Sven Smårs

Composting of Source-Separated Household Organics At Different Oxygen Levels: Gaining an Understanding of the Emission Dynamics

Source-separated household organics were composted in a reactor at three oxygen levels, 16%, 2.5% and 1% in the compost gas. Short-chained fatty acids were initially present in the compost material, and were also produced during the mesophilic phase at all three oxygen levels. This indicated that partial anaerobic conditions existed. No NH3 emissions occurred during the mesophilic phase due to acidic conditions. Composting at 2.5% and 1% O2 concentrations prolonged the mesophilic phase and reduced the microbial activity as compared to 16% O2. This led to delayed and decreased emissions of NH3. Nitrous oxide was not formed during thermophilic conditions. Methane, which was measured at 2.5% and 1% O2, was only found during thermophilic conditions. The emission of methane indicates that anaerobic conditions occurred during the thermophilic phase. The main reactions regulating pH during composting were outlined involving the ion species VFA, NH4+/NH3 and CO2/HCO3−/CO32−.

Improvement of the composting time for household waste during an initial low pH phase by mesophilic temperature control.

Earlier studies indicated that the activity in the initial phase of composting may be reduced when the temperature rises too fast under low pH conditions. A compost reactor experiment on household waste was designed to test whether the degradation time could be reduced by actively preventing the temperature from rising until the pH had reached a certain value. This experiment was performed by monitoring pH in the condensate from the cooled compost gas. The results from 3 + 3 runs with and without temperature control confirmed our hypothesis and a considerable reduction in composting time was achieved. One possible explanation for the results is that the microbes active in the low pH phase are hampered by high temperature. The abrupt rise in pH when the fatty acids are consumed seems to be a good marker of the point when temperature control can be discontinued.

Comparison of signature lipid methods to determine microbial community structure in compost

The microbial community structure changes substantially during the composting process and simple methods to follow these changes can potentially be used to estimate compost maturity. In this study, two such methods, the microbial identification (MIDI) method and the ester-linked (EL) procedure to determine the composition of long-chain fatty acids, were applied to compost samples of different age. The ability of the two methods to describe the microbial succession was evaluated by comparison with phospholipid fatty acid (PLFA) analysis on the same samples.Samples were taken from a 200-l laboratory compost reactor, treating source-separated organic household waste. During the initial stages of the process, the total concentration of fatty acids in compost samples treated with the EL and MIDI methods was many times higher than with the PLFA method. This was probably due to the presence of fatty acids from the organic material in the original waste. However, this substantial difference between PLFA and the other two methods was not found later in composting. Although the PLFA method gave the most detailed information about the growth and overall succession of the microbial community, the much simpler MIDI and EL methods also successfully described the shift from the initially dominating straight chain fatty acids to iso- and anteiso branched, 10 Me branched and cyclopropane fatty acids in the later stages of the process. Thus, the MIDI and EL extraction methods appear to be suitable for analysis of microbial FAME profiles in compost, particularly in the later stages of the process.

Effects of pH and microbial composition on odour in food waste composting

Highlights ► High odour emission from food waste compost was correlated to low pH. ► Microbes in high-odour samples included Lactic acid bacteria and Clostridia. ► For odour prevention, try high initial aeration rate and recycled compost as additive.

Activity and composition of ammonia oxidizing bacterial communities and emission dynamics of NH3 and N2O in a compost reactor treating organic household waste

Aims:  To monitor emissions of NH3 and N2O during composting and link these to ammonia oxidation rates and the community structure of ammonia oxidizing bacteria (AOB).

Nitrous oxide and methane emissions from food waste composting at different temperatures.

Emissions of methane (CH₄) and nitrous oxide (N₂O) from composting of source-sorted food waste were studied at set temperatures of 40, 55 and 67°C in 10 trials performed in a controlled environment 200L compost reactor. CH₄ and N₂O concentrations were generally low. In trials with 16% O₂, the mean total CH₄ emission at all temperatures was 0.007% of the mineralized carbon (C), while at 67°C this fraction was 0.001%. Total CH₄ production was higher in the 40°C trial and the limited oxygen (1% O₂) trial, with emissions of 0.029 and 0.132% of the mineralized C respectively. An early increase in N₂O production was observed in trials with higher initial nitrate contents. Increased CH₄ and N₂O production in trials at 40 and 55°C after 50% of the initial C was mineralized resulted in higher total greenhouse gas emissions. Overall, the global warming potentials in CO₂-equivalents from CH₄ emissions were higher than from N₂O, except for composts run at 67°C.

Greenhouse gas emission from covered windrow composting with controlled ventilation

Data on greenhouse gas (GHG) emissions from full-scale composting of municipal solid waste, investigating the effects of process temperature and aeration combinations, is scarce. Oxygen availability affects the composition of gases emitted during composting. In the present study, two experiments with three covered windrows were set up, treating a mixture of source separated biodegradable municipal solid waste (MSW) fractions from Uppsala, Sweden, and structural amendment (woodchips, garden waste and re-used compost) in the volume proportion 1:2. The effects of different aeration and temperature settings on the emission of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) during windrow composting with forced aeration following three different control schemes were studied. For one windrow, the controller was set to keep the temperature below 40 °C until the pH increased, another windrow had minimal aeration at the beginning of the process and the third one had constant aeration. In the first experiment, CH4 concentrations (CH4:CO2 ratio) increased, from around 0.1% initially to between 1 and 2% in all windrows. In the second experiment, the initial concentrations of CH4 displayed similar patterns of increase between windrows until day 12, when concentration peaked at 3 and 6%, respectively, in two of the windrows. In general, the N2O fluxes remained low (0.46 ± 0.02 ppm) in the experiments and were two to three times the ambient concentrations. In conclusion, the emissions of CH4 and N2O were low regardless of the amount of ventilation. The data indicates a need to perform longer experiments in order to observe further emission dynamics.