Biodegradable waste represents an ongoing challenge for modem societies
and developing cost effective, safe and sustainable alternatives to landfills are
essential. The implementation of the European Landfill Directive in the UK has
meant that there are clear targets for reducing the levels of biodegradable wastes sent to landfill. Consequently, there has been a sharp rise in the number of composting facilities in the UK, as composting is seen as a more sustainable management option for many biodegradable wastes. Green waste represents the most commonly composted waste stream in the UK and as part of a drive to conserve peat habitats, developing high quality peat replacements in horticulture represents a potential end market for these composts.
The effect of sterilizing the compost to improve plant growth and
establishment was studied (chapter 3). A reduction in phytotoxicity as a result of
sterilization meant the composts were comparable with the peat-based equivalents in the germination and plant growth trial, compared with the unsterilized equivalents.
Compost maturity represents an ongoing challenge with regard to assuring the quality of composts. The results from an earlier trial showed that in vitro compost tea disease suppression of three fungal plant pathogens occurred from a tea produced using immature compost (Appendix 1). Coupled with the rising interest in using composts and compost teas to suppress plant diseases we studied the potentially phytotoxic and allelopathic effects of compost teas (chapter 4). Four economically important plants were used in a germination bioassay to assess the phytotoxic and allelopathic conditions of mature and immature compost teas. Results showed that plant response
was highly variable and/or species specific, inferring that the PAS: 100 quality
assurance test of phytotoxicity in solid compost could be insufficient owing to only one plant species being used. Vermicomposting green waste is regarded as a way of adding value to the green waste compost and many studies have reported improved plant growth for vermicomposts. Chapter 5 addressed the lack of research on the water characteristics and physical stability of vermicomposts compared with standard green waste-derived compost and peat. Vermicompost was less hydrophobic at low moisture contents, compared with peat and green waste compost. The feedstock used to produce the vermicompost resulted in very different responses to drying and wetting, with paper pulp amended vermicompost shrinking irreversibly following drying. Using surfactants to alleviate the hydrophobicity of containerised media was
studied in chapter 6. Surfactants applied to peat and green waste compost successfully reduced hydrophobicity. This work and chapter 5 indicated that highly organic media represent a challenge when applying the theoretical and analytical methods developed for mineral soils for defining physical properties.
In summary, this work has demonstrated that green waste-derived compost is
highly heterogeneous but that there are options for improving the end quality and creating high quality peat replacements. Chapters 3, 4 & 5 indicated that the PAS: 100 Protocol for quality assurance has some potential shortcomings with regard to operational regulations and the parameter and tests used to provide quality assurance.