Monitoring and Detection with Active Venting Back-up It takes about two years for a landfill site to generate gases at a reasonably stable rate. The main constituents are methane and carbon dioxide with a little hydrogen and hydrogen sulphide, although there will be variation depending on conditions. Typically, 5m³ of methane is produced per tonne of waste each year and it may take 20 years for all of the gas to be evolved at which time nitrogen is generated. In most instances the venting below a new structure can be achieved with a passively vented gas dispersal layer, however where ground gas emissions are in greater concentrations the passive ventilation system will require an active backup system. Where a new development is on the threshold for an active venting system, the most economic approach to employ is to install the passive venting system as normal during the construction phase inconjunction with the duct work to accept the cabling and sampling nylons for the active system. The development will however require a gas detection and monitoring system with gas dectection sensors installed within critical areas of the new structure. These will monitor gas emission levels with readings sent back to a control / mimic panel which would generally be positioned in a security office or reception area. Typical control unit

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Post construction monitoring must be undertaken and the gas data reviewed in conjuction with the gas detection and monitoring log to ascertain if an active venting system requires installation. It is important to realise from a maintenance cost perspective in the long term it is more economic to operate an active assist system venting system. This is where gas detectors located at strategic positions will constantly monitor the gas emission levels and if the gas concentration levels rise above a preset trigger level a series of automated fans, situated within the vent terminations will be activated. These will draw fresh air into the underslab gas dispersal blanket dilute the ground bounre gases to reduced levels. If gas levels still rise an audible alarm will sound and the building will require evacuation. Don't Panic as active assist systems are designed to dilute and disperse gassing levels far in excess of evacuation trigger levels making building evacuations a rare occurence. Vent stack with inline fan Inline fan SEL is able to provide fully automated systems to sample and monitor the gas levels within and below new development.