At the beginning a high HRT (80 d), a moderate OLR (1.0 kgVSm3d1
) and a constant ammonia concentration of 6 g/kg wasapplied in order to establish a stable process. Within a period of 6 months HRT was slowly reduced from 80 d to 20 d keeping the
ammonia concentration constant. Although OLR increased from 1.0 to 2.5 kgVSm3 d
1 high specific methane yields (between 260 and 290 Nm 3
tCOD,cCH466–69 vol.%) and relatively low VFA concentrations (between 100 and 1000 mg/kg) were observed