- Retains anaerobic bacteria to perform sewage treatment and biogas generation
- Decouples hydraulic retention time (HRT) from solid retention time (SRT)
- Utilize gravitational sedimentation
- Returning some of the active organisms to the digester decreases digestion time
- Examples:
- Anaerobic activated sludge process
- Continuous stirred tank reactor (CSTR)
- Up-flow anaerobic sludge blanket (UASB) digestion