Microalgae for Bioenergy: Key Technology Nodes

Microalgae have increasingly gained research interest as a source of lipids for biodiesel production. The wet way processing of harvested microalgae was suggested and evaluated with respect to the possible environmental impacts and production costs. This study is focused on the three key steps of the suggested process: flocculation, water recycling, and extraction of lipids. Microalgae strains with high content of lipids were chosen for cultivation and subsequent treatment process. Ammonium hydroxide was tested as the flocculation agent and its efficiency was compared with chitosan. Determined optimal flocculation conditions for ammonium hydroxide enable the water recycling for the recurring microalgae growth, which was verified for the use of 30, 50, and 80% recycled water. For extraction of the wet microalgae hexane, hexane/ethanol and comparative chloroform/methanol systems were applied. The efficiency of hexane/ethanol extraction system was found as comparable with chloroform/methanol system and it seems to be promising owing to its low volatility and toxicity and mainly the low cost

A Comparative Analysis of Environmental Impacts of Operational Phases of Three Selected Microalgal Cultivation Systems

In recent years, microalgal biomass cultivation has been growing in importance, not only related to the production of alternative foods and nutritional supplements but also for its usage for energy purposes or as a natural solution for wastewater treatment. Regarding these cases, the practical potential associated with the circular economy is evident. However, this is not an option for microalgal food and supplements due to strict hygiene requirements for microalgae cultivation used for these purposes. Currently, the most common cultivation options for microalgae include phototrophic cascades, photobioreactors, and heterotrophic fermenters. Generally, the higher requirements for the purity of the resulting biomass, the higher the consumption of energy and nutrients needed. These are the main operational parameters that significantly shape the total environmental and economic performance of microalgae cultivation processes. The comparative Life Cycle Assessment (LCA) of environmental aspects in the operational phases of three selected cultivation systems, located in the Czech Republic and used for pure microalgae biomass production, confirmed that the impacts of these systems in the assessed categories are fundamentally dependent on the amount of electricity needed and nutrient consumption, as well as their sources. For this reason, the heterotrophic fermenter was evaluated as being the most damaging in the comparison of the three cultivation systems, while the phototrophic cascade showed a lower total environmental impact by 15% and the flat photobioreactor was lower still, by 95%, mainly due to energy production from biomass. The major impact categories observed were climate change, depletion of fossil fuels, human toxicity, and freshwater and marine ecotoxicity. The environmental impacts of microalgae cultivation systems could be further reduced if cycling practices, such as process water recycling and reprocessing of generated sewage sludge, were addressed

Platinum Nanoparticles Immobilized on Electrospun Membranes for Catalytic Oxidation of Volatile Organic Compounds

Structured catalytic membranes with high porosity and a low pressure drop are particularly suitable for industrial processes carried out at high space velocities. One of these processes is the catalytic total oxidation of volatile organic compounds, which is an economically feasible and environmentally friendly way of emission abatement. Noble metal catalysts are typically preferred due to high activity and stability. In this paper, the preparation of a thermally stable polybenzimidazole electrospun membrane, which can be used as a support for a platinum catalyst applicable in the total oxidation of volatile organic compounds, is reported for the first time. In contrast to commercial pelletized catalysts, high porosity of the membrane allowed for easy accessibility of the platinum active sites to the reactants and the catalytic bed exhibited a low pressure drop. We have shown that the preparation conditions can be tuned in order to obtain catalysts with a desired platinum particle size. In the gas-phase oxidation of ethanol, acetone, and toluene, the catalysts with Pt particle sizes 2.1 nm and 26 nm exhibited a lower catalytic activity than that with a Pt particle size of 12 nm. Catalysts with a Pt particle size of 2.1 nm and 12 nm were prepared by equilibrium adsorption, and the higher catalytic activity of the latter catalyst was ascribed to more reactive adsorbed oxygen species on larger Pt nanoparticles. On the other hand, the catalyst with a Pt particle size of 26 nm was prepared by a solvent evaporation method and contained less active polycrystalline platinum. Last but not least, the catalyst containing only 0.08 wt.% of platinum achieved high conversion (90%) of all the model volatile organic compounds at moderate temperatures (lower than 335 °C), which is important for reducing the costs of the abatement technology

A Comparative LCA of Aeroponic, Hydroponic, and Soil Cultivations of Bioactive Substance Producing Plants

Sustainable agriculture is currently trendy. It is supported not only for the urban environment but also as an innovation of conventional practices in order to increase the efficiency and quality of agricultural production. This study presents the results achieved within selected soil-less (hydroponic and aeroponic) systems. Then, it compares them, using the tool of comparative life cycle assessment (LCA), with the results of soil cultivation. The attention is directed towards biomass production and the content of bioactive substances, which can compensate for higher operating costs of soil-less cultivation systems. Coffea arabica has shown a significant increase of caffeine and theobromine contents, both in leaves and roots, as well as higher biomass yield during the aeroponic cultivation. On the contrary, Senecio bicolor evinced the results of a considerably increased growth in the hydroponic system, with no higher contents of alkaloid or flavonoids, except for the rutin concentration. The LCA results of the compared soil and soil-less systems showed that the consumption of fertilizers, diesel, and water in soil systems and of conventional electricity in aeroponics and hydroponics contributed mostly to their environmental burden. The major environmental impact categories are terrestrial ecotoxicity, human non-carcinogenic toxicity, and global warming. Therefore, in order to make the soil-less cultivation systems sustainable, these environmental aspects need to be considered deeply

Nickel Silicide Catalyst from Photovoltaic Waste for the Methanation Reaction

A technology designed for recycling photovoltaic (PV) cells at the end of their life was successfully used for the preparation of a nickel silicide catalyst. PV cells were mixed with magnesium scrap to produce magnesium silicide (Mg2Si), with almost total conversion under optimized conditions (400 °C, 5 Pa, 25 min), in a constructed semi-open tubular reactor. Subsequently, magnesium silicide was hydrolyzed by 25% phosphoric acid to produce a mixture of silicon hydrides, which were utilized as chemical vapor deposition (CVD) precursors for the preparation of a nickel silicide catalyst. The activity and stability of the prepared catalyst was repeatedly tested for methanation reactions. It was verified that the nickel silicide catalyst showed an approximately 20% higher activity for the methanation reactions compared to the commonly used nickel catalyst

Fenton Reaction–Unique but Still Mysterious

This study is devoted to the Fenton reaction, which, despite hundreds of reports in a number of scientific journals, provides opportunities for further investigation of its use as a method of advanced oxidation of organic macro- and micropollutants in its diverse variations and hybrid systems. It transpires that, for example, the choice of the concentrations and ratios of basic chemical substances, i.e., hydrogen peroxide and catalysts based on the Fe2+ ion or other transition metals in homogeneous and heterogeneous arrangements for reactions with various pollutants, is for now the result of the experimental determination of rather randomly selected quantities, requiring further optimizations. The research to date also shows the indispensability of the Fenton reaction related to environmental issues, as it represents the pillar of all advanced oxidation processes, regarding the idea of oxidative hydroxide radicals. This study tries to summarize not only the current knowledge of the Fenton process and identify its advantages, but also the problems that need to be solved. Based on these findings, we identified the necessary steps affecting its further development that need to be resolved and should be the focus of further research related to the Fenton process

Nickel Silicide Catalyst from Photovoltaic Waste for the Methanation Reaction

A technology designed for recycling photovoltaic (PV) cells at the end of their life was successfully used for the preparation of a nickel silicide catalyst. PV cells were mixed with magnesium scrap to produce magnesium silicide (Mg2Si), with almost total conversion under optimized conditions (400 °C, 5 Pa, 25 min), in a constructed semi-open tubular reactor. Subsequently, magnesium silicide was hydrolyzed by 25% phosphoric acid to produce a mixture of silicon hydrides, which were utilized as chemical vapor deposition (CVD) precursors for the preparation of a nickel silicide catalyst. The activity and stability of the prepared catalyst was repeatedly tested for methanation reactions. It was verified that the nickel silicide catalyst showed an approximately 20% higher activity for the methanation reactions compared to the commonly used nickel catalyst

Waste Feathers Processing to Liquid Fertilizers for Sustainable Agriculture—LCA, Economic Evaluation, and Case Study

The poultry meat industry generates about 60 million tons of waste annually. However, such waste can serve as a cheap material source for sustainable liquid fertilizers or biostimulant production. Moreover, its practical potential associated with the circular economy is evident. One of the options for waste feather reprocessing is to use a hydrolysis process, whose operating parameters vary depending on the waste material used. The better the quality of the waste feathers, the less energy is needed; moreover, a higher yield of amino acids and peptides can be achieved. These are the main operational parameters that influence the overall environmental and economic performance of the hydrolysis process. The assessment of process operational environmental aspects confirmed that the environmental impacts of hydrolysate production are highly dependent on the amount of electricity required and its sources. This fact influences the midpoint and the endpoint impacts on the observed environmental impact categories. It also minimizes the pressure associated with fossil resource scarcity and the related impact on climate change. During an economic evaluation of the process, it was found that the option of processing more fine waste, such as CGF, provided a 5% saving in energy costs related to the reduction in the cost per liter of hydrolysate of 4.5%. Finally, a case study experiment confirmed the fertilizing effect of the hydrolysate on pepper plants (biometric parameters, yield). Thus, the hydrolysate produced from the waste feathers can serve as a substitute for nitrate fertilizing, which is commonly drawn from raw fossil materials

Techno-Economic Analysis of Fluidized Bed Combustion of a Mixed Fuel from Sewage and Paper Mill Sludge

The treatment and disposal of sewage sludge is one of the most important and critical issues of wastewater treatment plants. One option for sludge liquidation is the production of fuel in the form of pellets from mixed sewage and paper mill sludge. This study presents the results of the combustion of pelletized fuels, namely sewage and paper mill sludge, and their 2:1 and 4:1 blends in a fluidized bed combustor. The flue gas was analysed after reaching a steady state at bed temperatures of 700–800 °C. Commonly used flue gas cleaning is still necessary, especially for SO2; therefore, it is worth mentioning that the addition of paper mill sludge reduced the mercury concentration in the flue gas to limits acceptable in most EU countries. The analysis of ash after combustion showed that magnesium, potassium, calcium, chromium, copper, zinc, arsenic, and lead remained mostly in the ash after combustion, while all cadmium from all fuels used was transferred into the flue gas together with a substantial part of chlorine and mercury. The pellets containing both sewage and paper mill sludge can be used as an environmentally friendly alternative fuel for fluidised bed combustion. The levelized cost of this alternative fuel is at the same current price level as lignite