Sludge
The residue or sludge from the fermentation process in the biogas digester is the main product of the
plant. It is a fertiliser of better quality than undigested waste because the nitrogen previously
unavailable to crops is transformed into water-soluble ammonia which is readily taken up by plants.
Experiments conducted locally and elsewhere have proved that sludge is a better fertiliser than
commercial, inorganic fertiliser. It improves crop yields when and if properly used, and farmers can
save considerable amounts of money that would otherwise be spent on purchasing commercial
fertilisers.
Chapter Five
TYPES OF PLANTS PROMOTED IN KENYA
Currently two major categories of biogas plants are being installed and promoted in Kenya. These are
the Batch Type and the Continuous Flow plants. The Batch Type plants have been installed mainly on
large coffee farms and the number of units installed by 1995 was estimated at thirty countrywide. The
Continuous Flow are more popular as they are suitable for use in small households. By the time the
survey was carried out it was estimated that about 850 domestic biogas Continuous Flow type of plants
had been installed in the country.
Batch Type plants
These are used on large farms where organic matter is available only during certain periods of the year.
They are particularly found on large coffee farms and are used to ferment coffee pulp that is available
twice a year.
Two operational plants were visited. One is located in Koru and was installed by Tunnel Technology
Limited for the Coffee Research Foundation.
The other is at Roma Lime Company Limited.
Feed material or slurry is added in one batch and left to ferment for a period. When it stops producing
gas, the slurry is removed from the plant and freshly loaded.
When a Batch plant is used, gas is not continuously available unless a big gas holder is incorporated or
several plants are used.
Continuous Flow plant
This type of plant requires the addition of feed material on a daily basis and produces gas at a constant
rate. Continuous Flow plants are useful in farms where organic material is available daily. When feed
material is added to the plant, an equal amount of sludge is discharged from the unit. It is discharged
into a sludge storage tank from where it can be directed to flow by gravity on to the farm. It can also be
removed manually. Two types of Continuous Flow plants are commonly disseminated in Kenya. These
are the Floating Drum and the Fixed Dome types.
Floating Drum plants
These plants have a large inverted drum which acts as a gas storage tank. They were designed and
developed in India but have been widely accepted in the developing world. They are easy to construct,
operate and are reliable. The Borda-sasse model is the most widely disseminated plant in Kenya. It has
performed very well under local conditions.
The Hutchinson type, built and disseminated by a local company, Tunnel Technology Limited, is
another model of the Floating Drum plant which has been available in Kenya since 1970. It has also
performed remarkably well.