Strategies to enhance production of microalgal biomass and lipids for biofuel feedstock

ABSTRACT

Microalgae have enormous potential as feedstock for biofuel production compared with other sources, due to their high areal productivity, relatively low environmental impact, and low impact on food security. However, high production costs are the major limitation for commercialization of algal biofuels. Strategies to maximize biomass and lipid production are crucial for improving the economics of using microalgae for biofuels. Selection of suitable algal strains, preferably from indigenous habitats, and further improvement of those ‘platform strains’ using mutagenesis and genetic engineering approaches are desirable. Conventional approaches to improve biomass and lipid productivity of microalgae mainly involve manipulation of nutritional (e.g. nitrogen and phosphorus) and environmental (e.g. temperature, light and salinity) factors. Approaches such as the addition of phytohormones, genetic and metabolic engineering, and co-cultivation of microalgae with yeasts and bacteria are more recent strategies to enhance biomass and lipid productivity of microalgae. Improvement in culture systems and the use of a hybrid system (i.e. a combination of open ponds and photobioreactors) is another strategy to optimize algal biomass and lipid production. In addition, the use of low-cost substrates such as agri-industrial wastewater for the cultivation of microalgae will be a smart strategy to reduce production costs. Such systems not only generate high algal biomass and lipid productivity, but are also useful for bioremediation of wastewater and bioremoval of waste CO₂. The aim of this review is to highlight the advances in the use of various strategies to enhance production of algal biomass and lipids for biofuel feedstock.     

Production and harvesting of microalgae for wastewater treatment, biofuels, and bioproducts

The integration of microalgae-based biofuel and bioproducts production with wastewater treatment has major advantages for both industries. However, major challenges to the implementation of an integrated system include the large-scale production of algae and the harvesting of microalgae in a way that allows for downstream processing to produce biofuels and other bioproducts of value. Although the majority of algal production systems use suspended cultures in either open ponds or closed reactors, the use of attached cultures may offer several advantages. With regard to harvesting methods, better understanding and control of autoflocculation and bioflocculation could improve performance and reduce chemical addition requirements for conventional mechanical methods that include centrifugation, tangential filtration, gravity sedimentation, and dissolved air flotation. There are many approaches currently used by companies and industries using clean water at laboratory, bench, and pilot scale; however, large-scale systems for controlled algae production and/or harvesting for wastewater treatment and subsequent processing for bioproducts are lacking. Further investigation and development of large-scale production and harvesting methods for biofuels and bioproducts are necessary, particularly with less studied but promising approaches such as those involving attached algal biofilm cultures.     

微藻在废水处理和生物质回收再利用方面的研究进展

随着经济的发展和人口的增加,环境污染和水资源短缺已经成为不可避免的全球性问题。基于微藻的废水处理技术不仅可以净化废水、解决环境污染问题,还可以利用废水中的营养元素合成生物质,现如今这种技术已经受到越来越多的关注。为了进一步提高废水处理效果、降低废水处理成本,有必要了解微藻去除废水中营养物质和污染物的机理,开发下游低成本收获技术,提升微藻高价值副产物的生产。本文综述了微藻去除碳、氮、磷、重金属、抗生素和有机物的机理和影响因素,总结了微藻的不同收获方式和微藻生物质在各个领域的应用。最后,分析了不同微藻共培养体系和微藻固定化技术的优缺点,并展望了微藻废水处理技术未来的发展方向。     

Flocculation of microalgae using cationic starch

Due to their small size and low concentration in the culture medium, cost-efficient harvesting of microalgae is a major challenge. We evaluated the potential of cationic starch as a flocculant for harvesting microalgae using jar test experiments. Cationic starch was an efficient flocculant for freshwater (Parachlorella, Scenedesmus) but not for marine microalgae (Phaeodactylum, Nannochloropsis). At high cationic starch doses, dispersion restabilization was observed. The required cationic starch dose to induce flocculation increased linearly with the initial algal biomass concentration. Of the two commercial cationic starch flocculants tested, Greenfloc 120 (used in wastewater treatment) was more efficient than Cargill C*Bond HR 35.849 (used in paper manufacturing). For flocculation of Parachlorella using Greenfloc 120, the cationic starch to algal biomass ratio required to flocculate 80% of algal biomass was 0.1. For Scenedesmus, a lower dose was required (ratio 0.03). Flocculation of Parachlorella using Greenfloc 120 was independent of pH in the pH range of 5 to 10. Measurements of the maximum quantum yield of PSII suggest that Greenfloc 120 cationic starch was not toxic to Parachlorella. Cationic starch may be used as an efficient, nontoxic, cost-effective, and widely available flocculant for harvesting microalgal biomass.     

Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review

Microalgae have the ability to mitigate CO₂ emission and produce oil with a high productivity, thereby having the potential for applications in producing the third-generation of biofuels. The key technologies for producing microalgal biofuels include identification of preferable culture conditions for high oil productivity, development of effective and economical microalgae cultivation systems, as well as separation and harvesting of microalgal biomass and oil. This review presents recent advances in microalgal cultivation, photobioreactor design, and harvesting technologies with a focus on microalgal oil (mainly triglycerides) production. The effects of different microalgal metabolisms (i.e., phototrophic, heterotrophic, mixotrophic, and photoheterotrophic growth), cultivation systems (emphasizing the effect of light sources), and biomass harvesting methods (chemical/physical methods) on microalgal biomass and oil production are compared and critically discussed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.     

微藻与其他微生物共培养的研究进展及应用

化石燃料的挖掘和燃烧导致环境污染以及气候变化.与化石燃料相比,微藻被认为是一种更有前途的生物柴油生产原料,它具有生长速度快、含油量高、不占用耕地的特点.尽管微藻被认为是生产第三代生物燃料的最佳生产者之一,但单独培养微藻容易污染且采收成本高,与化石燃料和传统可再生能源相比缺乏竞争力.利用微藻与其他微生物共培养能够实现自絮...     

Mechanism and challenges in commercialisation of algal biofuels

Biofuels made from algal biomass are being considered as the most suitable alternative energy in current global and economical scenario. Microalgae are known to produce and accumulate lipids within their cell mass which is similar to those found in many vegetable oils. The efficient lipid producer algae cell mass has been reported to contain more than 30% of their cell weight as lipids. According to US DOE microalgae have the potential to produce 100 times more oil per acre land than any terrestrial plants. This article reviews up to date literature on the composition of algae, mechanism of oil droplets, triacylglycerol (TAG) production in algal biomass, research and development made in the cultivation of algal biomass, harvesting strategies, and recovery of lipids from algal mass. The economical challenges in the production of biofuels from algal biomass have been discussed in view of the future prospects in the commercialisation of algal fuels.     

Nutrient removal and biomass production: advances in microalgal biotechnology for wastewater treatment

Owing to certain drawbacks, such as energy-intensive operations in conventional modes of wastewater treatment (WWT), there has been an extensive search for alternative strategies in treatment technology. Biological modes for treating wastewaters are one of the finest technologies in terms of economy and efficiency. An integrated biological approach with chemical flocculation is being conventionally practiced in several-sewage and effluent treatment plants around the world. Overwhelming responsiveness to treat wastewaters especially by using microalgae is due to their simplest photosynthetic mechanism and ease of acclimation to various habitats. Microalgal technology, also known as phycoremediation, has been in use for WWT since 1950s. Various strategies for the cultivation of microalgae in WWT systems are evolving faster. However, the availability of innovative approaches for maximizing the treatment efficiency, coupled with biomass productivity, remains the major bottleneck for commercialization of microalgal technology. Investment costs and invasive parameters also delimit the use of microalgae in WWT. This review critically discusses the merits and demerits of microalgal cultivation strategies recently developed for maximum pollutant removal as well as biomass productivity. Also, the potential of algal biofilm technology in pollutant removal, and harvesting the microalgal biomass using different techniques have been highlighted. Finally, an economic assessment of the currently available methods has been made to validate microalgal cultivation in wastewater at the commercial level.     

Methods of energy extraction from microalgal biomass: a review

The potential of algal biomass as a source of liquid and gaseous biofuels is a highly topical theme, The process operations for algal biofuel production can be grouped into three areas: growth, harvesting and energy extraction, with a wide range of combinations of unit operations that can form a microalgal biofuel production system, but as yet there is no successful economically viable commercial system producing biofuel. This suggests that there are major technical and engineering difficulties to be resolved before economic algal biofuel production can be achieved. This article briefly reviews the methods by which useful energy may be extracted from microalgae biomass: (a) direct combustion, (b) pyrolysis, (c) gasification, (d) liquefaction, (e) hydrogen production by biochemical processes in certain algae, (f) fuel cells, (g) fermentation to bioethanol, (h) trans-esterification to biodiesel, (i) anaerobic digestion.     

Microalgae potential as a biogas source: current status,restraints and future trends

In recent years, the world energy demands have had a recurrent increase. For this reason the alternative to the fossil fuel resources are trend topics in investigation. Microalgae have been extensively studied as a source of biofuels and as one of the most promising alternatives in this new framework. One of the possibilities of obtaining renewable energy from microalgae is biogas production using anaerobic digestion process. This process is considered a significant component for biofuels and waste management, since represent an opportunity for energy generation using different wastewater products; also, the economic viability of microalgae liquid biofuel production could be improved. However, the anaerobic digestion of microalgae biomass is still not optimized because of the numerous technical limitations such as the microalgae characteristics, low carbon:nitrogen ratio, ammonia toxicity and even salinity. The present review summarizes and compares information concerning to anaerobic digestion of microalgal biomass and future directions for research. Besides, specific operational factors and potential inhibitory parameters of the process are analyzed and compared. Additionally, the paper covers the state or art concerning in methane production enhancement from algal biomass.     

Effects of bacterial communities on biofuel-producing microalgae: stimulation,inhibition and harvesting

Despite the great interest in microalgae as a potential source of biofuel to substitute for fossil fuels, little information is available on the effects of bacterial symbionts in mass algal cultivation systems. The bacterial communities associated with microalgae are a crucial factor in the process of microalgal biomass and lipid production and may stimulate or inhibit growth of biofuel-producing microalgae. In addition, we discuss here the potential use of bacteria to harvest biofuel-producing microalgae. We propose that aggregation of microalgae by bacteria to achieve >90% reductions in volume followed by centrifugation could be an economic approach for harvesting of biofuel-producing microalgae. Our aims in this review are to promote understanding of the effects of bacterial communities on microalgae and draw attention to the importance of this topic in the microalgal biofuel field.     

Sustainable production of eicosapentaenoic acid-rich oil from microalgae: Towards an algal biorefinery

Utilization of sustainable natural resources such as microalgae has been considered for the production of biofuels, aquaculture feed, high-value bioactives such as omega-3 fatty acids, carotenoids, etc. Eicosapentaenoic acid (EPA) is an omega-3 fatty acid present in fish oil, which is of physiological importance to both humans and fish. Marine microalgae are sustainable sources of lipid rich in EPA, and different species have been explored for the production of EPA as a single product. There has been a rising interest in the concept of a multi-product biorefinery, focusing on the maximum valorization of the algal biomass. Targeting one or more value-added compounds in a biorefinery scenario can improve the commercial viability of low-value products such as triglycerides for biofuel. This approach has been viewed by technologists and experts as a sustainable and economically feasible possibility for the large-scale production of microalgae for its potential applications in biodiesel and jet fuel production, nutraceuticals, animal and aquaculture feeds, etc. In this review paper, we describe the recent developments in the production of high-value EPA-rich oil from microalgae, emphasizing the upstream and downstream bioprocess techniques, and the advantages of considering an EPA-rich oil-based biorefinery.     

The enhanced biomass and lipid accumulation in <Emphasis Type="Italic">Coccomyxa subellipsoidea</Emphasis> with an integrated treatment strategy initiated by brewery effluent and phytohormones

Brewery effluent (BE) as an appreciable and sustainable resource presented new possibilities in low-cost algal biomass production, whereas the relatively low essential macronutrients hindered extensive applications as growth medium for microalgae cultivation. The objective of this study was to investigate the feasibility of an integrated treatment strategy initiated by BE coupling phytohormones in augmenting biomass and lipid accumulation in Coccomyxa subellipsoidea. Results revealed that BE coupling synthetic 1-naphthaleneacetic acid (NAA) accomplished the favorable lipid productivity of 481.76 mg/L/days, representing 6.80- to 9.71-fold more than that of single BE as well as standard Basal media. BE coupling NAA feeding also heightened the proportions of C16–C18 fatty acids (over 96%) and mono-unsaturated C18:1 (approximate 45%) which were prone to high-quality biofuels-making. Such profound lipids accumulation might be attributable to that BE coupling NAA treatment drove most of metabolic flux (i.e. acetyl-CoA) derived from TCA cycle and glycolysis flowing into lipid accumulation pathway. Concurrently, the complete removal of total nitrogen and total phosphorus by C. subellipsoidea with assistance of NAA were easily complied with the permissible dischargeable limits for BE. These present results strongly demonstrated that BE coupling NAA was a potential feeding strategy in boosting algal lipid productivity and further provided great possibilities in linking affordable algal biomass production with high-efficient biological contaminants removal.     

絮凝微生物采收微藻的作用机制研究进展

微藻广泛分布于自然界,其易培养,生长快且应用价值高,普遍用于生物燃料、医学原料、优质食品源及畜牧养殖业等。近年来,通过对光生物反应器改造设计、高产藻株筛选、代谢通路基因改造等方法实现微藻产量的提高,而在微藻处理的下游过程的研究与创新不足,特别是微藻采收已经成为其产业发展的瓶颈。本文综述了絮凝法在微藻采收中的作用,重点讨论了絮凝微生物在微藻采收中的作用,并对絮凝微生物对微藻的絮凝机制进行广泛探讨,为絮凝微生物采收微藻提供理论依据。     

Membrane technology in microalgae cultivation and harvesting: A review

Membrane processes have long been applied in different stages of microalgae cultivation and processing. These processes include microfiltration, ultrafiltration, dialysis, forward osmosis, membrane contactors and membrane spargers. They are implemented in many combinations, both as a standalone and as a coupled system (in membrane biomass retention photobioreactors (BR-MPBRs) or membrane carbonation photobioreactors (C-MPBRs). To provide sufficient background on these applications, an overview of membrane materials and membrane processes of interest in microalgae cultivation and processing is provided in this work first. Afterwards, discussion about specific aspects of membrane applications in microbial cultivation and harvesting is provided, including membrane fouling. Many of the membrane processes were shown to be promising options in microalgae cultivation. Yet, significant process optimizations are still required when they are applied to enable microalgae biomass bulk production to become competitive as a raw material for biofuel production. Recent developments of the coupled systems (BR-MPBR and C-MPBR) bring significant promises to improve the volumetric productivity of a cultivation system and the efficiency of inorganic carbon capture, respectively.     

微藻采收方法的研究进展

微藻的生产过程可以实现能源生产、废水净化和CO2减排的高度耦合,在能源危机日益紧张、环境问题日趋严峻的今天,微藻的开发利用具有重要的研究价值和经济、社会效益。制约微藻产业化的瓶颈问题是采收成本过高,一种经济合理的采收方法不但可以大大降低生产成本,还可以奠定微藻产业化发展的基础。本文对目前应用较为普遍的微藻采收方法进行了介绍,重点阐述了絮凝法采收微藻,以期对微藻的低成本高效率采收以及产业化发展提供帮助。     

Extremophile Microalgae: the potential for biotechnological application

Microalgae are photosynthetic microorganisms that use sunlight as an energy source, and convert water, carbon dioxide, and inorganic salts into algal biomass. The isolation and selection of microalgae, which allow one to obtain large amounts of biomass and valuable compounds, is a prerequisite for their successful industrial production. This work provides an overview of extremophile algae, where their ability to grow under harsh conditions and the corresponding accumulation of metabolites are addressed. Emphasis is placed on the high-value products of some prominent algae. Moreover, the most recent applications of these microorganisms and their potential exploitation in the context of astrobiology are taken into account.     

Phycoremediation coupled production of algal biomass,harvesting and anaerobic digestion: Possibilities and challenges

Biogas produced from anaerobic digestion is a versatile and environment friendly fuel which traditionally utilizes cattle dung as the substrate. In the recent years, owing to its high content of biodegradable compounds, algal biomass has emerged as a potential feedstock for biogas production. Moreover, the ability of algae to treat wastewater and fix CO₂ from waste gas streams makes it an environmental friendly and economically feasible feedstock. The present review focuses on the possibility of utilizing wastewater as the nutrient and waste gases as the CO₂ source for algal biomass production and subsequent biogas generation. Studies describing the various harvesting methods of algal biomass as well as its anaerobic digestion have been compiled and discussed. Studies targeting the most recent advancements on biogas enrichment by algae have been discussed. Apart from highlighting the various advantages of utilizing algal biomass for biogas production, limitations of the process such as cell wall resistivity towards digestion and inhibitions caused due to ammonia toxicity and the possible strategies for overcoming the same have been reviewed. The studies compiled in the present review indicate that if the challenges posed in translating the lab scale studies on phycoremediation and biogas production to pilot scale are overcome, algal biogas could become the sustainable and economically feasible source of renewable energy.     

Algal biotechnology products and processes — matching science and economics

Several microalgae, such as species ofChlorella, Spirulina andDunaliella, are grown commercially and algal products such as -carotene and phycocyanin are available. The main focus of algal biotechnology continues to be on high value fine chemicals and on algae for use as aquaculture feeds. This paper provides the outline for a rational approach in evaluating which algae and which algal products are the most likely to be commercially viable. This approach involves some simple market analysis followed by economic modelling of the whole production process. It also permits an evaluation of which steps in the production process have the greatest effect on the final production cost of the alga or algal product, thus providing a guide as to what area the research and development effort should be directed to. An example of this approach is presented and compared with other models. The base model used here gives a production cost of microalgal biomass at about AS 14 to 15 kg^–1, excluding the costs of further processing, packaging and marketing. The model also shows that some of the key factors in microalgal production are productivity, labor costs and harvesting costs. Given the existing technology, high value products such as carotenoids and algal biomass for aquaculture feeds have the greatest commercial potential in the short term.This paper was presented at the Symposium on Applied Phycology at the Fourth International Phycological Congress, Duke University.     

Methods of downstream processing for the production of biodiesel from microalgae

Despite receiving increasing attention during the last few decades, the production of microalgal biofuels is not yet sufficiently cost-effective to compete with that of petroleum-based conventional fuels. Among the steps required for the production of microalgal biofuels, the harvest of the microalgal biomass and the extraction of lipids from microalgae are two of the most expensive. In this review article, we surveyed a substantial amount of previous work in microalgal harvesting and lipid extraction to highlight recent progress in these areas. We also discuss new developments in the biodiesel conversion technology due to the importance of the connectivity of this step with the lipid extraction process. Furthermore, we propose possible future directions for technological or process improvements that will directly affect the final production costs of microalgal biomass-based biofuels.