— Rubber tyres, the kind that lie at the bottom of rivers and at the back of junkyards the world over, could be ideal water filters says an environmental engineer at Penn State university in the US.
Yuefeng Xie says his crumb rubber suffers none of the problems of traditional water filtration systems and as a result can filter wastewater up to four times faster.
Traditionally, water filtration systems are made of particles of sand or anthracite stacked in a column. The particles are arranged so that the larger ones which leave larger gaps between them are at the top of the column and the smaller particles and therefore smaller holes are at the bottom. As a result, contaminants get filtered out from top to bottom in order of decreasing size.
The problem, says Xie, is that these systems clog up very quickly every couple days on average. Water is pushed through them backwards to clean them out, but this ruins the columns careful stacking as the large particles naturally settle to the bottom. Every subsequent filtration only uses the top of the column which therefore clogs up even faster. The filters are designed to last 20 years but after one backwash you get a filter you dont want, says Xie.
Xie and his team believe that crumbs of rubber, 1 to 2 millimetres across, are an ideal solution because the crumbs are compressible. As a result, regardless of how the filter is stacked the crumbs at the bottom of the column are always smallest because they are squashed by the weight of the column.
Xie, who obtained a patent for the rubber crumb filter in November 2005, says it works four times faster than conventional filters.
The suggestion is interesting and plausible, says Sean Moran, an environmental engineer who runs Expertise Limited in the UK. But I can see there being a lot of difficulties taking it from lab stage to full scale, he adds.
He points out that there are already filtering columns which maintain their size gradient even after backwashing by exploiting the density differences. These use large, low density particles of anthracite, on top of sand, with small, high density particles of garnet at the bottom.
Another problem in cleaning filtering systems is that sewage sludge sticks to the filter particles. Moran thinks the sludge is more likely to stick to rubber than to smooth hard grains of sand and coal.
He is also concerned that the rubber from old tyres might leach out toxic chemicals including heavy metals. Xies team is now taking feedback from experts, and running tests to make sure the rubber does not leach out heavy metals.