Operating the first stage reactor (60 m3) as a conventional UASB reactor at an HRT of 24 hr showed an average removal efficiency of 62% for CODtot during summer. The removal efficiency dropped to 51% during wintertime. However, the effluent suspended solids were stabilised with a VSS/TSS ratio around 0.50 all over the year. Moreover, the sludge developing in the one stage reactor is well stabilised and exerts an excellent settlability. Regular sludge discharge from the one stage UASB reactor had no significant effect on the performance in terms of CODtot removal efficiency; however, sludge discharge most likely resulted in a more stable performance of the system, as wash out of scum layer sludge would remain low. The removal of the stabilised solids from the effluent of the UASB reactor will provide an average total COD removal efficiency between 87-93%.