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Globally, waste generation is continually increasing, with landfill as the main destination for biological waste. Composting is a simple alternative for handling waste, but when poorly managed poses a risk of greenhouse gas emissions. The moisture content of substrate affects emissions of methane (CH4) and nitrous oxide (N2O) from composting, but the scale and mechanisms behind these effects are poorly understood. This study examined effects of different moisture levels (44–66%) on CH4, N2O and carbon dioxide (CO2) emissions during 20 days of composting food and garden waste under controlled conditions (55 °C, 16% oxygen) in a 200-L reactor. Total CO2 emissions were 400–500 g CO2-C kg−1 initial C. Total CH4 emissions were highest, 35 g CH4-C kg−1 initial C, for the wettest substrate (66% moisture) and decreased exponentially with declining moisture content, with the lowest total emissions, 0.04 g CH4-C kg−1 initial C, observed with the driest substrate. Total N2O emissions were negatively correlated with moisture content, decreasing from 1.2 g N2O-N kg−1 initial N at 44% moisture to 0.3 g N2O-N kg−1 initial N at 59%, but the wettest substrate (66% moisture) had the highest N2O emissions, 1.4 g N2O-N kg−1 initial N. NH4-N accumulated in the wettest material, suggesting that the increased N2O emissions were due to reduced oxygen availability. The results indicate potential to reduce greenhouse gas emissions from large-scale composting by adjusting the moisture content at different stages of composting, thus lowering its overall environmental impact. This finding can be used in guidelines for large-scale composting process to avoid moisture conditions causing large greenhouse gas emissions.

Mussels can be used as fertiliser in organic farming, particularly on stockless farms, which need external nutrients but have limited access to fertiliser products. Mussels are filter feeders and accumulate nutrients bound in e.g. phytoplankton. When harvested and used onshore, their accumulated nutrients are removed from the water, thus decreasing eutrophication. This LCA study assessed the environmental impact of mussel farming on the Swedish coast of the Baltic Sea for subsequent use as agricultural fertiliser. The functional unit was related to the agronomic value of the mussels, including plant-available nitrogen, phosphorus and liming effect. Flow of cadmium to soil was also assessed. In one scenario the mussels were composted and in another they were inertly stored to avoid nitrogen losses and other emissions from storage. Use of mussels on agricultural land fulfils three functions: fertilisation, liming and a reduction in eutrophication. The results clearly showed the importance of integrated assessment of these three functions. Due to significant nitrogen losses from the composting process, the compost scenario resulted in a higher impact on acidification, global warming potential and energy use than the inert storage scenario. Alternative techniques to composting for handling mussels should thus be developed, for example direct use of the mussels as fertiliser without prior storage, or inert storage. Energy use was strongly related to production of materials for mussel cultivation, so choice and life length of materials is important when introducing mussel farming. The cadmium content of mussels is relatively high, but they often supply less cadmium to the field than the fertiliser and liming products they replace. Reducing eutrophication by cultivating mussels for fertiliser proved more energy-efficient than reducing eutrophication in wastewater treatment plants.

Life cycle assessment methodology was used to compare conventional wheat production with a scenario where source-separated human urine replaced mineral fertilisers. A change-orientated perspective was used, including differences in capital goods between the scenarios. An optimal fertilising strategy regarding application time, technique and substitution of mineral fertiliser was demonstrated to be important for energy use, global warming and acidification. For reducing the energy use, a well designed collection system for urine also proved important, while recovery of the urine was essential for reducing eutrophication. Applying an agricultural perspective when evaluating the system highlighted potential conflicts regarding nutrient utilisation.