Can recovered road sweeping wastes provide a soil or soil amendment alternative? An investigation of temporal variability of physico-chemical parameters

By 2050, 90% of the Earth's topsoil is considered 'at risk' (according to the UN). Within the UK, more than 6 million hectares are at risk of compaction or erosion and soils remain one of the largest components of landfills (e.g. 28 million tonnes in 2016-55% of tonnage received). Hence, sustainable uses and solutions to maintain and restore soils are required. Street sweeping - a routine maintenance operation - handling diverse materials (e.g. grit, litter, leaves, glass, bitumen etc.), and thus containing potentially harmful elements, requiring landfill disposal (or incineration). However, with appropriate physico-chemical treatment, reclamation of gravel, sand, and fine residues (clay and silt) can be ensured. The latter making up approximately 30% of total solids collected, that could provide a 'circular economy' solution as soil or soil amendment, i.e. providing soil functions and essential ecosystem services (e.g. source of raw material, hosting biodiversity, carbon pool), thus, reducing the need for 'virgin material'. Consistent physical (e.g. moisture content, organic matter) parameters suggest 'good' soil properties, able to support plant growth. Chemical properties revealed 'urban' signature of contaminants, i.e. metal(loid) concentrations, whereas high levels of total petroleum hydrocarbons (TPHs), which in part, were attributed to treatment process chemicals. These were found to biodegrade by >70% during storage (using different remediation techniques). Although the material was outside of ranges to be certified as a top- or subsoil (BS 3882 or BS 8601), the suitability within 'public open space' (and 'commercial') surroundings was evident. This study provides the first long-term (over a 12-months period) physico-chemical characterisation of fine residues of recovered road sweeping material (or waste-derived material), following British Standards Institution (BSI) guidelines. Based on the determined characteristics, it aims to; (i) consider physico-chemical characteristics in an "urban soil" context (including temporal variability), to (ii) contextualise the material within soil British Standard specifications, e.g. for topsoil and subsoil, and (iii) evaluate their suitability for use as a soil/soil amendment in relation to human health screening values, e.g. 'soil guideline values' (SGVs), 'safety for use limits' (S4UL) and 'category 4 screening levels' (C4SL).