A biodegradation system used to treat the discharged concentrated liquid effluents from a tuna-processing unit is described. The study includes a characterisation of the industrial effluents, the design of a bioprocess treatment and the testing of its performance. The liquid effluent to be treated contained 1570 mg/l suspended matter, 1450 mg/l fats, a chemical oxygen demand (COD) of 11 100 mg/l and a biochemical oxygen demand (BOD) of 6600 mg/l. The designed system included a physical pre-treatment unit used to remove the fats and the suspended solids through a decanter, an anaerobic digester and an activated sludge aerated bio-reactor. The pre-treatment unit allowed the elimination of 40% of the suspended matter and fats expressed in terms of COD. The anaerobic system transformed 45% of the dissolved COD into methane gas with a production of about 0.25 m(3) methane per kg of degraded COD. The aerated treatment unit achieved 85% reduction of COD. An integrated system, combining the three steps, permitted the processing of 1.2 kg COD/m(3) per day and the removal of up to 95% of the COD with minimal energy consumption and minimal sludge production. (C) 2000 Elsevier Science Ltd. All rights reserved.