Algal Research 5 (2014) 95–102

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Algal Research

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Cultivation and anaerobic digestion of Scenedesmus spp. grown in apilot-scale open raceway

K.C. Tran a,⁎, J.L. Mendoza Martin a, S. Heaven a, C.J. Banks a, F.G. Acien Fernandez b, E. Molina Grima b

a Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UKb Department of Chemical Engineering, University of Almería, 04120 Almería, Spain

⁎ Corresponding author. Tel.: +44 2380 592845.E-mail addresses: kct1f10@soton.ac.uk (K.C. Tran), jlm

(J.L. Mendoza Martin), sh7@soton.ac.uk (S. Heaven), cjb@facien@ual.es (F.G. Acien Fernandez), emolina@ual.es (E. M

http://dx.doi.org/10.1016/j.algal.2014.06.0012211-9264/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2014Received in revised form 20 May 2014Accepted 2 June 2014Available online xxxx

Keywords:Micro-algaeScenedesmus spp.RacewayFlue gasAnaerobic digestionBiodegradability

Digestibility of a micro-algal mixture was evaluated by mesophilic anaerobic digestion in continuously-stirredtank reactors. The culture consisted primarily of Scenedesmus spp. continuously cultivated over a 6-month periodin a 100 m2 raceway reactor instrumented to record pH, dissolved oxygen and temperature. The racewayreceived supplementary carbon in the form of flue gas from a diesel boiler (10% CO2) injected into a 1-m deepsump to control pH in the range 7.8–8.0. Dilution was optimised to biomass productivity and gave valuesof 10–15 and 20–25 g total suspended solids (TSS) m−2 day−1 in winter (December–February) and spring(April–May), respectively. The culture for the anaerobic digestion trial was harvested in February by centrifuga-tion to give an algal paste containing 4.3% volatile solids (VS). Semi-continuous digestion at organic loading ratesof 2.00, 2.75 and 3.50 g VS l−1 day−1 gave volumetric biogas productions of ~0.66, ~0.83 and ~0.99 l l−1 day−1,respectively. Specific methane yield ranged from 0.13 to 0.14 l CH4 g

−1 VSadded with biogas methane content~62%. Overall the digestion process was stable, but only ~30% VS destruction was achieved indicating low biode-gradability, due to the short retention times and the recalcitrant nature of this type of biomass.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The finite nature of fossil fuel supplies combined with an increasingglobal demand for energy has led to substantial interest in developingrenewable biologically-produced carbon neutral fuel sources [1]. Inthe search for renewable biofuels, micro-algae have been consideredas an attractive source of bioenergy for several reasons: they have arapid growth rate and high biomass production, do not compete withterrestrial crops for arable land, have minimal needs for pesticidesor herbicides, and offer the potential for cultivation using wastewaternutrients [2–5].

The major disadvantages of micro-algal biomass are the cost andenergy requirements for growth and harvest, which currently make ituncompetitive with other biomass types [6]. Reduced costs of produc-tion are thus essential to the success of this technology. Open raceways,suggested more than fifty years ago by Oswald and Golueke [7,8],are still the only realistic large-scale engineered method available formicro-algal biomass production for biofuels: this is because of thelower initial investment compared to alternative systems, due to lowerconstruction costs, and the low operating costs [9–11]. The biomass

m1f10@soton.ac.uksoton.ac.uk (C.J. Banks),olina Grima).

productivity reported for this type of reactor is, however, lower thanthe required to achieve commercial bioenergy production [6].

Micro-algae can be utilised to produce different types of biofuels,and amongst these bio-methane appears increasingly attractive. Themain reason is that most of the macromolecules (i.e. protein, lipid,carbohydrate) in algal biomass can theoretically be converted to biogasthrough the anaerobic digestion process [12]. Algal residues after bio-diesel production can also be utilised to produce biogas [13].

Although the idea of using micro-algae as feedstock for biogasproduction is not new [14], this research area has seen an upsurge of in-terest in the past few years. Literature on anaerobic digestion of micro-algae is still limited, however, in part due to the limited availability ofsufficiently large quantities of feedstock material. This paper reportson a study of the semi-continuous digestion of a mixed micro-algalculture grown in an open raceway.

2. Materials and methods

2.1. Algal cultivation

2.1.1. BioreactorsThe algae were grown in experimental reactors at the Estación Ex-

perimental Las Palmerillas of Fundación CAJAMAR in El Ejido (Almería),Spain. The inoculum for the raceway was grown in a closed tubularfence-type photobioreactor constructed of 0.09 m diameter acrylic

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tubes with a total length of 400 m and stacked vertically to a height of2.2 m. The unobstructed space on either side of the reactor was 1.4 m,and the reactor was located in a greenhouse with a polythene coveringto diffuse direct sunlight (Fig. 1a). The volume to surface ratio was75 l m−2, and oxygen was stripped from the culture using coarse airbubbles in a vertical tower 3.5 m high and 0.4 m diameter, with a heatexchanger inside to control the reactor temperature. Pure CO2 wasinjected directly into the reactor to meet the growth needs of thealgae, with the addition based on pH control.

The open channel raceway reactor (Fig. 1b) used for long-term cul-tivation consisted of two 48 m channels, each 1 m wide and connectedby 180° bends at both ends to give a total surface area of 100 m2. Theraceway was constructed out of 3 mm white fibreglass and the culturewas circulated and mixed by a marine plywood paddlewheel with adiameter of 1.2 m driven by a 0.37 kW geared electric motor (EbarbaandWEG Iberica, Barcelona, Spain). In order to supply CO2 and enhancethe mass transfer capacity of the system, flue gas from a diesel boilerwas bubbled on demand through three membrane plate diffusers posi-tioned at the bottom of a sump (0.65m long × 0.90mwide × 1mdeep)located 1.5 m downstream of the paddlewheel. The raceway wasequipped with pH-T and dissolved oxygen (DO) probes (5083 T and5120, Crison, Barcelona, Spain) calibrated andmaintained in accordancewith the manufacturers' instructions and connected to transmitters(MM44, Crison, Barcelona, Spain). Information was collected by a dataacquisition card and software (LabJack U12 and Daqfactory AzeotechArizona USA, respectively). Solar radiation was measured on-site bymeans of a light sensor (Li-Cor, Pyranometer, PY 61654, USA). A full hy-draulic characterisation of the raceway and studies of O2 and CO2 masstransfers are reported elsewhere [15–17].

The total suspended solid content (TSS) of the raceway culture wasmeasured by vacuum filtration of a 250 ml sample through a pre-weighed glass fibre filter (MN 85/90, Macherey-Nagel, Spain) and

Fig. 1. Algal cultivation facilities. (a) Tubular fence-type photobioreactor used to growthe inoculum of Scenedesmus sp. (F.G. Acién Fernández et al., 2013) [40]. And (b) racewayreactor in which the main algal culture was grown.

subsequent drying of the filter at 80 °C for 24 h. Total solid (TS) contentof centrifuged algal material was measured according to StandardMethod 2540G [18].

2.1.2. Culture mediumThe culture medium used in both tubular photobioreactor and race-

way reactor was prepared in fresh water using commercial agriculturalfertilisers to give ionic concentrations additional to those naturallypresent in the fresh water source as follows (mmol l−1): NO−

3 9.49,Na+ 8.20, NH+

4 0.59, Cl−11.10, H2PO−4 1.00, Fe3+ 2.00, K+ 1.60,

Mn2+ 0.84, Ca2+ 5.00, and Zn2+ 0.56.

2.1.3. Seed culture, inoculation and raceway operationA seed culture of Scenedesmus spp. was grown in the tubular

photobioreactor in batch mode to a concentration of 2 g TSS l−1.2000 l of the seed culture was inoculated into the raceway, which hadpreviously been filled with 8 m3 of fresh culture medium. The finaloperating depth was 0.1 m, giving a total culture volume of 10 m3,and the paddlewheel rotational speed was set to give a liquid velocityin the channel of 0.27 m s−1. The raceway was initially operated inbatch mode for 11 days, until a biomass concentration of 0.9 g TSS l−1

was achieved. The culture was then switched from batch to semi-continuous mode, in which part of the culture was removed on eachweekday by gravity drainage and replaced by fresh culture medium torestore the raceway to its 0.1 mworking depth. The difference betweenthe volume harvested and the addedmediumwas taken as the loss dueto evaporation. The pH in the raceway was controlled between setpoints of 7.8 and 8.1 by bubbling exhaust gases (10% CO2, 18.1 ppmCO, 38.3 ppm NOx, and 0.0 ppm SOx) from a diesel oil-fired boiler(Tradesa, MOD SF 20, RA-GTI, TRADE, Italy) at 6 m3 h−1 (flow metreFR4500, Key Instruments, USA). Before injection into the raceway, theflue gas was cooled to air temperature and stored in a 1.5 m3 reservoirat a constant pressure of 2 bar (BUSCH, Mink MM 1104 BP, modelC-1000 compressor, Barcelona). Flue gas was only injected duringdaylight hours (solar radiation N 50 W m−2) as no CO2 demand wasexerted by the algae during the night.

Biomass concentration was measured daily, and average daily pro-duction (g TSS m−2 day−1) was calculated based on this concentrationand the harvested volume, taken over weekly periods. The culture wasmaintained from December until May to provide information on algalgrowth during winter and spring periods. The micro-algal culture usedin the AD experiment was harvested in batches from the racewaybetween 19 January and 3 February by continuous centrifugation (AlfaLaval Clara 15, LAPX 404 SGP-31G/TGP-61G). The batches of algalpaste had TS contents ranging from 7 to 12%, and were frozen immedi-ately on harvesting then shipped under refrigeration from Almeria(Spain) to Southampton (United Kingdom). On arrival the paste wasthawed and the batches mixed to give a homogeneous mass that wasdistributed into small containers and re-frozen immediately, thenthawed as required before use.

2.2. Anaerobic digestion

Six digesters, eachwith a working volume of 1.5 l, were operated fora period of 136 days (Fig. 2). These were constructed of PVC tube withgas tight top and bottom plates. The top plate was fitted with a gas out-let, a feed port sealed with a rubber bung, and a draught tube liquid sealthrough which an asymmetric bar stirrer was inserted with a 40 rpmmotor mounted directly on the top plate. Temperature was maintainedat 35 ± 0.5 °C by water circulating through an external heating coil.During semi-continuous operation digestate was removed throughan outlet port in the base plate, and feed added via the top plate. Gasproduction was measured using tipping-bucket gas counters with con-tinuous datalogging. Calibration of gas counters was checkedweekly bycollecting the gas from the outlet of the gas counter in a Tedlar bag (SKCLtd, Blandford Forum,UK); the volumewas thenmeasured accurately in

Fig. 2. Anaerobic digesters: (a) schematic cross-section and (b) photograph.

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a water displacement weight-type gasometer [19]. All gas volumesreported are corrected to standard temperature and pressure of 0 °Cand 101.325 kPa.

The inoculum was taken from laboratory-scale digesters that hadbeen operating with a feedstock of freeze-dried Scenedesmus spp.;its characteristics were: pH 7.5, TS content 6.17%, VS content 4.71%,total ammonia nitrogen (TAN) 2070 mg N l−1, and total alkalinity13.3 g CaCO3 l−1. The digestion feedstock composition is shown inTable 1. On day 1 of the digestion experiment, three pairs of digesterswere fed with the thawed micro-algal mixture at an organic loadingrate (OLR) of 2 g VS l−1 day−1. After 75 days, the OLR of one pair waskept unchanged, and the OLRs of the other two pairs were increasedto 2.75 and 3.5 g VS l−1 day−1 respectively. All six digesters were thenoperated for at least three hydraulic retention times at these OLR. Sys-tem performance was monitored by weekly measurement of digestatepH, TS and VS, TAN, total alkalinity, and volatile fatty acids (VFA).

pH was determined using a Mettler Toledo FE20/EL20 pH metrewith a combination glass electrode, calibrated in buffers at pH 4, 7 and9.2 (Fisher Scientific, UK). TS and VS were measured according toStandard Method 2540G [18]. Alkalinity was measured by titrationusing 0.25 N H2SO4 to endpoints of 5.7, 4.3 and 4.0 [20]. Total KjeldahlNitrogen (TKN) and ammonia were determined using a Kjeltech blockdigestion and steam distillation unit according to the manufacturer'sinstructions (Foss Ltd, UK). VFA concentrations were quantified in aShimazdu 2010 gas chromatograph (Shimadzu, UK). Calorific values(CV) were determined using a bomb calorimeter (CAL2k-ECO, SouthAfrica). Biogas composition was measured three times a week using aVarian CP-3400 gas chromatograph (Varian, UK). Elemental composi-tion (C, H, N) content was determined using a FlashEA 1112 ElementalAnalyzer, (Thermo Finnigan, Italy) based on themanufacturer's instruc-tions using methionine, L-cystine and sulphanilamide as standards.

Table 1Characteristics of the algal mixture after centrifugation.

Parameter Unit Algal mixture

TS g kg−1 WW 108.5b

VS g kg−1 WW 43.31b

TKN g kg−1 WW 3.77a

C % (on a VS basis) 45.5H % (on a VS basis) 9.0O % (on a VS basis) 35.9N % (on a VS basis) 8.7S % (on a VS basis) 1.0C/N – 5.25/1Calorific value MJ kg−1 VS 21.36

a Measured as dry matter.b Measured as wet matter.

Theoretical CV was calculated using the Dulong equation accordingto the method in Combustion File 24 [21]. Theoretical biogas composi-tion was calculated based on the Buswell equation [22]. The calorificvalue of methane was taken as 39.84 MJ m−3 at STP.

3. Results and discussion

3.1. Raceway cultivation

The method of carbonation of the culture medium in the racewaywas found to be adequate to maintain high micro-algal productivity inthe raceway. The CO2-rich flue gas injected at the bottom of the sumpgave turbulence in this zone and allowed almost complete dissolutionof the CO2 into the aqueous phase with only around 10% of CO2 enteringthe sump leaving in the exhaust gas, which had a CO2 content of b0.5%(Fig. 3a) (de Godos et al., 2014). By supplying flue gas ‘on demand’ itwas possible to control the raceway pH in the desired range of 7.8–8.1even during peak carbon demand in the mid-day period when solar ra-diation was at its highest (Fig. 3b).

During the batch start-up the average water temperature was13.6 °C, with a maximum daytime temperature of 18.4 °C, and wastherefore lower than the optimal growth temperatures reported forthe most common species [23,24]. The temperature increased in thetransition from winter to spring conditions and this had a pronouncedeffect on algal growth and on the evaporation rate. In December theaverage air temperature was around 15 °C with an evaporation rate ofaround 200 l day−1; whilst by the end of the experimental period inMay the temperature had risen to 25 °C with an evaporation rate of700 l day−1 (Fig. 4a). The evaporation rate was therefore between 2%and 7% of total reactor volume and a significant fraction of the totaldaily volume removed for harvesting: this loss was taken into accountwhen calculating the biomass yield.

Fig. 4b shows the total suspended solid (TSS) concentration in theraceway culture, which remained around 0.75 g l−1 for the first6 weeks of operation when the average water temperature was fairlyconstant. Once the temperature started to rise there was a corre-sponding increase in culture density, reaching ~1.4 g TSS l−1 by earlyApril. Further increases in temperature did not increase culture density,which in fact declined slightly as temperatures rose further in May.Fig. 4b also shows the solar efficiency, which was between 0.6 and 0.7g E−1, in the range for cultures that are photosynthetically active [25].Small variances in this parameter could be explained by changes inthe average irradiance due mainly to the variation in weather condi-tions from winter to spring; some variations, however, may also be asa result of changes in the cell size and concentration or pigment con-tent [26]. The higher temperatures seen at the end of the study periodresulted in a reduction in solar efficiency and TSS concentration as a

Fig. 3. CO2 demand on the sump during a daylight period: (a) pH control hysteresis in the range of 7.8–8.1 as maintained by flue gas injection (0 is flue gas valve closed and 1 is flue gasvalve opened). And (b) timing of flue gas injections and % of CO2 in gas exhausted to atmosphere.

Fig. 4. Parameters studied during the culture period. (a) Online culture temperature registered and daily evaporation in the raceway. (b) Daily analysis of total suspended solids and quan-tum yield. (c) Daily analysis of absorbance at 680 nm and turbidity. And (d) weekly average dilution and productivity in raceway algal culture.

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result of photoinhibition in part of the culture. Turbidity and absor-bance at 680 nm (Fig. 4c) were related to the culture TSS concen-tration (R2 = 0.613, n = 111 and R2 = 0.775, n = 108, respectively;p b 0.001 in both cases) showing that, as expected, absorption of lightin the culture increased with the culture density.

To determine the optimum dilution rate a trial-and-error approachwas adopted, hence the dilution rate was modified weekly. Fig. 4dshows the dilution rate expressed as the amount of fresh mediumadded as a percentage of the total reactor volume; and the productivityas themass of algae produced per unit area per day. The results indicatethat 43% of the variation in productivity can be attributed to the dilutionrate (R2 = 0.43, n = 23, p b 0.001), and also show how productivityincreased during the spring period from late February through to theend of May, reflecting the higher temperatures and longer days withmore solar radiation. When averaged over weekly periods, estimatedproductivities of 10–15 g TSS m−2 day−1 were achieved in winter(December–February) and of 20–25 g TSS m−2 day−1 during spring(April–May) (Fig. 4d). Maximumbiomass productivity was comparableto that reported in the literature, with values of 19 g TSSm−2 day−1 ob-tained by Ketheesan and Nirmalakhandan [10] in an airlift-driven race-way, and by Vonshak and Guy [27] in outdoor cultivation with naturalsunlight and a culture depth of 10 cm. Similar results were obtainedby Boussiba et al. [28] with CO2 addition and pH in the range 7.0–7.5.Yoo et al. [29] achieved 21.8 g m−2 day−1 for Scenedesmus spp. culti-vated for CO2 mitigation. These productivities have been suggested asindicating a good potential for fuel production [30]. The highest pro-ductivity was obtained in spring at a dilution rate of 20–25%. The incre-ment in productivity in Spring is due to the increasing intensity ofirradiance and the duration of daylight periods. The raceway designmay also have affected productivity during the winter period as theangle of inclination of the sun during these months is low, and the nar-row width of the raceway resulted in more than half of the culture sur-face being in the shadowof the freeboard of the channel atmid-day; thisarea was reduced as the sun's angle of inclination increased throughoutthe experimental period.

Fig. 5 shows continuousmonitoringdata for selected periods of oper-ation corresponding to batch start-up, first week of semi-continuousoperation, period of maximum productivity and the onset of reduced

Fig. 5. Selected results from continuousmonitoring of raceway culture during the experimental(d) reduced productivity.

productivity in early summer conditions (Fig. 5a, b, c andd respectively).During the batch start-up period (Fig. 5a) peak daily solar radiationwasbetween 300 and 550Wm−2 and bothwater temperature and DO con-centrations showed diurnal fluctuation, with day and night tempera-tures varying by up to 10 °C. A similar pattern was reflected in thefirst week of continuous operation (Fig. 5b), and during December DOconcentrations never rose above 200% saturation. During the entirestudy period pH remained within narrow limits, with only small in-creases during the dark period whichwere more pronounced in winterthan spring. The control of raceway pH by the addition of flue gas CO2

addition thus proved very effective, and the single carbonisation sumpwas adequate for its purpose. In the maximum productivity period(Fig. 5c), peak solar radiation was between 800 and 900 W m−2, andwater temperature peaked between 25 and 30 °C falling to around15 °C at night. DO concentrations during the day could reach 350%saturation. DO concentrations as high as 45 mg l−1 have been reportedin raceways [31,32], causing inhibition of photosynthesis and growth.By May water temperature had increased to over 30 °C in the middleof the day. This was associated with reduced productivity and lowerpeak DO concentrations of 200–250% saturation (Fig. 5d). In all casestemperature and DO peaks lagged behind peak solar radiation.

The accumulation of photosynthetic oxygen in raceways during op-eration at high irradiancies was one of the main potential problemsobserved, as this can lead to a loss of productivity due to inhibition.Accumulation was minimised by operating the gas exchange sumpwith flue gas rather than pure CO2 as this resulted in a greater gasvolume passing through the sump, which in turn strippedmore oxygenout of the culture medium [16]. The sump proved to be one of the mainzones in the raceway where oxygen could effectively be removed. Theother major problem was the increase in temperature between springand summer. As air temperatures increased in late May the watertemperature in the raceways rose above 35 °C resulting in verymarked effects on productivity as a result of thermal inhibition. Thiswas apparent as the dissolved oxygen concentration in the racewaywas reduced during the mid-day period along with a reduced demandfor CO2, both strongly indicate inhibition or even death of the culture.As the summer progressed it became increasingly difficult to maintainthe raceways in a stable condition.

period: (a) batch start-up, (b) firstweek semi-continuous, (c)maximumproductivity, and

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3.2. Anaerobic digestion trials

3.2.1. pHThe pH in all six digesters fluctuated between 7.07 and 7.52 (Fig. 6a).

From day 1 to day 75when all digesters were operating under the sameconditions, the daily pH values were almost identical: small day-to-daydifferences are therefore believed to reflect slightly different intervalsbetween sampling and pH measurement. From day 75 onward, therewas some variation between digesters at different OLRs, with thelower-loaded A1 and A2 operating at a slightly higher pH than A3–A6.Nevertheless, pH remained in the optimum range for the growth ofmethanogens in all cases.

3.2.2. TS and VSAs seen in Fig. 6b, in the first 28 days the digestate TS increased

gradually from 6% and stabilised at around 9.5%. This increase reflectedthe high proportion of inorganic matter (VS/TS = 0.40), which mayhave been in part due to a combination of wind-blown dust enteringthe raceway and high salinity due to surface evaporation. The digestateVS content decreased gradually from an initial value of 4.7% andstabilised at 3% by the end of experiment. Based on the feedstock VSconcentration of ~4.3%, the apparent VS destruction was low, withonly ~30% of VS degraded.

3.2.3. Alkalinity, ammonia and VFASimilar to the trend in digestate TS, in the first 28 days there was

an increase in total alkalinity, starting at 13 g CaCO3 l−1 and thenstabilising at around 35 g CaCO3 l−1 until the end of experiment. TheTAN concentration in all digesters rose slightly to around 2.4 g N l−1

in the first 14 days of operation then declined during the experimentalrun and stabilised at around 1.8 g N l−1 in all cases. The digestate TKNconcentration at the end of the run was around 4.1 g N l−1 indicatinga breakdown rate for organic nitrogen-containing compounds ofaround 44%, slightly above the VS destruction rate but still quitelow. Total VFA concentrations in the digestate were initially between100 and 250 mg l−1 and after day 75 remained consistently below

Fig. 6.Results from anaerobic digestion trial with raceway culture of Scendesmus sp. (a) Digestatproduction. Vertical dotted line indicates change in organic loading rates on day 75.

120 mg l−1. Despite the low VFA concentration, the IA/PA ratio roseduring the first 28 days then fluctuated between 1 and 4, a rangeoften considered to indicate potential instability [20]. The reason forthis high value for the IA/PA ratio under apparently stable conditionsis unknown.

3.2.4. Biogas production and methane yieldVolumetric biogas production (VBP) was low, with average

values over the last 30 days of the experiment of 0.66, 0.87 and1.06 l l−1 day−1 for OLR of 2.0, 2.75 and 3.5 g VS l−1 day−1 respectively(Fig. 6c). The specificmethane production (SMP)was almost unaffectedby the appliedOLR and in the sameperiod averaged around0.139, 0.133and 0.131 l CH4 g−1 VSadded at OLR 2.0, 2.75 and 3.5 g VS l−1 day−1,respectively (Fig. 6d). The average biogas methane content for alldigesters in this period was around 62%, slightly above the value of59% predicted by the Buswell equation [22]. This SMP was considerablybelow the specific methane yield of 0.220 l CH4 g−1 VSadded found ina 90-day batch test on the same feedstock (results not shown), butrelatively similar to the yield of around 0.15 l CH4 g−1 VSadded in thefirst 1.6 days of the batch test, corresponding to a change of slope inthe cumulative gas production curve. This indicates that only the readilydegradable proportion of the biomass is being digested effectively in thesemi-continuous trials.

The estimated calorific value based on the elemental compositionwas 21.87 MJ kg−1 VS, in reasonably good agreement with the mea-sured value of 21.36 MJ kg−1 VS. The energy recovered as methanetherefore corresponded to ~25% of the measured calorific value in thesemi-continuous trials at OLR 2.0, 2.75 and 3.5 g VS l−1 day−1, respec-tively. This compares to ~41% recovery in theBMP test, again confirmingthe poor degradability of the material. At the time of harvesting theaverage algal biomass productivity was 7.3 g TSS m−2 day−1, equatingto an energy output as rawmethane of 161MJ ha−1 day−1 at the max-imum SMP in semi-continuous digestion of 0.139 l CH4 g−1 VSadded.Using an average productivity of 10.3 g TSS m−2 day−1 over the winterto spring period and the BMP value formethane yield, this increases to a362 MJ ha−1 day−1 or 66 GJ ha−1 for the half-year period.

e pH, (b) TS and VS of digestate, (c) volumetric biogas production and (d) specificmethane

101K.C. Tran et al. / Algal Research 5 (2014) 95–102

The SMP in this study was similar to that found by Yen and Brune[33], but lower than reported by some others [7,34,35]. When a mixedalgal sludge consisting mainly of Scenedesmus and Chlorella spp. wasdigested at 10 days HRT and OLR of 2.0, 4.0 and 6.0 g VS l−1 day−1,Yen and Brune [33] reported methane yields of 0.09–0.14 l CH4 g−1

VSadded. Oswald and Golueke [7] reported a methane yield of 0.26 lCH4 g−1 VSadded from digestion of mixed algal cultures at 30 days HRT.Ras et al. [35] obtained a methane yield of 0.15 l CH4 g−1 VSadded for16 days HRT and 0.24 l CH4 g−1 VSadded from digestion of Chlorellavulgaris grown in a laboratory photobioreactor at 28 days HRT. Thereis considerable debate on the digestibility of micro-algal biomass, witha wide range of reported values [36]. The value reported here for amixed algal culture primarily consisting of Scenedesmus spp. is lowerthan the BMP value of 0.261 l CH4 g−1 VSadded for a laboratory cultureof Scenedesmus spp., and lower than that of 0.331 l CH4 g−1 VSadded forlaboratory-grown C. vulgaris run at the same time under the same testconditions [37]. The lower values found in the semi-continuous diges-tion compared to the BMP suggest that longer retention times couldgive slightly higher methane production, but pre-treatment of thistype of substrate may also be required to improve methane yield. Forexample, thermal treatment of micro-algae collected from a high-ratesewage stabilisation pond at 100 °C increased methane fermentationup to 33% [38]. Similarly, a 2.2-fold increase in methane production incomparison to untreated substrate was achieved when Scenedesmusbiomass grown in a laboratory bioreactor was pretreated at 90 °C [39].

4. Conclusions

The culture conditions in an open pond photobioreactor werestrongly related to the climatic conditions. Therefore, the correct choiceof location for this kind of reactors is essential if satisfactory productivityis to be achieved across the seasons. The biggest problem encounteredin operation of the raceways was the daytime accumulation of photo-synthetic oxygen in the culture: even during the period of relativelylow light intensity and temperature DO concentrations of 200% satura-tion occurred. At higher light intensities, DO concentrations of 350%saturation could have caused some reduction in productivity due to in-hibition of photosynthesis, and this problemwas compounded bywatertemperatures of up to 35 °C by late May.

Anaerobic digestion of the harvested raceway culture was possibleat a VS content of 4.3% and HRT as low as 12.4 days, as indicated by sta-ble volumetric and specific methane production and low VFA concen-trations. The methane yield and biodegradability of this mixture waslow, however, with a VS destruction of only about 30%. Possible reasonscould be the recalcitrant nature of this type of biomass, and the shortretention times used due to the high moisture content. It is suggestedthat either longer retention times or pre-treatment techniques may berequired in order to improve biodegradability and methane yield, ifthis mixture is to be grown and harvested at large scale as a feedstockfor biomethane production.

Acknowledgments

This study was supported by scholarships from the Government ofVietnam (Project 322) and the University of Southampton, with addi-tional funding from the EU FP7 All-Gas project no 268208. The assis-tance of the staff of the Estación Experimental Las Palmerillas fromFundación Cajamar is gratefully acknowledged.

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