Filtering out foul smell
METAL foundries pollute the atmosphere by throwing up dust and giving off bad ~dour. While the former is easy to remedy, the latter poses a challenge. The vat-shaped biofilters designed to solve the problem are filled with moistened wood chips inoculated with bacteria. The air from the exhaust flows via a sys- tem of pipes into the filters where it is doused with water vapour. A thin film of the precipitate subsequently forms on the surface of the wood chips. This solution is absorbed by the bacteria and converted into heat, carbon dioxide, water, and cellulose. Microbi1ll treatment removes 75 per cent of the offensive odour from the air and can clean 240,000 cu m of air in an hour.
Apart from comprising a mix of sol- vents and other organic substances, the pollutants also contain an invaluable substance -the catalyst amine -used in the manufacture of cores (an internal mould, filling the space to be left hollow in a casting). The chances of retrieving amine have so far beel1 rather poor because its concentration in the air is very small. As a result, around 60 per cent of the amines are wasted. A team of engineers from the Mercedes- Benz are now trying to counter this problem by developing a recycling system. This would convert the exhaust gas from being a large low-concentration stream to a smaller amine-rich stream. This can be achieved by forcing the amine-laden air - under high pressure into a membrane film tube placed inside a larger outer tube. With low pressure -surrounding the membrane, the amine enters the outer tube through the film, which is impermeable to air. Due to the difference in pressure, the amine gets concentrated in the outer tube and is then cooled to -40