Tru Design Carbon Filter For Holding Tank Vent- 90355A Carbon Filter is a useful addition to a holding tank vent pipe to reduce the odour of decomposing sewage. The activated carbon in the filter absorbs and converts the odours from your holding tank. Note: The Carbon Filter is not a substitute for good ventilation of the holding tank. It is designed to remove excess odour from a properly functioning holding tank. Holding tanks require good ventilation to ensure aerobic decomposition. Anaerobic