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 m 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, ammoniumand nitriteoxidising 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 gVS hr at the end of the active composting phase (after 3 weeks) and declined gradually to below 1 mg O2 gVS hr, 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.