Anaerobic digestion and agronomic applications of microalgae for its sustainable valorization

Prospects and challenges of thermal hydrolysis pretreatment of microalgae for enhancing bioenergy and resource recovery in anaerobic bioprocesses

Microalgae have emerged as a promising feedstock for bioenergy production through anaerobic digestion and fermentation, gaining significant attention due to their rapid growth rate, ability to adapt to diverse environments, and rich biochemical composition. However, the recalcitrant nature of the microalgal cell wall necessitates pretreatment to enhance the accessibility of intracellular components and improve overall bioenergy yields from anaerobic digestion/fermentation. Among the various pretreatment methods, the thermal hydrolysis process has proven to be a promising strategy for enhancing the efficiency of bioenergy recovery from microalgal biomass. The benefits of thermal hydrolysis pretreatment of microalgae include improved organic matter solubilization, enhanced digestibility, and increased product yields in subsequent anaerobic digestion/fermentation processes for biomethane, biohydrogen, and volatile fatty acids production. However, thermal pretreatment poses challenges, such as forming future research by-products like furfural and ammonia, which can adversely affect microbial activities and reduce process efficiency. Thus, addressing its associated challenges is critical for maximizing its effectiveness in bioenergy and resource recovery. This review provides a comprehensive analysis of these challenges and offers recommendations for future research, emphasizing the need for optimized pretreatment strategies for advancing the sustainable and efficient use of microalgae in bioenergy production.

Effect of Marine Microalgae Biomass (Nannochloropsis gaditana and Thalassiosira sp.) on Germination and Vigor on Bean (Phaseolus vulgaris L.) Seeds "Higuera"

The production of marine microalgae provides a sustainable solution for agriculture, acting as biostimulants to enhance seed germination, vigor, and early growth. In the present work, the parameters of pH, airflow, and dilution speed were established to produce biomass of two species of marine algae (Nannochloropsis gaditana and Thalassiosira sp.); in addition, its capacity to stimulate the germination of bean seeds was evaluated. The experimental treatments included three biomass concentrations (Cb) of both microalgae species (0.5, 1, and 1.5 g·L-1) and a control (distilled water) at two temperatures (25 and 35 °C). The rate, index, average time, time at 50% germination, and vigor were evaluated. The results indicated that the highest yield of microalgae biomass was obtained with D = 0.3 day-1 for N. gaditana and 0.2 day-1 for Thalassiosira sp. Microalgae biomass showed activity as a biostimulant on germination, improving the germination rate and reducing the germination time with better vigor for the seedlings at each of the evaluated concentrations.

Wastewater treatment by algae-based membrane bioreactors: a review of the arrangement of a membrane reactor, physico-chemical properties, advantages and challenges

Reducing wastewater contaminants is an emerging area of particular concern for many industrialized and developing countries in improving the ecological quality of their water sources. In this case, the use of algae-based microbial reactors for wastewater treatment has attracted increasing attention in recent years. The advantages of both conventional microbial membrane bioreactors (MBRs) and algae-based treatment are combined in algae-based MBRs. According to the literature, previous studies did not fully discuss the techniques and performance of algae-based bioreactor systems in the treatment of wastewater. In particular, little attention has been paid to the types of waste, their consequences, and the ways in which they are treated. This makes it more difficult to develop and scale up efficient systems to treat waste discharge from industry, agriculture, and urban areas. Thus, the objective of this study is to critically evaluate algae as a valuable biological resource for wastewater treatment, with the goal of reducing emerging contaminants and increasing the chemical oxygen demand (COD) in wastewater. The most common wastewater treatment techniques employed for addressing these wastes are examined together with a brief discussion on contaminants in wastewater. Furthermore, algae-based wastewater treatment arrangements, particularly hybrid configurations, are carefully studied in relation to techniques for removing contaminants using algae. After analysing the key physicochemical characteristics that affect the ability of algal-bioremediation to remove developing contaminants, the benefits of algal-bioremediation systems are compared to those of other techniques. Lastly, an investigation is conducted into the technological difficulties associated with employing algal-bioremediation systems to eliminate emerging contaminants.

Effect of pretreated and anaerobically digested microalgae on the chemical and biochemical properties of soil and wheat grown on fluvisol

In this study, the effects of the potential application of digestate as an agricultural fertiliser obtained from anaerobically digested microalgae treated by three pretreatment methods, namely alkaline hydrogen peroxide (AHP), high temperature and pressure (HTP), and hydrodynamic cavitation (HC) on some properties of soil, and wheat growth and yield were investigated. For this purpose, pretreated and anaerobically digested microalgae digestates alone or together with diammonium phosphate (DAP) as a chemical fertiliser were applied to soil for wheat growth. The highest dosage of AHP pretreated digestate combined with a half dose of DAP applied to soil was rich in nutrients as 0.25%N and 7.19 mg kg-1 compared to all groups. The properties of the soils were enhanced by applying the highest dosage (0.06 g kg-1) of microalgae digestate combined with a half dose of DAP. 0.02 g kg-1 dosage of HC pretreated digestate combined with a half dose of DAP also greatly improved nitrogen use efficiency indices by up to 104%. The soils' enzyme activities increased in wheat growth experiments by applying either raw or pretreated microalgae digestates. The soils' β-glycosidase, alkaline phosphatase, and urease enzyme activities increased to 1.38 mg pNP g-1 soil, 4.91 mg pNP g-1 soil, and 2.27 mg NH4-N 100 g-1 soil respectively by the application of highest dosage of HC pretreated digestate. The digestates did not have a toxic effect on wheat growth, it was determined that applied pretreatment processes did not cause significant changes in wheat plant height or wet and dry weight.

Incorporating saline microalgae biomass in anaerobic digester treating sewage sludge: Impact on performance and microbial populations

The aim of this study was to acclimate anaerobic prokaryotes to saline microalgae biomass. Semi-continuous experiments were conducted using two 1.5 L mesophilic reactors for 10 weeks, (hydraulic retention time of 21 days). The first reactor was solely fed with sewage sludge (control), while the second received a mixture of sewage sludge and microalgal biomass (80/20 %w/w) cultivated at 70 g·L-1 salinity. The in-reactor salinity reached after the acclimation phase was 14 g·L-1. Biomethane production was comparable between the control and acclimated reactors (205 ± 29 NmLMethane·gVolatileSolids-1). Salinity tolerance assessment of methanogenic archaea revealed that salinity causing 50% inhibition of methane production increased from 10 to 27 g·L-1 after acclimation. Microbial diversity analyses revealed notable changes in methanogenic archaea populations during co-digestion of saline microalgae biomass, particularly methylotrophic (+27%) and acetotrophic (-26%) methanogens. This study has highlighted the possibility of treating efficiently saline microalgae in co-digestion with sewage sludge in future industrial biogas plants.

Multi response surface optimization, Pareto analysis and kinetics study of microalgal post-treatment systems

High concentrations of nutrients are observed in the effluent of different wastewater treatment plants, while additional costs of post-treatment systems and low-value sludge are the main reasons for releasing such effluents. The present study aims to introduce an increased procedure for simultaneous nutrient recovery and biomass production using an algae-based post-treatment technique. The procedure has been utilized by two well-known strains (Scenedesmus dimorphus and Chlorella vulgaris) cultivated in different N/P ratios (16, 62, and 108) and trace metals (0, 50%, and 100%) in a synthetic meat processing wastewater as a model to investigate effects of the factors on microalgal cultivation and nutrient removal. Pareto statistical analysis and Multi Response Surface methodology were applied to determine the priority of factors and their optimum values, respectively. The unbalanced N/P ratio and lack of trace metals were introduced as two main reasons for the significant decrease of about 60% and 120% in nutrient removal and biomass production. The optimized procedure resulted in significant increases in the removal efficiencies where 90%, 83%, and 65% were achieved for ammonium, nitrate, and phosphate, respectively. Moreover, a 72% increase in biomass production was reported in the optimal points. The results of the Pareto analysis highlighted the significant superiority (about two times) of the trace metals in removal efficiencies. Finally, experimental data has also been modelled by Verhulst logistic model that successfully described the microalgae growth. This procedure showed promising results of microalgal systems to supersede the conventional post-treatment systems.

Differences and Similarities in Lipid Composition, Nutritional Value, and Bioactive Potential of Four Edible Chlorella vulgaris Strains

The microalga Chlorella vulgaris is a popular food ingredient widely used in the industry, with an increasing market size and value. Currently, several edible strains of C. vulgaris with different organoleptic characteristics are commercialized to meet consumer needs. This study aimed to compare the fatty acid (FA) and lipid profile of four commercialized strains of C. vulgaris (C-Auto, C-Hetero, C-Honey, and C-White) using gas- and liquid-chromatography coupled to mass-spectrometry approaches, and to evaluate their antioxidant and anti-inflammatory properties. Results showed that C-Auto had a higher lipid content compared to the other strains and higher levels of omega-3 polyunsaturated FAs (PUFAs). However, the C-Hetero, C-Honey, and C-White strains had higher levels of omega-6 PUFAs. The lipidome signature was also different between strains, as C-Auto had a higher content of polar lipids esterified to omega-3 PUFAs, while C-White had a higher content of phospholipids with omega-6 PUFAs. C-Hetero and C-Honey showed a higher content of triacylglycerols. All extracts showed antioxidant and anti-inflammatory activity, highlighting C-Auto with greater potential. Overall, the four strains of C. vulgaris can be selectively chosen as a source of added-value lipids to be used as ingredients in food and nutraceutical applications for different market needs and nutritional requirements.

Bioresource Upgrade for Sustainable Energy, Environment, and Biomedicine

We conceptualize bioresource upgrade for sustainable energy, environment, and biomedicine with a focus on circular economy, sustainability, and carbon neutrality using high availability and low utilization biomass (HALUB). We acme energy-efficient technologies for sustainable energy and material recovery and applications. The technologies of thermochemical conversion (TC), biochemical conversion (BC), electrochemical conversion (EC), and photochemical conversion (PTC) are summarized for HALUB. Microalgal biomass could contribute to a biofuel HHV of 35.72 MJ Kg-1 and total benefit of 749 $/ton biomass via TC. Specific surface area of biochar reached 3000 m2 g-1 via pyrolytic carbonization of waste bean dregs. Lignocellulosic biomass can be effectively converted into bio-stimulants and biofertilizers via BC with a high conversion efficiency of more than 90%. Besides, lignocellulosic biomass can contribute to a current density of 672 mA m-2 via EC. Bioresource can be 100% selectively synthesized via electrocatalysis through EC and PTC. Machine learning, techno-economic analysis, and life cycle analysis are essential to various upgrading approaches of HALUB. Sustainable biomaterials, sustainable living materials and technologies for biomedical and multifunctional applications like nano-catalysis, microfluidic and micro/nanomotors beyond are also highlighted. New techniques and systems for the complete conversion and utilization of HALUB for new energy and materials are further discussed.

Robust Control Based on Modeling Error Compensation of Microalgae Anaerobic Digestion

Microalgae are used to produce renewable biofuels (biodiesel, bioethanol, biogas, and biohydrogen) and high-value-added products, as well as in bioremediation and CO2 sequestration tasks. In the case of anaerobic digestion of microalgae, biogas can be produced from mainly proteins and carbohydrates. Anaerobic digestion is a complex process that involves several stages and is susceptible to operational instability due to various factors. Robust controllers with simple structure and design are necessary for practical implementation purposes and to achieve a proper process operation despite process variabilities, uncertainties, and complex interactions. This paper presents the application of a control design based on the modeling error compensation technique for the anaerobic digestion of microalgae. The control design departs from a low-order input–output model by enhancement with uncertainty estimation. The results show that achieving desired organic pollution levels and methanogenic biomass concentrations as well as minimizing the effect of external perturbations on a benchmark case study of the anaerobic digestion of microalgae is possible with the proposed control design.

Enhanced sustainable integration of CO2 utilization and wastewater treatment using microalgae in circular economy concept

Microalgae have been shown to have a promising potential for CO₂ utilization and wastewater treatment which still faces the challenges of high resource and energy requirements. The implementation of the circular economy concept is able to address the issues that limit the application of microalgae-based technologies. In this review, a comprehensive discussion on microalgae-based CO₂ utilization and wastewater treatment was provided, and the integration of this technology with the circular economy concept, for long-term economic and environmental benefits, was described. Furthermore, technological challenges and feasible strategies towards the improvement of microalgae cultivation were discussed. Finally, necessary regulations and effective policies favoring the implementation of microalgae cultivation into the circular economy were proposed. These are discussed to support sustainable development of microalgae-based bioremediation and bioproduction. This work provides new insights into the implementation of the circular economy concept into microalgae-based CO₂ utilization and wastewater treatment to enhance sustainable production.