By Maile Matsimela, Digital Editor at African Farming
At a recent field day held at Mme Moloto’s farm in Rooiwal, Pretoria (Gauteng Living Lab), Agricultural Research Council (ARC) researchers delivered a presentation that could reshape how South African farmers view their waste streams.
“So, for us we see waste as a resource,” explained Primrose Magama, a researcher at the ARC. “It’s not something that farmers should be getting rid of.”
Magama and her team focus specifically on finding ways to make organic waste generated by the farm sector useful to farmers.
A Decade of Research Bears Fruit
The presentation was the culmination of 10 years of focused research, made possible by collaboration with the Department of Agriculture.
Magama explained that biogas digesters are one of many technologies available to farmers for converting on-farm waste into valuable resources. Although the day’s focus was on digesters, Magama noted that their work is expanding to include biochar and biomass energy generation.
The Science and Economics of Biogas
Researcher Zikhona Buyeye took the technical lead, explaining that biogas is “a renewable energy source that is produced from the decomposition of organic matter in the absence of oxygen, and this is done in a process that is called anaerobic digestion”.
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Biogas, she said, consists of “50 to 70% methane, 30 to 45% carbon dioxide, as well as other small gases, but what we are looking for, the most important component that we want, is that methane”.
Common feedstocks for these systems include animal manure, food and organic waste, wastewater solids and industrial byproducts. “If you’ve got waste from your farming activities, instead of burning that waste or seeing it as a problem that you need to get rid of on the farm, you can actually use a biogas digester to convert it into energy,” Buyeye explained.
Multiple Technology Options
The researchers outlined several biogas technology options available in South Africa, each suited to different scales and applications:
Agama Systems: Prefabricated plastic tank digesters, approximately 6 cubic metres, buried underground. The estimated cost for small-scale pig or cattle farms ranges from R35 000 to R45 000.
Plug and Play Systems: Prefabricated, portable units ranging from two to 50 cubic metres, priced between R17 500 and R55 500. “If you have multiple points of use across your farm, this would be more suitable because it allows you to move it around,” Buyeye noted.
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Brick and Mortar Systems: Permanent installations of four to 10 cubic metres, costing R40 000 to R60 000. These systems “generally tend to be more durable” and are suited for long-term use.
Containerised Digesters: Larger-scale systems not necessarily buried, suitable for semicommercial operations or farmer cooperatives, with costs up to R11 million.
Industrial Scale: Large waste-to-energy digesters, such as the operational facility at Cape Town Biogas, with costs reaching R400 million.
Multiple Applications and Benefits
The applications for biogas extend far beyond simple cooking fuel. Buyeye outlined several key uses:
Cooking and Heating: Suitable for households with two to 20 pigs or one to four cattle, biogas can replace normal gas cylinders, paraffin and firewood. It makes for safer and cleaner cooking, and the operating costs are low.
However, she emphasised an important caveat: “Although it is technically feasible, we do not encourage using these digesters or … getting a digester [if] you have the [minimum number of] cattle or livestock that is needed because as we know, livestock can be a bit finicky. Cattle can die or they are slaughtered [or] sold.”
Electricity Generation: For small farms needing reliable power, biogas “can help with loadshedding. You can power your lights, pumps, fridges. It can cut your diesel generator costs.”
Combined Heat and Power: For larger operations, these systems generate both electricity and usable heat, making them useful for tasks such as cleaning pens, providing hot water and heating piglets, and are more efficient because they use both types of energy.
Processing Applications: These include “water heating, animal or carcass scaling, sterilising and small abattoirs”.
Waste Management: For all livestock farms, biogas systems help “reduce odours and [the amount of waste], improve cleanliness, prevent manure runoff and help with environmental compliance”.
The Daily Reality
Buyeye was candid about the operational requirements: “Biogas is not a technology that you can [pursue] part-time. If you are interested in it, there needs to be [a consistent supply of] work to feed [the system].”
The daily operation checklist includes consistently feeding manure, maintaining correct dilution, keeping foreign objects out, checking gas pressure, and monitoring inlets and outlets for blockages. “Digesters need daily feeding. You cannot have a digester that will work really well if it is not fed accordingly,” she emphasised.
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Safety Considerations
Given that biogas is flammable, the researchers outlined critical safety protocols:
Essential Safety Measures: Keep digesters away from flames, ensure proper ventilation, use pressure relief valves for larger systems and conduct regular safety checks.
Critical Prohibitions: No smoking around outlets, no fire exposure to digesters, preventing flooding of systems, avoiding long-term pipe clogs, and keeping children and animals away from gas pipes and valves.
Seasonal and Technical Challenges
The researchers acknowledged that “gas drops in winter without insulation”, something they’ve observed in their projects. Additionally, when converting biogas to electricity, “there’s actually quite a loss of energy”, making direct heating and cooking applications more efficient.
For electricity generation, systems may need hydrogen sulphide gas cleaning to prevent equipment damage. They also have higher upfront costs, and basic technical skills are required to operate them.
Environmental and Economic Benefits
Beyond energy production, biogas systems produce nutrient-rich slurry that can be used as fertiliser. The process also contributes to environmental protection by reducing greenhouse gas emissions and managing organic waste that would otherwise be “harmful to the environment”.
Magama noted that farmers can also “claim carbon credits from the way you utilise your waste”, adding another potential revenue stream.
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