photosynthetically oxygenated salicylate biodegradation in a continuous stirred tank photobioreactor

Photosynthetically Oxygenated SalicylateBiodegradation in a Continuous StirredTank Photobioreactor

Raul Munoz, Claudia Kollner, Benoit Guieysse, Bo Mattiasson

Department of Biotechnology, Center for Chemistry and Chemical Engineering,Lund University, P.O. Box 124, SE-221 00 Lund, Sweden;telephone: 46 46 222 9659; fax: 46 46 222 4713;e-mail: [email protected]

Received 15 August 2003; accepted 21 May 2004

Published online 19 August 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/bit.20204

Abstract: A consortium consisting of a Chlorella soroki-niana strain and a Ralstonia basilensis strain was able tocarry out sodium salicylate biodegradation in a continuousstirred tank reactor (CSTR) using exclusively photosyn-thetic oxygenation. Salicylate biodegradation depended onalgal activity, which itself was a function of microalgalconcentration, light intensity, and temperature. Biomassrecirculation improved the photobioreactor performanceby up to 44% but the results showed the existence ofan optimal biomass concentration above which dark res-piration started to occur and the process efficiency startedto decline. The salicylate removal efficiency increasedby a factor of 3 when illumination was increased from50–300 AE/m2�s. In addition, the removal rate of sodiumsalicylate was shown to be temperature-dependent, in-creasing from 14 to 27 mg/l�h when the temperature wasraised from 26.5 to 31.5jC. Under optimized conditions(300 AE/m2�s, 30jC, 1 g sodium salicylate/l in the feedand biomass recirculation) sodium salicylate was removedat a maximum constant rate of 87 mg/l�h, correspondingto an estimated oxygenation capacity of 77 mg O2/l�h(based on a BOD value of 0.88 g O2/g sodium salicylatefor the tested bacterium), which is in the range of the ox-ygen transfer capacity of large-scale mechanical surfaceaerators. Thus, although higher degradation rates wereattained in the control reactor, the photobioreactor is acost-efficient process which reduces the cost of aerationand prevents volatilization problems associated with thedegradation of toxic volatile organic compounds under aer-obic conditions. B 2004 Wiley Periodicals, Inc.

Keywords: photobioreactor; photosynthetic oxygenation;biodegradation; toxic organic pollutants

INTRODUCTION

Aerobic wastewater treatment is usually limited by the low

aqueous solubility of oxygen. Intense mechanical aeration

and/or bubbling are often necessary to achieve high pol-

lutant removal rates. This is expensive and problematic

because intense aeration can cause the formation of aerosols

containing microorganisms and toxic organic compounds

(Bell et al., 1993). In this regard, photosynthetic oxygen-

ation using microalgae is safer, because there is no risk of

volatilization due to air bubbling, and cheaper, because

sunlight is used as the main energy source (Borde et al.,

2003). More precisely, the microalgae produce the oxygen

required by the aerobic bacteria to mineralize organic

pollutants, which in turn use the carbon dioxide released by

the bacteria (Oswald, 1988). In addition, microalgae have

the ability to assimilate large amounts of nutrients (NO3�,

PO43�, NH4

+), adsorb heavy metals (Aziz and Ng, 1993;

McGriff and McKinney, 1972), and contribute to the re-

moval of pathogens due to the high oxygen tension and the

increase in pH associated with photosynthesis (Oswald,

1988). Although the association of microalgae and bacteria

is not novel (De-Bashan et al., 2002, 2004), more attention

should be paid to the potential of this association for

bioremediation processes. Algal biomass is also a valuable

product, which could serve to pay back part of the plant

operation cost (Laliberte et al., 1994).

Although microalgae have been used extensively for the

removal of nutrients and heavy metals from wastewater in

the treatment of domestic sewage (Gonzalez et al., 1997),

few authors have reported their use in the treatment of

toxic organic contaminants (Berthe-Corti et al., 1998; Borde

et al., 2003). Earlier researchers may have been discour-

aged by the limited knowledge of microalgae physiology

and cultivation methods. Yet, the increasing demand for

microalgae for aquaculture feed, the production of cos-

metics, or the CO2 fixation has led to many improvements

in the design of photobioreactors, ensuring more efficient

light utilization and better process control (Pulz, 2001;

Zhang et al., 2001). These advances should improve the

efficiency and stability of photosynthetically oxygenated

wastewater treatments.

The potential of an algal-bacterial microcosm consisting

of a Chlorella sorokiniana strain and a Ralstonia basilensis

strain for salicylate biodegradation in a continuously stirred

B 2004 Wiley Periodicals, Inc.

Correspondence to: B. Mattiasson

Contract grant sponsor: SIDA (Swedish International Development

Cooperation Agency)

photobioreactor was investigated. The influence of biomass

recirculation, light input, hydraulic retention time (HRT),

salicylate inlet concentration, and temperature on the pro-

cess efficiency was investigated. Salicylate was chosen as

a model contaminant as it is moderately toxic to microalgae

(Borde et al., 2003).

MATERIALS AND METHODS

Organisms and Culture Conditions

All experiments were performed using the following min-

eral salt medium (MSM) (g/l): KNO3, 1.25; MgSO4�7H2O,

0.625; CaCl2�2H2O, 0.1105; H3BO3, 0.1142; FeSO4�7H2O,

0.0498; ZnSO4�7H2O, 0.0882; MnCl2�4H2O, 0.0144; MoO3,

0.0071; CuSO4�5H2O, 0.0157; Co(NO3)2�6H2O, 0.0049;

EDTA, 0.5; KH2PO4, 0.6247; K2HPO4, 1.3251. The pH was

adjusted to 6.8 with KOH and the medium was autoclaved

before use (MgSO4�7H2O was autoclaved separately and

added to the sterile culture medium afterwards to avoid salt

precipitation). Sodium salicylate (Merck, Darmstadt, Ger-

many, min. 99.5%) at 1 g/l (unless otherwise specified) was

used as a model contaminant.

A Chlorella sorokiniana strain 211/8k was purchased

from the Culture Centre of Algae and Protozoa (Cam-

bridge, UK). Culture conservation and inoculum prepara-

tion were carried out according to Guieysse et al. (2002).

A Ralstonia basilensis strain (GenBank accession num-

ber AY047217; Borde et al., 2003) was used for salicylate

biodegradation. It was maintained in the mineral salt

medium described above with salicylate as the sole carbon

and energy source.

Reactors

The algal–bacterial reactor was set up using a magnetically

stirred, conical glass 600-ml vessel (Fig. 1). The algal–

bacterial reactor was inoculated with Ralstonia and

Chlorella strains at initial concentrations of 1.2 mg dw/l

and 51 mg dw/l, respectively. Light was provided by three

fluorescent lamps (Gelia E27, 15 W) in a triangular con-

figuration. When biomass was recirculated, a 110-ml cylin-

drical glass settler (3 cm internal diameter) was used for

sedimentation purposes and biomass was recirculated into

the photobioreactor from the conical bottom of the settler

(Fig. 1).

To serve as control, a similar reactor was inoculated

with Ralstonia only, at an initial concentration of 1.2 mg

dw/l and aerated from the bottom by air sparging. The

control reactor was operated at HRTs ranging from 3 h

to 2.6 days. Dissolved oxygen concentration (DOC) was

maintained above 1 mg/l by regulating the aeration rate.

The temperature of the control reactor remained constant

at 29jC, and the inlet sodium salicylate concentration

and agitation rate were set at 1 g/l and 500 rpm, respec-

tively (to achieve good agitation and consequently good

O2 transfer).

The reactors were initially started as batch cultures and

changed to a continuously fed mode once microbial growth

had started (visual observation). The DOC, temperature,

and illumination (when applied) were monitored daily. A

sample of 10 ml was withdrawn daily from valve A (Fig. 1)

to determine the absorbency, salicylate concentration, and

pH in the reactors.

Experimental Design and Statistical Analysis

Feasibility of a Continuous Photobioreactorfor Biodegradation Purposes

The algal–bacterial reactor was operated at HRTs ranging

from 1.5–4.7 days at 26.5jC and 80 AE/m2�s. The inlet

sodium salicylate concentration and agitation rate were set

at 1 g/l and 300 rpm, respectively.

Figure 1. Schematic setup of the experimental algal– bacterial photobioreactor. DOC, dissolved oxygen concentration; T, temperature; MSM, mineral

salt medium.

798 BIOTECHNOLOGY AND BIOENGINEERING, VOL. 87, NO. 6, SEPTEMBER 20, 2004

Optimization

Four series of experiments were conducted to study the

influence of biomass recirculation, light input, salicylate

inlet concentration, and temperature on salicylate removal

(Table I). The influence of biomass recirculation on the

reactor performance was investigated by allowing the ef-

fluent from the reactor to settle and continuously recircu-

lating part of the biomass in the reactor (Fig. 1). The

absorbency in the algal–bacterial reactor was controlled

by frequently removing portions of the biomass from

the settler (Valve B).

Final Optimized Process

A final experiment was conducted under optimized con-

ditions to determine the biodegradation capacity of the

system (Table I). The results represent the average and

standard deviation of values obtained from samples taken at

different times within each steady state. After each change

in one of the process parameters the reactors were allowed

to equilibrate, and once the salicylate outlet concentra-

tion was stable the reactor was monitored for a period of

at least 2 HRTs. During that period of time, between 3 and

7 samples (depending on the duration of the HRT) were

withdrawn, and each sample was considered to be a repli-

cate for this specific steady state. Results were analyzed

with one-way ANOVA with significance at P V 0.05.

Determination of the Maximum Specific GrowthRate of Ralstonia sp.

The maximum specific growth rate (Amax) of the Ralstonia

basilensis strain used was measured by cultivating the bac-

teria in a 2-L MultigGen fermentor (New Brunswick, NJ).

The reactor was operated at 500 rpm at room temperature

and aerated at 3 vvm with moist air to prevent oxygen

limitation. The reactor was filled with 1 L of MSM me-

dium containing 2 g/l sodium salicylate and inoculated with

18 mg/l Ralstonia culture. A 5-ml sample was taken ap-

prox. every hour from the liquid phase for absorbency and

salicylate concentration measurements. Fermentation was

performed in duplicate and each run was considered to be

a replicate.

Sampling and Analysis

Microbial density in the control and algal–bacterial

reactors was monitored by measuring the absorbency of

the culture broth at 550 and 620 nm, respectively. Absorb-

ency was measured with a UV-visible spectrophotometer,

Ultrospec 1000 (Pharmacia Biotech, Uppsala, Sweden).

When necessary, samples were diluted in order to maintain

the culture absorbency in the linear range (0.2–0.7). The

DOC and the temperature were determined using a

combined DOC and temperature sensor (Oxi 320, WTW,

Germany). The air flow was measured with a digital flow-

meter (Alltech, Deerfield, IL). Light input was measured

as PAR (photosynthetic active radiation) using a LI-170

sensor (LI-COR, Lincoln, NE). A Schott pH meter with a

3 M/KCL Schott electrode (Schott glas, Germany) was

used for pH determination.

For salicylate analysis, 1-ml samples were centrifuged

in microcentrifuge tubes for 10 min at 13,000g using a

centrifuge Biofuge 13 (Heraeus Instruments, Germany).

Portions of supernatant were then transferred to HPLC vials

for analysis. HPLC-UV analysis was performed using a

Varian 9010 liquid chromatograph (Varian, Walnut Creek,

CA) with a Varian ProStar 420 autosampler and a Varian

9050 variable wavelength monitor. The column used was a

Supelcosil LC-8, 5 mm (Supelco, Bellefonte, PA) (Munoz

et al., 2003b; Rahni et al., 1996). Samples were eluted

isocratically using a mobile phase composed of methanol,

water, and acetic acid (60:39:1, v:v:v) at a flow rate of

0.5 ml/min. UV detection was performed at 280 nm.

RESULTS

The absorbency measurements in this study were not very

reliable due to flock formation at long retention times and

because absorbency depends strongly on the chlorophyll

content of the microalgae, which is also a function of the

external light intensity and biomass concentration.

Table I. Influence of biomass recirculation, salicylate inlet concentration, light input, and HRT on

the photobioreactor performance.

Fixed parameters

Parameter Investigated Range

HRT

(days) T (jC)Light input

(AE/m�s)

Sodium

salicylate (g/l)

Max. estimated O2

capacity (mg/l�h)

Biomass (OD550) 1.8– 5.8 1.2 31.0 50 1 31.2

Light input (AE/m2�s) 50–300 0.8 30.6 1 25.2

Salicylate (g/l) 0.75– 2.00 1.5 31.4 150 24.7

HRT (days) 1.5– 4.7 26.5 80 1 17.6

1.2– 3.4 31.5 80 1 23.8

Optimization 0.3– 1.5 30.0 300 1 76.6

MUNOZ ET AL.: PHOTOSYNTHETICALLY OXYGENATED SALICYLATE BIODEGRADATION 799

Feasibility of a Continuous Photobioreactorfor Biodegradation Purposes

At 80 AE/m2�s and 26.5jC, complete salicylate removal

was observed in the algal–bacterial photobioreactor down

to an HRT of 2.7 days (Fig. 2). The salicylate removal

efficiency then decreased to 80% at 1.7 days and to 50% at

1.5 days. The absorbency in the algal–bacterial reactor also

decreased upon decreasing the HRT. The DOC remained

close to zero when the optical density at 550 nm (OD550)

was lower than 3 (HRT V 2.7 days). When the HRT was

reduced from 4.7 days to less than 2.7 days the pH de-

creased from 8.3 to values close to 7.

Sodium salicylate removal rates up to 181 mg/l�h were

recorded in the control reactor when aerated at 0.5 vvm

(Fig. 2). The absorbency in the reactor increased upon

reducing the HRT to 0.20 days (c4.8 h), but decreased at

shorter retention times. The pH decreased from pH 8 at an

HRT of 2.6 days to 7 at 0.12 days (c3 h).

Optimization

Influence of Biomass Concentration

Reactor performance improved with biomass recircula-

tion until the OD550 in the system reached a value of 2.4,

after which the salicylate removal rate started to decrease

(Fig. 3a). When the absorbency was greater than 2, the pH

increased from 6.8 to 7.3. In addition, the DOC remained

below 0.1 mg/l during the entire experiment.

Influence of Light Input

Salicylate removal increased with light input (Fig. 3b).

A total increase in removal efficiency from 24 F 2%

to 74 F 4% was achieved when the illumination was

Figure 3. Influence of the biomass concentration (a) and light input (b)

on the removal efficiency (squares) and biomass concentration (triangles)

in the algal–bacterial reactor.

Figure 4. Influence of the salicylate inlet concentration on the removal

efficiency (squares), removal rate (diamonds), and salicylate concentration

in the CSTR (circles).

Figure 5. Influence of HRT on the removal efficiency (squares) and

removal rate (diamonds) in the algal–bacterial reactor at 31.5jC (open

symbols) and at 26.5jC (closed symbols).

Figure 2. Influence of the HRT on the removal efficiency (squares) and

the removal rate (diamonds) in the control reactor (closed symbols) and in

the algal– bacterial reactor at 26.5jC and 80 AE/m2�s (open symbols).


increased from 50 to 300 AE/m2�s. The absorbency of the

culture increased significantly from 0.6 to 1.9 when the il-

lumination was increased from 50 to 300 AE/m2�s. The

DOC always remained below the threshold of 0.1 mg/l.

The pH values and temperature remained constant at 7

and 30.6jC, respectively.

Influence of Salicylate Inlet Concentration

Complete salicylate removal was observed at sodium salic-

ylate inlet concentrations up to 1 g/l (Fig. 4). The salicylate

removal efficiency decreased above this value. A max-

imum removal rate of 28 mg/l�h was achieved at 1.5 g/l

sodium salicylate inlet concentration (corresponding to

65% removal efficiency). The absorbency increased from

2.2 when sodium salicylate was supplied at 0.75 and 1 g/l to

3.5 when the inlet concentration was 1.5 or higher. The

DOC was always lower than 0.1 mg/l, except when sodium

salicylate was provided at 0.75 g/l, when it remained above

19 mg/l. The pH remained close to neutrality at the four

concentrations investigated.

Influence of Temperature

A maximum sodium salicylate removal rate of 19.9 mg/l�hwas achieved at 26.5jC, while at 31.5jC the maximum

removal rate reached 26.9 mg/l�h (Fig. 5). At both

temperatures the removal efficiency decreased sharply in

a very narrow interval of HRT. The DOC was below

0.3 mg/l, when the absorbency was lower than 3 (HRT V2.7 days in both cases), but increased above 10 mg/l at

higher absorbency.

The one-way ANOVA study of the process parameters

investigated showed that biomass recirculation, light input,

salicylate inlet concentration, and temperature were signif-

icant at P V 0.05 in regard to the response signal measured.

Optimized Process

Under optimized conditions the algal–bacterial microcosm

was able to degrade sodium salicylate at a maximum rate of

87 mg/l�h (Fig. 6). When the photobioreactor was ope-

rated at long HRTs (1.5 and 0.9 days) the process was

characterized by complete salicylate removal, high DOC

(f20 mg/l), and high absorbency (3.5 F 0.5 and 4.0 F 0.8,

respectively). As the HRT was reduced the removal effi-

ciency started to decrease, the pH dropped to values close to

neutrality, and the DOC decreased to zero. Complete bio-

mass recirculation was not sufficient to maintain absorb-

ency higher than 3 at the two shortest HRTs.

DISCUSSION

Photosynthetically oxygenated salicylate biodegradation

conducted under continuous illumination was efficient (re-

moval rates up to 87 mg/l�h) and stable (steady state main-

tained for several weeks) for over 1 year of operation (data

not shown). As reported by Borde et al. (2003), neither

photodegradation nor biotransformation of sodium salicy-

late by Chlorella sorokiniana caused the fate of the pol-

lutant in the present study.

When the photobioreactor was operated at long HRTs

the process was characterized by complete salicylate re-

moval, high values of absorbency, and high DOC, as shown

in Figure 2. At short HRTs the removal efficiency started to

decrease, the absorbency decreased to values close to 2, and

the DOC to levels below 0.5 mg/l, indicating that the

process was controlled by the oxygen supply and therefore

by the algal activity. In the same consortium for sodium

salicylate removal, it was reported that salicylate removal

was always limited by algal activity (Guieysse et al., 2002).

They also reported that when salicylate was present and

being degraded, the process was characterized by low O2

levels and CO2 accumulation, but once salicylate had been

completely degraded, CO2 levels fell quickly and O2 in-

creased. The same phenomenon occurred in the photo-

bioreactor, since at long HRTs the oxygenation capacity of

the system overcame the BOD requirement, leading to an

excess of oxygen produced.

The fact that algal activity controlled the salicylate re-

moval rate is clearly shown in Figure 2 at short HRTs,

because higher removal rates were recorded in the control

reactor than in the photobioreactor. In the control reactor

salicylate degradation was well described by a Monod

kinetic model. The decrease in salicylate removal rates

started to occur at a dilution rate of f0.33 F 0.01 h�1,

which is close to the maximum specific growth rate of

0.38 F 0.05 h�1 measured for Ralstonia sp. under batch

cultivation (data not shown).

The influence of the light intensity and biomass retention

on the process efficiency was therefore investigated. The

photobioreactor was tested at a low HRT, during which

salicylate removal was limited by oxygen supply.

Biomass recirculation clearly improved salicylate re-

moval (Fig. 3a), confirming that microalgae density also

limits the process at short HRT. This shows that the algae

at the steady state were not able to utilize all the irradiated

Figure 6. Influence of the HRT on the removal efficiency (squares) and

removal rate (diamonds) in the control reactor (closed symbols) and in the

optimized algal–bacterial reactor (open symbols).


light. Since photosynthetic oxygen production is propor-

tional to microalgae concentration (Li et al., 2002), an

increase in the microalgae density corresponds to removal

efficiency. However, an increase in the biomass density

above the optimum value led to a decrease in the process

efficiency because of the dark microalgal respiration that

consumes part of the photosynthetically generated oxygen

(Evers, 1991; Grobbelaar and Soeder, 1985). In addition,

sedimentation of the algal–bacterial biomass was rapid and

efficient. Thus, although unicellular green microalgae like

Chlorella species do not usually settle rapidly, the algal–

bacterial microcosms showed good sedimentation proper-

ties (visual observation), probably due to the presence of

bacteria, as previously reported by McGriff and McKinney

(1972). Hence, biomass retention is necessary and must be

carefully controlled.

Salicylate removal efficiency increased with increasing

the light intensity, as shown in Figure 3b. It shows the fact

that microalgal activity controlled the biodegradation pro-

cess. The removal efficiency depended hyperbolically on

the light intensity, with a sharper decrease at low light in-

tensities (Fig. 3b), which confirms the observations of

Janssen et al. (1999). Photoinhibition at high light intensity

(Ogbonna and Tanaka, 2000) was not observed in the pho-

tobioreactor, probably because low light intensities were

used during this study (<300 AE/m2�s).

Munoz et al. (2003) observed a decrease of 23% and

47% in the oxygen production by a Chlorella sorokiniana

strain in the presence of 500 mg/l and 1,500 mg/l sodium

salicylate, respectively. It means that reactor overloading

under continuous cultivation could rapidly lead to process

failure. Therefore, an additional experiment was carried out

to study the response of the system to changes in the pol-

lutant inlet concentration. The photobioreactor was oper-

ated at 0.75 g/l inlet sodium salicylate and at a long HRT

when the pollutant was completely removed. The salicylate

inlet concentration was then gradually increased at constant

HRT. If salicylate is not inhibitory to the algae, the removal

capacity should have remained constant in the reactor, even

under conditions of overloading. Such phenomenon was

not observed, and the removal efficiency decreased after

reaching an optimum of 28 mg/l�h at an inlet concentration

of 1.5 g/l.

Higher removal rates were obtained upon increasing the

process temperature from 26.5 to 31.5jC (Fig. 5). This is

in accordance with the results of Munoz et al. (2003a),

who reported temperature-enhanced biodegradation using a

symbiotic consortium consisting of a Chlorella sorokiniana

strain and a phenanthrene-degrading bacterium. Likewise,

Vonshak et al. (1982) showed that C. sorokiniana (a high-

temperature alga) increased oxygen production with in-

creasing temperature.

As shown in Figure 6, the optimized algal–bacterial

reactor did not exhibit as high degradation rates as the

control reactor. However the maximum oxygenation ca-

pacity was higher in the optimized experiment than in any

of the previously performed experiments (Table I). Based

on a BOD value of 0.88 g O2/g sodium salicylate reported

by Guieysse et al. (2002) it is possible to estimate the

maximum oxygen production rates in the photobioreactor

to be 77 mg/l�h (at an HRT of 0.5 days). In comparison,

Boon (1983) determined the maximum oxygenation rate

of mechanical surface aerators to be 125 mgO2/l�h. As-

suming that an efficiency of 1.2–2.4 kgO2/kWh is normal-

ly achieved in mechanically aerated ponds (Boon, 1983), a

power supply ranging from 0.104 to 0.052 kW/m3 would

be needed for conventional aerobic treatment. The amount

of oxygen that is supplied photosynthetically is equivalent

to a power consumption ranging from 0.064–0.033 kW/m3.

The energy savings in oxygen supply (since sunlight can

be used) and the increase in the process safety (no risk of

hazardous volatilization) makes photosynthetic oxygena-

tion attractive and practical.

The O2 and pH evolution in the optimized reactor was

similar to that observed in the first trial, with high DOCs

(f20 mg/l) and pH values at the highest HRT investigated

(1.5 and 0.9 days). The DOC was directly connected to

microbial activity and therefore process efficiency. Al-

though a DOC higher than 8 mg/l directly indicated com-

plete salicylate removal, the opposite is not supported by

the experimental data since total degradation was achieved

at low values of DOC. Hence, these results show that DOC

could be used as a monitoring parameter for photosynthet-

ically oxygenated wastewater treatments.

This study confirms the potential of algal–bacterial tech-

nology for the destruction of toxic organic pollutants and

especially volatile organic compounds because there is no

risk associated with pollutant volatilization. Although the

photosynthetic oxygenation rates did not match the oxygen

transfer rates in the control reactor, the safety and savings

derived from the elimination of air supply and incomes

from the biomass revalorization make this technology es-

pecially interesting in the treatment of toxic volatile or-

ganic compounds. Tank photobioreactors are usually

difficult to scale up due to the decrease in the surface/

volume ratio as the reactor size increases (Nielsen et al.,

2003), which drastically reduces the photosynthetic effi-

ciency of the system. However, other photobioreactor de-

signs such as tubular reactors or column reactors could be

more easily scaled up and should be tested (Borowitzka,

1999; Sanchez et al., 1999). Also, the constant progress

being made in the design of cost-efficient photobioreactors

for microalgae cultivation allows the range of application

of this technology to be broadened. Algal biomass retention

is necessary to improve process performance, but it should

be carefully controlled to avoid mutual shading limita-

tion. In order to avoid process failure due to microalgae

inhibition, simple control routines based on online O2

measurements can be implemented. Further studies on the

design of high-performance enclosed photobioreactors for

biodegradation purposes are needed. Future work will focus

on the scale-up of the process to demonstrate the practical

relevance of this technology for the biodegradation of toxic

volatile organic compounds.


We thank Dr. Xavier Borde for practical assistance, and Dr. Roberto

Romero (Division of Analytical Chemistry, Lund University) for

expert advice on the statistical analysis.

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