Background For reasons of economic efficiency, a considerable and increasing number of locations in rural areas in Austria cannot be connected to conventional wastewater treatment plants. As a consequence, wastewater needs to be treated on site under specific local conditions, which are characterised by pollution loads that are fluctuating over time. Nowadays, the operation and maintenance of such small wastewater treatment plants is done by the site owners themselves who are generally not professionals in the field of wastewater management. Experience over recent years has shown that technical problems can remain unresolved for weeks and even months. Consequently, rivers and ground waters are exposed to the increased risk of damage. Objectives The project would aim to develop an online measurement tool to avoid the long-term failure of decentralised small wastewater treatment systems. Such an aim is in keeping with the objectives of the Water Framework Directive (2000/60/EC). In addition, the project planned to install a central service station where data from the WWTPs could be sampled, evaluated and controlled. Other actions foreseen was the optimisation of maintenance plans for the plants; the achievement of the same operating performance of small and onsite WWTPs as large centralised WWTPs without considerable additional costs; and the dissemination of the results to professionals, experts, authorities and potential users. Results The project developed a new maintenance system for small wastewater treatment plants. This system enables an efficient maintenance service and will in the long run lead to a better purification performance of the wastewater treatment plants in rural areas. The maintenance system is based on an online data processing, which provides relevant performance parameter via the Internet. As a result, a centralised and efficient maintenance service is now possible. Travel costs, which can be high in rural areas, are substantially reduced and the water authorities are able to remotely monitor the performance of the plants. The installed plants are operating by the SBR-system (sequencing batch reactor-system) which is characterised by a single tank which is fed discontinuously. The new measurement system consists of two devices. By using a light emitting diode and a photodiode, the first device, the opto-sensor, measures the total solids in the wastewater, the settling velocity and the purification performance. The second device emits ultrasound in order to quantify the water level. It can also take a recording of the temperature. These parameters are sufficient for a reliable process control. However, additional chemical analysis will be carried out in parallel, but much less often. The environment will profit by avoiding long-term failure of decentralised small waste water treatment systems as the maintenance of such small plants is significantly improved. The project constructed plants that can serve towns with populations of around 1,000. Based on a calculation of standard loads the project reduced the daily emission of 60 kg BOD, 120 kg COD, 12 kg Nitrogen and 1,8 kg phosphorus to the environment. The project team also constructed and operats 17 new small waste water treatment plants of different sizes, between 270 and 5 person equivalents (PE). The project generated a large amount of interest among wastewater management companies.