A review of microalgae harvesting technology for biofuel production

86 Nguyen Thi Thanh Xuan A REVIEW OF MICROALGAE HARVESTING TECHNOLOGY FOR BIOFUEL PRODUCTION TỔNG QUAN VỀ CÔNG NGHỆ THU HOẠCH VI TẢO NHẰM SẢN XUẤT NHIÊN LIỆU SINH HỌC Nguyen Thi Thanh Xuan The Univers[.]

86 Nguyen Thi Thanh Xuan

A REVIEW OF MICROALGAE HARVESTING TECHNOLOGY

FOR BIOFUEL PRODUCTION

TỔNG QUAN VỀ CÔNG NGHỆ THU HOẠCH VI TẢO NHẰM SẢN XUẤT NHIÊN LIỆU SINH HỌC

Nguyen Thi Thanh Xuan

The University of Danang, University of Science and Technology; Email: nttxuan@dut.udn.vn

Abstract - Microalgae are receiving increasing attention worldwide

as an alternative and renewable source for energy production

Through various conversion processes, microalgae can be used to

produce many different kinds of biofuel, which include biodiesel,

bio-syngas, bio-oil, bio-ethanol, and bio-hydrogen However, large

scale production of microalgal biofuel, via many available

conversion techniques, faces a number of technical challenges

which have made the current growth and development of the algal

biofuel industry economically nonviable Many researchers

consider efficient harvesting is the major challenge for

commercializing micro-algal biofuel Algae can be harvested by a

number of methods; sedimentation, filtration flocculation, flotation,

centrifugation and or a combination of any of these This review

aims at collating and presenting an overview of current harvesting

technologies from microalgae for the production of biofuel

Electro-Coagulation Flocculation (ECF) is seen as a promising approach

for reducing the cost and energy input of microalgae harvesting for

biofuel However, choosing a harvesting method needed to be

considered with a microalgal strain in combination with a

considerable influence on the design and operation of both

upstream and downstream processes is an overall microalgal

biofuel production process

Tóm tắt - Vi tảo đang ngày càng nhận được sự quan tâm ở quy

mô toàn cầu cho mục tiêu sản xuất nhiên liệu sinh học và năng lượng thay thế Các loại nhiên liệu được điều chế từ vi tảo có thể

là biodiesel, biogas, bioethanol và bio-hydro Tuy nhiên, việc sản xuất loại nhiên liệu này ở quy mô lớn đang đối mặt với nhiều thách thức kinh tế kỹ thuật ngăn sản tiến trình thương mại hóa của nó Nhiều nhà nghiên cứu đều nhận định khâu thu hoạch là thách thức lớn nhất Các kỹ thuật thu hoạch vi tảo bao gồm lắng, lọc, keo tụ, tuyển nổi, ly tâm hoặc kết hợp nhiều phương pháp Nghiên cứu này đánh giá tổng quan các công nghệ thu hoạch vi tảo nhằm sản xuất nhiên liệu sinh học Phương pháp keo tụ tuyển nổi bằng điện phân dường như là công nghệ hứa hẹn chi phí và năng lượng thấp Tuy nhiên việc lựa chọn công nghệ thu hoạch nào cũng cần phù hợp với chủng vi tảo trong sự xem xét mọi yếu tố kinh tế kỹ thuật trong quá trình sản xuất nhiên liệu từ vi tảo

Key words - microalgae biofuels; microalgae harvesting;

sedimentation; flocculation; centrifugation; flotation; electro

coagulation

Từ khóa - vi tảo; nhiên liệu sinh học; thu hoạch sinh khối; lắng; lọc;

ly tâm; keo tụ; tuyển nổi; điện phân keo tụ

1 Introduction

An increasing energy demand and potential fossil fuel

depletion have become major concerns for people around the

world Furthermore, climatic changes and global warming

possibly caused by the emission of greenhouse gases have

become contemporary issues to be solved [1] Among the

renewable energy sources, such as solar, geothermal, wind,

hydropower, biomass is considered a renewable,

biodegradable, and carbon dioxide-neutral energy source

derived from plenty of resources, for example, agricultural

residue and waste, forestry waste, municipal solid and

industrial waste, terrestrial crops, and aquatic plants [2]

However, alternate energy resources akin to first generation

biofuels derived from terrestrial crops such as sugarcane,

sugar beet, maize and rapeseed place an enormous strain on

world food markets, contribute to water scarcity and

deforestation [3] Second generation biofuels derived from

lignocellulosic agriculture and forest residues and from

nonfood crop feedstocks address some of the above

problems; however there is concern over competing land use

or required land use changes [3] Therefore, based on current

knowledge and technology projections, third generation

biofuels specifically derived from microalgae are considered

to be a technically viable alternative energy resource that is

devoid of the major drawbacks associated with first and

second generation biofuels [4]

With the outstanding advantages of microalgae: high biomass yield (their growth rates tend to doubling as little

as 8 hours) and oil yield per unit area for some species are more than 20x higher than e.g for oil seed rape [5]; as well

as their high adaptability with a variety of culture media: freshwater, saltwater, brackish or nutrient-rich waste water

in particularly; combined with their consumption ability of large amounts of CO2 (1.8 tones of CO2 could create 1 tons

of algae biomass [6]), they have been highlighted as a feedstock of biofuels Algae based biofuels are not tainted with the ethical dilemmas of current, food-based biofuels: they do not require arable land; need not to compete for freshwater; do not threaten food security [7] These fuels produced from microalgae biomass can be bio-methane (by anaerobic digestion process), bio-ethanol (by fermentation), biodiesel (by trans-esterification of fatty acids in biomass), bio oil (by pyrolysis of biomass), bio-hydrogen (by fermentation and electro-hydro-genesis process) and green diesel (by pyrolysis of biomass associated with HDO process) [8],…

Despite its inherent potential as a biofuel resource, many challenges have impeded the development of algal biofuel technology to commercial viability that could allow for sustainable production and utilization Algae biofuels production process involves four major steps, including cultivation, harvesting, lipid extraction, and conversion

THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79).2014, VOL 1 87 Many researchers consider efficient harvesting is the major

challenge of commercializing micro-algal biofuels [9, 10,

11] The goal of this research is to identify and evaluate

several potentially viable harvesting methods that could be

incorporated into end-to-end algae biofuel production This

review provides useful information to help future

development of efficient and commercially viable

technology for microalgae-based biofuels production

2 Microalgae harvesting technologies

Efficient harvesting of biomass from cultivation froth

is essential for mass production of biodiesel from

microalgae The major techniques presently applied in the

harvesting of microalgae include centrifugation,

flocculation, filtration and screening, gravity

sedimentation, flotation, and electrophoresis techniques

[12] The cost of algae harvesting can be high, since the

mass fractions in culture broth are generally low, while the

size of micro-algal cells is small (most algae are below 30

µm) and the negative surface charge on the algae that

results in dispersed stable algal suspensions, especially

during the growth phase [12]

The selection of harvesting technique is crucial to

economic production of microalgal biomass and dependent

on the properties of microalgae, such as density, size, and

the value of the desired products [11]

2.1 Gravity sedimentation

Gravity sedimentation is a simple process of

solid-liquid separation under gravity This is a commonly

applied for separating microalgae in water and waste-water

treatment The major advantages of sedimentation are low

power requirement, low design cost and low manpower

requirement The sedimentation rates depend on cell

density, cell size [13], cell motility and type of water flow

Enhanced microalgal harvesting by sedimentation can be

achieved through lamella separators and sedimentation

tanks [9] In sedimentation the recoveries are too low for

large scale harvesting however sedimentation can be used

as one of the primary harvesting process to concentrate the

microalgae

2.2 Filtration

Filtration is a mechanical separation method, which

usually uses a bed of granular media or a porous membrane

Many types of filters have been used to harvest algae and

filtration has been found satisfactory at recovering relatively

large algal cells [14] but can be hampered by low throughput

and rapid clogging [15] Although there is a wide a variety

of filter designs, membrane filters can be simply classified

by the pore or membrane size; macro filtration >10µm,

micro-filtration 0.1–10µm, ultrafiltration 0.02–0.2µm and

reverse osmosis <0.001µm (fig 1) The pressure to force

fluid through a membrane, and therefore the operational

energy required, generally increases with reducing

membrane pore size [14]

Cloth media can also be used as in the case of a rotary

drum filter [15] The simplest mode of filtration is

dead-end filtration Dead -dead-end filtration of large quantities of

dilute algal suspension can only be achieved, using packed

bed filters (mixed media or sand) This type of filtration is

limited by the rheological properties of the microalgae as these form compressible cakes that easily blind filters

Figure 1 Filtration Threshold

(Source: http://www.novasep.com/technologies/Filtration.asp)

Two extensive reviews of the filtration of microalgae have concluded that filtration methods are suitable for micro-algae with larger cells, but inadequate to recover microalgal species with diameters of less than 10 µm [9, 14] Filter aids and flocculants would both appear to assist filtration and reduce equipment operational energy requirements, but at additional materials increase costs and they may need to be removed from the microalgal biomass and the spent microalgal growth medium Ultrafiltration is capable of the removal of small microalgae, but its use is limited by high energy input and low output microalgal suspension concentrations Flocculation and belt filtration has been successfully used in the water treatment industry

as an effective low-cost separation method for microbial biomass and could be a viable method for the large scale separation of micro-algae, but requires further investigation

2.3 Centrifugation

In centrifugation, gravity is replaced as the force driving separation by a much greater force Almost all types of microalgae can be separated reliably and without difficulty by centrifugation [15] Several types of continuous centrifuges are used in industrial processes Each variation uses a slightly different mechanism to separate dense materials from other materials [31] Depending on the particle size and the concentration of microalgal cells in a growth medium, there are different types of centrifuges to be used (fig.2) Disc stack centrifuges are the most common industrial centrifuge and are widely used in commercial high value algal product plants and algal biofuel pilot plants [16]

Fig 2 Centrifuge Application Graph Courtesy Alfa Laval [16]

88 Nguyen Thi Thanh Xuan The operational variables, such as, centrifugal force,

flow rate, biomass settling rate and settling distance, will

determine the efficiency of centrifugation [17]

Centrifugation is suitable for research, final thickening of

slurries and recovery of high value products However, at

a large scale, the use of centrifuges becomes more

problematic as the capital costs increase with scale

2.4 Flotation

Flotation is a gravity separation process in which air or

gas bubbles attach to solid particles and then carry them to

the liquid surface Chen et al [18] noted that flotation is

more beneficial and effective than sedimentation with

regard to removing microalgae Flotation can capture

particles with a diameter of less than 500 μm by collision

between a bubble and a particle and the subsequent

adhesion of the bubble and the particle [18] Based on

bubble sizes used in the flotation process, the applications

can be divided into dissolved air flotation (DAF), dispersed

flotation and electrolytic flotation

2.4.1 Dissolved air flotation

The DAF entails the pressure reduction of a water

stream that is presaturated with air at excess pressures to

produce 10–100 μm bubbles [9] Factors determining DAF

harvesting of microalgae include the pressure of the tank,

recycle rate, hydraulic retention time, and floating rate of

particle Chemical flocculation has been used with DAF to

separate microalgae [9] Microalgae autoflocculation using

dissolved oxygen which is produced photosynthetically

has also been studied after flocculation using alum or C-31

polymer, and about 80–90% microalgal removal was

obtained when about 16 mg/l microalgal float

concentration was used [19] Edzwald found that DAF

removed microalgae more effectively than settling,

although flocculation pre-treatment was required in the

former process [20]

2.4.2 Dispersed air flotation

Dispersed air flotation entails 700–1500 μm bubbles

formed by a high speed mechanical agitator with an air

injection system [19] Chen et al [18] compared dispersed

air flotation efficiencies for microalgae using three

collectors, and noted that the cationic

N-cetyl-N-N-N-trimethylammonium bromide (CTAB) effectively

removed Scenedesmus quadricauda, while the nonionic

X-100 and anionic sodium dodecylsulfate did not They

attributed these differences to changes in surface

hydrophobicity with collector adsorption

2.5 Flocculation

Flocculation is a process in which dispersed particles

are aggregated together to form large particles for settling

2.5.1 Autoflocculation

Flocculation can occur naturally in certain micro-algae,

in a process known as auto-flocculation, and micro-algae

may flocculate in response to environmental stress;

changes in nitrogen, pH and dissolved oxygen [21] As a

result of precipitation of carbonate salts with algal cells in

elevated pH, a consequence of photosynthetic CO2

consumption with algae, auto-flocculation occurs Hence,

prolonged cultivation under sunlight with limited CO2 supply assists autoflocculation of algal cells for harvesting Laboratory experiments also revealed that autoflocculation can be simulated by adding NaOH to achieve certain pH values [22]

2.5.2 Chemical coagulation

Microalgal cells carry a negative charge that prevents aggregation of cells in suspension The surface charge can

be neutralized or reduced by adding flocculants such as multivalent cations and cationic polymers to the broth Adding chemicals to microalgal culture to induce flocculation is a common practice in various solid–liquid separation processes as a pre-treatment stage, which is applicable to the treatment of large quantities of numerous kinds of microalgal species [23] There are two main classifications of flocculants according to their chemical compositions: (i) inorganic flocculants and (ii) organic flocculants/ polyelectrolyte flocculants Ideally, the flocculants used should be inexpensive, nontoxic, and effective in low concentration In addition, the flocculant should be selected so that further downstream processing

is not adversely affected by its use

a Inorganic coagulants

Multivalent metal salts are effective flocculants or coagulants The commonly used salts include ferric chloride (FeCl3), aluminum sulfate (Al2(SO4)3, and ferric sulfate (Fe2(SO4)3) The efficiency of electrolytes to induce coagulation is measured by the critical coagulation concentration, or the concentration required causing rapid coagulation Coagulation efficiency of metal ions increases with increasing ionic charge Multivalent metal salts such as alum have been widely used to flocculate algal biomass in wastewater treatment processes [14] The major problem was the very high amount of chemicals required, thereby producing a large quantity of sludge In addition, the end product is contaminated by the added aluminum or iron salts Dosages over 100 mg/l

of alum were required to achieve a 90% clarification of the waste pond effluent [24] Recovery of the alum by acidification was studied, but proved to be relatively unsuccessful However, cost chemical flocculation was estimated at roughly 40% lower than those of centrifugation [25]

b Organic flocculants

An alternative to using metal salts is the use of cationic polymers (polyelectrolytes) In addition to reducing or neutralizing the surface charge on cells, the polymer flocculants can bring particles together by physically linking one or more particles through a process called bridging Some researchers have demonstrated that the bridging mechanism also applies to flocculation of algal cells [25] The most effective flocculants for the recovery

of microalgae are cationic flocculants [26] Anionic and nonionic polyelectrolytes have been shown to fail to flocculate microalgae, which is explained by the repulsion existing between charges or the insufficient distance to bridge particles Cationic polymers doses of between 1 and

10 mg/ml can induce flocculation of fresh water algae;

THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79).2014, VOL 1 89 however, a high salinity of the marine environment can

inhibit flocculation by polyelectrolytes [25] In all cases,

the flocculation was reduced by increasing ionic strength

of the cell slurry The flocculation effectiveness of

polyelectrolytes depends on many factors, including the

molecular mass of the polymer, the charge density on the

molecule, the dose used, the concentration of the biomass,

the ionic strength and pH of the broth, and the extent of

mixing the fluid [26] Generally, high molecular weight

polyelectrolytes are better bridging agents Similarly, a

high charge density tends to unfold the polymer molecule,

improving its bridging performance and the ability to

neutralize the surface change on cells A high cell

concentration in the broth helps flocculation, because the

cell–cell encounters are more frequent in concentrated

suspensions Polymeric flocculants have been used

extensively for recovering microalgal biomass However,

in comparison with salts such as aluminum sulfate, cationic

polyelectrolytes may be less effective [25]

A combined flocculation process is a multistep

flocculation process using more than one type of

flocculant Flocculation by aluminum sulfate followed by

certain polyelectrolytes is effective in microalgal

harvesting [25] Biodegradable organic flocculants, such as

chitosan, are produced from natural sources that do not

contaminate the microalgal biomass [27] Muylaert et al

[28] demostrated the feasibility of using cationic starch for

flocculation of both freshwater microalgae and marine

water microalgae

2.6 Electrolytic flocculation process

Electrolytic flocculation works based on the movement

of microalgae to the anode in order to neutralize the carried

charge and then form aggregates The electrolytic

flocculation experiments are based on the principle of the

movement of electrically charged particles in an electric

field Microalgae have a negative surface charge which

causes them to be attracted by the anode during the

electrolysis of the algal suspension Once they reach the

anode they lose their charge which makes them able to

form algal aggregates As the electrolysis of water means

the production of hydrogen and oxygen gas at the

electrodes, the bubbles produced at the anode (oxygen) rise

to the surface taking with them algal aggregates or flocs,

which can be skimmed off easily The electrolysis leads in

this way to the flocculation and flotation of the algae at the

same time, without the usual addition of chemical

flocculants [29]

There are several benefits to using electrochemical

methods, including environmental compatibility,

versatility, energy efficiency, safety, selectivity, and cost

effectiveness [29], [30] This new technique consume

relatively little energy (2.1 kWh/kg of biomass harvested

for Chlorella vulgaris and 0.2 kWh/kg of biomass

harvested for Phaeodactylum tricornutum [31]) It is easy

to control and applicable this technique to various groups

of microalgae; and the most important, it results in an

efficient separation of the algae (harvesting efficiency

reached at 97.44% for Dunaliella salina [32]) Moreover,

since it is not contaminated with toxic flocculants the

harvested algal biomass can afterwards be used for different purposes such as algal feed and food Further research on electrolytic flocculation would have to look for application development at the industrial scale since only small scale experiments have been carried out up to now

3 Conclusions

Harvesting of microalgal biomass accounts for the highest proportion of energy input during production, but currently, there are no standard harvesting techniques Adaptation of technologies already in use in the food, biopharmaceutical and wastewater treatment sector may provide possible solutions

To produce microalgal biomass for biofuels, the scale

of production has to be increased and the cost of production decreased by at least an order of magnitude In particular, cost-effective harvesting is considered to be one of the biggest challenges to realize large-scale and low-cost production of microalgae biofuels The energy needed for harvesting microalgae from typical open pond systems (0.03%) using centrifugation was calculated to be 14 MJ/kg Dry Weight (MJ/kgDW) of microalgae [33] When energy applications are the only focus, this would imply that more than 50% of the total combustion energy (estimated at 25 MJ/kgDW) is to be invested in a one step harvesting process using centrifugation Moreover, if biodiesel (7÷10 MJ/kgDW) is the only product, this would result in unsustainable production of biofuels [34] This simple calculation demonstrates the urgent need for low-cost and energy-efficient harvesting methods Flocculation

is seen as a promising approach for reducing the cost and energy input of microalgae harvesting

Flocculation is a widely used technology in different fields of industry Development of an efficient flocculation technology for microalgae may yield major cost and energy savings in large-scale production of microalgal biomass

As a result of this, numerous studies have started to explore various approaches for flocculating microalgae Chemical flocculation processes are already frequently employed in water purification systems to remove microalgae or in wastewater systems to concentrate sludge Similar to those systems, flocculation efficiency, settling rate and concentration factor are important in the design of an efficient flocculation method for harvesting microalgae But in contrast to applications in other industries, the harvested biomass is the desired end-product during production of microalgae instead of the purified water Contamination of the biomass by chemical flocculants added is a critical evaluation criterion when developing flocculation technologies for harvesting microalgae, although the use of natural polymers may minimize this problem Contamination of the biomass with flocculation aids may interfere with downstream processing of the biomass (e.g lipid extraction for biodiesel conversion) Moreover, the addition of chemicals should not limit the reuse of the cultivation medium after harvesting The given arguments indicate that knowledge transfer from existing industries can certainly be interesting and useful, but needs

to be applied within the boundaries of the production process of microalgae biomass

90 Nguyen Thi Thanh Xuan

As alternative for alum, electro-coagulation

flocculation (ECF) was evaluated During electrolysis,

aluminum is dissolved in the medium at the anode and the

released Al formes hydroxides that cause flocculation An

advantage of ECF over the use of metal salts is that no

anions are released in the medium during this process ECF

was capable of flocculating microalgae in both freshwater

and marine media The proof of concept given in this study

for flocculation using ECF already have and will continue

to trigger new initiatives towards the integration of

flocculation in existing harvesting processes New types of

electrodes or process regimes (e.g polarity exchange) can

further lead to improved harvesting by ECF

The worldwide concerns about CO2 emissions and the

need for resource security have boosted decision makers to

increase research funding for projects concerning

renewable sources of energy Currently, microalgae are

considered to be one of the most promising new sources for

biofuels production The responsibility of researches lies in

the objective assessment of the potential of these new

approaches including fuel from microalgae

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(The Board of Editors received the paper on 01/06/2014, its review was completed on 19/06/2014)