Development of a microalgal process for bioenergy production, CO2 mitigation and wastewater treatment

Microalgae for wastewater treatment, CO2 mitigation and biofuels : dream or sustainable maid for all work?

Christof Holliger, Stephen Mackay, Eduardo Pereira Gomes, Jean-Paul Schwitzguebel

Over the last decade, intensive research has been carried out all over the world to explore the huge potential of microalgae for biofuel production, which could be coupled to the use of industrial flue gas to enhance CO2 fixation as a greenhouse gas mitigation strategy and to remove pollutants from various wastewaters. However, large-scale microalgae production for such purposes is not yet technically and economically feasible. Challenges are thus to increase the efficiency of both algae production and conversion into biofuel. The following parameters are critical for sustainable microalgal biomass cultivation: CO2 concentration; possible use of wastewater streams; light intensity and quality; photobioreactor design; batch, fed-batch or continuous growth conditions. The objective of our research was thus to assess the influence of these parameters on the quantitative and qualitative production of biomass, lipids and added-value chemicals, taking into account the probable antagonism between biomass and lipid production, often depending on the nitrogen supply. Several microalgal species were grown at laboratory scale to compare the yield of biomass and lipid production. In addition to the potential use of oil for biodiesel production, we are currently working towards demonstrating the technical feasibility of an innovative process, for syngas production via hydrothermal processing of microalgae. The process is envisioned as a closed-cycle with respects to nutrients, water and CO2, that are separated and reused for microalgae growth. The possible effect of increasing injection of CO2 in the growth medium was also investigated, with the aim to recycle CO2 from flue gas and enhance biomass production. On the other hand, to achieve the economical and ecological sustainability of algal biofuel production, it will be necessary to integrate it with the extraction of high-value products in a biorefinery process. Carotenoids have been proposed as added-value compounds that could contribute to make microalgal biofuel production economically feasible. Therefore, the viability and sustainability of extracting carotenoids before the hydrothermal treatment of the remaining biomass to produce syngas were investigated. Finally, harvesting and dewatering microalgae cells remains a technical challenge and typically contributes to 20-30% of biomass production costs and can represent up to half of the total cost of algal biofuels. The potential of co-culture of filamentous fungal species with microalgae in a lichenization process as a strategy to reduce the cost and energy consumption of harvesting and of the whole process seems promising and was thus investigated. In submerged cultures, filamentous microorganisms actually aggregate and grow as loosely packed pellets or compact granules. Microalgae cells were immobilized in these pellets and easily removed as an aggregate with the fungal cells. Pellet formation is strain specific and highly dependent on operational conditions during cultivation. This study was especially focused on lichen pellet formation during the co-culture of Chlorella sorokiniana and of an unidentified filamentous fungus, which was eventually characterized. While algae growth was optimal between pH 6 and 10, the highest pellet formation was observed in the pH range of 4-7, thus requiring a strict control of pH during the whole cultivation. The dense formed pellets did sediment in submerged cultures when the agitation was stopped and were easily harvested. The effect of such a co-cultivation on the production of added-value chemicals like carotenoids and on the biofuel potential of the remaining biomass is under evaluation, and will be compared to results obtained with cultivation of microalgae only. The proposed strategies may significantly reduce energy demands with regards to harvesting and dewatering of microalgal biomass and improve the sustainability of the whole process, but should be assessed at larger scale. Acknowledgements: The research was supported by the Swiss Secretariat for Education, Research and Innovation, in the framework of COST Action CM0903 (Utilisation of biomass for sustainable fuels and chemicals).

2014

Enhanced biofuel production from microalgae by adding CO2 to stimulate lipid biosynthesis (biodiesel) and by hydrothermal processing (syngas)

Gabriela Anca Haiduc, Christian Ludwig, David Pauli, Jean-Paul Schwitzguebel, Frédéric Vogel

Introduction: Microalgae cultures represent an attractive source of biomass for biofuel and chemicals production since they can have higher energy yields per area than conventional biomass crops and can be grown on marginal land using waste, saline, or brackish water. Their use can thus avoid the food-fuel competition for land, resulting in major economic and social benefits. At the present stage however, major developments are needed for making microalgae-to-biofuels processes technologically and economically feasible. One substantial technical challenge is the development of cost-effective processes for efficient conversion of microalgae biomass to biofuels. Our research aims to enhance the production of biodiesel and of bio-Synthetic Natural Gas (SNG) from microalgae. Experimental : Several species of microalgae are grown at laboratory scale to compare the yield of biomass and lipid production. In addition to the potential use of oil for biodiesel production, we are currently working towards demonstrating the technical and economical feasibility of an innovative process, “SunCHem”, for SNG production via hydrothermal (HT) processing of microalgae. The process is envisioned as a closed-cycle with respect to nutrients, water and CO2, that are separated and reused for microalgae growth, resulting in unprecedented energy efficiency for both algae and fuel production. The possible effect of increasing injection of CO2 in the growth medium is under investigation, with the aim to mitigate CO2 and enhance biomass production. Results and Discussion: Before the feasibility of the process can be demonstrated several key challenges stemming from its closed-nature need to be tackled. One major challenge is the recycling of nutrient-rich effluents, which upon continuous operation might become enriched in potential toxicants that in turn affect the growth of algae. We demonstrated that the presence of trace metals (Ni, Cr) in the recycled effluent as a result of reactor wall corrosion and catalyst leaching resulted in adverse effect on the growth of microalgae. The effect was directly proportional to the heavy metal concentration, and for the highest concentrations tested (Ni 25 ppm or Cr 5 ppm) complete inhibition of cell growth was observed. It is known that algae, like many other organisms, can adapt to extreme environments, including those with heavy metal contamination, and resulting mutants or strains should be more suited to survive under those particular conditions. The potential of developing and using metal-tolerant mutant microalgae as a biomass source for the SunCHem process is under investigation. Experiments are undertaken to develop Cr- and Ni- tolerant mutants by selective breeding and then comparing the mutants and the wild-type strains for their sensitivity to the heavy-metal contaminated HT effluent and the resulting biomass yields obtained in each case. It is hoped that the use of the metal-tolerant mutants or strains will contribute to the cost-effectiveness of the process by offering an efficient alternative to the use of the more costly setups otherwise needed for effluent clean-up, thus providing a way to avoid the reduction in the biomass yield upon continuous operation of the SunCHem process. On the other hand, metal-tolerant and accumulating microalgae could be used to treat industrial wastewater. Conclusions: Another aspect of utmost importance is the evaluation of the availability for algae of the nutrients contained in the recycled effluents from the process. To this end, microalgae growth experiments will be performed using as sole nutrient source the salt brine delivered from the HT step. The biomass composition with and without recycled input from the hydrothermal process will be characterized. These tests will provide information on the recycling efficiency and speciation of nutrients and will help to identify potential nutrient management issues. If there is significant loss of a given key nutrient, it will have to be replaced in the bioreactor. A candidate for loss through volatilization could be N. Subsequent studies will explore the role of growth media and especially that of nutrients such as N, P, and minerals. Algae growth experiments will be performed using different synthetic media compositions and varying nutrient levels to assess the tolerance of algae to changes in nutrient level and quality. The issue is of special relevance as the possibility of recycling and reusing all the nutrients, including N and P, is a sine qua non condition for closing the loop in the process.

2010

Enhanced biofuel production from microalgae by adding CO2 to stimulate lipid biosynthesis (biodiesel) and by hydrothermal processing (syngas)

Gabriela Anca Haiduc, Christian Ludwig, David Pauli, Jean-Paul Schwitzguebel, Frédéric Vogel

2010

Microalgae for wastewater treatment, CO2 mitigation and biofuels : dream or sustainable maid for all work?

Christof Holliger, Stephen Mackay, Eduardo Pereira Gomes, Jean-Paul Schwitzguebel

2014