Changes in the microbial communities during co-composting of digestates

Abstract

Anaerobic digestion is a waste treatment method which is of increasing interest worldwide. At the end of the process, a digestate remains, which can gain added value by being composted. A study was conducted in order to investigate microbial community dynamics during the composting process of a mixture of anaerobic digestate (derived from the anaerobic digestion of municipal food waste), green wastes and a screened compost (green waste/kitchen waste compost), using the COMPOCHIP microarray. The composting process showed a typical temperature development, and the highest degradation rates occurred during the first 14 days of composting, as seen from the elevated CO2 content in the exhaust air. With an exception of elevated nitrite and nitrate levels in the day 34 samples, physical-chemical parameters for all compost samples collected during the 63 day process indicated typical composting conditions. The microbial communities changed over the 63 days of composting. According to principal component analysis of the COMPOCHIP microarray results, compost samples from the start of the experiment were found to cluster most closely with the digestate and screened compost samples. The green waste samples were found to group separately. All starting materials investigated were found to yield fewer and lower signals when compared to the samples collected during the composting experiment.

Keywords: Anaerobic digestion; COMPOCHIP; Composting; Microarray; Nitrogen compounds.

Fig. 1

Schematic set up of the composting unit. Note: 1 – reactor lid, 2 – temperature sensors, 3 – manometer, 4 – sieve lid, 5 – insulation, 6 – water jacket, 7 – tipping device, 8 – control box, 9 – heating system, 10 – water pump, 11 – pressure reducer, 12 – mass flow meter with regulating valve, 13 – leachate collection, 14a, b – condensate collection, 15 – refrigerator, 16 – acidic traps, 17 – gas sampling unit, 18 – switching valve, 19 – liquid separator, 20 – air filter, 21 – measurement units for temperature, gas mass flow, pressure, carbon dioxide, 22 – electrotechnical unit for process regulation and data collection, c_CO2-, p_out-, V_out-, T_out.

Fig. 2

Temperature profile of composting process und samples taken during the process for microbiological analyses: ISM (initial substrate mixture), FC (fresh compost), SMC (slightly mature compost), MC (mature compost). Note: The arrows indicate turning and sampling days.

Fig. 3

Principal component analysis (PCA) loading plot obtained by principal component analysis, depicting the organisms responsible for community differences amongst the samples. The lengths of the vectors indicate their significance for compost differentiation (longer arrow length means higher SNR of probe).