Optimisation and assessment of different railway ballast cleaning systems

Spent railway ballast is a source of recycled aggregate. Recycling of aggregates contributes to sustainable development by reducing the volume of construction waste going to landfill, reducing transportation and reducing the impact of primary mineral extraction supplying primary aggregates. Railway ballast is renewed when it loses its geotechnical properties and is no longer able to support the track adequately and provide drainage. Alternatively, ballast is removed from locations where contamination, primarily by diesel, is unsightly and adds to the characteristic smell of a UK railway station. In this case ballast must first be cleaned before reuse as aggregate. Track-mounted systems exist to remove the ballast by vacuum and return it to the track after processing. Off- site systems are similar to traditional soil- and gravel- washing plants. An optimised cleaning system can represent savings in both time and money, producing less waste for processing and disposal and returning more materials to the marketplace. Such an approach is in keeping with the overall thrust of sustainable engineering. In this study, the primary factors of contact time, cleaner concentration and abrasive action were investigated for a surfactant- based cleaning agent (Biosolve®), applied to contaminated railway ballast using a laboratory-scale cleaning system. It was found that a 15-minute wash cycle incorporating a 1% surfactant solution concentration with abrasive action gave the optimum cleaning efficiency, reducing contamination by 86% from 17510 ±445 to 2525 ±345 mg kg-1. Several batches of contaminated ballast could be cleaned before significant reduction in cleaning efficiency was observed. Potential environmental impacts of surfactant and hydrocarbon residues were considered. The metal content and the biodegradability, with respect to the biochemical oxygen demand (BOD), of wastewaters generated were also measured.