Open access publications in the SLU publication database

Abstract

The objective of this thesis was to increase our knowledge of issues relevant to process problems in large-scale composting. The investigations focused on acid-related process inhibition and the relationships between temperature, aeration, evaporation and the scale of the process. Three manuscripts are summarised in the thesis proper. The first investigated composting at different scales; at full-scale, in a 2 m high reactor and in a one-litre vessel. The process in the reactor resembled the full-scale process, but the theoretical calculations showed that the heat losses from the reactor were large. About 0.45 m of glass wool would be necessary to produce similar thermal properties in the reactor as in the full scale plant. Accumulation of acids was studied in the second investigation. Different amounts of active compost were used as a starting culture in rotating three-litre reactors, which were fed daily with fresh waste and water. In reactors with a large amount of starting culture, more than four times the daily feed, a well-functioning process with high temperature, high CO2 production and high pH was established. In reactors with a starting culture less than twice the daily feed, the composting process failed. The temperature was below 42 °C and the CO2 production was small. In these reactors the pH was low and organic acids accumulated. It was concluded that acid inhibition of fed-batch processes can be avoided if sufficient amounts of a good starting culture are used. In the third investigation, the combined effects of temperature and pH on the degradation were studied. Small samples of compost from the initial acidic phase were treated with sodium hydroxide to raise the pH. This resulted in high respiratory activity in samples at all pH levels at 36 °C and in those with pH over 6.5 at 46 °C. However, at 46 °C the activity was very low in samples with pH below 6.0. This shows that a combination of high temperature and low pH can inhibit the composting process. The influence of the composting temperature on the evaporation was also analysed. Simulations showed that the difference in evaporation at different temperatures was very small for the same degradation, although there were large variations in airflow. Finally, addition of water to compost is discussed. It is often necessary to add water when composting energy-rich substrates, since otherwise the process may be halted due to drying before the compost has stabilised.