Failure of African digesters

Hi Everyone,

I'm new here and expected to read some comments about the high failure rate of biogas digesters in Africa. If you don't know about this I send you reports!

This seems to be due to the lack of slurry mixing so the gas flow drops off after a year or two. Mixing is possible but not it seems discussed!

I think I have an answer!

Graham Knight  BioDesign


Hello Graham,


I am designing my own digester now and am troubling with the idea of mixing.

Eventually I thought to clean out the digester every summer because with the heat it is easier to restart. But cleaning the digester would cause atleast 2-3 weeks of no production. And with my digester being an important role in my business plan, I think that would cause production troubles.

I was thinking to have a second small 100L digester next to my main digester (2000L) to kickstart it in case of a restart after cleaning. But I also am concerned it would increase too much the complexity for starting my first time digester.

What is your sugestion how to solve the mixing-problem?

Hi Shirokama,

I'm rather puzzled by your comments!

In Africa there are several types if digester but none are normally cleaned out. One reason being they are too large to even consider this option. As this is quite a complicated matter it might be best of you first describe your digesters.

In most larger digesters mixing is not very important for the first year of use! See below.


P.S. Those following this conversation might be interested in this summary of a recent report;

“Some degree of solids accumulation is inevitable in plug-flow and other flow-through systems due to limited mixing within these systems. This can be especially problematic when cattle manure is used as the primary feedstock due to the high lignocellulosic biosolid content in this waste. Indeed, these materials have a tendency to settle within the digester and are slow to degrade. As a result, the effective volume is reduced, and therefore the hydraulic retention time of the digester system is shortened, which leads to lower substrate conversion.

Mechanical mixing can be used to resuspend settled solids and thereby mitigate solids accumulation and clogging; however, it also promotes hydraulic short circuiting within the plug-flow digester, which can reduce substrate conversion and consequently methane yield.”



I just started my studies of biodigesters so my knowledge is still very limited. ( looking for a mentor :) ) I am going to use a 2000L water tank based on the IBC digester systems.

I thought biodigesters always needed to be cleaned out after a certain amount of time. Especially with biodigesters that don't have a mixing systems.

In my digester I was planning to put stones at the bottom to increase surface area for bacteria. I read this only one time but couldn't find more information about this. With an increased surface area bacteria would be able to digest more of the slow decomposing substances. Is this true? and would this be a good option for a 2000L biodigester?

The article what you presented is about 'One Cow, One Digester, and One Hour of Cooking per Day' and using a pipe diameter of 30cm, then I feel it is rather small compared to the big cleaning of tanks what I found on youtube and other articles?  But I think using a rope and dragging something over the bottom like that can be a good solution for that size of digesters.

Hi Shirokuma,

We seem to be on our own! No-one here seems to be interested in our concerns!

I cannot help you as my experience is very limited.


This forum is more quiet then I was thinking... .

What type of digester do you have if I may ask? 

Hey guys,

thanks for posting and commenting!

Some Rules first: Don't hijack threads. If someone writes about one topic, for example about african digesters, then stay on that topic and don't interfear with another topic. If you have a question about generel digester design -> start your own topic and wait for an answere. Sometimes it takes some time till you get an answere...

For the African design: I'm not an expert but this are plugflow system. You put something in on one side and something else comes out on the other side. Their greatest benefit is the long hydrolic retention time. Think of it as a very long gut. I'm not aware of problems with mixing as those digesters were mainly designed to transfer cow shit into biogas. 

for the Generl design: There are two different digester types: continouse and batch. In small scale operations and DIY continouse systems are prefered. They nearly never get emptied out. You put in food on one side and on the other you get the effluent. They are most of the time so called pluge flow systems as they need no mixing / stearing. - websites & marketing


A sludge pump of somekind that sucks from the slurry side and pumps it back to the entrance side. Do you think that can help also improve the mixing process? If this is done once a day/week/month to get everything mixed. Or would it ruin the 'gut effect'?

I heared back from a technician who is involved in the production of those type of biogas plants. He didn't experience that problem in long run trials. His suggestion was to make sure that the food / water mixture is more than 50/50. - websites & marketing

That sounds a lot more financially beneficial then paying electricity for a pump. :P

Whatever the reason most digesters stop working before the end of two years instad of 20+.

The maunfacturers and NGOs refuse to defend the situation so it is time that digesters that work commensurate with their cost were developed!

One answer is to fit a hand-driven mechanical mixer as in the Far East

a hand-driven mechanical mixer is probably one of the easiest soltions someone could think of. And it is probably the most feasible so far. The smaller the particles the Archae have to chew on the better. Still lignin content takes a lot of time to get decomposed therefore the size of the digester plays an important role. The Homebiogas system, at least their first version, has a hand-driven mechanical mixer - but it is a different style of digester.

There is already a lot of improvements of that African system going on. It is probably just that we are not that aware of. - websites & marketing

The problem with mosr commercial biogas sytems is the size of the gas storage.

To hold enough gas for an average family to just cook their meals, ignoring lights frig etc, requires a volume far greater than that on offer


gas storage is easy, you just need a bag in the size of the plant... or inner tubes... - websites & marketing