Opportunities for Switzerland to contribute to the production of algal biofuels : the hydrothermal pathway to bio-methane

Microalgae have a significant potential to be a sustainable source of fuel and thus are of interest in the transition to a sustainable energy system, in particular for resource-dependent countries such as Switzerland. Independence of fossil fuels, considerable reduction of CO2 emissions, and abandoning nuclear energy may be possible with an integrated system approach including the sourcing of biofuels from different types of biomass. Today, a full carbon-to-fuel conversion is possible, and has been recently demonstrated with an advanced hydrothermal technology. The potential to develop algal biofuels is viewed as high thanks to the possibility they offer to uncouple bioenergy from food production. Nevertheless, technological breakthroughs must take place before commercial production becomes a reality, especially to meet the necessary cost savings and efficiency gains in the algae cultivation structure. In addition, an integrated management of waste resources to promote the nutrient recovery appears today as imperative to further improve the economic viability and the environmental sustainability of algal production. We provide here a review that includes the global technological status of both algae production and their conversion into biofuels in order to understand first the added value of algal energy in general before we focus on the potential of algae to contribute specifically to the Swiss energy system to the horizon 2050. In this respect, the hydrothermal conversion pathway of microalgal biomass into synthetic natural gas (SNG) is emphasized, as research into this technology has received considerable attention in Switzerland during the last decade. In addition, SNG is a particularly relevant fuel in the Swiss context due to the existing gas grid and to the opportunity it offers to cover a wide spectrum of energy applications, in particular cogeneration of heat and electricity or use as a transport fuel in the growing gas car fleet

Fate and reuse of nitrogen-containing organics from the hydrothermal conversion of algal biomass

Hydrothermal (HT) conversion is a promising and suitable technology for the generation of biofuels from microalgae. Besides the fact that water is used as a “green” reactant and solvent and that no biomass drying is required, the technology offers a potential nutrient source for microalgae culture using an aqueous effluent very rich in essential inorganic nutrients. However, upon continuous and multiple recycling of this HT effluent, the recalcitrant organic fraction is likely to increase and may potentially attain toxic thresholds for microalgae use. In this work, we show the presence of recalcitrant N-containing organic compounds (NOC's) in the HT effluent. The most prominent NOC's in the extracts were carefully examined for their effect on microalgae, namely 2-pyrrolidinone and β-phenylethylamine (β-PEA). The first set of experiments consisted in testing these two substances at three different concentrations (10, 50 and 150 ppm) using three different microalgae strains: Phaeodactylum tricornutum, Chlorella sorokiniana and Scenedesmus vacuolatus. The confirmed half maximal inhibitory concentration (IC50) was approximately 75 ppm for all tested species. In the second set of experiments, P. tricornutum was grown using diluted HT effluent. Experimental conditions were set by adjusting the nitrogen concentration in the HT effluent to be equal to a known commercial medium. The concentrations of specific NOC's were lowered to concentrations of 8.5 mg/L 2-pyrrolidinone and 0.5 mg/L β-PEA after dilution. The growth of P. tricornutum using the diluted HT solution was kept constant with no evidence of inhibition or consumption of NOC's, as the concentration of the specific compounds remains the same before and after growth. Therefore, in order to avoid effects of accumulation of NOC's upon continuous recycling, the HT effluent was pumped through the existing hydrothermal gasification unit as a water clean-up step. The conversion of NOC's to ammonium was successfully achieved

Implementation of a genetic transformation strategy to improve the biological CO2 capture in Chlorella sp. and Scenedesmus sp.

The concept of producing renewable microalgae biomass using as a main nutrient source a secondary wastewater stream is presently under investigation. It is planned to purify the wastewater by the growth of algal biomass while producing a feedstock that can be used for the bioenergy sector. If important biomass density is achieved, this strategy will be adopted in a novel process named SunCHem (proposed at the EPFL-PSI SWT group) using hydrothermal treatment of microalgae to produce methane. By coupling the wastewater treatment, the capture of CO2 emissions and the production of methane through microalgae, we directly impact the sustainable use of carbon resources. In this work, we aim to develop a strategy for the genetic improvement of microalgae in order to enhance the accumulation of carbon inside the cells. The strategy contributes to efficiently obtain a significant change in the production of algal biomass and thus potentially, to increase the methane production yields. The strains Chlorella sp. andScenedesmus sp. have been used to apply the modification of genes involved in these metabolic pathways; in particular, the genes codifying for the glutamine synthetase (glnA). Chlorella sp. was isolated from the wastewater treatment plant and adapted for axenic culturing. These microalgae were also selected due to an appropriate carbon balance for their future hydrothermal conversion into methane