1

1

Investigation of the Effect of Growth from Low to High Biomass

Concentration inside a Photobioreactor on Hydrodynamic Properties of

Scenedesmus obliquus

Le, Evan1; Park, Chanwoo2; Hiibel, Sage.3

Abstract

An investigation on the effect of Scenedesmus obliquus’s growth from low to high

biomass concentration inside a horizontal tubular photobioreactor to determine the impact that it

has on hydrodynamic performances which will affect cost and production efficiency was done.

As the biomass concentration increased, the algal culture was found to remain Newtonian.

Additionally, the biomass concentration (expressed in optical density at 600 nm, OD600) was

found to have lower viscosity even at highest possible concentrations at OD600: 0.404 (1.372+/-

0.132 cp) compare to the Modified 3N Bold medium (1.408+/-0.0941 cp).Furthermore, the

total energy consumption does not appear to depend on the Scenedesmus obliquus biomass

concentrations, but on the medium it lives off of. The rheological properties of autotrophic algae

will not have significant impact on energy requirement until technology improves so that the

concentrations reach those of heterotrophic algae.

Subject Headings: Energy Systems Analysis, Alternative Energy Sources, Energy From

Biomass, Energy Storage Systems. 1Student, Dept. of Mechanical Engineering, University of Nevada, Reno, 89557 . E-mail: edule@ucdavis.edu

2Professor, Dept. of Mechanical Engineering, University of Nevada, Reno, 89557 . E-mail: chanwoo@unr.edu

3 Post-Doc, Dept. of Biochemistry and Molecular Biology, University of Nevada, Reno, 89557. E-mail:

shiibel@unr.edu

Table of Contents

2

2

I. Introduction…………………………..…………………………………………….…..…….4

II. Theory…………………………………………………………….…………………..…….5-6

III. Experimental Methods and Materials………………..……………….….………….….6-7

IV. Results………...………………………………………………………………….……….7-9

V. Discussion…………………………………….………………………………..………....10-11

VI. Conclusion……………………………………………………………… ……….................11

VII. 1omenclature………………………..…………………………………………………….12

VIII. References………………………….………...………….…………..……………………13

List of Figures

Figure 1: Behavior of non-1ewtonian Fluids…….……………………………………..5

Figure 2: Schematic of the Photobioreactor Used in the Investigation………….……..6

Figure 3: Biomass growth over time in Modified Bold 31 medium………..……….….7

Figure 4: Comparison of varying algae culture densities along with their associated

biomass concentrations……………....………………………….……………7

Figure 5: Comparison of varying algae culture viscosities along with their associated

biomass concentrations……………..……..……………..……………..…….7

Figure 6: Total Power consumed by the fluid through the acrylic tube at different

Reynolds 1umber assuming pump efficiency is 60%. …………………..…..9

Figure 7: Dependence of the Reynolds number on the algae culture flow rate at

different biomass concentration .........................................………………….9

3

3

Introduction It is today’s biggest challenges to develop clean alternative renewable energy fuel.

Solar, wind, fuel cells, and ethanol based sources all have severe limitations that prevent

them from meeting projected energy needs [1]. Algae biodiesel has been identified as the most

promising due to its quick growth rate, large oil yields, non-toxic, renewable, biodegradable,

and carbon neutral, and it does not compete with arable farm land compared to other

alternatives such as ethanol made from corn or sugarcane. It can also be grown in

environments unsuitable for agricultural crops such as the seashore, because of its greater

tolerance for salt and heat. Algae’s remarkably efficient physiology means that it requires less

sunlight, grows faster, and has more potential for genetic engineering.

In order to be a competitive alternative fuel, the algae biodiesel produce must cost less

than petroleum diesel. Currently, the price of crude oil is around $113 per barrel. At this price,

microalgal biomass with an oil content of 55% will need to be produced at less than around

$450/ton, which will depend mainly on the cost of producing the algae biomass[2]. One of the

main factors affecting the production cost is the selection of the host organism. Current criteria

in selecting the host organism for a successful algae biodiesel plant includes: (1) photosynthetic

efficiency to obtain high biomass yield from light, (2) biomass growth rate, (3) oil content of the

biomass, (4) temperature tolerance, (5) the value of biomass residue and byproducts, (6) the

ease of extraction, purification, and conversion process, sensitivity to high oxygen

concentration (7), resistance to photoinhibition (8), response to diurnal fluctuations (9),

amount of dark respiration(10), sensitivity to osmotic stress (11) [1, 3]. However, biodiesel

production at the moment is not profitable with the main challenge being scale up. Most of the

production cost in algae biodiesel plant utilizing photobioreactors comes from the energy

required to power the pumps and harvesting [4]. As a result, solutions to reduce the required

energy can significantly make algae biodiesel production more economically feasible. There

have been few researches on the maximum cell density and morphology’s impact on the

demanding auxiliary energy require for an algae biodiesel plant. Instead, the solution to the

auxiliary energy problem is focus mainly on the photobioreactor design[5]. Using the

microalgae strain that fulfill the traditional criteria in host selection as well as favorable

rheological properties in a photobioreactor with high biomass concentration can significantly

4

4

reduce the pump power consumption but maintain the algae overall productivity. This can

make algae biodiesel a more competitive alternative fuel.

The aim of this work is to investigate the effect of Scenedesmus obliquus’s (UTEX 1450)

growth from low to high biomass concentration inside a horizontal tubular photobioreactor,

one of the most widely used and investigated photobioreactors, to determine the impact that

they have on hydrodynamic performances (such as liquid flow rate, culture velocities, power

consumed, etc.) which will affect cost and production efficiency. The study will compare algae

cultures with varying biomass concentrations with the performances of the same reactor with

only water. Scenedesmus obliquus has been identified as promising source for algae oil content

[6]. Furthermore, it has a unique morphology, growing characteristics, and relatively large size

compare to other biofuel algae which are the main factors in affecting the viscosity of the

system [7].

Theory If a fluid is Newtonian, then the shear stress, ��� , should be proportional to the rate of

deformation (shear rate) ����:

��� ∝ ���� Eq. 1

The constant proportionality in Eq. 1 is the absolute (or dynamic) viscosity, . Thus in terms of

the coordinates of Eq.1, Newton’s law of viscosity is given for one-dimensional flow by:

��� = ���� Eq. 2

For gases, viscosity increases with temperature, whereas for liquids, viscosity decreases with

increasing temperature. Fluids in which shear stress is not directly proportional to deformation

rate are non-Newtonian. Non-Newtonian fluids commonly are classified as having time-

independent or time-dependent behavior. Some examples of non-Newtonian fluid is given

below (Fig 1). The curves shown indicate a few of the many types of non-Newtonian fluid

behavior which have been observed experimentally. The slope of these curves is often called

apparent viscosity, and denoted by ���. One of the simplest laws describing non-Newtonian

fluid behavior is the Ostwald-de Wael model (“power-law” fluid):

5

5

� = � ������

��� ����

Eq.3

When n=1, Eq. 3 reduces to Newton’s law viscosity with � = . If n < 1, it describes a

pseudoplastic fluid, and if n >1, the behavior is dilatant. Besides non-Newtonian and Newtonian

fluid, another way to identified types of fluids is through laminar and turbulent flows. Laminar

flow is characterized by smooth motion of one lamina of fluid past another, while turbulent

flow is characterized by an irregular and nearly random motion superimposed on the main

motion of the fluid. The two types of flow can be identified through the Reynolds number:

��� = ���� Eq. 4

The transition from laminar flow to turbulent in a pipe was found to be around 2100-4000. The

power to the fluid can be calculated with the following equation:

�� = ��ℎ Eq. 5

Where � the specific weight (kg/m3), Q is the volume flow rate (m3/s), and h is the water head

(m). By multiplying Eq. 5 by a coefficient of 9.81, Pc is expressed in watts (W).

Experimental Methods and Materials Horizontal turbular photobioreactor description

The experiment was carried out using a customized liquid loop (holding approximately

600 mL of liquid) with an inner diameter tube of 0.5 inches equipped with a flow meter and a

differential pressure transducer to collect pressure drops between the horizontal acyclic tube at

varying flow rates (Fig 2). Algae culture flow is produced using a mechanical pump. A

positive displacement pump is recommended to be used when circulating algae culture with a

mechanical pump as it minimized the hydrodynamic stress that would significantly reduced

algae productivity [9]. However, for the purpose of this experiment, a centrifugal pump is

suitable since the algae are not growing in the liquid loop. Biomass sedimentation in tubes is

prevented by maintaining highly turbulent flow. The liquid flow rate chosen to be study will be

based on the Reynolds’s number ranging from 0 to 5000. Increasing the Reynolds’s number

within the range was found to increase algae productivity [10].

Algal strain and cultivation conditions

Scenedesmus obliquus (UTEX 1450) was first cultivated in Modified Bold 3N medium

6

6

in 250 mL Erlenmeyer flasks (100 mL of culture) maintained on a flask shaker at a temperature

of 25℃ , under continuous illumination ( m2/s) provided by daylight fluorescent tubes. This was

cultivated for about two weeks before transferring into 2L flasks (1.5 L of culture). The 2L flasks

were grown under similar conditions without the shaker and under 18:6 light: dark cycle at

26:20 ℃ in a growth chamber. Additionally, sterile air was continuously pumped into the

culture. Biomasses concentrations were determined daily by measuring the optical density of

samples at 600 nm (OD600).

Hydrodynamic properties measurement

Density measurements of the algae culture for each corresponding OD600 were taken

along with the viscosity using a Gilmont® falling ball viscometer. Using the algae culture that

were grew in the growth chamber, the pressure drop was then measured using the liquid loop

with the OMEGA’S PX2300 Differential Pressure Transmitters and recorded at flow rates

ranging from 0 to 5 L/min in increments of 0.5 L/min.

Results Daily biomass growth rate of Scenedesmus obliquus in modified bold 3N were measured

by taking optical density reading at 600 nm (Fig 3). Based on the figure, there is a linear

increase in growth rate over the 26 days period instead of the ideal sigmoidal growth phase.

Poor mixing, and hence the slow growth, is likely the reason. Ten varying densities and

viscosities along with their associated biomass concentration (OD600 reading) were measured

and compared. Fig. 4 show a linear dependence of decreasing densities with increasing biomass

concentration. However, Fig. 5 shows no correlations between biomass concentration and

viscosities, which is unexpected. The higher biomass concentration is expected to have higher

viscosities. Despite this, the viscosities were always higher than that of water at 20 ℃. The

viscosities and densities of the Modfied Bold 3N medium and the lowest and highest collected

OD600 readings were also compared. Based on the graphs (Fig 3 & 4), there is little difference

in density even at high biomass concentration, with a slight decrease in density. The viscosities,

however, of the modified bold 3N medium appears to be higher than most of the algae culture

biomass concentrations including the highest OD600 reading (0.407). Ten measureable flow

rates for were taken ranging from 0.0 L/min to 5.0 L/min along with their associated pressure

7

7

drop and energy losses for biomass concentrations ranging from 0 to 0.407 OD600 Reading.

Based on Fig. 6, the comparison does not show any significance differences in power

consumption as the biomass concentrations increased. Any differences are most likely due to

errors. However, both the medium and the algae culture require more energy for the same

Reynolds number than water. In Fig. 7, a plot of Reynolds number against fluid velocity is

reported. Increasing the fluid velocity resulted in a higher Reynolds number in both the algae

cultures and the Modified Bold 3N medium as expected. Since this is a linear relationship, the

algae culture fluid remains Newtonian even as the biomass concentrations increased.

Discussion

The horizontal tubular PBR can usefully satisfy only medium level production demands.

A study by the University of Wageningen found that when comparing types of photobioreactors,

the main contributor to biomass production cost for the horizontal tubular photobioreactor was

the energy cost associated with the pump (46% of the overall cost) [11]. In fact, it requires the

most energy for its pumps compare to a raceway pond and a flat plate PBR. This type of

photobioreactor was used in the investigation to examine how significant the impact the

rheological properties of algae cultures can affect the overall energy consumption of an algae

biodiesel plant. Based on Fig. 7, increasing biomass concentrations does not affect the increase

in power consumption significantly or change the fluid to non-Newtonian. Any differences are

likely due to errors. Since Scenedesmus obliquus does not produce any byproducts, and any

possible aggregations due to its unique morphology and growing characteristics were quickly

dissociated due to the high turbulence, the largest influences on the power consumptions is due

to the medium it lives off. Even at high biomass concentration attainable of OD600: 0.404

(1.372+/-0.132 cp) with the current growing conditions, the viscosity was actually found to be

slightly lower than the pure medium (1.408+/-0.0941 cp). A different medium which has higher

viscosity or higher attainable biomass concentrations could be used to compare the effects the

mediums has on energy and overall cost and algae productivity. When comparing the highest

biomass concentrations with that of water at Reynolds number of 6000, it was found the

modified 3N bold medium requires 234.7% more energy overall. The difference becomes

significantly higher as the Reynolds number increase. Additionally, since the diameter of the

liquid loop is 0.5 inch, the energy required for pumping becomes more significant. The solar

8

8

collector tubes are generally 0.1 m or less in diameter. Tube diameter is limited because light

does not penetrate too deeply in the dense culture broth that is necessary for ensuring a high

biomass productivity of the photobioreactor [1]. However, a study found that for tube diameters

greater than 0.05 m, the mixing energy becomes a significant or dominant energy input, but

when the diameter is less than 0.25m it becomes negligible [12]. Increasing the size of the tubes

within the desire range can help reduce the energy consumptions but maintained the overall

desired algal productivity. Furthermore, the uses of static mixer can improve gas-liquid mass

transfer inside a turbular photobioreactors and give cells appropriate light/dark cycle frequency

without the using high energy input [13]. A study by Sánchez Mirón, A., et al. has even

suggested that the design of the horizontal turbular photobioreactor is inherently flawed, and

should be replace with the more energy-efficient vertical bubble columns [14]. Recent renewed

interest in closed photobioreactor technologies has increased the optimum biomass concentration

and therefore the flow resistance of the system. However, the rheological properties of

Scenedesmus obliquus, and most other popular biofuel algae species whose densities are often

similar or lower and whose morphologies are spherical (therefore, lower viscosity), does not yet

appear to be the main obstacle in reducing the energy consumption. Until the biomass

concentrations for autotrophic algae species is as high as those heterotrophic algae grown in

fermentors, the main culprit in energy pump consumptions will be the overall design of the

photobioreactors.

Conclusions The objective of this investigation was to determine whether the types of algal species

along with their maximum cell densities, sizes, and morphologies have significant affect on the

energy consumptions of the horizontal photobioreactor. By growing Scenedesmus obliquus with

varying biomass concentrations, it was possible to examine how the concentrations affects the

density, viscosities, and therefore the energy consumption required to keep the flow rate at

recommended Reynolds number for high algal productivity. As the biomass concentration

increased, the algal culture was found to remain Newtonian. Additionally, the biomass

concentration (expressed in optical density at 600 nm, OD600) was found to have lower

viscosity even at highest possible concentrations at OD600: 0.404 (1.372+/-0.132 cp) compare to

just the Modified 3N Bold medium (1.408+/-0.0941 cp). Furthermore, the total energy

consumption does not appear to depend on the Scenedesmus obliquus biomass concentrations,

9

9

but on the medium it lives off of. Hence, the main obstacle in reducing the energy consumption

is through the design of the photobioreactor as suggested in literatures [5, 15]. The rheological

properties of autotrophic algae will not have significant impact on energy requirement until

technology improves so that the concentrations reach those of heterotrophic algae. Through error

analysis, the largest source of error for the energy consumption calculation came from the

differential pressure transducer.

Reference 1. Chisti, Y., Biodiesel from microalgae. Biotechnology Advances, 2007. 25(3): p. 294-

10

10

306.

2. Chisti, Y., Biodiesel from microalgae beats bioethanol. Trends in Biotechnology,

2008. 26(3): p. 126-131.

3. Becker, E.W., ed. Microalgae: Biotechnology and Microbiology. 1 ed. Cambride

Studies in Biotechnology, ed. 1.H.C. Sir James Baddiley, I.J. Higgins, W.G. Potter.

1994, Cambridge University Press: 1ew York.

4. Asami Yamamura, S.W., Evans Le, Ke Rui Lei, Senior Design Project for

Biochemical Engineer 2009, UC Davis: Davis.

5. Lehr, F. and C. Posten, Closed photo-bioreactors as tools for biofuel production.

Current Opinion in Biotechnology, 2009. 20(3): p. 280-285.

6. Gouveia, L. and A.C. Oliveira, Microalgae as a raw material for biofuels production.

Journal of Industrial Microbiology & Biotechnology, 2009. 36(2): p. 269-274.

7. Carlozzi, P., A. Ena, and S. Carnevale, Hydrodynamic Alterations during

Cyanobacteria (<I>Arthrospira platensis</I>) Growth from Low to High Biomass

Concentration Inside Tubular Photobioreactors. Biotechnology Progress, 2005. 21(2):

p. 416-422.

8. Whitaker, S., Introduction to Fluid Mechanics 1ed. 1968, Malabar: Krieger

Publishing Company.

9. Gudin, C. and D. Chaumont, Cell fragility -- The key problem of microalgae mass

production in closed photobioreactors. Bioresource Technology, 1991. 38(2-3): p. 145-

151.

10. Grobbelaar, J.U., TURBULE1CE I1 MASS ALGAL CULTURES A1D THE ROLE

OF LIGHT-DARK FLUCTUATIO1S. Journal of Applied Phycology, 1994. 6(3): p.

331-335.

11. Wijffels, R.H. Biofuels from microalgae. [cited 2010 5/7/201; Available from:

http://www.rrbconference.com/bestanden/downloads/115.pdf.

12. Weissman, J.C., R.P. Goebel, and J.R. Benemann, PHOTOBIOREACTOR DESIG1

- MIXI1G, CARBO1 UTILIZATIO1, A1D OXYGE1 ACCUMULATIO1.

Biotechnology and Bioengineering, 1988. 31(4): p. 336-344.

13. Ugwu, C.U., J.C. Ogbonna, and H. Tanaka, Design of static mixers for inclined

tubular photobioreactors. Journal of Applied Phycology, 2003. 15(2-3): p. 217-223.

14. Sánchez Mirón, A., et al., Comparative evaluation of compact photobioreactors for

large-scale monoculture of microalgae. Journal of Biotechnology, 1999. 70(1-3): p.

249-270.

15. Molina Grima, E., et al., Photobioreactors: light regime, mass transfer, and scaleup.

Journal of Biotechnology, 1999. 70(1-3): p. 231-247.

11

11

Figure 1

Figure 1. Behavior of non-1ewtonian fluid.

Figure 2

Figure 2: Schematic presentation of the experimental set

pump, differential pressure transducer, and an excess tank. The inner diameters of

the tubes are 0.5 inches and the length between the two sensors is 15 inches. The

total volume is ~600 mL and the tube between two pressure sensors is acrylic, a

12

12

Figure 2: Schematic presentation of the experimental set-up showing the centrifugal

pump, differential pressure transducer, and an excess tank. The inner diameters of

the tubes are 0.5 inches and the length between the two sensors is 15 inches. The

olume is ~600 mL and the tube between two pressure sensors is acrylic, a

common photobioreactor material.

up showing the centrifugal

pump, differential pressure transducer, and an excess tank. The inner diameters of

the tubes are 0.5 inches and the length between the two sensors is 15 inches. The

olume is ~600 mL and the tube between two pressure sensors is acrylic, a

13

13

Figure 3

Figure 2: Scenedesmus obliquus biomass growth in Modified Bold 31 Medium over time

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 5 10 15 20 25 30

OD

60

0 R

ea

din

g

Time (day)

14

14

Figure 4 and 5

Figure 5: Comparison of viscosities at ! ℃ with their

corresponding biomass concentration through OD 600

readings

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

0.2 0.25 0.3 0.35 0.4

Vis

cosi

ty (

cp)

OD Reading (600 nm)

Figure 4: Comparison of densities at ! ℃ with their

corresponding biomass concentration through OD 600

readings

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

0.2 0.25 0.3 0.35 0.4

De

nsi

ty (

g/m

L)

OD Reading (600 nm)

15

15

Figure 6

Figure 7: Total Power consumed by the fluid through the acrylic tube at different Reynolds 1umber assuming pump efficiency is 60%. Cell

concentrations were expressed as OD600 reading.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 2000 4000 6000 8000 10000

To

tal

Po

we

r C

on

sum

ed

(J)

Reynolds Number

OD600 Reading: 0.239

OD600 Reading: 0.270

OD600 Reading: 0.323

OD600 Reading: 0.341

OD600 Reading: 0.407

Modified Bold 3N Medium

Water

16

16

Figure 7

Figure 8: Dependence of the Reynolds number on the algae culture flow rate at different biomass concentration measure in OD600 readings.

0

1000

2000

3000

4000

5000

6000

7000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Re

yn

old

s N

um

be

r

Scenedesmus Obliquus Culture (m/s)

OD600 Reading: 0.239

OD600 Reading: 0.270

OD600 Reading: 0.323

OD600 Reading: 0.341

OD600 Reading: 0.407

Modified Bold 3N Medium