Microaerobic conditions in anaerobic sludge promote changes in bacterial composition favouring biodegradation of polymeric siloxanes

Effect of toluene on siloxane biodegradation and microbial communities in biofilters

The removal of volatile methyl siloxanes (VMS) from landfill biogas is crucial for clean energy utilization. VMS are usually found together with aromatic compounds in landfill biogas of which toluene is the major representative. In the present study, two biofilters (BFs) packed with either woodchips and compost (WC) or perlite (PER) were used to study the (co-) removal of octamethyltrisiloxane (L3) and octamethylcyclotetrasiloxane (D4) from gas in presence and absence of toluene, used as a representative aromatic compound. The presence of low inlet toluene concentrations (315 ± 19 - 635 ± 80 mg toluene m-3) enhanced the VMS elimination capacity (EC) in both BFs by a factor of 1.8 to 12.6. The highest removal efficiencies for D4 (57.1 ± 1.1 %; EC = 0.12 ± 0.01 gD4 m-3 h-1) and L3 (52.0 ± 0.6 %; EC = 0.23 ± 0.01 gL3 m-3 h-1) were observed in the BF packed with WC. The first section of the BFs (EBRT = 9 min), where toluene was (almost) completely removed, accounted for the majority (87.7 ± 0.6 %) of the total VMS removal. Microbial analysis revealed the impact of VMS and toluene in the activated sludge, showing a clear selection for certain genera in samples influenced by VMS in the presence (X2) or absence (X1) of toluene, such as Pseudomonas (X1 = 0.91 and X2 = 12.0 %), Sphingobium (X1 = 0.09 and X2 = 4.04 %), Rhodococcus (X1 = 0.42 and X2 = 3.91 %), and Bacillus (X1 = 7.15 and X2 = 3.84 %). The significant maximum EC values obtained by the BFs (0.58 gVMS m-3 h-1) hold notable significance in a combined system framework as they could enhance the longevity of traditional physicochemical methods to remove VMS like activated carbon in diverse environmental scenarios.

Anaerobic digestion of process water from hydrothermal treatment processes: a review of inhibitors and detoxification approaches

Integrating hydrothermal treatment processes and anaerobic digestion (AD) is promising for maximizing resource recovery from biomass and organic waste. The process water generated during hydrothermal treatment contains high concentrations of organic matter, which can be converted into biogas using AD. However, process water also contains various compounds that inhibit the AD process. Fingerprinting these inhibitors and identifying suitable mitigation strategies and detoxification methods is necessary to optimize the integration of these two technologies. By examining the existing literature, we were able to: (1) compare the methane yields and organics removal efficiency during AD of various hydrothermal treatment process water; (2) catalog the main AD inhibitors found in hydrothermal treatment process water; (3) identify recalcitrant components limiting AD performance; and (4) evaluate approaches to detoxify specific inhibitors and degrade recalcitrant components. Common inhibitors in process water are organic acids (at high concentrations), total ammonia nitrogen (TAN), oxygenated organics, and N-heterocyclic compounds. Feedstock composition is the primary determinant of organic acid and TAN formation (carbohydrates-rich and protein-rich feedstocks, respectively). In contrast, processing conditions (e.g., temperature, pressure, reaction duration) influence the formation extent of oxygenated organics and N-heterocyclic compounds. Struvite precipitation and zeolite adsorption are the most widely used approaches to eliminate TAN inhibition. In contrast, powdered and granular activated carbon and ozonation are the preferred methods to remove toxic substances before AD treatment. Currently, ozonation is the most effective approach to reduce the toxicity and recalcitrance of N and O-heterocyclic compounds during AD. Microaeration methods, which disrupt the AD microbiome less than ozone, might be more practical for nitrifying TAN and degrading recalcitrant compounds, but further research in this area is necessary.

Microaeration promotes volatile siloxanes conversion to methane and simpler monomeric products

The ubiquitous use of volatile siloxanes in a myriad of product formulations has led to a widespread distribution of these persistent contaminants in both natural ecosystems and wastewater treatment plants. Microbial degradation under microaerobic conditions is a promising approach to mitigate D4 and D5 siloxanes while recovering energy in wastewater treatment plants. This study examined D4/D5 siloxanes biodegradation under both anaerobic and microaerobic conditions ( [Formula: see text]  = 0, 1, 3 %) using wastewater sludge. Results show that the use of microaeration in an otherwise strictly anaerobic environment significantly enhances siloxane conversion to methane. 16S rRNA gene sequencing identified potential degraders, including Clostridium lituseburense, Clostridium bifermentans and Synergistales species. Furthermore, chemical analysis suggested a stepwise siloxane conversion preceding methanogenesis under microaerobic conditions. This study demonstrates the feasibility of microaerobic siloxane biodegradation, laying groundwork for scalable removal technologies in wastewater treatment plants, ultimately highlighting the importance of using bio-based approaches in tackling persistent pollutants.

Are Si-C bonds cleaved by microorganisms? A critical review on biodegradation of methylsiloxanes

Methylsiloxanes, compounds that contain H₃C-Si-O subunits in their molecular structure, are emerging ubiquitous pollutants now detected in many environmental compartments. These compounds and generally Si-C bonds do not occur in living nature, but are industrially produced worldwide in millions of tons per annum and are widely used, resulting in their release to the environment. It is an open question whether or to what extent microorganisms are able to decompose these compounds. The presence of methylsiloxanes in many biogases adds to the economic relevance of this question. We here review and critically discuss, for the first time, the evidence obtained for and against degradation of methylsiloxanes by microorganisms, and in particular for microbial cleavage of Si-CH₃ bonds. As a result, no convincing demonstration of Si-C cleavage by native environmental microorganisms has been found.

Recent development of biodegradable synthetic rubbers and bio-based rubbers using sustainable materials from biological sources

Rubber is an amorphous hyperelastic polymer which is widely used in this modern era. Natural rubber is considered the ultimate rubber in terms of mechanical performance, but over the years, some limitations and challenges in natural rubber cultivation that could result in serious shortages in the supply chain had been identified. Since then, the search for alternatives including new natural and synthetic rubbers has been rather intense. The initiative to explore new sources of natural rubber which started during the 1940s has been reignited recently due to the increasing demand for natural rubber. The commercialization of natural rubber from the Parthenium argentatum and Taraxacum kok-saghyz species, with the cooperation from rubber product manufacturing companies, has somewhat improved the sustainability of the natural rubber supply chain. Meanwhile, the high demand for synthetic rubber drastically increases the rate of depletion of fossil fuels and amplifies the adverse environmental effect of overexploitation of fossil fuels. Moreover, rubber and plastic products disposal have been a major issue for many decades, causing environmental pollution and the expansion of landfills. Sustainable synthetic rubber products could be realized through the incorporation of materials from biological sources. They are renewable, low cost, and most importantly, biodegradable in nature. In this review, brief introduction to natural and synthetic rubbers, challenges in the rubber industry, alternatives to conventional natural rubber, and recent advances in biodegradable and/or bio-based synthetic rubbers are discussed. The effect of incorporating various types of biologically sourced materials in the synthetic rubbers are also elaborated in detail.

The Measurement, Application and Effect of Oxygen in Microbial Fermentations: Focusing on Methane and Carboxylate Production

Oxygen is considered detrimental to anaerobic fermentation processes by many practitioners. However, deliberate oxygen sparging has been used successfully for decades to remove H2S in anaerobic digestion (AD) systems. Moreover, microaeration techniques during AD have shown that small doses of oxygen may enhance process performance and promote the in situ degradation of recalcitrant compounds. However, existing oxygen dosing techniques are imprecise, which has led to inconsistent results between studies. At the same time, real-time oxygen fluxes cannot be reliably quantified due to the complexity of most bioreactor systems. Thus, there is a pressing need for robust monitoring and process control in applications where oxygen serves as an operating parameter or an experimental variable. This review summarizes and evaluates the available methodologies for oxygen measurement and dosing as they pertain to anaerobic microbiomes. The historical use of (micro-)aeration in anaerobic digestion and its potential role in other anaerobic fermentation processes are critiqued in detail. This critique also provides insights into the effects of oxygen on these microbiomes. Our assessment suggests that oxygen dosing, when implemented in a controlled and quantifiable manner, could serve as an effective tool for bioprocess engineers to further manipulate anaerobic microbiomes for either bioenergy or biochemical production.