In a sewage treatment plant that includes anaerobic digestion, ammonia-laden filtrate from anaerobic digestion is treated for breakdown of ammonia and optionally for removal of nitrogen, while at the same time thickening the waste activated sludge, preferably with membrane thickeners. This is efficiently done in a two stage process. Liquid filtrate from this subsystem can then be sent back to the head of plant or it can be disinfected. Preferably the filtrate discharged from the subsystem is denitrified, although in some plants it may be desirable to retain some of the nitrates for odor reduction at the head of plant, providing oxygen to neutralize hydrogen sulphide.