Methane potential and biodegradability of rice straw, rice husk and rice residues from the drying process.

Agricultural solid residues are a potential renewable energy source. Rice harvesting and production in Sancti Spíritus province, Cuba, currently generates residues without an environmentally sustainable disposal route. Rice residues (rice straw, rice husk and rice residues from the drying process) are potentially an important carbon source for anaerobic digestion. For this paper, rice residues were placed for 36 days retention time in anaerobic batch reactor environments at both mesophilic (37 °C) and thermophilic (55 °C) conditions. Biogas and methane yield were determined as well as biogas composition. The results showed that rice straw as well as rice residues from the drying process had the highest biogas and methane yield. Temperature played an important role in determining both biogas yield and kinetics. In all cases, rice straw produced the highest yields; under mesophilic conditions the biogas yield was 0.43 m(3) kg(VS)(-1), under thermophilic conditions biogas yield reached 0.52 m(3) kg(VS)(-1). In the case of the rice husk, the biodegradability was very low. Methane content in all batches was kept above 55% vol. All digested material had a high carbon:nitrogen (C:N) ratio, even though significant biodegradation was recorded with the exception of rice husk. A first-order model can be used to describe the rice crop residues fermentation effectively.

Methanogenic population dynamics during semi-continuous biogas fermentation and acidification by overloading.

AIMS: This study was to investigate the methanogenic community in a biogas reactor from start-up to acidification conditions. Furthermore, reliability and accuracy of the applied quantitative real-time PCR method (Q-PCR) was briefly evaluated. METHODS AND RESULTS: A mesophilic (37 degrees C), maize silage fed, continuously stirred tank reactor was surveyed. It was operated semi-continuously with increasing daily organic loading rates (OLRs) to reach acidification. Gas production and organic acid composition were measured. Methanogenic community structure was determined by 16S rDNA-based Q-PCR to estimate the abundance of key methanogenic micro-organisms. 16S rDNA of hydrogenotrophic Methanobacteriales was most abundant at OLRs of >or=3.7 g dry organic matter (DOM) l(-1) day(-1). By contrast, that of aceticlastic Methanosaetaceae predominated at lower OLRs but disappeared at OLRs of >or=4.1 g DOM l(-1) day(-1). At the same OLR, the propionate concentration increased dramatically indicating the acidification of the digester. Application of internal standards to examine Q-PCR's accuracy revealed that the detected amount of 16S rDNA may vary within one log cycle. CONCLUSIONS: These results suggest that the absence of Methanosaetaceae might be taken as biological indicator for process' instability. Inhibitory effects on Q-PCR analyses could not be determined based on the spiking experiments. SIGNIFICANCE AND IMPACT OF THE STUDY: In this study, reactors' microbiology was observed over time using Q-PCR. Insights into the abundance of different methanogens might be used to improve the performance of biogas reactors.