A recent news item in New Scientist highlighted a debate in California about the use of biodigesters on many of the state's dairy farms and their potential to reduce methane emissions. California hosts a large population of around 1.7 million dairy dairy cows and inevitably any changes in production systems initiate a political, as well as technical, debate.
The state government has awarded a significant grant of US$ 389 million to increase the adoption of biodigesters technology on farms and many are questioning whether this is the best use of government funds. The results look very promising, with 68% registering reductions and the overall result reducing methane emissions by around 25%. However, concerns have been raised about the potential for leaks with the new biodigesters, especially in the construction phase, becoming a source of leaked methane plumes.
Bio-digesters of course are not at all new: ruminants adopted this method of digestion millions of years ago and bio-digesters were developed to fill energy gaps during the Second World War. The principle is relatively simple; by fermenting plant material in an anaerobic environment, much energy can be extracted with methane being emitted and a residue of manure remaining. It has been shown that the ‘low carbon’ manure that remains after biodigestion emits 91% less methane than controls but the debate about the pros and cons of biodigester utilisation is technical, complex and sometimes confused.
As any clinical veterinarian will know, the rumen is a complex organ containing a population of microorganisms and relying on frequent mixing and the correct mix of plant material to function well. Bio-digesters are no different and the best outcomes result from good control of constituents and frequent monitoring of pH (acid levels). The pH is always important and so it proves with bio-digesters, with ancillary emissions of ammonia and sulphur dioxide, both of which are linked to the pH of the digester contents, having the potential for environmental contamination.
It is the leakage of methane that remains the core issue with some sources suggesting that more methane escapes than if bio-digestion were not utilised and others noting that surveillance methods are improving and that prompt follow up action can limit this potential hazard.
Of course, by burning the methane emitted and in many cases utilising the energy so released to heat water or power other processes on farms or in nearby industries, a positive energy benefit results. There is a market for biogas and it can be successfully sold and transported by tanker so providing additional economic benefits to farms. That this may incentivise dairy farm expansion is another argument contended by opponents.
The burning of methane produces carbon dioxide and water and so does not remove the greenhouse gas emission. It is argued that by lowering methane emissions, in effect by converting it to carbon dioxide, the short term negative impact of that gas on global warming is reduced; but the long-term impact of carbon dioxide,over hundreds of years, remains. This leads towards a debate on biological sources of methane versus fossil fuels, a topic which has been covered elsewhere. (See VetSalus article “Some Basic facts about methane.)
Biodigestion of farm manure is still relatively new and the potential to extract energy and lower methane outputs will be of interest to all veterinarians interested in reducing farm carbon footprints. The political debate, in California and elsewhere, will no doubt roll on but science will continue to modify the technology and the potential for a net benefit to farm carbon footprints seems obvious.