Bioconversion of syngas/waste gas components to produce ethanol appears to be a promising alternative compared to the existing chemical techniques. Recently, several laboratory-scale studies have demonstrated the use of acetogens that have the ability to convert various syngas components (CO, CO 2 , and H 2 ) to multicarbon compounds, such as acetate, butyrate, butanol, lactate, and ethanol, in which ethanol is often produced as a minor endproduct. This bioconversion process has several advantages, such as its high specifi city, the fact that it does not require a highly specifi c H 2 /CO ratio, and that biocatalysts are less susceptible to metal poisoning. Furthermore, this process occurs under mild temperature and pressure and does not require any costly pre-treatment of the feed gas or costly metal catalysts, making the process superior over the conventional chemical catalytic conversion process.The main challenge faced for commercializing this technology is the poor aqueous solubility of the gaseous substrates (mainly CO and H 2 ). In this paper, a critical review of CO-rich gas fermentation to produce ethanol has been analyzed systematically and published results have been compared. Special emphasis has been given to understand the microbial aspects of the conversion process, by highlighting the role of different micro-organisms used, pathways, and parameters affecting the bioconversion. An analysis of the process fundamentals of various bioreactors used for the biological conversion of CO-rich gases, mainly syngas to ethanol, has been made and reported in this paper. Various challenges faced by the syngas fermentation process for commercialization and future research requirements are also discussed.

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Bioprocesses have been developed as relatively recent alternatives to conventional, non-biological technologies, for waste gas treatment and air pollution control in general. This paper reviews major biodegradation processes relevant in this field as well as both accepted and major innovative bioreactor configurations studied or used nowadays for the treatment of polluted air, i.e. biofilters, one-and two-liquid phase biotrickling filters, bioscrubbers, membrane bioreactors, rotating biodiscs and biodrums, one-and two-liquid phase suspended growth bioreactors, as well as hybrid reactor configurations. Some of these bioreactors are being used at full-scale for solving air pollution problems, while others are still at the research and development stage at laboratory-or pilot-scale.

Nitrogen oxides (NOx) of environmental concern are nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ). They are hazardous air pollutants that lead to the formation of acid rain and tropospheric ozone. Both pollutants are usually present simultaneously and are, therefore, called NOx. Another compound is N 2 O which is found in the stratosphere where it plays a role in the greenhouse effect. Concern for environmental and health issues coupled with stringent NOx emission standards generates a need for the development of efficient low-cost NOx abatement technologies. Under such circumstances, it becomes mandatory for each NOx-emitting industry or facility to opt for proper NOx control measures. Several techniques are available to control NOx emissions: selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), adsorption, scrubbing, and biological methods. Each process offers specific advantages and limitations. Since bioprocesses present many advantages over conventional technologies for flue gas cleaning, a lot of interest has recently been shown for these processes. This article reviews the major characteristics of conventional non-biological technologies and recent advances in the biological removal of NOx from flue gases based on the catalytic activity of either eucaryotes or procaryotes, ie nitrification, denitrification, the use of microalgae, and a combined physicochemical and biological process (BioDeNOx). Relatively uncomplicated design and simple operation and maintenance requirements make biological removal a good option for the control of NOx emissions in stationary sources.

CO, H2, and CO2 are major components of syngas and some industrial CO‐rich waste gases (e.g. waste gases from steel industries), besides some additional minor compounds. It was recently shown that those gases can be bioconverted, by acetogenic/solventogenic bacteria, into ethanol and higher alcohols such as butanol, but also hexanol, through the so‐called HBE fermentation. That process presents some advantages over existing chemical conversion processes. This paper reviews HBE fermentation from C1‐gases after briefly describing the more conventional ABE (acetone‐butanol‐ethanol) fermentation from carbohydrates by Clostridium acetobutylicum, in order to allow for comparison of both processes. Although acetone may appear in carbohydrate fermentation, alcohols are the only major end‐metabolites in the HBE process with Clostridium carboxidivorans. The few acetogenic bacteria known to metabolize C1‐gases and produce butanol or higher alcohols are described. Clostridium carboxidivorans has been used in most cases. Bioconversion of the gaseous substrates takes place in two stages, namely acidogenesis (production of acids) followed by solventogenesis (production of alcohols), characterized by different optimal fermentation conditions. Major parameters affecting each bioconversion stage as well as the overall fermentation process are analyzed. Although it has been claimed that acidification is required in ABE fermentation to initiate the solventogenic stage, strong acidification seems to some extent not to be a prerequisite for solventogenesis in the HBE process. Bioreactors potentially suitable for this type of bioconversion process are described as well. © 2017 Society of Chemical Industry

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High-rate anaerobic treatment has emerged as a viable alternative for the treatment of many industrial and municipal wastewaters. A number of different process options have been reduced to practice, although some configurations are clearly more well developed than others. One common thread that links these various processes (principally the Anaerobic Filter, Upflow Anaerobic Sludge Blanket and Expanded/Fluidized Bed Reactors), is the ability to effectively separate solids and hydraulic retention times. This permits design to be based upon the degradative capacity of the anaerobes, not growth rate and results in reduction of treatment times from days (typical for conventional digester systems) to hours. This article compares and contrasts the principles of start-up and operation of these different high-rate anaerobic systems based upon laboratory research and full-scale operating experience gained over the past two decades. The application of anaerobic processes for treatment of certain toxic and hazardous waste streams is just beginning. The limited work performed to date and anticipated future needs for process monitoring and control are also presented in this article.

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