Aerobic treatment of liquid pig manure in Brittany, France : experiences and performances

French intensive livestock production has become increasingly concentrated in northwest France over the past 40 years. The region of Brittany which accounts for only 7% of French Agricultural land, is responsible for nearly 60% of French pig production plus poultry and milk. This concentration of livestock supports a strong food production and processing economy. One of the by-products of such intensive livestock farming is large quantities of manure. In Brittany, intensive biological processes for eliminating nitrogen and removing odour are being installed on individual farms. About 400 farm waste treatment plants have been built and are actually running, and there is potentially 6 to 700 additional units to be built in the near future. Aeration of the slurry induces nitrification and subsequent denitrification into the atmosphere as nitrogen gas. Remaining slurry is allowed to settle naturally and separates into an odourless liquid and sludge. The treated liquid, which is around 65% of the manure volume, contains only 2-7% of the original nitrogen, and only 10% of the phosphorus, and therefore can be recycled rapidly for irrigation with a reduced environmental risk.