Visit to two biogas plants in Monduli

As stated in the previous post, here are updated notes from my group's visit to two biogas plants in Monduli.




Visit to 2 biogas plants in Monduli


Mr. Donald Mari of KAMA was in Arusha, but he sent Mr. Elsante to meet us. Mr. Elsante took
us to two adjacent farms on steeply-sloping heavily-forested land, where he had constructed biogas
plants. They were both 6 m3 plants. Biogas contains 50-75% methane, 25-50% CO2, 0-10% N2, 0-1% H2, 0-3% H2S, 0% O2 (per Wikipedia).


In both cases, the only things visible were the concrete input tank, the man-hole-like cover over the
gas outlet, the man-hole-like cover over the outlet tank and the exit chute for the slurry, which was
collected in a hole in the ground. The input tank was double: the first chamber was dry, but was said
to be to store water, and the second was used for mixing before the plug was pulled and the mixture
flowed into the digester. In one case, vegetables were growing in the soil over the tanks: in the other,
there was nothing growing over the tanks. The water supply is municipal, so is plentiful. The first plant
was fed by 3 cows, which are ‘zero-grazed’ ie. kept in their shed, which had a concrete floor, all the
time, and dung was collected and added to the biogas system twice every day. There was a valve at
the gas outlet, and a connection for a pressure gauge (which would only be installed for diagnosis of a
problem). There was a water trap at the lowest point of the line to the house: it was said to be opened
once every two months in order to clear the pipe. Within the house, there was another valve, then the
gas was used to power a factory-made two-burner stove, which burned just like a gas stove in the US,
very controllably, as well as a mantle-type lamp, which gave very good light. Mr. Elsante said that the
price was 700,000 Shillings, ~ $450 [but it is not clear if that includes any subsidies]. The only reported problem had been a leak which once occurred near the lamp – it was detected by coating the pipes with soap solution, similar to how gas leaks are detected in the US. The residents of the house noted that the gas smells slightly like dung, which helped to keep insects away inside the house. They used the slurry for fertilizer; in addition, they had discovered that spraying a mix of filtered slurry and cow urine on to their plants was a good way to repel insects.


We did not spend as much time on our visit to the second plant, located at a neighboring farm. Their
system was also 6 m3, fed by 6 cows. The system was designed such that cow urine directly flowed into
a collection vessel. This system was also interesting in that the gas had to travel upwards, perhaps 10
feet, to reach the house; this did not cause a pressure problem.


In addition to these two 6 m3, 700k Shilling plants, Mr. Elsante mentioned that he had also built some 4 m3 plants, for which the price is 600k Shillings.


Both house-holders said that they were very pleased with the systems, which had been in operation for
about 1 year.


Neither household were Maasai. According to Kisioke, these farmers use a different breed of cow than
those prefered by the Maasai – these cows grow bigger, but are not suited to the nomadic lifestyle of
the Maasai. We discussed the implications with regards to building biogas systems for the Maasai.