By Daniel Dickinson
BBC News, Kilimanjaro region
It does not look particularly special; a tank, a few pipes and some foul smelling water, but a locally-built device is the talk of Leguruti village in Kilimanjaro region in northern Tanzania.
A bio-gas converter, the first of its type in East Africa, is being tested by the 120 members of the local coffee farmers' group.
Farmer Moses Urio hopes he will no longer have to buy diesel
The gas can be used instead of diesel to power machines used by the farmers to process the raw coffee they grow.
This bio-gas project is a part of a larger project funded by the Swiss government's State Secretariat for Economic Affairs (Seco) to boost the incomes of Tanzanian coffee farmers.
It is a simple idea. Waste water from the processing of the raw coffee beans is high in acid and it is this acid that micro-organisms like to feed on.
The by-product of the feeding process is methane gas, which can be used to drive engines.
The savings could be considerable. It costs around $1 an hour to power the pulper machine, so the farmers group could save around $4 a day or $28 a week.
The waste water is produced when the beans are pulped
"We're amazed that we can make power from water which we have always considered as useless and just thrown away," said farmer Moses Charles Urio.
The waste water comes from the group's pulper machine, which washes and prepares the green beans picked off the coffee bushes.
The water is collected and its pH tested. Then ash from cooking stoves in the village is added to ensure the optimal pH level of 6.5-8.5 is maintained.
"The process is straight-forward," explains Cyril Chimila from TechnoServe, the organisation which has installed the bio-gas converter.
"Waste water which would otherwise be thrown away is being used to create energy which costs the farmer nothing."
Once the gas is harvested it can be used to power the very pulper machine which provided the waste water in the first place, creating something approaching a virtuous power supply circle.
The bio-gas scheme may also benefit the environment as it helps prevent waste water from pulperies draining away into the surrounding area.
The coffee farmers previously had conflict with communities downstream of Leguruti village who have complained of foul odours in their drinking water.
Its high acidic content can also cause severe environmental damage, stunting crop growth and ruining the soil's fertility.
There are around 55 pulperies run by farmers groups in the northern and southern coffee growing areas of Tanzania, all of which could benefit from bio-gas. But with set-up costs of around $4,000, few will be able to afford it.
The Arabica coffee grown in Kilimanjaro region is amongst the best in the world
"We are happy to test this technology," said Mr Urio, "but if we had to pay to set it up it would be very difficult for us, even though we understand that it would take only a few years to repay the initial investment from the money we save from not buying fuel."
"Rising fuel prices and the unstable nature of world coffee prices threaten to reduce farmers' profits," says Seco's Emmanuel Maliti.
"The bio-gas converter will help farmers to make more money from their coffee."
The coffee farmers of Leguruti are eagerly anticipating the beginning of the coffee harvesting season later in the year.
They are hoping that waste water may really prove to be a valuable commodity.