Recycling: Beyond the glass ceiling

But as we shovel in the spent sauvignons and quaffed chiantis we are presenting the glass recycling industry with something of a challenge: we will soon be giving the industry far more material than it needs to turn it back into bottles and jars. And containers are not the only problem. Old TVs, computer monitors, windows and fluorescent tubes and even fibreglass also need to be turned into something useful rather than, as is the case now, chucked into the local landfill site.

The figures are impressive. Each year in the UK, around 2.4 million tons of glass bottles and jars are used – 400,000 tons by pubs, clubs and restaurants. Of the total, 770,000 tons is collected at bottle banks or from kerbside recycling schemes operated by local councils and commercial waste collectors. The remainder, around 1.6 million tons, is thrown out with the rubbish and is buried in landfill.

In addition, it is calculated that around 300,000 tons of flat glass – mainly old windows that are being replaced – also goes into the local dump. Spent fluorescent light tubes account for a further 20-30,000 tons, and at least 40,000 tons of glass in TVs and computer monitors are also thrown out. New EU regulations will shortly require these types goods to be recycled.

The organisation charged with ensuring that the UK does not sink beneath a sea of used glass is a Government-sponsored, not-for-profit company, Wrap – Waste Resources and Action Programme. It was set up early in 2001 to develop new, stable and sustainable markets for recycled materials.

"At the moment there is a hell of a lot of used glass going to landfill," says Wrap's Andy Dawe, the person responsible for the company's glass strategy. "There are Government targets to increase the amount that is recycled, and by 2006 we could be collecting twice as much as we are now."

With container glass – bottles and jars – the big problem is that of green bottles. These are mainly from wine and beer imports. "Plenty of clear glass is manufactured here, so used glass containers that are clear can go back to the factories for recycling," says Dawe. "But because we make less green and brown glass, we are reaching saturation on turning this back into bottles and jars. Furthermore, an increasing amount of glass being collected for recycling is mixed together as green, brown and clear, and this too creates problems for the recyclers. If things carry on as they are, there will be a surplus of used coloured glass in a few years." The solution, he says, is to find new ways to process used glass into products that have a high value.

One of the companies that Wrap is working with – Dryden Aqua in Edinburgh – provides water treatment services. The company is developing ways of using tiny glass particles for purifying water. One of the principal ways of cleaning water – be it in a swimming pool, a fish farm or in a water treatment works – is to filter it through a bed of sand. However, this has inherent problems. "The surface of a grain of sand is pitted and rough," says Dryden Aqua's managing director Howard Dryden. While these pits and grooves help in the filtration by snagging bacteria and algae, they also mean that the grains rapidly become populated by microbes, which cause the grains to clump together, bunging the system up. This requires frequent, intensive flushing to clean the filter.

Glass, however, can be ground into particles of less than a tenth of a millimetre width that have a very smooth surface. Smooth grains might seem unsuitable as a filtration medium in that they are less likely to trap particles. But they are surprisingly effective. "When you grind up the glass," Dryden explains, "you break molecular bonds. The effect of this is to leave spare electrons on the surface. The glass grains have a net negative electrical charge." This enables them to attract particles in the way that a balloon that has been brushed a few times against a sweater will stick to the ceiling.

A second effect then occurs. Glass is made green by the addition of salts of chromium, and brown by adding iron. Both metals have catalytic activity. They can split oxygen molecules into single, highly reactive oxygen "radicals". So in filters made from coloured glass grains, microbes are first drawn to the surface of the grains electrostatically and, once there, are zapped by oxygen radicals, which kill them.

"We think that if glass replaced sand as a filtration medium, the potential market could be in the region of 750,000 tonnes a year," says Dryden. "And we are interested only in coloured glass."

Meanwhile, at the Centre for Cement and Concrete at the University of Sheffield, Dr Ewan Byars, in collaboration with 30 UK companies, is finding new uses for waste glass in the concrete industry. Of particular interest is the possibility that finely ground glass could be useful as an additive in cement. "Cement chemistry is extremely complex," says Dr Byars. "Essentially, calcium, silicates and water react together to form calcium silicate hydrates, the building blocks on which concrete's strength and engineering properties develop."

There are a number of silica-based waste materials – pozzolans – that can be added to cement. Pozzolans may increase the cohesiveness and workability of concrete mixes. The Sheffield researchers have shown that powdered glass has pozzolanic activity, and could be a useful additive to cement without compromising its properties, and possibly improving them.

Larger particles of glass are being investigated as coarse aggregates for high-value concrete products. One company, for example, specialises in architectural masonry. Here, glass aggregate can be incorporated into the concrete and the surface given a fine polish to display the glass facets.

Over in Stoke-on-Trent, Dr Andy Smith is Technical and Research Manager in the Building Technology division of the research organisation Ceram. Dr Smith's team has demonstrated how finely milled glass can be added to clays used in brick making to save energy costs, reduce pollutants and produce bricks that are more resistant to damage from frost.

"Clay-based products such as bricks are made by forming the clay into the desired shape, drying it and then firing it at temperatures between 950C and 1200C," says Dr Smith. "During firing the boundaries around the individual grains of the clay minerals melt, causing the grains to fuse."

By adding powdered glass to the clay as a "fluxing agent", this melting process can occur at lower temperatures and in a shorter period of time. The glass melts and diffuses through the clay between the mineral grains where it initiates the localised melting process. Laboratory studies have shown that the firing temperature can be reduced by as much as 50C. "That is a saving of between 1 and 2 per cent in terms of raw energy," says Dr Smith. This helps to lower carbon emissions, and also decreases the emissions of other pollutants that arise at the higher firing temperatures.

Furthermore, the bricks that emerge from the process are tougher than those that have had no glass added. Principally they are more resistant to weathering by frost. The Ceram researchers are now scaling up the process to see if it is feasible in a factory, and Wrap is looking to establish a facility capable of processing 160,000 tons of glass powder a year that could be used in this application.

"Around eight million tons of raw materials are used in the manufacture of bricks each year," says Dr Smith. "If every manufacturer used 5 per cent glass, it would account for 400,000 tons of recycled glass."