Biogas production from microalgae grown in wastewater

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Biogas production from microalgae grown in wastewater: Effect of microwave pretreatment ARTICLE in APPLIED ENERGY · FEBRUARY 2013 Impact Factor: 5.61 · DOI: 10.1016/j.apenergy.2013.02.042

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Applied Energy 108 (2013) 168–175

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Biogas production from microalgae grown in wastewater: Effect of microwave pretreatment Fabiana Passos, Maria Solé, Joan García, Ivet Ferrer ⇑ GEMMA – Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya.BarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain

h i g h l i g h t s " Microwave irradiation enhanced the disintegration and digestibility of microalgae. " Algal biomass solubilisation increased by 800% with microwave pretreatment. " The main parameter influencing biomass solubilisation was the applied specific energy. " Increased biogas production rate (27–75%) and yield (12–78%) with pretreated biomass. " Linear correlation between microalgae solubilisation and biogas yield.

a r t i c l e

i n f o

Article history: Received 2 October 2012 Received in revised form 14 February 2013 Accepted 17 February 2013

Keywords: Anaerobic digestion Biofuel High rate algal pond Methane Renewable energy

a b s t r a c t The aim of this study was to evaluate the effect of microwave pretreatment on the solubilisation and anaerobic digestion of microalgae–bacterial biomass cultivated in high rate algal ponds for wastewater treatment. The microwave pretreatment comprised three specific energies (21,800, 43,600 and 65,400 kJ/kg TS), combining three output power values with different exposure times. Response surface analysis showed that the main parameter influencing biomass solubilisation was the applied specific energy. Indeed, a similar solubilisation increase was obtained for the same specific energy, regardless of the output power and exposure time (280–350% for 21,800 kJ/kg TS, 580–610% for 43,600 kJ/kg TS and 730–800% for 65,400 kJ/kg TS). In biochemical methane potential tests, the initial biogas production rate (27–75% increase) and final biogas yield (12–78% increase) were higher with pretreated biomass. A linear correlation was found between biomass solubilisation and biogas yield. It can be concluded that microwave irradiation enhanced the disintegration and digestibility of microalgae. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction During the last decade, there has been a growing interest in investigating the energy potential of biofuels obtained from microalgae cultures [1]. The high lipid content of microalgae makes them an alternative to terrestrial energy crops for biodiesel production. However, microalgae cultures and energy production are at an initial research phase. According to the literature, the cultivation of microalgae to produce biofuels has a number of requirements that limit its current implementation at industrial scale [2]. To make it economically feasible, massive biomass production and energy generation technologies must be addressed. The cultivation of certain specific strains of microalgae is not viable in economic and environmental terms, since freshwater and fertilizers are needed. In contrast, if microalgal biomass is ⇑ Corresponding author. Tel.: +34 934016463; fax: +34 934017357. E-mail address: [email protected] (I. Ferrer). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.02.042

grown as a by-product of high rate algal ponds (HRAPs) operated for wastewater treatment, the economic and ecological footprint are more realistic if used at large-scale [3,4]. High rate ponds are shallow, open raceway ponds, with continued mixing provided by paddle-wheels. This system works by a symbiosis between heterotrophic bacteria, which oxidize organic matter contained in wastewater, and the phytoplankton, which by photosynthesis consumes the CO2 derived from organic matter mineralization. Microalgae–bacterial biomass grown in this environment assimilates nutrients and subsequently, its separation from the final effluent eliminates nutrients from the wastewater [5]. Anaerobic digestion of microalgae was first studied in the 1950s [6,7]. These authors used microalgal biomass from HRAP, pointing out biomass separation from the liquor as a major limitation of the process. Up to date, the literature on microalgae digestion is very limited. The review by González-Fernández et al. [1] reports a specific methane production of 0.1–0.5 L CH4/g VS, with 60–80% CH4 in biogas, depending on process temperature (15–52 °C) and

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F. Passos et al. / Applied Energy 108 (2013) 168–175

hydraulic retention time (HRT) (3–64 days). This value is commensurate with biogas production from other substrates. For instance, the specific methane production of waste activated sludge, another by-product of wastewater treatment, ranges between 0.15 and 0.3 L CH4/g VS [8]; and that of lignocellulosic agricultural crops, such as maize, wheat, rice and sugarcane wastes, between 0.28 and 0.34 L CH4/g VS [9]. The complex cell wall structure of microalgae, composed by cellulose, hemicellulose and pectin, makes bacteria attack difficult [1]; suggesting that biomass pretreatment is necessary for the feasibility of microalgae anaerobic digestion [10]. The pretreatment of substrates to increase the anaerobic biodegradability has been the subject of intense research in recent years. Physical, chemical and biological processes have proven successful at improving the disintegration and anaerobic biodegradability of lignocellulosic biomass [11]. Furthermore, waste activated sludge pretreatment using mechanical, thermal and biological processes increased the specific methane production, leading to positive energy balances, and is currently applied in full scale facilities [12]. The few studies that have so far been conducted with microalgae show an increased methane yield after thermal and ultrasound pretreatments [13– 16]. The electromagnetic radiation of microwaves has also been investigated as a pretreatment process [17–19]. Microwaves are short waves of electromagnetic energy varying in a frequency from 300 MHz to 300 GHz, which can increase the kinetic energy of the water leading to a boiling state [17]. The quantum energy applied by microwave irradiation is not capable of breaking down chemical bonds, however hydrogen bonds are or can be broken [20]. Induction heating and dielectric polarization result in changes in the secondary and tertiary structure of proteins and cause cell hydrolysis. The polarization of macromolecules occurs by a consistent rotation through an alternating electric field. This process is influenced by microwave frequency, radiation time, biomass concentration and penetration depth [18]. The literature on microwave pretreatment of waste activated sludge under different conditions [18,21–25], shows how the process enhanced sludge solubilisation and/or cumulative gas production in anaerobic batch tests (Table 1). Microwave irradiation can produce both thermal and athermal effects. Some authors have compared microwave and thermal pretreatments, observing higher volatile solids and hemicellulose solubilisation [19] and biogas production [26] with the former. So far, the effect of microwave irradiation on microalgae remains unexplored. Thus, the objective of this study was to evaluate the effect of microwave pretreatment on the disruption and anaerobic biodegradability of microalgal biomass from HRAP for wastewater treatment.

2. Materials and methods 2.1. Microalgal biomass production system The experimental set-up was located at the laboratory of the GEMMA research group (Universitat Politècnica de Catalunya. BarcelonaTech, Spain). The system had been in operation since March 2010. The microalgae production system was composed of a hydrolytic up-flow sludge blanket reactor (HUSB), a high rate algal pond and a settler. Urban wastewater was pumped from a municipal sewer and stored in a tank (1.2 m3), which was continuously stirred to avoid solids sedimentation. From the storage tank, pretreated wastewater was conveyed to the primary treatment: a cylindrical PVC HUSB, with an internal diameter of 0.3 m, a total height of 1.9 m and an effective volume of 0.105 m3; operated at an HRT of 5 h. The sludge blanket inside the HUSB reactor was kept at a total volatile solids (VS) concentration below 10 g/L by periodical purge. A detailed description of the operation and performance of the HUSB reactor can be found in Pedescoll et al. [27]. The primary effluent of the HUSB reactor was stored in a 50 L regulation tank and pumped to the HRAP by means of peristaltic pumps. The experimental HRAP was a PVC raceway pond with a paddle wheel for stirring the mixed liquor. The HRAP had a nominal volume of 0.5 m3, a surface area of 1.5 m2 and a water depth of 0.3 m. The HRAP treated 62.5 L/day corresponding to an HRT of 8 days. Average surface loading rates were 24 g COD/m2 day and 4 g NH4–N/m2 day. The daily biomass production potential was calculated from Eq. (1), where TSS is the concentration of total suspended solids in the mixed liquor of the HRAP, A is the HRAP surface area and Q is the flow rate.

Biomass production ¼ ½TSS ðg=LÞ=A ðm2 Þ QðL=dÞ

ð1Þ

Microalgal biomass was harvested from the HRAP by means of a conventional settling tank, with a nominal volume of 0.01 m3 (0.16 days HRT). Purged biomass was then settled in laboratory Imhoff cones stored at 4 °C for 24 h. The HRAP performance was monitored by taking weekly samples at 2 PM from March 2011 to March 2012. Average characteristics of the HRAP influent, effluent and harvested biomass are summarized in Tables 2 and 3. 2.2. Microwave pretreatment To study the effect of microwave pretreatment on microalgal biomass a household type microwave (Samsung M1914, 2450 MHz frequency) was used. The output power range of the equipment was 100–1000 W (Microwave test procedure IEC705). Three target specific energies were applied: 21,800; 43,600 and 65,400 kJ/kg TS. Considering the following output power

Table 1 Effect of microwave pretreatment on activated sludge solubilisation and biochemical methane potential tests. Pretreatment conditions 14.3 min; 400 W; 102 °C; 2.3% TS 5 min; 800 W; 13,000 kJ/kg SS 5 min; 96 °C; 5.5% TS Progressive heating 1.2–1.4 °C/min; 175 °C 0.83 kJ/ml; 1000 W; 7–8% TS 1168 W; 90 °C; 4% TS

Batch test conditions 55 °C; 32 d 33 °C; 23 d 33 °C; 18 d 35 °C; 20–25 d 35 °C; 22 d

Results Increase Increase rate* Increase Increase

References of 17.9% in the CODs/COD ratio* of 311% in the VSs/VS ratio and no difference in biogas production and production

[18] [21]

of 143% in the CODs/COD ratio and 211% in the cumulative biogas production* of 74.3% in COD solubilisation and 34% in biogas production*

[22] [23]

Decreased solubilisation (CODs/COD) and increase of 15.4% in methane production* Increase of 2.5% in the CODs/COD ratio, 37% in the digestion rate and no impact on methane production*

[24] [25]

Note: TS: total solids, SS: suspended solids, VS: volatile solids, VSs: soluble volatile solids, COD: chemical oxygen demand, CODs: soluble chemical oxygen demand. Compared to control.

*

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Table 2 High rate algal pond performance from March 2011 to March 2012. Mean values (standard deviation) from samples taken at 2 PM. Parameter

Influent (HUSB primary effluent)

Effluent (HRAP filtrated effluent)

% Removal

pH Temperature (°C) DO (mg/L) Turbidity (NTU) TSS (mg/L) COD (mg/L) TKN (mg/L) NH4–N(mg/L)

6.8 (0.8) 22 (4) 0.5 (0.7) 60 (34) – 250 (57) 50 (12) 35 (11)

8 (1.1) 20 (7) 8 (4) 12 (9) 400 (130)* 80 (49) 20 (6) 2 (1.2)

– – – 87 – 68 60 95

Note: DO: dissolved oxygen, TSS: total suspended solids, COD: chemical oxygen demand, TKN: total Kjeldahl nitrogen, NH4–N: ammonia nitrogen. Not filtrated sample.

*

Y ¼ b0 þ b1 X 1 þ b2 þ X 2 þ b11 X 21 þ b22 X 22 þ b12 X 1 X 2

Table 3 Harvested microalgal biomass characteristics. Parameter

Mean value (standard deviation)

pH TS (% (w/w)) VS (% (w/w)) VS/TS (%) VSs (% (w/w)) VSs/VS (%) COD (g/L) CODs (g/L) CODs/COD (%) TKN (mg/kg) NH4–N (mg/L) Lipids (% (w/gTS)) Proteins (% (w/g TS)) Carbohydrates (% (w/g TS))

7.5 (0.8) 1.65 (0.2) 0.98 (0.2) 60 (0.9) 0.012 (0.002) 0.89 (0.11) 16.7 (0.42) 0.12 (0.02) 0.72 (0.44) 800 (180) 12 (7) 17.4 (1.50) 49.3 (1.23) 19.5 (1.87)

The polynomial function was estimated by the least squares method and checked with MatLab software. Experimental data were collected in triplicate. 2.4. Biochemical methane potential tests

Note: TS: total solids, VS: volatile solids, VSs: soluble volatile solids, COD: chemical oxygen demand, CODs: soluble chemical oxygen demand, TKN: total Kjeldahl nitrogen, NH4–N: ammonia nitrogen.

values: 300, 600 and 900 W; the time needed to achieve each target specific energy was calculated from the following equation:

Specific energy ðkJ=kg TSÞ ¼ ½power ðWÞ time ðsÞ=sample weight ðkg TSÞ ð2Þ For each pretreatment condition, a volume of 150 mL thickened microalgal biomass was pretreated in a beaker and cooled to room temperature. 2.3. Microlgal biomass solubilisation Biomass solubilisation was evaluated by the soluble to total volatile solids ratio (VSs/VS) and by the increase in this ratio with respect to the control (untreated biomass), calculated according to Eq. (3), were sub-indexes refer to samples before (o) and after pretreatment (p).

VSs =VS increaseð%Þ ¼ ½ðVSs =VSÞp ðVSs =VSÞo =ðVSs =VSÞo 100

ð4Þ

ð3Þ

The response surface methodology was used to analyse the relation between pretreatment conditions and biomass solubilisation by means of a second order polynomial expression, as proposed by other authors [18,21]. Experimental results were evaluated by fitting the second order polynomial expression (Eq. (4)), where Yi corresponds to experimental results (VSS/VS ratio); and X1 and X2 to the microwave output power (300, 600 and 900 W) and exposure time (1–9 min), respectively.

The anaerobic biodegradability of pretreated microalgal biomass was compared to the control (untreated biomass) by means of mesophilic (35 °C) batch tests. Pretreatment conditions are summarized in Table 4. The inoculum was mesophilic sludge from a full-scale anaerobic digester located in a municipal wastewater treatment plant near Barcelona (Spain). Each treatment was performed in duplicate. Serum bottles had a total volume of 160 mL and a useful volume of 100 mL. The concentration of substrate was 16.7 g COD/L, corresponding to 29.94 g microalgal biomass/bottle. As recommended by Cho et al. [28], the substrate/inoculum ratio was 0.5 g COD/g VS, corresponding to 43.48 g sludge/bottle. The bottles where flushed with Helium gas (He), sealed with butyl rubber stoppers and incubated at 35 °C until biogas production ceased. Biogas production was periodically determined by measuring the pressure increase with an electronic manometer (Greisinger GMH 3151). After each measurement gas was released until atmospheric pressure. Samples from the headspace volume were taken every 2–3 days, to determine biogas composition (CH4/CO2) by gas chromatography. Accumulated volumetric methane production (mL) was calculated from the pressure increase and methane content in biogas, expressed under standard conditions. The net values of methane production and yield were obtained by subtracting the endogenous production of the blank bottle. 2.5. Analytical methods All analyses were triplicated and results are given as mean values. The following parameters were determined from the influent and effluent of the HRAP: total suspended solids (TSS), total chemical oxygen demand (COD), ammonia nitrogen (NH4–N) and total Kjeldahl nitrogen (TKN), according to Standard Methods [29]. Water temperature and dissolved oxygen (DO) were measured in situ with an YSI 58 oxymeter. Turbidity was determined with a Hanna Microprocessor Turbidity Meter HI93703 and pH was determined with a Crison Portable 506 pH-meter. Microalgal biomass was characterised by the concentration of total solids (TS), VS, COD, CODs, NH4–N, TKN, determined according to Standard Methods [29]. The lipid content was determined by the Soxhlet extraction method [29]. A TKN to protein conversion factor of 5.95 was used [30]. Carbohydrates were determined by phenol– sulfuric acid method after acid hydrolysis and measured by spectrophotometry (Spectronic Genesys 8). Microalgae identification

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F. Passos et al. / Applied Energy 108 (2013) 168–175 Table 4 Microwave pretreatment conditions and microalgal biomass solubilisation. Trial

Target specific energy (kJ/kg TS)

Output power (W)

Exposure time (min)

Temperature (°C)

VSs/VS ratio

VSs/VS increase (%)

Control T1 T2 T3 T4 T5 T6 T7 T8 T9

– 21,800 21,800 21,800 43,600 43,600 43,600 65,400 65,400 65,400

– 300 600 900 300 600 900 300 600 900

– 3.0 1.5 1.0 5.0 3.0 2.0 9.0 4.5 3.0

– 50 58 56 81 86 92 95 98 98

0.0089 0.034 0.040 0.040 0.061 0.062 0.063 0.075 0.074 0.081

– 280 345 346 580 597 606 742 728 799

was carried out by microscopical examination (Nikon Optiphotpol, Japan). Batch test samples were analyzed for TS, VS, VSs, COD and CODs, according to Standard Methods [29]. The methane content in biogas and volatile fatty acids (VFA) were measured with a gas chromatograph (GC), following the procedure described by Ferrer et al. [31]. 3. Results and discussion 3.1. Microalgal biomass production The HRAP performance was monitored from March 2011 to March 2012 (Table 2). The HRAP influent corresponds to the primary effluent of the HUSB reactor, which had a turbidity (60 NTU) and COD (250 mgO2/L) within the typical range of a conventional primary effluent [27]. The pH in the HRAP (8.0) was always higher than in the influent (6.8), due to CO2 consumption by microalgal photosynthetic activity. The HRAP performed correctly during the whole experimental period with an HRT of 8 days: COD removal (68%), TKN removal (60%) and NH4–N removal (95%) efficiencies (Table 2) were in accordance with previous studies [5,32]. Indeed, García et al. [32] found a TKN removal efficiency of 73% and 57%, and a NH4–N removal efficiency of 97% and 91%, with an HRT in the HRAP of 7 and 4 days, respectively. The HRAP biomass production potential was 16 g TSS/m2/d, which fits in the range from 12 to 40 g TSS/m2/d reported by Park et al. [4]. However, harvestable biomass was approximately 9 g TSS/m2/d, since 45% of the produced biomass escaped from the settler. Up to date, microalgal biomass harvesting continues to be a bottleneck of this technology. The main properties of the produced biomass are summarised in Table 3. It was characterized by an organic content around 60% VS/TS, most of it in particulate form as indicated by the low VSs/VS (0.89%) and CODs/COD (0.72%) ratios. This means that almost all organic matter was not soluble and, therefore, retained inside microalgal cell walls. For this reason, the use of pretreatment techniques to disintegrate microalgal cell walls seems appropriate to improve the anaerobic digestion of this substrate. The biomass grown in the system was a consortia of microalgae and bacteria, especially from the group Chlorophyta. The main microalgae species identified in the mixed liquor were Scenedesmus and Chlorella. The average macromolecular composition of microalgal biomass was 49% proteins, 17% lipids and 20% carbohydrates. These results are commensurate with those found in pure cultures of Scenedesmus obliquus: 50–56% proteins, 12–14% lipids and 10–17% carbohydrates; and Chlorella vulgaris: 51–58% proteins, 14–22% lipids and 12–17% carbohydrates [33]. In microalgae pure cultures, higher lipids accumulation is obtained by applying techniques such as nutrient starvation or light deficiency. On the other hand, mixed cultures grown in HRAP for wastewater treatment tend to have lower lipid content, mainly because operating conditions are less controlled. For instance, González-Fernández

et al. [34] found a 3% lipid content for Scenedesmus sp. grown in swine slurry. 3.2. Solubilisation of microalgal biomass Microwave irradiation enhanced microalgae solubilisation under all pretreatment conditions assayed. The VSs content increased from 0.05% to 0.11%; corresponding to a VSs/VS increase between 280% and 800% depending on the microwave output power, exposure time and temperature reached (Table 4). To our knowledge, this is the first study dealing with the disintegration of microalgal biomass by microwave pretreatment. For the sake of comparison, literature results on the effect of microwave irradiation on waste activated sludge are summarised in Table 1. Almost all authors report an increase on sludge solubilisation; the solubilisation degree increases together with the applied specific energy, up to a limit value which is related to the sludge boiling point [21]. However, there is some controversy regarding the effect of the output power and exposure time. For the same specific energy (13,000 kJ/kg SS), Climent et al. [21] found higher VSs/VS increase with an output power of 800 W for 5 min (311%) compared to an output power of 400 W for 10 min (289%). On the other hand, Park et al. [18] found higher CODs/COD increase (17.5%) with a low output power and a long exposure time (400 W for 14.3 min) for the same target specific energy. In our study, both output power and exposure time influenced biomass solubilisation (Table 4). For the same exposure time (T1, T5 and T9) biomass solubilisation increased with the microwave output power. Similarly, for the same output power (i.e. T1, T4 and T7; or T2, T5 and T8; or T3, T6 and T9), biomass solubilisation increased with the exposure time. The combination of both variables represents the applied specific energy. Indeed, the solubilisation increase was similar for the same specific energy regardless of the output power and exposure time: between 280% and 350% for 21,800 kJ/kg TS, between 580% and 610% for 43,600 kJ/kg TS and between 730% and 800% for 65,400 kJ/kg TS (Table 4). Like this, the highest specific energy (65,400 kJ/kg TS) resulted in 21% higher solubilisation compared to the second specific energy (43,600 kJ/kg TS) and 58% higher solubilisation compared to the lowest specific energy (21,800 kJ/kg TS). As shown in Fig. 1, the solubilisation degree increased linearly with the applied specific energy, in accordance with previous results with sewage sludge [35]. Such an increase in readily digestible organic compounds should accelerate and/or increase the anaerobic biodegradability of microalgal biomass. For this reason, the effect of microwave irradiation on the methane production was evaluated in biochemical methane potential (BMP) tests. In order to represent the influence of pretreatment variables on the solubilisation degree, the following quadratic equation was determined by response surface analysis (Eq. (5)), where Yi corresponds to experimental results (VSs/VS ratio); and X are indepen-

Biomass solubilisation (VSs/VS ratio)

172


0,10

0,08

0,06

R2= 0,96

0,04

0,02

0,00 0

10000

20000

30000

40000

50000

60000

70000

Specific energy (kJ/kg TS) Fig. 1. Microalgal biomass solubilisation (VSs/VS) versus applied specific energy in the microwave pretreatment according to Eq. (7).

dent variables (output power and exposure time). The statistic parameters of the coefficients adjusted for the quadratic polynomial are summarised in Table 5.

VSs =VS ratio ¼ 2:579 108 X 2power 1:422 107 X 2time

Experimental results together with the model output are shown in Fig. 1. These results were found for specific energies from 21,800 to 65,400 kJ/kg TS. Notice that biomass solubilisation may reach an asymptote at higher specific energy values (i.e. 100,000 kJ/kg TSS) [21]. Previous studies point out a potential microwave thermal effect [19,26]. According to Hu et al. [19], the pretreatment of cattail (Typha latifolia) by microwave irradiation (500 W for 14 min) increased VS and hemicellulose solubilisation by 24.5% and 23.5%, respectively; while conventional heating at the same temperature (100 °C) increased VS and hemicellulose solubilisation by 15.4% and 7.4%, respectively. This highlights the microwave athermal effect. In our study, the temperature reached increased with the applied specific energy, which also led to the highest biomass solubilisation (Table 4). In order to consider the thermal effect, microalgae solubilisation with conventional heating at 55–95 °C was evaluated. After 1 h of pretreatment the solubilisation degree (0.027 VSs/VS (data not shown)) was lower than in all microwave pretreatment conditions, with exposure times below 9 min (0.034–0.081 VSs/VS) (Table 4). This suggests that microwave athermal effect enhanced biomass solubilisation with a shorter exposure time (1.0–9.0 min) if compared with conventional heating (1 h). 3.3. Biogas production in biochemical methane potential tests

þ 2:043 107 X power X time þ 6:726 105 X power þ 1:519 104 X time 1:627 102

ð5Þ

Previous studies have adjusted a quadratic equation for describing the influence of pretreatment conditions on biomass solubilisation, for instance the sludge thermal pretreatment [21] and microwave pretreatment [18]. However, in our quadratic model Pr values were all higher than 0.05. Since the interaction between output power and exposure time coefficient (Xpower Xtime) was the main factor influencing biomass solubilisation, a simplified model was proposed instead (Eq. (6)). In this simple model, both coefficient terms had Pr values lower than 0.05, indicating a significant fitting (Table 5).

VSs =VS ratio ¼ 3:589 107 X power X time þ 2:022 102

ð6Þ

As summarized in Table 5, R2 values were similar for the quadratic and simplified models (0.98 and 0.96, respectively), indicating that the interaction between output power and exposure time (Xpower Xtime) (i.e. simplified model) explained most of the variability. Moreover, the simplified model p-value (4.12 106) was much lower than the quadratic model p-value (9.29 103). According to this, the applied specific energy (Eq. (2)) was the main factor influencing biomass solubilisation. Therefore, the simplified model could be expressed in terms of specific energy (Eq. (7)) by introducing the total solids into Eq. (6), where Xspecific energy is the applied specific energy and XTS the total solids content in biomass.

VSs =VS ratio ¼ 3:589 107 X TS X specific energy þ 2:022 102

ð7Þ

Cumulative biogas production with pretreated and untreated microalgal biomass is shown in Fig. 2. All trials were completed within 46 days of incubation. The methane content was around 68% in all cases (Table 6). The initial biogas production rate was significantly higher (27– 75% increase) for all pretreatment conditions compared to the control (Table 6). The highest biogas production rate corresponded to biomass pretreated at 65,400 kJ/kg TS (the highest applied specific energy), followed by biomass pretreated at lower specific energies and untreated biomass (control). For instance, trial T9 had a production rate of 88 mL biogas/g VSday, while the control produced 57% of this value (50 mL biogas/g VSday). This suggests that microwave pretreatment may improve the methane production in anaerobic reactors operated at short HRT, reducing the required reactor volume and investment cost, although this hypothesis should be addressed in future research studies. The sonication of Scenedesmus biomass also increased the initial methane production rate, and has been pointed out as a way of reducing the HRT [14]. The final biogas yield was significantly higher (12–78% increase) for microalgae pretreated with all microwave conditions compared with the control (Fig. 2). The highest biogas yield was achieved by trial T9, 307 mL biogas/g VS (78% increase), followed by T8, 296 mL biogas/g VS (72% increase) and T7, 276 mL biogas/ g VS (60% increase); corresponding to the highest applied specific energy (65,400 kJ/kg TS) with output powers of 900 W, 600 W and 300 W, respectively (Table 5). The same pattern occurred for lower specific energies (43,600 kJ/kg TS and 21,800 kJ/kg TS), with somewhat lower biogas production increase (below 42% for all pretreatment conditions). For the same specific energy, the final bio-

Table 5 Comparison between quadratic and simplified model coefficients. Parameters

Estimate value Std. error t value Pr

Quadratic model

Simplified model

Intercept

X 2power

X 2time

Xpower

Xtime

Xpower Xtime

Intercept

Xpower Xtime

1.627e02 3.100e02 0.525 0.636

2.579e08 4.200e08 0.614 0.583

1.422e07 1.316e07 1.081 0.359

6.726e05 7.002e05 0.961 0.408

1.519e04 1.337e04 1.136 0.338

2.043e07 1.406e07 1.453 0.242

2.022e02 3.272e03 6.18 0.000454

3.589e07 2.805e08 12.80 4.12e06

Quadratic model: R2 = 0.98, p-value = 0.009289, F = 29.7; simplified model: R2 = 0.96, p-value = 4.122e06, F = 163.8.

173


desmus biomass thermal pretreatment at 90 °C and 33 days of incubation. More recently, different microalgae mixed cultures were pretreated using ultrasounds (10,000–57,000 kJ/kg TS), low temperature (55 °C) and thermal hydrolysis (110–170 °C) [16]. While the biological pretreatment at 55 °C had no impact on the methane yield, the thermal hydrolysis at 110 °C increased the methane yield by 62% compared to untreated biomass. These results can be compared with our study, where temperatures in the range of 95–98 °C increased biomass solubilisation by 730– 800% VSs/VS and the methane yield by 60–78% in respect to the control. Considering the positive effect of microwave irradiation in enhancing biomass solubilisation and anaerobic digestion, future research studies should evaluate the methane production in continuous reactors, and compare the energy balance of the process with and without pretreatment step. Fig. 2. Cumulative biogas production after microalgal biomass microwave pretreatment, under the conditions shown in Table 4.

gas yield was significantly higher with an output power of 600– 900 W compared to 300 W (Table 6), putting forward the effect of the output power on the methane production potential. The biogas production showed a linear correlation with the solubilisation degree (Fig. 3): the higher the biomass solubilisation, the higher the biogas production. A linear correlation between biogas production and solubilisation after thermal pretreatment was previously reported by other authors. In the case of sunflower oil cake, the methane production potential and organic content solubilisation after pretreatment showed a linear correlation (R2 = 0.87) for temperatures between 130 and 170 °C and acid addition between 0.74% and 5% [36]. A comparison between ozonation, sonication and thermal sludge pretreatment showed faster and/or higher biogas production depending on the amount of particles solubilised [37]. Our results with microalgae microwave pretreatment suggest that the higher the applied specific energy, the higher the organic matter solubilisation, biogas production rate and biogas yield. Bearing in mind the low soluble organic content of microalgal biomass (