Every year, the Household Waste Treatment Plant (UTOM) at Saint-Malo processes about 25,000 metric tons of waste, which is turned into 8,000 tons of compost.
The waste is deposited in a large cylinder, known as a stabilized bioreactor, the core element of the composting unit. This pre-fermentation stage tube is continuously rotated for three days at a temperature of 60° Centigrade, enabling the organic parts to decompose, leaving only a small fraction of the initial waste. A refining stage then follows, with the metals, glass, inert material and plastics being separated from the organic matter. This type of refuse, which represents less than 50% of the original waste entering the system, is then transferred to an energy recovery center. Before the start of a two-stage composting process, the fermentable waste is mixed with shredded green waste in order to help the circulation of air. For four to six weeks, this mixture is kept in fermentation tunnels where temperature, hygrometry and oxygen concentrations can be monitored and regulated. After being turned over, the mixture completes its aerobic decomposition phase in designated storage areas for a further three to four weeks. After sifting, the end product is then kept on a storage platform for several months before use.
The net result is that the center produces a high-quality compost that not only meets all the current norms in place for use as an organic additive for soils, but also meets the even stricter regulations laid down by CERAFEL, a French agricultural body that represents seven fruit and vegetable producer organizations.