Chlorella vulgaris cultivation in pilot-scale to treat real swine wastewater and mitigate carbon dioxide for sustainable biodiesel production by direct enzymatic transesterification

New insights into Chlorella vulgaris applications

Environmental pollution is a big challenge that has been faced by humans in contemporary life. In this context, fossil fuel, cement production, and plastic waste pose a direct threat to the environment and biodiversity. One of the prominent solutions is the use of renewable sources, and different organisms to valorize wastes into green energy and bioplastics such as polylactic acid. Chlorella vulgaris, a microalgae, is a promising candidate to resolve these issues due to its ease of cultivation, fast growth, carbon dioxide uptake, and oxygen production during its growth on wastewater along with biofuels, and other productions. Thus, in this article, we focused on the potential of Chlorella vulgaris to be used in wastewater treatment, biohydrogen, biocement, biopolymer, food additives, and preservation, biodiesel which is seen to be the most promising for industrial scale, and related biorefineries with the most recent applications with a brief review of Chlorella and polylactic acid market size to realize the technical/nontechnical reasons behind the cost and obstacles that hinder the industrial production for the mentioned applications. We believe that our findings are important for those who are interested in scientific/financial research about microalgae.

Strategies for livestock wastewater treatment and optimised nutrient recovery using microalgal-based technologies

Global sustainable development faces several challenges in addressing the needs of a growing population. Regarding food industries, the heightening pressure to meet these needs has resulted in increased waste generation. Thus, recognising these wastes as valuable resources is crucial to integrating sustainable models into current production systems. For instance, the current 24 billion tons of nutrient-rich livestock wastewater (LW) generated yearly could be recovered and valorised via biological uptake through microalgal biomass. Microalgae-based livestock wastewater treatment (MbLWT) has emerged as an effective technology for nutrient recovery, specifically targeting carbon, nitrogen, and phosphorus. However, the viability and efficacy of these systems rely on the characteristics of LW, including organic matter and ammonium concentration, content of suspended solids, and microbial load. Thus, this systematic literature review aims to provide guidance towards implementing an integral MbLWT system for nutrient control and recovery, discussing several pre-treatments used in literature to overcome the challenges regarding LW as a suitable media for microalgae cultivation.

A semi-continuous efficient strategy for removing phosphorus and nitrogen from eel aquaculture wastewater using the self-flocculating microalga Desmodesmus sp. PW1

The phosphorus content in eel aquaculture wastewater exceeds the discharge standard, and the amount of wastewater discharged is substantial. Therefore, there is an urgent need to explore an economical and efficient method of treating aquaculture wastewater. This study explored the use of Desmodesmus sp. PW1, a type of microalgae, to treat eel aquaculture wastewater. By optimizing the conditions, Desmodesmus sp. PW1 achieved a total phosphorus (TP) removal efficiency of 92.3%, as well as total nitrogen (TN) and ammonia nitrogen (NH4+-N) removal efficiency of 99%, using a photoperiod of 24:0, a temperature of 25 °C, and an inoculation amount of 15%. Furthermore, Desmodesmus sp. PW1 demonstrated a high self-flocculating efficiency (>90%) within 100 min of settling, which facilitated biomass recovery. Subsequently, a semi-continuous treatment process mode was established with a sewage renewal rate of 90%. The results showed that after four rounds of sewage renewal operations, the microalgae biomass in the sewage treatment system could be maintained between 160.0 and 220.0 mg/L, and the average removal rate of TP was 0.13 mg/(L * h). The lipid content of algae cells collected in the semi-continuous treatment system for eel aquaculture wastewater was as high as 36.5%, and the biodiesel properties met the biodiesel standards authorized by Europe and the United States. Overall, this study provides an economical and effective strategy for converting wastewater into high-value microalgae products.

Emerging Applications of Chlorella sp. and Spirulina (Arthrospira) sp

Chlorella sp. and Spirulina (Arthrospira) sp. account for over 90% of the global microalgal biomass production and represent one of the most promising aquiculture bioeconomy systems. These microorganisms have been widely recognized for their nutritional and therapeutic properties; therefore, a significant growth of their market is expected, especially in the nutraceutical, food, and beverage segments. However, recent advancements in biotechnology and environmental science have led to the emergence of new applications for these microorganisms. This paper aims to explore these innovative applications, while shedding light on their roles in sustainable development, health, and industry. From this state-of-the art review, it was possible to give an in-depth outlook on the environmental sustainability of Chlorella sp. and Spirulina (Arthrospira) sp. For instance, there have been a variety of studies reported on the use of these two microorganisms for wastewater treatment and biofuel production, contributing to climate change mitigation efforts. Moreover, in the health sector, the richness of these microalgae in photosynthetic pigments and bioactive compounds, along with their oxygen-releasing capacity, are being harnessed in the development of new drugs, wound-healing dressings, photosensitizers for photodynamic therapy, tissue engineering, and anticancer treatments. Furthermore, in the industrial sector, Chlorella sp. and Spirulina (Arthrospira) sp. are being used in the production of biopolymers, fuel cells, and photovoltaic technologies. These innovative applications might bring different outlets for microalgae valorization, enhancing their potential, since the microalgae sector presents issues such as the high production costs. Thus, further research is highly needed to fully explore their benefits and potential applications in various sectors.

Enhancement of carbon sequestration via MIL-100(Fe)@PUS in bacterial-algal symbiosis treating municipal wastewater

Bacterial-algal symbiosis (BAS) is a promising carbon neutrality technology to treat municipal wastewater. However, there are still non-trivial CO₂ emissions in BAS due to the slow diffusion and biosorption of CO₂. Aiming to reduce CO₂ emissions, the inoculation ratio of aerobic sludge to algae was further optimized at 4:1 on the base of favorable carbon conversion. MIL-100(Fe) served as CO₂ adsorbents was immobilized on polyurethane sponge (PUS) to increase the interaction with microbes. When MIL-100(Fe)@PUS was added to BAS in the treatment of municipal wastewater, zero CO₂ emission was achieved and the carbon sequestration efficiency was increased from 79.9% to 89.0%. Most genes related to metabolic function were derived from Proteobacteria and Chlorophyta. The mechanism of enhanced carbon sequestration in BAS could be attributed to both enrichment of algae (Chlorella and Micractinium) and increased abundance of functional genes related to PS I, PS II and Calvin cycle in photosynthesis.

Treatment and utilization of swine wastewater - A review on technologies in full-scale application

The management of swine wastewater has become the focus of attention in the farming industry. The disposal mode of swine wastewater can be classified as field application of treated waste and treatment to meet discharge standards. The status of investigation and application of unit technology in treatment and utilization such as solid-liquid separation, aerobic treatment, anaerobic treatment, digestate utilization, natural treatment, anaerobic-aerobic combined treatment, advanced treatment, are reviewed from the full-scale application perspective. The technologies of anaerobic digestion-land application is most appropriate for small and medium-sized pig farms or large pig farms with enough land around for digestate application. The process of "solid-liquid separation-anaerobic-aerobic-advanced treatment" to meet the discharge standard is most suitable for large and extra-large pig farms without enough land. Poor operation of anaerobic digestion unit in winter, hard to completely utilize liquid digestate and high treatment cost of digested effluent for meeting discharge standard are established as the main difficulties.

A biotechnological roadmap for decarbonization systems combined into bioenergy production: Prelude of environmental life-cycle assessment

Decarbonization has become a critical issue in recent years due to rising energy demands and diminishing oil resources. Decarbonization systems based on biotechnology have proven to be a cost-effective and environmentally benign technique of lowering carbon emissions. Bioenergy generation is an environmentally friendly technique for mitigating climate change in the energy industry, and it is predicted to play an important role in lowering global carbon emissions. This review essentially provides a new perspective on the unique biotechnological approaches and strategies based decarbonization pathways. Furthermore, the application of genetically engineered microbes in CO₂ biomitigation and energy generation is particularly emphasized. The production of biohydrogen and biomethane via anaerobic digestion techniques has been highlighted in the perspective. In this review, role of microorganisms in bioconversion of CO₂ into different types of bioproducts such as biochemical, biopolymers, biosolvents and biosurfactant was summarized. The current analysis, which includes an in-depth discussion of a biotechnology-based roadmap for the bioeconomy, provides a clear picture of sustainability, forthcoming challenges, and perspectives.

Bioremediation of zinc and manganese in swine wastewater by living microalgae: Performance, mechanism, and algal biomass utilization

The remediation effects of living Chlorella sp. HL on zinc and manganese in swine wastewater was investigated, and the responses of algal cells and the mechanism were explored. In the wastewater with Zn(II) concentration of 1.85 mg/L and Mn(II) of 1 or 6 mg/L, the highest removal of Zn(II) by Chlorella reached 86.72% and 97.16%, respectively, and the Mn(II) removal were 42.74% and 30.33%, respectively. The antioxidant system of cells was activated by a significant increase in superoxide dismutase and catalase enzyme activities and a significant decrease in malondialdehyde in the mixed system compared to the single system. The presence of Mn(II) could positively regulate the differentially expressed genes related to catalytic activity and metabolic processes between the single Zn system and the mixed systems, reducing the stress of Zn(II) on Chlorella and more favorable to chlorophyll synthesis. The heavy metal-containing microalgal biomass obtained has the potential as feed additives.

Competitive algae biodiesel depends on advances in mass algae cultivation

The aim of this review was to study why, despite large investments in research and development, algae biodiesel is still not price competitive with fossil fuels. Microalgal production was confirmed to be a critical cost item (84 up to 93 %) for biodiesel regardless of the production technology. Techno-economic assessment revealed the main cost drivers during mass cultivation. It is argued that a breakthrough in the cultivation efficiency of microalgae is identified as a necessary condition for achieving price-competitive microalgal biodiesel. The key bottlenecks were identified as follows: (1) light and O₂ concentration management; (2) overnight respiratory loss of oil. It is concluded that most of the research on microalgae biodiesel yields economically over-optimistic presumptions because it has been based on laboratory scale experiments with a low level of interdisciplinary overlap.

Pilot-scale remediation of rare earth elements ammonium wastewater by Chlamydomonas sp. YC in summer under outdoor conditions

This work evaluated the performance of real rare earth elements (REEs) wastewater purification and carbon dioxide (CO₂) fixation by Chlamydomonas sp. YC with pilot-scale airlift-photobioreactors (AL-PBRs), tubular photobioreactors (TB-PBRs) and raceway ponds (ORWPs) under high-temperature outdoor conditions in summer. The obtained results showed that Chlamydomonas sp. YC at 1 g/L oyster shell piece (OSP) and 3 % CO₂ had the highest biomass (1.9 g/L) and NH₄⁺-N removal efficiency (34.0 %) during the REEs wastewater treatment. Among the selected photobioreactors, Chlamydomonas sp. YC to treat real REEs wastewater at 3 % CO₂ under high-temperature outdoor conditions attained the highest biomass (2.3 g/L) in the TB-PBRs with the best NH₄⁺-N removal efficiency (43.0 %). Furthermore, the input cost and CO₂ net sequestration evaluation revealed that TB-PBRs was more economical photobioreactors to treat REEs wastewater and fix CO₂ by Chlamydomonas sp. YC, providing some vital scientific details for REEs wastewater and CO₂ fixation by microalgal biotechnology.

CO2 Levels Modulate Carbon Utilization, Energy Levels and Inositol Polyphosphate Profile in Chlorella

Microalgae have a growing recognition of generating biomass and capturing carbon in the form of CO₂. The genus Chlorella has especially attracted scientists' attention due to its versatility in algal mass cultivation systems and its potential in mitigating CO₂. However, some aspects of how these green microorganisms respond to increasing concentrations of CO₂ remain unclear. In this work, we analyzed Chlorella sorokiniana and Chlorella vulgaris cells under low and high CO₂ levels. We monitored different processes related to carbon flux from photosynthetic capacity to carbon sinks. Our data indicate that high concentration of CO₂ favors growth and photosynthetic capacity of the two Chlorella strains. Different metabolites related to the tricarboxylic acid cycle and ATP levels also increased under high CO₂ concentrations in Chlorella sorokiniana, reaching up to two-fold compared to low CO₂ conditions. The signaling molecules, inositol polyphosphates, that regulate photosynthetic capacity in green microalgae were also affected by the CO₂ levels, showing a deep profile modification of the inositol polyphosphates that over-accumulated by up to 50% in high CO₂ versus low CO₂ conditions. InsP₄ and InsP₆ increased 3- and 0.8-fold, respectively, in Chlorella sorokiniana after being subjected to 5% CO₂ condition. These data indicate that the availability of CO₂ could control carbon flux from photosynthesis to carbon storage and impact cell signaling integration and energy levels in these green cells. The presented results support the importance of further investigating the connections between carbon assimilation and cell signaling by polyphosphate inositols in microalgae to optimize their biotechnological applications.

Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO₂ capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.

Performance and mechanism of Chlorella in swine wastewater treatment: Roles of nitrogen-phosphorus ratio adjustment and indigenous bacteria

The effects of adjusting the nitrogen-phosphorus (N/P) ratio of wastewater and indigenous bacteria on swine wastewater treatment by Chlorella sp. HL were investigated. The optimal N/P ratio of Chlorella in swine wastewater was 20 by adjusting the phosphorus concentration. The participation of indigenous bacteria increased total extracellular polymeric substances content, which was beneficial to maintain the stability of the algal-bacterial consortium, and improved the algal density and the removal rate of total nitrogen, total phosphorus, and chemical oxygen demand by 47.8%, 24.0%, 30.7%, and 326.7%, respectively. Proteobacteria was the dominant phylum with the relative abundance of 71.58% in the algal-bacterial system at optimal N/P ratio, and Brevundimonas, Chryseobacterium, and Pseudomonas played positive roles in the establishment of symbiotic systems at the genus level. These results provide a theoretical basis for the construction of an efficient algal-bacterial symbiotic system in swine wastewater treatment and support for commercial scale-up.