Raw sludge (raw sludge): Primary precipitated sludge without sludge treatment. 

Secondary sedimentation of excess sludge or mixed sludge of both. 

Primary sedimentation sludge (primary sludge): sediment discharged from the primary sedimentation tank. 

The sediment discharged from the secondary sedimentation tank (or sedimentation zone) by the secondary sedimentation sludge (secondey sludge):. 

Flocs containing various aerobic microbial populations are propagated in activated sludge (activated sludge): aeration tank. 

Digested sludge (digested sludge): after aerobic or anaerobic digestion of sludge, the concentration of organic matter decreased to a certain extent, and tended to be stable. 

The reflux sludge (returned sludge): is separated from the secondary precipitation (or sedimentation zone) and returned to the activated sludge in the aeration tank. 

Excess sludge (excess activated sludge): activated sludge system discharges activated sludge from the secondary sedimentation tank (or sedimentation area). 

Sludge gas (sludge gas): is the gas produced by the decomposition of sludge during anaerobic digestion, which is mainly composed of methane and carbon dioxide, and a small amount of hydrogen, nitrogen and hydrogen sulfide. 

Commonly known as biogas.