Advertisement

Waste and Biomass Valorization

, Volume 7, Issue 6, pp 1397–1408 | Cite as

Biogas Production from Waste Microalgal Biomass Obtained from Nutrient Removal of Domestic Wastewater

  • Ozgul Calicioglu
  • Goksel N. DemirerEmail author
Original Paper

Abstract

In this study, a semi-continuous photobioreactor was operated for the investigation of nutrient removal efficiency of a unialgal culture, Chlorella vulgarıs. Maximum nitrogen and phosphorous removal efficiencies of 99.6 and 91.2 % were achieved in the photobioreactor. The microalgal slurry obtained from the effluent of the photobioreactor was subjected to biochemical methane potential assay, after application of heat, autoclave, and thermochemical pretreatments to improve anaerobic digestibility and biogas production. Evaluation of pretreatment options indicated that heat pretreatment is the most efficient method in terms of enhancing anaerobic digestibility, at the chemical oxygen demand (COD) loading of 19 ± 0.5 g L−1. This method increased the methane yield by 83.0 %, from 223 to 408 mL CH4 g VS added −1 , compared to untreated microalgal slurry reactor with the same COD value. Among reactors with 35 ± 1.5 g L−1 initial COD concentration, autoclave-pretreated microalgal slurry was found to yield the highest methane value of 356 mL CH4 g VS added −1 , which was 43.0 % higher than the value observed in the reactor fed with untreated microalgal slurry. The thermochemical pretreatment caused production of inhibitory compounds and resulted in lower biomethane production and COD treatment values, compared to untreated microalgae. Outcomes of this study reveal that coupled micro-algal and anaerobic biotechnology could be a sustainable alternative for integrated nutrient removal and biofuel production applications.

Keywords

Chlorella vulgaris Photobioreactor Anaerobic digestion Biogas Pretreatment 

Abbreviations

BM

Basal medium

BMP

Biochemical methane potential

CH4

Methane

COD

Chemical oxygen demand

DO

Dissolved oxygen

GC

Gas chromatograph

HPLC

High performance liquid chromatography

N

Nitrogen

NaOH

Sodium hydroxide

NH3

Free ammonia

NH4+

Ammonium ion

NH4+-N

Ammonium-nitrogen

NO3

Nitrate ion

NO3-N

Nitrate-nitrogen

OD

Optical density

P

Phosphorus

PAR

Photosynthetically active radiation (nm)

PO43−

Phosphate ion

PO43−-P

Orthophosphate-phosphorus

sCOD

Soluble chemical oxygen demand

SCP

Semi-continuous cultivation photobioreactor

S/X

Substrate-to-inoculum ratio

TAN

Total ammonifiable nitrogen

tCOD

Total chemical oxygen demand

TKN

Total Kjeldahl nitrogen

TN

Total nitrogen

TP

Total phosphorus

TS

Total solids

TSS

Total suspended solids

VFA

Volatile fatty acids

VDS

Volatile dissolved solids

VS

Volatile solids

VSS

Volatile suspended solids

vvm

Volume of gas per volume of broth per minute

Introduction

The limited nature and environmental disadvantages of fossil fuels have led to an increase in the efforts on finding renewable and sustainable resources to fulfill world’s growing energy demand [1, 2]. In this respect, the economic viability, social acceptance, and environmental sustainability of feedstock are of particular interest for the advances in biofuel research. Microalgae, therefore, could be a feasible alternative to partially meet future energy needs, as they could be grown in marginal-quality land and water with high biomass productivity. In addition, microalgae cultivation could be a tool for CO2 sequestration, which would yield a carbon–neutral fuel [3]. It has been reported that the life cycle impact of microalgae-derived biofuels is dominated by the cultivation phase, and the environmental and economic feasibility of the system could be improved by coupling biofuel production with wastewater treatment [3, 4, 5]. In fact, microalgae have been proven to be effective in the removal of nitrogen, phosphorus and metals from various waste streams [6, 7], including municipal [8, 9], agricultural [10, 11], and industrial [12, 13] wastewaters. Significant quantities of nutrient removal and biomass production reported in these studies demonstrated the feasibility of coupled microalgae cultivation and wastewater treatment processes [14]. In particular, Chlorella species (including Chlorella vulgaris) have been successfully used for nitrogen, phosphorus and chemical oxygen demand removal from wastewaters with a wide range of operating conditions [15].

Due to their high lipid content, microalgae have been mostly considered as a biodiesel feedstock. However, harvesting and processing costs currently limit large-scale applications of this process [16, 17]. Biomethane production can therefore be an energetically more favorable bioconversion pathway of microalgal biomass [17, 18, 19].

However, anaerobic digestion (AD) of raw microalgal biomass also face some limitations due to the rigid cell wall structure of microalgae, which is resistant for microbial degradation [20, 21]. For instance, the cell wall composed of polysaccharide and glycoprotein matrix [22] prevents Chlorella sp. cells from complete degradation in AD process [23]. In order to improve the biomethane yield obtained from AD of microalgae, prior disintegration of the cell wall structure is necessary. As such, disruption of the Chlorella sp. cell wall has been proposed as an important step for efficient AD of the biomass [24]. Numerous studies have focused on methods to enhance anaerobic biodegradability of microalgae [21, 25, 26], by means of physical, chemical and biological pretreatment [20, 26, 27]. Microalgal cultures were subjected to thermal pretreatment from 55 to 170 °C in various studies for enhanced biomethane production [28, 29, 30]. Thermal pretreatment at relatively low temperatures have been reported to be less energy intensive compared to other thermal pretreatment methods such as steam explosion, which are performed at relatively higher temperatures where the water is pressurized [23]. Another energy efficient thermal pretreatment method could be autoclave pretreatment for a relatively short duration. Alternatively, thermal pretreatment at alkaline pH levels, which have been proven as a viable alternative for enhancing biomethane production from other biomass sources such as waste activated sludge [31], could be applied to microalgal biomass.

The objective of this study was to investigate nutrient removal potential of green algae, C. vulgaris, from primary effluent of municipal wastewater treatment plants and anaerobic digestibility of the produced microalgal slurry to produce biogas. The effects of heat, autoclave, and thermochemical pretreatment methods on the improvement of anaerobic digestibility and biogas production were also evaluated.

Materials and Methods

Cultures

Unialgal culture of C. vulgaris (CCAP 211/11B) was obtained from Culture Collection of Algae and Protozoa, UK. Prior to semi-continuous nutrient removal tests, culture was enriched in Enhanced Bold’s Basal medium (3N-BBM+V) in batch mode (Fig. 1a), as reported elsewhere [32].
Fig. 1

Schematics of the experimental setup: a batch cultivation photobioreactor; b semi-continuous cultivation photobioreactor; c microalgae harvesting and pretreatment; d biochemical methane potential assays

Mixed anaerobic cultures were obtained from the anaerobic sludge digesters of Greater Municipality of Ankara Tatlar Domestic Wastewater Treatment Plant. Characterization of the seed culture is given in Table 1.
Table 1

TS, VS, VS as percent TS, and tCOD values of the anaerobic seed

Parameter

TS (g L−1)

VS (g L−1)

VS (%TS)

tCOD (g L−1)

Anaerobic seed

38.9 ± 0.566

13.3 ± 0.000

32.6

19.8 ± 0.012

Analytical Methods

TS, VS, TSS, VSS, tCOD, sCOD, and TKN values were determined according to Standard Methods [33]. TAN, NO3 -N, PO4 3−-P analyses were conducted using Lovibond test kit vials—Vario 535560 HR Ammonia, Vario 535600 LR Ammonia, VarioNitraX 535580, Vario 515810 LR Ortho-phosphate, respectively (Aqualytic, Germany).

Optical density was measured using macro-cuvettes and spectrophotometer (HACH, DR 2800) at 685 nm wavelength with 1 cm light path. Light intensity was measured using PAR device (Li-Cor, 250 A) and DO values were measured using Dissolved Oxygen Meter (Extech, 407510A).

During biochemical methane potential (BMP) assays, gas production of each reactor was measured by water displacement method [16]. Gas composition was quantified using a GC (Agilent 6890 N) equipped with a thermal conductivity detector and capillary column CP-Sil 8 (CP8752, Varian) to detect CH4 content. The temperatures of the oven, injector, and detector were 45, 100 and 250 °C, respectively. Helium was employed as a carrier gas at pressure of 4.11 psi.

VFA compositions including lactic, formic, acetic, propionic, iso-butyric, butyric and iso-valeric acids and their concentrations were determined using a HPLC device (Shimadzu, 20A, Kyoto, Japan) equipped with refractive index detector. A sample volume of 10 µL was used for each measurement. Oven temperature was adjusted to 66 °C and 0.085 M HPLC grade sulfuric acid solution with a flow rate of 0.4 mL min−1 was used as mobile phase. Total VFA concentrations were expressed in terms of total acetic acid (HAc) equivalences, which are calculated by division of each acid concentration by its molecular weight and multiplication of the result with the molecular weight of acetic acid.

Experimental Sets and Procedures

Semi-continuous Cultivation Photobioreactor (SCP)

A SCP was designed and used to determine nutrient removal potential of C. Vulgaris from domestic wastewater, as well as to obtain microalgal biomass to be used in the BMP test for biogas production (Fig. 1b). Details of the reactor design and operation were reported elsewhere [32].

Effluent of the SCP was collected and centrifuged at 4000 rpm for 30 min (Fig. 1c). Characterization of centrifuged microalgal slurry is given in Table 2.
Table 2

Characterization of untreated and pretreated microalgal slurries

Parameter

Untreated microalgae

Pretreated microalgae

Heat

Autoclave

Thermochemical

TS (g L−1)

33.3 ± 0.070

34.2 ± 0.283

30.3 ± 0.300

33.7 ± 0.990

VS (g L−1)

28.0 ± 0.070

28.4 ± 0.495

27.1 ± 0.236

27.3 ± 0.707

VS as %TS

84.1

83.0

89.4

81.0

TSS (g L−1)

32.2 ± 0.702

32.9 ± 0.318

28.0 ± 0.416

32.9 ± 0.076

VSS (g L−1)

26.8 ± 0.685

27.3 ± 0.177

23.2 ± 0.200

26.4 ± 0.150

VSS as %TSS

83.2

83.0

82.3

79.3

tCOD (g L−1)

42.9 ± 0.285

41.8 ± 0.289

40.7 ± 0.425

42.7 ± 0.236

TKN (g L−1)

3.33 ± 0.079

3.36 ± 0.040

2.97 ± 0.099

2.97 ± 0.079

TP (g L−1)

0.377 ± 0.014

0.348 ± 0.001

0.347 ± 0.000

0.400 ± 0.004

sCOD (g L−1)

0.209 ± 0.001

0.615 ± 0.021

6.63 ± 0.141

2.30 ± 0.127

TAN (g L−1)

0.285 ± 0.007

0.355 ± 0.021

1.02 ± 0.057

0.480 ± 0.028

PO4 3−-P (g L−1)

0.117 ± 0.005

0.173 ± 0.004

0.279 ± 0.005

0.272 ± 0.007

COD/TKN

12.9

12.4

13.8

14.4

Pretreatment

Microalgal slurry was subjected to three different pretreatment methods in 1 L autoclavable media bottles, each with 200 mL effective volume. Heat and autoclave pretreatment was performed at neutral pH. For thermochemical pretreatment, the pH of the slurry was adjusted to alkaline (12.0) by addition of 0.6 mL of 6 N NaOH. The bottles subjected to heat and thermochemical pretreatments were incubated at 121 °C for 120 min. Autoclave pretreatment was performed for five min at 121 °C and 5 psi. After the bottles were cooled down to room temperature, their pH were re-adjusted to neutral and the samples were refrigerated at 0 °C, prior to characterization and BMP setup. Characterization of microalgal slurries with pretreatment is given in Table 2.

Efficiencies of different pretreatment methods were evaluated by comparison of COD solubilization values. Solubilization of COD was calculated using Eq. (1) [34].
$$ \% \, Solubilization = \frac{{sCOD_{pretreated} - sCOD_{untreated} }}{{tCOD_{unterated} }} \times 100 $$
(1)
Solubilization effects of pretreatment methods were also expressed as volatile dissolved solids to volatile solids (VDS/VS) percent increase calculated by Eq. (2) [9]. Increase in sCOD was also calculated similarly (Eq. 3). The results of the calculations are provided on Table 3.
$$ VDS\;increase\,(\% ) = \frac{{\left( {\frac{VDS}{VS}} \right)_{pretreated} - \left( {\frac{VDS}{VS}} \right)_{untreated } }}{{\left( {\frac{VDS}{VS}} \right)_{untreated} }} \times 100 $$
(2)
$$ sCOD\;increase\,(\% ) = \frac{{\left( {\frac{sCOD}{tCOD}} \right)_{pretreated} - \left( {\frac{sCOD}{tCOD}} \right)_{untreated } }}{{\left( {\frac{sCOD}{tCOD}} \right)_{untreated} }} \times 100 $$
(3)
Table 3

VDS/VS ratio, percent VDS/VS increase, sCOD/tCOD ratio, percent sCOD/COD increase, and percent COD solubilization values of pretreatment types

Microalgal Slurry type

VDS/VS ratio

VDS/VS increase (%)

sCOD/tCOD ratio

sCOD/COD increase (%)

COD solubilization (%)

Untreated

0.007

0.005

Heat pretreated

0.038

420

0.015

203

1.0

Autoclave pretreated

0.144

1870

0.163

3250

15

Thermochemically pretreated

0.027

276

0.054

1010

5.0

BMP Assays

Batch reactors with 100 mL total and 71 mL effective volumes were performed for 66 days to determine the anaerobic degradability and biogas production potential of microalgal slurry harvested from photobioreactor effluents. If necessary, distilled water was used to complete reactor volume up to effective volume. Each reactor was seeded with 16 mL anaerobic sludge.

Six pairs of reactors were operated for BMP assay of pretreated algae. Two COD values, approximately 19.0 and 34.0 g L−1 were initially maintained in each reactor pair. COD/TKN values of reactors without BM which varied between 13.0 and 15.2, were lower than the optimum range of 25–35 [35] due to the composition of microalgae. Inoculum ratio (S/X) on VS basis ranged between 3.12 and 6.53.

The necessity of micro- and macro-nutrients supplementation for an optimum anaerobic microbial growth was also tested through BMP assays. For this purpose, a BM with following constituents was prepared (concentrations of the constituents are given in parentheses as mg L−1): MgSO4·7H2O (400), KCl (400), Na2S·9H2O (300), CaCl2·2H2O (50), FeCl2·4H2O (40), CoCl2·6H2O (10), KI (10), MnCl2·4H2O (0.5), CuCl2·2H2O (0.5), ZnCl2 (0.5), AlCl3·6H2O (0.5), NaMoO4·2H2O (0.5), H3BO3 (0.5), NiCl2·6H2O (0.5), NaWO4·2H2O (0.5), Cysteine (10), NaHCO3 (6000) [36]. Test and control reactors with identical COD values were performed with and without BM supplementation.

After addition of all the constituents, pH values of the reactors were set as 7.1 ± 0.2. Reactors were purged with nitrogen gas for 4 min and were capped with rubber septa and placed in a constant temperature room at 35 ± 1.0 °C for incubation with constant mixing at 125 rpm (Fig. 1d). Daily gas production was measured and gas compositions were analyzed. At the end of incubation period, final VFA compositions were quantified. Details of the reactors used in BMP assay are given in Table 4.
Table 4

Components of BMP assay reactors with pretreated microalgal slurry

Code

Algae

BM

S/X (g g−1)

C1

0.00

C2

+

0.00

A1a

Untreated

6.21

A1b

+

6.21

A2a

3.10

A2b

+

3.10

H1a

Heat 121 °C, pH = 7.0, 120 min

6.53

H1b

+

6.53

H2a

3.26

H2b

+

3.26

At1a

Autoclave 121 °C, pH = 7.0, 5 min

6.24

At1b

+

6.24

At2a

3.12

At2b

+

3.12

TC1a

Thermochemical 121 °C, pH = 12.0, 120 min

6.29

TC1b

+

6.29

TC2a

3.14

TC2b

+

3.14

Results and Discussion

Semi-continuous Cultivation Photobioreactor Operation

Prior to transfer into SCP, the maximum net specific growth rate during batch cultivation in 3NBBM+V for 56 days was observed as 0.39 day−1 through exponential growth phase (data not shown). This value is comparable to those reported by de Morais and Costa [37] where specific growth rate of C. vulgaris in basal medium fed with air varied between 0.12 and 0.25 day−1 for different reactor geometries. In the same study, maximum specific growth rate achieved was reported as 0.31 day−1 when carbon dioxide enriched air was supplied to the photobioreactor.

After enrichment up to an optical density of 3.92, the culture broth was transferred to SCP. TAN was effectively removed from wastewater, either by microbial uptake, or nitrification [38]. NO3 -N is consumed as secondary nitrogen source by microalgae, after TAN is preferentially utilized first [39]. Since the portion of the TAN converted into NO3 -N in the SCP could not be considered as treated, the nitrogen removal efficiencies were calculated relative to the difference between sum of TAN and NO3 -N concentrations in the influent wastewater and effluent of each feeding cycle. The maximum N removal efficiency of 99.6 % and PO4 3−-P removal efficiency of 91.2 % were achieved during SCP operation (data not shown) [32]. These observed values were consistent with the literature. For example, ammonium and phosphorus removal efficiencies of high rate algal ponds were reported as 89 and 49 %, respectively, by Green et al. [40]. Moreover, Li et al. [41] conducted batch, modified semi-continuous and continuous cultivation experiments of C. vulgaris with municipal effluent in which ammonia-N, total nitrogen and total phosphorus could effectively be removed by 98.0, 90.3–93.6 and 89.9–91.8 %, respectively.

Effect of Pretreatment on Substrate Characteristics

The results obtained clearly showed that all pretreatments increased the soluble COD concentrations of the microalgal slurry. The COD solubilization efficiencies of heat, autoclave and thermochemical pretreatment were 1, 15, and 5 %, respectively (Table 3). These results are in agreement with those reported by Alzate et al. [10] as 9 and 16 % after thermal hydrolysis of microalgae at 110 °C, 1.0 bar and 140 °C, 1.2 bar. However, in the same study, higher solubilization percentages of COD were reported for another type of microalgae as 38 and 39 %, which is higher than those reported in the present study. This difference is clearly due to the structural resistance of C. vulgaris against thermal pretreatment. Another reason for the low solubilization increase observed in the present study is due to the high solids loading during pretreatment. For example, Wang et al. [42] also subjected microalgae to thermochemical pretreatment by autoclaving and achieved up to 72 % solubilization; however, the initial concentration of algae was approximately one-fifth of the present study. In addition, the initial sCOD reported by Wang et al. [42] was also higher (31.9 %).

The final sCOD values were 615 ± 21, 6630 ± 141, and 2300 ± 127 mg L−1, respectively, for heat, autoclave, and thermochemical pretreatment. When compared to 2.3-fold sCOD increase reported by González-Fernández et al. [28] for Scenedesmuus sp. after thermal pretreatment at 80 °C, 203 % increase in sCOD/COD value of heat pretreated microalgal slurry is reasonable in this present study. However, as also pointed out by González-Fernández et al. [28] this value is lower than those reported by Appels et al. [43] as 2.9- and 25.6-fold increase in soluble organic matter in sludge, after thermal pretreatment at 70 and 90 °C for an hour. In the present study, the 3250 and 1006 % increase in sCOD/COD values achieved after autoclave and thermochemical pretreatment; however, are in agreement with the findings of Appels et al. [43]. The values obtained in this study are also comparable with the literature in terms of solubilization efficiencies of waste activated sludge which varied between 10.8 and 51.8 % after various pretreatment methods, such as heat and alkali [44].

Interestingly, VDS/VS increase, sCOD/COD increase, and COD solubilization percentages were not proportional (Table 3). The difference between these two ratios show that the end products in each pretreatment method were in different oxidation/reduction states. That is, autoclave and thermochemical pretreatments produced compounds mostly in reduced state and yielded higher COD. The highest VDS/VS increase value was observed in autoclave pretreated microalgal slurry as 1868 %. This value was followed by heat-pretreated slurry as 420 %. The lowest VDS/VS value was observed in thermochemically pretreated microalgal slurry as 276 %. However, all results can be compared to the outcomes of the study conducted by Passos et al. [9], revealing that microwave pretreatment of microalgae collected from wastewater treatment pond achieved 280–800 % VDS/VS increase. Thermochemical pretreatment caused lower VDS/VS increase than microwave pretreatment, whereas heat pretreatment is comparable to, and autoclave pretreatment is higher than the range given by Passos et al. [9].

Autoclave pretreatment appears to be the most effective method in terms of damaging the cell wall structure of microalgae, because (1) the increase PO 4 3 -P concentrations show that non-structural components of the cells could be solubilized, as phosphorus is found in nucleic acids, lipids, proteins, and the intermediates of carbohydrate metabolism of algae [14]; (2) the highest TAN concentration indicates higher extent of protein degradation [45] compared to those observed in other pretreatment methods. In turn, the highest VDS/VS increase, sCOD/COD increase, and COD Solubilization (%) values were achieved after this method. Compared to the heat and thermochemical pretreatment conditions provided at the same temperature value (121 °C), the contribution of pressure was significant in cell wall degradation. However, the duration of heat pretreatment is also a key factor in solubilization and could be improved by increasing the time of the exposure to heat. For example, Passos et al. [46] achieved 20-fold increase in soluble VS after thermal pretreatment at 95 °C, 10 h; although the increase by hydrothermal pretreatment (130 °C, 15 min) was only 9-folds. Thermochemical pretreatment, on the other hand, yielded the lowest sCOD/COD and VDS/VS increases. As stated by Hendriks and Zeeman [47], this inhibition in solubilization process could be observed due to the change of the cellulose structure to a form that is denser and thermodynamically more stable than the native cellulose [48].

In addition, each pretreatment causes formation and/or loss of different compounds, which could be carbonaceous derivatives of carbohydrates [47], or nitrogenous compounds such as ammonia and organic nitrogen [42]. Therefore, the COD/TKN values also varied among slurries subjected to different pretreatment methods. The substantial increase in COD/TKN value after autoclave and thermochemical pretreatments show loss of nitrogenous compounds, which might be due to successful degradation of proteins and consecutive ammonia loss. The effect of this process is observed higher on thermochemically pretreated slurry, as a result of the alkaline pH value, favoring ammonia stripping [49]. However, autoclave pretreatment resulted in carbonaceous compound loss as well, which can be inferred from the overall loss in solids content. Along with the decrease in COD/TKN ratio of heat pretreated microalgal slurry (Table 2), the carbon loss from the system might be due to hydrolysis of hemicellulose as a result of thermal processes. This causes a formation of acids that catalyze further break-up and potential loss of hemicellulose derived organics [47].

Biomethane Production and Anaerobic Treatment Efficiencies

All reactors produced biomethane (Figs. 2, 3); the amounts varied based on initial COD concentrations and presence of BM. In all reactors, biomethane production rates increased after day 10 and slowed down by day 25. Total volume of the biomethane produced was higher in reactors with higher initial COD (34 ± 1.5 g L−1). However, the methane yields of reactors with low COD loading (19 ± 0.5 g L−1) were very close to or higher than those observed in the reactors with 34 ± 1.5 g L−1 initial COD (Table 5). Autoclave and heat pretreated samples gave higher biomethane yields for both COD values, compared to untreated microalgal slurry. The methane contents of the reactors quantified at day 7, 14, 21, 28, 35 and 56 (data not shown) were used to calculate the average methane contents and methane yields of the reactors (Table 5). The maximum methane yield of 408 mL CH4 g VS added −1 was observed in Reactor H2b. This value is slightly higher than reported by Mussgnug et al. [7] as 387 mL CH4 g VS added −1 .
Fig. 2

Cumulative biomethane production data of pretreated microalgae reactors with 34 ± 1.5 g L−1 COD: a untreated algae and heat-pretreated algae; b untreated algae and autoclave-pretreated algae; c untreated algae and thermochemically-pretreated algae

Fig. 3

Cumulative biomethane production data of pretreated microalgae reactors with 19 ± 0.5 g L−1 COD: a untreated algae and heat-pretreated algae; b untreated algae and autoclave-pretreated algae; c untreated algae and thermochemically-pretreated algae

Table 5

Biogas and methane yields of reactors fed with pretreated microalgal slurry

 

Methane yield (mL CH4 g VS added −1 )

Average CH4 content (%)

A1a

249

65.7

A1b

180

63.6

A2a

223

57.2

A2b

198

63.9

H1a

291

68.0

H1b

290

70.0

H2a

393

67.8

H2b

408

71.8

At1a

332

69.7

At1b

356

74.3

At2a

398

66.9

At2b

348

68.9

TC1a

258

73.7

TC1b

208

68.6

TC2a

195

61.9

TC2b

230

67.3

Changes in the methane yields after pretreatment relative to untreated microalgal slurry differed among pretreatment types. The percent increase relative to A1a and A2a for pretreated microalgal slurry reactors is reported in Fig. 4. For reactors with 34 ± 1.5 g L−1 initial COD, it can be seen that the highest percent increase was observed in Reactor At1b, as 43.0 %, which is followed by Reactor At1a, with 33.3 %. It can be stated that autoclave pretreatment was the most effective method for relatively higher COD values, in terms of methane yield. This pretreatment type was followed by heat pretreatment, with 16.9 and 16.4 % increase in methane yields in reactors H1a and H1b, respectively. The negative effect of thermochemical pretreatment on methane yield was observed in Reactor Tc1b as 16.5 % relative decrease in the methane yield, whereas the slightly positive effect of only 3.6 % was achieved in Tc1a. Chen and Oswald [11] also reported a similar negative effect of high NaOH concentrations on biomethane production during thermochemical pretreatment. The reason for low biomethane productivity could be formation of inhibitory compounds during thermo-alkaline pretreatment conditions [24]. As shown in Fig. 4, for 19 ± 0.5 mg L−1 initial COD, the highest percent increase was observed in Reactor H2b, as 83.0 %, which was followed by Reactor At2a, with 78.5 %, H2a with 76.23 % and At2b with 56.1 %. It can be stated that heat pretreatment was the most effective method for relatively lower COD values, in terms of methane yield. This pretreatment type was followed by autoclave pretreatment. The lower performance observed in autoclave-pretreated microalgal slurry reactors under lower COD loading could be the due to the loss of organic material during pretreatment [50], occurrence of which is also supported by COD/TKN and solids data. The negative effect of thermochemical pretreatment on methane yield was also evident in Reactor Tc1a and Tc2a. The differences in the performances of different pretreatment methods on biomethane yields reveal that COD solubilization during pretreatment is not a direct indication of better anaerobic digestion performance [10]. Similarly, Samson and Leduy [51] reported a decrease in biomethane yield from 190 to 170 mL CH4 g VS−1 after ultrasound and mechanical pretreatment of Spirodela maxima biomass. In contrast, thermal pretreatment of Scenedesmus biomass at 70 and 90 °C produced similar organic material and ammonia release; however, the latter yielded higher biomethane concentrations, as the damage to the cell wall was higher.
Fig. 4

Percent biomethane yield increases in batch reactors fed with pretreated microalgal slurry: a percent biomethane yield increase in reactors with 34 ± 1.5 g L−1 initial COD relative to A1a; b percent yield increase in reactors with 19 ± 0.5 g L−1 initial COD relative to A2a

Overall performances of pretreated microalgal slurry in this study are comparable to the literature. Chen and Oswald [11] reported that heat pretreatment improves methane yield from anaerobic digestion of microalgae by 33 %. In another study conducted by Alzate et al. [10], thermal hydrolysis was found to be effective in biomethane production enhancement, increasing yield by 46–62 %.

Although the ammonia concentrations at the end of the anaerobic digestion process in pretreated microalgal slurry reactors were higher than observed in untreated microalgal slurry reactors, the biomethane yields were also higher (Table 5). Thus, it can be concluded that difficulty of cell disintegration negatively affected the anaerobic digestion of untreated microalgae more than elevated ammonia concentrations in the reactors. This was an expected result, since hydrolysis of the microalgal cell wall has been known to be problematic [9]. Similar results were reported by Schwede et al. [30] for anaerobic digestion of Nannochloropsis salina biomass, as cell disintegration of raw microalgal biomass was the limiting factor in anaerobic biodegradability.

Interestingly, the BM addition had negative or insignificant effect on cumulative gas production, regardless of initial COD concentrations and pretreatment method. This result can be related to the increases in buffer capacity and pH (Table 6). High pH conditions may have resulted in a “inhibited steady state”, during which the ammonia concentration rises up to levels high enough to cause process instability and temporary VFA accumulation [52].
Table 6

Initial and final pH, sCOD, TAN, PO4 3−-P and final VFA values in BMP reactors

 

pH

sCOD (mg L−1)

TAN (mg L−1)

PO4 3−-P (mg L−1)

Final VFA (mg HAc L−1)

Initial

Final

Initial

Final

Removal (%)

Initial

Final

Removal (%)

Initial

Final

Removal (%)

A1a

7.21

7.49

200

970 ± 14

−386

230

515 ± 49

−124

96.2

103 ± 5.0

−6.60

624

A1b

7.13

7.67

200

245 ± 7

−22.7

230

435 ± 21

−89.5

96.2

83.5 ± 0.7

13.2

91.9

A2a

6.95

7.22

127

105 ± 7

17.6

131

335 ± 9

−155

55.6

81.0 ± 0.0

−45.6

402

A2b

7.31

7.67

127

11.5 ± 1

91.0

131

260 ± 28

−98.1

55.6

46.5 ± 2.1

16.4

79.5

H1a

7.22

7.66

480

650 ± 42

−35.5

278

1540 ± 57

−177

135

117 ± 1.4

13.2

192

H1b

7.34

7.92

480

425 ± 35

11.4

278

890 ± 12

−220

135

45.5 ± 0.7

66.3

110

H2a

6.92

7.43

267

410 ± 14

−53.4

155

720 ± 14

−363

75.0

80.5 ± 0.7

−7.40

136

H2b

7.11

7.83

267

190 ± 14

28.9

155

455 ± 21

−193

75.0

54.0 ± 2.8

28.0

81.4

At1a

7.30

7.88

4630

1060 ± 7

77.2

737

1430 ± 41

−93.4

208

113 ± 0.0

45.7

109

At1b

7.33

8.06

4630

450 ± 57

90.3

737

1730 ± 17

−134

208

58.0 ± 1.4

72.1

57.5

At2a

7.31

7.5

2340

170 ± 14

92.7

385

815 ± 49

−112

112

89.5 ± 2.1

19.8

44.4

At2b

7.29

7.81

2340

125 ± 7

94.7

385

725 ± 21

−88.4

112

55.5 ± 3.5

50.2

19.6

Tc1a

6.9

7.44

1640

385 ± 21

76.6

364

680 ± 14

−86.7

203

122 ± 4.2

39.8

92.9

Tc1b

7.10

7.89

1640

565 ± 35

65.6

364

570 ± 17

−56.5

203

48.5 ± 0.7

76.1

49.8

Tc2a

7.14

7.12

849

211 ± 13

75.1

199

390 ± 28

−96.4

109

90.5 ± 0.7

16.9

60.6

Tc2b

7.32

7.74

849

280 ± 14

67.0

199

305 ± 11

−53.6

109

55.5 ± 1.4

48.6

56.8

Observed tCOD, TKN, TP values and removal efficiencies of the reactors are given in Table 7. Total COD removal efficiencies observed in pretreated microalgal slurry reactors were between 28.7 and 60.5 %, having a broader range than that of reactors fed with raw microalgal slurry, reported as 32–48.7 %. It can be stated that thermochemical pretreatment lowered COD removal efficiency of microalgae, whereas heat and autoclave pretreatment improved the efficiency. The highest COD removal efficiencies were observed in autoclave-pretreated microalgal slurry reactors. In these reactors, COD removal efficiencies were higher than that of reported by Ras et al. [53] as 33–51 %. However, Jegede [54] achieved 75–85 % COD removal using microalgae and cyanobacteria as substrates after autoclave pretreatment of substrates prior to anaerobic digestion process.
Table 7

Initial and final tCOD, TKN, TP, COD:N and N:P values in BMP reactors

 

tCOD (g L−1)

TKN (g L−1)

TP (g L−1)

COD:N (g:g)

N:P (g:g)

 

Initial

Final

Removal (%)

Initial

Final

Removal (%)

Ammonification (%)

Initial

Final

Removal (%)

Initial

Final

Initial

Final

A1a

34.1

21.5 ± 0

37.1

2.54

2.45 ± 0.020

3.6

12.3

0.339

0.334 ± 0.012

1.50

13.4

8.80

7.5

7.3

A1b

34.1

24.9 ± 0.880

27.0

2.54

2.38 ± 0.119

6.3

8.90

0.339

0.332 ± 0.003

2.10

13.4

10.5

7.5

7.2

A2a

19.3

12.2 ± 0.072

36.7

1.39

1.32 ± 0.040

5.4

16.2

0.209

0.205 ± 0.004

1.90

13.9

9.30

6.7

6.4

A2b

19.3

13.3 ± 0.144

31.2

1.39

1.23 ± 0.010

12

10.2

0.209

0.200. ± 0.007

4.20

13.9

10.8

6.7

6.1

H1a

33.3

18.4 ± 0.283

44.6

2.56

2.30 ± 0.040

10

62.5

0.319

0.333 ± 0.003

2.00

13.0

8.00

8.0

7.3

H1b

33.3

18.9 ± 0.964

43.1

2.56

2.24 ± 0.119

13

31.2

0.319

0.330 ± 0.002

2.80

13.0

8.40

8.0

7.2

H2a

18.9

9.64 ± 0.000

48.9

1.40

1.25 ± 0.020

11

51.8

0.199

0.202 ± 0.009

3.40

13.5

7.70

7.0

6.5

H2b

18.9

10.3 ± 0.170

45.3

1.40

1.43 ± 0.000

−1.9

23.5

0.199

0.200 ± 0.009

4.20

13.5

7.20

7.0

7.5

At1a

32.5

13.0 ± 0.555

60.2

2.29

2.31 ± 0.059

−0.8

43.7

0.339

0.338 ± 0.005

0.30

14.2

5.60

8.5

6.8

At1b

32.5

12.9 ± 0.144

60.5

2.29

2.27 ± 0.040

1.0

64.5

0.339

0.336 ± 0.005

1.00

14.2

5.70

8.5

6.7

At2a

18.5

7.45 ± 0.116

59.7

1.27

1.25 ± 0.059

1.6

50.0

0.209

0.205 ± 0.011

1.70

14.6

6.00

7.3

6.1

At2b

18.5

7.84 ± 0.000

57.6

1.27

1.25 ± 0.030

1.1

39.2

0.209

0.203 ± 0.013

3.00

14.6

6.30

7.3

6.2

Tc1a

33.9

23.8 ± 0.144

29.9

2.29

2.45 ± 0.139

−7.0

15.1

0.355

0.329 ± 0.012

3.10

14.8

9.70

6.5

7.1

Tc1b

33.9

24.2 ± 0.143

28.7

2.29

2.49 ± 0.119

−8.8

9.70

0.355

0.333 ± 0.014

2.00

14.8

9.70

6.5

7.2

Tc2a

19.2

13.5 ± 0.420

29.9

1.27

1.28 ± 0.069

−1.2

17.7

0.217

0.200 ± 0.008

4.50

15.2

10.5

5.8

6.2

Tc2b

19.2

13.5 ± 0.212

29.6

1.27

1.27 ± 0.040

−0.6

9.90

0.217

0.202 ± 0.011

3.20

15.2

10.6

5.8

6.1

In the reactors, ammonification ranged between 9.7 and 64.5 % (Table 6). These values are also similar to those reported by Ras et al. [53] as 19–68 %. For TKN and NH4 +-N values, negative removal data can be attributed to bioconversion of proteins into amino acids and to ammonia [55]. Although final ammonia concentrations in pretreated microalgal slurry reactors were higher than those fed with raw microalgal slurry, especially heat and autoclave pretreatment positively affected methane yields, COD removal, and ammonification. It can be concluded, that the main reason for low efficiencies in untreated microalgal slurry reactors was not ammonia toxicity, but difficulty in solubilization. Nevertheless, ammonia inhibition may be the cause of low biogas yields observed in the reactors, especially with BM supplementation, which leads to higher concentrations of free ammonia due to higher pH values.

It can be seen from Table 7 that TP removal efficiencies in pretreated microalgal slurry reactors were between 0.3 and 4.5 %. This is a result similar to those observed in untreated microalgal slurry reactors and is consistent with the literature [56]. Although initial N:P values in some reactors were slightly higher than optimum range, N:P values at the end of the BMP assay were all within the optimum range.

Inhibitory conditions observed in anaerobic digestion process can be indicated by final VFA concentrations above 100–300 mg HAc L−1 [57]. It can be seen from Table 6 that there has not been significant inhibition in methanogenic activities, considering VFA concentrations less than or equal to 191.8 mg HAc L−1. It can also be seen that final sCOD concentrations were in correlation with final VFA concentrations.

Conclusions

In this study, biomethane yields obtained from wastewater-derived microalgae were successfully improved after pretreatment. The highest microalgae solubilization was achieved by autoclave pretreatment, with 15 % increase in sCOD and 32.5-fold increase in VDS/VS ratio. Anaerobic digestibility and methane yield obtained from microalgae could be increased by both autoclave and heat pretreatment, and the highest methane production was obtained by autoclave pretreatment in reactors with higher COD loading (34 g L−1) and by heat pretreatment in reactors with lower COD loading (19 g L−1). On the other hand, thermochemical pretreatment at alkaline conditions may have caused production of inhibitory compounds, resulting in lower biomethane yields compared to untreated microalgae.

Based on the results of this study, utilization of microalgal cultures could be an alternative method for integrated biological wastewater treatment and bioenergy production. However, the sustainability of the system must be further evaluated by techno-economic analysis, in order to determine the net impact of the coupled process on water–energy nexus.

References

  1. 1.
    REN21. 2015: Renewables 2015 Global Status Report. Paris (2015)Google Scholar
  2. 2.
    Jung, H., Baek, G., Kim, J., Seung, S., Lee, C.: Mild-temperature thermochemical pretreatment of green macroalgal biomass: effects on solubilization, methanation, and microbial community structure. Bioresour. Technol. 199, 326–335 (2016)CrossRefGoogle Scholar
  3. 3.
    Mahdy, A., Ballesteros, M., González-Fernández, C.: Enzymatic pretreatment of Chlorella vulgaris for biogas production: influence of urban wastewater as a sole nutrient source on macromolecular profile and biocatalyst efficiency. Bioresour. Technol. 199, 319–325 (2016)CrossRefGoogle Scholar
  4. 4.
    Clarens, A.F., Resurreccion, E.P., White, M.A., Colosi, L.M.: Environmental life cycle comparison of algae to other bioenergy feedstocks. Environ. Sci. Technol. 44, 1813–1819 (2010)CrossRefGoogle Scholar
  5. 5.
    Murphy, C.F., Allen, D.T.: Energy-water nexus for mass cultivation of algae. Environ. Sci. Technol. 45, 5861–5868 (2011)CrossRefGoogle Scholar
  6. 6.
    Mata, T.M., Martins, A.A., Caetano, N.S.: Microalgae for biodiesel production and other applications: a review. Renew. Sustain. Energy Rev. 14, 217–232 (2010)CrossRefGoogle Scholar
  7. 7.
    Mussgnug, J.H., Klassen, V., Schlüter, A., Kruse, O.: Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotechnol. 150, 51–56 (2010)CrossRefGoogle Scholar
  8. 8.
    Holm-Nielsen, J.B., Al Seadi, T., Oleskowicz-Popiel, P.: The future of anaerobic digestion and biogas utilization. Bioresour. Technol. 100, 5478–5484 (2009)CrossRefGoogle Scholar
  9. 9.
    Passos, F., Solé, M., García, J., Ferrer, I.: Biogas production from microalgae grown in wastewater: effect of microwave pretreatment. Appl. Energy 108, 168–175 (2013)CrossRefGoogle Scholar
  10. 10.
    Alzate, M.E., Muñoz, R., Rogalla, F., Fdz-Polanco, F., Pérez-Elvira, S.I.: Biochemical methane potential of microalgae: influence of substrate to inoculum ratio, biomass concentration and pretreatment. Bioresour. Technol. 123, 488–494 (2012)CrossRefGoogle Scholar
  11. 11.
    Chen, P.H., Oswald, W.J.: Thermochemıcal treatment for algal fermentation. Environ. Int. 24, 889–897 (1998)CrossRefGoogle Scholar
  12. 12.
    Keymer, P., Ruffell, I., Pratt, S., Lant, P.: High pressure thermal hydrolysis as pre-treatment to increase the methane yield during anaerobic digestion of microalgae. Bioresour. Technol. 131, 128–133 (2013)CrossRefGoogle Scholar
  13. 13.
    Demirer, G.N., Chen, S.: Anaerobic biogasification of undiluted dairy manure in leaching bed reactors. Waste Manag 28, 112–119 (2008)CrossRefGoogle Scholar
  14. 14.
    Cai, T., Park, S.Y., Li, Y.: Nutrient recovery from wastewater streams by microalgae: status and prospects. Renew. Sustain. Energy Rev. 19, 360–369 (2013)CrossRefGoogle Scholar
  15. 15.
    Wang, L., Min, M., Li, Y., Chen, P., Chen, Y., Liu, Y., Wang, Y., Ruan, R.: Cultivation of green algae Chlorella sp. in different wastewaters from municipal wastewater treatment plant. Appl. Biochem. Biotechnol. 162, 1174–1186 (2010)CrossRefGoogle Scholar
  16. 16.
    Alcántara, C., García-Encina, P.A., Muñoz, R.: Evaluation of mass and energy balances in the integrated microalgae growth-anaerobic digestion process. Chem. Eng. J. 221, 238–246 (2013)CrossRefGoogle Scholar
  17. 17.
    Passos, F., Ferrer, I.: Microalgae conversion to biogas: thermal pretreatment contribution on net energy production. Environ. Sci. Technol. 48, 7171–7178 (2014)CrossRefGoogle Scholar
  18. 18.
    Tan, C.H., Show, P.L., Chang, J.S., Ling, T.C., Lan, J.C.W.: Novel approaches of producing bioenergies from microalgae: a recent review. Biotechnol. Adv. 33, 1219–1227 (2014)CrossRefGoogle Scholar
  19. 19.
    Wiley, P.E., Campbell, J.E., McKuin, B.: Production of biodiesel and biogas from algae: a review of process train options. Water Environ. Res. 83, 326–338 (2011)CrossRefGoogle Scholar
  20. 20.
    Demuez, M., Mahdy, A., Tomás-Pejó, E., González-Fernández, C., Ballesteros, M.: Enzymatic cell disruption of microalgae biomass in biorefinery processes. Biotechnol. Bioeng. 112, 1955–1966 (2015)CrossRefGoogle Scholar
  21. 21.
    González-Fernández, C., Sialve, B., Bernet, N., Steyer, J.P.: Impact of microalgae characteristics on their conversion to biofuel. Part II: focus on biomethane production. Biofuels. Bioprod. Biorefining. 6, 246–256 (2012)CrossRefGoogle Scholar
  22. 22.
    Domozych, D.S.: Algal cell walls. In: eLS. Wiley, Chichester (2011). doi: 10.1002/9780470015902.a0000315.pub3
  23. 23.
    Passos, F., Uggetti, E., Carrère, H., Ferrer, I.: Pretreatment of microalgae to improve biogas production: a review. Bioresour. Technol. 172, 403–412 (2014)CrossRefGoogle Scholar
  24. 24.
    Rodriguez, C., Alaswad, A., Mooney, J., Prescott, T., Olabi, A.G.: Pre-treatment techniques used for anaerobic digestion of algae. Fuel Process. Technol. 138, 765–779 (2015)CrossRefGoogle Scholar
  25. 25.
    Mendez, L., Mahdy, A., Timmers, R.A., Ballesteros, M., González-Fernández, C.: Enhancing methane production of Chlorella vulgaris via thermochemical pretreatments. Bioresour. Technol. 149, 136–141 (2013)CrossRefGoogle Scholar
  26. 26.
    Ometto, F., Quiroga, G., Pšenička, P., Whitton, R., Jefferson, B., Villa, R.: Impacts of microalgae pre-treatments for improved anaerobic digestion: thermal treatment, thermal hydrolysis, ultrasound and enzymatic hydrolysis. Water Res. 65, 350–361 (2014)CrossRefGoogle Scholar
  27. 27.
    He, S., Fan, X., Katukuri, N.R., Yuan, X., Wang, F., Guo, R.-B.: Enhanced methane production from microalgal biomass by anaerobic bio-pretreatment. Bioresour. Technol. 204, 145–151 (2016)CrossRefGoogle Scholar
  28. 28.
    González-Fernández, C., Sialve, B., Bernet, N., Steyer, J.P.: Comparison of ultrasound and thermal pretreatment of Scenedesmus biomass on methane production. Bioresour. Technol. 110, 610–616 (2012)CrossRefGoogle Scholar
  29. 29.
    Passos, F., García, J., Ferrer, I.: Impact of low temperature pretreatment on the anaerobic digestion of microalgal biomass. Bioresour. Technol. 138, 79–86 (2013)CrossRefGoogle Scholar
  30. 30.
    Schwede, S., Rehman, Z.-U., Gerber, M., Theiss, C., Span, R.: Effects of thermal pretreatment on anaerobic digestion of Nannochloropsis salina biomass. Bioresour. Technol. 143, 505–511 (2013)CrossRefGoogle Scholar
  31. 31.
    Vlyssides, A.G., Karlis, P.K.: Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion. Bioresour. Technol. 91, 201–206 (2004)CrossRefGoogle Scholar
  32. 32.
    Calicioglu, O., Demirer, G.N.: Integrated nutrient removal and biogas production by Chlorella vulgaris cultures. J. Renew. Sustain. Energy 7, 1–14 (2015)CrossRefGoogle Scholar
  33. 33.
    APHA: Standard Methods for the Examination of Water and Wastewater. APHA, Washington (2005)Google Scholar
  34. 34.
    Davidsson, A., La Cour Jansen, J.: Pre-treatment of wastewater sludge before anaerobic digestion—hygienisation, ultrasonic treatment and enzyme dosing. Vatten 62, 335–340 (2006)Google Scholar
  35. 35.
    Yen, H.W., Brune, D.E.: Anaerobic co-digestion of algal sludge and waste paper to produce methane. Bioresour. Technol. 98, 130–134 (2007)CrossRefGoogle Scholar
  36. 36.
    Clark, P.B., Hillman, P.F.: Enhancement of anaerobic digestion using duckweed (Lemna minor) enriched with ıron. Water Environ. J. 10, 92–95 (1996)CrossRefGoogle Scholar
  37. 37.
    de Morais, M.G., Costa, J.A.V.: Carbon dioxide fixation by Chlorella kessleri, C. vulgaris, Scenedesmus obliquus and Spirulina sp. cultivated in flasks and vertical tubular photobioreactors. Biotechnol. Lett. 29, 1349–1352 (2007)CrossRefGoogle Scholar
  38. 38.
    de Godos, I., Vargas, V.A., Guzman, H.O., Soto, R., Garcia, B., Garcia, P.A., Muñoz, R.: Assessing carbon and nitrogen removal in a novel anoxic–aerobic cyanobacterial–bacterial photobioreactor configuration with enhanced biomass sedimentation. Water Res. 61, 77–85 (2014)CrossRefGoogle Scholar
  39. 39.
    Becker, E.W.: Microalgae: Biotechnology and Microbiology. Cambridge University Press, Cambridge (2008)Google Scholar
  40. 40.
    Green, F.B., Lundquist, T.J., Oswald, W.J.: Energetics of advanced ıntegrated wastewater pond systems. Water Sci. Technol. 31, 9–20 (1995)CrossRefGoogle Scholar
  41. 41.
    Li, C., Yang, H., Li, Y., Cheng, L., Zhang, M., Zhang, L., Wang, W.: Novel bioconversions of municipal effluent and CO2 into protein riched Chlorella vulgaris biomass. Bioresour. Technol. 132, 171–177 (2013)CrossRefGoogle Scholar
  42. 42.
    Wang, Y., Guo, W., Cheng, C.L., Ho, S.H., Chang, J.S., Ren, N.: Enhancing bio-butanol production from biomass of Chlorella vulgaris JSC-6 with sequential alkali pretreatment and acid hydrolysis. Bioresour. Technol. 200, 557–564 (2016)CrossRefGoogle Scholar
  43. 43.
    Appels, L., Degrève, J., Van der Bruggen, B., Van Impe, J., Dewil, R.: Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion. Bioresour. Technol. 101, 5743–5748 (2010)CrossRefGoogle Scholar
  44. 44.
    Kim, J., Park, C., Kim, T.-H., Lee, M., Kim, S., Kim, S.-W., Lee, J.: Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95, 271–275 (2003)CrossRefGoogle Scholar
  45. 45.
    Costanzo, W., Jena, U., Hilten, R., Das, K.C., Kastner, J.R.: Low temperature hydrothermal pretreatment of algae to reduce nitrogen heteroatoms and generate nutrient recycle streams. Algal Res. 12, 377–387 (2015)CrossRefGoogle Scholar
  46. 46.
    Passos, F., Carretero, J., Ferrer, I.: Comparing pretreatment methods for improving microalgae anaerobic digestion: thermal, hydrothermal, microwave and ultrasound. Chem. Eng. J. 279, 667–672 (2015)CrossRefGoogle Scholar
  47. 47.
    Hendriks, A.T.W.M., Zeeman, G.: Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour. Technol. 100, 10–18 (2009)CrossRefGoogle Scholar
  48. 48.
    Pettersen, R.: The chemical composition of wood. Chem. Solid Wood 207, 1–9 (1984)Google Scholar
  49. 49.
    Tchobanoglous, G., Burton, F.L., Stensel, H.D.: Wastewater Engineering: Treatment, Disposal and Reuse. McGraw-Hill Inc., New York (2003)Google Scholar
  50. 50.
    Ariunbaatar, J., Panico, A., Esposito, G., Pirozzi, F., Lens, P.N.L.: Pretreatment methods to enhance anaerobic digestion of organic solid waste. Appl. Energy 123, 143–156 (2014)CrossRefGoogle Scholar
  51. 51.
    Samson, R., Leduy, A.: Influence of mechanical and thermochemical pretreatments on anaerobic digestion of Spirulina maxima algal biomass. Biotechnol. Lett. 5, 671–676 (1983)CrossRefGoogle Scholar
  52. 52.
    Montingelli, M.E., Tedesco, S., Olabi, A.G.: Biogas production from algal biomass: a review. Renew. Sustain. Energy Rev. 43, 961–972 (2015)CrossRefGoogle Scholar
  53. 53.
    Ras, M., Lardon, L., Bruno, S., Bernet, N., Steyer, J.-P.: Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris. Bioresour. Technol. 102, 200–206 (2011)CrossRefGoogle Scholar
  54. 54.
    Jegede, A.O.: Anaerobic digestion of cyanobacteria and chlorella to produce methane for biofuel. Int. J. Agric. Biol. Eng. 5, 1–8 (2012)Google Scholar
  55. 55.
    Demirer, G.N., Chen, S.: Two-phase anaerobic digestion of unscreened dairy manure. Process Biochem. 40, 3542–3549 (2005)CrossRefGoogle Scholar
  56. 56.
    Lusk, P.: Methane Recovery from Animal Manures The Current Opportunities Casebook. National Renewable Energy Laboratory, NREL/SR, 580-25145, Washington (1998)Google Scholar
  57. 57.
    Speece, R.E.: Anaerobic Biotechnology and Odor/Corrosion Control for Municipalities and Industries. Archea Press, Nashville (2008)Google Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Department of Civil and Environmental EngineeringThe Pennsylvania State UniversityUniversity ParkUSA
  2. 2.Department of Environmental EngineeringMiddle East Technical UniversityAnkaraTurkey

Personalised recommendations