M.A. Nason

Remediation of metal polluted mine soil with compost: co-composting versus incorporation.

Trace element contamination of post-industrial sites represents a major environmental problem and sustainable management options for remediating them are required. This study compared two strategies for immobilizing trace elements (Cu, Pb, Zn, and As) in mine spoil: (1) co-composting contaminated soil with organic wastes and (2) conventional incorporation of mature compost into contaminated soil. Sequential chemical extraction of the soil was performed to determine temporal changes in trace element fractionation and bioavailability during composting and plant growth. We show that mine spoil can be co-composted successfully and this action causes significant shifts in metal availability. However, co-composting did not lead to significant differences in metal partitioning in soil or in plant metal uptake compared with simply mixing mine spoil with mature compost. Both treatments promoted plant growth and reduced metal accumulation in plants. We conclude that co-composting provides little additional benefit for remediating trace-element-polluted soil compared with incorporation of compost.

Heavy metal fractionation during the co-composting of biosolids, deinking paper fibre and green waste.

Due to the introduction of the European Union Landfill Directive, composting has become a potentially viable disposal route for some organic wastes. As waste-derived compost is frequently added to soil to improve soil quality, it is important to quantify the environmental risk posed by potentially toxic elements contained within it. Here we used a sequential chemical extraction procedure to investigate the temporal dynamics of heavy metals (Cu, Zn, Pb and Ni) during the co-composting of biosolids, deinking paper fibre and green waste. Overall, composting over 26 weeks reduced the availability of Ni, had no effect on Pb and slightly increased the availability of Cu and Zn. We conclude that although the total Cu and Ni concentrations in the compost exceed legislative guidelines for land application, due to their recalcitrant nature within the compost, this compost posed very little threat to soil or plant quality if used in agriculture or land restoration.

FT-IR as an alternative method for measuring chemical properties during composting.

Chemical properties have been used as a way of following the composting process and compost maturity, however, their analysis is very time consuming as each must be separately determined. By developing a more rapid method to predict these properties, time and cost would be saved. This study investigates the use of Fourier Transform mid-Infrared Spectroscopy (FT-IR) for this purpose. FT-IR spectra and measured values of several chemical properties from a variety of compost mixtures were used to produce calibrated models using partial least-squares regression analysis which predicted the known chemical properties. These models displayed a range of accuracies that for most properties was more than sufficient to follow at least broad dynamic changes associated with maturation. The best calibrations were achieved for total C, total N, LOI, lignin, and cellulose with r(2) values within the range 56-77%. Some degree of calibration was achieved for available-P and NH(4)(+)-N, with r(2) values of between 40% and 57%. No useful calibration could be achieved for NO(3)(-) or pH.

Contaminated land clean-up using composted wastes and impacts of VOCs on land.

This paper describes experiments that demonstrate the effects and potential for remediation of a former steelworks site in Wales polluted with polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). Under field conditions, PAH-contaminated soil was composted in-vessel, with or without organic feedstocks, receiving forced aeration for 80 days followed by 4 months maturation. Treatments compared PAH removal in contaminated soil to contaminated soil mixed with three different organic waste mixes after composting and after composts were spread to land. After composting, PAH concentrations declined in all treatments, by up to 38%. Sixteen months after the composts were landspread and vegetation was established, only those containing contaminated soil with organic additions exhibited further PAH removal, by up to 29%. Composting resulted in a decline in the relative concentration of small PAHs, whereas the landspreading-vegetation phase saw a decline in the relative concentration of medium PAHs in two of the three composts exhibiting PAH removal. Under controlled glasshouse conditions, vegetated soil columns of differing depths were exposed to VOCs from beneath. VOC vapour affected both shoot and root growth and soil microbial activity; effects varied with distance from the VOC source. This work demonstrated that on-site remediation of aged PAH-contaminated land can be successfully initiated by in-vessel co-composting followed by land spreading and vegetation, within a practical timeframe.

RE-CREATING WOODLAND AND HEATHLAND ON SLATE WASTE IN WALES

We report on ecological restoration at Penrhyn, Europes largest slate quarry, which lies adjacent to Snowdonia National Park and a Site of Special Scientific Interest. Broadleaf woodland and heathland were targeted, to provide wildlife corridors to adjacent habitats of high conservation value. Young tree seedlings (six species) of local provenance were planted into slate pockets amended with nutrients (readily available mineral NPK or an organic mix of biosolids and paper sludge with an estimated five-year impact) and water retentive materials (clay overburden or polyacrylamide gel). Applying mineral NPK increased tree basal area by 70% in the first 18 months. The organic mix gave an increase of 130 % in tree basal area, promoted water retention and stimulated nutrient cycling. Both N-fixing tree species and non-fixers responded to fertilization and small-seeded species responded more than large-seeded ones. Tree basal area increased by 50% in the clay treatment compared with trees planted in bare slate ± gel. The transfer during quarrying of heath vegetation with associated peat to a site designated for restoration proved effective in establishing key heathland subshrubs. Bilberry re-sprouted easily from buried shoots whilst heather turf died, but within one year a flush of heather seedlings had germinated from the seedbank in the transferred peat. Three years on, there was complete ground cover of target heathland species, provided that sheep were excluded. Grazing by rabbits was beneficial in reducing grass and increasing heather cover. Where availability of heathland topsoil was limited, heathland brash was applied to clay-covered slate. Germination of heather proved slow and sparse, with no germination of bilberry.