In Rennes, studies are being conducted to combine livestock liquid manure methanization processes with a biological nitrogen removal process. The expected benefits are measured in terms of energy savings and preservation of river water quality.
The July 2006 increase in the price of electricity repurchase has given a boost to biogas production through methanization. This will delight farmers in western France (Brittany, the Loire Country, and Lower Normandy) who produce more than 50% of the country’s livestock manure. However, in the zones where livestock raising is the most intensive, the priority goes to removing organic nitrogen to limit water pollution. Yet anaerobic digestion to produce methane does not affect the stocks of nitrogenous matter responsible for the eutrophication of the aquatic environment.
An ambitious project
In Rennes, within the DIGESTAERO* project, a PhD dissertation is being conducted in collaboration with Breton industry leaders to combine the processes of methanization and biological nitrogen removal by nitrification and denitrification. These processes already exist in the food processing industry (distilleries, fabrication of dairy products), but they are not used in the manure treatment context. Indeed, to limit installation costs, the digesters should be able to be integrated into biological nitrogen removal units that already exist on the farm.
Moreover, the treatment unit should take into account inputs that vary substantially in terms of type and composition. For example, an effluent’s rate of biodegradable material varies from 30% to 40%.
Equip livestock breeders in 2009
First, an experimental pilot made up of two 100-liter tanks was developed to study the phenomena involved and then optimize the processes. At the same time, a numeric model was designed first to identify the important parameters and then to define the optimal industry for the process. Today, the tool provides a good response to the variations in the liquid effluent composition. A first industrial prototype should come out in 2009 so that it can be adjusted to full-size operation. This will be the last step before the industrial development of this new process.
Methanization: gains at all levels
With 300 million tons per year of animal manure from livestock breeding, France has one of the greatest potentials of agricultural biogas production in Europe. For example, the digestion of 1 m3 of pig liquid manure produces approximately 25 kWh as electricity and as much as heat. Since July 2006, each kilowatt is bought back up to 0.14 euros, instead of the 0.05 euros paid initially. These gains are also important for the environment since methanization can reduce greenhouse gas emissions from a livestock operation. Finally, odors are also reduced with manure management.
"What development model for the methanization of livestock effluents is needed in France?" This is the theme of the conference organized in Rennes by the Cemagref, INRA, the University of Southern Brittany, Valétec, and Odipure, to be held on 21 November 2008. The debate will center around the results of the DIGESTAERO project .
*DIGESTAERO, a project financed by the ANR, associates the Cemagref, the Narbonne INRA, the University of Southern Brittany, and two Breton industries: Valétec and Odipure.
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