Background The amount of sludge generated in the European Union has increased dramatically, amounting to 9 million tons of dry matter in 2005, compared to 5.5 million tons in 1992. This is mainly the consequence of the implementation of the 91/271/EEC Directive on Urban Waste Water Treatment. The sludge can be exploited in agriculture, a solution that the European Commission encourages. But up to now, only 53% of the total amount of sludge has been used as fertiliser. The reasons are, in some cases, the poor quality of the sludge in terms of public health and environment, or the reluctance of farmers in peri-urban zones, or more using intensive practices. The remaining 43% is disposed in landfills or incinerated. Reduction of sludge has therefore become a major issue for Member States. Objectives The project aimed to show the technical and economical feasibility of a new technique, based on ultrasound, to reduce the amount of sludge produced in a biological water treatment plant. The experiment has been carried out at full scale, treating a part stream of the return sludge of a small water treatment plant (6 500 inhabitants/equivalent). The technique was expected to reduce the quantity of sludge by at least 30%, therefore diminishing the pressure on the environment. It would also notably lower the amount of pollutants (nitrates and pathogens) in the discharged water. The data obtained were used to elaborate an expert system allowing the comparison of different scenarios of sludge reduction, treatment and disposal. Results The overall aim of the project was to show the technical and economical feasibility of a method to reduce the quantity of sludge (i.e., waste) produced in a biological water treatment plant by at least 30% without a transfer of the environmental pressure, to promote the use of this method and to inform and train its potential end-users. The method studied is the ultrasound treatment (‘US’) of the return sludge stream in a waste water treatment plant (WWTP) of 6 300 population equivalents. The project involved the following tasks: description of the initial state of the water treatment plant (operational, economical and environmental parameters); design and installation of the prototype ultrasound system; one-site installation; follow-up of operational, economical and environmental parameters, constant optimisation of the system (for 1.5 years); creation of an expert system allowing for a site specific technical/ economical/ environmental evaluation of different sludge treatment (including the ultrasound treatment); various dissemination activities. The results obtained provide a first evaluation of the effects of ‘US’ as an ‘at source treatment’ on the production of sewage sludge at real scale and also on its environmental impact. Sludge reduction was evaluated by comparing the measured amount of sludge produced to the theoretical amount (according to a well known equation) for periods without and periods with treatment. A sludge reduction of around 26% was demonstrated. However, this 26% value needs to be considered with care because of the high level of uncertainty of the measured amount of sludge. The ‘US’ treatment seems to have had no impact on the quality of the thickened sludge. Concerning the water treatment performance, it seems that the ‘US’ treatment impacts only the phosphorus concentration at the outlet of the WWTP. Even though the total C02 emissions seem to increase during the treatment, the total direct emission of greenhouse gases seems stable. Finally, no environmental pressures were observed. The treatment had no significant impact on the energy consumption of the WWTP as a whole. Thus the direct environmental benefits of ‘US’ treatment were demonstrated with the reduction of the quantities of sludge to be spread and therefore of the corresponding transport. The economical evaluation showed that the treatment is not cost effective for such a small waste water treatment plant with low sludge handling costs (€10 /ton). Extrapolation of the results showed that after some process modifications (treatment of thickened sludge and return of the treated sludge in the aeration basins) the process could be economically feasible for larger waste water treatment plants i.e. for a size of 100 000 population equivalent and with final sludge elimination costs > €11 / ton (DM 20%). An expert system (EcoBoue) was constructed. Linked with a database, the system allows a comparison of treatment, disposal and sludge reduction methodes from a technical, economical and environmental point of view. The system is based on a construction game in which each treatment procedure is represented by a unit operation. The user has the possibility to create treatment scenarios (combination on different unit operations) and to compare them on the basis of cost, emissions of greenhouse gases, etc. The system together with its user guide can be downloaded from the project website at: http://extranet.groupeirhenvironnement.com/soundsludge/ Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).