Supplier Information Pack
Algal Harvesting systems
15 Mar 2012
FILTRATION ................................................................................................................... 5
SETTLING AND SEDIMENTATION ............................................................................ 6
FLOCCULATION ........................................................................................................... 7
CENTRIFUGATION ....................................................................................................... 9
CELL DISRUPTION ..................................................................................................... 10
HIGH PRESSURE HOMOGENISATION .................................................................... 10
AUTOCLAVING, MICROWAVING, SONICATION AND BEAD-BEATING ........ 11
DEWATERING AND DRYING PROCESSES ............................................................ 12
This document is intended for the use of organisations that are interested in supplying
components and services for the algal industry.
It is presented for information and discussion purposes only and represents a snapshot
of the industry at the time of production and has been gathered from a number of
publicly available information sources.
Cranfield accepts no liability for any activity arising out of any inaccuracy in this
publicly available material.
Algae broth ready to harvest (0.3-0.5g/l)
Microalgae are the most rapidly growing algae species that are difficult to harvest
because they are very small (0.3-5gl-1) and are often motile unicells (2-40µm). Thus
there is a strong relationship between the development of the harvesting technique and
the selection of algae species for mass culture. Moreover, the selection of harvesting
techniques is dependent on the properties of microalgae, such as density and size (which
determines how easily the species can be settled and filtered), coupled with the value of
the desired product. The cost of harvesting (recovery of algal biomass) is also a
challenging issue because of a very low mass fraction in culture broth and negative
polarity, and thus accounts for 20-30% of the total cost of production (Brennan and
Owende, 2010; Chen et al., 2011).
Summary: In short, the main stimulating engineering demand rests in reducing the
energetic consumptions of separating or isolating the micro algal cells from the culture
medium which ultimately determines the cost of production. The cost of harvesting is
fundamental to the micro algal biomass production accounting for 20-30% of the total
cost. After aqueous cultivation of microalgae, dewatering to produce dry algal biomass
is a preparatory stage to deliver algae to the next stage (Khoo et al., 2011). The aim of
harvesting is to obtain slurry with at least 2-7% of algal suspension on a dry matter
basis (Singh et al., 2011).
1. Bulk harvesting. The aim is to separate the micro algal biomass from the bulk
suspension. Using this method, the total solid matter can reach 2-7% using a
concentration factor of 100-800 times by applying flocculation, flotation or
gravity sedimentation (Brennan and Owende, 2010).
2. Thickening. Filtration, centrifugation and ultrasonic aggregation harvesting
techniques are used in this method to concentrate the slurry. The process is more
energy intensive than bulk harvesting (Brennan and Owende, 2010).
The main harvesting techniques for microalgae are:
Filtration and related mechanical harvesting using filtration, by means of strong
membranes such as micro-screens and micro-strainers;
Settling and sedimentation – gravity and centrifugal sedimentation;
Chemical Methods – chemical and/or biological harvesting, such as by means of
flocculants e.g. auto flocculation, chemical coagulation, inorganic coagulants,
organic flocculants, combined flocculation, electrocoagulation and ultrasonic
Centrifugation – algae can also be harvested using centrifugation;
Flotation – froth flotation is a method whereby the water and algae are aerated into
froth, with the algae then removed from the water; and
Electrolytic method – such as the electrophoresis technique.
Different modes of filtration can be used to concentrate microalgal cells. The micro-
screen and micro-strainer are two screening devices for microalgae harvesting. The
principle is based on passing or retaining particles that are introduced onto a screen of a
given aperture size.
Types: micro-screens, micro-strainers, dead-end filtration, vacuum (pressure) filtration,
cross-flow filtration. Vacuum drum filters and chamber filter presses are the commonest
types of filtration applicable to fairly large microalgae.
Materials: Micro-strainers can be specified as a rotating filter with fine mesh screens
with frequent backwash. They are simple to operate, require low investment and have
high filtration ratios. Other modes of filtration include dead-end filtration, vacuum or
pressure filtration and cross-flow filtration. Dead-end filtration of large quantities of
algal suspension can only be achieved using packed bed filters (mixed media and sand)
and its application is limited to the removal of algae of low concentration due to the
rheological properties of microalgae which form compressible cakes and thus clog the
Throughput: Recovery of large microalgae, coupled with retaining the structure,
properties and motility of microalgae, are some of the merit parameters of vacuum
filtration and flow filtration compared to conventional filtration. Vacuum filters are able
to recover large amounts of microalgae, although they are less effective when applied to
organisms approaching bacterial dimensions. A recovery of 80% to 90% of freshwater
algae is achievable with tangential flow filtration. Moreover, micro-filtration is suitable
for fragile cells that require low trans-membrane pressure and low-cost flow velocity
conditions; and for the processing of low broth volumes (2m3 per day) membrane
filtration can be more cost effective compared to centrifugation. However, for large
scale production (>20m3 per day), centrifugation is more economical owing to
continuous membrane replacement (Brennan and Owende, 2010).
Microalgae and cynobacteria pose outstanding filtration challenges because most strains
considered for energy feedstock have cell diameters less than 10µm
(http://biomass.energy.gov). The conventional filtration process is suitable for the
harvesting of relatively large (>70µm) microalgae such as Coelastrum and Spirulina. It
cannot be used for microalgae specie (>30µm) such as Scenedesmus, Dunaliella and
Cost range: Filtrations are basically simple but potentially very expensive. Due to the
possibility of a scale up process associated with cross-flow filtration, such as its
capability for concentrating microalgae and can thus be use in downstream
fractionating. Decreasing the process volume by a factor of 100 will lower the cost of
disruption and fractionating stages downstream. Variables such as filter pore size, algae
aggregation rate, microalgae specie, filter materials etc. could play a significant role in
reducing the cost of filtration (Greenwell et al., 2010).
Energy balance: Dead-end filtration involves low energy consumption, but the
frequency of washing, coupled with loading, increases energy cost and reduces filter
effectiveness. Pressure or vacuum filtration attracts more energy cost, since power
consumption is in the order of 0.3-2Kw/h m-3 (Molina Grima et al., 2003). However,
Alabi et al. (2009) report energy requirement estimates for vacuum or pressure filtration
ranging from 0.2-0.88 kW/h m-3 to 0.1-5.9 kW/h m-3. Singapore experienced 0.56MJ
(cultivation + harvesting + dewatering) total life cycle energy of hypothetical integrated
PBR-raceway for the production of microalgae Nannochloropsis sp. conversion of
microalgal lipids to biodiesel via trans-esterification (Khoo et al., 2011
Issues: culture purity, blinding of filter materials and designs without washing
requirements are the major issues of filtrations.
Areas for development: For proper optimization of filters, the following
parameters should be understood: a) the filter pore size as relates to the algae specie and
algae aggregation rate; b) filter material also influences filtration and recovery. The
materials should control hyrophobicity and algae affinity with durability and blinding;
c) filtration design comprising dynamic and static filtering operations; d) power cost;
and e) recovering the algal mass from the filter (washing requirements).
SETTLING AND SEDIMENTATION
Types: Settling, sedimentation – gravity and centrifugal sedimentation.
Summary: Sedimentation is applicable to open pond systems and usually with algae
that have high intrinsic sedimentation rates in a water treatment process. Settling
characteristics are a function of the density and radius of algae cells and sedimentation
velocity, as defined by Stokes Law. Gravity sedimentation is mainly applied to
harvesting microalgae in waste water treatment because of the large volumes treated and
low value of the biomass generated (Brennan and Owende, 2010).
Material: Gravity sedimentation is simple with low cost energy requirements.
Sedimentation is a simple and very slow process (0.1-2.6cmh-1) and at high temperature
environment much of the biomass produced will deteriorate during the harvesting
process (Greenwell et al., 2009).
Throughput: Gravity sedimentation is suitable for large microalgae (ca.>70µm) such
as Spirulina. For algae with poorer sedimentation properties, a force flocculation is
induced through the addition of chemicals or culture flocculation. The flotation process
by sedimentation is, however, very fast, as it requires only a few minutes for
sedimentation. Capital and operating costs are also low, but the efficiency is poor in
shallow-depth ponds (Brennan and Owende, 2010; Singh et al., 2011).
Cost range: Sedimentation is performed in thickeners and clarifies standard
processes in water treatment plants. The capital and operating costs are low (Singh et
al., 2011). For recovery, centrifugation is preferred for harvesting high value
metabolites, but the process is rapid and energy intensive and depends on the settling
characteristics of cells, slurry residence time in the centrifuge, and settling depth
(Brennan and Owende, 2010; Chen et al., 2010).
Energy balance: Sedimentation of algal biomass coupled with centrifugation to 5%
solid biomass may require 2% of energy content.
Issues: Flocculation is normally incorporated to enhance the efficiency of gravity
sedimentation which depends on the density of micro algal particles. Sedimentation
alone is usually dismissed as a viable harvesting method.
Areas for development: Sedimentation velocity, sedimentation tanks, lamella
separators are the areas that need to be further explore.
1-3% mass solid
Summary: Following the poor sedimentation properties of some algae strains due to
inherent sizes and density, chemical methods can be used successfully for harvesting
purposes. Flocculation is the collection of dispersed particles into aggregated mass as a
result of pH adjustment, usually involving chemical additives. If aggregation is a result
of electrolytes addition it is regarded as coagulation, whereas aggregation as a result of
polymer addition is termed flocculation (Richmond, 2000).
Types: Auto-flocculation, bio-flocculation and chemical coagulation.
Chemicals additives such as carbonates and hydroxides (NaOH) can induce
physiochemical reaction between algae and promote auto-flocculation as a result of
carbonates precipitation in elevated pH effectively depleting photosynthetic CO2.
Culturing microalgae with another microorganism that promote sedimentation can also
induce bio-flocculation. Example is the use microbial flocculants for harvesting mass
culture of Chlorella vulgaris from Paenibacillys sp. AM49 was identified (Richmond,
2004, C.Y. Chen et al., 2011)
Organic, inorganic and polyelectrolyte flocculants can be added in various solid-liquid
separation processes to distinctive types of microalgae. Addition of inorganic
coagulants such as iron aluminium or iron based coagulant will disrupt the established
microalgae system to enabled harvesting.
Materials: organic, inorganic, polyelectrolyte and bio-flocculants.
Throughput: Organic flocculants can induce the efficient flocculation of fresh water
at low dosages, between 1 and 10mg/L. Naturally organic substances, such as Chitosan
(polymer of glucosamine), that are used for water treatment are also used as a flocculant
with varied concentration (40mg/L to 150mg/L) in different algal strains (Tetraselmis
chui, Thalassiosira pseudonana and Chaetoceros muelleri) due to their non-toxicity
effect. Cationic polyelectrolyte and aluminium sulphate are the most promising and
potentially tested flocculants. While non-ionic and anionic polyelectrolytes are regarded
as ineffective flocculants due to electrostatic repulsion interactions resulting in bridging
Cost range: Flocculation is a bulk harvesting technique used for separating biomass
from bulk suspension and therefore requires less energy than the thickening process.
However, chemical flocculation was labelled by the Aquatic Species Program (ASP) as
too expensive for the production of biofuels due to the requirement of large doses of
Energy balance: Electro-flocculation which uses electrically charged particles to
induce flocculation has efficiency of over 90% but with an energy consumption of 0.3
kW/hm-3 (Alabi et al., 2009).
Issues: The main issue associated with flocculation is toxicity of flocculants. The
flocculants should be non-toxic, inexpensive and effective in low concentration, and
should have a positive effect on the further downstream processing. Lack of clear
relationship between the amount of flocculation efficiency, dose and the algal
taxonomic group is another burning issue that raises concern. (Greenwell et al., 2010).
Areas for development: pH changes and variation, stability of the flocculants,
toxicity, chemical properties, biodegradability, selectivity with some algae strains, rate
(dosage), flocculating power, molar mass of the polymers to initiate flocculation
Advantages of flocculation include: the method is relatively cheap and contains high
volume processing. Its disadvantages include: the low solids content of the biomass
harvest (<10%) that often requires combining with other methods, (Molina Grima et al.,
2003; Moreno-Garrido, 2008).
Summary: Centrifugation is a thickening process used for harvesting microalgal
biomass. It appears as a semi-continuous or continuous process, which under the
centrifugal forces generated by spinning of suspended particles, separates and harvests
algal cells. Centrifugation is an established and industrial based suspension separation
technique that has been investigated in algal harvesting (Molina Grima et al., 2003).
Materials: Centrifugation uses gravitational force to achieve separation.
Throughput: Centrifugation is yet another preferred method of concentrating and
recovering algal cells due to the rapid nature of recovery of those cells, especially for
producing extended shelf-life concentrates for aquaculture hatcheries and nurseries
(Molina Grima et al., 2003). This can be achieved more rapidly if the gravitational field
to which the cells are subjected is increased, based on the Svarovsky equation
(Greenwell et al., 2010). However, Knuckey et al. (2006) mention that subjecting the
cells to high gravitational and shear forces can damage their structure. The extent of the
recovery of the biomass depends on variables such as the residence time of the cell of
the slurry in the centrifuge, the settling behaviour of the cell and the settling depth.
Controlling the flow rate can help in controlling the settling time in the centrifuge, while
the settling rate is tractable during the design operation of the centrifuge (Molina Grima
et al., 2003).
Cost range: The cost increases as the scale of production increases. Cost and energy
saving, needs to be realized before embarking on any large scale application. Basically,
capital and operation costs are high compared to sedimentation, but the efficiency of
centrifugation is also higher. Moreover, centrifugation could be used as a secondary step
to increase solid contents to 20%.
Generally speaking, actual harvesting cost, coupled with preferable harvesting
technology, depends on strain morphology and a few variables such as algal species,
growth medium, algae production system (open or closed system), end product and
production cost benefit analysis.
Energy balance: Centrifugation is energy intensive. Energy requirement
consumption for various types of centrifuge is estimated to range from 0.3-8kW/h m-3
(Alabi et al., 2009). Molina Grima et al. (2003) have reported energy costs of about
1kW/h m-3 for centrifuges.
Issues: Basically, the main disapproving issues of using centrifugations may include
high energy and cost requirements. Selectivity coupled with recovery of micro algal
biomass, using centrifuges, settling characteristics of the cells, residence time of the cell
slurry in the centrifuge and settling depth are very important issues. Comparative
performance of centrifugal methods of recovering micro algal biomass including costs
and energy cost, labour, relative harvesting cost and reliability should be considered.
Areas for development: Optimization requires knowledge of algae properties such
as algal size, cell wall sensitivity to shear force, ease of flocculation and oil content.
Summary: Cell disruption simplifies the process of recovery and release of
intracellular products from the microalgal cellular matrix essential for fuel conversion
processes. Strong relationships exist between the harvesting process, extraction and thus
the final fuel conversion process. Cell disruption is, therefore, an integral part of the
downstream process of unit operation to biofuel production (Halim et al., 2012). The
extraction yield increases with cell disruption methods and overall yield of biofuel
(biodiesel) will depend on both the disruption method and device employed (Amarol et
The efficiency of cell disruption toward lipids extraction in microalgae is specific to
microalgae species and extraction method(s) employed (Amarol et al., 2010).
Types: There are two types of cell disruption method; the physical or mechanical
methods (high pressure homogeniser, bead-beating, microwaving, autoclaving, ultra-
sonication, and cavitation) and non-mechanical methods such as osmotic shocks,
enzymatic hydrolysis, pyrolysis and physico-chemical methods in alkaline and acid
hydrolysis (NaOH, HCl, and H2SO4) in autoclaves.
HIGH PRESSURE HOMOGENISATION
Summary: Pressure homogenisation is an industry-established method for disrupting
bacteria in waste-activated sludge (Frank et al., 2011). The process of high pressure
homogenisation is a mechanical cell disruption method which pumps cell suspension to
a high pressure through a narrow opening of a valve before the cell suspension is
released into a chamber of a lower pressure (Halim et al., 2012).
High pressure homogenisers impressively enhance the bioavailability and the
assimilation of pigments from the cells (Molina Grima et al., 2003). The method is
temperature dependent – proteins are released at elevated temperatures (50°C)
(Richmond, 2004). Conditions of the broth pH, ionic strength and temperature, coupled
with medium components and final state of the desired product, influence the method
Throughput: High pressure homogenisation is widely employed to disrupt the
Haematococcus cell for use as fish feed. Molina Grima et al. (2003) report it as
preferable to alkalysis and enzymatic hydrolysis for the process of recovering
astaxanthin from encysted cells of Haematococcus pluvialis. The method yielded three
times as much astaxanthin, but the applicability of the method is not realistic for large
scale production. Moreover, the method is much more effective for microalgal strain
Chlorococcum sp. disruption on a laboratory scale, with an average of 73.8% of
initially intact cells than the sulphuric acid treatment (33.2%), followed by bead-beating
(17.5%) and ultrasonication (4.5%) (Halim et al., 2012). The efficacy of the method is
found at 500-850 bars and the kinetics followed a first order model. Increasing the
operating pressure will increase the cell disruption, which simply means that the method
is a direct function of the operating pressure. At 550 bars, the method is found to disrupt
the Chlorococcum cell at a lower density of the culture stock than at a higher density,
due to a higher kinetic energy absorbed by the cell particles (Halim et al., 2012).
GEA Niro Soavi (manufacturing homogenisers), report a 79% homogenisation of
Chlorella per pass at 600 bars and 2000L/h indicating 365kW/h/dry ton for two passes
at 10wt% solids (Frank et al., 2011). Stephenson et al. (2010), report a process model
of 22wt% from the decanter centrifuge, approximating 168kW/h/dry ton for
homogenisation. Frank et al. (2011), report that, it is quite remarkable to achieve such a
rate due to pumping difficulties and homogenisation efficiency.
Cost range: Different cost-effective homogenisers offer a reliable and consistent
Energy balance: Pressure homogenisation energy consumption of 750 kW/h/dry ton
for 4-7wt% solids was reported by EPA (2006). Moreover, Davis (2010) reports
200kW/h/dry ton with 90% disruption efficiency. Greenwell et al. (2010) report a
typical energy consumption of homogenisers (operating at 100-150MPa) in the order of
1.5-2 kW/h to produce a 95% protein release for 10 l of process fluid or about 1 m3 of
the original micro algal culture fluid, assuming a cell concentration factor of 100 by
End product: High value products, nutraceuticals,
Issues: Biochemical characteristics of extracted molecules, limitation of solvent use,
reproducibility, extraction yield, selectivity, protection of extracted molecules against
chemical degradation, dimensions, easiness and cost.
AUTOCLAVING, MICROWAVING, SONICATION AND BEAD-
Summary: Autoclaving at high temperature and pressure, using strong heated
containers for chemical reactions, and processing at high temperature and pressure, have
been reported in many literatures to break cells (Amarol et al., 2010). Microwaving that
breaks cells using the shocks generated by high-frequency waves were used for
vegetable oil extraction. Sonication has been used to disrupt microbial cells (due to the
cavitation effect) and bead-beating (based on high-speed spinning with fine beads), has
found application on both a laboratory and an industrial scale (Lee et al., 2010).
a) Ultra sonication: Vibra ultra sonicator, frequency (40KHZ), Acoustic power (1W-
b) Beat-beading: Glass beads (1mm diameter), low density stock culture (1:3 or 1:2 v:
v), batch operation.
Throughput: Lee et al. (2010) conducted works on the extraction of lipids from
three different types of microalgae (Botryococcus sp., Chlorella vulgaris and
Scenedesmus sp.) by comparing autoclaving, bead-beating, sonication, microwaving
and osmotic shock and concluded that the microwave method of cell disruption is the
most promising method for lipid extraction due to its easiness, simplicity and efficiency.
Botryococcus sp. showed the highest oleic acid productivity at 5.7mg L-1 d-1 meaning
that it is a promising algae species for hydrocarbon generation.
Cost range: No detailed cost range was found in the literature.
Energy balance: No detailed energy balance was found in the literature.
End product: Proteins release and fluid extractions, coupled with extractions of
metabolites such as astaxanthin, βeta-carotene and fatty acids from algal biomass.
Issues: Adoption for large scale disruption is still a problem. This stems from the fact
that the effectiveness of the cell disruption methodology depends heavily on the type of
species due to the variability in the cell wall across species and algal growth cycle. For
example Chlorella sp. and Scenedesmus sp. both have rigid cell walls with high
cellulose content; on the other hand, Spirulina lacks a rigid cell wall and therefore is
easier to disrupt.
Areas for development: Identifying the correct cell breakage procedure for
identification of biological factors such as cell wall strength, size and shape of the cells,
coupled with genetic modification of various traits associated with this process.
Summary: The target of the drying process is to extend the viability of the desired
product and prevent the degradation process of the harvested biomass slurry (15-25%
concentration) (Brennan and Owende, 2010). Simply put, the purpose of drying is so
that the algal biomass is converted to stable, storable product.
Types: Sun drying, spray drying, solar drying, drum drying, fluidized bed drying,
freeze drying and refractive window technology (Brennan and Owende, 2010). Other
types include flash drying, and rotary dryers. The selection of the drying process
depends on the scale of operation and the use for which the product is intended.
Throughput: a) Sun drying is a slow drying method, weather dependent, large
surface requirement with low cost and energy requirement; b) Spray drying is expensive
involving capital and high material costs, and is energy intensive and susceptible to
deterioration in the quality of some algal pigment. Spray drying can be used for the
extraction of higher value products (HVPs) (Brennan and Owende, 2010); c) Solar dryer
is associated with long drying times and is weather dependent but with low energy
d) Freeze drying: Valensa International use this type of drying system without the need
for cell disruption. Algal biomass is froze at -20°C and extracted within two days.
Freeze drying is relatively expensive for large scale operations
e) Drum drying is fast and efficient but is both cost and energy intensive;
f) Flash drying is achieved rapidly by spraying or injecting a mixture of dried and
undried material into a hot gas stream, and is mostly applied to waste water sludge
g) Rotary dryers use a sloped rotating cylinder to move the material being dried by
gravity from one end to another and are also widely used for heat drying of waste water
Cost range and energy balance: Drying has a major economic importance and as
such it constitutes 70-75% of the processing cost. Drying as a pre-treatment process is
not an economical process due to the involvement of high energy requirements. The
various systems for algae drying differ distinctly in the relative extent of capital
investment and energy requirements. The selection of drying method, therefore,
depends on the scale of operation and the intended use of the dried product. Moreover,
removal of 1kg of water by drying requires more than 800 kcal of energy which simply
means that any reduction of water content by dewatering techniques is paramount from
both cost and energy points of view. Consequently, the process of drying is often
skipped; oil extraction is combined with a biomass concentration step through
Basically, the energy and cost requirements of harvesting, dewatering and/or drying will
depend on the final microalgae concentration for the chosen extraction method which
underlines the fraction of the total energy cost of any algae-to-fuel process and
embodies available energy from microalgae.
End product: Algal biomass, biodiesel and HVPs.
Issues: The major issue in the drying process is the maximum solid concentration
requirements for the commercial process of lipid extractions from microalgae. Drying
and dewatering of algae may be more significant if algae are to be transported from sites
of harvest to distant processing plants.
Areas for development: The most significant area is to consider the various
important parameters discussed above in selecting the best harvesting and dewatering
technologies through cost benefit analysis and the development of innovative
technologies for harvesting and dewatering methods.