CPD Article: Venting External Drainage Systems with Active Carbon Filters

What is an active carbon filter for drainage? It is a two-way vent that uses active carbon as a filter to absorb the sewer gases (primarily hydrogen sulphide H₂S and methane CH4) which come out of a drainage vent.

Due to the common design of active carbon filters for drainage systems, they are restrictive vents, and therefore limited in application to prevent self or induced siponage of traps seals resultant from transient pressures in the drainage system. As such, they do not replace the need for open vents or Air Admittance Valves (AAVs) for venting drainage systems on their own.

The filters with the Maxi-Filtra can be used in combination with AAVs to provide the open vent requirement on the drainage system; the AAV will provide the fast air requirement of the drainage system, and the filter will provide the filtered relief to the slow positive pressure rise out of the drainage vent stack.

The active carbon filters work as an inlet and an outlet, and typically have an airflow of 5 litres per second in both directions at 250Pa (25mm Wg).

Being a two-way vent, the filter must be installed outside the building – the same as for an open vent. Over time the active carbon may become saturated and harmful sewer gases can pass through the filter and into the surrounding air. The filter cartridges typically have a life of over 2 years before needing replacement. High-use systems, or a pumped system, can reduce the life of the active carbon due to saturation. Typically on a domestic septic tank the cartridge is replaced when the tank is emptied of sludge, or there is an increase of sewer smells around the vent. When used for other applications it is recommended that the replacement of the cartridge is placed on a maintenance schedule.

Problem Solving

Active carbon filters are a good solution for solving issues with smelly vent pipes, either on a conventional system or septic tanks. They generally offer an immediate, easy fix solution, being fitted with a push-fit connector.

Wind Effect

Wind affects water trap seals, and can cause traps within the building to self-siphon – this usually occurs when wind gusts are over 35 k/mh. When the wind blows over the stack outlet, the air pressure in the pipe is lowered. This is due to the Bernoulli’s principle (“as the speed of the moving fluid (air) increases, the pressure within the fluid (air) decreases”). The lowered pressure in the pipe creates suction (negative transients) throughout the above ground drainage system. As the wind gusts, this causes the trap seals (typically closest to the open vent) to oscillate and can lead to the trap to self-siphon. We have all seen this occur when it is windy outside and have witnessed the water trap seals move! If the gust causes significant pressure (around +/- 400Pa (400mm Wg)) the water traps seals can be depleted.

If a building is affected by wind effect on a regular basis then an active carbon filter combined with an AAV can be used to cap the stack, and therefore protect the system from the wind effect.

Recirculation of Sewer Gases Back into the building

Part H of the Building Regulations requires that the open vents must be installed to external air at least 900mm above any opening into a building within 3 metres – this could be windows or air handling units. Also, with modern design, roof terraces are becoming more common.

Problems occurs when the vents are located too close to the opening; maybe through renovation work, or change of use. An active carbon filter filters combined with AAVs could be used to solve these issues.

Why Water Trap Seals Need Protecting

Water trap seals are the only barrier between the drainage system and the living space. Typically these have a water seal between 50mm and 75mm, but when subjected to pressures in the system of +/- 400Pa (40mm Wg) the traps can be subjected to siphonage. This removes the barrier, and allows potentially harmful sewer gases to enter the living space.

Sewer Treatment Plants and Septic Tanks

Active carbon filters are an ideal solution to vent the low level outlets on septic tanks. This prevents the need to vent the tanks back though the building, or away from areas around the tank.

As waste discharges in the tank it also draws air. When the tank starts to become full, the capacity of the tank to deal with the waste and the air becomes reduced. This can lead to slow discharges from appliances in the same way as experienced with a conventional system when there is a blockage downstream – the air has to go somewhere!

Providing a vent at the outlet of the tank provides a path for the air to leave the tank so that waste flow into the tank is normal.

The issue occurs with where to place this vent. As smells and sewer gases will come out of the tank, if the vent is at low level then the smells can disturb people in close proximity to it.

By placing the open vent with a longer pipe distance (10 meters) the open vent is less efficient, as the gasses from the tank may head back into the building drainage system, and if there is a depleted trap seal the gas will enter the building. Using an active carbon filter prevents this from happening as it provides a path for the air out of the tank at the outlet at low level, as well as filtering the unpleasant, and potentially harmful, sewer gases.

Maintenance and Ease of Installation

No specialist installation is required for this type of filter. They are commonly installed with a push-fit connector only, so you don’t even need to be skilled to secure the filter in place. They often have a cap which simply lifts off to enable easy access for filter cartridge replacement. In installations with high odour saturation, the carbon filter should be replaced more regularly than the recommended two years, i.e. before or when odour becomes noticeable.

The filters are commonly supplied with one carbon filter and a connector which fits standard UK pipe. An aluminium cover should ideally also be provided, which provides extra protection for the filter when it is installed outside in the open air – added insulation against extreme temperatures (-20°C to +60°C) and protection from animal/birds and the environment, i.e. inclement weather and the sun’s ultra-violet rays.

Written by Steve White
Technical Director DWV
High-Rise Building Solutions