An ultrasound (US)-assisted bioprocess for wastewater treatment for the food industry was developed in a three years EC-funded project (FAIR CT-3259). An improvement of biological activity by US was successfully demonstrated with lab scale reactors [Schläfer et al. Ultrasonics 38 (2000)]. Now a pilot scale optimisation with a 200 l bioreactor was carried out in co-operation with the Department of Industrial Acoustics, Technical University of Denmark and RESON A/S (Denmark). Batch experiments have been performed by measuring the degradation rate of organic compounds of wastewater. The process parameters were kept constant while varying the US-intensity. A significant increase of biological activity could be obtained only in a very narrow range of US-intensity. Very low US-power leads to a decrease of degradation rate down to the standard value if there is no US-intake. Implementation of too high level of US lead to drastically decreasing of the biological activity to a level far below the Standard value if there is no US-intake. The optimisation of this narrow US-power window needs to be carried out carefully for every investigated bioprocess. The most important aspect of the experimental results was that a more than 100% increase of the maximum biological degradation rate has been achieved by US-transduction with an optimum US-intensity of 1.5 W/l wastewater at 25 kHz. The developed bioprocess reduces the overall energy by use of low-energy US-irradiation below the cavitation level. The US-assisted biological degradation process would be presented together with its optimisation and the economical calculation.