Generating Biogas from Biomass

Farmers in the UK are still discussing the merits of anaerobic digestion, but in Italy things are moving on along. When the European Union first set out its environmental sustainability parameters, the Italian government responded with an incentive scheme to encourage small farms to generate electricity from biomass energy crops using anaerobic digestion. One of those farms was the Azienda Agricola Brusadino. It was approved under the scheme in the autumn of 2007 and Veolia Water Solutions & Technologies constructed the digestion plant and commissioned it in November 2008. 

Although the technology is sophisticated, the system is quite simple in concept. Maize silage is macerated with dilution water before two-stage digestion produces the gas, which is burnt in a gas turbine to generate electricity. A heat recovery system maintains the digester at a constant temperature of 55°C with excess heat being used for site heating. Digestate is stored in a 180,000m³ lagoon until it can be spread as a fertilizer on the fields where the maize is grown, closing the ecological loop.

Although it was originally designed to digest maize silage, the plant was subsequently modified to allow other liquid agricultural wastes, including slurry from the farm’s livestock, to be added to supplement the macerated maize feedstock. It now treats 60t/d of silage along with 39t/d of liquid farm waste to produce 11,300m³/d of biogas. With a capacity of 1000 kW, the plant produces 86.4t/d of high nutrient digestate, a stable and totally odourless fertilizer that satisfies most of the farm’s fertilizer requirements.

A major benefit of the process is the opportunity to re-use the digestate on the farm as a valuable "free" source of organic matter containing nutrients such as nitrogen, phosphorus and potassium. However, in order to re-use the digestate as an organic fertiliser farm it is necessary to monitor it to ensure that there are no toxic materials present, that the moisture content is in the right range for pumping, that the nutrient balance is correct for the different crops being cultivated and that it is odourless, sanitised and stable. All these goals have been achieved successfully by the Brusadino project’s anaerobic digestion process.

The Azienda Agricola Brusadino project demonstrates that anaerobic digestion of energy crops and agricultural wastes can be an effective means of generating electricity from a renewable source, along with a low cost means of waste disposal and fertiliser production. It also provides an opportunity to import wastes from food preparation and packing facilities at commercial gate fees, reducing the load to landfill. In addition, waste heat and carbon dioxide from the generator flue gas can be used in greenhouses to enhance plant growth. We believe that large individual farms or groups of smaller farms in the UK should now be benefiting from this kind of technology. As the Azienda Agricola Brusadino project has demonstrated, this really can be an opportunity from which all parties, including the environment, can benefit.