Waste Management xxx (2014) xxx–xxx
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Anaerobic digestion of microalgal biomass after ultrasound pretreatment Fabiana Passos a, Sergi Astals b, Ivet Ferrer a,⇑ a GEMMA – Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de CatalunyaBarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain b Advanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 17 March 2014 Accepted 8 June 2014 Available online xxxx Keywords: Algae Anaerobic biodegradability Biogas High rate algal pond Microalgae Wastewater
a b s t r a c t High rate algal ponds are an economic and sustainable alternative for wastewater treatment, where microalgae and bacteria grow in symbiosis removing organic matter and nutrients. Microalgal biomass produced in these systems can be valorised through anaerobic digestion. However, microalgae anaerobic biodegradability is limited by the complex cell wall structure and therefore a pretreatment step may be required to improve the methane yield. In this study, ultrasound pretreatment at a range of applied specific energy (16–67 MJ/kg TS) was investigated prior to microalgae anaerobic digestion. Experiments showed how organic matter solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6– 33%) were improved as the pretreatment intensity increased. Mathematical modelling revealed that ultrasonication had a higher effect on the methane yield than on the hydrolysis rate. A preliminary energy assessment indicated that the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication without biomass dewatering (8% VS). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Microalgae-based ponds for wastewater treatment were first developed in California in the 1950s (Oswald and Gotaas, 1957). This technology consists in shallow ponds (0.3–0.6 m) with constant mixing provided by paddle-wheels, where bacteria and phytoplankton grow in symbiosis. The symbiotic relationship between microalgae and heterotrophic bacteria is responsible for wastewater treatment, i.e. bacteria degrade organic matter, while consuming oxygen provided by microalgae photosynthesis. For this reason, high rate algal ponds (HRAP) are advantageous in respect to conventional activated sludge plants for wastewater treatment, since there is no need for artificial aeration, decreasing the energy demand of the wastewater treatment plant (WWTP). Like waste activated sludge in conventional WWTPs, microalgal biomass can be treated through anaerobic digestion, while recovering energy as biogas. Microalgae theoretical methane yield was estimated in the range of 0.48–0.80 L CH4/g VS (Sialve et al., 2009); nonetheless, experimental results have so far been limited to 0.05–0.31 L CH4/ g VS (González-Fernández et al., 2011). This is attributed to the characteristics of microalgae cells, in particular the complex cell wall structure, composed by slowly degradable compounds such ⇑ Corresponding author. Tel.: +34 934016463; fax: +34 934017357. E-mail address: [email protected] (I. Ferrer).
as cellulose. Lately, physical, chemical and biological pretreatment methods have been studied in order to disintegrate microalgae cells, solubilise the organic content, and increase the anaerobic digestion rate and extent. Thermal pretreatments have been the most widely investigated, already in continuous reactors, leading to net energy production (Schwede et al., 2013; Passos and Ferrer, 2014). Mechanical pretreatments have mostly been investigated in batch assays using pure microalgae cultures (Alzate et al., 2012; Cho et al., 2013), and not always showing positive results in terms of net energy production (Passos et al., 2013a). In the present study, microalgal biomass anaerobic digestion was investigated under ultrasound pretreatment. Ultrasounds consist in rapid compression and decompression cycles of sonic waves, which promote the formation of microbubbles inside the cells. Depending mainly on the applied specific energy, these microbubbles are compressed to their minimum and then implode, damaging the cell wall (Kim et al., 2013). Literature results on the effect of ultrasounds on microalgae anaerobic digestion are summarised in Table 1. So far, it has been shown that ultrasonication may increase organic matter solubilisation from negligible values to 60%, and microalgae methane yield from 6% to 90%. This high variability is due to two main factors: the applied specific energy and the different microalgae species studied in each case. The aim of this research was to evaluate for the first time the effect of ultrasonication on microalgal biomass grown wastewater treatment HRAP, in terms of organic matter solubilisation,
http://dx.doi.org/10.1016/j.wasman.2014.06.004 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Passos, F., et al. Anaerobic digestion of microalgal biomass after ultrasound pretreatment. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.06.004
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx
Table 1 Literature results on the effect of ultrasound pretreatment on microalgae solubilisation and methane yield in BMP tests.
a b
Microalgae species
Biomass concentration
Applied specific energy
Solubilisation increase
Methane yield increase
References
Clamydomonas sp., Scenedesmus sp. and Nannocloropsis sp. Acutodesmus obliquus and Oocystis sp. Microspora sp. Nannocloropsis gaditana Nannocloropsis gaditana after lipid extraction Chlorella sp. and Scenedesmus sp. Scenedesmus biomass
10 g TS/kg
10, 27, 40 and 57 MJ/kg TS
14%, 28%, 30% and 32%
14%, 14%, 14% and 12%
Alzate et al., 2012
10 g TS/kg 10 g TS/kg 10 g TS/kg 10 g TS/kg
10, 10, 10, 10,
24%, 48%, 53% and 60% 30%, 56%, 57% and 62% 17%, 17%, 19% and 21% 6%, 10%, 12% and 16%
6%, 8%, 13% and 13% 23%, 18%, 18% and 22% 6%, 9%, 14% and 20% 4%, 5%, 7% and 15%
Alzate Alzate Alzate Alzate
5 g VS/L 8 g COD/L
39, 117 and 234 MJ/kg VS 100.7 and 128.9 MJ/kg TS
2.1%, 7.8% and 13.4% 8%
6%, 10% and 15% 75% and 90%
Chlorella vulgaris and sludge
n.d.
0.2 MJ/La
80%
90%b
Cho et al., 2013 González-Fernández et al., 2012 Park et al., 2014
27, 27, 27, 27,
40 40 40 40
and and and and
57 MJ/kg TS 57 MJ/kg TS 57 MJ/kg TS 57 MJ/kg TS
et et et et
al., al., al., al.,
2012 2012 2014 2014
Result expressed as energy per volume. Result expressed as biogas.
anaerobic digestion kinetics and extent. To this end, microalgal biomass was harvested from a pilot HRAP treating real urban wastewater. Ultrasound pretreatment conditions (output power 50, 60 and 70 W and exposure time 10, 20 and 30 min) were aimed at evaluating the effect of low applied specific energies (16–67 MJ/ kg TS) as compared to previous works (100–200 MJ/kg TS). Furthermore, mathematical modelling was used to assess the effect of ultrasound pretreatment on the hydrolysis rate and methane yield. Finally, a preliminary energy assessment was estimated to compare the increase in biogas production with ultrasonication energy requirement.
probe, working with an operating frequency of 20 kHz. Sample temperature was not controlled through the pretreatment. Ultrasound pretreatment was carried out in Erlenmeyer flasks with a total volume of 150 mL and a liquid volume of 100 mL. Three output powers (i.e. 50, 60 and 70 W) along with three exposure times (i.e. 10, 20 and 30 min) were combined in order to attain nine different pretreatment conditions (Table 3). The applied specific energy was based on the TS content (Eq. (1)) and ranged from 16.0 to 67.2 MJ/kg TS, depending on the output power and exposure time.
Specific energy ðMJ=kg TSÞ ¼ ½Power ðWÞ Time ðsÞ=Sample weight ðg TSÞ=103
2. Material and methods
ð1Þ
2.1. Microalgal biomass Microalgal biomass was grown in a pilot HRAP treating real urban wastewater. The experimental set-up was located outdoors at the laboratory of the GEMMA research group (Universitat Politècnica de CatalunyaBarcelonaTech). The HRAP received the primary effluent from a settling tank which had a useful volume of 7 L and a HRT of 0.9 h. The HRAP consisted of a PVC raceway pond with a paddle wheel for mixed liquor stirring; it had a useful volume of 470 L and was operated with a HRT of 8 days. Average surface loading rates were 24 g COD/m2day and 4 g NH4-N/m2day. Microalgal biomass was harvested from a secondary settler with a useful volume of 9 L and a HRT of 9 h. Following, biomass was thickened by gravity in laboratory Imhoff cones at 4 °C for 24 h reaching a total solids (TS) concentration of 2.0–2.5% (w/w). Average characteristics of harvested biomass are summarised in Table 2. The main microalgae species composing the HRAP biomass were characterised using an optic microscope (Aixoplan Zeiss, Germany), equipped with a camera MRc5 and the software Axioplan LE. Classical specific literature was used for species identification (Bourrelly, 1966; Palmer, 1962). 2.2. Ultrasound pretreatment The ultrasonic device used was a HD2070 Sonopuls Bandelin Ultrasonic Homogenizer equipped with a MS 73 titanium microtip Table 2 Microalgal biomass and inoculum characteristics. Mean values (standard deviation). Parameter
Microalgal biomass
Inoculum
pH TS (g/L) VS (g/L) VS/TS (%) VSs/VS (%) COD (g/L)
7.23 (0.30) 18.74 (0.06) 11.92 (0.04) 63.6 (0.4) 1.27 (0.2) 19.0 (1.53)
7.36 (0.15) 33.10 (0.17) 23.3 (0.13) 70.2 (0.8) – 32.9 (0.26)
Organic matter solubilisation was evaluated to compare the effectiveness of ultrasound pretreatment. The solubilisation degree (%) was calculated as proposed previously (Alzate et al., 2012; Cho et al., 2013). In Eq. (2), VS stands for total volatile solids, VSs stands for soluble volatile solids and the sub-indexes refer to pretreated (p) and control (o) biomass.
Sð%Þ ¼ ½ðVSs Þp ðVSs Þo =½VS ðVSs Þo 100
ð2Þ
2.3. Biochemical methane potential (BMP) tests The anaerobic biodegradability rate and extent of pretreated and non-pretreated microalgal biomass were assessed in biochemical methane potential (BMP) tests. Digestate from a full-scale anaerobic reactor treating sewage sludge in a WWTP near Barcelona (Spain) was used as inoculum (Table 2). BMP bottles had a total volume of 160 mL and a useful volume of 100 mL. Each bottle had a concentration of 5 g COD/L of microalgal biomass (27.72 g) and 10 g VS/L of inoculum (42.92 g), corresponding to a substrate/inoculum ratio of 0.5 g COD/g VS (Passos et al., 2013b). Blanks containing only inoculum were used to correct background methane yield coming from digested sewage sludge and controls were used to quantify methane yield of non-pretreated microalgal biomass. Bottles where flushed with Helium gas (He), sealed with butyl rubber stoppers and incubated at 35 °C until biogas production ceased. Each pretreatment BMP was performed in duplicate, while control and blank were performed in triplicate. Biogas production was determined periodically by measuring the pressure increase with an electronic manometer (Greisinger GMH 3151). After each measurement, gas was released until atmospheric pressure was reached. Samples from the headspace volume were taken every 2–3 days, to determine biogas composition (CH4/ CO2) by gas chromatography.
Please cite this article in press as: Passos, F., et al. Anaerobic digestion of microalgal biomass after ultrasound pretreatment. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.06.004
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx Table 3 Anaerobic digestion of microalgae under different ultrasound pretreatment conditions. Trial
Output power (W)
Exposure time (min)
Applied specific energy (MJ/kg TS)
Solubilisation increase (%)
Hydrolysis constant (d1)
Methane yield (mL CH4/g VS)
Control T1 T2 T3 T4 T5 T6 T7 T8 T9
– 50 60 70 50 60 70 50 60 70
– 10 10 10 20 20 20 30 30 30
– 16.0 19.2 22.4 32.0 38.4 44.8 48.0 57.6 67.2
– 16 17 16 28 56 41 101 84 91
0.072 0.090 0.090 0.091 0.099 0.107 0.097 0.105 0.112 0.102
147.7 156.4 158.2 160.5 164.3 165.3 169.9 172.6 177.2 196.4
Accumulated volumetric methane production (mL) was calculated from the pressure increase and methane content in biogas, expressed under standard conditions. The methane yield was calculated by dividing the accumulated volumetric methane production by the VS content of microalgae in each trial (mL CH4/g VS). Net values of methane yield were obtained by subtracting the endogenous production of the blank trial.
2.4. Analytical methods All analyses were carried out in triplicate and results are given as mean values. Microalgal biomass and sewage sludge were characterised by the concentration of TS, VS and chemical oxygen demand (COD). Soluble total solids (TSs) and soluble volatile solids (VSs) were obtained after centrifugation (UNICEN20, 4200 rpm, 8 min, 20 °C) and filtration (glass fiber filter 47 mm and pore size 1 lm) of samples. All parameters were analysed according to standard methods (APHA, AWWA, WPCF, 1999). pH was measured with a Crison Portable 506 pH-meter. The methane content in biogas was analysed with a gas chromatograph (GC) equipped with a Thermal Conductivity Detector, according to the procedure described by Passos et al. (2013b).
2.5. Model implementation and data analysis Mathematical analysis of the BMP test was based on the IWA Anaerobic Digestion Model No. 1 (ADM1) (Batstone et al., 2002). Hydrolysis is considered the rate-limiting step of microalgal biomass anaerobic digestion process (Passos et al., 2014). When this is the case, substrate degradation can be modelled using first-order kinetics (Vavilin et al., 2008; Jensen et al., 2011). In Eq. (3), r is the reaction rate (mL CH4/L day), f is the substrate anaerobic biodegradability (), khyd is the first order hydrolysis rate constant (day1), and X is the microalgal biomass concentration (g COD/L).
r ¼ f khyd X
ð3Þ
From this equation, two key parameters in anaerobic digestion, namely anaerobic biodegradability (f) and hydrolysis rate (khyd), were adjusted to the BMP tests data to determine the 95% confidence limits (uncertainty surfaces) of both parameters. Model results allowed evaluating and comparing the effect of ultrasound pretreatment over the process kinetics and methane yield. The model was implemented in Aquasim 2.1d modified version, where parameter estimation and uncertainty analysis were performed simultaneously, with a 95% confidence limit (Batstone et al., 2009). Parameters uncertainty (f and khyd) was estimated based on a two-tailed t-test on parameter standard error around the optimum. The objective function was the sum of squared errors (v2), where average data from replicate BMP tests were used.
(3.2) (8.3) (1.4) (1.3) (2.5) (0.6) (2.1) (3.3) (1.6) (1.6)
3. Results and discussion 3.1. Effect of ultrasonication on microalgal biomass solubilisation Microalgal biomass solubilisation was enhanced after all ultrasound pretreatment conditions, between 16% and 100% (Table 3). For the lowest applied specific energy trials (T1, T2, T3), combining an exposure time of 10 min with an output power of 50, 60 and 70 W, biomass solubilisation was fairly low (16–17%). With longer exposure times of 20 and 30 min, biomass solubilisation increased along with the output power, i.e. 28–56% and 84–101%, respectively. On the whole, exposure time seemed to have higher impact on biomass solubilisation than output power. For instance, considering an output power of 50 W, biomass solubilisation was improved by 150% when increasing exposure time from 10 to 20 min; and by 400% when increasing exposure time from 10 to 30 min. The same trend was observed with an output power of 60 and 70 W (Table 3). As for other physical pretreatment methods (Park et al., 2014; Passos et al., 2013a), a positive linear correlation between the applied specific energy and microalgal biomass solubilisation was found (Fig. 1a). Indeed, the higher the applied specific energy, the higher the VS solubilisation. The solubilisation increase observed in our study was the highest reported so far after microalgae ultrasound pretreatment (Table 1). For instance, ultrasound pretreatment with a specific energy of 57 MJ/kg TS increased biomass solubilisation by 32%, when it was applied to a mixture of Clamydomonas sp., Scenedesmus sp. and Nannocloropsis sp.; and by 60% when it was applied to a mixture of Acutodesmus obliquus and Oocystis sp., or to Microspora sp. (Alzate et al., 2012). By applying a higher specific energy of 100.7 and 128.9 MJ/kg TS to Scenedesmus biomass, the solubilisation degree increased by only 8% (González-Fernández et al., 2012). In our study, for a range of specific energy between 24 and 33.7 MJ/kg TS, the solubilisation increase in respect to control was 84–101%. Such high results may be explained by the characteristics of microalgal biomass grown in wastewater treatment HRAP, which tends to be flocculated and, therefore, it contains more extracellular polymeric substances than microalgae grown at lab-scale under controlled conditions (Park et al., 2011). These particulate substances are also solubilised during the pretreatment step, along with microalgae and bacteria, increasing the concentration of soluble VS in comparison with non-pretreated biomass.
3.2. Effect of ultrasonication on microalgal biomass anaerobic biodegradability Microalgal biomass anaerobic digestion rate (khyd) and extent (methane yield) were improved under all ultrasound pretreatment conditions, according to BMP tests results (Table 3; Fig. 2). As shown in Table 3, non-pretreated biomass (control) reached a methane yield of 147.7 mL CH4/g VS. After pretreatment, the
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx
Organic matter solubilization (%)
100
(a)
80 R2 = 0.92
60
40
20
0 0
10
20
30
40
Applied specific energy (MJ/kg TS)
Methane yield (mL CH4/g VS)
200
(b) 190 R2 = 0.91
180 170 160 150 140 0
10
20
30
40
Applied specific energy (MJ/kg TS) Fig. 1. Correlation between the applied specific energy by ultrasound pretreatment and microalgal biomass solubilisation increase (a) and methane yield (b).
methane yield was increased by 6–33% (156.4–196.4 mL CH4/ g VS), with minor differences among pretreatment trials. In fact, with an applied specific energy below 30 MJ/kg TS, the methane yield increase did not exceed 20%. However, when applying a specific energy of 33.7 MJ/kg TS, the methane yield increased by 33%. Again, a positive linear correlation was found between the applied specific energy and microalgal biomass methane yield (Fig. 1b). The higher the applied specific energy, the higher the methane yield attained.
A positive correlation between the applied specific energy and methane yield was also found after ultrasound pretreatment of other microalgae species, such as Scenedesmus biomass and Chlorella vulgaris (González-Fernández et al., 2011; Park et al., 2014). However, the methane yield increase did not exceed 20% with a specific energy below 75 MJ/kg TS (Alzate et al., 2012; GonzálezFernández et al., 2012); whereas it increased by 80–90% with a specific energy of 100–200 MJ/kg TS (González-Fernández et al., 2012; Park et al., 2014). In our case, the methane yield increased by no more than 20–30%, probably due to the low specific energy applied (16–67 MJ/kg TS) as compared to previous results. Even if experimental results suggest that a higher specific energy (>100 MJ/kg TS) may be preferred; biogas production could be improved at the expense of unbalancing the energy input and output. For this reason, pretreatment techniques should always be evaluated from an energy perspective. The effect of pretreatment depends on the studied microalgae species, since the cell wall composition and structure affect the anaerobic biodegradability (Mussgnug et al., 2010). Microalgal biomass grown in HRAP for wastewater treatment is most commonly composed of a few adapted species, which predominate depending on environmental, operational and biological parameters (Park et al., 2011). In our study, microalgal biomass was mainly composed by Monoraphidium sp., Stigeoclonium sp. and the diatoms Nitzschia sp. and Amphora sp. Among these species, diatoms have a resistant cell wall, which may hamper anaerobic digestion. Moreover, Stigeoclonium sp. cells generally form flocs, which could also contribute to inhibit the degradation process. The effect of ultrasonication on microalgal biomass anaerobic digestion rate and extent was assessed through BMP tests mathematical modelling. Model outputs are shown in Fig. 3. In this graph, each circle bounds the 95% confidence region of each trial for the two parameters of interest: anaerobic biodegradability (f, x-axis) and hydrolysis rate (khyd, y-axis), and it represents all the possible solutions for f and khyd within the specified confidence level. Confidence regions for the control and pretreatment trials may help understanding the effect of ultrasonication on microalgae anaerobic digestion rate and extent. Indeed, both process kinetics and anaerobic biodegradability improved as the applied specific energy increased. This is evidenced by the way circles move to the right (increase in anaerobic biodegradability) and to the top of the graph (increase in process kinetics). Model results indicate that ultrasonication had a higher impact on microalgae anaerobic biodegradability than on process kinetics. This highlights that, although the pretreatment released soluble organic matter (up to 100% as compared to control), it may have assisted the final methane yield 0,18
200
150 Control T1 T2 T3 T4 T5 T6 T7 T8 T9
100
50
0 0
10
20
30
40
50
60
Digestion time (days) Fig. 2. Microalgal biomass methane yield after ultrasound pretreatment.
Hydrolysis rate (day-1)
Methane yield (mL CH4/g VS)
0,16 0,14
degrades faster ---->
4
Control T1 T2 T3 T4 T5 T6 T7 T8 T9
0,12 0,10 0,08 0,06
degrades more ---->
0,04 0,20
0,25
0,30
0,35
0,40
Anaerobic biodegradability (-) Fig. 3. Mathematical modelling of microalgal biomass BMP tests after ultrasound pretreatment. Each circle bounds the confidence region (95%) of each trial for the anaerobic biodegradability (f, x-axis) and the hydrolysis rate (khyd, y-axis).
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx Table 4 Energy assessment of microalgal biomass anaerobic digestion after ultrasound pretreatment. Trial
Energy output (Eo) (MJ/kg VS)
T1 T2 T3 T4 T5 T6 T7 T8 T9
2.80 3.38 4.12 5.35 5.67 7.15 8.02 9.50 15.69
Harvested biomass (1.2% VS)
Dewatered biomass (8% VS)
Energy input (Ei) (MJ/kg VS)
Energy ratio (Eo/Ei)
Energy input (Ei) (MJ/kg VS)
Energy ratio (Eo/Ei)
25.17 30.20 35.23 50.34 60.40 70.47 75.50 90.60 105.70
0.11 0.11 0.12 0.11 0.09 0.10 0.11 0.10 0.15
3.75 4.50 5.25 7.50 9.00 10.50 11.25 13.50 15.75
0.75 0.75 0.79 0.71 0.63 0.68 0.71 0.70 1.00
(or anaerobic biodegradability) more than the reaction rate (process kinetics). 3.3. Energy assessment An energy assessment of microalgal biomass ultrasonic pretreatment was carried out in order to get an insight into the viability of the process. Indeed, the energy input must be balanced by the extra methane produced as a result of implementing the pretreatment step. The energy input (Ei) corresponded to the specific energy applied to microalgal biomass, according to Eq. (1). However, in order to compare the Ei with the methane yield (mL CH4/g VS), the applied specific energy was recalculated for the VS content of pretreated biomass (MJ/kg VS). The energy output (Eo) was calculated from the difference between the methane yield of pretreated and non-pretreated biomass (control) (DPCH4) (Eq. (4)), where DPCH4 is the methane yield increase after biomass pretreatment (mL CH4/g VS) and n is the lower heating value of methane (35,800 kJ/m3CH4) (Metcalf et al., 2003). An efficiency of 90% on energy conversion was considered.
Eo ðMJ=kg VSÞ ¼ ½DPCH4 ðmL=gVSÞ nðkJ=m3 Þ 0:9=106
ð4Þ
In this manner, the feasibility of the process was estimated by the energy output to energy input (Eo/Ei) ratio (Table 4). A ratio higher than 1 indicates that the energy output from the extra methane generated is higher than the energy required for ultrasonication. As can be noticed, with harvested microalgal biomass thickened by gravity, the energy ratio was lower than 1 under all ultrasound pretreatment conditions (0.11–0.15). However, if biomass was dewatered reaching a concentration of 8% VS, a neutral energy ratio could be attained under optimal pretreatment conditions (70 W and 30 min). The reason for this is that the applied specific energy would decrease from 105.7 to 15.75 MJ/kg VS, by increasing biomass concentration from 1.2% to 8.0%VS. Indeed, for other mechanical pretreatments such as microwaves, biomass concentration has been pointed out as a crucial factor affecting the energy balance of the process (Passos et al., 2013a, 2014). In that case, the energy demand for biomass dewatering should also be taken into account, which is one of the challenges of full-scale microalgae biofuel production (Schwede et al., 2013). Besides, the methane yield of microalgae in BMP tests tends to be lower than in continuous reactors, due to the lack of acclimated biomass; i.e. 148 mL CH4/g VS in this BMP vs. 170 mL CH4/g VS in a continuous reactor at 20 days HRT (Passos et al., 2014). So, if ultrasound pretreatment was tested in continuous reactors, better results ought to be expected in terms of methane production and energy output, and possibly no such high biomass concentration would be required to attain a positive energy ratio. Additionally, the energy input would surely decrease by using pilot and fullscale equipment, which is more energy efficient than lab-scale
devices. In fact, ultrasonication has been applied to sewage sludge in full-scale WWTP leading to net energy generation (Perez-Elvira et al., 2009). Up to date, ultrasound pretreatment has only been applied to microalgae undergoing anaerobic digestion in BMP tests, which are useful to compare different pretreatment conditions, but not to estimate the methane yield that could be obtained upon continuous digester operation. Thus, the authors believe that future research should look at the effect of microalgae ultrasonication in continuous reactors, and if possible using pilot-scale equipment. 4. Conclusions Ultrasound pretreatment was studied with the aim of improving the anaerobic biodegradability of microalgal biomass grown in wastewater treatment HRAP. To this aim, output powers of 50, 60 and 70 W and exposure times of 10, 20 and 30 min were investigated prior to BMP tests. Experimental results showed how VS solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6–33%) improved with the pretreatment intensity, obtaining the best results with the highest applied specific energy (67 MJ/ kg TS). Mathematical modelling revealed that the pretreatment had a higher effect on the final methane yield than on the hydrolysis rate. Finally, a preliminary energy assessment indicated that the energy input required for ultrasound pretreatment was higher than the extra energy produced by the methane yield increase, when digesting harvested microalgal biomass thickened by gravity. However, positive energy balances could be reached by dewatering microalgal biomass to reach a VS concentration above 8%. Acknowledgements This research was funded by the Spanish Ministry of Economy and Competitiveness (Project BIOALGAS CTM2010-17846). Fabiana Passos is grateful to the Coordination for the Improvement of Higher Level Personal (CAPES) funded by the Brazilian Ministry of Education for her PhD scholarship. References Alzate, M.E., Muñoz, R., Rogalla, F., Fdz-Polanco, F., Perez-Elvira, S.I., 2012. Biochemical methane potential of microalgae: influence of substrate to inoculum ratio, biomass concentration and pretreatment. Bioresour. Technol. 123, 488–494. Alzate, M.E., Muñoz, R., Rogalla, F., Fdz-Polanco, F., Perez-Elvira, S.I., 2014. Biochemical methane potential of microalgae biomass after lipid extraction. Chem. Eng. J. 243, 405–410. Batstone, D.J., Keller, J., Angelidaki, I., Kalyuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A., Sanders, W.T., Siegrist, H., Vavilin, V.A., 2002. The IWA Anaerobic Digestion Model No. 1 (ADM1). Wat. Sci. Technol. 45, 65–73. Batstone, D.J., Tait, S., Starrenburg, D., 2009. Estimation of hydrolysis parameters in full-scale anerobic digesters. Biotechnol. Bioeng. 102, 1513–1520. Bourrelly, P., 1966. Les algues d’eau douce. Tome I: Les algues vertes. Édition N. Boubée & Cie, Paris.
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Cho, S., Park, S., Seon, J., Yu, J., Lee, T., 2013. Evaluation of thermal, ultrasonic and alkali pretreatments on mixed-microalgal biomass to enhance anaerobic methane production. Bioresour. Technol. 143, 330–336. González-Fernández, C., Sialve, B., Bernet, N., Steyer, J.P., 2011. Impact of microalgae characteristics on their conversion to biofuel. Part II: Focus on biomethane production. Biofuels, Bioprod. Biorefin. 6, 205–218. González-Fernández, C., Sialve, B., Bernet, N., Steyer, J.P., 2012. Comparison of ultrasound and thermal pretreatment of Scenedesmus biomass on methane production. Bioresour. Technol. 110, 610–616. Jensen, P.D., Ge, H., Batstone, D.J., 2011. Assessing the role of biochemical methane potential tests in determining anaerobic degradability rate and extent. Wat. Sci. Technol. 64, 880–886. Kim, J., Yoo, G., Lee, H., Lim, J., Kim, K., Kim, C.H., Park, M.S., Yang, J.-W., 2013. Methods of downstream processing for the production of biodiesel from microalgae. Biotechnol. Adv. 31, 862–876. Metcalf, Eddy, Tchobanoglous, G.; Burton, F. L.; Stensel, H. D., 2003. Wastewater Engineering, Treatment and Reuse. fourth ed., McGraw Hill Education. Mussgnug, J.H., Klassen, V., Schlüter, A., Kruse, O., 2010. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotechnol. 150, 51–56. Oswald, W.J., Gotaas, H.B., 1957. Photosynthesis in sewage treatment. Trans. Am. Soc. Civ. Eng. 122, 73–105. Palmer, C. M., 1962. Algas en los abastecimientos de agua. Manual ilustrado acerca de la identificación, importancia y control de las algas en los abastecimientos de agua. Editiorial Interamericana, Mexico. Park, J.B.K., Craggs, R.J., Shilton, A.N., 2011. Wastewater treatment high rate algal ponds for biofuel production. Bioresour. Technol. 102, 35–42.
Park, K.Y., Kweon, J., Chantrasakdakul, P., Lee, K., Cha, H.Y., 2014. Anaerobic digestion of microalgal biomass with ultrasonic disintegration. Int. Biodeter. Biodegrad. 85, 598–602. Passos, F., Solé, M., Garcia, J., Ferrer, I., 2013a. Biogas production from microalgae grown in wastewater: effect of microwave pretreatment. Appl. Energy 108, 168–175. Passos, F., García, J., Ferrer, I., 2013b. Impact of low temperature pretreatment on the anaerobic digestion of microalgal biomass. Bioresour. Technol. 138, 79– 86. Passos, F., Hernandez-Marine, M., Garcia, J., Ferrer, I., 2014. Long-term anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment. Water Res. 49, 351–359. Passos, F., Ferrer, I., 2014. Microalgae conversion to biogas: thermal pretreatment contribution on net energy production. Environ. Sci. Technol. 48, 7171–7178. Perez-Elvira, S.I., Fdz-Polanco, M., Plaza, F.I., Garralón, G., Fdz-Polanco, F., 2009. Ultrasonic pre-treatment for anaerobic digestion improvement. Wat. Sci. Technol. 60, 1525–1532. Schwede, S., Rehman, Z.-U., Gerber, M., Theiss, C., Span, R., 2013. Effects of thermal pretreatment on anaerobic digestion of Nannocloropsis salina biomass. Bioresour. Technol. 143, 505–511. Sialve, B., Bernet, N., Bernard, O., 2009. Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable. Biotechnol. Adv. 27, 409–416. Vavilin, V.A., Fernández, B., Palatsi, J., Flotats, X., 2008. Hydrolysis kinetics in anaerobic degradation of particulate organic material: an overview. Waste Manag. 28, 941–953.
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Anaerobic digestion of microalgal biomass after ultrasound pretreatment Fabiana Passos a, Sergi Astals b, Ivet Ferrer a,⇑ a GEMMA – Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de CatalunyaBarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain b Advanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072, Australia
a r t i c l e
i n f o
Article history: Received 17 March 2014 Accepted 8 June 2014 Available online xxxx Keywords: Algae Anaerobic biodegradability Biogas High rate algal pond Microalgae Wastewater
a b s t r a c t High rate algal ponds are an economic and sustainable alternative for wastewater treatment, where microalgae and bacteria grow in symbiosis removing organic matter and nutrients. Microalgal biomass produced in these systems can be valorised through anaerobic digestion. However, microalgae anaerobic biodegradability is limited by the complex cell wall structure and therefore a pretreatment step may be required to improve the methane yield. In this study, ultrasound pretreatment at a range of applied specific energy (16–67 MJ/kg TS) was investigated prior to microalgae anaerobic digestion. Experiments showed how organic matter solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6– 33%) were improved as the pretreatment intensity increased. Mathematical modelling revealed that ultrasonication had a higher effect on the methane yield than on the hydrolysis rate. A preliminary energy assessment indicated that the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication without biomass dewatering (8% VS). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Microalgae-based ponds for wastewater treatment were first developed in California in the 1950s (Oswald and Gotaas, 1957). This technology consists in shallow ponds (0.3–0.6 m) with constant mixing provided by paddle-wheels, where bacteria and phytoplankton grow in symbiosis. The symbiotic relationship between microalgae and heterotrophic bacteria is responsible for wastewater treatment, i.e. bacteria degrade organic matter, while consuming oxygen provided by microalgae photosynthesis. For this reason, high rate algal ponds (HRAP) are advantageous in respect to conventional activated sludge plants for wastewater treatment, since there is no need for artificial aeration, decreasing the energy demand of the wastewater treatment plant (WWTP). Like waste activated sludge in conventional WWTPs, microalgal biomass can be treated through anaerobic digestion, while recovering energy as biogas. Microalgae theoretical methane yield was estimated in the range of 0.48–0.80 L CH4/g VS (Sialve et al., 2009); nonetheless, experimental results have so far been limited to 0.05–0.31 L CH4/ g VS (González-Fernández et al., 2011). This is attributed to the characteristics of microalgae cells, in particular the complex cell wall structure, composed by slowly degradable compounds such ⇑ Corresponding author. Tel.: +34 934016463; fax: +34 934017357. E-mail address: [email protected] (I. Ferrer).
as cellulose. Lately, physical, chemical and biological pretreatment methods have been studied in order to disintegrate microalgae cells, solubilise the organic content, and increase the anaerobic digestion rate and extent. Thermal pretreatments have been the most widely investigated, already in continuous reactors, leading to net energy production (Schwede et al., 2013; Passos and Ferrer, 2014). Mechanical pretreatments have mostly been investigated in batch assays using pure microalgae cultures (Alzate et al., 2012; Cho et al., 2013), and not always showing positive results in terms of net energy production (Passos et al., 2013a). In the present study, microalgal biomass anaerobic digestion was investigated under ultrasound pretreatment. Ultrasounds consist in rapid compression and decompression cycles of sonic waves, which promote the formation of microbubbles inside the cells. Depending mainly on the applied specific energy, these microbubbles are compressed to their minimum and then implode, damaging the cell wall (Kim et al., 2013). Literature results on the effect of ultrasounds on microalgae anaerobic digestion are summarised in Table 1. So far, it has been shown that ultrasonication may increase organic matter solubilisation from negligible values to 60%, and microalgae methane yield from 6% to 90%. This high variability is due to two main factors: the applied specific energy and the different microalgae species studied in each case. The aim of this research was to evaluate for the first time the effect of ultrasonication on microalgal biomass grown wastewater treatment HRAP, in terms of organic matter solubilisation,
http://dx.doi.org/10.1016/j.wasman.2014.06.004 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Passos, F., et al. Anaerobic digestion of microalgal biomass after ultrasound pretreatment. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.06.004
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Table 1 Literature results on the effect of ultrasound pretreatment on microalgae solubilisation and methane yield in BMP tests.
a b
Microalgae species
Biomass concentration
Applied specific energy
Solubilisation increase
Methane yield increase
References
Clamydomonas sp., Scenedesmus sp. and Nannocloropsis sp. Acutodesmus obliquus and Oocystis sp. Microspora sp. Nannocloropsis gaditana Nannocloropsis gaditana after lipid extraction Chlorella sp. and Scenedesmus sp. Scenedesmus biomass
10 g TS/kg
10, 27, 40 and 57 MJ/kg TS
14%, 28%, 30% and 32%
14%, 14%, 14% and 12%
Alzate et al., 2012
10 g TS/kg 10 g TS/kg 10 g TS/kg 10 g TS/kg
10, 10, 10, 10,
24%, 48%, 53% and 60% 30%, 56%, 57% and 62% 17%, 17%, 19% and 21% 6%, 10%, 12% and 16%
6%, 8%, 13% and 13% 23%, 18%, 18% and 22% 6%, 9%, 14% and 20% 4%, 5%, 7% and 15%
Alzate Alzate Alzate Alzate
5 g VS/L 8 g COD/L
39, 117 and 234 MJ/kg VS 100.7 and 128.9 MJ/kg TS
2.1%, 7.8% and 13.4% 8%
6%, 10% and 15% 75% and 90%
Chlorella vulgaris and sludge
n.d.
0.2 MJ/La
80%
90%b
Cho et al., 2013 González-Fernández et al., 2012 Park et al., 2014
27, 27, 27, 27,
40 40 40 40
and and and and
57 MJ/kg TS 57 MJ/kg TS 57 MJ/kg TS 57 MJ/kg TS
et et et et
al., al., al., al.,
2012 2012 2014 2014
Result expressed as energy per volume. Result expressed as biogas.
anaerobic digestion kinetics and extent. To this end, microalgal biomass was harvested from a pilot HRAP treating real urban wastewater. Ultrasound pretreatment conditions (output power 50, 60 and 70 W and exposure time 10, 20 and 30 min) were aimed at evaluating the effect of low applied specific energies (16–67 MJ/ kg TS) as compared to previous works (100–200 MJ/kg TS). Furthermore, mathematical modelling was used to assess the effect of ultrasound pretreatment on the hydrolysis rate and methane yield. Finally, a preliminary energy assessment was estimated to compare the increase in biogas production with ultrasonication energy requirement.
probe, working with an operating frequency of 20 kHz. Sample temperature was not controlled through the pretreatment. Ultrasound pretreatment was carried out in Erlenmeyer flasks with a total volume of 150 mL and a liquid volume of 100 mL. Three output powers (i.e. 50, 60 and 70 W) along with three exposure times (i.e. 10, 20 and 30 min) were combined in order to attain nine different pretreatment conditions (Table 3). The applied specific energy was based on the TS content (Eq. (1)) and ranged from 16.0 to 67.2 MJ/kg TS, depending on the output power and exposure time.
Specific energy ðMJ=kg TSÞ ¼ ½Power ðWÞ Time ðsÞ=Sample weight ðg TSÞ=103
2. Material and methods
ð1Þ
2.1. Microalgal biomass Microalgal biomass was grown in a pilot HRAP treating real urban wastewater. The experimental set-up was located outdoors at the laboratory of the GEMMA research group (Universitat Politècnica de CatalunyaBarcelonaTech). The HRAP received the primary effluent from a settling tank which had a useful volume of 7 L and a HRT of 0.9 h. The HRAP consisted of a PVC raceway pond with a paddle wheel for mixed liquor stirring; it had a useful volume of 470 L and was operated with a HRT of 8 days. Average surface loading rates were 24 g COD/m2day and 4 g NH4-N/m2day. Microalgal biomass was harvested from a secondary settler with a useful volume of 9 L and a HRT of 9 h. Following, biomass was thickened by gravity in laboratory Imhoff cones at 4 °C for 24 h reaching a total solids (TS) concentration of 2.0–2.5% (w/w). Average characteristics of harvested biomass are summarised in Table 2. The main microalgae species composing the HRAP biomass were characterised using an optic microscope (Aixoplan Zeiss, Germany), equipped with a camera MRc5 and the software Axioplan LE. Classical specific literature was used for species identification (Bourrelly, 1966; Palmer, 1962). 2.2. Ultrasound pretreatment The ultrasonic device used was a HD2070 Sonopuls Bandelin Ultrasonic Homogenizer equipped with a MS 73 titanium microtip Table 2 Microalgal biomass and inoculum characteristics. Mean values (standard deviation). Parameter
Microalgal biomass
Inoculum
pH TS (g/L) VS (g/L) VS/TS (%) VSs/VS (%) COD (g/L)
7.23 (0.30) 18.74 (0.06) 11.92 (0.04) 63.6 (0.4) 1.27 (0.2) 19.0 (1.53)
7.36 (0.15) 33.10 (0.17) 23.3 (0.13) 70.2 (0.8) – 32.9 (0.26)
Organic matter solubilisation was evaluated to compare the effectiveness of ultrasound pretreatment. The solubilisation degree (%) was calculated as proposed previously (Alzate et al., 2012; Cho et al., 2013). In Eq. (2), VS stands for total volatile solids, VSs stands for soluble volatile solids and the sub-indexes refer to pretreated (p) and control (o) biomass.
Sð%Þ ¼ ½ðVSs Þp ðVSs Þo =½VS ðVSs Þo 100
ð2Þ
2.3. Biochemical methane potential (BMP) tests The anaerobic biodegradability rate and extent of pretreated and non-pretreated microalgal biomass were assessed in biochemical methane potential (BMP) tests. Digestate from a full-scale anaerobic reactor treating sewage sludge in a WWTP near Barcelona (Spain) was used as inoculum (Table 2). BMP bottles had a total volume of 160 mL and a useful volume of 100 mL. Each bottle had a concentration of 5 g COD/L of microalgal biomass (27.72 g) and 10 g VS/L of inoculum (42.92 g), corresponding to a substrate/inoculum ratio of 0.5 g COD/g VS (Passos et al., 2013b). Blanks containing only inoculum were used to correct background methane yield coming from digested sewage sludge and controls were used to quantify methane yield of non-pretreated microalgal biomass. Bottles where flushed with Helium gas (He), sealed with butyl rubber stoppers and incubated at 35 °C until biogas production ceased. Each pretreatment BMP was performed in duplicate, while control and blank were performed in triplicate. Biogas production was determined periodically by measuring the pressure increase with an electronic manometer (Greisinger GMH 3151). After each measurement, gas was released until atmospheric pressure was reached. Samples from the headspace volume were taken every 2–3 days, to determine biogas composition (CH4/ CO2) by gas chromatography.
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx Table 3 Anaerobic digestion of microalgae under different ultrasound pretreatment conditions. Trial
Output power (W)
Exposure time (min)
Applied specific energy (MJ/kg TS)
Solubilisation increase (%)
Hydrolysis constant (d1)
Methane yield (mL CH4/g VS)
Control T1 T2 T3 T4 T5 T6 T7 T8 T9
– 50 60 70 50 60 70 50 60 70
– 10 10 10 20 20 20 30 30 30
– 16.0 19.2 22.4 32.0 38.4 44.8 48.0 57.6 67.2
– 16 17 16 28 56 41 101 84 91
0.072 0.090 0.090 0.091 0.099 0.107 0.097 0.105 0.112 0.102
147.7 156.4 158.2 160.5 164.3 165.3 169.9 172.6 177.2 196.4
Accumulated volumetric methane production (mL) was calculated from the pressure increase and methane content in biogas, expressed under standard conditions. The methane yield was calculated by dividing the accumulated volumetric methane production by the VS content of microalgae in each trial (mL CH4/g VS). Net values of methane yield were obtained by subtracting the endogenous production of the blank trial.
2.4. Analytical methods All analyses were carried out in triplicate and results are given as mean values. Microalgal biomass and sewage sludge were characterised by the concentration of TS, VS and chemical oxygen demand (COD). Soluble total solids (TSs) and soluble volatile solids (VSs) were obtained after centrifugation (UNICEN20, 4200 rpm, 8 min, 20 °C) and filtration (glass fiber filter 47 mm and pore size 1 lm) of samples. All parameters were analysed according to standard methods (APHA, AWWA, WPCF, 1999). pH was measured with a Crison Portable 506 pH-meter. The methane content in biogas was analysed with a gas chromatograph (GC) equipped with a Thermal Conductivity Detector, according to the procedure described by Passos et al. (2013b).
2.5. Model implementation and data analysis Mathematical analysis of the BMP test was based on the IWA Anaerobic Digestion Model No. 1 (ADM1) (Batstone et al., 2002). Hydrolysis is considered the rate-limiting step of microalgal biomass anaerobic digestion process (Passos et al., 2014). When this is the case, substrate degradation can be modelled using first-order kinetics (Vavilin et al., 2008; Jensen et al., 2011). In Eq. (3), r is the reaction rate (mL CH4/L day), f is the substrate anaerobic biodegradability (), khyd is the first order hydrolysis rate constant (day1), and X is the microalgal biomass concentration (g COD/L).
r ¼ f khyd X
ð3Þ
From this equation, two key parameters in anaerobic digestion, namely anaerobic biodegradability (f) and hydrolysis rate (khyd), were adjusted to the BMP tests data to determine the 95% confidence limits (uncertainty surfaces) of both parameters. Model results allowed evaluating and comparing the effect of ultrasound pretreatment over the process kinetics and methane yield. The model was implemented in Aquasim 2.1d modified version, where parameter estimation and uncertainty analysis were performed simultaneously, with a 95% confidence limit (Batstone et al., 2009). Parameters uncertainty (f and khyd) was estimated based on a two-tailed t-test on parameter standard error around the optimum. The objective function was the sum of squared errors (v2), where average data from replicate BMP tests were used.
(3.2) (8.3) (1.4) (1.3) (2.5) (0.6) (2.1) (3.3) (1.6) (1.6)
3. Results and discussion 3.1. Effect of ultrasonication on microalgal biomass solubilisation Microalgal biomass solubilisation was enhanced after all ultrasound pretreatment conditions, between 16% and 100% (Table 3). For the lowest applied specific energy trials (T1, T2, T3), combining an exposure time of 10 min with an output power of 50, 60 and 70 W, biomass solubilisation was fairly low (16–17%). With longer exposure times of 20 and 30 min, biomass solubilisation increased along with the output power, i.e. 28–56% and 84–101%, respectively. On the whole, exposure time seemed to have higher impact on biomass solubilisation than output power. For instance, considering an output power of 50 W, biomass solubilisation was improved by 150% when increasing exposure time from 10 to 20 min; and by 400% when increasing exposure time from 10 to 30 min. The same trend was observed with an output power of 60 and 70 W (Table 3). As for other physical pretreatment methods (Park et al., 2014; Passos et al., 2013a), a positive linear correlation between the applied specific energy and microalgal biomass solubilisation was found (Fig. 1a). Indeed, the higher the applied specific energy, the higher the VS solubilisation. The solubilisation increase observed in our study was the highest reported so far after microalgae ultrasound pretreatment (Table 1). For instance, ultrasound pretreatment with a specific energy of 57 MJ/kg TS increased biomass solubilisation by 32%, when it was applied to a mixture of Clamydomonas sp., Scenedesmus sp. and Nannocloropsis sp.; and by 60% when it was applied to a mixture of Acutodesmus obliquus and Oocystis sp., or to Microspora sp. (Alzate et al., 2012). By applying a higher specific energy of 100.7 and 128.9 MJ/kg TS to Scenedesmus biomass, the solubilisation degree increased by only 8% (González-Fernández et al., 2012). In our study, for a range of specific energy between 24 and 33.7 MJ/kg TS, the solubilisation increase in respect to control was 84–101%. Such high results may be explained by the characteristics of microalgal biomass grown in wastewater treatment HRAP, which tends to be flocculated and, therefore, it contains more extracellular polymeric substances than microalgae grown at lab-scale under controlled conditions (Park et al., 2011). These particulate substances are also solubilised during the pretreatment step, along with microalgae and bacteria, increasing the concentration of soluble VS in comparison with non-pretreated biomass.
3.2. Effect of ultrasonication on microalgal biomass anaerobic biodegradability Microalgal biomass anaerobic digestion rate (khyd) and extent (methane yield) were improved under all ultrasound pretreatment conditions, according to BMP tests results (Table 3; Fig. 2). As shown in Table 3, non-pretreated biomass (control) reached a methane yield of 147.7 mL CH4/g VS. After pretreatment, the
Please cite this article in press as: Passos, F., et al. Anaerobic digestion of microalgal biomass after ultrasound pretreatment. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.06.004
F. Passos et al. / Waste Management xxx (2014) xxx–xxx
Organic matter solubilization (%)
100
(a)
80 R2 = 0.92
60
40
20
0 0
10
20
30
40
Applied specific energy (MJ/kg TS)
Methane yield (mL CH4/g VS)
200
(b) 190 R2 = 0.91
180 170 160 150 140 0
10
20
30
40
Applied specific energy (MJ/kg TS) Fig. 1. Correlation between the applied specific energy by ultrasound pretreatment and microalgal biomass solubilisation increase (a) and methane yield (b).
methane yield was increased by 6–33% (156.4–196.4 mL CH4/ g VS), with minor differences among pretreatment trials. In fact, with an applied specific energy below 30 MJ/kg TS, the methane yield increase did not exceed 20%. However, when applying a specific energy of 33.7 MJ/kg TS, the methane yield increased by 33%. Again, a positive linear correlation was found between the applied specific energy and microalgal biomass methane yield (Fig. 1b). The higher the applied specific energy, the higher the methane yield attained.
A positive correlation between the applied specific energy and methane yield was also found after ultrasound pretreatment of other microalgae species, such as Scenedesmus biomass and Chlorella vulgaris (González-Fernández et al., 2011; Park et al., 2014). However, the methane yield increase did not exceed 20% with a specific energy below 75 MJ/kg TS (Alzate et al., 2012; GonzálezFernández et al., 2012); whereas it increased by 80–90% with a specific energy of 100–200 MJ/kg TS (González-Fernández et al., 2012; Park et al., 2014). In our case, the methane yield increased by no more than 20–30%, probably due to the low specific energy applied (16–67 MJ/kg TS) as compared to previous results. Even if experimental results suggest that a higher specific energy (>100 MJ/kg TS) may be preferred; biogas production could be improved at the expense of unbalancing the energy input and output. For this reason, pretreatment techniques should always be evaluated from an energy perspective. The effect of pretreatment depends on the studied microalgae species, since the cell wall composition and structure affect the anaerobic biodegradability (Mussgnug et al., 2010). Microalgal biomass grown in HRAP for wastewater treatment is most commonly composed of a few adapted species, which predominate depending on environmental, operational and biological parameters (Park et al., 2011). In our study, microalgal biomass was mainly composed by Monoraphidium sp., Stigeoclonium sp. and the diatoms Nitzschia sp. and Amphora sp. Among these species, diatoms have a resistant cell wall, which may hamper anaerobic digestion. Moreover, Stigeoclonium sp. cells generally form flocs, which could also contribute to inhibit the degradation process. The effect of ultrasonication on microalgal biomass anaerobic digestion rate and extent was assessed through BMP tests mathematical modelling. Model outputs are shown in Fig. 3. In this graph, each circle bounds the 95% confidence region of each trial for the two parameters of interest: anaerobic biodegradability (f, x-axis) and hydrolysis rate (khyd, y-axis), and it represents all the possible solutions for f and khyd within the specified confidence level. Confidence regions for the control and pretreatment trials may help understanding the effect of ultrasonication on microalgae anaerobic digestion rate and extent. Indeed, both process kinetics and anaerobic biodegradability improved as the applied specific energy increased. This is evidenced by the way circles move to the right (increase in anaerobic biodegradability) and to the top of the graph (increase in process kinetics). Model results indicate that ultrasonication had a higher impact on microalgae anaerobic biodegradability than on process kinetics. This highlights that, although the pretreatment released soluble organic matter (up to 100% as compared to control), it may have assisted the final methane yield 0,18
200
150 Control T1 T2 T3 T4 T5 T6 T7 T8 T9
100
50
0 0
10
20
30
40
50
60
Digestion time (days) Fig. 2. Microalgal biomass methane yield after ultrasound pretreatment.
Hydrolysis rate (day-1)
Methane yield (mL CH4/g VS)
0,16 0,14
degrades faster ---->
4
Control T1 T2 T3 T4 T5 T6 T7 T8 T9
0,12 0,10 0,08 0,06
degrades more ---->
0,04 0,20
0,25
0,30
0,35
0,40
Anaerobic biodegradability (-) Fig. 3. Mathematical modelling of microalgal biomass BMP tests after ultrasound pretreatment. Each circle bounds the confidence region (95%) of each trial for the anaerobic biodegradability (f, x-axis) and the hydrolysis rate (khyd, y-axis).
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F. Passos et al. / Waste Management xxx (2014) xxx–xxx Table 4 Energy assessment of microalgal biomass anaerobic digestion after ultrasound pretreatment. Trial
Energy output (Eo) (MJ/kg VS)
T1 T2 T3 T4 T5 T6 T7 T8 T9
2.80 3.38 4.12 5.35 5.67 7.15 8.02 9.50 15.69
Harvested biomass (1.2% VS)
Dewatered biomass (8% VS)
Energy input (Ei) (MJ/kg VS)
Energy ratio (Eo/Ei)
Energy input (Ei) (MJ/kg VS)
Energy ratio (Eo/Ei)
25.17 30.20 35.23 50.34 60.40 70.47 75.50 90.60 105.70
0.11 0.11 0.12 0.11 0.09 0.10 0.11 0.10 0.15
3.75 4.50 5.25 7.50 9.00 10.50 11.25 13.50 15.75
0.75 0.75 0.79 0.71 0.63 0.68 0.71 0.70 1.00
(or anaerobic biodegradability) more than the reaction rate (process kinetics). 3.3. Energy assessment An energy assessment of microalgal biomass ultrasonic pretreatment was carried out in order to get an insight into the viability of the process. Indeed, the energy input must be balanced by the extra methane produced as a result of implementing the pretreatment step. The energy input (Ei) corresponded to the specific energy applied to microalgal biomass, according to Eq. (1). However, in order to compare the Ei with the methane yield (mL CH4/g VS), the applied specific energy was recalculated for the VS content of pretreated biomass (MJ/kg VS). The energy output (Eo) was calculated from the difference between the methane yield of pretreated and non-pretreated biomass (control) (DPCH4) (Eq. (4)), where DPCH4 is the methane yield increase after biomass pretreatment (mL CH4/g VS) and n is the lower heating value of methane (35,800 kJ/m3CH4) (Metcalf et al., 2003). An efficiency of 90% on energy conversion was considered.
Eo ðMJ=kg VSÞ ¼ ½DPCH4 ðmL=gVSÞ nðkJ=m3 Þ 0:9=106
ð4Þ
In this manner, the feasibility of the process was estimated by the energy output to energy input (Eo/Ei) ratio (Table 4). A ratio higher than 1 indicates that the energy output from the extra methane generated is higher than the energy required for ultrasonication. As can be noticed, with harvested microalgal biomass thickened by gravity, the energy ratio was lower than 1 under all ultrasound pretreatment conditions (0.11–0.15). However, if biomass was dewatered reaching a concentration of 8% VS, a neutral energy ratio could be attained under optimal pretreatment conditions (70 W and 30 min). The reason for this is that the applied specific energy would decrease from 105.7 to 15.75 MJ/kg VS, by increasing biomass concentration from 1.2% to 8.0%VS. Indeed, for other mechanical pretreatments such as microwaves, biomass concentration has been pointed out as a crucial factor affecting the energy balance of the process (Passos et al., 2013a, 2014). In that case, the energy demand for biomass dewatering should also be taken into account, which is one of the challenges of full-scale microalgae biofuel production (Schwede et al., 2013). Besides, the methane yield of microalgae in BMP tests tends to be lower than in continuous reactors, due to the lack of acclimated biomass; i.e. 148 mL CH4/g VS in this BMP vs. 170 mL CH4/g VS in a continuous reactor at 20 days HRT (Passos et al., 2014). So, if ultrasound pretreatment was tested in continuous reactors, better results ought to be expected in terms of methane production and energy output, and possibly no such high biomass concentration would be required to attain a positive energy ratio. Additionally, the energy input would surely decrease by using pilot and fullscale equipment, which is more energy efficient than lab-scale
devices. In fact, ultrasonication has been applied to sewage sludge in full-scale WWTP leading to net energy generation (Perez-Elvira et al., 2009). Up to date, ultrasound pretreatment has only been applied to microalgae undergoing anaerobic digestion in BMP tests, which are useful to compare different pretreatment conditions, but not to estimate the methane yield that could be obtained upon continuous digester operation. Thus, the authors believe that future research should look at the effect of microalgae ultrasonication in continuous reactors, and if possible using pilot-scale equipment. 4. Conclusions Ultrasound pretreatment was studied with the aim of improving the anaerobic biodegradability of microalgal biomass grown in wastewater treatment HRAP. To this aim, output powers of 50, 60 and 70 W and exposure times of 10, 20 and 30 min were investigated prior to BMP tests. Experimental results showed how VS solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6–33%) improved with the pretreatment intensity, obtaining the best results with the highest applied specific energy (67 MJ/ kg TS). Mathematical modelling revealed that the pretreatment had a higher effect on the final methane yield than on the hydrolysis rate. Finally, a preliminary energy assessment indicated that the energy input required for ultrasound pretreatment was higher than the extra energy produced by the methane yield increase, when digesting harvested microalgal biomass thickened by gravity. However, positive energy balances could be reached by dewatering microalgal biomass to reach a VS concentration above 8%. Acknowledgements This research was funded by the Spanish Ministry of Economy and Competitiveness (Project BIOALGAS CTM2010-17846). Fabiana Passos is grateful to the Coordination for the Improvement of Higher Level Personal (CAPES) funded by the Brazilian Ministry of Education for her PhD scholarship. References Alzate, M.E., Muñoz, R., Rogalla, F., Fdz-Polanco, F., Perez-Elvira, S.I., 2012. Biochemical methane potential of microalgae: influence of substrate to inoculum ratio, biomass concentration and pretreatment. Bioresour. Technol. 123, 488–494. Alzate, M.E., Muñoz, R., Rogalla, F., Fdz-Polanco, F., Perez-Elvira, S.I., 2014. Biochemical methane potential of microalgae biomass after lipid extraction. Chem. Eng. J. 243, 405–410. Batstone, D.J., Keller, J., Angelidaki, I., Kalyuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A., Sanders, W.T., Siegrist, H., Vavilin, V.A., 2002. The IWA Anaerobic Digestion Model No. 1 (ADM1). Wat. Sci. Technol. 45, 65–73. Batstone, D.J., Tait, S., Starrenburg, D., 2009. Estimation of hydrolysis parameters in full-scale anerobic digesters. Biotechnol. Bioeng. 102, 1513–1520. Bourrelly, P., 1966. Les algues d’eau douce. Tome I: Les algues vertes. Édition N. Boubée & Cie, Paris.
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