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TWIRLS: Treating Waste for Restoring Land Sustainability

Site Restorations - Urban Brownfield site - Shotton

Producing compost from six different combinations of organic wastes at a brownfield former steelworks site in Shotton, Flintshire

In June 2005, the TWIRLS project began composting 1000 m3 of wastes at Shotton, Flintshire, Wales. Wastes (green waste, de-inking paper fibre and tertiary treated sewage sludge) were delivered to site, mixed according to six different combinations designed to give a target initial C:N ratio between 20 and 35, and composted for 80 days in EcoPOD® in-vessel composters.

Left, feedstock wastes are accurately weighed and mixed using a cattle-feed mixer wagon.

The experiment had several aims: to demonstrate that quality assurance criteria can be satisfied using mixed wastes; to follow the changing chemistry of the composts over time; and to produce composts with a range of different physical and chemical characteristics to use in grassland establishment trials. All of the composts met criteria detailed in the BSI PAS 100 processing standards in terms of temperatures, pathogen kill and final amounts of potentially toxic elements.

Right, perforated plastic aeration pipe is fed into the recyclable LDPE EcoPOD® composting bags and connected to timed aeration fans powered by a diesel generator.




Assessing the potential of six different composts applied at three rates to promote mesotrophic grassland establishment at a brownfield site

Following three months maturation, the six composts produced at Shotton were applied to land using a farm spreader wagon. Composts were applied to the bare site at rates of ca 0, 250 or 500 t ha-1.  The composts were not incorporated with the sandy soil on this occasion as the dimensions of the plots (16 m2) were too small. Half of each plot received a mixture of seeds of 24 species present in the target MG5b habitat (Emorsgate Wildflower Seeds, Norfolk) at a rate of 40 kg ha-1.  

Left, newly established mesotrophic grassland trial plots at Shotton showing zero, low and high rate of application of six different composts according to a randomised complete block design.

Right, floristic surveys carried out after 18 months identified 15 of the 24 sown MG5b grassland species, some of which were already present at the site. The frequency of some species (principally Achillea millefolium and Festuca rubra) was significantly greater in sown plots. In general it was clear that biomass was greater on the highest rate of application of compost, reflecting improvements in soil water-holding capacity as well as nutrient supply. 

The composts differed in their potential to support vegetation similar to the target MG5b community. In terms of the frequency of indicator species for the target vegetation type, 100% green waste compost performed worst and compost produced from 35:30:35 green waste : sewage : paper fibre performed best. Long-term monitoring is necessary to follow development of the plant communities and see how initial amounts of nutrients and compost water-holding capacity determine progress of the plots toward the target MG5b vegetation.