Renewable resources are abundant and widely distributed globally.
A number of commercial technologies are available to harness these resources promoting a shift toward more self-sufficient farms.
In Zimbabwe biogas is a renewable source of energy increasing in exploitation. It is produced by the action of certain bacteria on organic matter in the absence of oxygen, a process referred to anaerobic respiration.
This process, popularly called the “biogas process” generates a mixture of methane and carbon dioxide — the biogas.
A biogas plant (or anaerobic digester) releases and uses the stored energy from the plants and animal manures.
On-farm biogas production facilities typically utilise manure as the main substrate, but other materials such as food processing waste and crop residues can be added to increase biogas production.
When planned optimally, agricultural biogas plants perfectly fit into farming and soil nutrition cycles.
The anaerobic digestion of manure, agricultural by products and energy crops not only provides electricity and usable heat — it also produces a high quality organic fertiliser and soil conditioner.
The nutrients contained in the substrate are broken down during the biogas conversion thereby increase the fertiliser quality of the digested product on the arable land.
Plants can access the nutrients in digestate much easier than in untreated manure.
Depending on the type of energy production, the biogas is normally utilised on-site to produce one of the following: space heat (gas furnace); renewable electricity (engine + generator); renewable natural gas (gas scrubber).
The volume and nutrient content of the feedstock essentially remain constant during the anaerobic digestion process.
However, nutrients do change for during the process, potentially increasing fertiliser use efficiency.
Anaerobic digestion also produces valuable secondary products such as liquid fertiliser and livestock bedding material, as well as potentially marketable environmental attributes such as carbon offsets.
A biogas production facility is typically comprises the following components: pre-storage tanks and/or pads; grinder/mixer; reactor tank; biogas storage; gas utilisation equipment; heat exchanger unit; liquid-solid separator and post storage tanks and/or pads.
A majority of the small-scale agricultural biogas production facilities are operated at mesophilic temperatures (25°C to 35°C).
Thermophilic temperatures (49°C to 60°C) are applied in medium and larger scale biogas
production facilities with co-digestion when some of the inputs are from a non-agricultural origin.
Higher temperatures are usually required for more stringent sanitation.
The footprint of an on-farm anaerobic digester depends on the scale of the facility. An average-sized on-farm biogas system, including the digester and biogas utilisation equipment, will occupy less than a 1/4 hectare of space.
Typically, a digester can be easily integrated into farm landscapes.
The anaerobic digestion process breaks down volatile organic compounds, which reduces odour if due diligence is practised for pre-storage of the feedstock, especially with non-agricultural wastes.
Often, one of the main objectives of installing an on-farm anaerobic digestion system is to reduce odours, thereby facilitating good neighbour and community relations.
The process also generates other environmental benefits such as the reduction of greenhouse gases, pathogens, and the viability of weed seeds.
It also reduces the potential for water pollution because it decreases biological oxygen demand.
On-farm anaerobic digestion has the potential to generate energy security for the host farm, diversify farm income, and increase rural investment and employment opportunities.
Negative impacts include the potential for increased ammonia emissions.
All anaerobic digestion projects should include provisions to mitigate such concerns.
For example, adding a floating cover to the post-storage tank will reduce ammonia emissions and pumping manure through pipes to a digester will reduce the need to transport waste.
Nutrient management planning is recommended with anaerobic digestion to ensure the integrity of the nutrient balance of the farm, especially if off-farm waste is being digested together with manure.
The writer is Engineer Tapuwa Justice Mashangwa, a young entrepreneur based in Bulawayo, Founder and CEO of Emerald Agribusiness Consultancy. He can be contacted on +263 739 096 418 and email: tjmashangwa