Abstract
Microalgae have attracted significant attention worldwide as one of the most promising feedstock fossil fuel alternatives. However, there are a few challenges for algal fuels to compete with fossil fuels that need to be addressed. Therefore, this study reviews the R&D status of microalgae-based polyculture and biocrude oil production, along with wastewater treatment. Mixotrophic algae are free to some extent from light restrictions using organic matter and have the ability to grow well even in deep water-depth cultivation. It is proposed that integrating the mixotrophic microalgae polyculture and wastewater treatment process is the most promising and harmonizing means to simultaneously increase capacities of microalgae biomass production and wastewater treatment with a low land footprint and high robustness to perturbations. A large amount of mixotrophic algae biomass is harvested, concentrated, and dewatered by combining highly efficient sedimentation through flocculation and energy efficient filtration, which reduce the carbon footprint for algae fuel production and coincide with the subsequent hydrothermal liquefaction (HTL) conversion. HTL products are obtained with a relatively low carbon footprint and separated into biocrude oil, solid, aqueous, and gas fractions. Algae biomass feedstock-based HTL conversion has a high biocrude oil yield and quality available for existing oil refineries; it also has a bioavailability of the recycled nitrogen and phosphorus from the aqueous phase of algae community HTL. The HTL biocrude oil represents higher sustainability than conventional liquid fuels and other biofuels for the combination of greenhouse gas (GHG) and energy return on investment (EROI). Deep water-depth polyculture of mixotrophic microalgae using sewage has a high potential to produce sustainable biocrude oil within the land area of existing sewage treatment plants in Japan to fulfill imported crude oil.
Highlights
This study proposes a considerable improvement on the upstream process, the best selection of an appropriate concentration, and the advantages and usefulness of biocrudeoil production through hydrothermal liquefaction (HTL) from wastewater-grown algae polyculture
Most of the studies strongly suggested that polyculture outperforms the best mono-culture by performing multiple functions simultaneously, such as producing resilient communities, enabling efficient nutrient uptake and resisting population crashes and undesirable invaded species, and showing less tendency to trade-off between desirable functions [70,71,72,73,74,75], and only few reports indicated that polyculture does not outperfor m the most pro-ductive monoculture in producing biocrude oil [71] and biomass [72]
It is commonly recognized that integrating wastewater treatment and microalgae biomass production has multiple benefits, including the circular use of freshwater, reducing the biofuel production cost, and mitigating the impact of contaminated water on the living environment and ecosystems
Summary
As a true alternative to fossil fuels, an ideal organism produced in large quantities of. Microalgae biomass production in monoculture is more costly and energy-intensive than terrestrial energy crop production [12,13] For these reasons, it is necessary to develop technology to produce stable algae-based fuels throughout the year in temperate regions with large environmental fluctuations and limited land area to make them competitive with fossil fuels [14]. The realistic view of its amount of land use highlights the recent deep water-depth polyculture of mixotrophic microalgae combined with wastewater treatment process This suggests that it is the most promising way to simultaneously increase capacities of algal biomass production, achieving wastewater treatment with high perturbation resilience and biocrude oil production, showing a high potential to meet the current demand of crude oil in. Japan [14,25,26,27,28]
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