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Highlights in BioScience             

ISSN: 2682-4043 

DOI:10.36462/H.BioSci.20209                                                                                                          
 

 

Highlights in BioScience                                                                                                                                                                           July 2020| Volume 3 

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Page 1 of 8 

                                           

 

Review Article 

 

Open Access 

 
 

 

 

 

1Biofuel Research Laboratory, Department of 

Microbiology, School of Life Sciences, 

Central University of Tamil Nadu, Thiruvarur, 

Tamil Nadu 610 005, India. 
2Department of Microbiology, PRIST, 

Institution Deemed to University, Thanjavur – 

613 403, India. 
3Biomass Refining Lab, Department of 

Renewable Energy Engineering, Tamil Nadu 

Agricultural University, Coimbatore 641 003, 

Tamil Nadu, India. 
 

Contacts of Authors  

 

 

* To whom correspondence should be 

addressed: Suchitra Rakesh 

 

Citation: Rakesh S, Kumar J T, Sri B, 

Jothibasu K, Karthikeyan S. 2020.  Sustainable 

cost-effective microalgae harvesting strategies 

for the production of biofuel and 

oleochemicals. Highlights in BioScience 

Volume 3. Article ID 20209. dio:10.36462/ 

H.BioSci.20209 

Received:  May  30, 2020 

 

Accepted:  July 3, 2020 

 

Published: July 7, 2020 
 

Copyright: © 2020 Rakesh et al. This is an 

open access article distributed under the terms 

of the Creative Commons Attribution License, 

which permits unrestricted use, distribution, 

and reproduction in any medium, provided the 

original author and source are credited. 

 

Data Availability Statement: All relevant data 

are within the paper and supplementary 

materials 

 

 

Funding: This work is financially supported by 

the SERB DST project (EEQ/2018/001463) 

 

Competing interests: The authors declare that 

they have no competing interests. 

Sustainable Cost-Effective Microalgae Harvesting 

Strategies for the Production of Biofuel and Oleochemicals 

 
Suchitra Rakesh

1
*, Jayakumar Tharunkumar

1
, Bhavya Sri

1
, Karuppaiyan 

Jothibasu
2
 and Subburamu Karthikeyan

3
 

 

 

 

 

Abstract 

      Microalgae have been explored for sustainable production of biofuel 

and chemicals. Microalgae is promising feed stock for  the production of 

several oleochemicals. It has the ability to utilize a variety of low cost 

feed stocks, accumulated large quantities of lipids and variety of value 

added products in their biomass. One of the major obstacles associated 

with the conversion of algae into value-added products is harvesting. The 

harvesting of algae is the most problematic area due to its low 

sedimentation rate, low biomass concentration, and high capital costs. 

Harvesting of algae is carried out by different physical, chemical, 

mechanical, biological, and electrolytic methods such as sedimentation, 

centrifugation, microstraining, dissolved air flotation, electrolytic 

flotation, chemical flocculation, bioflocculation, autoflocculation, 

Filtration. This review highlights the various methods of microalgae 

harvesting with advantages and future perspective of sustainable and 

cost-effective harvesting of microalgae.  

 

Keywords: Microalgae, Harvesting, Autoflocculation, Sedimentation, 

Bioflocculation. 

 

 

Introduction  

Algae are simple aquatic photosynthetic microorganisms that live in saline 

and freshwater environments. Sunlight plays a key role in the harvesting of the 

algae. They can grow at any place where enough sunlight is found. Apart from 

sunlight, the chemical composition of algae includes proteins, carbohydrates, 

lipids, and nucleic acids. Algae are known as third- generation biofuels due to 

their higher biomass production capacity compared to other cereal- based crops 

[1]. They are one of the important sources of food, animal feed, medicinal 

products, and most important oil for fuels [2]. Cost-effective production of 

microalgae biomass for value added products like biodiesel is usually restricted 

due to poor efficiency in essential processes like cultivation, harvesting and lipid 

extraction [3]. Harvesting of microalgae involves two consecutive steps i.e., Bulk 

harvesting and thickening. Bulk harvesting involves flocculation, floatation, and 

sedimentation, in which solid biomass is separated from suspensions. Whereas, 

thickening involves filtration, centrifugation [4]. The present paper foccuses on 

recent techniques involved in harvesting microalgae and to assess their technical, 

economical and application potention. 

 



 

 

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Algal Metabolites 

Algae like Macrocystis, Laminaria, and Ascophyllum 

produce alginic acids. Alginate has the potential to make a 

highly viscous solution and chelating properties, so they are 

widely used in the pharma and food industries. Alginates in 

textile industries are used for sizing cotton yarn [5]. Agar 

derived from macroalgae is used in making desserts, jellies, 

candies, frozen foods, etc. due to its semi-solid nature and 

jellying properties. Agarose, as one of the byproducts of 

algae, is used significantly in molecular biology and many 

biomedical fields for the production of tablets, capsules, and 

anticoagulants [6]. Many of the algae are used for 

aquaculture feed like Nannochloropsis, Chlorella, 

Tetraselmis, Phaeodactylum, Pavlova, etc [7]. For hundreds 

of years, microalgae have been used as nutrient supplements 

as it is a rich source of Carbohydrates, protein, enzymes, 

vitamins, and minerals. There are a variety of pigments 

produced by microalgae like Chlorophyll, Phycobiliproteins, 

Carotenoids, β-carotene, Astaxanthin, Lutein, Canthaxanthin 

[8]. Vitamins like Biotin, pantothenate, riboflavin, folic 

acid, nicotinic acid are produced by microalgae [9]. 

Harvesting of Microalgae 

Harvesting of algae is a two-step process that includes 

cultivation and separation of the algae. The separation of the 

microalgae can be of mechanical, chemical, electrical, and 

biological methods. Harvesting by using cost-effective 

procedures is the most difficult phase in algal biofuel 

production [4]. The techniques applied for microalgae 

harvesting are filtration, gravity sedimentation, 

centrifugation, microstraining, floatation, flocculation, etc., 

[10-11] and sometimes a combination of one or two 

methods. An ideal harvesting method must be effective for 

more algal species, which gives a higher biomass 

concentration with minimal costs, energy, and 

maintenance.
 

Centrifugation 

Centrifugation is the ideal process for harvesting of the 

microalgae because of its quickest and effective procedure 

to remove particles from the algal suspension without any 

additive agents. During the process of centrifugation, large 

shear forces are exerted and lyse the algal cells releasing 

algal oil into the medium. Later to which the oil recovery 

has been done [2]. Chlorella vulgaris shows a very high 

recovery of biomass and oil yield when subjected to the 

centrifugation process [12]. Pahl et al. [13] have examined 

various centrifuges like disk stack centrifuge, tubular 

centrifuge, nozzle-type centrifuge, decanters, perforated and 

imperforated basket centrifuges, hydrocyclones for 

microalgae separation.  Disc stack centrifuges are 

exclusively used in industrial high-value product recovery 

from algae but recorded poor energy return as more energy 

is exhausted than produced [14].  Decanter centrifuge is 

highly efficient but involves high energy consumption, 

whereas hydrocyclone centrifuges can be used with less 

maintenance but due to its high energy and capital cost, 

restricted its use for large scale purposes [15-16].  

Gravity Sedimentation 

In Gravity sedimentation, the gravitational forces 

separate the solids and liquids from one another depending 

on their size and density. The factors affecting 

sedimentation are particle size and density, pH, temperature, 

the intensity of light, and aging of the cells [17]. The rate of 

sedimentation is directly proportional to the difference in 

densities of the particles. If there is a high difference in 

densities between solids and liquids, the sedimentation rate 

is faster and in case of less difference, the sedimentation rate 

is slower [15; 18]. Sedimentation requires low cost and less 

energy. Sedimentation is mostly species-specific, slow 

separation, and low final concentration. The Chlorella sp. 

cultured at pH=10.5 and sedimentation time of 12 h showed 

effective harvesting from 1.01g/L initial concentration to 

3.95g/L [19]. The Centrifugation method is not appropriate 

for the cost-effective harvesting of microalgae like Chlorella 

sp. due to its small cell size. The application of a 

combination of various harvesting techniques is necessary 

for enhanced recovery efficiency. A study suggests that the 

granulation of the biomass obtained from microalgae 

(Chlorella sp. cells) via gravity sedimentation with 

filamentous microalgae is a possible solution for effective 

recovery [20].  

Microstraining 

In microstraining, drum rotates slowly in the partially 

submerged condition in the trough of suspended algal cells. 

A mesh act as a screen that captures large particles of algae, 

whereas microalgae still pass via mesh and were not able to 

be harvested [21]. It is a simple cost-effective method, but 

due to low harvesting efficiency, it is not desired much.   

Filtration 

The problems associated with the use of the filtration 

process are the small size of the algal cells, their shape 

[spherical], and the gelatinous and other extracellular 

materials of the algal cells. These results in the poor 

filtration causing plugging of the filters [22]. The particles 

in the suspension are introduced onto the screen of different 

sizes, the particles either pass through the filters or retained 

on the screen depending upon their size [23]. Filtration 

techniques can be classified into microfiltration, macro-

filtration, ultrafiltration, and reverse osmosis (Figure 1). 

 
Figure 1: The filteration technique for microalgae 

harvesting. 



 

 

Rakesh et al., 2020                                                                                                                            Sustainable cost-effective microalgae harvesting strategies 

 

 

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Many types of filters and membranes with different 

sizes are available for easy filtration. This method is suitable 

for only large algal cells; smaller cells may clog in the filter 

pores [24]. Filtration is the process of separation of 

microalgal biomass from the broth medium by passing the 

suspension through the permeable membrane [25]. 

Membrane filtration technology for harvesting microalgae 

comprises osmotic and pressure-driven processes. The 

pressure-driven membrane process involves hydraulic 

pressure to force biomass passage via the permeable 

membrane for solids separation from liquid [26].  

Common membrane filter designs include 

microfiltration (0.1-10µm), macro filtration (10µm), 

ultrafiltration (0.01-0.1 µm), nanofiltration (0.001-0.01µm), 

tangential flow filtration, vacuum filtration, dead-end and 

cross-flow filtration based on the size of the filter [27-28]. 

Osmotic pressure is used to separate the microalgal biomass 

and liquid medium in osmotically driven membranes; this 

includes reverse and forward osmosis. However, reverse 

osmosis has lower water flux, reverse draw solute flux, and 

fouling process compared to forward osmosis [29-30]. The 

criteria for the selection of membrane material involve 

characteristics of species, biomass concentration, 

hydrophilicity, Hydrophobicity, surface charge, flow 

parameters, etc. The typical materials used for polymer 

preparation of membrane are polyethersulfone 

polyvinylpyrrolidone, polyvinyl chloride, cellulose acetate, 

polyvinylidene fluoride, polyacrylonitrile, polyethersulfone, 

polyamide, etc [26; 28]. 

Recent studies have reported the use of ceramic-based 

material for membrane in the form of a diatomite dynamic 

membrane (DDM) [31]. One of the studies has reported that 

a mixture of sand, starch, and kaoline can be used for the 

novel ceramic membrane production for microalgae 

concentration and can overcome difficulties like high 

shrinkage, low porosity, high thickness, poor flexibility, 

small pore size, etc [32]. Other membrane developments 

include electromembrane [33], steel-use-stainless membrane 

coated with selective polymers, etc [34]. Research on the 

application of membrane processes for microalgae 

harvesting is mostly restricted to lab-scale and lab-scale 

performance does not guarantee the same performance on a 

large scale too. Hence, a study on a larger scale membrane-

based harvesting of microalgae is pre-requisite to focus on 

the near future [26]. 

Floatation 

In this process, tiny air bubbles are introduced which 

in turn transport the solid particles suspended in the system 

to the surface. Later, the floating solid particles from the 

surface can be removed by skimming or other methods. 

Dissolved Air Floatation: High pressure-recycled water 

which is saturated with dissolved air is introduced into the 

chamber. This creates the release of microbubbles, and they 

get attached to the suspended algal particles causing them to 

float on the surface [35]. Electrolytic floatation involves the 

formation of gas bubbles by electrolysis [15]. It is more 

rapid than the sedimentation method. This method is 

species-specific and requires high capital and operational 

costs. Flotation involves gravity separation via gas bubbled 

through a microalgal suspension, due to which soil particles 

get attached to gaseous molecules and can be skimmed off 

from the surface [36].  

Flotation processes are classified based on bubble 

generation methods, these involve dispersed air flotation, 

dissolved air flotation, dispersed ozone flotation, electrolytic 

flotation, jet flotation, etc [37].  Dissolved air flotation 

involves the use of bubbles of mean size 40 μm with range 

10 to 100 μm, under high pressure with air dissolved in 

water and later atmospheric pressure is released in the unit.  

The air bubbles aggregate are released with the microalgae 

particles and float on the surface which is later skimmed off 

[36; 38]. Dissolved Air Floatation of Chlorella sorokiniana 

after coagulation with four different coagulants: aluminum 

sulfate, ferric chloride, tanfloc, and zetag showed the 

maximum efficiency of 98.4, 94.5, 95.4, and 96.7% at 8 

cm·min
−1

, respectively [39].  

In dispersed air flotation method bubbles ranging from 

700-1500 µm size are produced by continuous passage of air 

stream via high-velocity mechanical fomenter. This process 

requires less energy; whereas the equipment cost is 

relatively quite high. The recovery percentage of microalgal 

cells has been raised to 90% with surfactants by increasing 

the hydrophobicity of the microalgae cells and this will 

facilitate the attachment of bubbles to microalgae cells [37]. 

Dispersed air flotation of Chlorella saccharophila with 

Cationic Trimethyl-Ammonium Bromide (CTAB) added as 

a surfactant was investigated as an effective harvesting 

technique for to be used for the production of bio-diesel 

[40]. 

In the Electrolytic floatation process, the gas bubbles 

are produced by electrolysis of water. The investigation of 

electrolytic floatation of microalgae Dunaliella salina 

showed similar harvesting efficiency as well as energy 

consumption while a graphite plate was used as the anode, 

and different materials and forms of cathodes (stainless steel 

plate, perforated stainless steel plate and graphite plate) 

[41].Dispersed Ozone Flotation (DOF) uses ozone gas 

instead of atmospheric air to produce charged bubbles that 

promote the flotation of microalgal cells on the surface. 

Ozone oxidizes the organic contents of the effluent and also 

improves the quality of water by lowering the turbidity [25; 

28].  However, the dispersed ozone flotation technique is 

not desired for large scale purposes, due to contamination 

issues and high operating prices [42].  

Flocculation 

Flocculation is the aggregation of the microalgal cells 

to enhance the size of the particle that eases the 

sedimentation and recovery of the microalgae. The settling 



 

 

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velocity and the concentration factor are the key parameters 

of the flocculation process. Usually, the negative charge 

present on the surface of the microalgal cells prevents its 

aggregation in the suspension [43]. This negative charge can 

be neutralized by adding flocculating agents to initiate 

flocculation in the microalgal cells. The flocculating agents 

should be non-toxic, inexpensive, easily available, and 

effective at low concentrations [44]. 

Multivalent metal salts 

Ferric chloride [FeCl3], Aluminium sulfate [Al (SO4)3, 

Alum], and ferric sulfate [Fe (SO4)3] are the multivalent 

metal salts used to initiate flocculation in the microalgal cell 

suspension. It is the phenomena where positively charged 

ions are used to neutralize or reduce the negative charge on 

the surface of the algae to form flocs [45-46]. The addition 

of these compounds may negatively affect the recycling of 

the medium and also the quality of the product. The metal 

salts possess a positive charge to interact with the negative 

charge present on the algal surface and result in floc 

formation [43]. The study compared and evaluated 

harvesting microalgae Arthrospira maxima through 

flocculation by the addition of CaCl2 as a flocculant and/or 

pH increase above 10 using NaOH showed an effective 

harvesting technique [47]. 

Biodegradable organic flocculation 

Biopolymers like guar gum, starch, alginic acid, and 

chitosan are used as organic flocculants for microalgae 

harvesting. These biopolymers may not contaminate the 

algal suspensions [48-49]. Chitosan has been shown to be an 

effective biopolymer for harvesting microalgae without any 

toxic effect on the harvested algae.
 

Bioflocculation 

Bioflocculation is the harvesting of microalgae without 

any addition of flocculating agents. Bioflocculation involves 

other microorganisms having the capability of flocculation 

in the medium, as shown in Figure 2. 

 

Figure 2: Bioflocculation process in microalgae harvesting. 

The flocculating fungi or bacteria or algae are 

harvested along with the non-flocculating microalgae in the 

medium. Extracellular polymeric substances secreted by 

organisms such as bacteria or fungi or algae act as 

bioflocculants. These bioflocculating agents can increase the 

growth rate of the algae in the system [44; 50]. The 

flocculating microorganisms do not require any special 

cultivation conditions and their presence don’t interfere with 

the downstream process of algal lipids. bThe study showed 

that the co-flocculation of Citrobacter freundii and 

Chlorella pyrenoidosa (bacteria: microalgae ratio was 1.6:1) 

showed the maximum flocculation efficiency of 97.45% and 

Mucor circinelloides and Chlorella pyrenoidosa 

(microalgae: fungi ratio was 333:1) showed the maximum 

flocculation efficiency of 92.08% [51]. 

Autoflocculation 

Few autoflocculating microalgae can form flocs 

without any addition of flocculating agents. 

Autoflocculation can be defined as the interaction between 

the surface molecules of the algae and the surrounding 

medium or interaction between the microalgae among 

themselves. The algal cells aggregate and form into larger 

flocs. The flocs induce the sedimentation of the algae [52]. 

The microalgae produce extracellular polymeric substances 

[EPS], which plays a key role in the autoflocculation. When 

this EPS production reaches its maximum level, the 

microalgae tends to undergo flocculation by settling down 

itself. Autofloculation is largely dependent on algal species 

and pH manipulation [53]. Chlorococcum sp. at pH 12 has 

been reported with a harvesting efficiency of 94% [50]. In 

the case of Scenedesmus obliquus, autoflocculation 

efficiency was improved from 10.4 to 33.2%, when the pH 

increased from 7 to 10 [54]. A study on auto-flocculation of 

Ettlia sp. driven by polysaccharides had a positive impact on 

harvesting efficiency of 91 ± 2.7%; whereas, auto-

flocculation of Chlorella sp. driven by polysaccharides had 

a negative impact on harvesting efficiency of 51 ± 1.3%. 

This study revealed that autoflocculation is species-

dependent and for each species harvesting protocol will vary 

[55].  

Electrolytic flocculation 

In electrolytic flocculation, electrodes are placed in the 

culture, and the current is supplied to run across the 

electrodes. The negatively charged algal cells migrate 

toward the positive charge of the electrode causing 

aggregation [23]. This method is more effective than 

chemical flocculation [56]. The study evaluated the 

harvesting of Scenedesmus sp. using Electro-Coagulation–

Flocculation (ECF) showed the effective harvesting 

efficiency (>99%) under optimal conditions [57]. Thus, this 

technique could be well-suited harvesting technique for 

biofuel production.  

Polymeric flocculation 

In polymeric flocculation, long-chain polymers are 

used to bind the algal cells by bridging the gaps between the 

cells, causing aggregation and settling. Polymers with a 

positive charge like polyethylene amine and 

polyacrylamides bind with negatively charged cells of 



 

 

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microalgae in the suspension and result in floc formation 

[18]. The harvesting of freshwater microalgae viz., 

Chlorella vulgaris and marine algae viz., Phaeodactylum 

tricornutum using a cationic polyacrylamide flocculant, 

showed 100% and 90% biomass recovery, respectively. The 

optimal flocculant concentration for Chlorella and 

Phaeodactylum was 18.9 and 13.7 mg/g of dry algal 

biomass, respectively [58].  

Physical flocculation 

Sometimes the flocculating agents may contaminate 

the algal suspension; to avoid such problems physical forces 

were induced. One such example is sonication. In this 

process, high-frequency sound waves are introduced into the 

algal suspensionthe cells charge negative and will move to 

the anode where they get neutralized and form into 

aggregates. Always high-frequency sound waves should be 

used, as low-frequency sound waves may cause rupture of 

algal cells [59].  

Magnetic separation using nanoparticles 

Magnetic separation using nanoparticles has recently 

gained attention for harvesting microalgae due to its 

potential over traditional harvesting techniques [60]. In this 

technique, magnetic nanoparticles are tagged to the 

suspended microalgal cells and are recovered from the 

culture broth via external magnetic field [42]. Microalgal 

harvesting is carried out by two types of magnetic 

nanoparticles i.e., naked and surface functionalized. In 

naked magnetic nanoparticles, a wide range of microalgal 

strains is recovered based on its specific surface area, 

biocompatibility, super-paramagnetism, etc [61-62]. In the 

case of surface-functionalized magnetite, there are two 

strategies for tagging polyelectrolyte, "attached-to" and 

"immobilized" strategy. In the "attached-to" approach, the 

surface of microalgal cells is coated with a polymer binder 

that helps attach with the magnetic particles. In the case of 

"immobilized-on" strategy surface of the uncoated magnetic 

particles is functionalized with a polyelectrolyte that aids the 

binding with the algal cells [63]. 

The harvesting efficiency of microalgae can be 

improved by surface coating of magnetic nanoparticles with 

various polymers, surfactants, etc [64]. A recent study has 

confirmed that the harvesting efficiency of Scenedesmus sp 

is improved when different Fe3O4 -based nanoparticles are 

used as adsorbents. Cetyl-trimethyl-ammonium bromide-

coated Fe3O4 nanoparticles, polyethyleneimine coated Fe3O4 

nanoparticles, amino-propyl-tri-ethoxy-silane functionalized 

Fe3O4 nanoparticles. All the synthesized magnetite-based 

nanoparticles showed high potential efficiency for 

microalgae harvesting [60].  

Concluding Remarks 

Various methods for microalgae harvesting were 

reviewed. Centrifugation is a rapid and effective method that 

required high capital and operational costs. Other methods 

like floatation, filtration techniques, and other 

electrochemical techniques require high capital cost. 

Autoflocculation and bioflocculation are found to be 

inexpensive and effective dewatering techniques for algal 

harvesting. Autoflocculationhas a high sedimentation rate 

without any addition of the flocculants. The autoflocculation 

can be enhanced by a high aeration rate, CO2 concentration, 

and nitrogen levels. Bioflocculation is also an efficient, eco-

friendly, and cost-effective algal harvesting method (Table 

1). 

 

 

Table 1: Various microalgal harvesting technique advantages and disadvantages. 

Sl. No. Harvesting Method Advantages Disadvantages References 

1 
Centrifugation  Rapid and efficient cell harvesting 

 Handles most of the microalgae cells 

 High operational cost 
[2] 

2 

Sedimentation  Reduces cost and energy  Mostly used for  non-motile algal cells  

 Species specific 

 Low separation rate 

[15, 17, 18] 

3 

Filtration  Filters and membranes of different sizes 

are available which makes the process 

of harvesting easy and rapid 

 Species dependent 

 Mostly suitable for large algal cells 

 Fowling and clogging becomes the main 

problem when using filtration as the harvesting 

method 

[22-24] 

4 
Floatation  Rapid when compared with 

sedimentation 

 Species specific 

 High capital and operational costs 
[15, 35] 

5 

Chemical flocculation  Low cost 

 Simple and rapid harvesting 

 no energy requirements are needed 

 Removal of chemical flocculants 

 Sometimes the chemicals are toxic to the algae [43, 45, 46] 

4 

Bioflocculation  eco-friendly and cost effective algal 

dewatering technique. 

 No energy inputs are required 

 The bioflocculants [bacteria/fungi/algae] used in 

the flocculation sometimes causes contamination [44, 50] 

5 

Autoflocculation  Effective 

 High sedimentation rate with no energy 

inputs 

 Changes in cellular composition 

[44, 52] 

6 

Magnetic Separation  Short time period for microalgal 

harvesting. 

 Reusability of the culture medium 

 Nanoparticles are costly. 

 quire special recycling eruipment  [63] 



 

 

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Acknowledgements 

This work is financially supported by the SERB DST 

project (EEQ/2018/001463)  

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