Farm Size Digesters

Probably the biggest challenge in the biogas industry in South Africa is to find an affordable biogas solution for the average small to medium size farming operation - with the main challenge being balance between capital cost and return on investment.  The minute a farmer has access to a reasonable amount of organic waste, the capital cost of the biogas plant (Civil work, digester tanks, biogas generator and mechanical & electrical installations) tend to start running into the millions of Rand range.  The return on investment is directly dependant on the replacement cost of Eskom bought electricity and for these projects this normally only shows a payback period of 8-10 years, making it a risky and unattractive investment for the average farmer.  It is only when the biogas plant has a capacity to generate in the order of 0.5MW when the payback period is potentially reduced to around the 5 -7 year mark.  These numbers are only indicative and should not be taken as gospel – there are simply too many variables that will influence the eventual viability of such a plant.

BiogasSA has developed 2 options that could potentially provide a solution into this market segment:  the large Biobag and the Floating Digeste

a) The Large Biobag

BiogasSA can provide Biobags up to 50m3 in size and if the installation is done by the farmer, the costs can be kept low enough to show a 2-3 year payback period.  These type of projects could typically generate enough biogas to provide energy for the staff accommodation (stoves for cooking, gas lights, space heating, water heating and potentially enough biogas to run a small generator to provide electricity to charge cell phones and run TV’s).  Alternatively the biogas could be used to run a generator to provide electricity to the main house.

Unfortunately it is impossible to provide a generic solution when it comes to biogas as there are simply too many variables (type of feedstock, quantity, temperature, mixing, heating, geographical location, etc, etc) any one of which could either result in doubling or halving the biogas production.  
Just to provide some numbers, the following can be used as a rough indication:

A 50m3 Biobag Digester could potentially produce 25m3 of biogas from the manure of:

  • 200 grown pigs
  • 35 cattle
  • 3500 chickens
  • 700kg vegetable waste

Note: manure is the full daily production of each animal, no straw or other bedding material.

Again, as indication only, 1m3 of biogas will provide the following:

  • One hour cooking time on a 2-plate biogas stove
  • 20min hot water from a 6l/min biogas geyser
  • 15 hours lighting on single biogas light
  • 1.5kWh of electricity by running a biogas generator
Two 50m3 Biobags installed for a pig farmer











b)    Floating Plug Flow Digester

BiogasSA is in the process of developing a medium size digester built from either concrete or brick & mortar, covered by a ‘floating’ type gas dome.  The design again allows for most of the construction and installation work to be done by the farmer, thus keeping the costs as low as possible.  
The main digester is a rectangular construction with a inlet and outlet manhole very similar to those of the Biobag installation, just on a much larger scale.  The edge of the dome is anchored in a water channel that runs around the structure, which also serves as gas trap.  This type of structure allows for practical installation of both heating as well as mixing options as pipes can easily be installed through the concrete or brick walls.  The dome can also easily be removed should it become necessary to gain access to the inside of the digester for maintenance purposes.

BiogasSA installed its first pilot Floating Plug Flow Digester at an Aids Clinic in Johannesburg and is currently busy with a 100m3 Floating Digester for the Cane Growers Association in Durban.


Pilot Floating Plug Flow Digester in Johannesburg
Installed 200m3 Floating Plug Flow Digester in China