Specially designed carrier materials are available, usually made of polyethylene or polypropylene. They are highly voided to reduce the risk of clogging and have high specific surfaces between 100 and 200 m² per m³ carrier material.
- Bio-effluents with moderate organic load: 5-10 kg COD/m³
- When non-granular or non settleable sludge is expected
- When available area is limited.