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Due to legislation (EU nitrate directive and national application), treatment of pig slurry for nitrogen removal is largely used in Brittany (France). Among available techniques, biological aerobic treatment represents more than 90% of the farm treatment units. This kind of treatment generates new products (solid phase from separation of raw slurry, supernatant and sludge) which could be managed by the farmer. A survey on the built units was performed and allowed to identify four main types of treatment units: (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 mass balances for nitrogen, phosphorus, copper and zinc. Between 60 and 70 % of the nitrogen was removed in gaseous form. Phosphorus, copper and zinc were accumulated mainly in the sludge resulting from separation of aerated slurry (60-90%). This accumulation led to a high concentration of copper and zinc up to 160 and 340 g/ton, respectively. 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 content.