Laboratory- and Pilot-Scale Cultivation of Tetraselmis striata to Produce Valuable Metabolic Compounds

Marine microalgae are considered an important feedstock of multiple valuable metabolic compounds of high biotechnological potential. In this work, the marine microalga Tetraselmis striata was cultivated in different scaled photobioreactors (PBRs). Initially, experiments were performed using two different growth substrates (a modified F/2 and the commercial fertilizer Nutri-Leaf (30% TN-10% P-10% K)) to identify the most efficient and low-cost growth medium. These experiments took place in 4 L glass aquariums at the laboratory scale and in a 9 L vertical tubular pilot column. Enhanced biomass productivities (up to 83.2 mg L^-1 d^-1) and improved biomass composition (up to 41.8% d.w. proteins, 18.7% d.w. carbohydrates, 25.7% d.w. lipids and 4.2% d.w. total chlorophylls) were found when the fertilizer was used. Pilot-scale experiments were then performed using Nutri-Leaf as a growth medium in different PBRs: (a) a paddle wheel, open, raceway pond of 40 L, and (b) a disposable polyethylene (plastic) bag of 280 L working volume. Biomass growth and composition were also monitored at the pilot scale, showing that high-quality biomass can be produced, with important lipids (up to 27.6% d.w.), protein (up to 45.3% d.w.), carbohydrate (up to 15.5% d.w.) and pigment contents (up to 4.2% d.w. total chlorophylls), and high percentages of eicosapentaenoic acid (EPA). The research revealed that the strain successfully escalated in larger volumes and the biochemical composition of its biomass presents high commercial interest and could potentially be used as a feed ingredient.

Manufacturing and Application of 3D Printed Photo Fenton Reactors for Wastewater Treatment

Additive manufacturing (AM) or 3D printing offers a new paradigm for designing and developing chemical reactors, in particular, prototypes. The use of 3D printers has been increasing, their performance has been improving, and their price has been reducing. While the general trend is clear, particular applications need to be assessed for their practicality. This study develops and follows a systematic approach to the prototyping of Advanced Oxidation Processes (AOP) reactors. Specifically, this work evaluates and discusses different printable materials in terms of mechanical and chemical resistance to photo-Fenton reactants. Metallic and ceramic materials are shown to be impracticable due to their high printing cost. Polymeric and composite materials are sieved according to criteria such as biodegradability, chemical, thermal, and mechanical resistance. Finally, 3D-printed prototypes are produced and tested in terms of leakage and resistance to the photo-Fenton reacting environment. Polylactic acid (PLA) and wood-PLA composite (Timberfill^®) were selected, and lab-scale raceway pond reactors (RPR) were printed accordingly. They were next exposed to H2O2/Fe(II) solutions at pH = 3 ± 0.2 and UV radiation. After 48 h reaction tests, results revealed that the Timberfill^® reactor produced higher Total Organic Carbon (TOC) concentrations (9.6 mg·L^-1) than that obtained for the PLA reactor (5.5 mg·L^-1) and Pyrex^® reactor (5.2 mg·L^-1), which suggests the interference of Timberfill^® with the reaction. The work also considers and discusses further chemical and mechanical criteria that also favor PLA for 3D-printing Fenton and photo-Fenton reactors. Finally, the work also provides a detailed explanation of the printing parameters used and guidelines for preparing prototypes.

Comparison of the Performance and Microbial Community Structure of Two Outdoor Pilot-Scale Photobioreactors Treating Digestate

This study aimed at examining and comparing the nutrient removal efficiency, biomass productivity and microbial community structure of two outdoor pilot-scale photobioreactors, namely a bubble column and a raceway pond, treating the liquid fraction of an agricultural digestate. Bacterial and eukaryotic communities were characterized using a metabarcoding approach and quantitative PCR. The abundance, composition, diversity, and dynamics of the main microbes were then correlated to the environmental conditions and operational parameters of the reactors. Both photobioreactors were dominated either by Chlorella sp. or Scenedesmus sp. in function of temperature, irradiance and the nitrogen compounds derived by nitrification. Other species, such as Chlamydomonas and Planktochlorella, were sporadically present, demonstrating that they have more specific niche requirement. Pseudomonas sp. always dominated the bacterial community in both reactors, except in summertime, when a bloom of Calothrix occurred in the raceway pond. In autumn, the worsening of the climate conditions decreased the microalgal growth, promoting predation by Vorticella sp. The study highlights the factors influencing the structure and dynamics of the microbial consortia and which ecological mechanisms are driving the microbial shifts and the consequent reactor performance. On these bases, control strategies could be defined to optimize the management of the microalgal-based technologies.

Challenges and opportunities for microalgae-mediated CO2 capture and biorefinery

Aquacultures of microalgae are frontrunners for photosynthetic capture of CO2 from flue gases. Expedient implementation mandates coupling of microalgal CO2 capture with synthesis of fuels and organic products, so as to derive value from biomass. An integrated biorefinery complex houses a biomass growth and harvesting area and a refining zone for conversion to product(s) and separation to desired purity levels. As growth and downstream options require energy and incur loss of carbon, put together, the loop must be energy positive, carbon negative, or add substantial value. Feasibility studies can, thus, aid the choice from among the rapidly evolving technological options, many of which are still in the early phases of development. We summarize basic engineering calculations for the key steps of a biorefining loop where flue gases from a thermal power station are captured using microalgal biomass along with subsequent options for conversion to fuel or value added products. An assimilation of findings from recent laboratory and pilot-scale experiments and life cycle analysis (LCA) studies is presented as carbon and energy yields for growth and harvesting of microalgal biomass and downstream options. Of the biorefining options, conversion to the widely studied biofuel, ethanol, and manufacture of the platform chemical, succinic acid are presented. Both processes yield specific products and do not demand high-energy input but entail 60-70% carbon loss through fermentative respiration. Thermochemical conversions, on the other hand, have smaller carbon and energy losses but yield a mixture of products.