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Anaerobic Digestion: the potential of organic waste

September 23, 2015

The ‘systemic approach’ is a founding pillar of the circular economy. It aims to consider the interactions and dependencies of every contributing factor in the production process rather than treating each one as a separate entity. Nature’s own example of this is the ecosystem, in which the interdependence between land and the maintenance of the food-chain stabilises biological activity.

Agriculture, along with the entire food-chain, depends on the conservation of ecosystems. Global initiatives are being developed which emulate this continual circulation of resources and as a result, there has been growing interest towards the benefits of anaerobic digestion over the last few years. This new bioprocess uses the by-products and waste from agriculture and food industries for biogas production, rethinking the existing approach which was simply the practice of ‘burying’ or ‘spreading’.

Lead image by Flickr user usepagov
Lead image by Flickr user usepagov

1. A multitude of ‘by-products’ and waste are gathered for use in anaerobic digestion. These include vegetative waste, most slaughterhouse waste, animal waste from agriculture and organic househould waste, and other sources will be able to be used in the future. The findings of a project by Biodecol launched in 2009 by the Cemagref in Rennes found that combining these resources for collective use encourages an economy of scale and produces a much better yield of biogas. This goes hand in hand with ‘Grenelle II’, a law that came into force in January 2012, which has meant the food-production industry and the restaurant industry – between them producing 120 tonnes of waster per year – are required to meticulously sort their recycling for this reason.

2. Anaerobic digestion plays a major role in this whole process, and consequently is very complex to set-up. It requires a very precise chemically-balanced environment where microorganisms can be added to begin digesting the feedstock anaerobically. A lack of oxygen induces the organic matter to start fermenting, and this produces methane and heat. Once purified, the methane becomes biogas and a source of renewable energy has been created. This biogas can either be put into the natural gas network or used to produce heat and electricity in cogeneration plants. We can now see that biogas is a viable alternative to fossil fuels which makes real use of unwanted organic waste. By 2020, nearly 1300 new facilities will be built in France – this has the potential of powering 800,000 homes with renewable energy.

3. Once the bigoas is extracted from the digester, there is a large amount of organic matter which remains – the ‘digestate’. This crude product still contains nearly all its major mineral elements N – Nitrogen, P – Phosphorous, K – Potassium and other ‘trace elements’ which are two key ingredients in agricultural fertiliser. In order to use these newly formed fertilisers in the most efficient way, the constituent parts are separated to create two liquid parts and one solid part which will be composted.

4. One of the liquid extractions contains 80% nitrogen and the other is a mixture of ammonia water and more than 90% potassium. These fertilisers are just as easy for farmers to use and are absorbed easily by their crops, and most importantly, they replace the usual fertiliser supplied by the chemical industries who are intensive users of both materials and fossil fuels energy. This approach however, must take into account the requirements of the crop and soil. Using the liquid by-product of anaerobic digestion to restore natural capital to soil is a step forward in finding a way to produce fertiliser from a waste resource, keeping nutrients in a cycle

5. The extracted solid part of the process is rich in organic matter and phosphorous. It undergoes one final process of composting so that it stocks an optimum amount of stable humus, necessary for the health of the crop. Once applied to the crop, the compost will help to prevent erosion in the soil by retaining a healthy amount of organic matter.

Amongst other advantages, this process dramatically reduces the volume of rubbish, and the normal odour associated with raw waste is eradicated through anaerobic digestion. It is also worth noting that this could be an opportunity for agricultural businesses to take a new direction in their exploits by developing their own treatment facility following examples in Germany and Scandinavia. A document by the Comité Biogas Régions describes the various initiatives developed within the EU.

One potential hazard that has been suggested is the excess of harmful minerals in the different phases when forming of the digestate. A preliminary study by Fabienne Muller for ADEME predicts that these fertilisers do not emit any more ammonia than traditional fertilisers. In fact, if inserted deep enough into the soil, very little nitrate and ammonia emission will occur.

Existing biogas plants show that there is still some progress to be made. Fabienne Muller and his field teams, taking samples from places where biogas fertiliser has been used, show there are certain organic pollutants such as dioxins, pesticides, PCB and heavy metals – mercury and lead which have not managed to be filtered out in the extraction process.

As with all new developments, this is a stumbling block that can be overcome. We must remember that this whole process relies on the initial input of waste material – if this already contains harmful or toxic matter, then we cannot expect to produce a pure, toxin-free fertiliser in the end result. It requires both the industry and consumer to change existing habits in order to achieve a safer outcome.

Conclusion

Biogas shows real promise for future implementation. It offers the ideal situation where a continual circulation of resources can be successful in the long-term rather than offering a temporary solution to the problem.

This process has yet another advantage. It gives a second-life to materials that would otherwise be considered as waste, giving them value when they would have had none. The result of using this previously unwanted waste provides a source of renewable energy in the form of biogas and organic fertiliser made from a veritable non-fossil-fuelled production method. In order to allow anaerobic digestion to meet its full potential, we must have a standardisation process which encourages the redirection of waste to landfill to waste to re-use.