A novel reactor concept has been developed that combines Ca2+ removal and BOD conversion under mesophilic (30 degreesC) as well as under thermophilic (55 degreesC) conditions. Soluble Ca2+, present in many industrial wastewaters, precipitates as a result of forced aeration in the aerobic bioreactor. The CaCO3 precipitates, in turn, act as a carrier material for biofilm growth. This resulted in dense rapidly settling sludge granules covered with a biofilm on the surface. These sludge characteristics facilitate the application of hydraulic retention times of 1 h while maintaining a high biological activity in the reactor.