Supplier Information Pack Algal Harvesting systems

lyricalwillingMécanique

22 févr. 2014 (il y a 3 années et 4 mois)

111 vue(s)

School of
Engineering

15 Mar 2012

Supplier Information P
ack


Algal Harvesting systems


Contents

HARVESTING

................................
................................
................................
.................

4

FILTRATION

................................
................................
................................
...................

5

SETTLING AND SEDIM
ENTATION

................................
................................
............

6

FLOCCULATION

................................
................................
................................
...........

7

CENTRIFUGATION

................................
................................
................................
.......

9

CELL DISRUPTION

................................
................................
................................
.....

10

HIGH PRESSURE HOMOGENISATION

................................
................................
....

10

AUTOCLAVING, MICROWAV
ING
, SONICATION AND BEAD
-
BEATING

........

11

DEWATERING AND DRYING PROCESSES

................................
............................

12



Note


This document is intended for the use of organisations that are interested in supplying
co
mponents 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 ar
i
sing out of any inaccuracy

in this
publicly available material.






HA
RVESTING



Algae broth ready to harvest

(0.3
-
0.5g/l)


Introduction:


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
-
40
µ
m). 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 (reco
very 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 rest
s

in reducing the
energetic consumptions of separating or isolating the micro

a
lgal cells from the culture
medium which ultimately determines the cost of production
. The cost of harvesting is
fundamental to the m
icro

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 th
e 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 coagula
nts,
organic flocculants, combined flocculation, electrocoagulation and ultrasonic
aggregation;



Centrifugation


algae can also be harvested using centrifugation;



Flotation


froth flotation is a method whereby the water and algae are aerated into
froth, w
ith the algae then removed from the water; and



Electrolytic method


such as the electrophoresis technique.

F
ILTRATION

Different modes of filtration can be used to concentrate micro
a
lgal cells.
The m
icro
-
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:

m
icro
-
screens,
m
icro
-
strainers, dead
-
end filtration, vacuum (pressure) filtration,
cross
-
flow f
iltration. Vacuum drum filters and chamber filter press
es

are the commonest
types of filtration applicable to fairly large microalgae.


Characteristics



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
filt
ers.


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 a
ble
to recover large amount
s

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 (2m
3

per day) membrane
filtration can be more cost effective compared to centrifugation. However, for
large
scale production (>20m
3

per day), centrifugation is more economical owing to
continuous membrane replacement

(Brennan and Owende, 2010).

Microalgae and
c
ynobacteria

pose outstanding filtration challenges because most strains
considered for energy feedstock have cell diameters less than 10
µ
m

(
http://biomass.energy.gov)
.

The c
onventional filtration process is suitable for
the
harvesting of relatively large (>70
µ
m
) mic
roalgae such as

Coelastrum
and

Spiruli
na
.
It
cannot be used for microalgae specie (>30
µ
m)
such as

Scenedesmus, Dunaliella
and

Chlorella.


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
. D
ecreasing 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
-
2
Kw/
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
micr
o
algal lipids to biodiesel via trans
-
esterification

(Khoo
et al.
,

2011


I
ssues:
culture purity, blinding
of
fi
lter 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)
t
he filter pore size
as relates to
the algae specie

and

alga
e

aggregation rate
;

b)
f
ilter material also influence
s

filtration and recovery
. The
materials should control hyrophobicity and algae affinity with durability and blinding;
c)
f
iltration 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
S
tokes
L
aw. Gravity sedimentation is ma
inly appl
ied

to
harvesting microalgae in waste water treatment because of the large volumes treated and
low value of the biomass generated (Brennan and Owende, 2010).


Characteristics


Material:

Gravity sedimentation is simple with low cost energy

requirem
ents
.

Sedimentation is a simple and very
s
low 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.>70
µ
m) such
as
Spirulina.

For

algae with

poorer sedimentation

properties
, a force flocculation is
induced through the addition of chemicals or culture flocculation.

The f
lotation process
by sedimentation is, however, very fast,
as
it require
s only a

few minutes for
sedimentation. Capital and operating cost
s

are also low, but the efficiency is poor in
shallow
-
depth pond
s

(Brennan and Owende, 2010
; Singh
et al
., 2011
).


Cost range:
Sedimentation is performed in thickeners and clarifie
s standard
processes in water treatment plants. The capital and operating cost
s

are low (Singh
et
al
., 2011).
F
or 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.



I
ssues:
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 de
velopment:
S
edimentation

velocity, sedimentation tanks,

lamella
separators

are the areas that need to be further explore.

FLOCCULATION




1
-
3% mass solid


S
ummary
:
Following the poor sedimentation properties of some algae strain
s

due
to
inherent si
z
es and

density, chemical methods can be used succe
ss
fully 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 electr
olytes 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

c
hemical coagulation
.


Auto
-
flocculation

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 CO
2
.


Bio
-
flocculation

Culturing microalgae with a
nother 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)


Chemical coagulation

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 dis
rupt the established
microalgae system to enabled harvesting.


Characteristics


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 substance
s,

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
-
ioni
c and anionic polyelectrolyte
s

are regarded
as ineffective flocculants due to electrostatic repulsion interactions resulting in bridging
mechanism
s
.


Cost range:
Flocculation is a bulk harvesting technique
used for se
parating biomass
fr
o
m 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
chem
icals.


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).


I
ssues:
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.
L
ack of clear
relationship betw
een 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 hi
gh
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)
.

CENTRIFUGATION

Summary:
Centrifugation is a thickenin
g process used
for
harvesting microalgal
biomass. It appear
s

as a semi
-
continuous or continuous process
, which

under the
centrifugal forces generated by spinning of suspended particles, separate
s

and harvest
s

algal cells.

Centrifugation is an

established a
nd

industrial based suspension separation
technique that has been investigated in algal harvesting (Molina
Grima
et al.,

2003)
.


Characteristics


Materials
:
Centrifugation uses gravitational force to achieve separation.


Throughput:

Centrifug
ation

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 an
d 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 flo
w 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
.
C
ost a
nd energy
saving
,
needs to be realized before embarking
on any

large scale application
.

Basically,
capital and operation cost
s 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 harvesti
ng
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. E
nergy requirement
consumption for various types of centrifuge is estimated to range fr
o
m 0.3
-
8
kW
/
h m
-
3

(Alabi
et al.
,

2009).
Molina Grima
et al.

(2003)

have reported energy costs

of about
1kW
/
h m
-
3

for centrifuges.


I
ssue
s
:

Basically, the main disapproving

issues of using centrifugations may include
high e
nergy and cost
requirements.

S
electivity
coupled with

recovery of micro

algal
biomass
,
using centrifuges,
settling characteristics of the cells, residence time of the cell
slurry in the centrifuge and sett
ling 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:

Optimizat
ion requires knowledge of algae properties such
as algal size, cell wall sensitivity to shear force, ease of flocculation and oil content
.

CELL DISRUPTION

Summary:

Cell disruption simplifies the process of recover
y

and release of
intracellular products fr
o
m
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 pr
ocess of unit operation to biofuel production (Halim
et al
.,

2012).
The
e
xtraction yield increases with cell disruption methods

a
nd overall yield of biofuel
(biodiesel) will depend on both
the
disruption method and device employed (Amarol
et
al.
,

2011).

The e
fficiency of cell disruption toward lipids extraction in microalgae is specific to
microalgae species and extraction method(s) employed (Amaro
l

et al.
,

2010).


Types:
There are two types of cell disruption method
; the physical or mechanical
methods (
h
igh

pressure homogeniser, bead
-
beating, microwav
ing
, 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 H
2
SO
4
) in autoclave
s
.

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
h
igh pressure
homogenisation is a mechanical

c
ell 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 bi
oavailability and the
assimilation of pigments from the cells

(Molina Grima
et al.
,

2003). The method is
temperature dependent



proteins
are
release
d

at elevated temperatures (50
°
C)
(Richmond, 2004). Conditions of the broth

p
H, ionic strength and
temperature
,

coupled
with medium components and final state of the desired product
,

influence the method
(Najafpour, 2007).


Throughput:

High pressure h
o
mogenisation 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
, b
ut the applicability of the method is
not realistic
for
large
scale production. Moreover, the method is much more effective for microalgal strain
Chlorococcum sp.

d
isruption
on a

laboratory scale
,

with an average of 73.8% of
initial
ly

intact cells
than

the
sulphuric acid treatment (33.2%)
,

fo
llowed by bead
-
beating
(17.5%) and

u
l
trasonication (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
e
t 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
process
.


Energy balance:

Pressure homogenisation energy

consumption of 750 kW
/
h/dry

ton
for 4
-
7wt% solids was reported by EPA (2006). Moreover, Davis (2010) rep
ort
s

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

m
3

o
f
the original micro

algal culture fluid
,

assuming a cell concentration factor of 100 by
mass.


End product:

High value products
, nutraceuticals,


I
ssues:

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, MICROWAV
ING
, SONICATION AND BEAD
-
BEATING

Summary:

Autoclaving
at h
igh t
emperature and pressure
,

us
ing strong heated
containers for chemical reactions
,

and processing at high temperature and pressure
,

have
been

reported in many literatures

to break

cells (Amaro
l

et al.
,
2010
). Microwav
ing

that
break
s

cells using the shocks gen
erated 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
labor
atory and
an
industrial scale (Lee
et

al
., 2010)
.




Types

a)

Ultra sonication: Vibra ultra sonicator, frequency (40KHZ), Acoustic power (1W
-
130W)

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, microwav
ing

and osmotic sho
ck 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

extract
ions
,

coupled with extractions of
metabolites such as astaxanthin,
β
eta
-
carotene and fatty acids from algal biomass.


I
ssues:

Adoption for large scale disruption is still a problem. This stems from the fact
that the effectiveness of the cell disruption

methodol
ogy 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

lack
s

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 mod
ification of various traits associated with this process.

DRYING PROCESSES

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,
s
pray drying,

solar dry
ing
, drum drying, fluidized bed drying,
freeze drying and refractive window te
chnology (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 metho
d,

weather dependent, large
surface requirement with low cost and energy requiremen
t
; b) Spray drying is expensive
involving capital and
high
material cost
s
, 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

(HV
P
s)

(Brennan and Owende, 2010); c) Solar dryer

is

associated with long drying times

and is

weather dependent but with low energy
requirement;

d) Fre
eze 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
(
http://www.algaeindustrymagazine.com
);

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 ho
t gas stream
,

and is mostly applied
to
waste water sludge
drying;

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
sludge.


Cost ran
ge

and

e
nergy balance:

Drying
has
a major economic

importance

and

as
such it constitute
s

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 energ
y

requirements.
The s
election 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 energ
y

point
s

of view. Consequently, the proce
ss of drying is often
skipped; oil extraction is combined with
a
biomass
concentration step through
centrifugation.

Basically, the energ
y

and cost requirement
s

of harvesting, dewatering and/or drying will
depend on the final microalgae concentration for th
e chosen extraction method
which
underline
s

the fraction of the total energy cost of any algae
-
to
-
fuel process and
embodie
s

available energy from microalgae.


End product:

Algal biomass,

b
iodiesel and
HV
P
s
.


I
ssues:

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 processin
g 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
techno
logies for harvesting and dewatering methods.