Reusing Road Sweepings and Stormwater Sediments: Reducing Heavy Metal Leaching

Every year millions of dollars are spent by councils and road authorities landfilling thousands of tonnes of street sweepings and catchpit sediments. In addition to street sweepings, the last decade has seen a large increase in the number of storm water treatment devices, which as part of ongoing maintenance, will significantly increase the amount of contaminated road derived sediment (RDS) requiring costly landfill disposal. With numerous examples of successful RDS reuse applications in North America (landfill reductions waste by 90-95%); there is growing interest in NZ to implement environmentally responsible reuse options as an alternative to landfill disposal. A major challenge is to adequately stabilize the heavy metals (namely copper and zinc) to address environmental concerns regarding heavy metal mobilization. Compost was ineffective at stabilising zinc, but reduced copper leaching by up to 40% for RDS mixtures amended with 15% compost (dry weight). Phosphate amendment was largely ineffective and actually increased heavy metal mobilisation in most instances. Using an alkaline modifier to increase the leachate pH to 7.5-8 was found to be the most effective way of immobilising zinc (60-99% reduction), while a combination of compost (15%) and alkaline modifier (5-10%) reduced copper and lead leaching by ca. 60% and 50%, respectively. Based on potentially relevant international guidelines, carcinogenic PAH levels in Christchurch RDS would require a 2- to 3-fold dilution with clean material to ensure the most stringent guidelines were met (namely carcinogenic PAHs). Potential municipal wastes streams that could be used to incorporate RDS include greenwaste (composting) and wastewater treatment plant biosolids.