Microbial community dynamics and stability assessment during green waste composting

Green waste (leaves, grass and shredded twigs) was composted for 5.5 months in a wooden
composting bioreactor, of 1 m3 capacity, using both passive convective aeration, facilitated by
the design of the bioreactor, and manual turning. Temperature was measured daily, while
samples were analysed for moisture content, volatile solids, pH value and electrical
conductivity, all of which showed a typical variation for the type of composted material and
process.
For the assessment of compost stability, a simple automated respirometric technique was
applied (SOUR test), which utilised a dissolved oxygen probe to measure changes in the
oxygen concentration in an aqueous compost suspension, under conditions ensuring
optimum microbial activity and maximum reaction rates. Results were compared with other
stability tests, such as the dehydrogenase activity and the germination index (GI) of cress
seeds and correlated with the changes in population size of different microbial groups (total
aerobic and spore forming heterotrophs, ammonium- and nitrite- oxidising bacteria,
actinomycetes, filamentous fungi, yeasts and cellulolytic bacteria).
Both the SOUR test and the GI indicated increasing compost stability with processing time
and gave significant correlation with compost age and with each other. SOUR reached a
value of 4 mg O2 g-1VS hr-1 at the end of the active composting phase (after 3 weeks) and
declined gradually to below 1 mg O2 g-1VS hr-1, a value indicative of stable composts, after
about 3 months of processing. GI increased from 30% for the raw material, a value indicating
high phytotoxicity, to about 80% at the end of the active phase and fluctuated around this
value thereafter. Dehydrogenase activity fluctuated during the process, with the highest
values being measured during the active composting phase. However, high values were also
recorded towards the end of the maturation phase, following a long period of stabilisation at
low values. The parameter did not correlate with compost age or SOUR and GI and thus did
not seem suitable for monitoring green waste compost stability.
Although at lower levels compared to other substrates, such as manures and biowaste, the
population size of different microbial groups did not seem to be a limiting factor in green
waste composting. The numbers of most microbial groups increased after the end of the
active composting phase, indicating that microorganisms multiplied rapidly as temperatures
felled, although for some groups population counts declined again towards the end of the
maturation period, possibly indicating the exhaustion of specific substrates.