Product-oriented composting: from open to closed bioconversion systems.

The thesis recapitulates and defines what composting is and presents a summary of four primary research papers, two experimental and two theoretical. The themes represent steps from scientific reductionism to studies on system-level complexities, i.e. from cultivation and composting experiments to a holistic approach to sustainable management of solid and liquid organic waste, focusing on effecient recycyling of plant nutrients. This work motivate a paradigm shift where 'composting' became 'aerobic bioconversion' and 'compost' became 'biofertiliser'. In the cultivation experiments, crop response to compost was studied and a novel method for calculating compost amendment rate was developed. The results showed that compost from source separated municipal solid waste, when mixed with peat, can give similar or higher yields than a number of peat-based cultivation media. Composts from centrally sorted waste are not acceptable for cultivation purposes. The cultivation experiments were stimulating for research on composting methods. For composting experiments on standard substrate, with and without additives, static and rotated bioreactors were constructed. The effects of internal and external factors on the process of microbial transformation were investigated. A high degree of efficiency, a short retention time, and increased control of the bioconversion are some of the advantages of using bioreactors. A thought-provoking comparison of economical assessments between present and future methods of organic waste management was made. Completely closed local bioconversion systems for plant nutrient recovery were suggested. The impact of the whole system - including collection, transport, and treatment by microbial transformation on the environment and economy was estimated. At the end follows an assessment of the potential hidden in organic waste, which can be used as renewable raw material for efficient plant nutrient recycling and for conservation of bioenergy in organic matter. By microbial transformation in decentralised facilities, the solid and liquid organic wastes and residues can be upgraded to valuable biofertilisers and biogas. Proper management of organic material by closed bioconversion will bridge the gap between industrial properity and biological requirements. Present polluting systems can be counteracted by environmentally friendly technology.