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Mussel protein recovery using dissolved air flotation
Journal of Food Engineering 5 (1986) 135-151
Mussel Protein Recovery Using Dissolved Air Flotation C. R. Holland
and M. Shahbaz
Department of Chemical Engineering, The Queen’s University of Belfast, 21 Chlorine Gardens, Belfast BT9 5DL, Northern Ireland
ABSTRACT A protein recovery process has been developed to recover protein from sea mussels. Chitosan and ?c-carrageenan are shown to be good flocculating agents for the recovery of mussel protein, efficiencies of up to 90% being reported. Design data are given to enable other dissolved airflotation systems to be designed.
INTRODUCTION Protein is a necessary component of any diet, and as the world population increases so does the demand for this nutrient for man and animals. Inevitably this increased demand has led to intensive searches for cheap sources of dry, protein-rich concentrate. Feeding trials on poultry (Hopgood, 1978), using mussel protein, have demonstrated that such high-protein concentrates can replace equal weights of herring and soya meal proteins without significant change in live weight gain, feed conversion and mortality. Because of the fecundity, rate of growth and wide distribution of highly successful cultures of mussels, they offer an important alternative source of protein. Around the coastlines of Northern Ireland extensive natural beds of mussels exist. On the map of Northern Ireland (Fig. 1) some of these beds are marked, examples being Belfast Lough, Carlingford Lough and the Lough Foyle. In Lough Foyle alone, estimates of bed masses in excess of 11000 t of full-size mussels have been made. 135 Journal of Food Engineering
0260-8774/86/$03.50
- 0 Elsevier Applied
Publishers Ltd, England, 1986. Printed in Great Britain
Science
C. R. Holland, M. Shahbaz
136
Lough Foyla
’ Carllngford
Fig. 1.
Studied mussel locations (underlined)
in Northern Ireland
The mussels consist of 6-7% w/w edible flesh and continuity of supply is ensured after harvesting as mussel beds are self-renewing. This paper reports ongoing research into the development of a continuous pilot plant for the production of a protein concentrate from fresh sea mussels. THE PROCESS Figure 2 is a block diagram of the continuous process consists of three separate stages: (i) grinding; (ii) separation; and (iii) protein recovery.
system developed.
The
137
Mussel protein recovery using dissdlved air-flotation
Air
Fig. 2.
-----1
Sattmti;
‘,l
Block diagram of the unit operations
L.-_._
in the continuous
Coaplmt
system developed.
The grinding stage involves the size-reduction of whole mussels using a hammer mill. The product consists of finely divided mussel flesh (approximately 20 pm) and crushed shell particles of approximately 2.0 mm size. The separation of flesh from shell is by hydraulic classification in a ‘grit tank’. The liquid effluent contains the protein in the proportions of 40% insoluble and 60% soluble protein. The protein is recovered by a combination of chemical precipitation and physical separation using continuous dissolved air flotation. The various stages of the process are dealt with in more detail below. (i) Grinding stage Fresh whole mussels obtained from Lough Foyle (Fig. 1) are sizereduced in a laboratory-scale Apex Hammer Mill. The housing is approximately 10 mm thick and has an internal diameter of O-230 m. Twelve O-06-m-long steel hammers are fitted on the horizontal shaft. A 3 hp motor drives the unit at approximately 4000 rpm. The screen employed is stainless steel, 1-O mm thick, having sixteen 6-O mm diameter holes per 10 cm2 of screen surface. The bulk of the shell is reduced to approximately 2.0 mm size particles, with a small propor-
C. R. Holland, M. Shahbaz
138
70__ N a, .z Y 9 0 $ .? g
+
Shell
Firm
60__
SO__
40__
30__
0
10
20
30 Particle
Fig. 3.
Size distribution
40 Diameter
50
60
70
60
p
of the extracted material in the stream from the classification to the flotation units.
tion of fines, whose size distribution is given in Fig. 3. The flesh is ground very fine, i.e. 80% w/w less than 20 pm, except for some fibrous material which remained in lengths of a few millimetres. The milled product is transferred to a storage hopper in preparation for the shell/protein separation. (ii) Separation
stage
The separation of the shell and flesh particles is by difference in density. The specific gravity of the shell particles is 2.63 while that of the flesh is very close to unity. The system is designed along the lines of a hydraulic classifier and similar to the grit tank system used in sewage works (Babbet and Baumann, 1958). This technique of separa-
Mussel protein recovery using dissolved airflotation
139
tion depends upon the principle of free settling of the denser mussel shell relative to the lighter tissue. The result is a suspension of tissue with accumulated shell at the bottom of the separation channel. The unit is shown diagrammatically in Fig. 4. The circulation pump has a maximum capacity of 150 litres mm1 giving a horizontal average fluid velocity of O-0625 m s-l. The settling velocity of the smallest shell fraction was measured in a vertical column to be O-02 m s-l, which
Cruehed MkJMPlO IJI
Retention Tank
Concentrate
Flotation
Chombar
Effluant
M Fresh
Water
Supply
Mono’ Pump Fig. 4.
Saturator 'L___l
Schematic diagram of the pilot plant.
140
C. R. Holland, M. Shahbaz
with a channel depth of 0.4 m requires a settling time of 20 s, and a channel length of 1.25 m. This was increased to 2-O m, to provide a safety margin. Liquid is discharged from the channel through a vertical slit i&o a retention tank. A 16-mesh basket is installed at the top of the retention tank for the collection of large mussel tissue fibres which would obstruct flow meters and the pump. The crushed mussel is fed to the grit tank by means of a vibratory feeder. As the volume of the water in the system is known, the concentration of protein in it may be controlled by the mass of mussels fed into the tank. When the desired concentration is achieved, a continuous bleed stream is removed from the circulating stream by means of the four-way valve. This stream is fed to the flotation unit for protein recovery. (iii) Protein recovery stage After separation of the shell by hydraulic classification the majority of the protein (60-70%) is in solution or colloidal suspension.Further treatment for the recovery of the soluble and colloidal fractions involves chemical treatment selectively to precipitate, coagulate and flocculate the proteins in suspension and solution. The dominant mechanism depends upon the chemical chosen. A general term ‘flocculation’ will be used to describe the chemical treatment process. The next recovery stage is a physical solid-liquid separation employing the technique of dissolved air flotation. A continuous dissolved air flotation unit was constructed based on the design of Bratby and Marais (1978). The unit, fabricated from 12 mm thick Rrspex’ sheet, is 1.4 m long, 0.1 m wide and 0.5 m deep. The 0.1 m diameter inlet is fitted at the centre of one end and a compartment for receiving the clarified effluent with an overflow weir to drain is situated at the other end. The unit, shown in Fig. 4, is divided into two sub-units. One is for the flocculation process and the other for the flotation operation. Flocculation The flocculation sub-unit is divided into three similar chambers, linked to provide residence times of up to 25 min. Stage 1 The influent into this stage is monitored by a rotameter having a range of 0.4-5.0 litres min- ‘. The acidity is measured by a pH meter
Mussel protein recovery using dissolved air flotation
141
and controlled by the addition, via a peristaltic pump, of 10% v/v sulphuric acid solution. The ‘flocculant’ was also added at a controlled rate by means of another peristaltic pump. The process of flocculation is initiated by rapid stirring with a 5 cm wide by 7.5 cm long paddle rotating at 200 rpm. Stage 2 At this stage flocculation is enhanced by slow stirring. This section promotes intermediate flocculation whereby small floes are formed continuously. Stage 3 No stirring is employed in this section but further floe growth takes place. The outlet, which is at the bottom of the stage, leads directly into the main chamber of the dissolved air flotation unit. Flotation The object of the flotation sub-unit is to flotate the suspended floes by the injection of water containing dissolved air under pressure. The chamber is O-8 m in length, 0.1 m in width and O-5 m in depth. It is designed to treat a maximum hydraulic loading (defined as the total volumetric flow into the flotation unit divided by the horizontal crosssectional area) of 0.2 m min-‘, a value recommended by Masterson and Pratt (1958). The flotation section of the unit has a surface area of 0.08 m2. The unit can therefore be operated with flows of up to 16.0 litres min-I. Flow into the flotation sub-unit is under the dividing wall between Stage 3 and the flotation sub-unit, through a gap of 0.05 x O-1 m towards the dissolved air inlet. Two baffles inclined at 60” to the horizontal were installed. The first, O-08 X 0.1 m in size, is situated at the base O-1 m from the inlet to the flotation sub-unit. Its purpose is to prevent reverse flow within the flotation vessel. The second inclined baffle, 0.2 x O-1 m in size, is similarly situated 0.15 m from the inlet gap. The purpose of this baffle is to direct the flow up towards the surface of the vessel. The clarified effluent leaves the flotation sub-unit via a 0.05 x O-1 m gap, leading to the outlet weir which controls the depth of liquid in the flotation unit. A dissolved air flotation system requires a supply of water, supersaturated with air, to the flotation vessel. To achieve dissolution of air in water at an elevated pressure a packed tower is used, with which
C. R. Holland, M. Shahbaz
142
very high levels of saturation can be obtained. In addition, on an industrial scale the system provides the smallest operating cost per unit of saturation level achieved. The superiority of packed towers for this application is well documented (Bratby and Marais, 1975). The pilot saturation unit is shown in Fig. 5. The packing consists of a O-5 m depth of 25 mm Raschig rings. Compressed air, at 5 bar absolute pressure, is introduced at the top of the unit along with mains water from a ‘Mono’ pump. A liquid level control system, using high and low level probes to activate the ‘Mono’ pump, maintained the quantity of saturated liquid in the base of the unit within predetermined limits. The saturation performance is shown in Fig. 6. In industrial systems the feed to the saturation unit would be clarified effluent from the flotation unit. The ratio of this stream to the
Pressure
go”ge
Relief
Pressure
from Mono-pump
Volva to Flotation Air
Supply
Liquid
-
Distributor
SO cm of Roochig ring to On-Off
pocking
Swttch
ond Mono-pump
2s cm
/ Elm%-odes *
Fig. 5.
Packed-column
saturator.
Unit
Mussel protein recovery using dissolved air flotation
al
-’
m
-’
70
-'
143
60504030-
im
200
300
4oc
500
600
700
am
soa
1000
Voter Flow Rate ql/rein
Fig. 6.
Variation
in percentage
saturation of the water with its flowrate from the saturator.
untreated feed in this system is known as the recycle ratio. Although in the pilot system no recycle occurs, the term is retained and defined as the ratio of the flow rates of air-saturated water and untreated feed to the flotation unit.
MATERIALS
AND EXPERIMENTAL
METHODS
In order to investigate the continuous performance of the dissolved air flotation unit, two chemical reagents were used: chitosan and carrageenan. Bough (1976) reported that chitosan, an extract from shrimp and crab wastes, could be used as a coagulant in treating waste waters from meat processing industries. A 05% w/w chitosan solution was prepared by dissolving dry, white flakes of chitosan in 5% v/v acetic acid.
C. R. Holland, M. Shahbaz
144
Carrageenan is a hydrocolloid extracted from seaweed. A solution of 0.5% w/w Ic-carrageenan was used in this work. A systematic experimental programme was undertaken to optimise the various process variables, influent and chemical flowrates and recycle ratio, for maximum protein recovery. The protein contents of product concentrates and effluent solutions were assessed by Kjeldahl analysis and Biuret test, respectively.
RESULTS
AND DISCUSSION
Preliminary batch sedimentation work had established the optimum pH for maximum protein recovery using chitosan and rc-carrageenan as 6.4 and 4.5, respectively (Shahbaz, 1983).
100 90
5
20 10
1
01 0
.5
1
1.5
2
2.5
Influmt
Fig. 7.
Protein
3
Flow Rate
3.5
4
4.5
l/min
recovery vs. influent flowrate for Ic-carrageenan chitosan (pH = 6.4), both at 50% recycle ratio.
(pH = 4.5) and
Mussel protein recovery using dissolved air flotation
145
To investigate the effect of influent flowrate on the performance of the unit, a range of flowrates from 1-O to 4-O litres mine1 at intervals of 0.5 litre min-’ was used. The recycle ratio was kept constant at 50%, the pH at the optimum values, and chemical feedrates were 5 and 10 ml min-’ for chitosan and Ic-carrageenan respectively (based upon initial batch trials; the effect of flocculant feedrate variation was ascertained subsequently). The results shown in Fig. 7 were obtained after a time of 80 min had elapsed for each run ensuring that steady-state conditions existed. For chitosan, the maximum percentage recovery (80%) occurs at a flowrate of 1-O litre min-’ whereas the maximum production rate is obtained at higher flowrates of 1.5-2-O litres min-‘. A compromise flowrate was used of 1.5 litres min-‘, giving a recovery of 75%. The results for carrageenan showed an improved performance (Fig. 7). At an influent flowrate of 2.0 litres min-l maximum
100
90
90
70
I
N
z-- 60
9 8
--
B
50--
.z 3P
40
--
30
--
20
--
+
Chitosan Influant rate Recycle ratio pH = 6.4
= 1.5 l/min = 50 X
K-Carrageenon Influent rote Recycle Ratio pH = 4.5
= 2.0 l/min = 50 %
a n
10 t 0
I
I I
I
I 1
I I
I I
I I
I
I
I
0
5
10
15
20
25
30
35
40
45
Fiocculont
Fig. 8.
Concentrotlon
Protein recovery vs. flocculant concentration the flotation unit.
I 50
mg/l
in the flocculation
section of
C. R. Holland, M. Shahbaz
146
values of rate of production and percentage recovery (92%) are achieved. The protein contents of the concentrates corresponding to either precipitant remained relatively constant, at about 40% w/w. The two flowrates, 15 and 2-O litres mm-‘, were then used with a constant 50% recycle ratio to determine the influence of variation in flocculant dosage (Fig. 8). The optimum precipitant dosages found are, for x-carrageenan, 25 mg litre-’ (10 ml mm-‘) with an average protein recovery of 90% (Fig. 8), and for chitosan 20 mg litre-’ (6 ml rninS1) with an average protein recovery of 79%. The rate of production using rc-carrageenan is 928 mg min-’ compared to 8.14 mg min-’ using chitosan. Its improved production rate and percentage recovery clearly establishes the superiority of carrageenan. Qualitatively, the floes produced by both chemicals appeared similar in size, density and ability to withstand the effects of flow and flotation.
100
T
90
90
70 60
50
+
Chltoson !nfluentrote = 1.5 l/mm Flocculentrote = 6 nl/min pH = 6.4
O
K-Carrooeenan InfluenTrote = 2.0 l/min Flocculentrote = lOml/mln pH = 4.5
40
30
20 10 0
I 20
+A \
I I
I I
1
I I
t
I
30
40
50
60
70
80
RecycleRatio X
Fig. 9.
Protein recovery dependence
on the recycle ratio using Ic-carrageenan chitosan.
and
Mussel protein recovery using dissolved airpotation
147
The optimum recycle ratio (Fig. 9) is the same for both flocculants at 50%. In all tests, the total influent solids concentration was kept at a value in the region of 3.5 g litre-‘. The evidence suggests that the optimum recycle ratio is a property of the separation system rather than of the particular flocculant. The importance of the recycle ratio is that it determines the air/solids ratio, A,, within the system. The value is calculated (Bratby and Marais, 1978) from: A = 19.5 PRf s Ci
(1)
where A, = air/solids ratio (mg mg-l), P = absolute pressure in the saturator (atmosphere), R = recycle ratio, Ci = influent concentration (mg litre-‘) and f = fraction of saturation of air in water. Using data for
100
90
5
+
Chltcson Chitown rate / Inf luent rate = 0.004
0
K-Corrogeenan K-bra eenon rote I Influent rote = 0.005 pH = 4. 2
0
I .004
Fig. 10.
I 1
.006
I I
.006
I I
. 01
I I
.012
I I
I
r
.014
.016
Air / SoliL
Ratio,&
I
.019
Protein recovery vs. air/solids ratio using x-carrageenan
I I
I I
.02
.022
and chitosan.
1 .024
C. R. Holland, M. Shahbaz
148
the recycle ratio, protein recovery, influent concentration and the above equation a plot of percentage protein recovery against air/solids ratio was made for the two flocculants (Fig. 10). Similar values for the optimum air/solids ratio were obtained, O-014 for chitosan and O-0135 for Ic-carrageenan, showing that A, is independent of the flocculant used and is a characteristic of the system. For the system to operate at the optimum A,, it is possible to relate the desired recycle ratio to the influent solids flowrate (mg min-‘) and produce a family of curves for given influent flowrates (litre min-‘) (Figs 11 and 12). As it had been established that the optimum recycle ratio was 50%, best performance should be obtained at around this value, say 0.4-0.6. Consideration must be given to the fact that the total flowrate into the flotation unit (influent plus recycle) should not exceed 5.0 litres min-‘, otherwise, it was found, the effluent solids
1
+
.9
.a .7
1.0 lhfn
q
1.5 lhln
b
20
0
2.5 Ihin
X
3.0 Ihin
lhln
/
I
,
/
I
,
/
/
,
,'
,
/
.6
.5 .4 .3 .2 .l 0
!
1000
I I
I I
I I
I /
t I
I I
2ooo
3000
4000
5000
8000
7000
1
BOO0
Solida Fla Rate qhin
Fig. 11. The relationship between recycle ratio, influent solids loading (mg min-I) and flowrate (litre min-' ) using chitosan in the process operating at the optimum air/ solid ratio of 0*014.
149
Mussel protein recovery using dissolved airflotation
content increased. Within these limitations, a prediction can be made, for any given influent solids flowrate, of the appropriate influent flowrate for the system to operate at a recycle ratio within the range 0.4-0.6 and at the optimum air : solids ratio. For maximum protein recovery, the ratios of the flocculant flowrate to the untreated influent flowrate for chitosan and rc-carrageenan were 0.004 and 0.005, respectively. Taking an average value of 0.0045 then the required precipitant flowrate for any influent flowrate can be calculated. Naturally, for other concentrations, adjustments would have to be made to the precipitant/influent ratio. The solids concentration of most influents will be measured in mg litre-‘. So, for a given solids content, it is important to be able to predict the most appropriate influent flowrate, in litre min-‘, to obtain optimum protein recovery. The relationship between recycle ratio and
1
+
.9
0
.8
D
0
X
.7
1.0 1.5 20 2.5 3.0
lhin Vain llmin l&n Ihin
I
I
/
/
/ . /
.6
.
.5
<’ .I
.’
.4
c
c
c
.3
_
_ -
-
.2 .1 0
! 1000
I 1
2000
1 I
3ooa
1 I
4000
I I
5ooo
I I
mm
I I
7am
I eooo
Fig. 12. The relationship between recycle ratio, influent solids loading (mg min-‘) and flowrate (litre min-I) using Ic-carrageenan in the process operating at the optimum air/solid ratio of 0.0135.
C. R. Holland, M. Shahbaz
lnfluentTotal SolidsConcentration mgA
Fig. 13.
Data for optimum recycle ratio based upon influent solid concentration (mg litre-’ ) at optimum conditions for chitosan and x-carrageenan.
influent solids concentration is given in Fig. 13. This graph was computed using the optimum A, values for both flocculants and the result is a linear relationship: R = 14.5 x lo-’ C; For any flocculating agent, an estimate of the appropriate ratio for a given influent concentration to obtain maximum recovery can be made.
(2) recycle protein
CONCLUSIONS Chitosan and x-carrageenan both successfully operate as flocculating agents in a dissolved-air flotation system to recover soluble and suspended insoluble proteins. Recovery efficiencies of up to 90% are obtained for K-carrageenan, marginally less for chitosan. The air/
Mussel protein recovery using dissolved airjlotation
151
solids ratio is a characteristic property of the system rather than of the particular flocculant used, enabling a generalised equation to be used to estimate the required optimum recycle ratio to be used for a given solids loading, for this system.
REFERENCES Babbet, H. E. and Baumann, E. R. (1958). Sewerage and Sewage Treatment, John Wiley and Sons Inc., New York. Bough, W. A. (1976). Chitosan, a polymer from seafood waste for use in treatment of food processing waste and activated sludge. Process Biochemistry, 11(l), 13. Bratby, J. and Marais, G. V. R. (1975). Saturator performance in dissolved air (pressure) flotation. Water Research, A, 929-36. Bratby, J. and Marais, G. V. R. (1978). Solid/Liquid Separation Equipment Scale Up, ed. D. B. Purchas, Uplands Press, London, pp. 155-89. Hopgood, A. P. (1978). Protein recovery. Eff. Wut. J., 18,356. Masterson, E. M. and Pratt, J. W. (1958). Application of pressure flotation principles to process equipment design. In: Biological Treatment of Sewage and Industrial Wastes, Vol. 2, eds J. McCabe and W. Eckenfelder, Reinhold Pub. Corp., New York, pp. 223-40. Shahbaz, M. I. A. (1983). Design optimization for protein recovery from effluents using dissolved air flotation. MSc Thesis, The Queen’s University of Belfast.
Mussel Protein Recovery Using Dissolved Air Flotation C. R. Holland
and M. Shahbaz
Department of Chemical Engineering, The Queen’s University of Belfast, 21 Chlorine Gardens, Belfast BT9 5DL, Northern Ireland
ABSTRACT A protein recovery process has been developed to recover protein from sea mussels. Chitosan and ?c-carrageenan are shown to be good flocculating agents for the recovery of mussel protein, efficiencies of up to 90% being reported. Design data are given to enable other dissolved airflotation systems to be designed.
INTRODUCTION Protein is a necessary component of any diet, and as the world population increases so does the demand for this nutrient for man and animals. Inevitably this increased demand has led to intensive searches for cheap sources of dry, protein-rich concentrate. Feeding trials on poultry (Hopgood, 1978), using mussel protein, have demonstrated that such high-protein concentrates can replace equal weights of herring and soya meal proteins without significant change in live weight gain, feed conversion and mortality. Because of the fecundity, rate of growth and wide distribution of highly successful cultures of mussels, they offer an important alternative source of protein. Around the coastlines of Northern Ireland extensive natural beds of mussels exist. On the map of Northern Ireland (Fig. 1) some of these beds are marked, examples being Belfast Lough, Carlingford Lough and the Lough Foyle. In Lough Foyle alone, estimates of bed masses in excess of 11000 t of full-size mussels have been made. 135 Journal of Food Engineering
0260-8774/86/$03.50
- 0 Elsevier Applied
Publishers Ltd, England, 1986. Printed in Great Britain
Science
C. R. Holland, M. Shahbaz
136
Lough Foyla
’ Carllngford
Fig. 1.
Studied mussel locations (underlined)
in Northern Ireland
The mussels consist of 6-7% w/w edible flesh and continuity of supply is ensured after harvesting as mussel beds are self-renewing. This paper reports ongoing research into the development of a continuous pilot plant for the production of a protein concentrate from fresh sea mussels. THE PROCESS Figure 2 is a block diagram of the continuous process consists of three separate stages: (i) grinding; (ii) separation; and (iii) protein recovery.
system developed.
The
137
Mussel protein recovery using dissdlved air-flotation
Air
Fig. 2.
-----1
Sattmti;
‘,l
Block diagram of the unit operations
L.-_._
in the continuous
Coaplmt
system developed.
The grinding stage involves the size-reduction of whole mussels using a hammer mill. The product consists of finely divided mussel flesh (approximately 20 pm) and crushed shell particles of approximately 2.0 mm size. The separation of flesh from shell is by hydraulic classification in a ‘grit tank’. The liquid effluent contains the protein in the proportions of 40% insoluble and 60% soluble protein. The protein is recovered by a combination of chemical precipitation and physical separation using continuous dissolved air flotation. The various stages of the process are dealt with in more detail below. (i) Grinding stage Fresh whole mussels obtained from Lough Foyle (Fig. 1) are sizereduced in a laboratory-scale Apex Hammer Mill. The housing is approximately 10 mm thick and has an internal diameter of O-230 m. Twelve O-06-m-long steel hammers are fitted on the horizontal shaft. A 3 hp motor drives the unit at approximately 4000 rpm. The screen employed is stainless steel, 1-O mm thick, having sixteen 6-O mm diameter holes per 10 cm2 of screen surface. The bulk of the shell is reduced to approximately 2.0 mm size particles, with a small propor-
C. R. Holland, M. Shahbaz
138
70__ N a, .z Y 9 0 $ .? g
+
Shell
Firm
60__
SO__
40__
30__
0
10
20
30 Particle
Fig. 3.
Size distribution
40 Diameter
50
60
70
60
p
of the extracted material in the stream from the classification to the flotation units.
tion of fines, whose size distribution is given in Fig. 3. The flesh is ground very fine, i.e. 80% w/w less than 20 pm, except for some fibrous material which remained in lengths of a few millimetres. The milled product is transferred to a storage hopper in preparation for the shell/protein separation. (ii) Separation
stage
The separation of the shell and flesh particles is by difference in density. The specific gravity of the shell particles is 2.63 while that of the flesh is very close to unity. The system is designed along the lines of a hydraulic classifier and similar to the grit tank system used in sewage works (Babbet and Baumann, 1958). This technique of separa-
Mussel protein recovery using dissolved airflotation
139
tion depends upon the principle of free settling of the denser mussel shell relative to the lighter tissue. The result is a suspension of tissue with accumulated shell at the bottom of the separation channel. The unit is shown diagrammatically in Fig. 4. The circulation pump has a maximum capacity of 150 litres mm1 giving a horizontal average fluid velocity of O-0625 m s-l. The settling velocity of the smallest shell fraction was measured in a vertical column to be O-02 m s-l, which
Cruehed MkJMPlO IJI
Retention Tank
Concentrate
Flotation
Chombar
Effluant
M Fresh
Water
Supply
Mono’ Pump Fig. 4.
Saturator 'L___l
Schematic diagram of the pilot plant.
140
C. R. Holland, M. Shahbaz
with a channel depth of 0.4 m requires a settling time of 20 s, and a channel length of 1.25 m. This was increased to 2-O m, to provide a safety margin. Liquid is discharged from the channel through a vertical slit i&o a retention tank. A 16-mesh basket is installed at the top of the retention tank for the collection of large mussel tissue fibres which would obstruct flow meters and the pump. The crushed mussel is fed to the grit tank by means of a vibratory feeder. As the volume of the water in the system is known, the concentration of protein in it may be controlled by the mass of mussels fed into the tank. When the desired concentration is achieved, a continuous bleed stream is removed from the circulating stream by means of the four-way valve. This stream is fed to the flotation unit for protein recovery. (iii) Protein recovery stage After separation of the shell by hydraulic classification the majority of the protein (60-70%) is in solution or colloidal suspension.Further treatment for the recovery of the soluble and colloidal fractions involves chemical treatment selectively to precipitate, coagulate and flocculate the proteins in suspension and solution. The dominant mechanism depends upon the chemical chosen. A general term ‘flocculation’ will be used to describe the chemical treatment process. The next recovery stage is a physical solid-liquid separation employing the technique of dissolved air flotation. A continuous dissolved air flotation unit was constructed based on the design of Bratby and Marais (1978). The unit, fabricated from 12 mm thick Rrspex’ sheet, is 1.4 m long, 0.1 m wide and 0.5 m deep. The 0.1 m diameter inlet is fitted at the centre of one end and a compartment for receiving the clarified effluent with an overflow weir to drain is situated at the other end. The unit, shown in Fig. 4, is divided into two sub-units. One is for the flocculation process and the other for the flotation operation. Flocculation The flocculation sub-unit is divided into three similar chambers, linked to provide residence times of up to 25 min. Stage 1 The influent into this stage is monitored by a rotameter having a range of 0.4-5.0 litres min- ‘. The acidity is measured by a pH meter
Mussel protein recovery using dissolved air flotation
141
and controlled by the addition, via a peristaltic pump, of 10% v/v sulphuric acid solution. The ‘flocculant’ was also added at a controlled rate by means of another peristaltic pump. The process of flocculation is initiated by rapid stirring with a 5 cm wide by 7.5 cm long paddle rotating at 200 rpm. Stage 2 At this stage flocculation is enhanced by slow stirring. This section promotes intermediate flocculation whereby small floes are formed continuously. Stage 3 No stirring is employed in this section but further floe growth takes place. The outlet, which is at the bottom of the stage, leads directly into the main chamber of the dissolved air flotation unit. Flotation The object of the flotation sub-unit is to flotate the suspended floes by the injection of water containing dissolved air under pressure. The chamber is O-8 m in length, 0.1 m in width and O-5 m in depth. It is designed to treat a maximum hydraulic loading (defined as the total volumetric flow into the flotation unit divided by the horizontal crosssectional area) of 0.2 m min-‘, a value recommended by Masterson and Pratt (1958). The flotation section of the unit has a surface area of 0.08 m2. The unit can therefore be operated with flows of up to 16.0 litres min-I. Flow into the flotation sub-unit is under the dividing wall between Stage 3 and the flotation sub-unit, through a gap of 0.05 x O-1 m towards the dissolved air inlet. Two baffles inclined at 60” to the horizontal were installed. The first, O-08 X 0.1 m in size, is situated at the base O-1 m from the inlet to the flotation sub-unit. Its purpose is to prevent reverse flow within the flotation vessel. The second inclined baffle, 0.2 x O-1 m in size, is similarly situated 0.15 m from the inlet gap. The purpose of this baffle is to direct the flow up towards the surface of the vessel. The clarified effluent leaves the flotation sub-unit via a 0.05 x O-1 m gap, leading to the outlet weir which controls the depth of liquid in the flotation unit. A dissolved air flotation system requires a supply of water, supersaturated with air, to the flotation vessel. To achieve dissolution of air in water at an elevated pressure a packed tower is used, with which
C. R. Holland, M. Shahbaz
142
very high levels of saturation can be obtained. In addition, on an industrial scale the system provides the smallest operating cost per unit of saturation level achieved. The superiority of packed towers for this application is well documented (Bratby and Marais, 1975). The pilot saturation unit is shown in Fig. 5. The packing consists of a O-5 m depth of 25 mm Raschig rings. Compressed air, at 5 bar absolute pressure, is introduced at the top of the unit along with mains water from a ‘Mono’ pump. A liquid level control system, using high and low level probes to activate the ‘Mono’ pump, maintained the quantity of saturated liquid in the base of the unit within predetermined limits. The saturation performance is shown in Fig. 6. In industrial systems the feed to the saturation unit would be clarified effluent from the flotation unit. The ratio of this stream to the
Pressure
go”ge
Relief
Pressure
from Mono-pump
Volva to Flotation Air
Supply
Liquid
-
Distributor
SO cm of Roochig ring to On-Off
pocking
Swttch
ond Mono-pump
2s cm
/ Elm%-odes *
Fig. 5.
Packed-column
saturator.
Unit
Mussel protein recovery using dissolved air flotation
al
-’
m
-’
70
-'
143
60504030-
im
200
300
4oc
500
600
700
am
soa
1000
Voter Flow Rate ql/rein
Fig. 6.
Variation
in percentage
saturation of the water with its flowrate from the saturator.
untreated feed in this system is known as the recycle ratio. Although in the pilot system no recycle occurs, the term is retained and defined as the ratio of the flow rates of air-saturated water and untreated feed to the flotation unit.
MATERIALS
AND EXPERIMENTAL
METHODS
In order to investigate the continuous performance of the dissolved air flotation unit, two chemical reagents were used: chitosan and carrageenan. Bough (1976) reported that chitosan, an extract from shrimp and crab wastes, could be used as a coagulant in treating waste waters from meat processing industries. A 05% w/w chitosan solution was prepared by dissolving dry, white flakes of chitosan in 5% v/v acetic acid.
C. R. Holland, M. Shahbaz
144
Carrageenan is a hydrocolloid extracted from seaweed. A solution of 0.5% w/w Ic-carrageenan was used in this work. A systematic experimental programme was undertaken to optimise the various process variables, influent and chemical flowrates and recycle ratio, for maximum protein recovery. The protein contents of product concentrates and effluent solutions were assessed by Kjeldahl analysis and Biuret test, respectively.
RESULTS
AND DISCUSSION
Preliminary batch sedimentation work had established the optimum pH for maximum protein recovery using chitosan and rc-carrageenan as 6.4 and 4.5, respectively (Shahbaz, 1983).
100 90
5
20 10
1
01 0
.5
1
1.5
2
2.5
Influmt
Fig. 7.
Protein
3
Flow Rate
3.5
4
4.5
l/min
recovery vs. influent flowrate for Ic-carrageenan chitosan (pH = 6.4), both at 50% recycle ratio.
(pH = 4.5) and
Mussel protein recovery using dissolved air flotation
145
To investigate the effect of influent flowrate on the performance of the unit, a range of flowrates from 1-O to 4-O litres mine1 at intervals of 0.5 litre min-’ was used. The recycle ratio was kept constant at 50%, the pH at the optimum values, and chemical feedrates were 5 and 10 ml min-’ for chitosan and Ic-carrageenan respectively (based upon initial batch trials; the effect of flocculant feedrate variation was ascertained subsequently). The results shown in Fig. 7 were obtained after a time of 80 min had elapsed for each run ensuring that steady-state conditions existed. For chitosan, the maximum percentage recovery (80%) occurs at a flowrate of 1-O litre min-’ whereas the maximum production rate is obtained at higher flowrates of 1.5-2-O litres min-‘. A compromise flowrate was used of 1.5 litres min-‘, giving a recovery of 75%. The results for carrageenan showed an improved performance (Fig. 7). At an influent flowrate of 2.0 litres min-l maximum
100
90
90
70
I
N
z-- 60
9 8
--
B
50--
.z 3P
40
--
30
--
20
--
+
Chitosan Influant rate Recycle ratio pH = 6.4
= 1.5 l/min = 50 X
K-Carrageenon Influent rote Recycle Ratio pH = 4.5
= 2.0 l/min = 50 %
a n
10 t 0
I
I I
I
I 1
I I
I I
I I
I
I
I
0
5
10
15
20
25
30
35
40
45
Fiocculont
Fig. 8.
Concentrotlon
Protein recovery vs. flocculant concentration the flotation unit.
I 50
mg/l
in the flocculation
section of
C. R. Holland, M. Shahbaz
146
values of rate of production and percentage recovery (92%) are achieved. The protein contents of the concentrates corresponding to either precipitant remained relatively constant, at about 40% w/w. The two flowrates, 15 and 2-O litres mm-‘, were then used with a constant 50% recycle ratio to determine the influence of variation in flocculant dosage (Fig. 8). The optimum precipitant dosages found are, for x-carrageenan, 25 mg litre-’ (10 ml mm-‘) with an average protein recovery of 90% (Fig. 8), and for chitosan 20 mg litre-’ (6 ml rninS1) with an average protein recovery of 79%. The rate of production using rc-carrageenan is 928 mg min-’ compared to 8.14 mg min-’ using chitosan. Its improved production rate and percentage recovery clearly establishes the superiority of carrageenan. Qualitatively, the floes produced by both chemicals appeared similar in size, density and ability to withstand the effects of flow and flotation.
100
T
90
90
70 60
50
+
Chltoson !nfluentrote = 1.5 l/mm Flocculentrote = 6 nl/min pH = 6.4
O
K-Carrooeenan InfluenTrote = 2.0 l/min Flocculentrote = lOml/mln pH = 4.5
40
30
20 10 0
I 20
+A \
I I
I I
1
I I
t
I
30
40
50
60
70
80
RecycleRatio X
Fig. 9.
Protein recovery dependence
on the recycle ratio using Ic-carrageenan chitosan.
and
Mussel protein recovery using dissolved airpotation
147
The optimum recycle ratio (Fig. 9) is the same for both flocculants at 50%. In all tests, the total influent solids concentration was kept at a value in the region of 3.5 g litre-‘. The evidence suggests that the optimum recycle ratio is a property of the separation system rather than of the particular flocculant. The importance of the recycle ratio is that it determines the air/solids ratio, A,, within the system. The value is calculated (Bratby and Marais, 1978) from: A = 19.5 PRf s Ci
(1)
where A, = air/solids ratio (mg mg-l), P = absolute pressure in the saturator (atmosphere), R = recycle ratio, Ci = influent concentration (mg litre-‘) and f = fraction of saturation of air in water. Using data for
100
90
5
+
Chltcson Chitown rate / Inf luent rate = 0.004
0
K-Corrogeenan K-bra eenon rote I Influent rote = 0.005 pH = 4. 2
0
I .004
Fig. 10.
I 1
.006
I I
.006
I I
. 01
I I
.012
I I
I
r
.014
.016
Air / SoliL
Ratio,&
I
.019
Protein recovery vs. air/solids ratio using x-carrageenan
I I
I I
.02
.022
and chitosan.
1 .024
C. R. Holland, M. Shahbaz
148
the recycle ratio, protein recovery, influent concentration and the above equation a plot of percentage protein recovery against air/solids ratio was made for the two flocculants (Fig. 10). Similar values for the optimum air/solids ratio were obtained, O-014 for chitosan and O-0135 for Ic-carrageenan, showing that A, is independent of the flocculant used and is a characteristic of the system. For the system to operate at the optimum A,, it is possible to relate the desired recycle ratio to the influent solids flowrate (mg min-‘) and produce a family of curves for given influent flowrates (litre min-‘) (Figs 11 and 12). As it had been established that the optimum recycle ratio was 50%, best performance should be obtained at around this value, say 0.4-0.6. Consideration must be given to the fact that the total flowrate into the flotation unit (influent plus recycle) should not exceed 5.0 litres min-‘, otherwise, it was found, the effluent solids
1
+
.9
.a .7
1.0 lhfn
q
1.5 lhln
b
20
0
2.5 Ihin
X
3.0 Ihin
lhln
/
I
,
/
I
,
/
/
,
,'
,
/
.6
.5 .4 .3 .2 .l 0
!
1000
I I
I I
I I
I /
t I
I I
2ooo
3000
4000
5000
8000
7000
1
BOO0
Solida Fla Rate qhin
Fig. 11. The relationship between recycle ratio, influent solids loading (mg min-I) and flowrate (litre min-' ) using chitosan in the process operating at the optimum air/ solid ratio of 0*014.
149
Mussel protein recovery using dissolved airflotation
content increased. Within these limitations, a prediction can be made, for any given influent solids flowrate, of the appropriate influent flowrate for the system to operate at a recycle ratio within the range 0.4-0.6 and at the optimum air : solids ratio. For maximum protein recovery, the ratios of the flocculant flowrate to the untreated influent flowrate for chitosan and rc-carrageenan were 0.004 and 0.005, respectively. Taking an average value of 0.0045 then the required precipitant flowrate for any influent flowrate can be calculated. Naturally, for other concentrations, adjustments would have to be made to the precipitant/influent ratio. The solids concentration of most influents will be measured in mg litre-‘. So, for a given solids content, it is important to be able to predict the most appropriate influent flowrate, in litre min-‘, to obtain optimum protein recovery. The relationship between recycle ratio and
1
+
.9
0
.8
D
0
X
.7
1.0 1.5 20 2.5 3.0
lhin Vain llmin l&n Ihin
I
I
/
/
/ . /
.6
.
.5
<’ .I
.’
.4
c
c
c
.3
_
_ -
-
.2 .1 0
! 1000
I 1
2000
1 I
3ooa
1 I
4000
I I
5ooo
I I
mm
I I
7am
I eooo
Fig. 12. The relationship between recycle ratio, influent solids loading (mg min-‘) and flowrate (litre min-I) using Ic-carrageenan in the process operating at the optimum air/solid ratio of 0.0135.
C. R. Holland, M. Shahbaz
lnfluentTotal SolidsConcentration mgA
Fig. 13.
Data for optimum recycle ratio based upon influent solid concentration (mg litre-’ ) at optimum conditions for chitosan and x-carrageenan.
influent solids concentration is given in Fig. 13. This graph was computed using the optimum A, values for both flocculants and the result is a linear relationship: R = 14.5 x lo-’ C; For any flocculating agent, an estimate of the appropriate ratio for a given influent concentration to obtain maximum recovery can be made.
(2) recycle protein
CONCLUSIONS Chitosan and x-carrageenan both successfully operate as flocculating agents in a dissolved-air flotation system to recover soluble and suspended insoluble proteins. Recovery efficiencies of up to 90% are obtained for K-carrageenan, marginally less for chitosan. The air/
Mussel protein recovery using dissolved airjlotation
151
solids ratio is a characteristic property of the system rather than of the particular flocculant used, enabling a generalised equation to be used to estimate the required optimum recycle ratio to be used for a given solids loading, for this system.
REFERENCES Babbet, H. E. and Baumann, E. R. (1958). Sewerage and Sewage Treatment, John Wiley and Sons Inc., New York. Bough, W. A. (1976). Chitosan, a polymer from seafood waste for use in treatment of food processing waste and activated sludge. Process Biochemistry, 11(l), 13. Bratby, J. and Marais, G. V. R. (1975). Saturator performance in dissolved air (pressure) flotation. Water Research, A, 929-36. Bratby, J. and Marais, G. V. R. (1978). Solid/Liquid Separation Equipment Scale Up, ed. D. B. Purchas, Uplands Press, London, pp. 155-89. Hopgood, A. P. (1978). Protein recovery. Eff. Wut. J., 18,356. Masterson, E. M. and Pratt, J. W. (1958). Application of pressure flotation principles to process equipment design. In: Biological Treatment of Sewage and Industrial Wastes, Vol. 2, eds J. McCabe and W. Eckenfelder, Reinhold Pub. Corp., New York, pp. 223-40. Shahbaz, M. I. A. (1983). Design optimization for protein recovery from effluents using dissolved air flotation. MSc Thesis, The Queen’s University of Belfast.