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A major problem in anaerobic waste-water treatment results from difficulties with the mixing of sludge, when it has settled after periods of standstill. A new method for reactivating those plants is discussed in this paper. In a technical anaerobic plant (volume 9600 m3) a control problem led to a breakdown of the process, connected with a drastic increase of the reactor COD. The load to the reactor was temporarily stopped to remove the inhibiting conditions. Restart with low loading rates and addition of new sludge did not lead to the expected increase in performance. The main problem was to suspend the sludge layer, which was settled on the bottom of the reactor. For activating the sludge layer feed “shots” were added up to three times per day. These shots consisted of volume streams of high loaded waste-water, up to five times bigger than the normal stream at that time and lasting for 1 h. The shots provided the sludge layer much better with substrate and caused biogas formation which itself led to a suspension of bacterial flocs. The same strategy was applied again successfully for reactivating a second anaerobic 10,000 m3 reactor.