Harvesting marine algae for biodiesel feedstock

Dinh Trinh Thanh Xuan

Department of Environmental Engineering, National University of Singapore Singapore 117576

ABSTRACT

Harvesting is one of the key processes that determine the feasibility of algal biodiesel

production. This paper proposes a cost-effective system to harvest Nannochloropsis sp., using

ferric chloride for coagulation and air sparging for flocculation. For an influent concentration of

0.5g algal dry biomass per liter, the system can achieve up to 90% removal efficiency. Optimum

conditions are ferric chloride dosage of 0.18 mg/L and air sparging at 2 L/min for 2 min,

followed by unagitated settling for 10 min.

1. Introduction

The compromise between harvesting efficiency and cost is a critical problem in algal

biodiesel production. Evidently, poor harvesting process not only is a waste of manufacturing

material, but also poses a threat to the environment as high algal concentration in effluent may

cause eutrophication. However, due to the small size of micro-algae (2-30μm) and its dilute

concentration in race-way ponds (approximately 0.5-1.0g dry biomass per liter), effective

harvesting methods can be very costly. The process is estimated to contribute up to 20-30% of

the total cost, and thus, harvesting optimization has been emphasized as one of the key factors

determining the feasibility of algal biodiesel development in the future (Sheehan et al, 1988).

There are a number of possible methods for harvesting algae, including centrifugation,

filtration, electro-flocculation and coagulation. Centrifugation seems to be the most efficient yet

is too costly and therefore not suitable for mass biomass production. Similarly, filtration is not a

practical solution because of the formation of a filter-cake, which substantially increases head

loss and requires frequent maintenance.

Electro-flocculation, on the other hand, has been proven to effectively remove up to 95%

of algae in fresh water (Poleman et al, 1997). Nevertheless, the efficiency in harvesting marine

algae has not been tested. One reason is the high normality of seawater, which would compete

for positive charges from an electrode source, thus increase the current level required to

destabilize algae to form flocs. Furthermore, there is a possibility of cell oxidation that leads to

undesirable changes in lipid profile and final product quality.

In comparison with all the above methods, coagulation has many advantages. Unlike

centrifugation and filtration, coagulation is a practical technique that has been widely used for

algal removal in wastewater treatment and some industrial mass manufacture (Moraine et al,

1980). The process is relatively fast, with a reasonable cost and high efficiency. Besides, since

coagulation uses additional cations such as Fe3+ and Al3+ to destabilize algae for floc formation,

the problem of cell oxidation is minor. Therefore, with regard to algal biodiesel production,

coagulation should be the most appropriate method for harvesting.

This paper reviews our experience in using coagulation to harvest algae from laboratory

race-way ponds. The work focuses on three main targets: dosage optimization, types of

mechanical mixing and the effects of time period on flocculation and sedimentation.

2. Materials and Methods

Algal growth conditions

The marine algae selected for study was Nannochloropsis sp. isolated from Singapore

seawater. Firstly, algae were cultured in 10 liter bottles with Guillard F/2-Si medium for 15

days. After that, they were diluted and transferred into an indoor race-way pond where glycerol

was added as fixed carbon source. On photoperiod, continuous illumination was provided by

fluorescent light tubes at 6000 lux. After 1 week growing in the race-way pond, the culture

reached stationary phase and was collected for experimentation.

Calibration curve of cell density vs optical density

Optical density (OD) was used to represent the algal concentration in solution before and

after the experiments. Hence, the very first step was to construct a calibration curve of cell

density vs OD. Cell numbers were counted using a hemecytometer and OD was measured at

wavelength 680 nm by a Hitachi 2900 UV-visible spectrophotometer.

Flocculation experiments

There were two types of mechanical mixing studied: paddle-driven flocculation and air

sparging. The conventional paddle flocculation was provided by a lab-scale Stuart Scientific

SW1 Flocculator, with the blades immersed to 4 cm from the bottom of 500 ml jars. The

procedures followed Sukenik’s tests4: after coagulant addition, solutions were stirred at 80 rpm

for 2 min, then at 20 rpm for 30 min, and then left unagitated for 30 min for settling.

Air sparging was produced by letting the air supplied from a HP air pump go through an

air stone (diameter 3cm and height 2cm) to form micro bubbles. Between the pump and the air

stone connected a TSI 4000 flow meter to monitor air flows. There were various tests with

different agitating and settling times conducted to specify air sparging.

Coagulation experiments

Dosage optimization was tested using two inorganic coagulants, i.e. ferric chloride and

aluminum sulfate. Concentrations of ferric chloride and aluminum sulfate used in stock solution

were 0.0600 mg/ml and 0.0565 mg/ml, respectively. Different amounts of chemicals were added

in after ionic strength and pH had been adjusted to 0.7 and 5.0, respectively. After a settling

period, samples were taken specifically at the 200 ml position for OD measurement.

3. Results and discussion

Calibration curve of cell density vs optical density

Fig 1: Calibration curve of cell density vs optical density

Optical density

Cell density (million/ml)

The calibration curve shows a strong linear relationship between cell density and optical

density (R2 = 0.9932). Hence, the efficiency of coagulation tests can be directly calculated from

the OD data as follows.

Efficiency = 1 - finalODinitialOD

Eq 1

It should be noted that the calibration curve is only valid for OD between 0.075 and

1.675. For other values beyond this range, cell density may not be directly proportional to OD,

thus equation 1 will not be applicable.

The calibration curve may change from time to time during the algal lifetime. In

different growth phases, variation in algal biological activity may lead to changes in chlorophyll

concentration inside the cells. Therefore, even though cell numbers may be relatively constant,

there would be dissimilar OD results because the OD measurement was based on absorption of

chlorophyll by visible light. In this study, the calibration curve was constructed on the same day

as coagulation experiments in order to minimize any errors when estimating cell density from

OD data.

Effect of coagulant on harvesting efficiency

The study focuses on two coagulants: ferric chloride and aluminum sulfate (alum),

which are most common in use for coagulation in water treatment. After different amounts of

coagulants were added, all samples were agitated using the same air sparge rate of 2 ml/min for

2 min. The results are shown in Fig 2 and Fig 3 below.

Fig 2: Effect of ferric chloride concentration on harvesting efficiency

Time (minutes)

Harvesting efficiency

(%)

Fig 3: Effect of Alum concentration on harvesting efficiency

Comparing the two figures, it is obvious that ferric chloride is more efficient for

harvesting Nannochloropsis sp. The optimum dosage of ferric c hloride, 3ml/L (equivalent to

0.18 mg/L) gives 90% efficiency after 10 min settling, while the optimum dosage of alum 2ml/L

only gives 55% efficiency in the same conditions. Therefore, in all experiments following on

effects of air sparging duration and types of mechanical mixing on harvesting efficiency, only

ferric chloride was used.

The two figures also show that after 2 min settling, the change in harvesting efficiency

with respect to time was relatively small where the difference between the harvesting efficiency

at 2 min and 10 min is only 1% for ferric chloride, and 5% for alum. Hence, if ferric chloride is

used in practical algal biodiesel production, the settling duration can be reduced to 2 min to

maximize harvesting capacity.

Effect of air sparging duration on harvesting efficiency

To study the effect of agitating duration on harvesting efficiency, experiments were

conducted with various air sparge times of 1 min, 2 min, 3 min and 4 min. All samples were

pretreated with the same amount of coagulant, i.e. 3ml ferric chloride per liter solution. After

mixing, samples were left to settle for 5 min before being collected for OD measurement. The

harvesting efficiency obtained for each sparge time is demonstrated in Fig 4 below.

Time (minutes)

Harvesting efficiency

(%)

Fig 4: Effect of air sparging time on harvesting efficiency

Figure 4 shows that the optimum air sparging duration for harvesting Nannochloropsis

sp. by ferric chloride coagulation is 2 min, with an efficiency of 89%. Lower than 2 min will be

too short for algae to form flocs. However, a longer mixing time up to 3-4 min may destabilize

the flocs or result in too much micro bubble exposure which tends to bring the flocs up to the

surface instead of settling; hence, decreasing the overall harvesting efficiency.

Effect of air sparging flow rate on harvesting efficiency

Fig 5: Effect of air sparge flow rate on harvesting efficiency

The effect of air sparge flow rate on harvesting efficiency is demonstrated in Fig 5,

where the highest efficiency, approximately 90%, was obtained for an air flow rate of between

1.5 L/min to 2.5 L/min after 10 min settling. For air flow rates less than 1 L/min, the mixing was

too weak to provide effective chemical dispersion or suitable velocity gradients for floc

formation. In contrast, too high sparging flow rates leads to overexposure to bubbles that tend to

float algal flocs, thus negatively impacting on the consequent settling process. As shown in the

Time (minutes)

Harvesting efficiency

(%)

Harvesting efficiency

(%)

Sparing time (minutes)

figure, using a 2.5 L/min flow rate required up to 6 min of settling to achieve 89% efficiency,

while it only took approximately 2 min using 1.5 L/min and 2 L/min flow rates to achieve the

same result. Furthermore, shorter settling time implies a higher harvesting capacity for biodiesel

production, and lower air sparging flow rates are also beneficial in terms of operational costs.

Effect of mechanical mixing types on harvesting efficiency

The study compared two types of mechanical mixing: conventional paddle-driven

flocculation and air sparge flocculation. Samples were pretreated with the same amount of

coagulation, i.e. 3 ml ferric chloride per liter solution. For paddle-driven flocculation, the

procedure followed Sukenik’s experiment as described in Materials and Methods section. Air

sparge flocculation was based on flow rate of 2 L/min for 2 min agitating, then a settling period

of 10 min before OD measurement.

From fig 6, it can be noted that the harvesting efficiencies obtained from paddle-driven

and air sparge flocculation were 85% and 89%, respectively. Although the difference in

effectiveness of the two mixing types is relatively small, the total retention time (agitating time

+ settling time) for paddle-driven flocculation was 62 min compared to 12 min for air sparging.

Hence, with regard to harvesting capacity, air sparging flocculation is more favorable.

Fig 6: Effect of mixing types on harvesting efficiency

Harvesting efficiency

(%)

Mechanical types of mixing

4. Conclusion

We have developed a cost-effective procedure to harvest Nannochloropsis sp. for algal

biodiesel production. The selected coagulant is ferric chloride with an optimum dosage of 18

mg/L algal solution. With an air sparge flow rate of 2 L/min flow rate for 2 min agitating and 10

min settling, the process achieved up to 90% efficiency with a total retention time of 12 min.

5. Acknowledgement

I would like to thank Dr Jeff Obbard for the opportunity to participate in UROP and Probir Das

for his guidance.

6. References

[1] Sheehan, J., Dunahay, T., Benemann, J. & Roessler, P (1988). A look back at the US

Department of Energy’s Aquatic Species Program – Biodiesel from Algae.

Hyperlink: http://www.nrel.gov/docs/legosti/fy98/24190.pdf

Retrieved on 8th November, 2008.

[2] Poleman, E., De Pauw, N. & Jeurissen, B. 1996. Potential of electrolytic flocculation for

recovery of micro-algae. Resources, Conservation and Recycling 19 (1997) 1-10.

[3] Moraine, R., Shelef, G., Sandbank, E., Bar-Moshe, Z. & Shvartzbard, L. Recovery of

sewage borne algae: flocculation and centrifugation techniques. In: Algae biomass, G. Shelef

and C. J. Soeder (eds). Elsevier/North Holland, Amsterdam, 1980, pp. 531-46.

[4] Sukenik, A., Bilanovic, D. & Shelef G. 1988. Flocculation of microalgae in brackish and sea

waters. Biomass 15, pp. 187-199.