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Biogas digesters: How feasible are they?

AGAMA Biogas did a feasibility assessment of biogas digesters at ten pig units on three piggeries, on request of the Premier Pork Producers (PPP).
At the organisation’s AGM (2008) , Greg Austin from AGAMA Biogas gave feedback on the assessment. He said the study should be used as an initial guide to assessing at a high level whether the farm in question warrants a more in-depth analysis, leading to design and implementation.He said more efficient and sustainable farming methods should be looked at, especially because of the current difficult financial position in which farmers find themselves, mostly due to the inflexible market price and rising input costs.
What is biogas?
Biogas is a clean, renewable, naturally produced and under-utilised source of energy. It is produced through an anaerobic biological process of conversion utilising any organic material (e.g. animal manure, urine and other agricultural residues).
The process breaks down the organic material and generates methane (CH4), similar to natural gas, which can be used for heating, cooling, electricity production, motorised vehicle fuel, etc. This biogas can be easily and safely piped, bottled, sorted, compressed and even liquefied.
What does a digester do?
A digester:

  • converts volatile solids to methane
  • reduces the pollutant levels in the waste, as measured by the Chemical Oxygen Demand or COD
  • reduces the odour from the waste
  • converts nitrogen to ammonia
  • can provide pathogen reduction
  • produces methane rich biogas
  • produces valuable fertiliser.

How does it work?
Through the action of bacteria, anaerobic digestion is the natural decomposition of organic matter in an anaerobic environment. The process requires at least two steps, namely Acidogenesis (organics converted to acetate and H2) and Methanogenesis (acetate and H2 converted to methane). These steps rely on a diverse and interdependent group of bacteria to complete the process.
Benefits to farmers
The benefits of biogas are endless. Not only is odour controlled in this way, it also is a source of energy, as well as soil fertility and fertiliser. Flies and weeds are controlled and reduced levels of Phosphorus are released into the environment. It also leads to pathogen destruction and carbon and other environmental credits. It also has value in terms of public relations.
Types of digesters
The types of digesters used depend on feedstock types, feedstock total solids concentration, facility location and management structure. Other factors include bedding materials, land availability and budget.
Getting value
To optimise the location of the digester for gravity flow, planning a digester around the proximity of the digester to gas use, the use of the nutrients flowing from the digester, and existing waste management practice are important. The output of the system can be optimised by finding thermal applications for the gas and finding heat applications if electricity is generated. It utilise the nutrients in the digestate, both solid and liquid.
Type of information reviewed

  • How many animals? How big are they and how much feed do they consume?
  • What is the volume of water used?
  • Other components of waste, e.g. bedding?
  • What type of flushing system is used?
  • What is the current energy usage and cost?
  • Are there any economic considerations?

Cost benefit analysis

  • The NPV represents the total value of the project in today’s Rand value. A project should only be developed if the NPV is positive. The higher the value, the higher the value of the project will be to the farmer.
  • The IRR is a measure of the return on the investment made, over time. Typically, the investor (farmer) should look for an IRR greater than a return on investment that can be made in the open market, i.e. a project with an IRR >15% should be considered an attractive prospect.
  • The BCR is a ratio that compares the net benefits (revenues) from a project against the net expenditures (outgoings) for a project. The BCR must be >1 for a project to be deemed appropriate, and the higher the BCR, the more attractive the project is.
  • The B/EVEN is the period of time measured in years where the cumulative net difference between income and expenditure for the project turns form being negative to being positive.

CBA assumptions
Assumptions made are that a project has a lifetime of 15 years, the inflation rate is eight percent and the discount rate also eight percent. It is expected that electricity as well as fertiliser will increase with 15% per annum. Carbon revenues do not increase annually, while the maintenance cost increase eight percent per annum.
Farm Biogas is being considered as a way to make large farms environmentally acceptable. There is tremendous interest in the farm community, as large farms and animal concentration increases.
Biogas is a viable option for distributed generation of farms. It appears to be economically viable in many instances, although a careful feasibility and design process is required.
The study has shown that the larger the farm (number of animals) the more financially viable it becomes.  It has also indicated that developing the project as a carbon project invariably makes the project financially viable. One of the big drawbacks with carbon finance is the additional costs associated with developing the carbon aspects of the project. Valuing and using the nutrient content of the digestate, against today’s chemical fertiliser costs, means that a biogas project pays for itself within two years.
It is recommended that farmers that have a need to manage odours, slurry, generate energy or are simply planning a new pig unit, should seriously consider placing biogas digester technology at the centre of their planning process.
For more info contact Greg Austin and 021-7013364.

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