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.

Effects of differing temperature management on development of Actinobacteria populations during composting.

Actinobacteria are believed to play a major role in organic matter degradation and humification processes in composts. In this study, the effects of different temperature regimes on the succession of Actinobacteria populations during composting were investigated in a laboratory reactor. Phospholipid fatty acid (PLFA) was used to investigate quantitative changes in the overall microbial biomass and community structure, and in the size of Actinobacteria populations. Qualitative changes were determined using PCR-DGGE (denaturing gradient gel electrophoresis) and sequencing of 16S rRNA genes with Actinobacteria-specific primers. The peak in total microbial biomass was roughly twice as high and delayed in trials where the maximum temperature was 40 degrees C, compared to those where it was 55 or 67 degrees C. There was a shift from members of Corynebacterium, Rhodococcus and Streptomyces at the onset to species of thermotolerant Actinobacteria in the cooling phase, e.g. Saccharomonospora viridis, Thermobifida fusca and Thermobispora bispora. In conclusion, temperature was an important selective factor for the development of Actinobacteria populations in composts, and they constituted a substantial part of the community in the later compost stages.

Anaerobic digestion of alfalfa silage with recirculation of process liquid.

Process liquid recirculation initially stimulated one-phase anaerobic digestion of alfalfa silage in two semi-continuously fed and stirred tank reactors. Thus, with increased pH, alkalinity and stability it was possible to increase the organic loading rate to 3 g VS L(-1) d(-1), as compared to 2.25 g VS L(-1) d(-1) in a control reactor without recirculation. However, the recirculation of liquid eventually caused an accumulation of organic and inorganic substances, leading to an inhibition of hydrolysis and methanogenesis. This inhibition of microbial activity was prevented in one of the processes by replacing 50% of the recirculated process liquid with water during the second half of the operation period. A multiple linear regression model of principal components using seven input variables explained the variance in output variables nearly as well as the original model using all 23 measured input variables. The results show that it is necessary to adjust the degree of liquid recirculation to reach an optimal process.

Reassessing PCR primers targeting nirS, nirK and nosZ genes for community surveys of denitrifying bacteria with DGGE.

We re-evaluated PCR primers targeting nirS, nirK and nosZ genes for denaturing gradient gel electrophoresis as a tool to survey denitrifying community composition in environmental samples. New primers for both nirS and nosZ were combined with existing primers, while for nirK the previously published F1aCu:R3Cu set was chosen for denaturing electrophoresis. All three sets yielded amplicons smaller than 500 bp and amplified the correct fragment in all environmental samples. The denaturing gradient gel electrophoresis worked satisfactorily for nirK and nosZ, but not for nirS. This was probably due to the multiple melting domains in this particular nirS fragment. From the excised and sequenced bands, only sequences related to the target genes were detected and tree analysis showed that the selected primers acted as broad range primers for each of the three genes. By use of the new nirS primers it was demonstrated that agricultural soil harbours a substantial diversity of nirS denitrifiers.

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.