Microbial oil production from industrial waste streams and biogenic resources, such as biomass hydrolysates, is emerging as a sustainable alternative to use of fossil and vegetable oils. However, the carbon sources of these substrates are typically diluted, leading to low product concentrations and, therefore, high fermentation and downstream processing costs. In this study, high-cell-density yeast oil production with a defined medium, which imitated the sugar composition of a diluted substrate, a typical microalgal biomass hydrolysate, is carried out on a 50 L scale using a membrane bioreactor (MBR) consisting of a microfiltration unit suited for industrial application. The process was run on a semi-continuous mode to reduce operational costs. Oleaginous yeast Cutaneotrichosporon oleaginosus was used as a biocatalyst and lipid production was induced by phosphate deficiency in the medium with a C/P ratio of 3515 g g⁻¹. In this way, high cellular lipid contents of up to 76.5% (w/w) of dry cell mass, an average lipid yield of 32% (w/w), and a lipid space–time yield (STY) of up to 8.88 g L⁻¹ d⁻¹ were achieved with final high cell densities of up to 116 g L⁻¹ dry biomass. Furthermore, use of a defined medium and elemental analysis of the yeast cells and yeast oil enabled drawing an accurate carbon mass balance of the production system. Carbon conversion efficiencies—fraction of total carbon supplied in the form of sugars converted into lipids at the end of the process—of up to 61.5% were achieved from diluted substrates using the MBR with total cell retention. Considering these results, it is concluded that utilization of an MBR on a semi-continuous mode would be very reasonable for yeast oil production, enabling high productivities with diluted sugar substrates.

Lipid production using microalgae is challenging for producing low-value-added products. Harnessing microalgae for their fast and efficient CO₂ fixation capabilities may be more reasonable since algal biomass can be utilized as a precursor for various products in a biorefinery approach. This study aimed to optimize the productivity and efficiency of Microchloropsis salina biomass production in open thin-layer cascade (TLC) photobioreactors under physical simulation of suitable outdoor climate conditions, using an artificial seawater medium. Continuous operation proved to be the most suitable operating mode, allowing an average daily areal productivity of up to 27 g m⁻² d⁻¹ and CO₂ fixation efficiency of up to 100%. Process transfer from 8 m² to 50 m² TLC photobioreactors was demonstrated, but with reduced daily areal productivity of 21 g m⁻² d⁻¹ and a reduced CO₂ fixation efficiency, most probably due to increased temperatures at midday above 35 °C. An automated overnight switch-off of the circulation pumps was implemented successfully, reducing energy and freshwater requirements by ~40%. The ideal conditions for continuous production were determined to be a dilution rate of 0.150–0.225 d⁻¹, pH of 8.5, and total alkalinity of 200–400 ppm, facilitating efficient pilot-scale production of microalgal biomass in TLC photobioreactors.

Microalgae are a promising biomass source because of their capability to fixate CO₂ very efficiently. In this study, the potential of Microchloropsis salina biomass as a feedstock for fermentation was explored, focusing on biomass hydrolysis by employing various mechanical and chemical cell disruption strategies in combination with enzymatic hydrolysis. Among the mechanical cell disruption methods investigated on a lab scale, namely ultrasonication, bead milling, and high-pressure homogenization, the most effective was bead milling using stainless-steel beads with a diameter of 2 mm. In this way, 87–97% of the cells were disrupted in 40 min using a mixer mill. High-pressure homogenization was also effective, achieving 86% disruption efficiency after four passes on a 30–200 L scale using biomass with 15% (w/w) solids content. Enzymatic hydrolysis of the disrupted cells using a mixture of cellulases and mannanases yielded up to 25% saccharification efficiency after 72 h. Acidic hydrolysis of undisrupted cells followed by enzymatic treatment yielded around 30% saccharification efficiency but was coupled with significant dilution of the resulting hydrolysate. Microalgal biomass hydrolysate produced was determined to have ~8.1 g L⁻¹ sugars and 2.5% (w/w) total carbon, as well as sufficient nitrogen and phosphorus content as a fermentation medium.