Assessment of Compost Stability using Single and Mixed Culture in a Static Semi-Closed Fed-Batch Reactor

Abstract

Different microbial inoculums are expected to affect the degradation and stability of different compost materials. The results tend to vary more significantly in an open system due to the varied ambient conditions (temperature, moisture and air-borne microbial). To minimise such variation and avoid low reproducibility of the data, the present work was performed in a static semi-closed fed-batch reactor (SSCFBR) for better control of the composting process. The thermophilic Bacillus coagulans (BC) and the commercial effective microorganisms (EM) were used as the microbial inoculum (MI) in the SSCFBR. Distilled water was used in place of the MI for the control experiments. The mixture of chicken dung (CD), wooden husk (WH), and rice husk (RH) were used as the compost materials. The MI was inoculated into the compost bed in the SSCFBR (15 L working volume) at the same initial optical density (OD) of 0.8 and the initial volume of 800 mL with a forced aeration of 0.4 L/min. The parameters for the assessment of compost stability included the temperature, moisture content, oxygen uptake rate (OUR) and carbon dioxide emission rate (CER). BC showed comparable or better results to that achieved by the commercial EM. The temperature profile for both composts showed similar patterns where the highest peak of temperature was recorded within 1 d of composting (slightly higher for BC, 55 ± 0.8 °C and 47± 0.8 °C for EM). Such thermophilic temperature profile has not been observed in the control. The initial moisture content was set similar for all composts (44.5 ± 0.7 %). The moisture content declined slightly different for both MI compost (to 39.0 ± 0.6 % for BC and 36.6 ± 1.2 % for EM). Both composts showed very similar trend in the OUR. A higher evolution rate of carbon dioxide (CO₂) was observed for the compost with BC for the first three days (0.06 ± 0.014 g/mol) as compared to that by EM (0.02 ± 0.007 g/mol). On the 4^th d of composting, the production of CO₂ in EM compost showed a near constant value (0.008 ± 0.002 g/mol) but the BC compost showed the rate of 0.06 ± 0.01 g/mol the 5^th d and declined to 0.02 ± 0.004 g/mol on the 7^th d. For both composting cases, the highest rate of CO₂ was observed at the highest peak of temperature. The SSCFBR has been designed and could be used to facilitate the assessment of compost stability in a closed system with improved reproducibility of the data for composting.