Combining different sources of biowaste to produce quality biogas

Combining nitrogen treatment and effluent energy production

In Brittany, intensive pig farming produces 10 to 12 million tons of slurry each year. This manure has a high nitrogen content and excessive spreading on fields pollutes superficial water bodies that serve for drinking water and causes eutrophication of rivers. Since 1991, the European nitrates directive has attempted to reduce the pollution of water by nitrates from farms. Farmers are now required to process their effluents before they may be used as fertiliser.

The goal is to reduce the nitrogen content by up to 70% using the biological nitrification-denitrification process. At the same time, methanisation would appear to be an effective means of extracting value from the slurry. That is why Cemagref in Rennes with INRA in Narbonne and Université Bretagne Sud set up the Digestaero project (2006-2009) with funding from ANR (National research agency). The goal is to combine the mandatory nitrogen treatment with a process to make use of the energy potential in farm effluents.

That is not simple because the two processes require very different conditions. Nitrogen treatment necessitates the presence of oxygen whereas methanisation must be carried out in the absence of oxygen (anaerobic digestion). After three years of work, an experimental reactor was produced to understand the phenomena and subsequently optimise the processes. A digital model was designed to identify the major parameters and then determine the optimum process conditions.

The Evalor and Odipure companies were brought on board for system industrialisation, promotion and technical monitoring. There are over 300 nitrogen-treatment plants in Brittany and they constitute the initial targets for Digestaero because they are already equipped with nitrogen-treatment equipment and storage capacities, thus offering reduced investment costs. This innovative technical solution means energy production will be almost sufficient to cover energy outlays.

Different substrates for a more effective process

One of the research results produced by Digestaero was that animal waste has relatively low potential for biogas production. The new Biodecol project launched in 2009 addresses methane production using numerous different sources of fermentable waste, including waste from crop silage, agri-food industries and households, in order to ensure the profitability and usefulness of methanisation. The research deals with the quality of the various substrates and different mixes in view of optimal energy production. The goal is to create charts presenting the best mixes for use by operators throughout the industry. In addition to the technical aspects, Biodecol also looks into the environmental impacts of methanisation. In conjunction with the Brittany Chamber of agriculture, full-scale tests have been carried out to monitor gas emissions during storage and spreading, and to determine potassium, phosphorus and nitrogen contents by comparing the various mixes. This project also includes a sociological dimension in that it attempts to mobilise all the concerned players in the region to ensure the viability of the process.

Industrial companies and local governments supply the production unit with waste and consume the produced heat and electricity. The project, initially launched on the scale of individual farms, has now reached the regional scale and all the concerned parties are brought in right from the design stage. The Ille et Vilaine departmental council has taken notice of the Biodecol project and will take into account the research results concerning territorial management of waste and methanisation in its waste-management plan.