Tired tyres supply beating demand
IT’S unclear how many old tyres are discarded annually in the Waikato but it’s estimated about four million a year are worn out nationally, generating about 36,000 tonnes of waste.
So it’s a fair bet to say the Waikato total is in the hundreds of thousands. Recycling initiatives can’t keep pace with the volumes of old tyres generated so most end up in landfills.
Old tyres can sometimes be put to good use on farms but it pays to be careful in the way they’re used as they can cause environmental damage.
Using them on silage stacks is a good use that has little potential to harm the environment.
But, some farmers are using old tyres as part of farm drainage systems where the tyres can cause gradual and insidious pollution of water. This pollution is largely unseen and often not obvious visually.
When a tyre comes into contact with water some of the constituents in the tyre can leach into the surrounding environment.
Tyres contain natural and synthetic rubbers, carbon black, oils and resins, sulphur, textiles, steel and assorted chemicals that have been left over from the manufacturing process.
Chemicals used to manufacture tyres include solvents, agents used to activate, accelerate, slow or terminate rubber vulcanisation, curing agents, adhesives and antioxidants.
Two key classes of contaminants in tyre rubber are zinc and polycyclic aromatic hydrocarbons.
Ordinarily, with only one or a small number of tyres, the size of this leaching effect is quite small and may not be noticeable. But in cases where a reasonably large number of tyres are brought together or water is allowed to pool inside a tyre over an extended period, the strength and toxicity of tyre rubber leachate can become quite significant.
Research on the environmental impacts of tyre leachate reveals that:
Concentrated tyre leachate is toxic to some fish species, insect life and microbial organisms.
Most of this toxicity is thought to be associated with the leached zinc. Concentrations of zinc in tyre rubber are about 10,000 to 15,000 mg/kg (parts per million).
Tyre leachate can contain high levels of several other metals and a range of manmade organic substances. Of these, compounds called ‘‘benzothiazoles’’ are regarded as being diagnostic of the presence of tyre rubber in the environment.
Some contaminants that leach from tyres do not degrade but instead tend to build up in the surrounding soil or nearby sediment.
Physically, tyres are also not designed to serve as sections of drainage pipe. In some cases, poorly designed drainage structures using tyres can damage the habitat of aquatic organisms and impede or block the passage of native and other fish species.
For these various reasons, Environment Waikato does not allow the use of tyres in farm drainage applications.
Similarly, stockpiling of tyres greater than the number needed for silage stacks is not allowed as it can cause the sorts of leaching problems outlined above and also be a fire risk Leaching occurs when rainwater runs over and through the stockpile.
Some of the leached chemicals can enter waterways and others can gradually build up in the soil.
Over the longer term, this gradual build-up of contaminants in soil under and near large tyre stockpiles has the potential to create a contaminated site.
Burning tyres also burn to produce a thick toxic smoke. To protect human health, the National Environmental Standard for air quality prohibits burning of tyres in the open area.
Tyre stockpiles can also be breeding grounds for pests, such as rats and mosquitoes.
Tread: Carefully with tired tyres.