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As more and more people move to urban areas, bringing ever-greater needs for energy and taking an increasing toll on the environment, biowaste looks like an attractive resource whose management must be incorporated into the sustainable development of territories. That is why partners in the European project DECISIVE have developed appropriate technology for reaping all the benefits of biowaste and run full-scale tests of a micro-AD plant at territorial level.
Proven robust technology, local energy, and a biopesticide to boot
Several micro-AD processes (supplied by two different manufacturers) with biogas recovered by Sterling engines[1] have been tested over several years. The DECISIVE consortium, notably through INRAE’s lab in Rennes, has also developed a new micro-AD process that consumes less energy. This new process consists of two phases: first, a solid biowaste is introduced into a cylinder that acts as a piston, where the waste breaks down thanks to a recirculating liquid called leachate. The products from the disintegration extracted by the leachate are then transformed into biogas in an adjacent tank. When the cylinder is removed, a solid residue, digestate, is retrieved that can be post-treated and re-used. The DECISIVE project tested fermentation on solid substrate in Spain with a view to manufacturing a biopesticide.
When it comes to nutrients, the consortium showed that biowaste, which has high methane-generating potential, helps produce a digestate that contains mineral nitrogen that can be used in fertilisers. However, research still needs to be carried out on returning the digestate to the soils, because while the biowaste is treated[2] before undergoing methanization, reusing animal residue from food waste requires strict supervision for health reasons. What happens to pathogens in biowaste management networks is therefore a major subject of study.
An unprecedented full-scale test in a dense urban area
After three years of assessing local biowaste flows and research into the most appropriate technology for their recovery, a pilot demonstration site was set up in Ecully[3] in the greater Lyon region in November 2019. The micro-AD unit, built by a French-Chinese company and surmounted by a solar panel, was designed to recycle a minimum of 50 tonnes of biowaste per year – or about one tonne per week - to close the energy loop. It was designed to produce biogas, 80% of which was used to heat the unit and provide hot water needed for purifying, and 20% of which was used to generate electricity for an urban farm. This unprecedented pilot site in a dense urban setting was planned to be fuelled mainly by biowaste from the restaurant and catering sector (collective catering, traditional restaurants, and waste from neighbouring retail premises). In the urban farm, the goal was initially to recover biowaste collected within a radius of 5km, and the site was sized to produce 20m3 of biogas per day and 1 to 2 kWh in cogeneration, and to deposit the digestate into nearby fields. Despite the pandemic, which slowed down waste collection, 4.3kg of organic material per m3 of reactor was recovered thanks to the micro-AD unit, producing 15m3 of biogas per day.
The concept of decentralised organisation tried, tested and true
From a technological point of view, the installation of a micro-AD unit in a dense urban space has proved efficient, even attracting new players. The organisation of a short circuit of recycled organic material also ultimately demonstrates the potential for creating jobs. The technology, which can easily be replicated, has also proven to be adaptable since it can be transported as needed. The micro-AD unit at Ecully is now set up in Narbonne, and a new site in Italy capable of treating up to 400 tonnes of waste per year is currently being installed in a mountainous zone.
A study carried out by a German partner is looking into how dedicated people are to sorting biowaste in their homes. The results of this sociological study in households of varying socio-economic profiles will help fine-tune policies related to managing waste at the source.
Beyond technological processes, the success of such decentralised waste management programmes requires the commitment of local players (local authorities, restaurant owners and caterers, suppliers, farmers, etc.) and the cooperation of ordinary citizens.
To see the record of the final conference and download the presentations: http://www.decisive2020.eu/news-item/large-audience-gathers-for-the-decisive-final-conference-and-its-main-conclusions/
*The act of 17 August 2015 on energy transition for green growth.
**INRAE, coordinator, SUEZ, Refarmers (France); UAB, ENT, Aeris, ARC (Spain); AU (Denmark); TUHH, GEOMAR (Germany), ITS, AT (Italy), ACR+, PSUtec (Belgium).
[1] A Stirling engine is an external combustion cogeneration engine. Temperature differences that alter the internal gas pressure in the engine trigger a piston whose mechanical energy moves a coil, in turn generating electricity. The external combustion allows for a minimum purification of the biogas before combustion on the external burner. Traditional cogeneration engines are internal combustion engines. Here too, the biogas must be purified before combustion to prevent the engine from being damaged.
[2]Treatment consists of subjecting waste to temperatures of 70°C for one hour after grinding to a maximum of 12mm-particles.
[3] At CFPH, a public horticulture training and promotional centre in Ecully, near Lyon, in collaboration with Refarmers, a local company specialising in urban farming.
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What is methanization? Methanization (or anaerobic digestion) is a biological process whereby organic material disintegrates without oxygen. Carried out by micro-organisms, the breakdown of organic substrata in a closed and controlled environment allows for the maximum production of a gas called biogas, which is rich in methane. If the process involves injections into natural gas networks, the biogas that is produced is purified (the other gases in the mix are extracted so that only methane remains) in order to obtain biomethane that has the exact same properties as natural gas. At the end of the process, a pasty residue, called digestate, is obtained. Digestate is a by-product of methanization that contains fertilising elements (nitrogen, phosphorous, potassium) initially introduced in the AD unit via substrata. Like effluents from livestock farming, this digestate is an organic fertiliser that can be used in growing crops. |