Catalytic Iron Filters are generally used where high concentrations of hydrogen sulphide are present in sewage and sludge treatment facilities. They are ideally applied as a first stage of a two- stage odour control unit where a dry chemical system is installed as the second or “polishing” stage. Random media of rusty iron are packed into a vessel to remove hydrogen sulphide from a ventilation air stream. With an accurate air humidification system, correctly sized air residence time and regeneration regime, removal efficiencies in excess of 60% can be achieved in a single column.