A farm-scale activated sludge treatment plant for separated pig slurry was operated with four different aeration cycles during a year with the objective of optimising the process and producing an effluent with low nutrient and pathogen concentrations. The high TC nitrogen-removals observed (over 95%) appeared to be FC related to the denitrification achieved through the intermittent operation of the aerator. It was necessary to supply, on. average, about 120 kWh/day (i.e. 10 kWh/pig) in order to obtain a treated effluent with mean concentrations of 31.6 mg N-NH4/l and 77.6 mg NO3- N/l The removal of phosphorus was more variable. Percentage removals ranged from 20 to 90%. Concentrations between 25 and 60 mg PO4-P/l were obtained in the treated effluent. From the experimental data and available observations it could not be concluded whether the removal mechanism for P was predominantly chemical or biological. Mean percentage removals of metals were about 90% for Cu and 85% for Zn. Although the TS value of raw slurry indicated a large dilution of slurry (dilution factor of about 4), the initial bacterial concentrations were relatively high. About 2-3 logarithmic units reductions were observed but concentrations of the order of 10(6) faecal coliform and 10(7) faecal streptococci were still found in the final effluent.