More aerobic ways to farm

Haase maize anaerobic digester Author: Alex Marshall, Clarke Energy Ltd October 2007

Like it or not, livestock farming continues to grow. Alisa Bryce explains how anaerobic digesters could help cut greenhouse gas emissions.

When we think green house gases we think global warming. The words global warming and climate change often conjure up images of smoggy filled cities crammed with exhaust emitting cars. Whilst there is truth to the impact of modern life on the climate, there are also seemingly innocent sources of emissions.

Agriculture is one such example. Agriculture contributes 16% of Australia’s gas emissions, second only to the energy sector (75%). Of this 16%, livestock contribute to about 70% of Australia’s agricultural emissions. The predominant livestock emissions are enteric fermentation (fermentation that takes place in the digestive system of ruminants) and manure.

Between 1990 and 2007, livestock related emissions declined by 7.5%. This reduction was predominantly a function of changes in stock numbers due to the fluctuating market, rather than smarter farming techniques to reduce emissions. Therefore, an upward trend in the ruminant industry is likely to increase the number of animals, and hence emissions.

To support world populations, the agricultural industry will continue to grow. And as such, emissions are expected in increase. Although Australia’s population is declining, other populations such as China, Mexico and the United Arab Emirates are expanding. And it is these countries which buy our produce. Whilst agriculture in Australia only accounts for 3% of the GDP, agriculture accounted for 35% of Australia’s merchandise exports from 2004 to 2008, compared to imports a fifth of this value.

Smarter farming
Increasing agricultural production doesn’t necessarily mean clearing more land for farming, but smarter farming techniques. Maximising production, increasing yield and above all, sustainable agriculture. Sustainable farming has been, in some minor form, a part of agriculture since agricultural practices began. The technique of inter-cropping to stave off weeds and pests was at the forefront of native American agriculture. Yet the importance of the techniques and intense research have only been the focus for the last 30 or so years.

Today, common and simple measures such as fallow stages, crop rotation, planting of leguminous crops, no-till farming, retention of native vegetation, water use efficiency and stubble retention are widely adopted practices halting the demise of the already degraded Australian land. These initiatives are, for the most part, easily adopted with little cost to the producer.
But there are newer technologies making their way onto the world’s technology carpet.

Anaerobic digestion is just one example. Anaerobic digesters essentially work by reducing waste to gas, solids and liquid stream. During  anaerobic digestion, aerobic micro-organisms ferment biodegradable matter to a variety of usable products, the most popular the biogas mixture of methane and carbon dioxide.

The use of this technology is becoming more popular, particularly in the United States and Europe, where below freezing temperatures cause a sizeable electricity bill. With the ability to not only produce their own heat and power, but to also sell excess power back to the electricity company, anaerobic digesters are gaining considerable favour.

The beauty of the process is that each of the by-products can be utilised in some way. Biosolids can be used for bedding or as a soil amendment, and the liquid stream as a fertiliser or if treated, for animal consumption. As the process removes the odour from the waste, the solids and liquid stream can be spread during the warmer summer months without the resulting unpleasant smells. Finally, if production is on a large enough scale, the process can provide enough biogas power to run the property. In the case of excess, this can be sold back to the power company for a profit.

The biogas produced offsets carbon dioxide emissions by displacing fossil fuel combustion i.e. reducing the dependence on fossil fuel for energy. As with any alternate energy, anaerobic digestion reduces the use of the finite and continually depleting fossil fuel stores.

Berrybank Farm Piggery, Victoria, is home to 15,000 pigs, producing 275,000 liters of sewage effluent each day. After installing a Total Waste Management System in 1989, the farm is now saving $435,000 a year by converting the effluent into biogas and fertiliser.  The process consists of seven steps, from waste collection to biogas conversion to heat and electricity.

Once collected, the waste is subjected to sedimentation to remove grit, thickened, then sent to the primary and secondary digesters. Here the waste is broken down into the gas, liquid and solid forms. Scrubbers, trappers and dehumidifiers then remove sulphur from the biogas, before it can be used for electricity and heat. Through this process, Berrybank Piggery produces a daily output of 2900 kW of electricity, equivalent to $125,000 per year.

An example of outputs and use from anaerobic digestion

Why such a low uptake?

As a seemingly self-sustaining process, the question persists—why doesn’t every ruminant property in the country have a digester? The high initial cost is probably the biggest factor holding back this technology. The capital start up required is close to $400,000, not including the costs of maintenance and general day to day running expenditures. Berrybank Piggery spent $2 million dollars setting up their Total Waste Management System. However the costs can be recovered in as little as five years, providing the scale of operations is large enough.

Risk is the second critical issue with the technology. Biogas produced is typically 40% methane and 60% carbon dioxide, but small amounts of water vapor, hydrogen sulfide, carbon monoxide and nitrogen are also produced. As biogas does not contain any oxygen, asphyxiation is a potential danger, as well as the danger of fire and explosion. The hydrogen sulfide converts to corrosive sulfuric acid at low temperatures, and engines must be designed accordingly to cope. And overall, it’s a biological process. Changes in the system, such as animal feed, can upset the process.

Anaerobic digesters are not suitable for every ruminant enterprise. The system relies on waste being easily available. In intensive enterprises, such as feedlots, where waste is easily collected from the one point source, the labour required to run the digester is minimised. However if the animals are kept across a vast area of land, collation of waste will be labour intensive.

Naturally occuring process

The concept itself is not new. Anaerobic digestion is a process which occurs naturally, and is well known in the bottom of ponds or lagoons. The process has also been used for over a century to process sewage biosolids. As such, anaerobic digestion is not limited to ruminant farm use. Any composting can be essentially utilised for biogas production, from vegetable and wine process, other livestock such as chickens and pigs and municipal waste.

Though relatively unknown now, anaerobic digesters, in some form, are filtering their way into the Australian market. For now, yes, it’s expensive, but the same thing was said about solar panels 40 years ago. Further research and development fine tunes the processes and technology, resulting in a safer, cleaner and often less expensive product.

Alisa Bryce is an Environmental Scientist with the URS Corporation.

A good diet…

There are other methods available to reduce livestock gas emissions, such as controlling diet quality and quantity. The higher the fiber content of the feed, the higher the gas emissions. Cattle grazing low quality pasture are likely to emit higher amounts of methane and cattle on greener higher quality pasture. This highlights the importance of high quality agriculture, and sustainable farming to maintain quality land.