text: Trickling may have a variety of types of filter media used to support the biofilm. Types of media most commonly used include coke, pumice, plastic matrix material, open-cell polyurethane foam, clinker, gravel, sand and geotextiles. Ideal filter medium optimizes surface area for microbial attachment, wastewater retention time, allows air flow, resists plugging, is mechanically robust in all weathers allowing walking access across the filter, and does not degrade.  Some residential systems require forced aeration units which will increase maintenance and operational costs.
  Synthetic filter media may pose a significant risk of flammability as demonstrated in Christchurch, New Zealand in May 2022 when two large trickling filters filled with plastic filter bales caught fire. The resultant smell had a significant impact on many city residents and this event put out of action a significant proportion of the sewage treatment capacity.