Microbial volatile organic compounds as novel indicators of anaerobic digestion instability: Potential and challenges

Microbial volatile organic compounds produced during the anaerobic digestion process can serve as potential indicators of microbial community stability

The principal challenge in monitoring anaerobic digestion (AD) processes lies in the fact that conventional operational parameters (pH, volatile fatty acids, etc.) frequently lag behind the actual microbial performance. This study proposes microbial volatile organic compounds (mVOCs) as a novel indicator reflecting the species-specific characteristics of microorganisms and evaluates their ability to indicate the stability of AD processes in response to organic loading rate (OLR) perturbations. Both gradual and pulsed increases in OLR concentration resulted in an instantaneous change in production, which was also reflected in the variation of mVOCs. Furthermore, alkanes, alkenes, alcohols, aldehydes, ketones, and aromatic compounds exhibited a more pronounced response to OLR perturbations. Additionally, these mVOCs presented a strong co-occurrence with specific bacterial genera (Syntrophomonas, Defluviitoga, and Fastidiosipila) and archaeal genera (Methanobacterium, Methanoculleus, and Methanosarcina), thus positioning them as crucial producers of mVOCs. This suggests that mVOCs could function as biochemical indicators of AD performance and facilitate the understanding of what mVOCs are produced, and who the contributing microorganisms are. Further research is required to elucidate the resolution of mVOCs regarding other AD process perturbations, as well as to establish the optimal measurement frequencies and reference values.

Microbial Volatile Organic Compounds as Microecological Stability Indicators in Response to Temperature Changes during Anaerobic Digestion

The use of traditional early warning indicators for monitoring anaerobic digestion (AD) is limited, owing to their inability to sensitively reflect microbial performance. Microbial volatile organic compounds (mVOCs) possess distinctive species-specific characteristics, functioning as a communication mechanism between microorganisms at two distinct levels: interspecific and intraspecific, and they can be potential indicators of microbial performance. Therefore, we conducted time-series and comparative nontarget analyses using headspace solid-phase microextraction and high-resolution mass spectrometry to investigate temporal variations in mVOC responses to temperature changes during AD. In total, seventy-six differential mVOCs, playing crucial roles in important material and energy metabolism, signaling, and membrane transport processes throughout AD, were investigated after gradual and pulsed temperature perturbations. These differential mVOCs exhibited high co-occurrences with bacteria and archaea based on 16S rRNA gene sequencing analysis. The study findings suggest that mVOCs can serve as explanatory indicators during AD monitoring, bridging AD process stability, and anaerobic microecology. However, further verification is necessary, owing to the wide range of mVOC types involved.

Microbiome regulation for sustainable wastewater treatment

Sustainable wastewater treatment is essential for attaining clean water and sanitation, aligning with UN Sustainable Development Goals. Wastewater treatment plants (WWTPs) have utilized environmental microbiomes in biological treatment processes in this effort for over a century. However, the inherent complexity and redundancy of microbial communities, and emerging chemical and biological contaminants, challenge the biotechnology applications. Over the past decades, understanding and utilization of microbial energy metabolism and interaction relationships have revolutionized the biological system. In this review, we discuss how microbiome regulation strategies are being used to generate actionable performance for low-carbon pollutant removal and resource recovery in WWTPs. The engineering application cases also highlight the real feasibility and promising prospects of the microbiome regulation approaches. In conclusion, we recommend identifying environmental risks associated with chemical and biological contaminants transformation as a prerequisite. We propose the integration of gene editing and enzyme design to precisely regulate microbiomes for the synergistic control of both chemical and biological risks. Additionally, the development of integrated technologies and engineering equipment is crucial in addressing the ongoing water crisis. This review advocates for the innovation of conventional wastewater treatment biotechnology to ensure sustainable wastewater treatment.

Evaluation of traditional and machine learning approaches for modeling volatile fatty acid concentrations in anaerobic digestion of sludge: potential and challenges

This study evaluates models for predicting volatile fatty acid (VFA) concentrations in sludge processing, ranging from classical statistical methods (Gaussian and Surge) to diverse machine learning algorithms (MLAs) such as Decision Tree, XGBoost, CatBoost, LightGBM, Multiple linear regression (MLR), Support vector regression (SVR), AdaBoost, and GradientBoosting. Anaerobic bio-methane potential tests were carried out using domestic wastewater treatment primary and secondary sludge. The tests were monitored over 40 days for variations in pH and VFA concentrations under different experimental conditions. The data observed was compared to predictions from the Gaussian and Surge models, and the MLAs. Based on correlation analysis using basic statistics and regression, the Gaussian model appears to be a consistent performer, with high R2 values and low RMSE, favoring precision in forecasting VFA concentrations. The Surge model, on the other hand, albeit having a high R2, has high prediction errors, especially in dynamic VFA concentration settings. Among the MLAs, Decision Tree and XGBoost excel at predicting complicated patterns, albeit with overfitting issues. This study provides insights underlining the need for context-specific considerations when selecting models for accurate VFA forecasts. Real-time data monitoring and collaborative data sharing are required to improve the reliability of VFA prediction models in AD processes, opening the way for breakthroughs in environmental sustainability and bioprocessing applications.

A comprehensive study on anaerobic digestion of organic solid waste: A review on configurations, operating parameters, techno-economic analysis and current trends

The excessive discharge and accumulation of solid organic waste into the environment is of severe concern across the globe. Thus, an efficient waste management system is important to mitigate health risks to humans, minimize harmful impacts on the environment, and ensure a sustainable ecosystem. The organic waste is converted into value-added products either using microorganisms or heat energy; these methods are commonly known as biochemical and thermochemical techniques. The biochemical process has the advantage of higher selectivity of the products and lower processing temperatures. The principal conversion processes of this category are fermentation and anaerobic digestion (AD). This review article focuses on AD, a potential method for treating organic waste and creating a variety of products with added value. Here we present the digestibility of various organic wastes, the role of microorganisms, the decomposition process, co-substrates, digester designs, biogas yields, by-products, environmental impacts, and overall techno-economical effectiveness of the process. Further, this review offers insights into new directions for AD for waste treatment and future research without compromising the overall feasibility and environmental sustainability.