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Microalgae-based technology is widely utilized in wastewater treatment and resource recovery. However, the practical implementation of microalgae-based technology is hampered by the difficulty in separating microalgae from treated water due to the low density of microalgae. This review is designed to find the current status of the development and utilization of microalgae biogranulation technology for better and more cost-effective wastewater treatment. This review reveals that the current trend of research is geared toward developing microalgae-bacterial granules. Most previous works were focused on studying the effect of operating conditions to improve the efficiency of wastewater treatment using microalgae-bacterial granules. Limited studies have been directed toward optimizing operating conditions to induce the secretion of extracellular polymeric substances (EPSs), which promotes the development of denser microalgae granules with enhanced settling ability. Likewise, studies on the understanding of the EPS role and the interaction between microalgae cells in forming granules are scarce. Furthermore, the majority of current research has been on the cultivation of microalgae-bacteria granules, which limits their application only in wastewater treatment. Cultivation of microalgae granules without bacteria has greater potential because it does not require additional purification and can be used for border applications.

Microalgae are promising alternatives to mitigate atmospheric CO 2 owing to their fast growth rates, resilience in the face of adversity and ability to produce a wide range of products, including food, feed supplements, chemicals, and biofuels. However, to fully harness the potential of microalgae-based carbon capture technology, further advancements are required to overcome the associated challenges and limitations, particularly with regards to enhancing CO 2 solubility in the culture medium. This review provides an in-depth analysis of the biological carbon concentrating mechanism and highlights the current approaches, including species selection, optimization of hydrodynamics, and abiotic components, aimed at improving the efficacy of CO 2 solubility and biofixation. Moreover, cutting-edge strategies such as gene mutation, bubble dynamics and nanotechnology are systematically outlined to elevate the CO 2 biofixation capacity of microalgal cells. The review also evaluates the energy and economic feasibility of using microalgae for CO 2 bio-mitigation, including challenges and prospects for future development., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

To suffice the escalating global energy demand, microalgae are deemed as high potential surrogate feedstocks for liquid fuels. The major encumbrance for the commercialization of microalgae cultivation is due to the high costs of nutrients such as carbon, phosphorous, and nitrogen. Meanwhile, the organic-rich anaerobic digestate which is difficult to be purified by conventional techniques is appropriate to be used as a low-cost nutrient source for the economic viability and sustainability of microalgae production. This option is also beneficial in terms of reutilize the organic fraction of solid waste instead of discarded as zero-value waste. Anaerobic digestate is the side product of biogas production during anaerobic digestion process, where optimum nutrients are needed to satisfy the physiological needs to grow microalgae. Besides, the turbidity, competing biological contaminants, ammonia and metal toxicity of the digestate are also potentially contributing to the inhibition of microalgae growth. Thus, this review is aimed to explicate the feasibility of utilizing the anaerobic digestate to cultivate microalgae by evaluating their potential challenges and solutions. The proposed potential solutions (digestate dilution and pre-treatment, microalgae strain selection, extra organics addition, nitrification and desulfurization) corresponding to the state-of-the-art challenges are applicable as future directions of the research., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Microalgae-based biodiesel has gained widespread interest as an alternative energy source. Low-cost microalgae harvesting technologies are important for economically feasible biodiesel production. This study investigated, for the first time, the impact of adaptation period and height to diameter (H/D) ratio of a reactor on the growth and self-flocculation of microalgae, without the addition of bacteria. Six reactors were grouped into three sets of experiments, and each reactor was operated for 30 days at similar operating conditions (volume exchange ratio = 25% and settling time = 30 min). In set 1, two 8-L reactors, H5a (H/D ratio: 5) and H8a (H/D ratio: 8), were operated under batch operation. In set 2, reactors H5b and H8b were operated as sequential batch reactors (SBRs) without an adaptation period. In set 3, the reactors H5c and H8c were operated as SBRs with an adaptation period. The findings showed a threefold improvement in biomass productivity for the higher H/D ratio (H8c) and a reduction in biomass loss for microalgae. The H8c reactor exhibited 95% settling efficiency within 5 days, in comparison to 30 days for the H5c reactor. This study demonstrated that a higher H/D ratio and the introduction of an adaptation period in SBR operation positively influences growth and self-flocculation of enriched mixed microalgae culture.

The accumulation of conventional petroleum-based polymers has increased exponentially over the years. Therefore, algae-based biopolymer has gained interest among researchers as one of the alternative approaches in achieving a sustainable circular economy around the world. The benefits of microalgae biopolymer over other feedstock is its autotrophic complex to reduce the greenhouse gases emission, rapid growing ability with flexibility in diverse environments and its ability to compost that gives greenhouse gas credits. In contrast, this review provides a comprehensive understanding of algae-based biopolymer in the evaluation of microalgae strains, bioplastic characterization and bioplastic blending technologies. The future prospects and challenges on the algae circular bioeconomy which includes the challenges faced in circular economy, issues regard to the scale-up and operating cost of microalgae cultivation and the life cycle assessment on algal-based biopolymer were highlighted. The aim of this review is to provide insights of algae-based biopolymer towards a sustainable circular bioeconomy., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

This study aims to produce hydrochar from high-ash low-lipid Chlorella vulgaris biomass via hydrothermal carbonization (HTC) process. The effects of hydrothermal temperature and retention time with respect to the physicochemical properties of hydrochar were studied in the range of 180-250 °C and 0.5-4 h, respectively. It was found that the hydrothermal temperature had resulted in a significant reduction of hydrochar yield as compared to the retention time. The raw microalgal biomass was successfully converted into an energy densified hydrochar via an optimized HTC reaction, with higher heating value (HHV) of 24.51 kJ/g, which was approximately two-times higher than that of raw biomass. In addition, the overall carbon recovery rate and energy yield were in the range of 53.2-86.4% and 46.9-76.6%, respectively. The high quality of the produced hydrochar was further supported by the plot of van Krevelen diagram and combustion behaviour analysis. Besides, the aqueous phase collected from HTC process could be further used as nutrients source to cultivate C. vulgaris, in which up to 70% of the biomass yield could be attained as compared to the control cultivation condition. The reusability of the aqueous phase collected from HTC process as an alternative nutrients source to cultivate microalgal indicated the feasibility and positive integration of HTC process in microalgal biofuel processing chain., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Algae biomass comprises variety of biochemicals components such as carbohydrates, lipids and protein, which make them a feasible feedstock for biofuel production. However, high production cost mainly due to algae cultivation remains the main challenge in commercializing algae biofuels. Hence, extraction of other high value-added bioproducts from algae biomass is necessary to enhance the economic feasibility of algae biofuel production. This paper is aims to deliberate the recent developments of conventional technologies for algae biofuels production, such as biochemical and chemical conversion pathways, and extraction of a variety of bioproducts from algae biomass for various potential applications. Besides, life cycle evaluation studies on microalgae biorefinery are presented, focusing on case studies for various cultivation techniques, culture medium, harvesting, and dewatering techniques along with biofuel and bioenergy production pathways. Overall, the algae biorefinery provides new opportunities for valorisation of algae biomass for multiple products synthesis., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Utilization of macroalgae biomass for bioethanol production appears as an alternative source to lignocellulosic materials. In this study, for the first time, Amberlyst (TM)-15 was explored as a potential catalyst to hydrolyze carbohydrates from Eucheuma cottonii extract to simple reducing sugar prior to fermentation process. Several important hydrolysis parameters were studied for process optimization including catalyst loading (2-5%, w/v), reaction temperature (110-130°C), reaction time (0-2.5 h) and biomass loading (5.5-15.5%, w/v). Optimum sugar yield of 39.7% was attained based on the following optimum conditions: reaction temperature at 120°C, catalyst loading of 4% (w/v), 12.5% (w/v) of biomass concentration and reaction time of 1.5h. Fermentation of the hydrolysate using Saccharomyces cerevisiae produced 0.33 g/g of bioethanol yield with an efficiency of 65%. The strategy of combining heterogeneous-catalyzed hydrolysis and fermentation with S. cerevisiae could be a feasible strategy to produce bioethanol from macroalgae biomass., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Culturing of microalgae as an alternative feedstock for biofuel production has received a lot of attention in recent years due to their fast growth rate and ability to accumulate high quantity of lipid and carbohydrate inside their cells for biodiesel and bioethanol production, respectively. In addition, this superior feedstock offers several environmental benefits, such as effective land utilization, CO(2) sequestration, self-purification if coupled with wastewater treatment and does not trigger food versus fuel feud. Despite having all these 'theoretical' advantages, review on problems and issues related to energy balance in microalgae biofuel are not clearly addressed until now. Base on the maturity of current technology, the true potential of microalgae biofuel towards energy security and its feasibility for commercialization are still questionable. Thus, this review is aimed to depict the practical problems that are facing the microalgae biofuel industry, covering upstream to downstream activities by accessing the latest research reports and critical data analysis. Apart from that, several interlink solutions to the problems will be suggested with the purpose to bring current microalgae biofuel research into a new dimension and consequently, to revolutionize the entire microalgae biofuel industry towards long-term sustainability., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Microalgae is widely utilized in water purification technology, biofuel production as well as in the synthesis of biopolymers. However, the separation of microalgae from its growth medium is the major issue that restricts the harvesting processes due to microalgae's lower density. In this study, the effect of environmental factors on microalgae granules development was investigated. Results from response surface methodology (RSM) revealed that maximum growth rate and biomass productivity were obtained at a photoperiod of 23.9 h, aeration rates of 2.4 L/min, inoculum concentration of 11 % and pH 6 respectively. The mixed liquor volatile suspended solids (MLVSS) value of the initial seed microalgae increased from 50 mg/L to 2978 mg/L after 70 days of cultivation. Microalgae productivity in this study is approximately 2 times faster than the standard suspension growing method. The microalgae settleability was significantly enhanced, as indicated by a major reduction in the sludge volume index (SVI 30) from an initial value of >3000 to 58.51 mL/g, accompanied by an increase in settling velocity from virtually 0 to 19.2 cm/h and total harvesting time of 15 min. After 70 days of operation, smoothly settled, compact algal granules with largest sizes of 1.90 mm were obtained. [Display omitted] • Optimum growth at photoperiod 23.9 h, aeration 2.4 L/min, inoculum concentration 11 % and pH 6 • Algae concentration increases from 1.49 mg/L to 2978 mg/L after 70 days cultivation. • Algae settling velocity increases from 0 to 19.2 cm/h and total harvesting time of 15 min. • Algae concentration and productivity were 2 times faster than standard suspension growing method. [ABSTRACT FROM AUTHOR]

There has been an increase in the amount of focus placed on microalgae-based technology in recent years, due to its great performance in wastewater treatment, nutrient restoration, and manufacture of alternative biofuels or other high-value commodities. However, the difficulties of harvesting biomass hinder the deployment of microalgae-based technology for continuous wastewater treatment and resource recovery. This article analysed the worldwide dynamics of microalgae bio-granulation research from 2011 to 2023 based on a bibliometric analysis of 250 publications extracted from the Scopus database. The research progression, prominent contributors in the subject, and latest developments and research gaps were determined using the Scopus database. The results show a rapid increment in the annual publications, with China as the most active nation in this study field, followed by India, United State, Taiwan, Japan, South Korea and Malaysia. The research trend in microalgae bio-granulation show most research focusing on the incorporation of microalgae-bacteria symbiosis for granulation. However, there are almost no research on development of denser single-species microalgae granules without bacteria for easy harvesting via self-flocculation. Besides that, till today there are no study that investigate the effect of organic nutrient (glucose) and inorganic nutrient (sodium bicarbonate) on microalgae granulation. In addition, there are lack of studies use Chlorella Vulgaris microalgae species in bio-granulation process. An outline of possible future research was given, with a focus on technology used, nutritional media and environmental studies, and the microorganism selection. • Development of denser single-species microalgae granules without bacteria for rapid harvesting by self-flocculation. • Effect of organic and inorganic nutrients on the microalgae granules growth. • Enrichment of Chlorella vulgaris sp. microalgae via bio-granulation method for wastewater treatment. [ABSTRACT FROM AUTHOR]

Utilizing compost derived from waste offers an alternative nutrients source to cultivate microalgae for biofuel production. This is because compost is a low-cost nutrients source which contains high amount of nutrient composition that suitable for microalgae cultivation. Thus, in the present work, plant-based compost and animal waste-based compost were used to cultivate Chlorella vulgaris . The results shows that C. vulgaris favoured to grow at the following condition: 50 ml amount of goat waste-based compost, under pH of 9, and illuminated continuously for 15 days using fluorescent light. Based on these cultivation conditions, 30% of lipid was successfully extracted from the dried microalgae biomass. [ABSTRACT FROM AUTHOR]

Pilot-scale cultivation of Chlorella vulgaris in a 100 L sequential baffled photobioreactor was carried out in the present study. The highest biomass yield attained under indoor and outdoor environment was 0.52 g/L and 0.28 g/L, respectively. Although low microalgae biomass yield was attained under outdoor cultivation, however, the overall life cycle energy efficiency ratio was 3.3 times higher than the indoor cultivation. In addition, negative energy balance was observed in producing microalgae biodiesel under both indoor and outdoor cultivation. The minimum production cost of microalgae biodiesel was about RM 237/L (or USD 73.5/L), which was exceptionally high compared to the current petrol diesel price in Malaysia (RM 3.6/L or USD 1.1/L). On the other hand, the estimated production cost of dried microalgae biomass cultivated under outdoor environment was RM 46/kg (or USD 14.3/kg), which was lower than cultivation using chemical fertilizer (RM 111/kg or USD 34.4/kg) and current market price of Chlorella biomass (RM 145/kg or USD 45/kg). [ABSTRACT FROM AUTHOR]

Abstract: Cultivating microalgae for biodiesel production is a new and emerging research field that has the potential to revolutionize the renewable energy industries. However, up to now, comprehensive study on the conversion efficiency of microalgae lipid to biodiesel is still inadequate. In the present study, reaction optimization studies were carried out to convert high viscosity microalgae lipid (extracted from Chlorella vulgaris) to biodiesel by using sulfuric acid (H2SO4) as catalyst. In order to attain 95% fatty acid methyl ester (FAME) content, the optimized reaction conditions were as follows: methanol to lipid molar ratio of 180, catalyst concentration of 35wt.%, temperature of 60°C and reaction time of 6h. Due to the high requirement of methanol and catalyst concentration, attempt was made to improve the reaction conditions by introducing a co-solvent into the reaction mixture. It was found that tetrahydrofuran (THF) could significantly enhance the reaction rate by homogenizing the reaction mixture. The optimum reaction conditions with a co-solvent were found to be less extreme: methanol to THF to lipid molar ratio of 60:15:1, catalyst concentration of 21wt.%, temperature of 60°C and reaction time of 3h. [Copyright &y& Elsevier]

Abstract: Current interest on growing microalgae for CO2 mitigation has received overwhelming response from researches, mainly due to their high photosynthetic rate that allow them to bio-fix CO2 more efficient than terrestrial crops. In the present study, two types of carbon source, namely CO2 bubbling and sodium bicarbonate (NaHCO3) were used to grow freshwater microalgae Chlorella vulgaris. It was found that CO2 bubbling could accelerate 5.2 times higher biomass productivity compared to NaHCO3, however, significantly lower in term of CO2 removal efficiency. Interestingly, when NaHCO3 was supplied as carbon source, C. vulgaris preferred to utilize free CO2 molecules at acidic cultivation condition (pH 4) instead of bicarbonate ions at alkaline conditions (pH 8.5) with CO2 removal efficiency of 82.5–99%. To overcome the limitation of low solubility of CO2 in water, sequential photobioreactors were successfully developed in this study that gave five times more CO2 removal efficiency than a single photobioreactor. Furthermore, this study also explored the potential of re-utilizing the water used for cultivation (without additional treatment) for subsequent cycle of cultivation and the result showed a higher biomass productivity and CO2 fixation rate as compared to fresh water. The lipid extracted from C. vulgaris supplied with air/CO2 or NaHCO3 were mostly unsaturated fatty acids, which are suitable for biodiesel production. [Copyright &y& Elsevier]

Abstract: Worldwide concern on the negative effect of climate change towards human and environment has synergized the development of CO2 capture technologies. Currently, culturing of microalgae for CO2 bio-fixation has gained a huge momentum due to their high photosynthetic rate that allows bio-fixation of CO2 more efficient than terrestrial plants. In addition, lipid from microalgae biomass can be converted to biodiesel; a renewable fuel that emits less CO2 compared to fossil-diesel when combusted. However, several recent life cycle assessment (LCA) studies have revealed that enormous energy input is required to cultivate microalgae and also for the harvesting and drying processes. The energy required (in the form of electricity) is normally generated from burning coal or natural gas that emits substantial amount of CO2 to the atmosphere and this could entirely eliminate all the positive effect of culturing microalgae for CO2 bio-fixation and biofuel production. Thus, in this review, critical assessment and comparative study of CO2 bio-fixation rate by microalgae and CO2 emission rate during cultivation and processing of microalgae biodiesel were carried out. In addition, the prospects and limitations of using flue gas to culture microalgae and several possible strategies to enhance CO2 utilization by microalgae will also be discussed extensively. [Copyright &y& Elsevier]

Abstract: Cultivating microalgae at industrial scale for biodiesel production required substantial amount of mineral fertilizer, typically nitrogen and phosphorus. In fact, the production of mineral fertilizer implies the usage of energy and fossil fuels resulting to unsustainable practise in a long term. On the other hand, organic fertilizer which is derived from food waste, biomass or manure also contains high value of nutrients that can support microalgae growth. Hence, in the present study, the potential of using organic fertilizer as an alternative nutrient source to cultivate Chlorella vulgaris was investigated. Under the supplement of organic nutrients, it was found that C. vulgaris grown favourably under the following conditions: initial nitrate content of 26.67mg/L, 24h of continuous illumination and pH of 5. Nevertheless, slow growth rate was observed when cultivating C. vulgaris under open environment; a reduction of 27% was recorded in comparison with controlled environment. On the other hand, it was possible to reutilize the water to re-cultivate C. vulgaris. This observation reflects the high adaptability of C. vulgaris towards the surrounding environment and suitability to be grown under outdoor conditions. Total lipid of 18.1% from C. vulgaris biomass was successfully extracted and the fatty acids methyl ester profile was proven to be suitable for making biodiesel. [Copyright &y& Elsevier]

Abstract: In the present study, the use of immobilization technology to cultivate microalgae in entrapped matrix gel beads was demonstrated. Since the gel beads are denser in water, the beads can be easily collected through simple filtration method and hence, simplifying the overall separation process. Various parameters were investigated to optimize the growth rate of immobilized microalgae and the optimum conditions were obtained as: alginate to microalgae volume ratio of 0.3, Ca2+ concentration of 2%, organic nutrients concentration of 50mL (equivalent to 13.09mg/L nitrate), initial culture pH of 4 and photoperiod of 24h. Using this optimum culture condition, 0.50mg biomass/bead was attained on the 10th day of cultivation. Apart from that, this study also attempted to co-immobilize nutrients into microalgae beads in order to minimize free cell culture (microalgae cells that are released into the culture medium due to rupturing of beads) and to reduce water consumption. Through this approach, it was found that microalgae biomass yield increased to 0.67mg/bead within a shorter culturing time (5 days) with insignificant amount of free cell culture detected. Furthermore, lipid extracted from immobilized microalgae biomass has high potential for biodiesel production due to the similarity of fatty acid profile with other oil bearing crops. [Copyright &y& Elsevier]

Abstract: Palm oil industry is one of the leading agricultural industries in Malaysia with average crude palm oil production of more than 13million tonne per year. However, production of such huge amount of crude palm oil has consequently resulted to even larger amount of palm oil mill effluent (POME). POME is a highly polluting wastewater with high chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in which can caused severe pollution to the environment, typically pollution to water resources. On the other hand, POME was identified as a potential source to generate renewable bioenergies such as biomethane and biohydrogen through anaerobic digestion. In other words, a combination of wastewater treatment and renewable bioenergies production would be an added advantage to the palm oil industry. In line with the world''s focus on sustainability concept, such strategy should be implemented immediately to ensure palm oil is produced in an environmental friendly and sustainable manner. This review aims to discuss various technologies to convert POME to biomethane and biohydrogen in a commercial scale. Furthermore, discussion on using POME to culture microalgae for biodiesel and bioethanol production was included in the present paper as a new remedy to utilize POME with a greater beneficial return. [Copyright &y& Elsevier]

Palm oil mill effluent (POME) is the wastewater produced during the palm oil sterilization process, which contains substantial amounts of nutrients and phosphorous that are harmful to the environment. High BOD and COD of POME are as high as 100,000 mg/L, which endanger the environment. Effective pre-treatment of POME is required before disposal. As microalgae have the ability of biosorption on nutrients and phosphorous to perform photosynthesis, they can be utilized as a sustainable POME treatment operation, which contributes to effective biofuel production. Microalgae species C. pyrenoidosa has shown to achieve 68% lipid production along with 71% nutrient reduction in POME. In this study, a brief discussion about the impacts of POME that will affect the environment is presented. Additionally, the potential of microalgae in treating POME is evaluated along with its benefits. Furthermore, the condition of microalgae growth in the POME is also assessed to study the suitable condition for microalgae to be cultivated in. Moreover, experimental studies on characteristics and performance of microalgae are being evaluated for their feasibility. One of the profitable applications of POME treatment using microalgae is biofuel production, which will be discussed in this review. However, with the advantages brought from cultivating microalgae in POME, there are also some concerns, as microalgae will cause pollution if they are not handled well, as discussed in the last section of this paper. [ABSTRACT FROM AUTHOR]

• Sequential flow photobioreactor system was used to maximize CO 2 fixation efficiency. • The C. vulgaris growth was promoted by optimizing the cultivation parameters. • CO 2 removal efficiency in a sequential-flow system increased by 3.78-fold. • Highest lipid content was attained for microalgae grown in the spent medium. Biological CO 2 fixation by microalgae coupled with value-added product creation appears to be a sustainable approach in microalgae-based industry. However, the major constraint of this process is the low solubility of CO 2 in water which limits CO 2 fixation ability by the microalgae. Therefore, the current work aimed to improve the CO 2 fixation efficiency of Chlorella vulgaris through the introduction of sequential-flow bubble column photobioreactor system. The influence of various cultivation parameters towards the growth, carbon fixation efficiency, lipid accumulation, and fatty acid profile of C. vulgaris were explored. Experimental results showed that microalgae cultivated under optimized conditions attained the highest biomass productivity (0.83−0.86 g/L/day) as compared to the control conditions. Meanwhile, the highest lipid content (50.4 % for SFB-PBR1) was recorded when the microalgae were cultivated in the spent medium. In addition, the CO 2 fixation efficiency by microalgae in the sequential-flow photobioreactor system under the optimized condition was 3.78-fold higher than a single column mode (control). In short, the present work had demonstrated that cultivation conditions and photobioreactors configuration were equally important for an effective carbon capture process via microalgae cultivation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Recovering valuable carbohydrate compounds from microalgae biomass residues. [•] Optimization of hydrolysis conditions to obtain low-molecular-weight maltodextrin. [•] Maltodextrin obtained was resistant to enzymatic hydrolysis. [•] Maltodextrin obtained was found similar as commercial-grade maltodextrin. [Copyright &y& Elsevier]

The rapid depletion of fossil fuels and ever-increasing environmental pollution have forced humankind to look for a renewable energy source. Microalgae, a renewable biomass source, has been proposed as a promising feedstock to generate biofuels due to their fast growth rate with high lipid content. However, literatures have indicated that sustainable production of microalgae biofuels are only viable with a highly optimized production system. In the present study, a cradle-to-gate approach was used to provide expedient insights on the effect of different cultivation systems and biomass productivity toward life cycle energy (LCEA), carbon balance (LCCO 2) and economic (LCC) of microalgae biodiesel production pathways. In addition, a co-production of bioethanol from microalgae residue was proposed in order to improve the economic sustainability of the overall system. The results attained in the present work indicated that traditional microalgae biofuels processing pathways resulted to several shortcomings, such as dehydration and lipid extraction of microalgae biomass required high energy input and contributed nearly 21 to 30% and 39 to 57% of the total energy requirement, respectively. Besides, the microalgae biofuels production system also required a high capital investment, which accounted for 47 to 86% of total production costs that subsequently resulted to poor techno-economic performances. Moreover, current analysis of environmental aspects of microalgae biorefinery had revealed negative CO 2 balance in producing microalgae biofuels. Unlabelled Image • A comprehensive LCA of three different microalgae biofuel systems were compared. • Effect of different biomass productivity on the LCA study was assessed. • Negative energy balance was attained due to a number of energy-intensive processes. • High production cost has led to an uneconomical microalgae biofuel system. • CO 2 emission has surpassed the amount of carbon captured at the cultivation stage. [ABSTRACT FROM AUTHOR]

Cultivation of microalgae using wastewater has received considerable attention around the globe as a platform to remove inorganic nutrients from the wastewater and producing microalgae biomass for biodiesel production. This can be done by converting freely available nutrients from the wastewater (particularly nitrogen, N and phosphorus, P) into microalgae biomass with concomitant carbon dioxide (CO 2 ) sequestration via photosynthesis process. In fact, microalgae biodiesel could help to reduce greenhouse gas (GHG) emissions to the atmosphere through the replacement of fossil diesel as it is made from renewable resources. Despite the various benefits to produce microalgae biodiesel, a number of technical hurdles such as complex processes, requirement of high energy input during cultivation and dewatering (harvesting steps) need to be addressed for commercialization purpose. Besides, additional pre-treatment step is necessary to disrupt the microalgae cell wall to enhance lipid extraction efficiency. Hence, this paper is aims to reveal in-depth analysis and discussions on the process of microalgae-based wastewater treatment as well as microalgae cells disruption technology for lipid extraction. The paper also includes several new directions for technological improvements to improve commercialization potential of microalgae biodiesel. [ABSTRACT FROM AUTHOR]

In the present study, cultivation of Chlorella vulgaris was extensively explored under semi-continuous approach to enhance its growth. The microalgae were cultivated in a photobioreactor supplemented with compost derived from chicken waste as an alternative nutrients source in both batch and semi-continuous cultivations. It was found that Chlorella vulgaris grew well up to 17 cycles of semi-continuous cultivation based on the following conditions for each cycle (3 days per cycle): 30% (v/v) removal of cultivation medium and 0.04 L/L of chicken compost in cultivation medium of pH 3. The average biomass productivity attained through these conditions was 0.0736 g/L/day, which was higher than batch cultivation (0.0568 g/L/day). Besides, the average total lipid content from each cycle under semi-continuous cultivation was maintained in the range of 25–35 wt%. The success of lipid extraction from the microalgae biomass was evidenced by the result of Fourier Transform Infrared Red (FT-IR) analysis that revealed lower peak intensity of carbon, especially in the range 2809–3012 cm −1 after lipid extraction. It was also worthwhile to mention that the fatty acid methyl ester (FAME) compositions of Chlorella vulgaris consisted mainly of C16:0 and C18:3 that were not significantly altered during the semi-continuous cultivation. Both saturated and unsaturated fatty acids in the harvested biomass accounted for approximately 26.3% and 73.3%, respectively, within 5 cycles of the cultivation. [ABSTRACT FROM AUTHOR]

Owning to the presence of essential mineral nutrient content in wastewater, cultivation of microalgae using wastewater sources provides an alternative and sustainable solution for biodiesel production. Hence, the potential of using domestic wastewater as nutrient source to cultivate Chlorella vulgaris was presently studied. It was found that the microalgae was favoured to grow in domestic wastewater under the conditions of 0.02 v/v of wastewater, initial pH of 3, and 0.03 v/v of initial amount of microalgae seed with 24 h of continuous illumination. Under these conditions, a high lipid content of 32.7% was embedded within the microalgae biomass. From the analysis of fatty acid methyl ester (FAME) profile, the extracted microalgae lipid was suitable for biodiesel production. The existing growth kinetic models were able to predict the growth of Chlorella vulgaris using the domestic wastewater as nutrients source. The fair model fitting was however limited to contaminant-free conditions, where the growth decays of the microalgae was negligible. [ABSTRACT FROM AUTHOR]

[Display omitted] • Extraction of antioxidative biostimulant from onion waste was optimized via RSM. • 1.76 g/L of biomass yield was achieved using very low microalgae inoculum. • Microalgae lipid content was sustained at 19.42 % without major loss. • The biostimulant showed high DPPH (78.65 %) and ABTS (73.68 %) inhibition. In this study, a novel and cost-effective method for using onion peel as a biostimulant is presented to overcome the challenge of low microalgae biomass productivity. The study successfully optimized and modelled the extraction parameters, including solid-to-solvent (SS) ratio, temperature, and time, using the response surface methodology (RSM) tool. The results indicated a strong correlation between the extraction parameters and microalgae biomass concentration (BC) and lipid content (LC), with R2 values of 0.9440 and 0.9638, respectively. The optimum biostimulant extraction conditions were SS ratio of 0.006 g/mL, temperature of 90 °C at 1.00 h, attaining BC and LC of 1.76 g/L and 19.42 %, respectively, without any negative impact on LC. Findings on bioactivities of biostimulant recovered at optimal extraction conditions supported its biostimulation capacity on microalgae, with reasonable total phenolic content (TPC) of 9.1094 ± 0.748 mg GAE/g dry weight, total flavonoid content (TFC) of 3.1307 ± 0.210 mg RE/g dry weight, and antioxidant activity (AA) of 78.65 ± 3.421 % and 73.68 ± 9.608 % for DPPH and ABTS inhibition, respectively. The results of this study suggest that biostimulant derived from onion waste has great potential for promoting rapid proliferation of microalgae even with extremely low inoculum cells. These findings present a promising roadmap for high-density microalgae farming, and the developed RSM model can be utilized for process optimization while improving the efficiency, quality, and cost-effectiveness of microalgae industry. [ABSTRACT FROM AUTHOR]

Derivation of biofuel from microalgae biomass has been widely researched in the past few decades. Microalgae is capable of producing 58,700 litres oil per hectare that can generate 121,104 litres biodiesel per hectare, which seemingly a promising transition over conventional fossil fuels. Nevertheless, economic sustainability of commercial scale production of microalgae biomass is still in shadows of doubt, especially the cultivation and harvesting process. Apparently, the microalgae cultivation system has evolved from traditional open pond to various modern photobioreactor (PBR) designs. However, with regards to tubular and flat panel PBRs as the most ubiquitous systems for biofuel production at commercial level, extensive discussion on reactor configurations and design betterment was presented in this review, along with precise technical comparison on cost and energy requirements for the cultivation systems. This review intended to serve as guideline for long term adoption of these well-established cultivation technologies in biofuel plants given the numerous economic benefits. Besides that, in attempt to lower the harvesting cost, potential use of various waste biomass as bioflocculants to recover microalgae biomass was introduced in this review. This article also deliberates direction on potential policy interventions to produce microalgae biofuel in a more sustainable and cost-effective manners in near future. • Microalgae-derived biofuel is the potential substitute over fossil fuel. • Cultivation and harvesting steps are the bottlenecks to produce microalgae biofuel. • Wise selection of cultivation technology ensures sustainable biofuel production. • Bioflocculants from waste biomass are cheap, non-toxic, easily extracted. • Suitable policy framework ensures successful microalgae biofuel commercialization. [ABSTRACT FROM AUTHOR]