Feasibility of microalgae cultivation system using membrane-separated CO2 derived from biogas in wastewater treatment plants

Biomass and Bioenergy 106 (2017) 191e198

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Research paper

Feasibility of microalgae cultivation system using membraneseparated CO2 derived from biogas in wastewater treatment plants Yugo Takabe a, *, 1, Shuji Himeno b, Yuji Okayasu a, Mizuhiko Minamiyama a, 2, Toshiya Komatsu b, Kouhei Nanjo b, Yukiyo Yamasaki a, Ryuji Uematsu a a

Materials and Resources Research Group, Innovative Materials and Resources Research Center, Public Works Research Institute, 1e6 Minamihara, Tsukuba, Ibaraki, 3058516, Japan Department of Civil and Environmental Engineering, Nagaoka University of Technology, 1603e1, Kamitomiokamachi, Nagaoka, Niigata, 9402188, Japan

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2017 Received in revised form 8 September 2017 Accepted 11 September 2017

Utilising CO2 from biogas as a carbon source can introduce energy production systems incorporating microalgae cultivation into wastewater treatment plants (WWTPs). In this study, the effects of utilising membrane-separated CO2 (MSC; CO2 content: 983 ± 11 dm3 m
3)dobtained from biogas using a newly developed separation systemdon indigenous microalgae cultivation with treated effluents was investigated. Assuming model cultivation systems, energy balance analysis was conducted to evaluate the feasibility of MSC utilisation and its superiority over flue gas CO2 (FC) from power plants (PPs). Scenarios 1 and 2 used generated biogas of 11.6 and 116 dm3 s
1, and produced MSC (carbon based) were 0.802 and 8.02 g s
1, respectively. Experiments comparing MSC (suspended solids (SS): 166 ± 23 mg dm
3; higher heating value (HHV): 21.1 ± 0.46 kJ g
1) with commercial CO2 (SS: 176 ± 16 mg dm
3; HHV: 21.2 ± 0.90 kJ g
1) revealed no negative effects on microalgae activity, biomass production, or energy content. Energy revenue of the MSC utilisation system was 27.0 MJ per 1 kg injected MSC, which is greater than the energy costs (18.2 and 17.1 MJ kg
1 in scenarios 1 and 2, respectively). MSC utilisation is energetically feasible, and energetically superior to FC utilisation when the PP-to-WWTP distance is over 0.6 and 1.9 km in scenarios 1 and 2, respectively. © 2017 Elsevier Ltd. All rights reserved.

Keywords: CO2 Biogas Membrane separation Wastewater treatment plant Indigenous microalgae Energy balance analysis

1. Introduction Wastewater and treated effluent in municipal wastewater treatment plants (WWTPs) have abundant nutrients such as nitrogen and phosphorus. Cultivation of microalgae, a promising third-generation feedstock for biofuel production, has been extensively investigated with primary settled wastewater [1] and treated effluent [2]. Biodiesel (i.e. fatty acid methyl ester (FAME)) with lipid-rich-specific microalgae species, such as Nannochloropsis sp. (FAME content: 210 mg g
1) and Heterosigma sp. (196 mg g
1) [3], is one of the most attractive biofuel productions via cultivation; meanwhile, Sturm et al. [4] evaluated the energy balance in

* Corresponding author. E-mail address: [email protected] (Y. Takabe). 1 Present address: Graduate School of Engineering, Tottori University, 4e101 KoyamaeMinami, Tottori, 6808552, Japan. 2 Present address: Water Quality Research Team, Public Works Research Institute, 1e6 Minamihara, Tsukuba, Ibaraki, 3058516, Japan. http://dx.doi.org/10.1016/j.biombioe.2017.09.004 0961-9534/© 2017 Elsevier Ltd. All rights reserved.

outdoor open cultivation, which is the cheapest method of largescale microalgae biomass production [5], with treated effluent and an energy recovery system, and suggested direct combustion of cultivated microalgae may be suitable when lipid content is low (100 mg g
1). CO2 is an essential carbon source for microalgae growth, and intermittent injection of CO2 via pH control in microalgae cultures can successfully enhance biomass and energy production through microalgae cultivation [1,2]. Takabe et al. [2] operated two 380 dm3 outdoor open ponds, with effective surface areas and depths of 1.52 m2 and 0.25 m respectively, in which indigenous microalgae were cultivated. One pond had commercial CO2 (CC) injection to maintain the pH at 8.0; the other did not. Total carbon and energy production in the former was 18.0 g m
3 and 0.722 MJ m
3 higher than in the latter. Securing energetically and economically favourable CO2 sources is important. Utilisation of flue gas CO2 (FC) from power plants (PPs) has been investigated. He et al. [6] showed that on-off pulse FC injection promoted Chlorella growth and provided cost-effective algal cultivation, and FC utilisation had no negative

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influence on the growth of microalgae (Chlorella sp.), compared to cultivation with pure CO2 [7]. Direct pumping of FC should also be economical [8]. Scenedesmus sp. was cultivated in a 20 m3 raceway reactor with an effective surface area and depth of 100 m2 (two 50 m channels with 1 m width) and 0.2 m, respectively, combined with a sump (depth: 1 m, length: 0.65 m, width: spanned the full width of the channel (1 m)) with intermittent injection of FC (CO2 content: 106 dm3 m
3) to maintain the culture pH at 8, and 96% removal of injected CO2 was obtained [9]. Campbell et al. [10] revealed that utilising FC from an adjacent PP via pipelines is favourable with respect to greenhouse gas emissions when compared to utilisation of pure liquefied CO2 delivered by truck. Meanwhile, energy costs in delivery and direct blower injection of FC from a PP near a cultivation pond by a blower (22.2 Wh per 1 kg CO2) [8] accounted for 65% of the total energy costs in microalgae cultivation systems [11]. Anaerobic digesters in WWTPs generate biogas. Median and 95th percentile biogas generation in Japanese WWTPs (n ¼ 269) was 15.6 and 109 dm3 s
1, respectively [12]. Utilisation of the CH4 in biogas has been extensively investigated, such as for power generation via biogas engines in which siloxane-free CH4 is used because of the production of abrasive microcrystalline silica, which results in serious biogas engine damage, from siloxane combustion [13,14]. Meanwhile, the CO2 in biogas has not been utilised, even though on-site utilisation of biogas CO2 as a carbon source can reduce the energy demand of cultivation systems. Researchers have investigated direct usage of biogas CO2 in microalgae cultures, as well as tried to purify CH4 for biomethane usage, such as in natural gas networks. H2S inhibited the activity of the protein responsible for electron transport between photosystems II and I [15]. Meanwhile, compared to cultivating Chlorella sp. with artificial biogas consisting of ambient air and CO2, injection of artificial biogas with H2S to a culture (H2S content: 50 cm3 m
3; H2S injection rate to the culture: 250 mm3 m
3 s
1) resulted in no decrease in growth rate [16]. Approximately 25 mg dm
3 of free ammonia in a culture decreased photosynthesis of Scenedesmus obliquus by 50% [17]. Injection of artificial biogas with CH4 (CH4 content: 200 dm3 m
3; CH4 injection rate: 1 dm3 m
3 s
1) to a culture resulted in a 4% reduction in biomass concentration; meanwhile, a decrease in biomass concentration of 26% was observed following injection of the biogas whose CH4 content was 600 dm3 m
3 [16]. Installation of an external absorption column, coupled with a microalgae cultivation reactor, can optimise CO2 dissolution and removal of CH4 [18]. However, the effects of dynamic pH conditions in the absorption column on microalgal growth must be investigated [19]. Therefore, further improvements to system design and operational conditions are needed to optimise energy production via microalgal growth with direct use of biogas. Membrane separation of biogas has several advantages, including low capital cost, high energy efficiency and relative ease of operation and control [20]. Sasaki et al. [21] developed a new two-stage separation system for CO2 and CH4 from biogas with commercial hollow fibre membrane modules made from polyimide (NM-B01A, Ube Industries, Japan), the most suitable commercial membrane for biogas separation [20]. The system achieved 37.6% CO2 recovery with CO2 content of 987 dm3 m
3 via low injection pressure (absolute pressure: 1.0 MPa). Therefore, utilisation of membrane-separated CO2 (MSC) as a carbon source for microalgae is promising. However, existence of technical problems in cultivation through MSC utilisation, specifically growth inhibition, and feasibility of the MSC utilisation system must be investigated. In this study, indigenous microalgae were cultivated with treated effluents. Substantial biomass can be obtained with a hydraulic retention time (HRT) of 172.8 ks in indigenous microalgae cultivation with treated effluent [22]. The contribution of

zooplankton dry mass to that of total biomass in microalgae cultivation with treated effluent was small (at most 2.9%) [23]. In addition, the particulate organic carbon (POC) of bacteria accounted for approximately 5% in total POC in a high rate algal pond (HRAP) [24]. Because the organic matter concentration in treated effluent was lower than that in raw wastewater, it was likely that the contribution of bacteria to total biomass in microalgae cultivation with treated effluent was lower than that in the HRAP (5%). Therefore, it was likely that the cultivated biomass with treated effluents mainly consisted of indigenous microalgae and their detritus. The objective of this study was to investigate the effects of MSC utilisation on indigenous microalgae cultivation via comparative cultivation experiments with MSC and CC with high CO2 content (999.5 dm3 m
3). In addition, energy revenues and costs were estimated in a model cultivation system to evaluate the feasibility of the MSC system. Energy costs of the MSC system were compared to those of FC systems.

2. Materials and methods 2.1. Treated effluent and biogas Treated effluent and biogas were collected from WWTP 1, which is located in Niigata Prefecture, Japan and has an operational capacity of 475 dm3 s
1. Raw wastewater, mainly comprised of domestic wastewater, flowed through a separated sewage system and was treated by a pseudo-anaerobic aerobic process (aeration tank HRT: 39.2 ks). Treated effluents from a final sedimentation tank were collected in 20 dm3 poly tanks and stored at 4  C under ambient air and dark conditions before use. The treated effluents were used for cultivation without any nutrient adjustment. Biogas was generated via mesophilic anaerobic digestion (35  C) in which only the sludge from WWTPs was treated. The effective volume of the digestion tank was 9.5 dam3, and the digestion time, volatile solid loading rate and biogas CH4 content were 3.54 Ms, 9.7 mg m
3 s
1 and 590 dm3 m
3, respectively. The biogas was dry desulphurized and treated by the membrane separation system described in Section 2.2.

2.2. Membrane separation system Membrane separation was conducted following Sasaki et al. [21] (Fig. 1(a)). After raising the biogas pressure with a compressor (2BSN10, Mikuni Kikai Kogyo Co. Ltd., Japan), the biogas was first passed through an air dryer (RDG-22C, Anest Iwata, Japan) to remove moisture and then columns packed with 3A molecular sieves (Nacalai Tesque Inc., Japan) and activated carbon (MESOCOAL, Cataler, Japan) to remove siloxane, assuming CH4 usage in biogas engines. The system was operated to obtain a dew point temperature of
55  C of the biogas, measured by a Hygrometer Mark 1 (Tekhne, Japan), after passing through the columns packed with 3A molecular sieves and activated carbon. The biogas was then injected into the membrane modules. The gas (MSC) that permeated through the second module of the membrane separation system was packed in two gas tanks, each with a volume of 47 dm3, at 1 MPa. The CO2, CH4, H2S and NH3 contents in the gases (i.e. biogas after dry desulphurization, MSC and ambient air in the experimental indigenous microalgae cultivation room) as received were measured.

Y. Takabe et al. / Biomass and Bioenergy 106 (2017) 191e198

193

Fig. 1. (a) Membrane separation conditions and (b) MSC utilisation system in model indigenous microalgae cultivation.

2.3. Indigenous microalgae cultivation Indigenous microalgae, the microalgae that naturally grew in the reactor, were cultivated in a growth chamber (LPHe350SP, NK Systems, Japan). Microalgae species were not artificially introduced for the cultivation. The temperature and photon flux density of the white fluorescent lamps were 25  C and 130 mmol m
2 s
1, respectively, with a light/dark cycle of 43.2 ks. Two glass reactors (volume 2.4 dm3) were set in the chamber, while the reactor heads were open to the ambient air. The effective volume and height of the culture in each reactor were 2.0 dm3 and 18.7 cm, respectively. Each culture was mixed using a magnetic stirrer. Once daily, 1.0 dm3 of the culture was replaced by the same volume of treated effluent to obtain an HRT of 172.8 ks, following [22]. After collection, part of the collected culture was filtered. The unfiltered and filtered cultures were stored at 4  C under ambient air and dark conditions before they were analysed for the water qualities described in Section 2.4. MSC and CC were directly injected into the bottom of the cultivation reactor through an air stone without an external absorption column, which made it easier to control the injection for optimised energy production via microalgae growth. Following [2], CO2 was intermittently injected by a pH controller (NPH-660NDE, Nissin, Japan) to maintain the culture pH at 8.0. MSC and CC were packed in a 10 dm3 aluminium gas bag and injected via a peristaltic pump (FPC100, AS ONE, Japan) at 330 mm3 s
1. The reactors used for the experiments with MSC and CC were designated R1 and R2, respectively. The mass balance of carbon in R1 was calculated to evaluate the assimilation of injected CO2 by the indigenous microalgae. The culture (1 dm3) was replaced by treated effluent, after which the reactor was left open for approximately 1.8 ks in the chamber to allow the indigenous microalgae to acclimatise to the environment and the culture pH to reach 8.0. After collection of 100 cm3 of the culture, the reactor head was sealed with a plastic cover. A 1 dm3 aluminium gas bag was connected to a hole in the plastic cover and

the gas in the reactor headspace was closed off from the air. Sealed cultivation was conducted for 32.4 ks under lighted conditions. Carbon concentration in the culture and CO2 concentration in the headspace were measured before and after cultivation. A 2 dm3 aluminium gas bag was used for MSC injection into the sealed cultivation. The amount of carbon injected as CO2 was calculated based on the difference in the volume of the gas bag before and after the experiment, as well as the CO2 content in the MSC. 2.4. Analysis Dissolved oxygen (DO), suspended solids (SS), chlorophyll a (Chl. a), total nitrogen (TN), dissolved total nitrogen (DTN), total phosphorus (TP), dissolved total phosphorus (DTP) and inorganic carbon (IC) were continuously measured in the cultivations. Particulate total nitrogen (PTN) and particulate total phosphorus (PTP) were calculated as the difference between TN and DTN and between TP and DTP, respectively. IC, total organic carbon (TOC) and dissolved organic carbon (DOC) were measured in the sealed experiment. POC was calculated as the difference between TOC and DOC. DO was measured with a 55 dissolved oxygen meter (YSI, USA). Standard methods were used to determine SS [25] and Chl. a [26]. TN, DTN, TP and DTP were measured with a TRAACS2000 (Bran þ Luebbe, Germany), whereas IC, TOC and DOC were measured with a TOC-VCPH analyser (Shimadzu, Japan). A GF/C membrane (pore size: 1.2 mm; Whatman, USA) was used for Chl. a. analysis, whereas a GF/B membrane (pore size: 1 mm; Whatman, USA) was used for analysis of SS and other water quality parameters in the dissolved phase. During periods with stable SS, cultures were continuously collected for 259.2 or 345.6 ks in a 3 dm3 plastic bottle that was stored at 4  C under ambient air and dark conditions to count each microalgae species and measure higher heating value (HHV) and FAME content in the produced biomass. Microalgae species were counted using a microscope (BH-2, Olympus, Japan) after 12.5 dm3 m
3 glutaraldehyde fixation, according to a standard

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method [27]. To measure HHV and FAME content, cultures were centrifuged for 1.2 ks at 19.6 km s
2 (H-7000UL, Kokusan, Japan). After supernatant removal, the residue was collected and frozen (
20  C), then lyophilised (FDU-2100, Tokyo Rikakikai Co. Ltd., Japan). HHV was measured with a bomb calorimeter (OSK 150, Ogawa Sampling, Japan). One-step direct transesterification was used to measure FAME content, following Griffiths et al. [28]. FAME compounds were identified using gas chromatography (GC; 6890 Series Plus, Agilent Technologies, USA)/mass spectrometry (5973 Network, Agilent Technologies, USA). The GC was equipped with a DB-23 capillary column (Agilent Technologies, USA). The injector was operated in splitless mode at 250  C, with 1 mm3 injection volume. The carrier gas (helium) was controlled at 16.7 mm3 s
1. The oven temperature was 50  C for 120 s; raised to 160  C at a rate of 167 m C s
1, raised to 200  C at a rate of 50 m C s
1, and held for 300 s; and finally raised to 230  C at a rate of 50 m C s
1 and held for 600 s. Ionisation was achieved by electronic impact at 11.2 aJ. CO2 and CH4 were measured using a GC-thermal conductivity detector (GC-2014, Shimadzu, Japan). H2S was measured using a GC-flame photometric detector (GC-2014, Shimadzu, Japan), according to a standard method [29]. NH3 was measured in accordance with a standard method [30]. The distribution of the measured values is presented as average ± standard deviation. 2.5. Energy balance analysis The energy revenues and costs of MSC utilisation in model cultivation systems in WWTPs were calculated. Two biogas generation scenarios (1 and 2 for 11.6 and 116 dm3 s
1 respectively) in model WWTPs were assumed, similar to the median and 95th percentile of biogas generation in Japanese WWTPs [12]. The biogas temperature was set to 35  C. All biogas was assumed used in MSC production. MSC weight is expressed as carbon basis in this study, and according to calculations, produced MSC were 0.802 and 8.02 g s
1 in scenarios 1 and 2, respectively, based on CO2 content in the biogas (396 dm3 m
3) and CO2 recovery via the membrane separation system (36.9%), which was affected by the injection pressures to the first and second membrane modules [21], as described in Section 3.1. 2.5.1. Microalgae cultivation system In this study, it was assumed that indigenous microalgae were continuously cultivated in the outdoor raceway reactors. The FAME content in the indigenous microalgae, described in Section 3.3.2, was much lower than that in the lipid-rich-specific microalgae [3], and the cultivated microalgae were used for direct combustion in this analysis. Assuming that (1) the increase in the total carbon in the pond with CO2 injection [2] is attributed to the injected CO2 and (2) the injected CO2 dissolution rate into the culture is the same as the CO2 removal rate in Ref. [9], then 18.8 g of CO2-C injection is required for 1 m3 of culture. The microalgae culture was assumed to require CO2 injection during the day (43.2 ks) but not at night (43.2 ks). Considering the produced MSC and required CO2 injection for 1 m3 of culture, the amounts of treated effluent used for microalgae cultivation in scenarios 1 and 2 were 85.5 and 855 dm3 s
1, respectively. When all generated sludge in a WWTP is used for anaerobic digestion, 100 dm3 of biogas is generated from 1 m3 of WWTP inflow [13]. Therefore, inflow to the model WWTPs was calculated as at least 116 dm3 s
1 and 1.16 m3 s
1 in scenarios 1 and 2, respectively, and the required amounts of treated effluent for microalgae cultivation were covered.

70% of the energy content can be recovered from combustion [4] and the required CO2 injection amount, thus the energy revenue via MSC injection was set at 27.0 MJ per 1 kg injected MSC. 2.5.3. Energy costs for MSC utilisation Energy costs include equipment manufacture, construction and power consumption. The required equipment and constructions for MSC utilisation are shown in Fig. 1(b), including compressors, air dryers, siloxane removal system, membrane modules, gas holders to store MSC and blowers to inject MSC to the cultivation system. The pressure of each gas is expressed as absolute pressure. The compressors worked day and night. Atmospheric and MSC temperatures were assumed 25  C. MSC produced during the night was assumed used during the following day, thus the capacity of the gas holders was set to store generated MSC for 43.2 ks. Based on the frequency of CO2 addition in the raceway reactor [9], it was assumed that MSC is injected (i.e. the blowers work) for 300 s at intervals of 1.8 ks during the day. MSC injection pressure was set at 0.121 MPa. The energy costs for equipment manufacturing and construction are shown in Table 1. Expenses for manufacturing and construction are from previous studies or quotations from manufacturers. Differences in equipment or construction capacities occurred between the references and the values used in this analysis, so expenses were corrected using Eq. (1) [13]. The expense in US dollars ($) was calculated based on a conversion rate of Japanese yen to US dollar (1 $ ¼ 110 yen).

 Xa ¼ Xr

Ya Yr

0:6 (1)

where Xa is the expense of equipment and construction in this analysis ($), Xr is this expense in the references ($), Ya is the capacity of equipment or construction in this analysis (m3 s
1 or m3) and Yr is this capacity in the references (m3 s
1 or m3). Energy costs are calculated by multiplying the expense for equipment manufacture or construction by energy unit per expense (MJ $
1). Energy units are taken from an input-output table (IO table) and an energy balance table for Japan [31]. Energy costs are divided by their lifetime and calculated per second. The electric power of compressors and blowers was calculated using Eq. (2) [33], and the coefficients are listed in Table 2. Considering loss in generation and transmission, 3.6 MJ of electrical energy at the receiving end required 9.48 MJ of primary energy at the generating end [37]. Considering the working time of the compressor, blowers energy consumptions were calculated.

E ¼

Ps Qd g ge1

  Pd Ps



ðge1Þ

g

e1

1

ha

(2)

where E is electric power (W), Ps is suction pressure (Pa), Pd is discharge pressure (Pa), Qd is discharge flow (m3 s
1), g is specific heat ratio and ha is adiabatic efficiency. The electric power of the air dryer was set at 0.262 and 2.62 kW based on the power of a product whose expense is referenced in this study. Each energy cost (MJ s
1) was divided by the amount of produced MSC (kg s
1), and the energy costs to utilise 1 kg of MSC were calculated. 3. Results and discussion 3.1. Gas composition

2.5.2. Energy revenues by MSC utilisation Based on the enhanced energy production by CO2 injection [2],

The CO2 contents (n ¼ 3) in the biogas and MSC were 396 ± 6

Y. Takabe et al. / Biomass and Bioenergy 106 (2017) 191e198

195

Table 1 Energy costs for equipment manufacture and construction. The expense and energy cost per expense in USA dollars ($) were calculated based on a conversion rate of Japanese yen to USA dollar (1 $ ¼ 110 yen). 1 year (y) was calculated as 31.5 Ms. Equipment and construction

Scenario Energy cost (J s
1)

Manufacture of compressors

1 2 Manufacture of air dryers 1 2 Siloxane removal system 1 construction 2 Manufacture of membrane modules 1 2 Gas holder construction 1 2 Manufacture of blowers 1 2

294 1170 10.6 42.1 525 2090 501 2000 47.6 189 22.4 89.3

Energy cost per expense [31]

Expense

Value (MJ $
1)

Segment of IO table

Value ($)

References

Value (y)

References

4.96

Pump and compressors

[13]

15

[13]

4.30

Refrigerators and conditioning apparatus Other special industrial machinery

Quotation from manufacturers Quotation from manufacturers [13]

15

4.08

15

[13]

10.5

High functionality resins

[32]

Other civil engineering and constructions Pump and compressors

Quotation from manufacturers [13]

7.5

4.71

28 000 112 000 1230 4870 57 700 230 000 11 400 45 100 15 900 63 400 2140 8520

50

[13]

Quotation from manufacturers

15

[13]

4.96

Table 2 Coefficients to calculate electric power of compressors and blowers. Equipment

Coefficient

Scenario

Value

Unit

Reference

Compressor

Ps Pd Qd

1 1 1 2 1 1 1 1 1 2 1 1

101 000 1 000 000 0.0116 0.116 1.35 0.78 101 000 121 000 0.0200 0.200 1.30 0.70

Pa Pa m3 s
1

e e e

e e Pa Pa m3 s
1

[34] [35] e e e

e e

[36] [35]

g ha Blower

Ps Pd Qd

g ha

and 2 and 2

and and and and

2 2 2 2

and 2 and 2

and 983 ± 11 dm3 m
3, respectively, whereas the CH4 contents (n ¼ 3) were 599 ± 13 and 9.02 ± 4.67 dm3 m
3, indicating high purity of CO2 in the MSC. The CO2 recovery in the biogas was 36.9%. H2S and NH3 contents (n ¼ 2) in the biogas were 1.9 and 3.1 cm3 m
3, and 0.12 and 0.27 cm3 m
3, respectively, and <0.2 cm3 m
3, and 0.20 and 0.38 cm3 m
3 in the MSC, respectively. CO2 was detected in the ambient air (n ¼ 3; 470 ± 80 cm3 m
3), but other chemicals were not.

3.2. CO2 assimilation by indigenous microalgae The carbon mass in each medium before and after the sealed cultivation experiment is described in Fig. 2. The injected volume of MSC during the sealed experiment was 230 cm3. The CO2 injection

Life time

rate to the culture was 3.3 cm3 m
3 s
1. Carbon in the form of DOC and IC remained similar throughout the experiment, whereas that in the form of POC was 6.27 times higher after the experiment than before, owing to indigenous microalgal growth. The CO2 assimilation ratio (%) of indigenous microalgae was calculated using Eq. (3).

Ac ¼

Cpa eCpb Cc

(3)

where Ac is the CO2 assimilation ratio; Cc is the carbon in injected CO2-C (mg); and Cpb and Cpa are the POC-C (mg) before and after the experiment, respectively. The assimilation ratio by the indigenous microalgae was 91.8%. The assimilation ratio of Scenedesmus sp. at 8.9 cm3 m
3 s
1 of CO2 injection rate [9], similar to the injection rate in the current study, was calculated as 77% using Eq. (3), and the assimilation ratio in the current study was higher than that in Ref. [9]. The carbon concentration of influent water in the raceway reactor was 20 mg dm
3 lower than that of the culture in the raceway reactor, with 12% of the injected CO2 being consumed to arrange the carbon concentration in the culture [9]. In the current study, the sealed experiment started when the acclimated culture attained a pH of 8.0, with the IC of the culture being similar before and after the experiment. Therefore, consumption of injected CO2 by the IC arrangement of the culture was smaller, which contributed to the higher assimilation ratio. Meanwhile, in the current study, it was revealed that IC in the treated effluent was higher than that in the culture (Table 3), indicating the possibility of high assimilation of injected CO2 in indigenous microalgae cultivation with treated effluents.

3.3. Effects of MSC utilisation

Carbon amount (mg)

250 200

Unknown C in injected CO CO2 2

150

C in head space gas POC in reactor

100

DOC in reactor

50 0

IC in reactor

1 Before experiment

2 After experiment

Fig. 2. Carbon mass balance in sealed experiment.

3.3.1. Indigenous microalgal growth Changes in SS, Chl. a and the ratio between PTP and SS in each culture over time are shown in Fig. 3. Constant SS was observed in both R1 and R2 from day 16. The distribution of measured water quality parameters in the treated effluents and in R1 and R2 from day 16 are shown in Table 3. Despite constant SS, Chl. a in both reactors decreased sharply from days 14e19. As described in Section 1, it was likely that biomass produced by indigenous microalgae cultivation with treated effluents mainly consisted of microalgae and their detritus. The ratio of PTP SS
1 indicates the phosphorus content in the microalgae. The PTP SS
1 clearly decreased from days 12e16, with constant values being observed from day 16. Powell et al. [38]

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Y. Takabe et al. / Biomass and Bioenergy 106 (2017) 191e198

Table 3 Distribution of measured parameters.

SS (mg dm
1) Chl. a (mg dm
1) IC (mg dm
1) Nitrogen species (mg dm
1)

Phosphorus species (mg dm
1)

DO (mg dm
1) HHV (kJ g
1) FAME content

Total (mg g
1) Relative content (-)

R1 R2

Chl. a

R1 R2

C16:0 C16:1 C18:0 C18:1n9c C18:2n6c C18:3n3 Others

PTP SS−1

R1 R2

0.03

2.5

2 0.02

1.5

1

0.01

0.5 0

0.00

PTP SS −1 (mg mg−1)

SS

SS ( 100) (mg dm−3) Chl. a (mg dm−3)

TN DTN PTN TP DTP PTP

0 3 6 9 12 15 18 21 24 27 30 Time (d) Fig. 3. Changes in SS, Chl. a and PTP SS
1 in each culture over time.

reported that microalgae store phosphorus as polyphosphate to act as a phosphorus source for growth; the stored polyphosphate is utilised under depleted phosphorus conditions. In addition, Fernandes et al. [39] reported that limitation of nutrients, including phosphorus, led to decreases in pigment content (chlorosis) in Parachlorella kessleri. Considering half saturation constant of nitrogen (25 mg dm
3) [22], the nitrogen in R1 and R2 was abundant (Table 3). Therefore, phosphorus depletion around day 14 likely resulted in chlorosis and consumption of stored phosphorus, a phenomenon that terminated around day 16. A Welch's t-test (significance level 0.05) revealed no significant differences in SS (p ¼ 0.162) and Chl. a (p ¼ 0.491) between R1 and R2. Based on these results, MSC injection has no significant negative effect on either indigenous microalgae activity or biomass production by growth. Much lower CH4 (33 mm3 m
3 s
1), H2S (<670 mm3 hm
3 s
1) and NH3 injection rates to the culture (1 mm3 dam
3 s
1) than the previous studies [16,17] were contributed to the lack of significant negative effects. Cell number ratios (n ¼ 2) of Chlorophyceae (R1: 96% and 97%; R2: 96% and 97%) and Scenedesmaceae (R1: 73 and 88%; R2: 84% and 94%) to total cell number were high, and Chlorophyceae, specifically Scenedesmaceae, dominated in both reactors. In the treated effluent, cell number ratios (n ¼ 1) of Bacillariophyceae

Treated effluent

R1

All parameters (n ¼ 2)

HHV and FAME content (n ¼ 3); others (n ¼ 7) (average ± standard deviation)

4.4 and 9.5 <0.01 23.5 and 27.5 16.7 and 29.5 16.0 and 29.0 0.5 and 0.7 0.521 and 0.595 0.375 and 0.518 0.077 and 0.146 e e e e e e e e e e

166 ± 23 1.27 ± 0.24 13.9 ± 2.3 16.4 ± 0.8 5.89 ± 0.45 10.5 ± 0.8 0.537 ± 0.050 0.023 ± 0.009 0.513 ± 0.043 16.6 ± 2.1 21.1 ± 0.46 40.1 ± 14.5 0.470 ± 0.055 0.0838 ± 0.0085 0.0603 ± 0.0231 0.146 ± 0.024 0.110 ± 0.022 0.0929 ± 0.0237 0.0373 ± 0.0161

R2

176 ± 16 1.27 ± 0.21 12.4 ± 1.0 15.8 ± 1.0 5.18 ± 1.12 10.7 ± 1.0 0.531 ± 0.045 0.022 ± 0.009 0.509 ± 0.038 15.7 ± 1.3 21.2 ± 0.90 48.3 ± 7.1 0.465 ± 0.105 0.0809 ± 0.0099 0.0678 ± 0.0344 0.143 ± 0.051 0.117 ± 0.053 0.0970 ± 0.0579 0.0290 ± 0.0332

(69%) and Chlorophyceae (26%) to total cell number were high, and Scenedesmaceae was also found (0.59%). Scenedesmaceae dominance was also seen in indoor indigenous microalgae cultivation at the same temperature and photon flux density as in the current study, with treated effluent from a different WWTP, in which the conventional activated sludge process was applied, and CO2 injection maintained culture pH at 8.0 [2]. 3.3.2. Energy values of produced biomass HHV and FAME content and composition of the produced biomass are shown in Table 3. HHVs of biomass produced in both reactors were the same, revealing that MSC utilisation had no negative effects on energy production by whole indigenous microalgae cells. Previous indoor cultivation with treated effluent from different WWTPs revealed that the HHV of cultivated biomass, also dominated by Scenedesmaceae, was 21.0 kJ g
1 [2], a value similar to those in the current study. There was no significant difference in FAME content between R1 and R2 (p ¼ 0.237; by Welch's t-test), and FAME composition in R1 and R2 was similar (principal component: C16:0). Meanwhile, FAME content was much lower than the lipid-rich-specific microalgae species [3]. 3.4. Feasibility and superiority of the MSC utilisation system The energy revenues and costs of the MSC utilisation system are

Energy revenues and costs (MJ kg–1)

Parameter

30

Power consumption of blowers

Energy revenues 20

Manufacture of blowers Gas holder construction

Manufacture of membrane modules Siloxane removal system construction

10

Power consumption of air dryers Manufacture of air dryers Power consumption of compressors

0

Scenario 1

Scenario 2

Manufacture of compressors

Fig. 4. Energy revenues and costs of MSC utilisation system.

Y. Takabe et al. / Biomass and Bioenergy 106 (2017) 191e198

shown in Fig. 4. Total energy costs were 18.2 and 17.1 MJ kg
1 for scenarios 1 and 2, respectively. Energy consumption by compressors was the main energy cost, accounting for 84.9% and 90.1% of the total costs in scenarios 1 and 2, respectively. Energy revenues were 1.48 and 1.58 times higher than energy costs in scenarios 1 and 2, respectively; thus the MSC utilisation system is energetically feasible. Assuming CO2 content and temperature of cooled flue gas from liquefied natural gas PPs are 100 dm3 m
3 [40] and 25  C, respectively, then 16.3 and 163 dm3 s
1 of flue gas is required to provide the same amount of CO2 by MSC utilisation in scenarios 1 and 2, respectively. It is assumed that energy consumption by the compressors that deliver flue gas from the PP to the WWTP is the only energy cost in FC utilisation. To decrease energy costs of FC utilisation below those for MSC, the maximum pressure to deliver flue gas through pipelines was calculated as 0.81 and 0.74 MPa for scenarios 1 and 2, respectively, based on Eq. (2) with adiabatic efficiency of 1.4 [36] and requirement of 8.68 MJ in primary energy at the generating end to 3.6 MJ in electrical energy at the receiving end due to the loss in generation [37]. The flow rate of flue gas at the inlet was set at 10 m s
1 [41], and the pipeline inner diameter was calculated as 1.6 and 5.3 cm for scenarios 1 and 2, respectively. Transport distance of flue gas according to pressure was calculated using Eq. (4) [42].

L ¼

  2 eP 2 10K 2 D5 Pin out Qf2 Sg 2

(4)

where L is pipeline length (km), K is flow rate coefficient, D is pipeline inner diameter (cm), Qf is flow rate (m3 h
1), S is specific gravity, Pin is inlet pressure (MPa) and Pout is outlet pressure (MPa), and 1 h (h) was 3.6 ks. The coefficients were set as follows: K ¼ 52.31 [42] and S ¼ 1.02, assuming that N2 and CO2 contents in flue gas are 900 and 100 dm3 m
3 [40], respectively. If Pout is 0, then L is calculated as 0.6 and 1.9 km in scenarios 1 and 2, respectively. When the PP-toWWTP distance was above these pipeline lengths, more energy costs were required in the compressors. In addition, other energy costs, including FC cooling, were neglected in the calculation. Therefore, MSC utilisation is energetically superior to FC utilisation when the PP-to-WWTP distance is above 0.6 and 1.9 km in scenarios 1 and 2, respectively. 4. Conclusions In this study, it was demonstrated that utilisation of MSC derived from biogas has no negative effects on biomass and energy productivity by indigenous microalgae, and MSC is a useful carbon source for cultivation. The energy balance analysis suggested that a energetically feasible microalgae cultivation system with MSC in WWTPs was developed. Depending on the PP-to-WWTP distance, MSC utilisation is recommended over FC utilisation. Acknowledgements This study was supported by the Gesuido Academic Incubation to Advanced Project/Ministry of Land, Infrastructure, Transport and Tourism. The authors are grateful to the staff at the wastewater treatment plant for providing samples. References [1] D.L. Sutherland, C. HowardeWilliams, M.H. Turnbull, P.A. Broady, R.J. Craggs, The effects of CO2 addition along a pH gradient on wastewater microalgal photo-physiology, biomass production and nutrient removal, Water Res. 70

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