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Essay paper on "A Review on microalgae and Bacteria for municipality wastewater Treatment" due kesho in kesho midnight please will give you a tip on this ) Minimum number of 20 pages excluding the pages for table content, list of figures, tables, equations, etc. and references This Essay is for “ Bioprocess Engineering ” class in Bioengineering program Essay topic: A Review on microalgae and Bacteria for municipality wastewater Treatment This essay should have less than 5% in plagiarism Minimum number of 20 pages excluding the pages for table content, list of figures, tables, equations, etc. and references Minimum of 100 citations All references should be listed under the “References” section and provided in PDF format highlighting the parts used in the essay Include figures, tables, charts, equations, etc in the essay Quality of the material and writing informative essay not any info that increase the number of the words Clear ideas with sequential paragraphs Strictly follow the same format as of "attached essay sample" The attached sample for the Table of content is only for "your reference" but wasn’t written for this essay I will follow the attached grading scheme to evaluate this essay, so please follow it to evaluate the essay as well SAMPLE Table of Content (TOC) “ different essay topic” 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Chemistry and biochemistry of carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Biosynthesis of carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chemical synthesis of carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Significance of carotenoids to human health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Advantages and disadvantages of microalgae as acarotenoid source . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Current technologies for carotenoid production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .\ . . . . . 6. Technologies of microalgae cultivation for carotenoid production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Cultivation systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Cultivation strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Stress-driven adaptive evolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Metabolic engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Down stream processing for carotenoids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Harvesting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1. Physical methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2. Chemical methods for cell harvesting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Cell disruption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1. Mechanical disruption methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2. Non-mechanical disruption methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1. Conventional solvent extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2. Super-/sub-critical solvent extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3. Other extraction methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.4. Wet extraction. . . . . . . . . Essay’s Marking Scheme Items Percent Chapter introduction, background and/or significance etc. (correct, clear and easy to follow?) 5% Is the subject review comprehensive - covering all essential area? - Is all area covering well and properly organized -Sections and Subsections have proper connections and include connecting statements from one paragraph to another 5x2=10% Presentation, analyses and discussions (critical review with additional analyses) -Clear, logical, with enough explanation -Good connections in each paragraph and sections -Critical analysis and points by authors during discussion -Analysis and discussed the results of Figures and Tables -Include Equations (Kinetics/analytical area) - Similarity index less than 10 % (Include Similarity index report) 5x6=30% New points, arguments, and inputs · New points and findings · Arguments and input comments · Rephrase sentences in logical and proper grammatical used 5x3=15% Figures, tables & Charts  -Clear and informative, enough to support the presentations  --Insert all figures and Tables in appropriate place in the texts(Inside the texts not in the last part) - Include the reference name at the end of title -Analysis and Discuss all the presented Figures and Tables -Include modified or adopted tables/figures -Include charts drawn by Author -Include Process Schematic diagram/flow diagram drawn by Author 5x7=35% Report format (correct?) (Title, table of content, sections with proper titles, nomenclature, reference citation)-All the text use Times new roman 12 Fonts 1.5 line space. 5% Additional comments: [submit separate page(s) of written comments] TITLE PAGE TABLE OF CONTENTS Contents TITLE PAGE 1 TABLE OF CONTENTS 3 LIST OF FIGURES 5 LIST OF TABLES 6 LIST OF EQUATIONS 7 Abstract 8 1.0. Introduction 9 2.0. Microalgae harvesting method 10 2.1. Common harvesting technology 10 2.1.1. Centrifugation 10 2.1.2. Sedimentation 11 2.1.3. Flocculation 11 2.1.4. Flotation 13 2.1.5. Filtration 14 2.2. New Emerging Microalgae Biomass Harvesting Techniques 15 2.2.1. Flocculation using magnetic microparticles 16 2.2.2. Flocculation by natural biopolymer 17 2.2.3. Electrical approach 18 3.0. Extraction and Analysis of Lipid from Microalgae Biomass 20 3.1. Lipid extraction 21 3.1.1. Mechanical extraction 21 3.1.2. Chemical/solvent extraction 23 3.1.3. New emerging green solvents systems and process intensification techniques for lipids extraction from microalgae 25 4.0. Heterogeneous transesterification catalysts 29 4.1. Solid Bases Transesterification 33 4.2. Solid Acids Transesterification 35 4.3. Heterogeneous transesterification of algae oil 36 5.0. Reactors 44 5.1. Influence of reactor design and operating conditions 44 6.0. Conclusions 51 References 54 LIST OF FIGURES Figure 1: Flowsheet for biodiesel production from microalgae. Some intensified process techniques highlighted may reduce some downstream steps as it would render the dewatering step unneeded. i.e. MAE – Microwave assisted extraction (MAE), Enzyme assisted extraction (EAE), Ultrasound assisted extraction (UAE), Surfactant assisted extraction 27 Figure 2:Flow sheet of an oscillatory baffled reactor and it mixing features. Also illustrating the solid acid catalyst PrSO3H-SBA-15 undergoing no oscillation but sedimentation and or with about 4.5Hz oscillation traped in the baffles. Figures exuracted from (Eze et al., 2013) 47 Figure 3: Diagram of membrane reactors for producing biodiesel in transesterification reaction through (a) Solid acid catalyst and (b) base catalysts. 49 LIST OF TABLES Table 1: Performance comparison of flotation techniques 14 Table 2: Performance comparison of filtration methods 15 Table 3: Performance of flocculation using biopolymer 17 Table 4: performance comparisons for microalgae biomass harvesting by various electrical methods operated in just 1 hour 19 Table 5: Reported catalyst used for heterogenous transesterification reaction on various feedstocks 30 Table 6: The effect of calcination temperature on the performance of WO 3 /ZrO 2 catalyst (Jothiramalingam & Wang, 2009). 39 Table 7: Literature review on biodiesel production via heterogenous catalyst 41 LIST OF EQUATIONS Equation 1: Chemical equation showing production of biodiesel from any bio oil 32 Equation 2: Reaction mechanism of transesterification via base catalyst (denoted Y) in the equation. 33 Abstract The dwindling rate of our fossil fuel reserves and general believe of major contribution of CO2 emissions which is linked to the climate change due to the burning of such carbon sources in engines either for locomotion or power generation have geared both the academic and industrial research towards the new routes for renewable and sustainable fuels. However, microalgae as one of the third-generation biomass feedstock has recently been proven to be one of the best option considered employable for biodiesel production. But one of the crucial challenges not yet explicitly attended to is the method of harvesting, lipids extraction and heterogenous catalyst for transesterification reaction for oil conversion to biodiesel. Herein. we reported several techniques for microalgae biomass harvesting, both the conventional and new emerging ones, as this helps in building ideas for improvement in the field. We also present a critical review on the work done on areas of lipid extraction from microalgae biomass and its conversion to biodiesel through heterogenous catalysis. it covers the progress made in this fields from the last decade, available systems for heterogenous catalysis, mechanism of the reactions and optimal process conditions. Lastly, we discuss on the reactors employed in the transesterification, effects of reactors design and way forward. 1.0. Introduction In our world, today, the demand for energy is on the increase. Fossil fuel reserves are running out. The persistent fluctuation and increase in price of fossil fuels and its adverse effect on the ecosystem through the emission of greenhouse gases makes it imperative that we seek alternative, sustainable and more environmental friendly energy source. The demand for safe alternative sources of energy such as biofuel is more pressing than ever before. Top on the list of such sustainable renewable energy is the feedstock energy source. A good source of feedstock energy is the Microalgae commonly referred to as the third-generation feedstock (Patrícya et al., 2014; Wawrik & Harriman, 2010). Various bio-products, such as biofuel and bio-hydrogen can be manufactured from Microalgae. Microalgae also have higher biomass yield and lower carbon footprint requirement compared to other plants (Besson & Guiraud, 2013; Farooq et al., 2015). In making microalgae based biofuel production commercially available and economically viable, the challenge of what method to use in harvesting microalgae and lipid extraction must be taken into consideration. Most harvesting methods can be capital intensive, uneconomical, and produce some level of environmental pollution. Harvesting microalgae can require as high as 20-30% of total biomass production budget (Grima et al., 2003; Mata et al., 2010; Verma et al., 2010) or 50% of the cost of producing biofuel (Muradov et al., 2015). Also, it has been proposed that world usage of biodiesel could increase to two or three times in most part of the globe by year 2020 and numerous influencing factors have not been fully addressed. Bio-oils or oils as the origin implies contains a key compound called triglyceride esters which when react with any monohydric alcohol (i.e. methanol) would form a group of compounds called mono-alkyl esters i.e. biodiesel. However, scientist around the world suggested the use of lower monohydricalcohols (i.e. CH3OH to C3H8O), without any explicit justification of which gives the best performance and requirements in terms of its viscosity as specified by ASTM or related international agencies. Similarly, finding the best catalyst and optimized operating reaction conditions is of a great challenge to biofuel industries. Homogenous catalysis offers faster biodiesel production with even moderate reaction conditions, but faced with recovery or separation problem after the transesterification process. These above highlighted challenges have triggered scientists at both industries and academics to seek alternative means, while emphasizing on the feedstock flexibility, and green catalytic systems. Lately, both microalgae and microalgae emerged as a better option for the biodiesel feedstock. Also, some green catalyst discussed herein emerged as a better candidate heterogenous catalysis for the transesterification process. We therefore present a critical review in these areas and lastly, the reactors involved. 2.0. Microalgae harvesting method Show et al., (2015) inferred in their work that, when considering a preferred harvesting procedure two important issues must first be determined; the attributes of the microalgae considered and the condition of their growth. The efficiency of any harvesting method chosen will depend largely on the specie of microalgae, size of the microalgae, its morphology and composition of the medium employed. Some harvesting techniques commonly used are centrifugation, flocculation, filtration and sedimentation. 2.1. Common harvesting technology 2.1.1. Centrifugation Centrifugation is the application of a centrifugal force of higher intensity than the gravitational force to increase the rate of separation a suspension. Common centrifugation methods include Solid-bowl decanter, nozzle-type centrifuge, Hydro-cyclone, and solid ejecting disc (Milledge & Heaven, 2013). The challenge with the methods above despite their efficiency at harvesting majority of the microalgae cell types are high energy, capital and operational cost required to carry them out (requirement) (Grima et al., 2003; Milledge & Heaven, 2013; Yuan et al., 2009).Centrifugation can harvest averagely between 12-25% of microalgae biomass with an energy consumption of 50-75kW (Milledge & Heaven, 2013). The only justification for this high cost and large amount of energy is that sufficient biofuel of over 90% of the microalgae biomass must be harvested. 2.1.2. Sedimentation Sedimentation is the process by which solids are separated from liquids by capitalizing on differences in the density of the solids to obtain an effluent of clear liquid (Milledge & Heaven, 2013). Most wastewater treatment facilities use sedimentation for sludge treatment. Sedimentation is the most cost effective and least complicated method for harvesting microalgae biomass, especially heavy microalgae suspensions. Two difficulty with using sedimentation for microalgae biomass harvesting is that for solids with little difference in their densities the process can be excruciatingly slow and the dry solids concentration of microalgae biomass that can be harvested is about 0.5-3% (Grima et al., 2003; Milledge & Heaven, 2013; Yuan et al., 2009). Golueke & Oswald, (1965) reported that using alum as a coagulant in a flocculation-sedimentation process an of 85% microalgae biomass harvest was achieved. 2.1.3. Flocculation Flocculation seldom used alone but in conjunction with other methods (Brennan et al., 2010) such as coagulation-flocculation and flotation-flocculation. Flocculation through aggregation improves particle size of microalgae suspension and speeds up rate of suspension settling (Mata et al., 2010; Milledge & Heaven, 2013). Auto-flocculation, physical flocculation, bio-flocculation and physio-chemical flocculation are four types of flocculation commonly in use. Auto-flocculation usually cuts the flow of carbon dioxide (CO2) to the microalgae system when the pH of the culture is above 9, therefore the microalgae flocculate on its own (Vandamme et al., 2013). Although auto-flocculation can be a slow, unreliable process and also requires the presence of calcium and magnesium ions many researchers have published results of up to 90% microalgae recovery harvest (Baya et al., 2016; Gerardo et al., 2015; Milledge & Heaven, 2013; Ras et al., 2011). Bio-flocculation is a technique in which microorganism are used in the treatment of wastewater. Such microorganisms include fungi and bacteria (Gerardo et al., 2015; Vandamme et al., 2013). Bio-flocculation is a technique yet to be fully comprehended but it is well documented that it improves the abilities of microalgae to form in suspension (Salim et al., 2011; Zhou et al., 2013). Zhou et al., (2013) reported almost 100% success harvesting microalgae cells of Chlorella vulgaris UMN235 by employing palletization-assisted bio-flocculaton. Their further recommended in their study that adding 20g/L glucose and of spores in BG-11 medium is desirable for palletization. Two locally isolated fungi being used as bio-flocculants are; Aspergillus sp. UMN F01 and Aspergillus sp. UMN F02. More so, a bio-flocculant from Bacillus licheniformis CGMCC 2876 was discovered to be an excellent harvester with 96% efficiency. This level of efficiency results from reduction in the negative charge of Desmodesmus sp. to about zero surface charge (Ndikubwimana et al., 2015). From the findings of the studies above, it can be clearly seen that bio-flocculation in conjunction with other techniques such as flotation and electrical approach can be used to solve the challenge of high quantity of bio-flocculants and time required to efficiently harvest high amount of microalgae using bio-flocculation. 2.1.4. Flotation Microalgae have low densities; this characteristic can be explored during harvesting using flotation method (Gerardo et al., 2015; Show et al., 2015). Air bubbles enhance the movement of microalgae particles upwards. Microalgae cells become hydrophobic when surfactant or coagulants are added into the system. Addition of surfactant or coagulants expands the mass transfer between the air and microalgae particles improving particles separation (Gerardo et al., 2015; Uduman et al., 2010). Some readily available surfactants in use are aluminum sulfate (Al2(SO4)3), iron (III) sulfate (Fe2(SO4)3), cetyltrimethylammonium bromide (CTAB), chitosan, and iron (III) chloride (FeCl3). High efficiency harvesting rates between 70 and 99% of microalgae biomass by flotation has been reported in some studies (Aulenbach et al., 2010; Barrut et al., 2012; Coward et al., 2014; Show et al., 2015). Flotation harvesting method requires low initial equipment cost and shorter period compared to others (Gerardo et al., 2015; Show et al., 2015). The choice of surfactants and its effect on reusability of culture and biofuel manufacturing has only been reported by few researchers as of today. Recent studies have shown an increase in the production of biomass and support for the growth of C. vulgaris – an outstanding discovery using iron (III) chloride (FeCl3) (Farooq et al., 2015). The challenge with this process is that ferric acid which is a residue of the iron (III) chloride has a negative impact on the oxidative stability of biodiesel, upon completion of the harvesting process it has to be separated from the biomass. It is worthy of mention that studies by Farooq et al., (2015); Kim et al., (2011); Kim et al., (2013) have implied that cytotoxicity from residual alum from the alum used as surfactant inhibits the growth of microalgae. Hence the effect of surfactants need to be properly studied and understood before harvesting microalgae to be used for production of biofuel and culture recyclability. Other flotation techniques have been reported. Some of these are; dispersed air, micro-flotation, foam flotation, dissolved air, vacuum gas, electro-flotation, and ozone flotation. A comparison of these techniques is displayed in Table 1. Table 1: Performance comparison of flotation techniques Flotation Techniques Strains Harvesting efficiency (%) References Foam flotation Tetraselmis sp. 93 (Garg et al., 2014) Chlorella sp NR (Coward et al., 2014) Microflotation Dunaliella salina 99 (Hanotu et al., 2012) Vacuum gas Mixed culture 23 (Barrut et al., 2012) Dissolved air Mixed culture 90 (Phoochinda & White, 2003) C. zofingiensis 91 (X. Zhang et al., 2014) Column flotation Chlorella sp 90 (Liu et al., 2006) Flocculation flotation C. vulgaris 93 (Lei et al., 2015) Dispersed air Spirulina platensis 80 (Kim et al., 2005) Electro-flotation Mixed culture NR (Sandbank, 1979) Ozone flotation Microcystis 90 (Benoufella et al., 1994) *NR – Not reported 2.1.5. Filtration Filtration is the separation of a solid-liquid mixture of microalgae using a semi-permeable membrane with small pores that allow the passage of the liquid but retains the solid microalgae (Gerardo et al., 2015; Show et al., 2015). Microalgae with low density such as Chlamydomonas sp., Chlorella sp. and Scenedesmus sp. can easy have their biomass harvested using filtration method (Gerardo et al., 2015; Rickman et al., 2012; Show et al., 2015). One problem with filtration however is that clogging and fouling brought by settled cells that can reduce the solid content due to low volumes of liquid that is able to pass through the filter used (Huang et al., 2012; Show et al., 2015). Two filtration setups are common, which are; the dead-end and the tangential flow. Dead-end setup made up of cartridge filtration, horizontal filter press, belt filter and vacuum drum filter are carried out in batch modes. These methods can harvest 5-37% mean solid content. Cross-flow filtration which is an alternative name for tangential flow filtration was created to overcome the challenge of fouling and decrease the accumulation of cake layer such that the filtration time is accelerated (Gerardo et al., 2015). Shear movement (Morineau-Thomas et al., 2002; Nurra et al., 2014), back-flushing (Baerdemaeker et al., 2013), supplementation of coagulant (Hwang et al., 2013), and alteration of membrane surface (Baerdemaeker et al., 2013) have been reported by many studies as ways of minimizing membrane fouling. Table 2 is a summary of the outcome obtained from using each filtration method in microalgae biomass harvesting. Table 2: Performance comparison of filtration methods Filtration Techniques Strains Harvesting efficiency (%) References Vacuum filter Coelastrum sp. NR Belt filter Mixed culture NR (Grima et al., 2003) Ultrafiltration Scenedesmus quadricauda NR (Zhang et al., 2010) Chlorella sp. 94 (Hwang et al., 2013) Deep-bed filtration Mixed culture NR Vacuum filter Spirulina sp. NR (Goh, 1986) Ultrafiltration Dunaliella sp. 99 (Mixson et al., 2014) *NR – Not reported 2.2. New Emerging Microalgae Biomass Harvesting Techniques Many scientific studies have been carried out to enhance microalgae biomass harvesting, which include reducing the energy requirement and operational cost. Recent approaches being developed to harvest microalgae biomass include flocculation employing magnetic microparticles (Seo et al., 2015; Vergini et al., 2015), flocculation using natural biopolymer (Banerjee et al., 2014; Rahul et al., 2015), sedimentation involving polymers (Zheng et al., 2015), magnetic membrane filtration (Bilad et al., 2013) as well as electrical approaches which includes electro-coagulation-filtration (ECF) (Gao et al., 2010) and electrochemical harvesting (ECH) (Misra et al., 2015). 2.2.1. Flocculation using magnetic microparticles Seo et al., (2015) examined four different magnetic particles, namely pure copper, iron (III) nitrate nonahydrate, copper/carbon composite and polyvinylpyrrolidone (PVP). Vergini et al., (2016) also examined the application of iron oxide magnetic microparticles (IOMPs). Both studies reported microalgae biomass harvesting using flocculation with the various magnetic particles aforementioned (Seo et al., 2015; Vergini et al., 2015). By using various methods and different materials these magnetic particles can be coated. Some coating methods are polyol method, sol-gel processing, thermal reduction, and spray pyrolysis (Athanassiou et al., 2006; Grouchko et al., 2009; Yan et al., 2007; Yanhui et al., 2009; Jacob et al., 2006; Jung et al., 2011; Li & Liu, 2009; Li et al., 2011; Qian et al., 2012; Wang & Asefa, 2010; Xu et al., 2003). A 99% harvesting efficiency of Chlorella sp. KR-1 was achieved by Seo et al., (2015) using polyvinyl-pyrrolidone (PVP) and iron nitrate with 0.8 () ratio at 10mg/mL. Vergini et al., (2015) on their part reported a 91% efficiency in harvesting D. tertiolecta using iron oxide magnetic microparticles (IOMPs) during 5 min of harvesting. The harvesting procedure was carried out for a certain period using cylindrical neodymium magnets (NdFeb) as an external magnetic field. Judging from the reports by Seo et al., (2015) and Vergini et al., (2015), a conclusion can be reached; that flocculation using magnetic particles brings about cost effectiveness, as far as the magnetic microparticles can achieve full functionality via one-step synthesis. It is worthy of note that harvesting efficiency is affected by the dose of each particular magnetic flocculant. Vergini et al., (2015) reported a minimal increase of 16% when the concentration if IOMPs changed from 6.2mg/L (75%) at the beginning to 62mg/L (91%). Furthermore, Seo et al., (2015) in their study reported that increasing the dose of PVP/Fe (0.33) while reducing the ratio had resulted in reduced harvesting efficiency (31.4%). 2.2.2. Flocculation by natural biopolymer Biopolymers are organic flocculants manufactured by microalgae, bacteria, and plants haven undergone the process of cationization using (3-chloro-2-hydroxypropyl) trimethylammonium chloride (HPTAC). Polymeric substances which are a kind of organic flocculant when linked with different colloidal particles to produce floc formation have been described as efficient harvesters. An optimized dosage of 60mg/L of cationic inulin resulted in 88.61% harvesting efficiency within 15 min was reported by Rahul et al., (2015) while harvesting isolated Botryococcus sp., this study further showed that cationic inulin is a potent flocculant. In another study by Banerjee et al., (2014), the potency of cationic cassia gum to harvest Chlamydomonas sp. CRP7 and Chlorella sp. CB4 was reported. An harvesting efficiency of 93% was reported for Chlamydomonas sp CRP7 within 15 min when a 80 mg/L optimized flocculant dosage was used. On the other hand, the harvesting efficiency for Chlorella sp. CB4 was 92% within 30 min of introducing a 35 mg/L dosage of optimized flocculant. Finally, these studies showed a similar trend when the flocculant efficiency increased and the chlorophyll content decreased. Table 3 contains a summary of the performance of flocculation using biopolymer. Table 3: Performance of flocculation using biopolymer Biopolymer Strains Optimum concentration (g/L) Harvesting efficiency (%) References bCatinic cassia gum Chlorella sp. CB4 0.035 92 (Banerjee et al., 2014) aCatinic cassia gum Chlamydomonas sp. CRP7 0.08 93 (Banerjee et al., 2014) aCatinic inulin Botryococcus sp. 0.06 88.61 (Rahul et al., 2015) a – run time = 15 min, b – run time = 30min 2.2.3. Electrical approach Various researchers have worked on the use of electric approach in biomass harvesting. Methods such as electro-coagulation-filtration (ECF) (Syafaini et al., 2017), magnetically induced membrane filtration (MMV) (Bilad et al., 2013) and electrochemical using non-sacrificial electrodes (Misra et al., 2015) have been studied. The electrical approach for biomass harvesting is environmentally friendly, not restrained to certain microalgae species only, safe, and cost effective. A comparative study on the treatment of water containing microalgae using aluminium and iron electrodes by Gao et al., (2010) in an electro-coagulation-filtration (ECF) system reported that aluminum electrodes performed better than the iron electrode for harvesting. Micro-cystis aeruginosa, as seen in their harvesting efficiencies which was about 100% for aluminum and 78.9% for iron. The study further showed that increasing operating time resulted in increased harvesting efficiency. Gao et al., (2010) also studied the impact of initial pH on the harvesting efficiency, the best efficiency was observed for pH values between 4 and 7 where efficiency of 100% was observed. The alkaline broth resulted in the lowest percentage efficiency. As the initial pH values increased from 8 to 9, and then 10, the harvesting efficiency reduced from 99% to 90% and then 87.2% respectively. To improve the shear-rates at the liquid-membrane interface membrane vibrating system was used. From the result of polyvinyldene fluoride (PVDF) at two different porosities of 9% and 12% w/w, PVDF-12 presented a greater efficiency for harvesting Phaeodactylum tricornutum and C. vulgaris, which was higher than 97% (Bilad et al., 2013). Reduced harvesting efficiency was observed when using PVDF-9, this was suspected to be a result of the shortcoming of the membrane itself. Misra et al., (2015) reported the use of electrochemical method in the harvesting of Scenedesmus obliqus FR75119.1 using carbon plates. This study was interested on the effects of applied current, initial pH and electrolyte addition. When a current of 1.5 A was supplied for half an hour, the electrochemical harvesting (ECH) attained 55.4% efficiency. This finding was in consonance with that by Gao et al., (2010) in which the harvesting efficiency relied on the applied current and conductivity of the broth. Adding electrolytes, for instance sodium chloride (NaCl) would increase conductivity and reduce power consumption (S. Gao et al., 2010). The highest harvesting efficiency reported was 83% when 6 g/L of NaCl was added. The study further buttressed the report that initial pH within the acidic range for example pH 5 produced the highest harvesting efficiency of 73% while initial pH in the alkaline region between7–9 had produced a lower harvesting efficiency in the ECH system. From the aforementioned studies, it can be concluded for the electrical method, the alkaline broth had the worst performance and that addition of electrolyte was vital in making the process economically sustainable. Table 4 shows a summary for performance comparisons for microalgae biomass harvesting by various electrical methods. Table 4: performance comparisons for microalgae biomass harvesting by various electrical methods operated in just 1 hour Electrical approach Strains Initial pH Harvesting efficiency (%) References Electrochemical harvesting Scenedesmus obliqus 9 65 (Misra et al., 2015) 7 66 5 73 Electro-coagulation-floatation Microcystis aeruginosa 10 87 (S. Gao et al., 2010) 9 90 8 99 4 100 3.0. Extraction and Analysis of Lipid from Microalgae Biomass Lipids are a major component of microalgae. Microalgae can contain between 2-60% lipids out of its total dry weight depending on the condition of growth and the type of specie being examined. Lipids extracted from microalgae have attracted major interest for their fatty acids and triglycerides which can be converted into alcohol esters through esterification. The oils derived through this process are called ester fuels, when blended with diesel stocks these oils have produced efficiency of about 30% without degrading engine performance (Razzak et al., 2013). A group of researchers have successfully demonstrated a modified engine that can run on 100% of these fuels (Xu et al., 2008). Microalgae serve as special chemical production sites using the instrument of photosynthesis. After over four decades of studying these microorganisms, they finally displayed their abilities to produce different varieties of complex compounds and fossil fuels. The extraction of fatty acids from microalgae or lipid synthesis which usually results in hydrocarbons with 16 to 22 carbon chain length requires that oxygen be available (Hu et al., 2008). Lipid content and distribution in the cell are affected by some elements, some of which are; (a) CO2 concentration (b) temperature (c) intensity of light, (d) the nutrient concentration and (e) the relative proportion of salt - salinity. In algae, carbon compounds such as triglycerides-which are nonpolar lipids act as reservoirs of energy. However, the function of cell and chloroplast membrane formation is that of two polar lipids - phospholipids and glycolipids which are found inside the cell (Razzak et al., 2013). In as much as biodiesels can be manufactured from conversion of polar lipids, conventionally the non-polar triglycerides such as feedstocks are still the favored option. Therefore, non-polar lipids remain the most important algal product of interest. The conditions under which an algae grows during the growth phase will determine greatly the lipid concentration and productivity (Razzak et al., 2013). The total lipid content can differ between species beginning at 4.5% for very low and 80% for very high (Xu et al., 2008). 3.1. Lipid extraction We can not emphasize enough the fact that lipid extraction from microalga to be used as biofuel is not only a strenuous process, but it is both energy intensive and cost demanding. To convert microalgae into diesel fuel that research into better ways for the extraction from dry biomass of lipids and further refining is pivotal. The challenge here is that the specific condition to achieve this is yet to be fully laid out (Chisti, 2007, 2008). To extract lipid from biological cell, chemical means, physical means or a combination of both means are employed. Cell disruption is mostly needed for releasing lipid and sugar contents within the cells of microalgae, to be used in the production of biodiesel and ethanol. Mechanical cell disruption can lead to extraction of an immense amount of lipid from microalga when accompanied by chemical solvent extraction. Furthermore, literatures have reported other mechanical processes which include: freezing, bead milling, osmotic shock, homogenization, high pressure, sonication. Also alkali and organic solvent extraction which is a chemical procedure was also advanced. (Chisti, 2007, 2008; Razzak et al., 2013). 3.1.1. Mechanical extraction When a dense suspension of microalgae or other organisms is vigorously stirred together with the purpose of extracting the lipid in the mills cell disruption occurs. Cell disruption could happen to the suspended cells when they come into contact with energetic glass beads with powerful crushing ability during stirring. Hopkins (Razzak et al., 2013) reported that vigorous agitation by several small sized glass beads or ceramic beads was observed while mixing the cells and the beads. A bead mill is a simple arrangement of an enclosed grinding chamber having at its center a shaft that rotates. The discs fixed to the shaft transfer energy of motion to the tiny beads within the enclosure, …