Microalgae Harvesting and Processing

8/18/2019 Microalgae Harvesting and Processing

1/71

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


2/71

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


3/71

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


4/71

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.


5/71

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


6/71

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


7/71

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


8/71

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.


9/71

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


10/71

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


11/71

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


12/71

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 .


13/71

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


14/71

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


15/71

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


16/71

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


17/71

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


18/71

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 .


19/71

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),


20/71

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 .

15


21/71

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.

16


22/71

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


23/71

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.

18


24/71

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

19


25/71

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


26/71

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

21


27/71

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

22


28/71

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

23


29/71

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

24


30/71

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


31/71

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

26


32/71

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

27


33/71

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

28


34/71

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 .

29


35/71

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 .

30


36/71

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) .

31


37/71

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 .

32


38/71

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


39/71

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 .

34


40/71

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

Documents

Microalgae Harvesting and Processing