Total phosphorus and dissolved ortho-phosphate was monitored in three different farm-scale biological aerobic treatment plants. For each step of treatment, a mass balance was established. Samples of raw slurry, solid products, aerated slurry, liquid effluent and sludge were analysed for their concentration in total phosphorus. Liquid products were analysed for their dissolved ortho-phosphate content. Three major streams result from the treatment system : separate solids, sludge and liquid effluent which represent 5% , 15-40% and 75-83% of the raw slurry, respectively. A mechanical separation step prior aeration allowed to export 25-30% of total phosphorus as organic fertiliser. A large amount of total phosphorus (e.g. 60-70%) was located in sludge. Phosphorus remaining in liquid effluent is about 15-25%. Raw slurry separation and sufficient aeration allowed to concentrate phosphorus in sludge. Insufficient aeration led to release phosphorus as dissolved ortho-phosphates within liquid effluent. Improvements of biological aerobic treatment to enhance phosphorus removal and/or recycling ability are considered.