at the necessary gas outlet. There is also very little and very simple constructional work involved, and few places where the digester could break or leak slurry. Since this reactor type is, however, less efficient than the tube type, you would need a very large digester to have enough gas, which means a higher initial investment, but if the robust design would make it last longer, that might pay off in the end. Then of course we would also have the problem mentioned before with a less efficient killing off of pathogens. But the design is definitely worth trying! (In the next project)

Just a comment on the picture: The slurry level of the inlet would not be that high! It would be as high as the level of the outlet, except maybe just after feeding, while the feed would slowly sink in and displace the slurry already on the inside.

13    Appendix C: More pictures

Early discussions, constructing and happy days

Lecture to the Anyiko Women’s Groups
Study visit to Tobias Omoro, biogas digester constructor in Maseno area

That yellow bucket is actually a biogas digester!...
The biogas flames come out through the many holes
The tool to fix radios with, after heating it in the flame
A used up light bulb will get turned into a biogas lamp
And the residue will fertilize the crops
Late construction work                              .

Testing for leaks (Yeah, it leaked)
The last day – Party day!

The kitchen staff of the school
Karolina is leaving lovely Homeground and beautiful Kenya…


…for this time!
Categories: ,


Post a Comment

Recent Posts

Related Posts Plugin for WordPress, Blogger...


Featured Posts