Biological aerobic treatment of pig slurry in France: nutrients removal efficiency and separation performances

Four types of biological aerobic treatment units were identified in France: (1) intermittent aeration without any separation, (2) intermittent aeration followed by sedimentation of aerated slurry, (3) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and sedimentation of aerated slurry and (4) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and mechanical separation of the aerated slurry. Among the 26 treatment units built in Brittany (June 1999), three were studied for 8 weeks. This monitoring period allowed to establish nutrient distribution for nitrogen, phosphorus, copper and zinc. Between 60 and 70 % of the nitrogen was removed in gaseous form. Mechanical separation of the raw slurry (press-auger) concentrated 25-30% of total phosphorus, 10% of total nitrogen and 8-12% of heavy metals (copper and zinc) into a solid fraction representing 4-5% of the raw slurry on a weight basis. Separation of the treated slurry using sedimentation or mechanical separation concentrated phosphorus and heavy metals in the sludge (60-90%). This separation could lead to a high content of copper and zinc in the sludge up to 160 and 340 g/ton, respectively. Consequently, the use of this product on the farm as organic fertilizer led to an overload with regard to agronomic requirements and export was difficult due to the high copper and zinc contents. The clarified supernatant obtained by separation of the treated slurry could be irrigated with a reduced environmental risk. Concerning the biological reactor, the variation of the volumetric load owing to the variation of raw slurry characteristics was identified as the main parameter influencing the treatment efficiency.