Algal Research 7 (2015) 86–91

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High-rate biomethane production from microalgal biomass in aUASB reactor

Boris Tartakovsky ⁎, Frederique Matteau Lebrun, Serge R. GuiotNational Research Council of Canada, 6100 Royalmount Ave., Montreal, QC H4P 2R2, Canada

⁎ Corresponding author.E-mail address: Boris.Tartakovsky@cnrc-nrc.gc.ca (B. T

http://dx.doi.org/10.1016/j.algal.2014.12.0042211-9264/Crown Copyright © 2014 Published by Elsevie

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 August 2014Received in revised form 25 November 2014Accepted 2 December 2014Available online xxxx

Keywords:MicroalgaeAnaerobic digestionBiomethaneUASB reactor

Biomethane production from the microalga Scenedesmus sp. AMDD was demonstrated in an upflow anaerobicsludge bed (UASB) reactor. A full factorial design experiment was used to identify the effect of organic loadingrate (OLR) of algal biomass and hydraulic retention time (HRT) on the volumetric rate of methane production.At an HRT of 4 days and an OLR of 3.23 gTVS LR−1 d−1 corresponding to an influent microalgae concentration of12 g total volatile solids (TVS) L−1, the volumetric rate of CH4 production reached 0.6 LSTP L−1

R d−1. A methaneyield of 0.18–0.2 L per gTVS of fed microalgae was estimated. A stable performance was observed throughout 3months of UASB reactor operation. Due to the short HRT and the good performance of UASB reactor, operatedat influent microalgae concentrations in a range of 4–12 gTVS L−1, this reactor type is suitable for coupling withphotobioreactors equipped with gravitational settlers.

Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction

Anaerobic digestion of microalgae was extensively analyzed andconcluded to represent an essential step in the development of anintegrated and cost-efficientmicroalgae-based process of CO2 capturingand methane production [1–3]. Algal biomass can contain varyingconcentrations of lipids with lipid accumulation associated withnitrogen stress. Although nitrogen stress can be controlled to achievebalanced growth and lipid production [4], even moderate nitrogenlimitation may lead to a decreased volumetric rate of CO2 consumption.At the same time, protein content increases in fast-growing cultures,which can be successfully used for biomethane production [5]. Overall,algae cultivation under N non-limiting conditions followed by theanaerobic digestion process offers the advantage of a high-rate CO2

sequestration process combined with the production of a renewableenergy source, such as methane. Furthermore, the anaerobic digestatecould be further utilized, e.g. as a fertilizer or for animal bedding.

Former studies related to CH4 production from whole algae weremostly concerned with estimating the CH4 yield from different (micro)algal strains in bottle tests but several studies have demonstrated CH4

production in continuously stirred tank reactors, CSTRs [6–8]. It wasdemonstrated that CH4 production in a CSTRwith a degradation efficien-cy of 40–50% requires the reactor to be operated at a hydraulic retentiontime (HRT) of 15–30 days [6,8]. The long retention times and highconcentration ofmicroalgae feed needed are a result of the slow algae hy-drolysis, which leads to ammonium-related toxicity [6]. Also, H2S-relatedinhibition ofmethanogenesiswas observed [8]. Amuch shorter retention

artakovsky).

r B.V. All rights reserved.

time of 2.2 dayswas reported for biomethanization tests performed in anupflow anaerobic sludge bed (UASB) reactor [9]. However, UASB reactorsare typically used to treat wastewater rather than solid organic wastes,since the accumulation of solids in the sludge bed might lead to reactorfailure.

In this work we investigate CH4 production in a UASB reactor frommicroalgal biomass (Scenedesmus sp. AMDD) with the objective ofoptimizing key operating parameters such as upflow velocity, influentalgae concentration and hydraulic retention time.

2. Materials and methods

2.1. Analytical methods

The volatile fatty acid (VFA) concentrations were analyzed in anAgilent 6890 gas chromatograph (Wilmington, DE, USA) equippedwith a flame ionization detector. The method details are provided inTartakovsky et al. [10]. Analytical measurements of total suspendedsolids (TSS), suspended solids (SS), total volatile solids (TVS), and vola-tile suspended solids (VSS) were carried out according to StandardMethods (APHA, [11]). The pH of the reactor effluent was measuredusing a standard pH probe and a pH meter (Accumet AB15, FisherScientific, Pittsburgh, PA, USA).

O2, N2, CH4, and CO2 concentrations in the biogas were determinedby a gas chromatograph (Agilent technologies 7820A GC system,Wilmington, DE, USA) coupled to a thermal conductivity detector,with argon as the carrier gas. The gas-phase concentration of H2S wasalso measured by a gas chromatograph (Photovac Voyager, PerkinElmer, Waltham, MA, USA) equipped with a PID detector (Column C).The column temperature was 60 °C and the carrier gas was air. The

87B. Tartakovsky et al. / Algal Research 7 (2015) 86–91

biogas flow was measured by a MilliGasCounter (Ritter ApparatebauGmbH, Germany) with a measuring resolution of 1 mL.

The destruction of whole algal cells in the anaerobic reactor wasevaluated using the cell count technique. The reactor influent and efflu-ent samples were periodically collected and examined microscopicallyfor the presence of whole (undamaged) algal cells. The samples werediluted between 10 and 100 times in order to obtain an appropriate num-ber of cells for viewing. A Leitz Laborlux S microscope (Leitz Wetzlar,Germany) with a 400× magnification was used for this analysis, pairedwith a Petroff-Hausser counting chamber slide.

2.2. Anaerobic activity tests

The specific substrate activities were determined in a series of bottletests by measuring the depletion of a non-limiting concentration of asingle substrate over time by the seed sludge or the sludge aliquotsampled from the reactor. The specific activity is used as an indicatorof the relative content of target trophic group within the biomass. Thetarget trophic group is defined by the substrate used in the test (e.g.glucose for acidogens, acetate for acetotrophic methanogens, hydrogenfor hydrogenotrophic methanogens, and sulfate for sulfate-reducingbacteria). Sludge samples were strained of any liquid and then addedto the assay bottles to obtain an initial concentration of 5 g VSS L−1,except for H2 activity tests, where an initial concentration of 2 g VSSL−1 was used.

All activity tests were conducted in triplicate using 120 mL serumbottles. At the startup, each bottle contained anaerobic biomass and0.5 mL of cysteine-sulfide reducing solution (1.25%). The bottles werethen topped up to 20 mL with 0.05 M phosphate buffer and flushedwith an 80% N2/20% CO2 gas mixture and placed in an incubator/shaker(Excella E24 model, New Brunswick Scientific, Enfield, CT,USA) at 35 °Cand at 100 rpm (except for the H2 assay, which was conducted at400 rpm to avoid mass transfer limitations). The tests were initiatedby injecting acetate (3 g L−1), glucose (2 g L−1), or H2 (239 kPa, 80%H2, 20% CO2) for acetoclastic methanogenic, fermentative, orhydrogenotrophic activity tests, respectively. For the sulfate-reducingtest, apart from a nutrients solution, which contained sulfate, a 1.6 Msolution of lactate was added as a carbon source. Also, the bottleswere supplemented with iron as well as sodium ascorbate and sodiumthioglycollate solutions [12]. Specific activity was estimated by dividingthe substrate consumption rate by the amount of VSS in the bottle.Moredetails on the specific activity determination can be found in Arcandet al. [13].

2.3. Biological methane potential

The assay performed was based on a standard wastewater BMPassay found in Cornacchio et al. [14]. An inoculum to substrate ratio of

Table 1Operating conditions during upflow velocity and full factorial design tests.

Test # Test description Upflowm h−1

UV 2 2 m h−1 upflow velocity 2UV 3 3 m h−1 upflow velocity 3UV 1 1 m h−1 upflow velocity 1UV 2R 2 m h−1 upflow velocity (repeat) 2FD 4 Factorial design #4 2FD 3 Factorial design #3 2FD 1 Factorial design #1 2FD 2 Factorial design #2 2FD 5 Factorial design #5 2FD 6(1R) Factorial design #6 (repeat) 2FD 7 Optimization step #1 2FD 8 Optimization step #2 2REP Startup repeat 2

2:1 was used based on a TVS concentration of the seed sludge andalgae in order to ensure sufficient biomass for proper degradation.

The same granular seed sludge used in the reactor was also used forthe bottle tests. Each 500 mL test bottle contained 20 g of seed sludge,2 mL of phosphate buffer and defined media, and 0.5 ml Na2S (1.25%).Liquid volume was completed to 100 mL with either distilled water ora combination of water and algal feed or reactor effluent, both solutionshaving a TVS value of approximately 16 g L−1. Recipes for the differentsolutions can be found in Frigon et al. [15]. All bottles were flushedwithN2/CO2 gas during preparation and then capped and sealed to insure ananaerobic environment. Bottles were then placed in an incubator/shak-er (Excella E24model, New Brunswick Scientific, Enfield, CT, USA) at 35°C and 100 rpm for the length of the test; usually around 35 days, oruntil methane production stopped.

2.4. UASB reactor setup and operation

Reactor experiments were carried out in a 3.5 L upflow anaerobicsludge bed (UASB) reactor with an external recirculation loop. Reactortemperature was maintained at 35 °C using a water jacket with anadjustable temperature. The reactor was inoculated with anaerobicgranular sludge (Lassonde Inc., Rougemont, QC, Canada) to obtain aninitial VSS concentration of about 40 g L−1. For the duplicate test, thereactor was emptied of anaerobic sludge, washed with water andre-inoculated using the same quantity of the anaerobic granular sludgeas in the initial inoculation procedure.

During reactor tests, the reactorwas continuously fedwith algal feedand bicarbonate buffer solutions, except for the first two weeks ofreactor operation, when a synthetic wastewater solution containing(in g L−1) pepticase 25; (NH4)2CO3, 2.5; urea, 2; NaCl, 1.75; CaCl2·2H2O,1; K2HPO4, 1; MgSO4·4H2O 0.3, was used. During the duplicate test thereactor was fed with the same algal feed and bicarbonate solutions as inthe first test. The bicarbonate buffer contained 2.72 g L−1 of NaHCO3

and 3.47 g L−1 of KHCO3. At high organic loads the buffer strengthwas increased to 5.44 g L−1 of NaHCO3 and 6.94 g L−1 of KHCO3 tomaintain the pH between 7.0 and 7.5. Algal feed was prepared using aconcentrated Scenedesmus AMDD paste containing 22–24% TVS, whichwas diluted to obtain the target influent concentration of algae. The hy-draulic retention timewas controlled by adjusting the bicarbonate flowrate. The upflow velocity was controlled by adjusting the flow rate inthe external recirculation loop. Table 1 describes all the tested combina-tions of influent algae concentration, HRT, and upflow velocity.

3. Results and discussion

UASB reactors are typically used for biomethane production fromwastewaters with low content of organic solids, while continuouslystirred tank reactors (CSTRs) are preferred for biomethanization oforganic wastes with large solid content. Accumulation of non-degradable

HRTDay

InfluentgTVS L−1

OLRgTVS LR−1 d−1

1.82 2.0 1.041.82 2.1 1.051.82 2.2 1.051.82 1.9 1.051.81 1.8 1.121.85 4.0 2.243.67 8.8 2.253.67 4.0 1.122.77 5.1 1.803.91 8.7 2.204.07 12.0 3.237.29 26.0 3.573.0 7.0 2.19

1.E+08

1.E+09

1.E+10

1.E+11

feed reactor

(UV2)

reactor

(UV3)

effluent

(UV2)

effluent

(UV3)

cell

coun

t, ce

ll g-

1TV

S

Fig. 1. Cell counts of reactor influent, reactor sludge and effluent.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

UpV 2 Upv 3 UPV 1 UpV 2 FD 4 FD 3 FD 1 FD 2 FD 5 FD 6 FD 7 FD 8

met

hane

flow

, L L

R-1d-1

A

88 B. Tartakovsky et al. / Algal Research 7 (2015) 86–91

solids in the sludge bed of a UASB reactor, which features a solidretention time that is much longer than its hydraulic retention time,eventually leads to a reduced performance and reactor failure. For thisreason, CSTRs are considered more suitable for the biomethanizationof whole algal biomass. Nevertheless, it can be hypothesized thatowing to the small size ofmicroalgae, accumulation of undigested solidsin the sludge bed of a UASB reactor could be avoided by maintaining asufficiently high upflow velocity to washout the undigested algalbiomass. This approach was verified by UASB reactor operation atthree progressively increasing upflow velocities, as shown in Table 1.In each test, the algae accumulation in the sludge bed was estimatedby comparing the influent, sludge bed, and effluent counts of whole(undigested) algal cells.

Table 2 gives biogas production rates and methane yields for eachupflow velocity test described in Table 1. The corresponding cell countsare shown in Fig. 1. Overall, no significant changes in biogas productionwere observed at the three tested upflow velocities. Cell counts at theend of each test showed no accumulation of undigested algae. Thesemeasurements were in agreement with the observed stable CH4

production. Sludge bed contained 2%–9% of whole microalgae cells,while the effluent contained 25%–30% of whole cells, as compared tothe influent counts. Algae cell counts in the sludge bed declined whenthe upflow velocity was increased to 3 m h−1. Nevertheless, cell countsremained low at all tested upflow velocities, at less than 10% of theinfluent value. Considering that an excessive upflow velocity mightlead to the washout of active anaerobic granular biomass, while at lowupflow velocities biogas production decreases due to poor mixing, anupflow velocity of 2 m h−1 was chosen for the following experiments.This value is common in UASB reactor operation [13].

Interestingly, cell counts showed that the effluent contained notmore than 30% of undigested whole algal cells, while a comparison ofTVS values in the influent and effluent streams suggested that onaverage only 50% of the algal biomass fed to the reactor was digested.This implies that although anaerobic digestion is capable of initiatingthe breakdown of microalgal cells, not all of the cell components wereeasily digested. In particular, algae cell walls contain complex polymers,which could be poorly hydrolysable under anaerobic conditions [16]. Inaddition, microscopic examination of the influent samples showed thatalgal cells were intact. The feed solution was prepared by diluting analgal paste, which contained 22–24% TVS. The paste was prepared bycentrifuging the photobioreactor effluent. Apparently, this proceduredid not result in significant cell wall damage. It can be hypothesizedthat the UASB reactor operation using fresh algae, e.g. after a gravita-tional settler, might lead to similar performance.

Following initial tests aimed at selecting an acceptable upflowvelocity for UASB operation, the impact of two key process parameters,HRT and organic load (i.e. influent concentration of algae) onbiomethane production was studied. A full factorial design experimentwas planned at two levels (Statistics Toolbox of Matlab R2010a,

Table 2Biogas production and CH4 yields.

Test # CH4 flowLSTP L−1

R d−1CH4

%Effluent tCODg L−1

CH4 yieldL gTVS−1 fed

UV 2 0.10 ± 0.020 80.0 0.20 0.09 ± 0.02UV 3 0.146 ± 0.006 80.0 1.76 0.12 ± 0.01UV 1 0.166 ± 0.006 78.3 1.72 0.14 ± 0.01UV 2R 0.140 ± 0.006 77.5 1.67 0.15 ± 0.02FD 4 0.086 ± 0.003 78.3 1.23 0.08 ± 0.02FD 3 0.289 ± 0.003 70.2 0.92 0.13 ± 0.02FD 1 0.540 ± 0.009 63.5 2.16 0.22 ± 0.01FD 2 0.163 ± 0.03 69.2 4.72 0.15 ± 0.01FD 5 0.274 ± 0.037 74.6 5.12 0.15 ± 0.03FD 6 0.374 ± 0.017 70.1 5.15 0.18 ± 0.01FD 7 0.580 ± 0.043 67.1 8.12 0.18 ± 0.02FD 8 0.626 ± 0.003 65.6 10.24 0.17 ± 0.01REP 0.337 ± 0.014 73.1 n/a 0.14 ± 0.01

Mathworks, Natick, MA, USA) as described in Table 1. Each operatingpoint described in this table was maintained for at least 6 HRTs toensure steady-state conditions at the end of the test. The results of thefull factorial design tests are given in Table 2 and Fig. 2. Based on theseresults the following regression models were obtained to describe CH4

flow (FCH4, in LSTP L−1R d−1) and CH4 yield (YCH4, in LSTP gTVS−1 fed)

dependence on HRT and OLR.

FCH4 ¼ 0:043þ 0:052 X1X2 R2 ¼ 0:91� �

ð1Þ

YCH4 ¼ −0:069þ 0:045 X1X2 R2 ¼ 0:92� �

ð2Þ

where X1 is the hydraulic retention time (days) and X2 is the organicload (g d−1).

The methane flow response surface corresponding to Eq. (1) isshown in Fig. 3A. It suggests that an increase in both organic load (i.e.

0.00

0.05

0.10

0.15

0.20

0.25

UpV 2 Upv 3 UPV 1 UpV 2 FD 4 FD 3 FD 1 FD 2 FD 5 FD 6 FD 7 FD 8

met

hane

yie

ld, L

g-1

TVS

fed

B

Fig. 2. Steady-state values ofmethane production (A) andmethane yield (B) in upflowve-locity and factorial design experiments.

0

20

40

60

80

100

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30 35

CH

4 co

ncen

trat

ion,

%dL,noitcudorp

sagoiB

-1

time, days

biogas

concentration

algal feed starts

Fig. 4. CH4 production and CH4 concentration during the reproducibility test carried out atan HRT of 3.5 days and an OLR of 2.2 gTVS L−1

R d−1.

89B. Tartakovsky et al. / Algal Research 7 (2015) 86–91

influent algae concentration) and HRT, increases the volumetric CH4

production, as can be also seen from Eq. (1). The CH4 yield regressionmodel suggested a similar dependence (Eq. (2)), although the optimaloperating point might not be necessarily the same. Following thesetests, a gradient ascendmethod was used to maximize CH4 production.Two additional experiments were performed. These experiments aredescribed in Table 1 as tests FD7 and FD8 with corresponding resultsincluded in Fig. 3B. The tests confirmed that in order to maximizebiogas production while maintaining an acceptable CH4 yield (e.g.0.18–0.2 L g−1

COD fed), the HRT should be increased whenever theinfluent algae concentration is increased. At the highest tested influentalgae concentration of 26 gTVS L−1 the methane production reached0.63 LSTP L−1

R d−1, although HRT had to be increased to 8 days. Withthis respect, the optimal operating point corresponded to an HRT of4 days and an influent concentration of 12 gTVS L−1 (FD7 in Tables 1and 2). CH4 yield corresponding to this operating point (0.18 ±0.02 L gTVS−1 fed) was slightly lower than in FD1 (0.22 ± 0.01 L gTVS−1

fed). Also, this operating point was in agreement with the correspond-ing regression model (Eq. (1)) value, while the second optimizationstep resulted in lower than expected biogas production. Furtheroptimization of the operating conditions can be pursued by carrying outa three level design experiment centered at FD7 to obtain a quadraticregression model. In the same way, methane yield optimization can beachieved by a factorial design experiment centered at FD1.

When compared with CH4 production from the same strain ofmicroalgae in a CSTR, where the highest CH4 production rate of 0.07 L

1.121.8

2.253.3

0.0

0.5

1.0

1.5

2.0

2.5

1.85 2.77 3.67 4.07 7.29

met

hane

flow

, L d

-1

HRT, day

1.12

3.3Factorial designGradient ascend

B

1.121.92

2.840.0

1.0

2.0

3.0

4.0

5.0

1.85 2.71 3.57 4.43 5.29 6.14 7.00 OLR

, g L R-1

d-1

met

hane

flow

, L d

-1

HRT, day

A

Fig. 3. (A) Response surface of the biogas flow rate model Eq. (1) and (B) experimentalvalues of biogas flow obtained in the full factorial design experiment and during thegradient ascend optimization steps.

L−1R d−1 was observed at an HRT of 16 days and an influent algae

concentration of 11.4 gTVS L−1 [8], the UASB reactor operation resultedin a 9 fold improvement in terms of volumetric performance. The CH4

yield was similar in both reactor types, with an average yield of0.19 L g−1

COD fed. A similar methane yield of 0.15–0.24 gTVS L−1

corresponding to a 50% conversion efficiency was observed by Raset al. [7] in a CSTR fed with Chlorella vulgaris biomass and operated atan HRT of 16–28 days. Also, an UASB reactor fed with Scenedesmusobliquus and operated at anHRT of 2.2 days showed close to 50% conver-sion efficiency [9]. In this case a CH4 production rate of 0.3 L L−1

R d−1

was observed under mesophilic conditions.Overall, the UASB reactor was operated onmicroalgal feed for a total

of 140 days with a stable CH4 production at all tested operating modes.Nevertheless, the reproducibility of CH4 production from microalgalbiomass was confirmed by re-inoculating and restarting reactor opera-tion. Upon reactor re-inoculation, it was operated at an influent concen-tration of 7.0 g L−1 and anHRT of 3 days, which is close to the operatingconditions during the FD5 test (Table 1). Fig. 4 shows the observeddynamics of biogas production and CH4 concentration in the biogasduring reactor startup. Stable CH4 production was achieved after 2weeks of operation, which was similar to the startup period duringthe first reactor experiment. Also, the biogas production observed inthe replicate test was similar to that obtained in test FD5, whichwas carried out at a slightly shorter HRT (0.34 LSTP L−1

R d−1 vs

0

50

100

150

200

250

300

350

400

0 10 20 30 40

gL

m(noitcudorp

4H

Cevitalu

muC

TVS-1

)

Time, days

effluent

feed

Fig. 5. Cumulative CH4 production from whole algae and UASB reactor effluent in BMPtests.

0

40

80

120

whole sludge seed sludge

CH

4pr

oduc

tion

(mg

g VSS

-1d-

1 )

B

0

20

40

60

80

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120

140

160

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whole sludge seed sludge

H2

cons

umpt

ion

(mg

g VSS

-1d-1

) A

0

100

200

300

400

500

whole sludge seed sludge

SO

4co

nsum

ptio

n (m

g g V

SS

-1d-

1 )

D

0

250

500

750

1000

1250

whole sludge seed sludge

gluc

ose

cons

umpt

ion

(mg

g VSS

-1d-

1 )

C

Fig. 6. Hydrogenotrophic (A), acetoclastic methanogenic (B), fermentative (C), and sulfate-reducing (D) metabolic activities observed in batch activity tests.

90 B. Tartakovsky et al. / Algal Research 7 (2015) 86–91

0.27 LSTP L−1R d−1 in REP and FD5 tests, respectively, Table 2). Further-

more, methane production in the replicate test was in a reasonableagreement with the regression model (Eq. (1)) prediction of0.38 LSTP L−1

R d−1 for this set of operating conditions.In addition to the reactor experiments described above, sludge

samples withdrawn at the end of the first reactor test were used tocarry out the biochemical methane potential (BMP) and batch activitytests. In particular, the BMP test was performed to evaluate the com-pleteness of algal biomass digestion in the UASB reactor. Consequently,CH4 production from fresh algal feed and theUASB reactor effluentwerecompared. Fig. 5 shows that after 35 days of incubation CH4 productionin the bottles containing the reactor effluent was four times lower thanin the bottles containing fresh algal feed. This implies that the UASBreactor with a retention time of only 4–8 days achieved close to 80%conversion efficiency.

Batch activity tests were carried out using reactor sludge sampleswithdrawn on day 140 of reactor operation. These tests were aimed atcomparing initial trophic activities with the activities established inthe reactor fed with microalgae for a prolonged period of time. Ascompared to the inoculum sludge, acetoclastic and hydrogenotrophicmethanogenic activities slightly decreased (Fig. 6A,B). Most changeswere observed in the fermentative (glucose consumption) activity,which considerably decreased (Fig. 6C), and in sulfate-reducing activity,which increased several folds (Fig. 6D). A significant increase of thesulfate-reducing activity and elevated (above 1000 ppm) levels of H2Sin the biogas were previously observed in the CSTR fed with the samestrain of microalgae [8]. However, H2S levels in the UASB reactor biogasremained below 50 ppm, apparently due to lower influent concentra-tion of algal feed and, accordingly, lower sulfate concentration in thereactor liquid.

4. Conclusions

Whole microalgae biomethanization was demonstrated in a UASBreactor operated with an HRT of 4–8 days and an influent microalgae

concentration of 3.7–12 g gTVS L−1. A factorial design experimentwas used to identify the effect of OLR and HRT on the CH4 productionrate and to increase the rate of methane production. A volumetricproduction rate of 0.57 LSTP L−1

R d−1 was obtained at an HRT of4 days and an influent concentration of 12 gTVS L−1, correspondingto an OLR of 3.23 gTVS LR−1 d−1. This compares favorably withpreviously reported volumetric rate of CH4 production of0.07 LSTP L−1

R d−1 observed in a CSTR reactor operated at an HRTof 16 days and fed with the same microalgal biomass. The accumula-tion of undigested algal biomass was not observed after 140 days ofUASB reactor operation. Also, batch activity tests demonstrated thepresence of a robust anaerobic microbial consortium in the sludgebed. Owing to the relatively low influent concentration of microalgalbiomass, ammonium accumulationwas avoided and the H2S concen-tration in the biogas remained relatively low. Overall, a CH4 yield of0.18–0.2 L gTVS−1 fed and the degradation efficiency close to 50% wereestimated.

UASB reactor operation at a relatively low influent concentrationof micro-algae enables the use of a gravitational settler after thephotobioreactor. A regression model of biomethane productionsuggested that the HRT should be increased proportionally to theinfluent microalgae concentration, in order to maintain optimal reactorperformance. This regression model could be used for process designoptimization and for controlling the reactor HRT in case of influent com-position variations. While the current study demonstrated high-ratebiogas production from whole microalgae, biogas production fromdefatted algae might also be possible and might contribute to processfeasibility [1].

Acknowledgment

We would like to thank Crystal Whitney, Scott MacQuarrie, andPatrick McGinn (NRC-Halifax) for providing Scenedesmus sp. AMDDbiomass.

91B. Tartakovsky et al. / Algal Research 7 (2015) 86–91

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