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SERI/STR 231 2396
UC Category:
61a
DE84013036
Microalgae Harvesting and
Processing Literature
Review
A
Subcontract
Report
G
Shelef
A
Sukenik
M
Green
August
1984
Technion
Research
and Development
Foundation
l td.
Haifa
Israel
Prepared under Subcontract No.
XK 3 03031 01
SERI
Technical Monitor:
Robins P
Mcintosh
Solar Energy Research Institute
A Division of Midwest
Research
Institute
1617
Cole Boulevard
Golden
Colorado
80401
Prepared for the
U S Department of
Energy
Contract
No
E AC02 83CH
10093
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Printed in the
United
States of
merica
Available from:
National Technical Information Service
U S Department of Commerce
5285 Port Royal Road
Springfield VA 22161
Price:
Microfiche A01
Printed Copy A04
NOTI
This repor t was prepared as an
account
of
work
sponsored by the
United
States
Government Neither the United States nor the United States Department of Energy
nor any of their employees
nor
any of their contractors subcontractors or their
employees makes any warranty express or implied or assumes any legal liability
or responsibility for the accuracy completeness or usefulness of
any information
apparatus product or process disclosed or represents that its use would not
infringe privately owned rights
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FOR WOR
This
report
is a literature rev ie w on mic ro al ga l
harvesting
and processing
submitted
as
partial ful fi ll me nt of subcontract XK-3-03031-01. The work was
performed
under
subcontract
to SERI with funds provided by
the
Biomass Energy Technology Division of
the
U.S.
Department
of Energy.
~ t ~ ~ / ~
Robins P. McIntosh
Coordinator
Aquatic Species Program
tein
Coordinator
ram
Office
Stari1eYR .
tillirector
Solar Fuels
Research
Division
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SU RY
Objective
The objective
of
this
report
is to
present
a discussion of
the literature
review
performed
on methods of harvesting microalgae.
iseussion
There is no single
best
method of harvesting mieroalgae. The choice of
preferable
harvesting technology depends on
algae
species, growth medium,
algae
production, end
product, and production
cost benefit
Algae
size
is an
important
factor since low-cost
filtration
procedures are
presently
applicable only for harvesting fairly
large
microalgae. Small microalgae should be
flocculated into larger bodies
that
can be
harvested
by one of
the
methods mentioned
above. However,
the
cells mobility
affects the
flocculat ion process, and addit ion
of
nonresidual oxidants to
stop
the mobil ity should be considered to aid flocculat ion.
The decision between
sedimentation
or
flotation
methods depends on
the
density
dif ference between the algae cell and the growth medium. For oil-laden
algae
with low
cell density, flotation technologies should be considered. Moreover, oxygen
release
from
algae cells and oxygen
supersaturation
conditions in growth medium support
the
use of
flotation methods.
high-quality algae
are
to be produced for human consumption, continuous
harvesting
by
solid
ejecting
or
nozzle type
disc
centrifuges
is recommended. These
centrifuges
can
easily be cleaned and
sterilized
They
are
sui table for all types of microalgae,
but their
high
operating
costs should be compared with the
benefits
from their use.
Another basic
criterion
for
selecting
the suitable
harvesting
procedure is
the
final
algae
paste concentration
required for
the nex t
process. Solids
requirements
up
to
30 can be
attained
by established
dewatering
processes.
For
more
concentrated
solids, drying
methods are required.
The various sys tems for algae drying
differ
both in
the extent of capital
investment
and
the
energy requirements Selection of
the
drying method depends on
the scale
of
operation
and the use for which the dried product-is intended.
onclusions
The
literature
review on microalgae harvestfng technologies does
not reveal
any
revolutionary conceptual advances since
the first
comprehensive study done by Golueke
and Oswald (1965). Never theless , opt imiz ing various
trains
of
processes
can
not
only
reduce
the
cost, but can render
the
whole
scheme
economically feasible. The exist ing
literature is not conclusive enough to propose such optimal train of
harvesting
processes,
and the continued work of
the
Technion Group on this
project
will try to establish
these
optimal processes.
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1 Introduction
T LE OF CONTENTS
2 The stability of microalgae with
respect
to their
separability
from aqueous suspensions
2 1
2 2
Colloidal character of microalgae suspension
Algae
sedimentation rate
4
4
9
3
Flocculation of microalgae
4
Algae harvesting technologies 16
4 1 iltration screening straining
18
4 2
Sedimentation
31
4 3 lotation
34
4 4
entrifugation
40
5
6
Algal drying
Summary and conclusion
v
49
56
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1.
INTHooucnorl
Mass culture of microalgae carl be ~ r t i e to atta in d i f l e r e n t
obj ect i ves
such
a s:
production
o f hydrocarbons, proteins,
var i
ou s
o r
gan
i
c subs
t
an c e s
wastewater treatment, s o l a r energy
conversion
or
combination of ~ h e above.
An b l g a l
ni
s s
cu
I
t ur e is
t
t a l n ab l e
in outdoor ponds un rer
su
i
t ab I e cl i m at i c
c o n d itio n s .
High rate a l g a l pond HHAP is an or en
photosynthetic r
eact.or
wh i ch is
operated
for mass
blbBl
cu I
t
ur e and
intended
to
maY/mize al gal production
pe r
uni t
o f a re a . I t consi st s
t
he
ref or e of
a
sh a
l l o w
rc.:lCe-way
or meande r Lng
channel
pend Where
mixing
is
provided to keep the
algae
in
suspension.
Tile op
e r
a t i
onal
kr.owhot, an d th e sc ien t i f ic
b ac k g
ro u
nd
of micro-
algae production in
HRAP
are well based on long experience. The
scien t i f ic
foundamentals,
th e oper
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defined
s a l
t
s wLich are
di
ssoLved
in water,
the aLgal
separ
a
t i
cn and
concentrat ion is essent ia l for the further processing
steps
according
to the
desired
end
produc
t ,
The
resultant algal
I r
ee
culture
med
i
ur,
be
recycled,
af ter
nutrients
addition, into
the
/lEAP.
iviicroalgae by their
swall size
(5-50
their
negatively
charged surfaces
and
in
some cases their
mobil i ty, f orrn st ao
l e
suspensions and
hereby
di f f icu l t ies in their separation ane
recovery
(Tenny
e
t.
a
I .• 1fj6i3).
Technological
solut ions for
s epar at ion o f
algae
from
that
st.abl e
suspensions should
be
gi ven in tLE
processing
sequence
of mi c roa I gae production Fig
1 1
The
term a lg ae harvesting refers to the concentration of fa ir ly
diluted (ca. O.02-0.06X T ~ S
algae
suspension unti l
a
slurry or paste
containing 5 2 5 ~
m
more is
ob
t a i ued ,
As
i t is
i
nd i
cat.ed in
Fig. 1 1 such
concentrated slurry is attainable by one
step
harvesting ~ r o c e s s or by
two
s t e ~ process
consisting
of harvesting
step
which b rings the
algal slurry to
2-7 TSS, dnd
dewatering step
wlose end product is an
algal
past{;: of 1 ~ 2 5 TSS. The concentration
of
the
resultant algal paste
or
slurry greatly influences the
sub sequen tia l p roce ss ing steps as drying or organic
extraction
(Hahn 1978
1980).
substances
The methods and
devices
I-. hich
ore suitable for microalgCie
s ep ar at.ion from HR P effluent depend on the
al
gae
species,
the
production system
and
the objectives
of the
final
product.
l iohn
1980, Docd , 19bO).
This review
deals with separation
and
processing
n.e
tt ods
of
microuLg
ae
from the pond eff luent .
The s
t
ab
i
Li
t.y
of
micr-oa Lgae suspensions, and
the principles
which may be used to
2
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One
s te p c on ce ntr ati on
Two s t ep s c o nc ent ra t io n
b
2 10
Algal
cake
a 15
5
drying
Processing
ex t rax t ion
1
r r
I
~
I
I
I
dewater ing
Algal
Slurry
harvesting I
~ I a 2
I
b
100
200
14
L
High Rate
Algae ff luen t
Product ion s tep
a 0.02
0.06
a
a lga l
concen t ra t ion
TSS
b concen t ra t ion
fac to r
Fig
1. 1
-Schematic
presen ta t ion
of
a lga l
product ion an d process ing.
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and other microorganisms
separation
and
overcome i t
technologies
during
the
for
algae
separation step, are
discussed.
Updated
processing are, c r i t i c ~ l l y rev iewed. Promissing
technologies are
recommended
fo r
further
improvement end
application
for
oil
laden
~ l c r o l e
separation.
Tfi STJWILITY
OF
MICRO LG E WIT RESPECT
TO
T IR SEP R ILITY
RO QUEOUS SUSPENSIONS
The HR P effluent c on sis ts o f a culture medium
containing
micro-
algae biomass which form stable suspension.
There are
two factors
which affect the s tabi l i ty
of
that suspension:
a
algal
surface
repulsion forces.
electr ic
charge
which causes the
development of intercel lular
b) tiny
cel l
diffiensions and cel l density close to
that
of
the medium cause slow cel l sinking ra te .
2.1
The
colloidal
character
of
an
algal
suspension.
Both
th e
electr ic repulsion
interactions
between algal cel ls and
cel l
interactions
with
t he sur rounding
water
con tri bu te t o
th e
s tabi l i ty
of the
algal suspension (Tenny
et a l
1968 . Most
of
the p lank toni c
algae
are characterized
as
negatively
charged
surfaces.
The intensity
of
that charge
is
a
function of algal
species, ionic
strength
of medium,
pH
nd other environmental
conditions lves
1959
Hegewald 1972 . The sources of
the
algal
surface
electr ic charge a re : i on iz at ion of ionogenic
functional groups at
the
algal cel l wall
Golueke
Oswald
1970)
and
selective
adsorption of ions from the culture medium (Shaw
1969).
4
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The electric
s tate of
a
surface depends
on the spat ial dis-
tr ibution of free charges ions) in i t s neighborhood
(Stumm
Morgan 1981) and
is idealized as
an electrochemical double
layer.
One
layer
is
described
as
a
fixed
charge
attached
to
a
part icle surface and is called the Stern
layer.
The other is
called Gouy layer or diffuse layer which
contains
an
excess
of
counter
ions ions of opposite sign
to the fixed charge)
and a
defic i t of co-ions of the same
sign as the fixed
charge). The
dis tr ibution of ions
and
potential at solid
solution
interface is
described in Figure 2.1.1.
::loUd-solution
Interface
®
e
I
I
I
Potential
f ul\
Concentration rnM
Counter ions
+
or
Co-ions
-
Fig. 2.1.1 - The dis tr ibution of ions and potentia l a t so lid
solution interface.
5
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Neithe r the potential at th e
surface
1 ;
nor
the Stern
a
potent ia l 1 ;_ ,nor the pote ntia l a t the
border
of Stern and
diffuse layer can be direct ly measured. Instead, the zeta
potential - the potent ia l measured at the shear plane that
separates the solid
surface
and the mobile l iquid) , is the one
generally
used and
is
obtained by simple electrokinetic methods.
The
zeta
potential is assumed to be
equal to ;d
although i t is
not necessari ly
correct.
A simplified formulation valid for small potentials) shows
th e potential decreases
exponentially
with the distcJOce
2.1.1
where 1 ; is the potential at a distance X and K is the reciprocal
of the
double
l ayer thi ckne ss and is
defined
by equation
2.1.2.
where Z is
the
charge
of
counter ion whose
concentration is
n
o
k is bolzman
constant,
Kelvin
temperature
and e is a basic
charge.
The above equations show that the
electr ic
pote nti al a t
a
gi ven distance in th e d if fu se d layer is affected by the valency
of th e counte r ion and
i t s
concentration. Compression
of the
l tri
double
layer
is attainable
either by
i nc reas ing the
counter ion
concentration
or
by
using
counter
ions
of higher
6
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valency.
The
interaction
between
colloidal
part icles
are
affected by
the
electr ic
repulsion
forces
on one hand and
attrac t ion
forces
of Van-der-Waals on
the
other hand. The combined
effect
of
those
two energies
i s
shown in
Figure 2.1.2.
There
i s
a potential
barrier to be overcome i f attachment is
to
be
attained.
I t
can
be
exceeded
by
th e kinetic
energy of the part icles
or
alternati
vely
by th e reduction of the energetic barried. This is done by
compressing the
double
layer
increasing
K through addition of
electrolytes to the solution
or ions
of
higher
valences.
I
I
I
Double-layer repulsion,V
R
Resultant potent ial of
interact ion,V
T
Potent ia l
bar r ie r
Part ic le distance
der
ls
at t ract ion V
Fig. 2.1.2 - Combined
effect
of electr ic repulsion and
Van-der-Waals attraction energy Ref. tu
Morgan 1981 .
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Although the double layer
theory i s
o f
g r e a t
t h e o r e t i c a l
importance, i t s us e i s res tr ic ted to cases
where
specific
chemical
interactions
do
not
p l y
r ol e
in
col l oi d
s t bi l i ty
O Melia 1978). D est abi l i zat i on of
colloidal suspension
as
a
r e s u l t o f specific
chemical i nt er act i on
i s
at t ai nabl e
by the
presence
of
pol yel ect r ol yt es
or
p o ly h yd r ox y c o mp l ex e s.
Hydrolysis of
metal ions fo r example Fe H 0)3.+ and
2 6
A I H ~ O 3 + )
i s
d es crib ed as a
stepWise
consecutive
replacement
of
6
H
molecules in the hydration s h e l l by
ions Stumm
C Melia, 1968), according to the scheme shown in F ig . 2 . 1 . 3 . The
e f f e c t s
of
ferr ic
and 21uminium
s l t s
are
b ro ug ht a bo ut
by
the i r
hydrolysis
products
and
not
tr y
th e sim ple aqua-m etal ion them-
sel ves. Over dose of the h yd ro xo co mp l ex es
ca n
res t bil ize
d is p e r s io n s a r e v e r s a l
of
the charge
of th e colloidal
p a r t i c l e s
Fig.
2 . 1 . 3 - Stepwise
conversion of a
posi t i ve
aluminium
io n
in to
negative on e
Ref: Stumm and
O Melia
1968).
8
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Organic polymers, usually those of quite high m o l e u l ~ r
weight
are considered
as
good
flocculants.
The
polymeric
f locculat ion
is explained by bridging model
says that
a polymer can attach
i t s l to the surface of a
colloidal
particle by several segments
being remainder
segments extended into solutions. These segments
are
then
able to attach
on
vacant si t s
on
other particle
forming
a three
dimensional
floc network Gregory 1979
Destabilization
and flocculation of algal
suspension is
an
important
procedure in most of the various algal separation
process and
is
described separately in a
following
section.
2.2 Sinking rate
of
microolgae.
Planktonic algal cell can be
considered as
a body which fal ls in
aqueous
medium and
is affected
by the
g r ~ i t y force
on one hand,
and drag
forces
on th e other.
Within
a short time this body
exceeds
constan t s inking velocity which is described by Stokes
law
eq . 2.2.1
v =
9
2.2.1
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where V is the f l l in g velocity, g -
gravity
force,
d - p a r t i c l e
diameter, p and pI th e d en sity of the me iul l and the
p r t icle
respectively, and
r is
the medium
v is c o s ity .
~ o r d i n e to
Eq.
2.2.1,
the
f a l l i n g
vel oci t y
of
a ~ o d y
decreases e i t h e r by increasing medium viscosity o r red uc ing
th e
cell-medium
d e n si ty d if fe re n c e
or
by decreasing
c e l l diameter.
Stokes cl l
s
; ipplicatJle or
s pher i cal
bodies
and any di
ver s i ty
from
s p h e ric ity reduces
th e sin kin g r a t e , inversely, to the
c o e ffic ie n t of
form r e s i s t e n t ¢
v
2 . 2 . 2 )
while ¢ is a
dimensionless
parameter and
cal cul at ed
from the
r
a tio of
t he
sinking
rate
o f sphere of the
same
diameter
and
t h a t
of
the ~ t u l
bOdY.
The sinking vel oci t y of planktonic algae in nat ur al habitat
is
disturbed
y
c e l l mobility,
water
turbulence and
upwelling
caused by winds and
temperature str t if ic t ion
Hutchinson,
1967).
P la nk to ni c a lg ae in
ecosystem
reduce
t h e i r sinking ra te
by the f oLl
ow.i
ng
methods:
a)
mo tility ,
b)
reducing
c e l l
dimensions, c) increament of the
drag
forces as in Scenedesmus
species
which contain
s e ta
(Conway
Trainer 1973),
d)
reducing
c e l l density
as
in many blue
green algae
which contain ga s
vacuoles
(Fogg
1975, Pearl
Ustach
1982).
I n8 re as in g o f al gal c e l l sedimentation rate
ca n
be obtained
by increasing c e l l dimensions,
i e
by c e l l s aggregation into
10
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large body.
This principle is
applied in algal separation
processes where chemical flocculants
are
added and cause large
alg al flo es which set t le ra pid ly to
the
container bottom.
Alternatively, t iny a ir
bubbles which may adsorbe to
the already
formed
algal floes
will reduce dramat ica lly the floc
density
and
cause
the
floc to f loat . I nc reas ing the gravity force
will
increase the sed imen ta tion
rate
of algal cells and is attainable
by p p l y i ~ g centrifugal forces on
algal suspensions.
3.
MICROALGAE
FLGCCULATION
Add ition o f
chemiCals to algal cultures
in
order to induce
algae
flocculation is a routine p rocedu re in various
separation technologies
as: sedimentation
Friedman e t
81. 1977,
Mohn 1980 ,
flotation
Moraine
e t
a1.
1980 , f i l t rat ion ohn 1980 1978) and
centrifugation
Golueke
Oswald 1965, Moraine et a l 1980 .
Therefore, a brief discussion is dedicted herein to a lg al flo cc ula tio n
methods and flocculants.
The various h e m i ~ l s which were studied as a lgal f loccu lant s can
be broadly divided into two groups: a
inorganic agents including
polyvalent metal ions as Al+
3
and Fe+
3
which form
polyhydroxy
complexes
a t
suitable pH as shown in fi gu re 2 .1 .3 . Lime Ca OH Z
flocculation is a common technique in
water
and
wastewater
treatment.
t involves
raising th e pH with lime to
the
point
at which
Mg OH 2 is
formed and acts as
ultimate
flocculant Folkman Wachs 1973,
Friedman e t
a l
1977 .
b)
Polymeric
organic
flocculants
which
may
be
anionic,
cationic
and non
.ionic.
The term
polyelectrolyte
is
generally used to describe polymeric flocculants
inclUding
the
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nonionic speci es,
s y n t h e t i c
and natural polymers (Stumm Ivlorgan 1981)
as i s shown in
Table
3.1.
Various fl occul ant s were
evaluated e i t h e r
by
batch flocculation
~ x p e r i m e r . t s
J a r
t e ~ t s or by p i l o t scal e apparatus. Table 3.2
summarizes
th e d i f f e r e n t
fl occul ant s which Here t e s t e d for al gal
flocculation and t h e i r operating conditions,
primarily pH
and
optimal
dose as
reported in the l i t e r a t u r e .
Alum, Al
2
SOL; 3 x 18
H
2
°
o r
other
s a l t s
of aluminium
were used
as
flocculants in many branch and
field
scale experiments Golueke
Oswald 1965, I lcGarry e t a l , 1970, Hor am e e t a l , 1980). Ferri c
s u l f a t e was used too,
but
found to be
i n f e r i o r
in comparison with
alum,
regarding
the
optimal
dose,
pH
and
th e q u a l i t y
of the r e s u l t a n t
water
and
s l u r r y bare
e t
a1 . 1975, r ;oraine
e t a1 .
1980) •
Table 3.1 - Some ;jynthetic and Natural Polymeric Floc c ula nts .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - - - - - - - - - -
- - - - -
NONlONIc NIONI
TIONI
a ) Synthetic polymers
r : ~ H - C H 2 - 1
L
ONH
n
polY
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FLOCCULANT
TYPE
OPTIMAL DOSE
OPTIMAL
pH
TESTING
SCALE NOTSS
REFERENCES
mg/l
A12 S04)3 18
H
O
Polyvalent
metal
80-250
5. 3
5 .6
Sedimentation
was t
ewat
e r Moraine e t
a l 1980
io n
I
flotat ion batch
system Friedman e t
a l 197
EXI>er.
Pi l o t
s c a l e
experiments
sULfate
Polyvalent
m,:,tal
50-90 -
9. 0 batch an d
pi lot
c l e a n an d
Funk et
a l
1968
io n
flotat ion
u n i t s wastewater Bare
e t a l
1975
s ys tems
t r e a t m e n t
p o s i t ively
charged 500-700
10.5-11.5
batch
sedimenta- wa s t e v a t e r
Folkman
Wachs
197
Ng OH)
m e ta l h y dr ox i de
t io n experiments
systems
Friedman e t
a l 197
precipi tates
polymers
P u ri fl o c
35
3. 5
batch
wastewater
Moraine
e t
a l
syscems
Zetay 51
polyethylene
amine
10.
>
9
batch
Dow
M
Polyethylene
amine
10
4
·7 batch
clean Ti l t o n
e t
a l
system
Dow
C31
Polyamine
1
5 2
- 4
batch
c l e a n
Tenny
e t
a l
system
Chitosan
d i a c e t y l a t e d
polymer
100
8 .4
batch
clean
Venkataraman e t
a l
of chi t in
sy s tem
1980
Table
3. 2
D i f fe r en t f l oc c ul an t s an d their
optima pH
an d
dose)
f or a lg ae f lo c c u la tio n .
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Although
good
clar t f ication of
algal pond effluent-
has been
achieved
by lime t reatment Folkman
Wachs
1973,
Shelef e t a l
1978,
Fr-Ledman et a I ,
19 (7)
that f locculant is restr ic ted to cultures which
contain magnesium
concentration
o v ~ 10
mg/l
and the resul tant sludge
consisted more 0 - lime t han of a lgae containing up
to
25
calcium.
Organic polymers
were tested as
algal
f locculant on hatch scale.
Only the c
at.i
on
i
c polymers were
found as ef f ic ien t
flocculants Tenny
e t 81. 1968,
Tilton
e t
a
l ,
1972,
Hor-a
I ne e t
a1. 1980).
In
addition
polymers
n
be used
in
conjunction
with
alum or ferr ic sul fate
to
improve the separat ion process , while
anionic
polymers improve 1 ime
flocculation (Friedman
e t
a l 1977).
Tenny
e t a l 1968)
and Tilton e t a l
1972) explained
~ l g l
polymeric flocculation by
adsorption
and bridging model and
studied
few
parameters
which
affect
th e
phenomena.
w
molecular
w i g ~ t
cationic polymers ei ther do not cause any flocculation or are required
in very h igh concentrat ions. At higher molecula r weight
polymers the
optimal dose will
decrease
with increasing
molecular
weight, however,
very
high molecular
weight polymer will rev er sed th e algal surface
charge and
stabi l ize
the suspension (Til ton et
a1 .
1972). The
hydrogen
ion
concentration as
well as
medium e lec t ro l i te concentration
influence
th e
surfa-.oe charge density of
th e
a lg al s ur fa ce ,
th e
degree
of ionization charge
density
and th e e xte ntio n
of the
polymer and
subsequently
the
whole f locculat ion process.
Variations in
algal
concentrations (algal surface area) would influence the
concentration
of
polyelectrolyte required
for
a given degree of flocculation and
there i s a
def ini te stechiometry
between algal concentration and
polymer
dosage
for
algal
f loccul a t
i
on Tenny et
a1 .
1969),
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The
chemical
composition
of algal
medium
may
affect th e
flocculation optima
1
e. dose and pH .
For
lime
treatment
process
where
Mg OH)2 precipi tates
and
act
as
a
flocculant,
i t
was found
that
the
higher d isso lved organ ic substanceS (measured by
,COD)
in
the,
algal
suspension,
the
higher
was
the dose
of
Mg OH)2
r equ.i r-ed
for
good
flocculation of
the
algae
(Folkman Wachs 1973 .
Inhibit ion of
flocculation
processes caused by
the presence
of dissolved
organic
substance of biologic origin was
observed
by other invstigations as
well (Hoyer Bernhardt
1980, Narkis
Rebhun 1981 .
On th e
other
hand,
Tenny et a l , 1969 showed
that
algal
exocellular
organic
substances decrease the
optimal
f locculant dose
during
th e
early
declining growth phase, whereas accumulation of
these
substances
during the
la te growth s tages i nc reases the
optimal
dose evidently due
to the
organics which
serve as protective colloid.
Moraine
et
l 1980 pointed out that th e
soluble P0
4
concentration is an important factor which
influence
the alum optimal
riose.
The
required
dose of alum may be
described
by
2 .3 .1
where
is
th e
alum
dose
m
(PO-
3
th e soluble
phosphate
4 S
mt- ;,
TSS, the
suspended solid concentration gil
and k
is
alum
specific
dose
mmole Al+
3/gTS8.
The coefficient k should be a fun ction o f
effluent
character is t ics . However, i t was
not correlated with
such
parameters
as
alkal ini ty, NIl
3
BOD, but weakly correlated with temperature and
algal
type
Shelef et a1. 1981 .
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The many variables which affect th e flocculation process make th e
prediction
of
th e
operational
conditions impossible and they should be
evaluated by bench scale experiments as ja r t e s t
The
apparant
spontaneous
floc
formation
and
settling
of
micro-
algae
has
been mentioned in
th e l i t e r a t u r e
fo r two decades. The
phenomenon was termed autoflocculation .
In
some cases
this
phenomenon
is
associated
with elevated
pH due
to
photosynthetic O
2
consumption
corresponding
with
precipitation o f i no rg an ic precipitates
mainly
calcium
phosphate which
cause th e flocculation
Sukenik
Shelef
in press . Aside from
this
coprecipitative
autoflocculation,
th e formation
of algal aggregates
can
also
be due
to:
a) excreted
organic macromolecules Pavoni et
al e
1974,
enemann
et ale 1980), b)
inhibited release
of
microalgae
daughter c e l l s
Arad et al e 1980) and
c) aggregation between microalgae and bacteria Kogura et al e 1981).
4.
LG E
H RVESTING
TECHNOLOGIES
Solid-liquid
separation
processes can be clas s ified
into
two
kinds
of separation.
Svarovsky 1979a). In
th e f i r st , t he
liquid
i s
constrained in a vessel and particles can move freely within the
liquid. Sedimentation and flotation fall into
this
category. In th e
second
kind, the particles are
constrained
by a
permeable
medium through which th e liquid can flow. F i l t r a t i o n and
screening
can
f i t
this definition.
Fig.
4.1 shows further
sub-divisions
within
both
of
these categories. Density difference between th e solids and
th e liquid
are
needed
fo r
gravity or centrifugal
sedimentation.
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separa t ion
Liquid Const ra ined ,
pa r t i c l e s f ree
Par t i c l e s
cons t r a i ned ,
Flo ta t i on
d i spe r sed - a i r ,
dissolved a i r ,
e l e c t r o l y t i c
Gravl ty
sed imenta t ion
t
i c
k
e ne r s
c l a r i f i e r s
Fixed wal l
Chydrocyclones
Cent r i fugal
sed imenta t ion
Cake
f i l t r a t i on
vacuum,pressure ,
cen t r i f uga l
Ro ta ti ng w a ll
Csedimenting
cen t r i fuges
Deep-bed f i l t r a t i on
sand and coke
Screening
dewa te r in
vibra t ing
screens
Figure
4 : Class i f i c a t ion of common
i n d u st ri a l s o li d -l iq u i d separa t ion t echniques
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4.1
4.1.1
Background
F i l t r a t i on
an d Screening
F i l t r a t i on
and sc re en In g p ro ce sse s
both
s epa ra t e so l ids from
l i qu ids
by
pas ng s u s p e n s t.on t hr ou gh p er meab le medium t ha t r e ta ins
t.he
so l i d s .
r e e n ~ n g
The
pr inc ip l e
of screen ing is in t roducing par t i c l es onto a screen
of i
ve
n aper tu re s i z e .
The p ar t i c le s e ith er
pass
through
or co l l e c t
on
the
screen according
to t h e i r
s ize . Although th i s method i s
used
pr imar i ly for so Li
d-is o l
i
d s e pa r a
t i
on
i s a l so
used
fo r
s ol id l iq uid
sepa r a t ion . For
algae harves t Ing
two
s c re e ni ng d e vi ce s w ere e mp lo ye d:
f f i lcrostralners and
vibra t ing
screen
f i l t e r s .
F i l t r a t i on
III a l l f i l t r a ti o n
a
pressure
drop
must
be appl ied
across the
medium
in o rd er
to
force i
d
to flow th rough. Depending
upon
the
r e q u t
re
d magnitude of p r e s s u r e drop
on e
or more of the fo llow ing
dr iv ing
force may
bf employed: grav i ty ,
vacuum,
pressure
or
cen t r i f uga l .
Two bas ic
types of f i l t r a t i on
are used:
I .
Surface f i l t e r s
in which
the so l ids
are d epo si te d t
n the
form
of
a cake on
the face of
a th in
f i l t e r
medium. As
soon
as a
l ayer
o f
cake appears
en the f i l t e r face, depos i t ion sh i f t s to
cake i t s e l f
an d the medium ac ts only as
a
suppor t .
As
the
cake grows,
the
r
e s i s tanc e to flow i.nc r e as e s . Thus, for a cons t ant
pressure
drop the
feed
ra te
dec l ine s .
I I .
Depth
f i l t e r s
deep bed i l
r r
a t i on in which the so l ids are
depos i ted
within
thE f i l t e r medium.
The
problem
with
us ing f i l t r a t i on to c l a r i f y algae pond ef f luen t
IS
tha t
a
media
f ine
enough
to
r e t a i n
a l l
the
algae
tend
to b l ind
r ap id ly ,
requ i r ing f requent
backwashing. As a r e su l t f i l t e r s i ze
has
F i l t e r s desc r ip t ion and
opera t ion mode were taken
from
Sol ids Li qu i
d s
Separa t ion
by
1 Svarovsky, Chemical
Eng. July
2, 1979.
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to be increased an d
so l id
co n ten t of the
biomass s t
ream decreases .
However, the search for e f f e c t i v e an d e f f i c i e n t means of
f i l t r a t i on
con t inues
due
to
i t s
p o t e n t i a l ad va nta ges I n
reduced
co s t and energy,
and
avoidance
o f chemlcals an d t h e i r
impact on feed
q u a l i t y .
Se ve r a I
f
i l t r a t ion m ethods h ave
been t e s t ed
with vary ing degrees
of success .
D iscussions of f i l t r a t i on and screen lng procedures fo r a lg ae
h a rv es t i n g
are
presen ted
in
th e f ollowin g sec t ions .
4 .1 .2
F i l t r a t i o n
an d Screening Devices
4 .1 .2 .1
Pressure
F i l t e r s
In
pressure
f i l t e r s
the dr iv ing
force
for f i l t r a t i on i s the l iquid
pressure
developed
by
pump i.n g
or
by
the
force
of
gas
pressure in
the
feed vesse l .
Pressure
f i l t e r s can t r e a t feed with
concen t ra t ions
up to
10 sol i s . Pressure
1
t
e r s may
be grouped i n to two ca tegor i e s
pla te and f rame
f i l t e r
presses
an d
pressure
vesse l s
con ta in ing
f i l t e r
elements .
(Sv
a r
ov sk
-
1979) .
In
th e
c o n v e n t i o n a l
p la te a n d f r a m e p re s s
a sequence
o f
pe r f o r a t ed square or r ec t an g u l a r pla t e s a l te rna t ing with hollow frames
i s mounted on
su i tab le suppor t s and
pressed toge ther with
hydr au l i c or
screw-drawn
rams. The pla t e s
are
covered
w i
t
h a
f i l t e r c l o t h . The
s lu rry is pumped in to th e frames
an d the
f i l t r a t e
i s drained from the
p l a t e s .
Th e second ca tego ry of
pressure
f i l t e r s inc ludes
a
number o f
ava i l ab le
designs
t ha t fea tu re
a pressure
v es s e l
con ta in ing
f i l t e r
elements
such
as rotary drum pressure
f i l t e r s cy l indr i ca l e l ement
f i l t e r s
v e r t i c a l tank v e r t i c a l l ea f
f i l t e r s hor i zon ta l
tank v e r t i c a l
l e a f
f i l t e r s and
h o ia on ta l le af
f i l t e r s .
Mohn (1 98 0) tester:
f ive
d i
f f
e r e n
t
pressure
f i l t e r s
fo r
Colast rum
h a rv es t i n g :
Chamber
f i l t e r
pr e ss
Bel t
pr e ss
Pressure Suct ion
F i l t e r
Cyl
i ndr i c
S i eve
an d Fi
I
t e r
Baske t . His r
e.su
I
t s
are shown In
t able
4 . 1 . 1 . Fi n a l TSS concen t ra t lons were in the range of 5 to 27 and
the
i n i t i a l
concen t ra t ion was 0.1 . Based on
energy
cons ide r a t ion
r e l i a b i l i ty and
concen t ra t
ing c apab
i
l i ty
the chamber f i l t e r press the
c
y
I
i
nd r i.c
SIeve
and the f i l t e r baske t were recommended
as
pote t en t
f i l t e r i n g sys tems. The
b e l t
f i l t e r press was
not
recommended because
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Table
4.1.1
Devices
fo r
harvest ing
through
pressure f i l t r a t ion Mohn 1980
N
Device
Chamber f i l t e r press
B el t P re ss
P res sure Suct ion
i l te r
Cylindric s i.eve
pressure ~ u s e d
by
rotators
Fi Her basket
T8S
of
th e
concentrate
22 27
8
6
7.5
n e r g ~ Consumed
pe r
m
0.88
kWh
15 ppm
Flocc.
0.5 kWh
0 .3 kWh
0 .2 kWh
Algae Species
Coelastrum
Coelastrum
Coelastrum
Caelastrum
Cae l a s t rum
Remarks
Discontinuous
method,
very
r e l i abi l i ty
Continuous method , n eed
preconcentrating
of
Flocculant, low r e l i abi l i ty
Discontinuous Method,
good
r
e l i ab
i
l i ty
Continuous method, good
r e l i abi l i ty
Discontinuous method, fo r
preconcentrat ing,good re l i
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the cake obtained without f loccu lan t s to
the preconcen t ra te was not
dense enough.
Pressure
suct ion
f i l t e r
was not
recommended because of
i n s u f f i c i e n t
information
on opera t iona l expenses and because
of
low
f i l t r a t i o n
r a t io
and high
inves tment
cos t s .
4.1.2.2
Vacuum F i l t e r s
In vacuum f i l t e r s [he
dr iv ing
force for f i l t r a t i o n r e s u l t s from
th e
ap p c a t i.on
of
a
suc t ion
on the f i l t r a t e s ide
of
the medium.
Although the t heor e t i c a l
pressure drop avai l ab le for vacuum f i l t r a t i o n
is 100 kP a 1n pr ac t i c e i t
is
l imi ted to 70 or 80 kPa.
In app l ica t ions
where
the
propor t
ion of f ine
p a rt i c l e s
i.n the feed s lu r ry 1S low,
r e l a t ive ly
cheap vacuum f i l t e r s
can yie ld
cakes
with
mois ture
conten ts
comparable
to those of
pressure
f i l t e r s .
Fur thermore t
h
s
category
includes the only
t r u ly
cont inuous f i l t e r s b u i l t in la rge s ize s th a t
can provide fo r
washing, drying
and o th e r
process requirements .
Vacuum f i l t e r s
are usual ly
c l a s s i f i e d
as e i t h e r batch operated or
cont inuous
Svarovsky
1979 . The two most common
batch-vacuum
f i l t e r s
are the
vacuum-leaf
f i l t e r
and the
vacuum-Nutsche or batch
bed f i l t e r .
Both
are t nexp ens r.ve
and
very v e r sa t i l e
and
can cope
with
f requent changes In process
condi t ions .
The
vacuum-leaf , or
Moore
f i l t e r
cons i s t s simply of
a number
of
r ec t an g u l a r leaves manifo lded together
and
connected to vacuum.
The
l eaves which
are ca r r i ed by
an
overhead c r.arie during
the
f i l t r a t i o n
sequence,
are dipped success ive ly n a
feed s lu r r y t ank where
the
f i l t r a t i on t akes
place
a h old ing tan k where washing occurs
and
a
cake rece iv ing conta1ner where
cake
discharge
is performed,
usual ly
by
back-blowing.
Simple des ign genera l f l e x ib i l i t y
and good
separa t ion of
the
mother
l iquor
and
th l
wash are the important vi r tue s o f v ac uum -le af
f i l t e r s .
On the
other
hand, they are
also
l abor i n t ens ive r e qu i. re
s u b s t a n t i a l f loor space, and in t roduce
the danger
of the cake f a l l ing
o ff during t r anspor t .
Vacuum-Nutsche f i l t e r s cons i s t
of
cy l indr i ca l
or rec tangu lar
t anks
div ided
t
n t o two compartments by a h o r i zo n t a l medium supported by a
f i l t e r
p la te .
Vacuum i s
applied to
the
lower
compartment,
from which
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the
f i l t r a t e i s
c o l l e c t e d .
The cake
removed manual ly or
sometimes
by i e s lu r ry ing .
These f i l t e r s
are par t i cu la r ly
advantageous
when neccessary
to
keep
the batches
sepa r a t e
and
when
ex tens ive
washing
i s
r eq u i r ed .
They siwple
in
des ign but laborious in
cake
discharge .
They
are
prone
to high amounts of wear du e
to
the digg ing ou t opera t ion.
Throughputs
are
l imi t ed . Varia t ions on t h i s kind
of
f i l t e r are :
double t i pp ing
pan
f i l t e r s hor i zon ta l r o t a t i n g pan f i l t e r s and
hor i zon ta l
r o t a r y t i l t in g p a n
f i l t e r s .
Vacuum
b e l t f i l t e r s Another of f spr ing o f the pan f i l t e r was
the
h o r i z o n t a l b e l t
f i l t e r a
row
of vacuum arranged
along
the path
of
an end le ss be l t
f i l t e r c lo th . No longer used t h i s type has been
superseded by the ho r i zon ta l e nd Les s c lo th vacuum f i
I t e r
which
resembles a
be l t conveyor
in
appearance.
The top
s t rand of the c lo th
The
bot
torn
r e tu rn
s used fo r
f i l t r a t i o n
cake w ashing an d
dry ing .
s t rand
i s
fo r t rack ing an d washing of the c la th .
Horizon ta l be
1 t f i 1t e r s are c l a s s i f i e d accord ing to the method
employed to suppor t the f i l t e r medium.
One common s ~ g n i s typ i f i ed
by
a rubber be l t mounted in ten sio n.
The
be l t
s grooved to provide
drainage
toward
i t s cen te r .
Covered
2
with
c lo th
the
b e l t
has
ra i sed
edges
to
con ta in
the
reed
s lu r ry and
i s dragged over s ta t ionary vacuum
boxes
located
a t
the
b e l t cen te r .
Wear caused by f r i c t ion between the
be l t
and. the vacuum chamber i s
redueed by
using
replaceable
secondary wear be l t s made o f a su i tab le
mater ia l
such
as
PTFE t e r y l ene
e t c .
These f i l t e r s
are
ava i l ab le
in la rg e
capac i t i es
with
areas up
to
2
m or more. They can be run a t very high b e l t
speeds
when handl ing
f a s t f i l t e r i n g m a t e r i a l s such
as m i n e r a l s l u r r i e s .
The ma n
disadvantages of
rub:
e r b e l t f i l t e r s are the h igh replacement cos t of
the b e l t s the r e l a t ive ly low vacuum l eve l s and l im i ta t ions on
the
proper t ies
of
the rubber
in
c e r t a i n so lven t s .
Another type of h o r i z o n t a l b e l t f i l t e r uses
r ec ip r oca t ing
vacuum
t r ays
mounted
under a cont inuous ly
t r ave l ing
f i l t e r c lo th .
The t r ays
move
forward with the
c loth
as
long
as the vacuum
i s
appl ied and
re turn
q uick ly to t he i r or ig ina l pos i t i on a f t e r the vacuum i s r e l ea sed .
This
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overcomes the
problem
o f
f
r i c t ion between the be l t
and
the
t r ay s ,
because there
no
r e l a t ive
movement
between
them
while the vacuum i s
being app l ied .
The
mechanics of th i s f i
t e r
are
r a the r
complex
and
x p n s ~ v
however
and
s o l v en t s c a n
be
used .
ava i l ab le .
r e
qu
i
r e
i n t ens ive
maintenance . A
range
of
Widths
up to
2 m and
a r ea s
up
to
40 are
The
index ing-c lo th
machines
are a
f u r the r
development along
these
l i n e s .
In
these , the vacuum t r ay s
a re
s t a t i ona ry , and the
c loth
r s
indexed by means of a
r ec ip roca t ing discharge
r o l l .
During
the t ime
th e
vacuum i s app l ied ,
the c lo th i
s
s ta t ionary
on the
vacuum t r ays .
When
the
vacuum r s
cu t o ff
and vented , the
discharge
r o l l
advances
r ap id ly ,
moving the
c loth
fo rward
500 rn
The
cycle
then repea ted .
As with
r ec ip roca t ing- t r ay
types ,
the c loth
can be washed
on both
s i de s .
Cake
discharges by grav i ty a t the be l t
s
end when
t r ave l s
over
th e d is ch arg e r o l l .
The
major advantages of th i s f i l t e r
are
i t s simple
design
and low
maintenance cos t s . The main
disadvantage
i s the d i f f i c l u t y
of
handl ing
very f a s t - f i l t e r i n g mate r i a l s
on a l a rge
sca l e .
Rotary vacuum f i l t e r
All
of
the
vacuum f i l t e r s covered so fa r ,
with
th e
excep t ion
of
th e v acu um -le af f i l t e r , use
a
hor i zon ta l
f i l t e r i ng
sur face
top
f eed .
This
ar rangement
of f e r s
the
fo l lowing
advantages :
1.
Gravi ty f i l t e r i ng
can take
p la ce b efo re the
vacuum
i s app l ied .
In many
cases
th i s
may prevent
excess ive
b lin din g o f
the
c lo th .
2. Heavy or coar se mater ia l s can be f i l t e r ed withou t problems due
to
s e t t l i ng .
3. F ine -pa r t i c l e pene t r a t ion through the
medium can
be to le ra ted
because th e f i l t r a t e can be re -cyc led back onto the be l t .
Coarse
mate r i a l
separa ted
there can
then
serve as a pre-coa t .
4 .
Top-feed f i l t e r s
are i d ea l
fo r
cake washing cake dewater ing ,
and o the r process opera t ions such as
l each ing .
5. A h ig h d eg re e of con t ro l can be exerc i sed over cake formation.
Allowances can be
made
fo r changed
feeds
and/or d i f f e r en t cake-qua l i ty
r equ i rements .
This
pa r t i cu l a r l y
t rue of
many
ho r i zon t a l - be l t
f i l t e r s .
With
these
un i t s
the r e l a t ive propor t ions of the
be l t
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a l loca ted
to f i l t r a t i on , washing, dry ing , e
t
c . ,
as
wel l
as
the be l t
speed and vacuum
qua l i ty , can
be
a lt er ed e as il y
to su i t process
changes .
There
are , however,
two
major drawbacks:
1. requi re
la rge
f loor
area .
2. Their cap i t a l cos t i s
high.
With the
excep t ion of
the indexing be l t
f i l t e r ,
a sav1ng
1n
i n s t a l l ed cost of about 25
can
be made by sub t i tu r ing a ro ta ry-drum
f i l t e r . But the cos t of doing
so
is
los ing
many of the above-ment1oned
advantages.
The rotary-vacuum f i t e r in par t i cu l a r
the
rotary-vacuum-drum
f i l t e r
or
RVDF i s st the most popular vacuum
f i l t e r
today .
The drum ro ta tes s lowly about i t s hor izon ta l aX1S and 1S
pa r t i a l l y submerged in a
s lu r ry
r ese rvo i r . The p er fo ra te d s urfa ce o-f
the
drum 1S
divided in to
a
number of
shal low, lo n gi tu d in a l s ec ti on s
about 20
r
deep.
Each
sec t ion 1S an i nd iv idua l
vacuum
chamber,
connected
through
piping
to a
cen t ra l
ou t l e t valve a t
on e
end
of
the
drum.
The drum
sur face
i s
covered
with a c lo th
f i l t e r
medium
and
the
f i l t r a t i on
tak es place
as each
sec t i on
i s
submerged in
the feed
s lu r ry .
Fi l t r a t i on
can
be followed by dewatering,
washing
and
maybe a lso
dry ing .
In use are severa l d i f f e r en t systems
of
cake discharge , a l l
o f which can
be
ass i s ted
by a1r blowback: s imp le -k ni fe d is ch ar ge ,
advanc ing-kni fe discharge with precoat f i l t r a t i on ) , be l t
or
s t r ing
d i s c ~ a r g e
and
ro l l e r discharge .
Mohn 1980)
t e s ted
f ive d1f fe ren t vacuum
f i l t e r s
fo r the f a rves t ing
of coelastrum:
vacuum
drum f i l t e r
not
precoated ,
vacuum drum
f i l t e r
precoa ted with pota to s t a rch , suc t i on f i l t e r , be l t
f i l t e r
an d
f i l t e r
t h i ckene r .
His
resu l t s
are
shown in
table
4 .1 .2 .
Fina l
TSS
concent ra t ions were 10 the range
of
5 to 37
and the
i n i t i a l
concen t ra t i on
was 0.1 . Based on
energy cons idera t ion , r e l i a b i l i t y and
concent ra t ing
capab i l i ty
the precoated vacuum drum f i l t e r , the suc t i on
f i l t e r and
the be l t
f i l t e r were
recommended.
The precoa ted f i l t e r
can
a l so be
used
fo r harves t ing of t iny microalgae l i ke
Scenedesmas.
The
none precoated vacuum drum f i l t e r had a low r e l i a b i l i t y . Afte r 15
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Table 4 2
TSS of
the
e v ~ c e s
for harves t ing
through vacuum
f i l t r a t ~ o n
Energy
Consumed
3
Device
concen t ra te
Algae Species
Remarks
None
precoated 18
5. 9
kWh
Coelas t rum
Continuous
method, low
vacuum drum
f
i l t e r
r e l i a b i l i t y
Pota to s ta rch
7
Coelas t rum
Continuous method
precoa ted vacuum
Scenedesmus
drum f i l t e r
Suc
t i
on
f i l t e r
vacuum by a :. m
water
column
Bel t
f i l t e r
F i l t e r th ickener
9.5
5-7
0.1 kWh
0.45 kWh
1.6
kWh
Coelas t rum
Coelas t rum
c e n e d e ~ m u s
Coelas t rum
Discontinuous method
Continuous
method fo r
preconcen t ra t ion ; good
r e l i a b i l i t y
Discontinuous method fo
p r e c o n c e n t r a t ~ o n
method
low re
Li
ab i
l t
i.y
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minutes f i l t r a t ion tIme the f I l t e r cloth
was consis tently clogged.
Vacuum f I l t ra t ion without
precoat
was found to be ineffective for the
accelera ted pond eff luent
in the
Technion. (Shelef
-
1981).
The
f i l t e r
thickeners
were
not
recommended
because
of low
density
of the concentrate 0-7 TSS), low f i l t ra t ion velocity high energy
demand, and
inconsistent
re l i ab i l I ty .
Dodd (1972) was the f i r s t to harvest microalgae by a be l t f i l t e r
precaated
with
eucapyptus and
pine
k ra ft s fi be rs .
The
use of
a
precoat
was
found
to cause undes
i
r ab Le operat ional complexity
and increased
cos ts .
Flne
weave cloth ra the r than the pre coated f i l t e r
was
invest igated
In Singapore Dodd
1980)
.
This method required
a
re la t ively
low
n ~ r y
Input
and
no chemicals.
I
t was
found
to be
eff Ic ien t when
harvesting
the larger speCIes
of algae such as
Micractinium, but had problems of
bl inding
with the sm aller sp ecie s
such as Ch
l o r e
1.18
I ts capi ta l costs are higher than dissolved
a
i
r
f loata t ion
but
the
operat ing expenditures are
the lowest
of
any
harvesting technique wi t
h
the exception of
natural
se t t l ing .
1980).
4.1.2.3
Microstrainers
Dodd -
Microst ralners c
on
sr
s
t
of a rotary
drum
covered
by a
s t ra in ing
f ab r i c s ta in le ss s te el
or polyester .
A
backwash spray col lec ts
the
par
t
i
c
l.e
s
ont.o
an
ax i
a through. The
un i
t .
cost of micros t ra ining is
low, from
5 to
15 /10
61
depending on
sc a l e and some spec i f i c
a
s s un.pt ions.
Belwmann
e t a
l
1978). For
larger
algae even lower
costs may be
achieved.
Advantages
of
microstra iners
are:
simple
function
and construct ion, easy
operat ion,
low investment, neg1igable
a t t r i t ion
due to
absence
of quickly movIng
par ts low
energy
consumption,
and high
f i l t r a t ion
ra t ios .
Problems
encounl erpd wit h
micros
t r a iners include
incomplete sol ids
removal and di f f icu l ty in handling
sol ids
f luctuations . These p roblems
may be
pa r t i a l l y
overcome
by
v a r y r ng
the
speed
of
ro t a t i on .
(Middlebrooks, P orc ella e t
al
-
1974). Another problem associated with
micros
t r a iners
is the buildup of
bac te r ia l
and
algae
slime on the
microfabric .
This
growth
may be inhibi ted by USIng
ul t rav io le t
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r r
ad
a t ion equipment .
However,
m i c r o s
t
r a
ne r s may r e
qu i. r
e per iodic
c lean ing .
Micros t ra iners have
been widely
used the
e l imina t ion
of
par t
i cu l a t e
matter
from ef f luen ts of sewage works, Bod en Steinberg -
1969, Diaper
1969,
Henisch 1974) and
in removal of algae from water
supp l ies
Berry
1966). Despi te t h i s , they have usual ly
fa i led when
appl ied to
oxidat ion pond ef f luen ts Golueke and
Oswald
1965) . Van
Vurren 1960) r ep o rt ed s uc c es sf ul removal of Micract in ium from
algae
ponds in South
Afr ica ,
but ,
when
an
unce l lu l a r s t r a i n
of Schenedesmus
and ch lore l l a overtook the
ponds ,
algae removal became very poor. At
the
i s t i t u t e for biotechnology
in
Dortmund micros
t r a ine r s
were
found to
concen t r a t e
Coelastrum proboscideum
to
about 1.5 TSS. Mohn 1980) .
Ope r a t i o n a l
exp en s e s
amoun t ed to about
0.02/m
3
a t
an
ene rgy
consump t i on o f 0 .2 kWh/m
3 .
Koopman , Ben emann
and Oswald
1978
achieved
some success c l a r i f y ing
high
r a te pond e f f l u en t with
r o t a t i ng mic ros t r a ine r w ith c on tin uo us backwash.
T heir success
was
l i m ~ t e d
to
ef f luen t of ponds dominated by a lgae
growing
in
cenobia
such
as
Micract in ium and Schenedesmus, s ince
the
f ines t screens
ava i lable
to
them
a t t ha t t ime
had ~ m
openings , and they
were
not able
to
main ta in
the
dominance
of such a
populat ion for
long
even
by recyc l ing most
of
the
s ep ar at ed a lg a e.
Tes t s the Technion with a prototype
rota ry
micros t r a i n e r
equipped
with
~ m
nylon
mesh gave s imi la r
r e su l t s ,
and s ~ n c e
f re qu en tly t he re
were
algae presen t as s ingle ce l l s or
smal ler
cenob ia ,
c l a r i f i c a t i on was not c o n s ~ s t e n t l y sa t i s f ac to ry
Shelef 1981)
.
Recent ly
polyes t e r screens as
f ine
as
have become ava i lab le .
Cravens
and Lauri tch 1980, Cravens and Kormanik 1978,
Kormanick
and C rav en s
1978). Wittman
and Cravens 1980 have
reported success
c l a r i f y i ng
s t a b i l i z a t i o n l agoon
e f f l u en t
such r o t a ry
microscreens ,
reducing TSS
from
up to 80
mg/l
to
20
mg/l
or
l e s s .
C l a r
i.c
a
t
on with
mi
c r o s t r a
ne
r s l
u
in the
Technion
ponds
e f f l u en t
Shelef
1981) showed a good algae removal from th e F ran cea
Micract in ium pond, and
qui te poor
c l a r i f i c a t i on
fo r
the
ChIare l la
pond.
The
di f fe rence was
evident ly
du e
to
the
di f fe rence
in
s ize
of the
algae
in each
pond.
Whereas th e F ran cea were
completely
re ta ined even by the
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6\l screen and served as f i l t e r - a id fo r smaller
algae
present , the
Chlore l la passed even through
the 1 \l
screen,
al though the i r
diami te r
exceeded
1um
More
exper iments should be done
to
determine whether
the re
was
a
problem
of screen
s iz e co ntro l,
passage
of
smal l
ce l l
fragm ents, or
whether Ch1ore11a rea l ly was not re ta ined by such a
screen.
Continuous
opera t ion
may overcome par t of the
problem
by
bui ld ing up
and maintaining a
contro l led
precoat
l ayer
of
algae.
4.1 .2 .4
~ b r t i n g Screen
Fi l t e r s
Vibrat ing
screen
f i l t e r s are used r
n
many
in du st r ie s l ik e the
paper or food indus t ry .
I t
i s also used 1.n municipal sewage
plants
to
concent ra te
sewage
Lledtke 1977). At Sede Boker Ben
Gurion
Univers i ty ,
I s r ae l ,
Vibrat ing
screens
are used
fo r
separa t ing
Spiru1ina.
At
Dortmund Coelastrum was
harvested by
v ib ra t ing
screen
f i l t e r s
ohn
1980).
Discont inuously
harves t ing
increased
the
TSS to 7-8
and continuous
operat ion
i
nc
r e as e d the TSS to
but
the
former
complicated
th e removal of
the
s lu r ry .
4.1 .2 .5 Cart r idge
F i l t e r
These
are
f i l t e r s
tha t
use
an
eas i ly
replaceable
car t r idge made
of
paper , c
lo th
or
various
membranes h avin g p ore s ize down to 0 .2 The
suspens1.on
S simply pumped, sucked, or
gravi ty fe d through
the
f i l t e r .
In order to
keep
down
the frequency
of
car t r idge replacement ,
car t r idge f i l t r a t i on is almost always l imited
to
pol i sh ing
of
l iqu ids
with so l ids contents
less
than 0.01 by weight .
4 .1 .2 .6 Deep-bed F1. l t ra t ion
The
par t i c le s recovered
i n a depth
f i l t e r are
genera l ly
smal le r
than the pores .
Hence, they pass in to
the
medium
and
are co l l ec ted
within the
bed
by s ev e ra l d epo si ti on
mechanisms.
Deep-bed f i l t ra t1 .on
is most
often
operated
as a
batch
process .
During
the
opera t ing sequence
the
f i l t e r
wi l l
exh ib i t
a gradual
i nc
r e
as
e
i
n pressure
drop as
the
par t i c l e s
are
depos i ted .
When the
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pressure
drop reaches the
maximum
ava i l ab le ,
the f i l t e r must be
taken
out o f s e r v c e fo r c lean ing . This i s usual ly done by reverse
flow
b ackwas h
i.ng
Deep bed f i l t e r s were o r ig ina l l y developed
fo r
potab le
wate r
t rea tment ,
where
they served
as
the
f in al po lish ing s tep .
More
nd more -:Ire be ing C pplied to i ndus t r i a l wastewater t r ea tmen t .
Re
i.no
Lds
e t
a l 979 ) , an d Ha r r
i.s
e t a l 978) r e r t ed succes s fu l
c l a r i f i c a t i o n
o f
s t ab i l l z a t i on
pond
e f f l u en t
by
in te rmedia te
sand
f i l t r a t i on , but
TSS couc e u t r a
t
on
o f t he i r ef f luen t
d id
not reach 100
mg l ,
and
was
only 30 mg l on average .
With
Technion acce le ra ted pond
e f f l u en t sand,
f i l t e r s
clogged
with in 15 rmnut e s and
f i l t r a t i on ra te
has f a l l en
c lo se to
zero .
In te rmi t ten t sane
f i l t r a t i on
was
t es ted as
a process
to upgrade
ex i s t i ng wastewater t r ea tment
f a c i l i t i e s In
U tah . M id dlebrooks an d
Marshal l - 1974,
Marshal l
an d Middlebooks - 1973). The r e su l t s
showed
good ef f luen t
qua l i t y :
5 mg l O and l ess than 5
mg l
suspended
so l ids
concne t r a t ion . Only l a rge algae
can
be harves ted by
deep
bed
l
t. r
a t
ou by s epa ra t i ng the
dr ied
cake
from
the sur face of the bed.
Smal le r algae pene tra te in to the
medium and
can not
be separa ted
e f f i c i enL ly .
4.1.2.7
Cross-Flow
Ul t r a - f i l t r a t i o n
SUF)
The
cross
flow u l t r a f i l t r a t i on
system
developed by the I s r a e l
Desa l i na t i on
Engineering Zarch in Process} Ltd.
was adopted
fo r
t rea tment of algae sewage pond e f f l u en t s
In co l l abora t ion with
the
Tec hn io n Env ir onmen ta l R es ea rc h Center
in
o rder to provide a one-s tage
un i t of
opera t ion fol lowing
the algae
ponds
t ha t
would produce
high
q ua li ty e ff lu en t fo r reuse
on
the on e
hand,
and produce an algae
concen t r a te fo r f u r the r u i t l i z a t i on
as
a source
of
animal pro te ins on
the o the r
hand.
Up
to
20 fold concen t ra t ion
of the algae
had
been
reached with very hlgh
4ua l i t y
c l a r i f i ed
e f f l u en t , but
the
high
energy
requi rements made th i s
method
uneconomical .
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4.1.2.8
Magpe t ic Separat ion
High gradient magnetic
f i l t r a t i on HGMF
fo r environmental
purposes was
used
in the pas t fo r
suspended
par t i c l e s
removal and
the
removal
of
l. e
...lvy me
t
a l s from
wastewaler . Bi t ton e t a l
-
1974, de
Lature - 1973, Okamato - 1974, Okudo
e t
a l - 1975 . Alg ae r emaov al
by
HGMF
was t e s ted by
Mitche l l
e t a l 1977
and Yedidia
e t a l
1977 .
The
methods i s based
on
suspend ing magnet ic p a r t i c l e s u sua l l y Fe
30
4
magnet i t.e in
the
so lu t ion . Thes e magnet i c par t ic le s were coagula ted
with the a lgae and the so lu t ion
was
then passed through a
magnetic
f ie ld
focused
on a
porous
screen which re ta ined
the
magnet i c looes .
Bi t ton e t
a l -
1975
repor ted
algae removal e f f ic iency of between 55
and 94 by counts from f ive
Flor ida
l akes by
use
of alum as a
f locculant and a commercial magnetic
f i l t e r .
Yadidia
e t
a l
1977
t he i r
batch
experiments
achieved
algae
removal above
9 with 5-13 ppm
FeCI
as p r ~ m r y f l o c cu l an t
and
500-1200 ppm magne t i t e F e
30
4
as a
magne tic s ee d
for l abora to ry
prepared and pond-grown a lgae s u s p e n s ~ o n s
Reliab le cos t es tim ate s fo r
commercial p lan t s
are not ava i l ab le
as
present .
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f loes have a
This
equat ion
pa r t i c l e s i s
4 .2
Gravl ty
Sedimenta t ion
Gravl ty s ed lmen ta ti on i s a process of so l i d - l i qu id s ep ara tio n th at
s epa ra t e s a feed
suspens ion
ln to a s lu r ry of hlgher concen t ra t ion and
an
e f f luen t
of
subs t an t i a l l y
l iquid
Sv ar ov sky 1979b ).
To
remove
pa r t i c l e s which have r easonab le s e t
t.Li
n g
ve loc i ty
from a
SUSpEPSlon,
grav i ty
sed imenta t lon under f ree
or
hinder ing se t t l l ng lS
s a t i s f a c to ry .
However,
to remove
f lne
pa r t i c l e s
with a
d iam eter o f
a
few
microns
an d
fo r prac t i c ab le oper a t l un f loccula t ion should
be
induced to form l a rge r
pa r t i c l e s which possess a
reasonable
se t t l i ng
ve l oc i t y .
GraVi ty s e d
im e
u
t.
a r t o n of non - f l o ccu l a t ed
qua l l t a t i v e ly descr ibed by Stokes Law equat ion
2 . 2 . 1 ) ,
is not
ap
p Lt cac l e fo r f loccu la ted pa r t i c l e s
s
i nc e the
compl ica ted s t r uc tu r e
and
c on ta in c on si de ra bl e amount of
water ,
t hu s ,
mak
i ug the d r ame t e r shape an d d
en
s i t
y of the
f l oc u nd ef in ab le and
th e
s e t t Li ng mec h
an i s
m c omp c
at e
d
McCabe
Smith 1975),
Sed imen t a t i on
p r oc e s s e s
a re p r ima r i l y
d iv l ded
l n t o
a )
c l a r i f i c a t i o n
where
the c l a r l t y of the
overf low lS
of
prlmary
impor tance
an d i eed s
us
pe n
on
1S usual ly
d i l u t e and
b) th icken ing
w h e r e th ick
und
e r
f
l ow 1S
the
ma i n
purpose an d
the feed s lu r ry i s
u
su
a Ll
y
more c o
nc
e n t r at ed
Sv
ar ov s k
y
1979b) .
The f i r s t process was
sugges ted
fo r
algae
separa t10n
Mohu
1980,
1978,
Eisenberg
e t a l
1981,
Venkataraman e t a l 1980, Suk e n i k
She1ef in press ) while
th e
second
process was only
men t ioned as a
po
s s t b i 1
i
ty fo r algae s l u r r y
c oric cn t
r
a t i on p r oc e s s Mohn
1980) .
4 2 Cla r i f i c a t
IJ sed imenta t ion
tank or pond
On l y few
r epo r t s on algae sed lmenta t ion in pond wi thout any
f loccu la t ion process publ l shed . Koopman e t a t
1980)
used
o La t
10 n
o f t a eu l
La
l ive ox i d a t ion pond from i n f low
feed
to
promote
water
c La r
i c a
ti.on
The
use
of f i l l -and-draw op e r a t
i
on fo r secondary
pond al lowed
s
i
g n a c a n
t rernov
a I o f
a lgae from f acu l ta t ive
oxida t ion
pond
e
f
I l
ue
n t
but the process
r equ i red a
cyc
l e
of two to
th ree
weeks
Benemann
e t
a1
1980) .
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Such
secondary ponds were used fo r a lgae s e t t l ing from
high ra te
oxidat ion
pond
e f f luen t Aden Lee
1980,
Benemann e t a l 1980) .
11
c l a r i f i ed
e f f luen t
an d algae
s lu r ry
up to 3 TSS were obta ined a t the
secondary
ponds due
to a lgae
au to f loccu la t ion
which
enhanced
the
s
a t
t i ng ve l,o . i t y .
The aut
o
f l.oc cu
l a
t
i.cn
mechanism
in
these c
as
e s
IS
unc lear Eisenberg e t
a l
1981)
and
IS ev iden t ly d i f f e r en t from the
coprec ip i ta t ive au to f loccu la t ion
p ro ce ss su gg es te d
by Sukenik
Shelef
in p re s s .
Mohn 1980)
repor ted on
f lo cc ul an t a dd iti on to a
se t t l i ng tube in
o rde r to
promote
algae sedimenta t ion.
This
process
was
opera ted
d iscon t inuous ly at In te rva l s
of
20 min. per batch an d a lg ae s us pe ns io n
was
concent ra ted
to
1.5
TSS.
Algae separa t Ion by sedimenta t ion
tanks
or tubes IS
cons idered
as
an inexpensive process ,
howev
e r , without f loccu la t ion i ts r e l ia b il it y
IS low Table 4.2).
Algae
au to f loccu la t ion phenomena shouold
be
s tud ied an d well u nd erstoo d before
one
can incorpora te these na tu r a l
processes
In
sedimenta t ion
tank and use i t as an
inexpens ive r e l i ab l e
a a e separa t ion method fo r
primary
concen t ra t ion .
4.2.2
L ~ e l l type c l a r i f i ca t ion tank
In th i s type
of c l a r i f i e r , f l a t inc l ined
p la tes are used
in a
se t t l i ng
tank to
promote s ol id s c on ta cti ng
and
se t t l i ng
along
and down
the
p la t e s . Corrugated and
o ther p la te conf igura t
ions can
a l so be
used. The p la tes
s lopes
ensure the
downgliding
of the sediments in to a
sump from
wich they
are
eas 11y removed by pumping
Svarovsky 1979b,
Mohn
1980) .
This type
of c l a r i f i e r was
used by
Mohn
1980)
fo r a lgae
separa t ion .
Algae were co ncen tra ted to
1.6
TSS
and add i t ion
of
f loccu lan t was
requi red
when t iny algae as Scenedesmus suspensIon
was
fed to the separa to r .
Opera t iona l
r e l i a b i l i t y of th i s
method
was
f a i r
and add i t iona l concen t ra t ion of
algae
s lu r ry
was r equ i red .
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Table
4.2
Comparison of
mlcroalgae
harvest ing by
Gravi t y s ed imen t at ion
methods
DeVIce
l ri f ic tion tank
Lamel la t yp e
sedimentat ion tank
F loccu l t
io n
i.n
conjunct ion
with
s ed iment at io n t an k
FInal
s lu r ry
concentrat ion
TSS
0.5 3
1. 5
1. 5
Relat ive
energy
required
very low
very
low
high
Re
I
a
b
t
y
poor
f i r
good
RecammendabL2 fo r
algae s ize l roup
a b
a b
a b
Remarks
f loccu l n
required
f loccu l n
requi
re d
t iny alga
a - Ch l o r e l La
type
t
ny algae
b - Coelas trom; Microac tin lum type
grouped
algae
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4.2.3
Thickener
Grav i ta t iona l
th ickener
may be
used
fo r f ina l
concent ra t ion
of
a lgae s lu r ry
with
or w ithout
addi t ion
of f loccu lan t s However, there
1S no
repor t
in the l i t e r a tu r e deal ing
with
t h i s device fo r a lgae
t.h i.cs.eni.ng
4.2.4 f lo r cu la t ion fol lowed by grav i ty c l a r i f i c a t i on
Golueke
Oswald
(1965) 1n a p10neer1ng
study
suggested
t ha t
f loccu la t ion process followed by
grav i ty
c l a r i f i c a t i on as
i t
1S
prac t iced
in waste
water t reatment
plants
Metca l f
Eddy
- 1974), i s a
r e l i ab le method fo r a lgae sepa r a t ion They
used
alum as a f locculant
and a f t e r
gravi ty c l a r i f i c a t
ion removed up
to
85
of
the
suspended
biomass from the h igh ra te oxida t ion pond. Various algae spec ies could
be
separa ted
by
th i s
r e l i ab le
method
to
give
an
algae
s lu r ry
of
1.
5
TSS
Table
4.2). A compar1son of
th i s separa t ion
method with
f loccu la t ion f lo t a t ion method
Moreine
e t a1. 1980, Friedman e t a1.
1977) shows
t ha t
the l a s t one has an advantage
of
very
sharp
optima fo r
c l a r i f i c a t i on
4.3 Flo ta t ion
F lo ta t io n is a
grav i ty
separa t ion process based on the
at tachment
o f a1r or
gas
bubbles
to
sol id
pa r t i c l e s
which then
are
car r ied
to
the
l iqu id sur face
and
accumulate as f l o a t which tan
be
skimmed of f
The success of f lo t a t ion
depends
on the i n s t ab i l i t y
of
the
suspended
pa r t i c l e s
The
lower
the i n s t ab i l i t y the higher the a i r
par t i c l e contac t
The
at tachment
of
an a
r bubble to a pa r t i c l e
depends
on a i r sol id
and
aqueous phases con tac t angle
and
i s
described
by
equat ion 4.3.1
4.3.1
where
°1S the i n t e r f a c i a l tension between
a1r
so i l
AS),
water so i l
(WS) and the water a1r (WA), t i s the con tac t angle formed between the
1
a1r w ater boundary and the
water
so l id boundary.
I f 0AS W the con tac t angle
i s
g rea te r
than
zero and the
a1r
bubble adheres
to
the so l id The l a rger the con tac t angle the grea te r
the tendency of a ir to adhere .
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Substances which are
e f f ec t ive
in changing i nt e r f ac i a l t e n s i.on s
sur face
ac t ive agen t s , may be
used
to
modify
the i n t e r f a c i a l ten sion of
the so l id and to
change
the c on ta ct a ngle .
The f l o t a t i on processes a re c l a s s i f i ed according to the method of
b
ul.b
l e p r odc c
t i
o-i : d
s o l
ve
d
a i
r t lo ta t i on , e le c tr o ly t i c
f l o t a t i on
e l ec t r o f l o t a t i on
and
disper sed
a i r
f lo ta t ion Svarovsky 1979b,
McCabe
Smith 19 74 ,
Metca l f
Eddy
1975 .
In sp i te of ea r ly works Golueke Oswald
1965
an d Levin e t
a1
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