Biological desulfurization of biogas: A comprehensive review on sulfide microbial metabolism and treatment biotechnologies.

Hydrogen sulphide (H2S) removal from biogas is of high relevance as it damages combustion engines used for heat and power generation and causes adverse public health and environmental effects. Biological processes have been reported as a cost-effective and promising approach to desulfurize biogas. This review presents a detailed description of the biochemical foundations of the metabolic apparatus of H2S oxidizing bacteria, namely chemolithoautotrophs and anoxygenic photoautotrophs. The review focuses on the current and future applications of biological processes for biogas desulfurization and provides insights into their mechanism and main factors influencing their performance. The advantages, drawbacks, limitations, and technical improvements of the biotechnological applications currently based on chemolithoautotrophic organisms are covered extensively. Recent advances, sustainability and economical aspects of biological biogas desulfurization are also discussed. Anoxygenic photoautotrophic-bacteria-based photobioreactors were herein identified as useful tools to improve the sustainability and safety of biological biogas desulfurization. The review addresses gaps in the existing studies concerning the selection of the most suitable desulfurization techniques, their benefits and consequences. The research is useful for all stakeholders involved in the management and optimization of biogas and its findings are directly applicable in the development of new sustainable technologies for biogas upgrading processes on waste treatment plants.

Biogas bioconversion into poly(3-hydroxybutyrate) by a mixed microbial culture in a novel Taylor flow bioreactor.

Biogas-based biopolymer production represents an alternative biogas valorization route with potential to cut down plastic pollution and greenhouse gas emissions. This study investigated for the first time the continuous bioconversion of methane, contained in biogas, into poly(3-hydroxybutyrate) (PHB) by a mixed methanotrophic culture using an innovative high mass-transfer Taylor flow bioreactor. Following a hydrodynamic flow regime mapping, the influence of the gas residence time and the internal gas recirculation on CH4 abatement was assessed under non nutrient limiting conditions. Under optimal operational conditions (gas residence time of 60 min and internal gas recycling ratio of 17), the bioreactor was able to support a CH4 removal efficiency of 63.3%, a robust CH4 elimination capacity (17.2 g-CH4 m-3h-1) and a stable biomass concentration (1.0 g L-1). The simultaneous CH4 abatement and PHB synthesis was investigated under 24-h:24-h nitrogen feast/famine continuous operation. The cyclic nitrogen starvation and the Taylor flow imposed in the bioreactor resulted in a relatively constant biomass concentration of 0.6 g L-1 with PHB contents ranging from 11 to 32% w w-1 (on a dry weight basis), entailing an average PHB productivity of 5.9 g-PHB m-3 d-1 with an associated PHB yield of 19.8 mg-PHB g-CH4-1. Finally, the molecular analysis of the microbial population structure indicated that type II methanotrophs outcompeted non-PHB accumulating type I methanotrophs, with a heterotrophic-methanotrophic consortium enriched in Methylocystis, Hyphomicrobium, Rubinisphaeraceae SH PL14 and Pseudonocardia.

Production of solid biofuels from organic waste in developing countries: A review from sustainability and economic feasibility perspectives.

The current increase in the world population and its energy demand promotes the study and implementation of cleaner energy forms since the traditional energy recovery systems are seriously affecting the environment. Biofuels and especially biomass or solid biofuels represent a sustainable energy source for developed and developing countries. This review aims to discuss the characteristics and advantages of solid biofuels, analyse the pretreatments and thermal treatments required to recover energy, and compare them with traditional fossil fuels. Other areas such as the sustainability and economic feasibility of solid biofuels are likewise addressed by explaining frequently used tools to evaluate the environmental impact as Life Cycle Assessment (LCA). Comparatively, more recent methodologies are examined as efforts for accomplishing sustainability in the biofuel industry, namely Life Cycle Sustainability Assessment (LCSA) and certification schemes like the Roundtable on Sustainable Biomaterials (RSB), the Inter-American Development Bank Sustainability Scorecard, and initiatives like the Roundtable for Sustainable Palm Oil (RSPO). Finally, it was revealed that the economic feasibility and competitiveness of solid biofuels differ among developing countries but represent a notable contribution to their energy matrix.