Greenhouse gases and odour emissions from composting.

Composting is the most widely used biological treatment method for recovering the plant nutrients of organic waste. Composting is an aerobic process and it is well known that mismanagement, e.g. insufficient ventilation, can seriously affect the turnover and performance of the process. The turnover is also strongly inhibited if the operation temperature rises above 40°C before the pH has increased above 6. Such a combination of high temperature and low pH also seriously increases the odour emission potential of the compost operation as the initial high odorous stage is prolonged until the pH increases. When the pH increases beyond 6.5, the odour potential rapidly decreases. The pH development can be accelerated by good cooling and ventilation, and/or by addition of alkaline amendment, e.g. recycling of alkaline compost or addition of wood ash. When the compost is turned, the structure of the compost matrix, and thus the aeration, is improved which decreases odour generation and also the proportion of anaerobic processes and thus the emissions of methane.Studies of food waste home composts show that these have a much higher moisture content (70-80%) compared to what is normally recommended for composting (40-65%). The common assumption has therefore been that methane emissions from home composts are greater than from large scale municipal composts. However, recent studies in both Sweden and Denmark show that the methane emissions from home composts usually are smaller than from large municipal composts. Furthermore, both studies indicate that the emissions seem to increase if the composts are frequently turned, quite contrary to what was expected. The emissions also seem to increase if the feeding is too high. This means that the capacity of the home compost should not be exceeded and that the efforts for its operation can be kept low, as turning does not seem to be an advantage, at least from the point of view of greenhouse gas emissions.