Nigerian Converts Septic Tank into a BioReactor
Olatubosun Obayomi Adeleke reports on his progress in converting a septic tank into a biogas reactor at a guest house in Abuja, Nigeria. The major idea and inspiration of this effort is that septic tanks can be converted over to bioreactors for a very minimum cost in Nigeria to provide energy and fertilizer for the gardens.
The focus of the project is to demonstrate how a biogas facility can be developed using local organic waste to produce electricity. This innovative project if developed into a best practice could potentially be a low cost way to increase power reliability in regions like Nigeria where power outages are an common event.
About the Digester and the Process
A particular kind of reactor; an Upflow Anaerobic Sludge Blanket design is used. The UASB is basically a system that uses the build up (that’s sludge blanket) of solids granules in wasteflows to filter solids. It works at very low pressures as the flow of incoming effluent forces the effluent forward into the system. The solids particles are digested by bacteria as they flow through the sludge blanket. As the bacteria digest them, they release gas (biogas), which flow to the top of the digester and is then piped out as a energy source. When the solids granules are fully digested (about 30 days), they are discarded by the mat/sludge blanket in much the same way an animal excretes what it now sees as spent matter or waste. These granules then mixed again with the effluent and flow out into outflow pipe of the digester for further processing. In this particular configuration he is using a Horizontal design. It works the same way with the vertical UASBs, but the flow of wastes is horizontal while the gas still flows vertically. It has a baffle to retard wastes for a longer period to form the slugde banket as the vertical cone does in the vertical UASB.
Converting Septic Tanks to Transform Waste into Resources
Normally the septic tank encourages growth of pathogens and drains directly into the ground through what are called Lateral Lines (a series of pipes diffusing the septic tank effluent flow into the ground). By converting the septic tank into biodigester BOD is reduced by 60%. The second chamber is designed to expose effluent to sunlight to enable a further reduction in BOD while draining. The digester opening will be sealed with cement and will only be opened for repairs.
Collection of Solids in the Manhole for use in the Gardens
In these kind of digesters, there is a element called a Manhole. It is a box on the side and in some digester it is the point of solids collection. The solids can then be used to fertilize gardens. After the waste is processed in the digester, it enters the manhole where the solids settle at the bottom. A pipe then links the manhole to the drainage chamber where the effluent is allowed to settle into the ground.
Pre-mixing the Waste
All pipes from the residences of the 8 residence addition to the resort will be connected directly to the digester system. However the occupants will not provide enough biomass production to satisfy the digester and so a nearby farm has been selected as a source for animal waste that will be added and mixed with the human excrement and kitchen wastes. In the system there is a separate, premix chamber where the farm waste will be added. The mason is seen working on the premix chamber (Picture 19). By Obayomi’s estimate the waste mixture will be: 10% Human excrement; Kitchen waste 10%; and animal wastes 80%. The plan is to supply the wastes in bags in a dried state, to avoid odor in transportation.