Nutrient Recycling of Spent Biomass and Lipid-Extraction Wastewater in the Production of Thraustochytrids

Abstract

One of the most important challenges for the emerging sector of microalgae biotechnology is reducing the production costs by improving the efficacy of resource use, while minimizing environmental repercussions of the process. An avenue for accomplishing this goal is implementing a novel strategy of nutrient recycling, whereby the effluent wastewater produced is recycled internally for subsequent cultures. This concept is the foundation of this thesis where the prospect of nutrient recycling, and all of the corollaries associated with it, are explored in the context of the production process at Mara Renewables Corporation. Due to the vast majority of valuable nutrients residing in the lipid-extracted hydrolysate, that liquid waste material was the primary focus of the secondary fermentations conducted. Initial flask-scale secondary fermentations revealed that Thraustochytrium sp. T18 could achieve improved levels of biomass and lipid production in recycled hydrolysate with full media supplements (20.48 g cells L-1 and 40.9% (w/w) lipid) compared to the control (13.63 g cells L-1 and 56.8% (w/w) lipid). Furthermore, results indicated that the proteinaceous content of the recycled hydrolysate can offset the need to supply fresh nitrogen in a secondary culture, without any detrimental impact upon the produced biomass. In fact, the experiments employing the recycled hydrolysate with no nitrogen addition accumulated 14.86 g L-1 of biomass in 141 hours with 43.3% (w/w) lipid content in comparison to the control which had 9.26 g L-1 and 46.9% (w/w), respectively. Using the protocols developed in the flask experiments the nutrient recycling strategy was then applied to larger, laboratory bench top fermentors. In recycled hydrolysate, T18 amassed 63.68 ± 1.46 g L-1 of biomass with 50.8 ± 1.20% (w/w) of lipids compared to 58.59 g L-1 of biomass and 69.4% (w/w) in fresh media. When recycling the same hydrolysate for a tertiary fermentation, biomass reached 65.27 ± 1.15 g L-1 in 90 hours and 62.2 ± 0.3% (w/w) of lipids. The positive findings for nutrient recycling are an important opportunity that can aid in mitigating waste production and reducing costs as heterotrophic microalgae production progresses toward commercialization.