Passive Vent Filters: How they work
Passive vent filters fall into the broad category of carbon filters. They are very effective to remove organic compounds from the air, which eliminates the odours those compounds cause.
- Activated carbon causes odour to stick to its open surface
- A passive vent filter uses this chemical attraction to filter odorous particles out
- They are often used in municipal sewer systems
What is activated carbon and how does it work?
Carbon filters are made by treating carbon with oxygen, so that the oxygen atoms open up millions of pores between the carbon atoms. The net result is a huge surface area of carbon atoms within a very small block of material. Activated Carbon is the term given to carbon treated with oxygen in this way.
When other organic gases pass through the activated carbon bed, carbon attracts the molecules causing them to stick to the open surfaces.
This chemical attraction is called adsorption. Once the molecules stick to the carbon site, they are trapped there - using up that site. When all the sites are used, the carbon filter is spent and must be replaced.
What is a passive vent filter?
Passive vent filters rely on a natural flow of air to filter the odorous particles out. They do not use fans or any other method to produce an air flow. Passive filters have an extremely low pressure drop so that there is little resistance to the movement of air across the activated carbon bed.
OdourPro uses two distinct blends of activated carbon to target specific gases. The combination of Acticarb EA 1000 granular vapour activated carbon, and Acticarb EA 1000K pelletised impregnated activated carbon target H₂S gases, mercaptans and other volatile organic compounds (VOCs). Custom mixtures can be created to target the specific range of compounds that are of concern for customers.
It is also important to note that passive filters come in multi cartridge systems, which increase their capacity. The independent plenum base ensures that air is evenly distributed through the bed thus maximising efficiency.
Where are passive vent filters used?
Passive vent filters are often used on municipal sewer systems to reduce odours and improve relations with neighbours.
Every time air releases from a sewer system, the potential for odours to affect people is high. Passive vents fit directly onto air valves to purify the air as it emerges from the sewer system.
In action: how a passive vent filter is used at a sewer pump station
Sewer pump stations receive raw waste into a well. When the level rises, a pump automatically switches on and pumps the waste into the discharge line. Sewer pipes carry it to the next collection point or treatment plant.
Pump stations have vents which allow air to enter the well when the waste is being pumped out, and to exit the well while it is filling up. This air exiting the pump station well can contain highly odorous compounds. Passive filters are ideal for pump station vents because they allow air flow in both directions.
Waste leaving a pump station is normally pumped to a higher level via a rising main into an end chamber. Rising main manholes are another source of odour and passive vents operate here in the same way as on pump station wells. Air relief valves are safety devices that release air downstream of the pump station in the case of pressure surges or water hammer in the system.
Maintenance and monitoring
Breakthrough indicators on passive vent filters indicate whether the activated carbon is spent. Replacement of the cartridge will prevent odour complaints. OdourPro also provides telemetry solutions, which give real time information about the presence of odorous gases and the effectiveness of odour treatment equipment.
Contact OdourPro for more information about passive vent filters
OdourPro has extensive experience in passive vent filter systems. We use customised carbon beds to maximise absorption of odours. Our technical experts and proven solutions can help your business overcome the challenges of odours emitted from sewers.
OdourPro on 21 November 2016
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