Influence of activated sludge derived-extracellular polymeric substance (ASD-EPS) as bio-flocculation of microalgae for biofuel recovery

Strategies and challenges to enhance commercial viability of algal biorefineries for biofuel production

The rise in energy consumption would quadruple in the coming century and the, existing energy resources might be insufficient to meet the demand of the growing population. An alternative and sustainable energy resource is therefore needed to address the fossil fuel deficiency. The utility of microalgae strains in the aspect of biorefinery has been in research for quite some time. Algal biorefinery is an alternate way of renewable energy however even after decades of research it still suffers from commercialization bottlenecks. The current manuscript reviews the scenarios where the innovation needs an ignition for its commercialization. This review discusses the prospects of up-scale cultivation, and harvesting algal biomass for biorefineries. It narrates algal biorefinery hurdles that can be solved using integrated technology approach, life cycle assessment and applications of nanotechnology. The review also sheds light upon the ties of algal biorefineries with its economic viability.

Microalgae and biogas: a boon to energy sector

The global energy generation market immensely depends on fossil fuels which balances our survival on this planet. Energy can be called as the "master element" for our daily needs, starting from household power supply, agricultural purpose, automobile and transportation, industrial workload to economic and research domains. Fuel switching initiatives are being adapted by environmentalist and scientists to bring a novel sustainable source of energy. An environment and renewable alternative to fossil fuels are a must. Over the years, the world has shifted toward generating green fuels immensely. One such potential alternative to fossil fuels are biogases. Being versatile and renewable in nature, it has drawn immense attention globally. Despite having such potentials there exist some major drawbacks which mainly deal with the starting material. One such source for biogases can be microalgae. Microalgae based biogas production can produce huge amount of energy and that has been implemented by many foreign countries and their companies. Despite being in use in many countries, there are issues which needs to be addressed which will overall improve the biogas potential from microalgae even more. This review mainly focuses on generation of biogas from microalgae as a feedstock which are very economical and sustainable in its nature, presenting improvement strategies which can be impended to boost the over biogas sector globally.

A review on the sustainable procurement of microalgal biomass from wastewaters for the production of biofuels

The feasibility of microalgal biomass as one of the most promising and renewable sources for the production of biofuels is being studied extensively. Microalgal biomass can be cultivated under photoautotrophic, heterotrophic, photoheterotrophic, and mixotrophic cultivation conditions. Photoautotrophic cultivation is the most common way of microalgal biomass production. Under mixotrophic cultivation, microalgae can utilize both organic carbon and CO2 simultaneously. Mixotrophic cultivation depicts higher biomass productivity as compared to photoautotrophic cultivation. It is evident from the literature that mixotrophic cultivation yields higher quantities of polyunsaturated fatty acids as compared to that photoautotrophic cultivation. In this context, for economical biomass production, the organic carbon of industrial wastewaters can be valorized for the mixotrophic cultivation of microalgae. Following the way, contaminants' load of wastewaters can be reduced while concomitantly producing highly productive microalgal biomass. This review focuses on different aspects covering the sustainable cultivation of different microalgal species in different types of wastewaters.

Exopolysaccharides from marine microbes with prowess for environment cleanup

A variety of both small and large biologically intriguing compounds can be found abundantly in the marine environment. Researchers are particularly interested in marine bacteria because they can produce classes of bioactive secondary metabolites that are structurally diverse. The main secondary metabolites produced by marine bacteria are regarded as steroids, alkaloids, peptides, terpenoids, biopolymers, and polyketides. The global urbanization leads to the increased use of organic pollutants that are both persistent and toxic for humans, other life forms and tend to biomagnified in environment. The issue can be addressed, by using marine microbial biopolymers with ability for increased bioremediation. Amongst biopolymers, the exopolysaccharides (EPS) are the most prominent under adverse environmental stress conditions. The present review emphasizes the use of EPS as a bio-flocculent for wastewater treatment, as an adsorbent for the removal of textile dye and heavy metals from industrial effluents. The biofilm-forming ability of EPS helps with soil reclamation and reduces soil erosion. EPS are an obvious choice being environmentally friendly and cost-effective in processes for developing sustainable technology. However, a better understanding of EPS biosynthetic pathways and further developing novel sustainable technologies is desirable and certainly will pave the way for efficient usage of EPS for environment cleanup.

New concept of biodiesel production using food waste digestate powder: Co-culturing algae-activated sludge symbiotic system in low N and P paper mill wastewater

This paper aims to demonstrate an innovative process for the conversion of food waste digestate (FWD) powder into biofuel. The effects of different doses of FWD are investigated on microalgae-activated sludge (MAS) in treating pulp and paper mill wastewater (PPW) which generally contains insufficient nitrogen and phosphorus. FWD was added to adjust the initial N:P molar ratio in MAS at various levels (8:1 to 15:1). The highest Auxenochlorella protothecoides biomass achieved was 1.67 gL^-1 at a 13.45:1 N/P molar ratio of PPW. After 10 days of cultivation, Auxenochlorella protothecoides-activated sludge system removed 91.7 %, 74.6 %, and 91.5 % of total nitrogen, phosphorus, and sCOD respectively at D_0.836 gL^-1 DD. The highest lipid productivity was reported as 41.27 ± 2.43 mg L^-1 day^-1. Fatty acid methyl ester (FAME) analysis showed the presence of an appreciable percentage of balanced saturated and unsaturated fatty acids i.e. palmitic, oleic, and linoleic acid, rendering its potential as a feedstock for biodiesel production. Activated sludge induced flocculation of Auxenochlorella protothecoides was measured. The whole process establishes an effective means of circular economy, where the secondary source of recyclable nutrients i.e. FWD will be used as a source of N and P in PPW to obtain algal biodiesel from a negative value industrial wastewater.

A review on biodiesel production from microalgae: Influencing parameters and recent advanced technologies

Microalgae are the important part of carbon cycle in the nature, and they could utilize the carbon resource in water and soil efficiently. The abilities of microalgae to mitigate CO₂ emission and produce oil with a high productivity have been proven. Hence, this third-generation biodiesel should be popularized. This review firstly introduce the basic characteristics and application fields of microalgae. Then, the influencing parameters and recent advanced technologies for the microalgae biodiesel production have been discussed. In influencing parameters for biodiesel production section, the factors of microalgae cultivation, lipid accumulation, microalgae harvesting, and lipid extraction have been summarized. In recent advanced technologies for biodiesel production section, the microalgae cultivation systems, lipid induction technologies, microalgae harvesting technologies, and lipid extraction technologies have been reviewed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.

Continuous antibiotic attenuation in algal membrane photobioreactor: Performance and kinetics

The attenuation of 10 mixed antibiotics along with nutrients in a continuous flow mode by four freshwater microalgae (Haematococcus pluvialis, Selenastrum capricornutum, Scenedesmus quadricauda, and Chlorella vulgaris) was examined in membrane photobioreactors (MPBRs). At lab-scale, consistent removal of both antibiotic and nutrient was shown by H. pluvialis and S. quadricauda, respectively. The system exhibited better performance with enhanced removal at HRT 24 h compared to 12 h and 48 h. The highest removal efficiency of antibiotics was observed in H. pluvialis MPBR, with the mean antibiotic removal values of 53.57%- 96.33%. Biodegradation was the major removal pathway of the antibiotics in the algal-MPBR (AMPBR), while removal by bioadsorption, bioaccumulation, membrane rejection, and abiotic was minor. Then, the bacterial feature was studied and showed significant influence from system hydrodynamics. The kinetics of continuous flow antibiotic removal followed Stover-Kincannon and Grau second-order models, which revealed great potential of AMPBR to withstand antibiotic load. The latter coupled with the computational fluid dynamic simulation was successfully applied for the residual antibiotic prediction and potential system optimization. Overall, these results provide an important reference for continuous flow antibiotic removal using AMPBR.

CO2-triggered switchable solvent for lipid extraction from microalgal biomass

This study proposed a novel and energy-efficient method using switchable polarity solvents for lipid recovery from microalgae. Different from the existing methods, use of switchable polarity solvents does not require the fractional distillation for separation of lipid and solvent by only converting the polarity of the solvent after lipid extraction. When a non-polar amine solvent reacts with CO₂, amino group (NH) can be transformed to a polar form, i.e. carbamate (NHCOO^-). Nuclear magnetic resonance (NMR) spectrum indicated that only secondary amines are convertible to a polar compound of carbamate after CO₂ treatment. The polarity switching potential of each amine candidate was quantitatively evaluated by normalized polarity energy (E_T^N). Dipropylamine (DPA) showed the greatest change in E_T^N from 0.452 to 0.789 kcal/mol (E_T^N of water = 1.0 kcal/mol) before and after CO₂ treatment. DPA is a potential polarity switchable solvent capable of achieving an excellent lipid extraction yield of 7.51% from tested microalgal biomass (Chlorella vulgaris) with 9.16% of total lipid content and 95.5% fatty acid methyl esters (FAMEs) content. Furthermore, the used solvent could be recovered at the high efficiency of 84.0%. With a significant polarity switchability from nonpolar amine to carbamate in the presence of CO₂, DPA, a secondary amine, could be suggested as a suitable solvent used for both extraction of lipids with a higher FAMEs content from microalgae and separation of lipid by only adding CO₂.

Recent Advances of Microalgae Exopolysaccharides for Application as Bioflocculants

Microalgae are used in flocculation processes because biopolymers are released into the culture medium. Microalgal cell growth under specific conditions (temperature, pH, luminosity, nutrients, and salinity) provides the production and release of exopolysaccharides (EPS). These biopolymers can be recovered from the medium for application as bioflocculants or used directly in cultivation as microalgae autoflocculants. The optimization of nutritional parameters, the control of process conditions, and the possibility of scaling up allow the production and industrial application of microalgal EPS. Therefore, this review addresses the potential use of EPS produced by microalgae in bioflocculation. The recovery, determination, and quantification techniques for these biopolymers are also addressed. Moreover, other technological applications of EPS are highlighted.

Wastewater based microalgal biorefinery for bioenergy production: Progress and challenges

Treatment of industrial and domestic wastewater is very important to protect downstream users from health risks and meet the freshwater demand of the ever-increasing world population. Different types of wastewater (textile, dairy, pharmaceutical, swine, municipal, etc.) vary in composition and require different treatment strategies. Wastewater management and treatment is an expensive process; hence, it is important to integrate relevant technology into this process to make it more feasible and cost-effective. Wastewater treatment using microalgae-based technology could be a global solution for resource recovery from wastewater and to provide affordable feedstock for bioenergy (biodiesel, biohydrogen, bio-alcohol, methane, and bioelectricity) production. Various microalgal cultivation systems (open or closed photobioreactors), turf scrubber, and hybrid systems have been developed. Although many algal biomass harvesting methods (physical, chemical, biological, and electromagnetic) have been reported, it is still an expensive process. In this review article, resource recovery from wastewater using algal cultivation, biomass harvesting, and various technologies applied in converting algal biomass into bioenergy, along with the various challenges that are encountered are discussed in brief.

Simultaneous harvesting and extracellular polymeric substances extrusion of microalgae using surfactant: Promoting surfactant-assisted flocculation through pH adjustment

In this research, the use of four different types of surfactants on biomass harvesting and extracellular polymeric substances (EPS) extrusion of Chlorella sorokiniana sp was investigated. The synergy between cationic surfactants and pH was tested to improve flocculation efficiency through the combined mechanism of charge neutralization, bridging and sweeping. Zeta potential and microscopic images were used to gain mechanistic understanding. The harvesting efficacy correlated positively with the biomass zeta potential and the surfactants alkyl-chain length; i.e., CTAB (88%) > DTAB (66%) > triton X-100 (41%) > SDS (11%). When the pH increased from 8 to 12, the harvesting efficiency was improved 12% and 39% for CTAB and DTAB, respectively. More interestingly, pH adjustment dramatically reduced the optimal dosages of CTAB and DTAB from 400 to 50 and 1000 to 300 mg/L, respectively. All selected surfactants could successfully release high value components of EPS such as protein and polysaccharide.

Co-culture of microalgae-activated sludge for wastewater treatment and biomass production: Exploring their role under different inoculation ratios

In this study, mixed culture (microalgae:activated sludge) of a photobioreactor (PBR) were investigated at different inoculation ratios (1:0, 9:1, 3:1, 1:1, 0:1 wt/wt). This work was not only to determine the optimal ratio for pollutant remediation and biomass production but also to explore the role of microorganisms in the co-culture system. The results showed high total biomass concentrations were obtained from 1:0 and 3:1 ratio being values of 1.06, 1.12 g L^-1, respectively. Microalgae played a dominant role in nitrogen removal via biological assimilation while activated sludge was responsible for improving COD removal. Compared with the single culture of microalgae, the symbiosis between microalgae and bacteria occurred at 3:1 and 1:1 ratio facilitated a higher COD removal by 37.5-45.7 %. In general, combined assessment based on treatment performance and biomass productivity facilitated to select an optimal ratio of 3:1 for the operation of the co-culture PBR.

Aging forming process of Chlorella vulgaris growing medium and its cultivation inhibition mechanism

To investigate the possibility of culture medium reuse in large-scale industrial microalgae cultivation for the alleviation of the massive water requirement pressure, the aging forming process of Chlorella vulgaris growing medium was explored and the aged medium's inhibition mechanisms on cell growth were inspected. The results demonstrated that when the medium was continuously reused, the collected maximal C. vulgaris biomass decreased. After the fourth medium reuse, the maximal biomass concentration was only 55 ± 1.1% of that in the fresh medium, which indicated the gradual aging of the medium. Furthermore, the composition variation of the released organic secretions during the culture medium reuse was monitored and the results showed that high concentrations of fatty acids (FAs), including palmitic acid, stearic acid, and small amounts of polysaccharides, were accumulated. Further investigation indicated that the obtained maximal biomass of C. vulgaris has a negative relationship with the manually added initial FA concentrations in the medium which suggested that the accumulated FAs in the medium probably were the main C. vulgaris growth inhibition factor. The inhibition effect of FAs on C. vulgaris was mainly achieved via impacting the cells' photosynthesis efficiencies to destroy the intracellular antioxidant system.