Pushing out the boundaries of air quality monitoring

Cameron Stathers, Sales Manager for Casella ETi, outlines how technology will revolutionise the routine boundary measurement of methane at landfill sites as an extremely cost effective and reliable alternative to the traditional hand-held or open path techniques.

Modern landfill sites are now subject to much stricter standards. Landfill gas is the natural by-product of the decomposition of organic waste in landfills and is composed mainly of carbon dioxide and methane. As one of many steps taken to prevent pollution, the UK Environment Agency has set limits of methane at site boundaries. So how do companies meet these limits? And how can methane be effectively measured?

As part of its Pollution Prevention Control (PPC) operating permit, the Environment Agency (EA) set the Waste Recycling Group (WRG) a boundary limit of 10 parts per million (ppm) at one of their operational landfill sites. Up until now the company made regular manual checks to meet this requirement. This method proved both time consuming and expensive as it was performed 24 hours a day, seven days a week.

After an initial three month trial using a NEO Laser System, WRG discovered the benefits of having continuous real-time measurement of low level methane concentrations and decided to invest in this advanced gas monitoring technology. Although the concept of open path monitoring is not new, this was the first time that such an advanced laser based system was used on a UK landfill site.

Laser operation

The NEO Laser System operates by having a combined transmitter/receiver unit securely mounted on a tripod or other solid structure. The laser beam is then projected up to a distance of 500 metres to a special un-powered reflector that bounces the beam directly back to the combined transmitter/receiver unit. Here the data is logged. As the system is completely optically based, signal processing is almost instantaneous and the minimum two second response time can catch even the most rapid fluctuations in methane levels.

This monitoring system measures real time variations in methane concentrations. Unlike older, traditional “broadband” infra-red based systems, the NEO uses a technique known as “single line spectroscopy” to make the measurement. The laser is able to produce a beam of light that is 2-300 times narrower then the traditional infra-red systems. This very narrow wavelength is able to pick out and isolate specific individual “absorption lines” that are generated by the methane gas. This very narrow light produced by the laser prevents any cross interferences from occurring during the measurement process. Hence it is extremely specific to methane and does not generate false readings from other common hydrocarbon or atmospheric gases that also display very strong absorption characteristics in this part of the infrared spectrum.

Other benefits

For the site operator the implication of this type of spectroscopy is that the system only measures methane so that no false results are ever recorded. Checks are now undertaken automatically and have reduced the need for manual checks. The system is also extremely sensitive to background methane levels with a detection limit of only 0.01ppm because of the special signal processing undertaken by the instrument. The NEO also displays very little long term measurement drift, hence there is no longer any need to undertake regular manual time consuming calibrations. The NEO system is completely solid state with no moving parts. This means that it is extremely reliable with no regular consumables or long term running costs.

WRG has already invested in a Wind Speed & Direction system supplied by Casella and can use its existing “Online Pro” software to combine methane concentrations with meteorological information. The company can then produce directional “pollution rose” graphical plots in order to study the effects of wind direction on methane concentrations at the site boundary.

With the technology that is available both the meteorological information and the methane data can also be sent directly to the site control room via a real time radio link. This means that instantaneous readings are always available to the site management and they can automatically see when any increases in methane concentrations are occurring.

A special SMS alarm texting system is also currently under development by Casella. With this new system a customer defined alarm message can be sent automatically from the NEO Lasergas instrument to a mobile phone as soon as an alarm condition is breached. Hence any site manager can be confident that their PPC permit conditions for methane are being complied with.

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