Various types of anaerobic units that have been developed are as follows:
- Upflow anaerobic filters packed with either pebbles, stones, PVC sheets, etc. as media to support submerged biological growths (fixed film). The units are reported to work well but a likely problem is accumulation of solids in the interstices.
- Downflow anaerobic filters packed with similar media as above but not to be confused with usual trickling filters which are aerobic. In the anaerobic units, the inlet and outlet are so placed that the media and fixed film stay submerged.
- UASB type units in which no special media have to be used since the sludge granules themselves act as the 'media' and stay in suspension. These are commonly preffered.
- Fluidized bed units filled with sand or plastic granules are used with recirculation under required pressure to keep the entire mass fluidized and the sludge distributed over the entire reactor volume. Their power consumption is higher.