Sniffing out emissions

A new system that uses a sensing technology more commonly found inside power plants is to monitor bad odours and methane gas escaping from landfill sites.

The device was developed by researchers from ManchesterUniversity, and will help waste management companies to track nuisance emissions and odours, according to the project team.

Based on an array of metal oxide sensors the system can detect the tell-tale signature of certain organic compounds in the air. It monitors the change in conductivity of the metal oxide as gas is absorbed by its surface. The sensor was developed by Prof Krishna Persaud from the university’s chemical engineering department.

‘The technology itself has been around a while but this is the first time it has been used in an environmental capacity,’ said Persaud. ‘By deploying an array of these instruments around a landfill site we can get continuous, real-time monitoring of gas and odour emissions.’

Similar sensors have traditionally been used in gas alarm systems to monitor dangerous emissions in power plants.

The active part of the sensors is made mainly from tin oxide, which is heated via a main power supply to 300ºC. At this temperature the metal oxide behaves like a semiconductor and the change in conductivity can be monitored.

Existing methods for detecting high concentrations of gases and odours are time-consuming and inefficient, said Persaud. Handheld detectors, known as flame ionisation detectors (FID) use a flame which reacts with combustible gases in the air which, although accurate, must be carried around the site. Waste management companies also hire teams of volunteers to smell samples of air to detect high levels of unpleasant odours.

According to Persaud the advantage of the new sensor system is that it will deliver accurate data readings, continuously and in real-time, enabling a much more accurate picture of gas emissions.

One of the challenges the research team faced was developing the devices so that they require as little maintenance as possible. Persaud said: ‘Many of the gases that are emitted can be corrosive, so the sensors and electronics have to be packaged in such a way that they are protected.’

The device also uses its means of collecting samples to protect itself from the long-term effects of the gases. ‘The sampling strategy is a push-pull approach, which mimics the way a nose works,’ said Persaud. ‘It sucks in a sample of air at regular intervals across the sensor and then expels it through a filter so that the sensor is cleaned after every sample.’

Each device is also equipped with a GPS transmitter, which downloads the data to a remote computer where it is stored and analysed. In the future the software will be further developed so that a contour map of the emissions in a single area is automatically produced from the collected data.

If gas emissions are above an acceptable level, landfill site operators can then take steps, such as capping and lining areas of the site with layers of plastic and clay.

A full test of the technology is currently underway at Brookhurst Wood landfill site near Gatwick airport with five devices positioned around the perimeter.

Persaud said that a number of UK and US water companies have shown a strong interest in the technology as it can also be used to monitor sewage works and water treatment plants.