Mono-combustion of sewage sludge is conducted currently only in few big incineration plants with a capacities of more than 50,000 t/a each. The technology used for the combustion and cleaning of the flue gas is quite complex and an economic downscale is quite difficult. Therefore, small wastewater treatment plants do not burn the sludge on-site but send it to one of the big plants or to a waste incineration plant respectively a coal power plant for co-firing. But the transport of wet sludge, which contains approx. 75 wt% water, is not economic. Especially co-firing is critical from a sustainable point of view, as the fertilizer phosphorous, which is contained in the sludge, is lost in the ash. Therefore, simpler but just as clean technologies should be developed to increase the amount of on-site combusted sludge. A key factor, in technical and economical respect, is the flue gas cleaning system, which represents more than half of the apparatus of a conventional plant and it is also responsible for the biggest amount of operational costs. In the current research project a flue gas scrubbing system for simultaneous removal of HCl, SO2 and NO was developed. Effluent water from a wastewater treatment plant is used as scrubbing liquid in a combined quench/spray scrubber system. Afterwards the scrubbing liquid is sent back to the wastewater treatment plant for regeneration in bioreactors. The scrubber system was installed and tested at the site of the sludge incineration plant Bad Vöslau. Experiments with varying liquid-to-gas ratios, several gas and liquid temperatures and different scrubbing liquid recirculation rates have been carried out to investigate the capabilities of the new scrubber system. As the flue gas composition was measured simultaneously before and after the scrubber system, the removal rates could be determined for each operational state. In these experiments removal efficiencies of up to 100% for HCl, 95% for SO2 and 55% for NO were achieved. It was found that an increased liquid-to-gas ratio and the scrubbing liquid recirculation has a positive effect. A major finding was that the NO removal efficiency depends on the nitrite content of the fresh scrubbing fluid. During the absorption process NO can react with nitrous acid or nitrite and form N2O3, which has a much higher solubility compared to NO. This behavior makes high NO removal rates possible without preliminary oxidation of NO to NO2 in the gas phase. Based on the experiences from the experimental series and the investigated absorption mechanism, it was found that the system is suitable for the intended purpose and can be used as a single flue gas treatment system after a dust removal system. With an eye on the effluent composition of the wastewater treatment plant, this system is appropriate also for other sludge incineration plants located on or near a wastewater treatment plant.