Fixed beds in anaerobic operation (e.g. digestion with methane production) can be run in recycle mode to avoid gas building up in the packing. With low gas production rates, recycling is not needed, and a gradient of concentration may be established. In contrast to fluidised beds, no hydrodynamic requirements have to be fulfilled, except for even distribution of the flow over the entire cross-section. It should be remarked that as with gas absorbers or other packed bed operations any maldistribution of the liquid flow will result in poor performance (e.g. channelling). Plugging of the beds may occur as rapidly as every 1 or 2 days and this makes periodic cleaning necessary. Injecting air and using a higher liquid flow rate for a short period will normally dislodge excess biomass. This sudden burst of biomass in the effluent has to be separated (e.g. by sedimentation) and collected for further treatment.