Combined yeast and microalgal cultivation in a pilot-scale raceway pond for urban wastewater treatment and potential biodiesel production

Pilot-scale microalgae cultivation and wastewater treatment using high-rate ponds: a meta-analysis

Microalgae cultivation in wastewater has been widely researched under laboratory conditions as per its potential to couple treatment with biomass production. Currently, only a limited number of published articles consider outdoor and long-term microalgae-bacteria cultivations in real wastewater environmental systems. The scope of this work is to describe microalgal cultivation steps towards high-rate algal pond (HRAP) scalability and identify key parameters that play a major role for biomass productivity under outdoor conditions and long-term cultivations. Reviewed pilot-scale HRAP literature is analysed using multivariate analysis to highlight key productivity parameters within environmental and operational factors. Wastewater treatment analysis indicated that HRAP can effectively remove 90% of NH4+, 70% of COD, and 50% of PO43-. Mean reference values of 210 W m-2 for irradiation, 18 °C for temperature, pH of 8.2, and HRT of 7.7 are derived from pilot-scale cultivations. Microalgae biomass productivity at a large scale is governed by solar radiation and NH4+ concentration, which are more important than retention time variations within investigated studies. Hence, selecting the correct type of location and a minimum of 70 mg L-1 of NH4+ in wastewater will have the greatest effect in microalgae productivity. A high nutrient wastewater content increases final biomass concentrations but not necessarily biomass productivity. Pilot-scale growth rates (~ 0.54 day-1) are half those observed in lab experiments, indicating a scaling-up bottleneck. Microalgae cultivation in wastewater enables a circular bioeconomy framework by unlocking microalgal biomass for the delivery of an array of products.

Microalgal-bacterial consortia for the treatment of livestock wastewater: Removal of pollutants, interaction mechanisms, influencing factors, and prospects for application

The livestock sector is responsible for a significant amount of wastewater globally. The microalgal-bacterial consortium (MBC) treatment has gained increasing attention as it is able to eliminate pollutants to yield value-added microalgal products. This review offers a critical discussion of the source of pollutants from livestock wastewater and the environmental impact of these pollutants. It also discusses the interactions between microalgae and bacteria in treatment systems and natural habitats in detail. The effects on MBC on the removal of various pollutants (conventional and emerging) are highlighted, focusing specifically on analysis of the removal mechanisms. Notably, the various influencing factors are classified into internal, external, and operating factors, and the mutual feedback relationships between them and the target (removal efficiency and biomass) have been thoroughly analysed. Finally, a wastewater recycling treatment model based on MBC is proposed for the construction of a green livestock farm, and the application value of various microalgal products has been analysed. The overall aim was to indicate that the use of MBC can provide cost-effective and eco-friendly approaches for the treatment of livestock wastewater, thereby advancing the path toward a promising microalgal-bacterial-based technology.

A review of lipid accumulation by oleaginous yeasts: Culture mode

Microbial lipids have attracted considerable interest owing to their favorable environmental sustainability benefits. In laboratory-scale studies, the factors impacting lipid production in oleaginous yeasts, including culture conditions, nutrients, and low-cost substrates, have been extensively studied. However, there were several different modes of microbial lipid cultivation (batch culture, fed-batch culture, continuous culture, and other novel culture modes), making it difficult to comprehensively analyze impacting factors under different cultivation modes on a laboratory scale. And only few cases of microbial lipid production have been conducted at the pilot scale, which requires more technological reliability assessments and environmental benefit evaluations. Thus, this study summarized the different culture modes and cases of scale-up processes, highlighting the role of the nutrient element ratio in regulating culture mode selection and lipid accumulation. The cost distribution and environmental benefits of microbial lipid production by oleaginous yeasts were also investigated. Our results suggested that the continuous culture mode was recommended for the scale-up process because of its stable lipid accumulation. More importantly, exploring the continuous culture mode integrated with other efficient culture modes remained to be further investigated. In research on scale-up processes, low-cost substrate (organic waste) application and optimization of reactor operational parameters were key to increasing environmental benefits and reducing costs.

A state of the art review on the co-cultivation of microalgae-fungi in wastewater for biofuel production

The microalgae have a great potential as the fourth generation biofuel feedstock to deal with energy crisis, but the cost of production and biomass harvest are the major hurdles in terms of large scale production and applications. Using filamentous fungi to culture targeted alga for biomass accumulation and eventually harvesting is a sustainable way to mitigate environmental impacts. Microalgal co-culture method could be an alternative to overcome limitations and increase biomass yield and lipid accumulation. It was found to be the high feasibility for the production of biofuels from fungi and microalgae using wastewater. This article aimed to state the synergistic approaches, their culture protocols, harvesting procedure and their potential biotechnological applications. Additionally, algal-fungal consortia could digest cellulosic biomass, potentially reducing operating costs as part of industrial need. As a result of co-cultivation, biofuel production could be economically feasible owing to its excellent ability to treat wastewater and be eco-friendly. The implications of the innovative co-cultivation technology have demonstrated the potential for further development based on the policies that have been supported and implemented.

Treatment of Liquid Digestate by Green Algal Isolates from Artificial Eutrophic Pond

The ability of aquatic microalgae to treat the liquid digestate obtained from the anaerobic digestion of plant waste was investigated. Microalgae were isolated from natural environment for a laboratory-scale cultivation and were then used to remove nutrients and organic contaminants from the liquid digestate. It was shown that the microalgae consortia (Tetradesmus obliquus, Microglena sp., Desmodesmus subspicatus) could reduce nitrogen, phosphates, and total COD by up to 70%, 57%, and 95%, respectively. A new algae genus Microglena was isolated, which in a consortium with Tetradesmus obliquus and Desmodesmus subspicatus exhibited a high efficiency in the removal of both organic contaminants and nutrients from the liquid fraction of digestate.

Municipal Landfill Leachate Treatment and Sustainable Ethanol Production: A Biogreen Technology Approach

Sustainable material sources are an important agenda to protect the environment and to meet human needs. In this study, Scenedesmus sp. was used to treat municipal landfill leachate via batch and continuous cultivation modes to protect the environment and explore sufficient biomass production for bioethanol production using Saccharomyces cerevisiae. Physicochemical characteristics of leachate were determined for the phases before, during, and after the process. Batch and continuous cultivation were used to treat raw leachate to determine optimum conditions for treatment. Then, the biomass of Scenedesmus sp. with and without sonication was used as a substrate for ethanol production. Sonication was carried out for biomass cell disruption for 20 min at a frequency of 40 kHz. Through batch cultivation mode, it was found that pH 7 was the optimum condition for leachate treatment. Continuous cultivation mode had the highest removal values for COD, phosphorus, and carbohydrate, namely 82.81%, 79.70%, and 84.35%, respectively, among other modes. As for ethanol production, biomass without sonication with 9.026 mg·L⁻¹ ethanol, a biomass concentration of 3.300 µg·L⁻¹, and pH 5 were higher than biomass with sonication with 5.562 mg·L⁻¹ ethanol, a biomass concentration of 0.110 µg·L⁻¹, and pH 5. Therefore, it is evident that the leachate has the potential to be treated by Scenedesmus sp. and converted to bioethanol in line with the concept of sustainable materials.

A review on co-culturing of microalgae: A greener strategy towards sustainable biofuels production

There is a growing global recognition that microalgae-based biofuel are environment-friendly and economically feasible options because they incur several advantages over traditional fossil fuels. Also, the microalgae can be manipulated for extraction of value-added compounds such as lipids (triacylglycerols), carbohydrates, polyunsaturated fatty acids, proteins, pigments, antioxidants, various antimicrobial compounds, etc. Recently, there is an increasing focus on the co-cultivation practices of microalgae with other microorganisms to enhance biomass and lipid productivity. In a co-cultivation strategy, microalgae grow symbiotically with other heterotrophic microbes such as bacteria, yeast, fungi, and other algae/microalgae. They exchange nutrients and metabolites; this helps to increase the productivity, therefore facilitating the commercialization of microalgal-based fuel. Co-cultivation also facilitates biomass harvesting and waste valorization, thereby help to build an algal biorefinery platform for bioenergy production along with multivariate high value bioproducts and simultaneous waste bioremediation. This article comprehensively reviews various microalgae cultivation practices utilizing co-culture approaches with other algae, fungi, bacteria, and yeast. The review mainly focuses on the impact of several binary culture strategies on biomass and lipid yield. The advantages and challenges associated with the procedure along with their respective cultivation modes have also been presented and discussed in detail.

Recent advances in microalgae-based remediation of industrial and non-industrial wastewaters with simultaneous recovery of value-added products

The ability of microalgae to grow in a broad spectrum of wastewaters manifests great potentials for removing contaminants from effluents of industries and urban areas. Since the post-treatment microalgae biomass is also a significant source of high-value products, microalgae-based wastewater treatment is an economical and sustainable solution to wastewater management. Adding more value, the integration of microalgae with living/non-living materials looks more promising. Microalgae-based treatment technology has certain limitations like high operational costs, problematic harvesting, large land requirements, and hindrance in photosynthesis due to turbid wastewater. These challenges need to be essentially addressed to achieve enhanced wastewater remediation. This review has highlighted the potential applications of microalgae in contaminant removal from wastewaters, simultaneous resource recovery, efficient microalgae-based hybrid systems along with bottlenecks and prospects. This state-of-the-art article will edify the role of microalgae in wastewater remediation, biomass valorization for bio-based products, and present numerous possibilities in strengthening the circular bioeconomy.

Insights into bioflocculation of filamentous cyanobacteria, microalgae and their mixture for a low-cost biomass harvesting system

Cyanobacteria and microalgae are considered as interesting feedstocks for either the production of high value bio-based compounds and biofuels or wastewater treatment. Nevertheless, the high costs of production, mainly due to the harvesting process, hamper a wide commercialization of industrial cyanobacteria and microalgae based products. Recent studies have found in autoflocculation and bioflocculation promising spontaneous processes for a low-cost and environmentally sustainable cyanobacteria and microalgae biomass harvesting process. In the present work, bioflocculation process has been studied for three different inocula: filamentous cyanobacteria, microalgae and their mixture. Their cultivation has been conducted in batch mode using two different cultivation media: synthetic aqueous solution and urban wastewater. The removal of nutrients and flocculation process performance were monitored during the entire cultivation time. Results have proved that bioflocculation and sedimentation processes occur efficiently for filamentous cyanobacteria cultivated in synthetic aqueous solution, whereas such processes are less efficient in urban wastewater due to the specific characteristics of this medium that prevent bioflocculation to occur. Besides different efficiencies associated to cultivation media, this work highlighted that bioflocculation of sole microalgae is not as effective as when they are cultivated together with filamentous cyanobacteria.

Biorefinery-Based Approach to Exploit Mixed Cultures of Lipomyces starkeyi and Chloroidium saccharophilum for Single Cell Oil Production

The mutualistic interactions between the oleaginous yeast Lipomyces starkeyi and the green microalga Chloroidium saccharophilum in mixed cultures were investigated to exploit possible synergistic effects. In fact, microalga could act as an oxygen generator for the yeast, while the yeast could provide carbon dioxide to microalga. The behavior of the two microorganisms alone and in mixed culture was studied in two synthetic media (YEG and BBM + G) before moving on to a real model represented by the hydrolysate of Arundo donax, used as low-cost feedstock, and previously subjected to steam explosion and enzymatic hydrolysis. The overall lipid content and lipid productivity obtained in the mixed culture of YEG, BBM + G and for the hydrolysate of Arundo donax were equal to 0.064, 0.064 and 0.081 glipid·gbiomass−1 and 30.14, 35.56 and 37.22 mglipid·L−1·day−1, respectively. The mixed cultures, in all cases, proved to be the most performing compared to the individual ones. In addition, this study provided new input for the integration of Single Cell Oil (SCO) production with agro-industrial feedstock, and the fatty acid distribution mainly consisting of stearic (C18:0) and oleic acid (C18:1) allows promising applications in biofuels, cosmetics, food additives and other products of industrial interest.

The algal trophic mode affects the interaction and oil production of a synergistic microalga-yeast consortium

The use of non-food feedstocks to produce renewable microbial resources can limit our dependence on fossil fuels and lower CO₂ emissions. Since microalgae display a virtuous CO₂ and O₂ exchange with heterotrophs, the microalga Chlamydomonas reinhardtii was combined with the oleaginous yeast Lipomyces starkeyi, known for their production of oil, base material for biodiesel. The coupled growth was shown to be synergistic for biomass and lipid production. The species were truly symbiotic since synergistic growth occurred even when the alga cannot use the organic carbon in the feedstock and in absence of air, thus depending entirely on CO₂-O₂ exchange. Since addition of acetate as the algal carbon source lowered the performance of the consortium, the microbial system design should take into account algal mixotrophy. The mixed biomass was found be suitable for biodiesel production, and whereas lipid production increased in the consortium, yields should be improved in future studies.