Bioresource Technology 171 (2014) 203–210
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment Jesper Olsson a,⇑, Xin Mei Feng b, Johnny Ascue b, Francesco G. Gentili c, M.A. Shabiimam d, Emma Nehrenheim a, Eva Thorin a a
The School of Business, Society and Engineering, Mälardalen University, Box 883, SE-721 23 Västerås, Sweden JTI – Swedish Institute of Agricultural and Environmental Engineering, Box 7033, SE-750 07 Uppsala, Sweden Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden d Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microalgae were co-digested with a
sewage sludge mixture in batch studies. Highest CH4-yield, of 408 ± 16 Ncm3 g VS1 was reached in 37 °C with 37% microalgae. In 55 °C the same increase in methane yield with microalgae added could not be seen. The microalgae seems to be easily degraded with a short lag phase in the batches.
a r t i c l e
i n f o
Article history: Received 8 June 2014 Received in revised form 14 August 2014 Accepted 16 August 2014 Available online 23 August 2014 Keywords: Biogas production Co-digestion Microalgae Sewage sludge
Microalgae unit
0 % microalgae, 100 % sludge mixture 12 % microalgae, 88 % sludge mixture
Primary sludge
Waste activated sludge
37 % microalgae, 63 % sludge mixture
Methane yield (CH4 g VS-1) from batch test
100 % microalgae, 0 % sludge mixture Anaerobic digestion - thermophilic conditions
a b s t r a c t In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic codigestion of microalgae and undigested sewage sludge did not give the same synergy. Ó 2014 Published by Elsevier Ltd.
Anaerobic digestion of sewage sludge from municipal wastewater treatment is a common stabilization method for biosolids. The combustible biogas produced in the process is considered to be a ⇑ Corresponding author. Tel.: +46 21 10 70 35; fax: +46 21 10 14 80.
http://dx.doi.org/10.1016/j.biortech.2014.08.069 0960-8524/Ó 2014 Published by Elsevier Ltd.
Anaerobic digestion - mesophilic conditions
25 % microalgae, 75 % sludge mixture
1. Introduction
E-mail address: [email protected] (J. Olsson).
Methane yield (CH4 g VS-1) from batch test
renewable energy source thereby making the expansion of biogas production systems an important contributor to the global conversion from fossil fuels to renewable energy systems (Tchobanoglous and Burton, 2002). The demand for biogas continues to grow and it is therefore important for municipal wastewater treatment plants (WWTP) to maximize biogas production. One way to do this is by using microalgae cultivated with wastewater as a co-substrate in the digestion process. Microalgae can be cultivated in a treatment
204
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
step in photo bioreactors or microalgae ponds in the final polishing wastewater treatment in the WWTP (Larsdotter, 2006) or as a treatment for nutrient rich side streams like reject water from sludge dewatering (Ficara et al., 2014; Rusten and Sahu, 2011). Simultaneous nutrient recovery, water treatment and biomass production are thus possible. Successful cultivation of microalgae in wastewater has been demonstrated in recent studies (Odlare et al., 2011; Su et al., 2011). Rusten and Sahu (2011), Ficara et al. (2014) and Ma et al. (2014) have evaluated the process of cultivation of microalgae from reject water, and have found that microalgae remove nitrogen and phosphorous from the reject water, consequently reducing the impact of returning side streams to the main stream of the treatment plant. Microalgae have also been used to successfully reduce nutrients in nutrient rich digested piggery wastewater (Wang et al., 2013a). According to Wang et al. (2013b), waste activated sludge (WAS) can be a suitable co-substrate with microalgae. Co-digesting a microalgae mix containing Chlorella sp. with varying amounts of waste activated sludge (WAS), 59–96% in mass of VS-content, the biogas yield is improved which is 73–79% larger than the gas yield from pure algae digestion sets. Krustok et al. (2013) have shown in an anaerobic batch experiment with municipal food waste and harvested microalgae cultivated in lake water that the biogas production is improved after addition of microalgae. The experiment has been carried out with fermentation bottles where 0%, 12%, 25% and 37% of the food waste is replaced with harvested microalgae. During the first 25 days of fermentation the replacement of 12% food waste with microalgae has given the highest biogas production rate. A possible explanation for the synergetic effect of co-digesting microalgae with other substrates is an optimization of the C/N-ratio. Since anaerobic digestion is inhibited when using microalgae alone as a substrate due to the high protein content in the algae biomass (Brune et al., 2009; Wiley et al., 2011) other carbon rich substrates can enhance the C/N-ratio. The use of co-digestion of substrates to stabilize the digestion process by improving the C/N-ratio has been suggested by several authors (Brune et al., 2009; Khalid et al., 2011; Mata-Alvarez et al., 2011). Yen and Brune (2007) have reported increased gas yields from co-digestion of microalgae with waste paper from recycling bins by adjusting the low C/N-ratio in microalgae. The results have shown that an optimum C/N-ratio for co-digestion of algal sludge and wastepaper is in the range of 20/1–25/1. The anaerobic co-digestion between corn silage and the marine microalga Nannochloropsis salina have also shown a better process stability and enhanced biogas yields by microalgae addition (Schwede et al., 2013). This positive influence in co-digestion according to Schwede et al. (2013) can be explained by an optimized C/N-ratio but also enhanced alkalinity and addition of important trace elements for the digestion. The enhanced alkalinity that microalgae bring to the co-digestion has also been mentioned by Formagini et al. (2014). In this study vinasse from ethanol production was co-digested with microalgae. The alkalinity of the algae suspension is higher but not enough to keep the pH –level stabilized to a neutral level together with the vinasse (Formagini et al., 2014). Although there have been recent development in the field of co-digestion of microalgae with other substrates, there is still a lack of knowledge regarding co-digestion of microalgae with a representative mix of sewage sludge for a municipal WWTP in both thermophilic- and mesophilic conditions. This would be important knowledge for further studies and development of systems for using microalgae as a co-substrate together with traditional wastewater treatment and anaerobic digestion in full scale applications. Earlier studies in the area of co-digestion between sewage sludge and microalgae have only focused on co-digesting the microalgae and the biological waste activated sludge (WAS). However, the
experiments have only been made in mesophilic conditions (Ficara et al., 2014; Wang et al., 2013b). This study was designed to investigate cultivation of indigenous microalgae from a nearby lake and in municipal wastewater and using it as a co-substrate in anaerobic digestion with undigested sewage sludge in various proportions. The methane potentials of the different mixtures were evaluated in anaerobic batch tests in mesophilic and thermophilic conditions. The aim of the study was to investigate whether co-digestion of microalgae and sewage sludge were more efficient for biogas production than digestion of the sewage sludge alone. 2. Methods 2.1. Substrates and inocula To get a broad approach in the study three different microalgae cultures were used as a co-substrate to the undigested sewage sludge. The inoculums used in the BMP-experiments were all taken from digesters at waste water treatment plants so that the micro culture were already adapted to the undigested sewage sludge. 2.1.1. Microalgae A – wet Microalgae A were cultivated in a water sample from Lake Mälaren taken in mid-June 2012. Cultivation began on the day of the water sampling, without any prior preservation or storage step. Batch cultivation was set up in two 120 dm3 glass aquariums each containing 10.5 dm3 lake water and 21.5 dm3 tap water. A modified version of the nutrient mix Jaworski’s medium (3.5 dm3) as described by Odlare et al. (2011) was added to each aquarium in order to ensure sufficient growth of microalgae. The aquariums were placed in a room with constant light. Light intensity at the beginning of the cultivation period was measured at 7000 lux (100 lmol photons m2 s1). The microalgae culture were all stored at 2 °C until the start of the anaerobic batch tests. 2.1.2. Microalgae B – wet Microalgae B were cultivated in a water sample taken in midDecember 2012 at the same place in Lake Mälaren. The batch cultivation setup was the same as for microalgae A. 2.1.3. Microalgae C – dry A third microalgae culture was cultivated in municipal wastewater in a photo bioreactor placed on the roof of the power plant in Umeå, northern Sweden (63°52). This culture was a mixture of natural green freshwater algal species and was grown in a 650 dm3 open natural light photo bioreactor for 5 days in August 2012. The photo bioreactor was constructed following the open ponds principle, and water flow was generated by a mechanical device (paddles). A metal supporting structure held the photo bioreactor above the roof surface. The reactor was made from thin fiberglass in order to allow light penetration on all surfaces. The municipal wastewater influent was collected at the local wastewater treatment plant (Umeva, Umeå) and transported once a week to the power plant station. A 1 m3 tank was used for transportation and for partial settling of the influent (Axelsson and Gentili, 2014). Treated flue gases from the local combined heat and power plant (Umeå Energi, Umeå), which burns municipal and partly industrial solid wastes were pumped from the smokestack and bubbled into the algae culture through a ceramic tubular gas diffuser (Cole-Parmer, USA) at approximately 3 dm3/min. The bubbling was stopped at night. The length of the night varied from 4 h 45 min in the beginning of August to 8 h 40 min at the end of the same month (Axelsson and Gentili, 2014).
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
2.1.4. Undigested sewage sludge The substrate to be co-digested with the microalgae in the anaerobic batch tests were undigested sewage sludge collected from the municipal WWTP in Västerås, central Sweden. The process configuration at the WWTP consists of: 1. Mechanical treatment with screens, sand grit and presedimentation 2. Biological treatment with an activated sludge process. In the biological treatment a poly-electrolyte is also added between the aeration and the sedimentation to lower the amount of outgoing phosphorous. The undigested sewage sludge was a representative mixture of primary sludge from the pre-sedimentation and a poly-electrolyte treated waste activated sludge from the biological treatment. The sludge sample was taken directly after the gravimetric thickening step of the mixed sludge and transported in 25 dm3 containers. It was then stored at +2 °C prior to the experiments. Undigested sewage sludge D was taken in mid-June 2012 and undigested sewage sludge E was taken in mid-December 2012, prior to the respective experiment. 2.1.5. Inocula for anaerobic digestion The different inocula used in the experiments were: F – Mesophilic digester at the municipal WWTP in Västerås from 2012. G – Mesophilic digester at the municipal WWTP in Västerås from 2013. H – Thermophilic pilot digester at the municipal WWTP in Uppsala from 2013. In order to ensure degradation of the remaining easily degradable organic matter and to remove dissolved methane, the inocula were stored with an anaerobic headspace for 10 days prior to the start of each experiment. Mesophilic inoculum were incubated at 37 °C and the thermophilic inoculum was incubated at 55 °C according to the method described by Angelidaki et al. (2009). Anaerobic digestion batch tests using a substrate with known theoretical methane potential were used to evaluate the activity of the inocula. In this study cellulose were used with a theoretical methane potential of 415 Ncm3/g VS. Inocula are not suitable to use if the yield is less than 70% of the substrate’s theoretical potential. This value is based on the combined experience of the reference group and authors in the unique study of Carlsson and Schnürer (2011).
205
to 6 g dm3) were added to flasks and the volume was made up to 0.07 dm3 with tap water. For the mesophilic and thermophilic experiment with microalgae B and C 0.5 dm3 conical flasks were used. In this case the working volume in each flask was 0.398 dm3, the added substrate was equivalent to 1.4 g VS (3.5 g VS dm3) and added inoculum G and H was equivalent to 2.8 g VS (7.0 g VS dm3), respectively. Different substrate mixtures were prepared for both the mesophilic and the thermophilic experiments by replacing undigested sludge with the cultivated microalgae. The algae concentrations were chosen based on the previous study made by Krustok et al. (2013). All substrate mixtures and blanks (inoculum only) were run in triplicate and incubated on a rotary shaker at 130 rpm (model: Orbital shaker 4535/4536) for 35 days incubations time in the experiments with microalgae A and for 57 days incubation time in the experiments with microalgae B and C. All the substrate mixtures are presented in Table 1. 2.3. Analytical procedure Gas production was determined by measuring pressure in the flasks using a pressure gauge (model: GMH 3111) equipped with a pressure sensor (GMSD 2BR, 1000 to 2000 mbar), while a gas sample was taken for methane content analysis. Pressure was normalized to take into account the volume of gas under standard conditions (Ncm3) i.e. at atmospheric pressure and at 0 °C. The frequency of pressure measurements and sampling depended on the biogas production rate. Methane content was analyzed by gas chromatography (PerkinElmer Arnel Clarus 500; column: 7’’ HayeSep N 60/80, 1/8’’ SF; FID Detector 250 °C, carrier gas: helium, flow 31 cm3/min, injector temperature: 60 °C; injection using Headspace sampler Turbo Matrix 110). Methane production was estimated based on the total biogas pressure, the gas volume, methane content and the gas temperature. The estimated methane production from the inoculum was subtracted from the total methane production. Methane production was calculated relative to the amount of VS added to each bottle under standard conditions (Ncm3). The calculated BMP were determined by combining the measured BMP-results from the anaerobic batch test on pure algae and on pure sewage sludge (mix. no. 9, 13, 17, 19, 23 and 27) (see Table 1). Confidence intervals (p = 0.05) were calculated using Microsoft Excel to indicate statistically significant differences between treatments. 2.4. Estimation of the theoretical BMP
2.1.6. TS- and VS-content in the substrates and inocula The content of total solids (TS) and volatile solids (VS) in the microalgae slurries, the undigested sewage sludge and the inocula were determined using standard techniques (APHA, 1995). 2.2. Anaerobic batch test Anaerobic batch experiments were conducted to determine the methane potential of different mixtures of microalgae cultures and undigested sewage sludge according to the protocol described by Dererie et al. (2011) with substrate:inoculum ratio 1:2 based on VS. The mesophilic experiment (37 °C) with microalgae A and undigested sewage sludge was performed in 1 dm3 conical flasks. Flasks were filled with substrate equivalent to 2.1 g VS (at 3 g VS dm3 loading rate) and inoculum F equivalent to 4.2 g VS (6 g VS dm3) and the total volume was made up to 0.7 dm3 with tap water. For the thermophilic experiment (55 °C) with microalgae A anaerobic digestion was performed in 0.1 dm3 conical bottles. Substrate equivalent to 0.21 g VS (corresponding to 3 g dm3) and inoculum F equivalent to 0.42 g VS (corresponding
The composition of a substrate influences the biogas production and the methane content of the biogas. The important parameters of the substrate are TS, VS and the nutrient composition. The organic fraction of the substrate can be divided into lipids, carbohydrates and protein. Approximate methane yield for lipids are 1.0 m3 kg VS1, for proteins 0.53 m3 kg VS1 and for carbohydrates 0.38 m3 kg VS1 (Carlsson and Schnürer, 2011). In order to estimate the theoretical methane potential of the microalgae and the sewage sludge, the substrates were analyzed for lipids, protein and carbohydrates. The amount of lipids was determined by SBR analysis (Schmid-Bondzynski-Ratslaff) according to standard method No. 131 from the Nordic Committee of Food Analysis (NMKL, 1989). The Kjeldahl method for nitrogen analysis was used to determine the protein content. The nitrogen content was multiplied by 6.25, which is the conversion factor for protein in food samples (Salo-väänänen and Koivistoinen, 1996). Ammonium content was determined using a standard technique (APHA, 1995). Carbohydrates could then be calculated according to Eq. (1).
206
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Table 1 Description of substrate mixtures and control substance in the anaerobic batch experiments. Mix. comp. no.
Temp. (°C)
Micro-algae A (%)
Micro-algae B (%)
Micro-algae C (%)
Sewage sludge D (%)
Sewage sludge E (%)
Control subst. (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
37 37 37 37 55 55 55 55 37 37 37 37 37 37 37 37 37 37 55 55 55 55 55 55 55 55 55 55
– 12 25 37 – 12 25 37 – – – – – – – – – – – – – – – – – – – –
– – – – – – – – – 12 25 37 100 – – – – – – 12 25 37 100 – – – – –
– – – – – – – – – – – – – 12 25 37 100 – – – – – – 12 25 37 100 –
100 88 75 63 100 88 75 63 – – – – – – – – – – – – – – – – – – – –
– – – – – – – – 100 88 75 63 – 88 75 63 – – 100 88 75 63 – 88 75 63 – –
– – – – – – – – – – – – – – – – – 100 – – – – – – – – – 100
Carbohydrate content ½W% ¼ 100 H2 O content ½W% Inorganic matter ½W% lipids ½W% protein ½W% NHþ4 ½W% 3
ð1Þ 1
Theoretical methane potentials (Ncm CH4 g VS) were then calculated from the lipids, protein and carbohydrates content and their methane potentials. 2.5. Kinetic model of biogas production
through a 100 lm filter to remove extracellular water and then dried at 70 °C for 24 h. The dominant algae belonging to the genus Scenedesmus were identified morphologically using light microscopy. Algae belonging to the genus Scenedesmus grow well in municipal wastewater according to Doria et al. (2012). Since all microalgae substrates in the batch tests contained Scenedesmus it is possible to have similar mixtures of microalgae and sewage sludge in anaerobic digestion in possible future full-scale applications. 3.2. Co-digestion of microalgae with undigested sewage sludge
The biogas production was modelled using the modified Gompertz equation (Zhu et al., 2009) (Eq. (2)).
BG ¼ BGP expðexp ½Rm e=BGP ðk tÞ þ 1Þ 3
ð2Þ 1
BGP is the biogas yield potential (Ncm g VS ), Rm = maximal daily biogas yield (Ncm3 g VS1 d1), k = bacteria growth lag time (d), e = mathematical constant (2.718), t = digestion time (d), BG = cumulative biogas yield (Ncm3 g VS1). The constants k, BGP and Rm were determined from the experimental data using the MS Excel Solver Toolpak. 3. Results and discussion 3.1. Characteristics of microalgae Microalgae A were harvested from the water surface of the aquariums after 20 days of cultivation. Scenedesmus and Chlorella vulgaris were identified in the microalgae mixture by microscopic examination. As microalgae B had a slower growth it was harvested after 55 days of cultivation. Scenedesmus and C. vulgaris were also identified in these samples by microscopic examination. The microalgae B were harvested by centrifugation at 5000g for 15 min. Microalgae C were harvested hourly by sedimentation in a semi continuous regime. Once harvested the algae paste was filtered
The TS and VS of substrates and inocula are presented in Table 2. The chemical composition of microalgae B, microalgae C and undigested sewage sludge E are also presented in the same table. Fig. 1 shows the results from the anaerobic batch experiment after 35 days with mixture no. 1–8 (see Table 1). The highest measured BMP was reached with mesophilic digestion of 12% microalgae A and 88% sewage sludge (mixture no. 2) with approximately 3% higher BMP compared to flasks with sludge alone. The same effect could not be seen in the results from the experiment in thermophilic conditions. In this study the inoculum for 55 °C was the same as the one for the mesophilic conditions, therefore it might not be adapted to 55 °C which can explain the low BMP in thermophilic conditions. The results from the anaerobic batch experiment with mixture no. 9–17 and 19–27 were presented in Fig. 2. The highest measured BMP in mesophilic conditions was reached in the mixture no. 12 with 63% undigested sewage sludge E and 37% microalgae B. The BMP in this sample was 408 ± 16 Ncm3 CH4 g VS1, 23% higher than the BMP from 100% undigested sewage sludge E (mix. no. 9). This difference was statistically significant. Samples with other substrate ratios in the same temperature had also tendencies to have higher methane levels than 100% undigested sewage sludge, but the differences were not statistically significant. Samples incubated in thermophilic conditions did not show the same tendencies to have an increase in BMP with addition of
207
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Table 2 The content of TS and VS in the microalgae slurries, the undigested sewage sludge and the inocula (average values from duplicates) and the chemical composition of microalgae B and C, and sewage sludge E. Substrates
TS (%)
VS (%)
VS/TS (%)
Microalgae A Microalgae B Microalgae C Sew. sludge D Sew. sludge E
6.05 4.30 90.0 6.75 3.50
3.09 3.01 58.5 3.66 2.70
51 70 65 54 77
2.70 3.10 6.40
1.46 1.80 3.44
54 58 53
Inocula Mes. digested sludge F Mes. digested sludge G Thermo. digested sludge H
Protein (% of TS)
Carbo-hydrates (% of TS)
C-tot (g kg1)
N-tot(g kg1)
7.36 2.99
25.95 25.91
36.49 30.88
17 306
1.8 39
11.34
25.28
43.47
12
1.3
Lipids (% of TS)
350 Measured BMP (mesophilic conditions)
300 Measured BMP (thermophilic conditions)
[Ncm3 CH4 g VS-1]
250 200 150 100 50 0 1, 5
2, 6 3, 7 [Mixture number]
4, 8
Fig. 1. Ncm3 CH4 g VS1 for co-digestion of microalgae A and undigested sewage sludge D, mixture no. 1–8 (±confidence interval, p = 0.05).
700
Theoretical BMP Measured BMP (mesophilic conditions) Measured BMP (thermophilic conditions) Calculated BMP (mesophilic conditions) Calculated BMP (thermophilic conditions)
[Ncm3 CH4 g VS-1]
600 500 400 300 200 100 0 9, 19
10, 20 11, 21 12, 22 13, 23 14, 24 15, 25 16, 26 17, 27 [Mixture number]
Fig. 2. BMP-comparison between different mixture no. with algae B and C (±confidence interval, p = 0.05).
microalgae B or C to the undigested sewage sludge. The mixture no. 20, with 88% undigested sewage sludge E and 12% microalgae B, had the highest BMP. The BMP in these samples measured 388 ± 83 Ncm3 CH4 g VS1, 7% higher than in flasks containing 100% undigested sewage sludge E (mix. no. 19), but this difference was not statistically significant. Comparison of the methane production by co-digestion of algae and undigested sewage sludge indicated that the microalgae and sewage sludge in the co-substrates had a synergetic effect in mesophilic conditions. The measured BMP was higher than the calculated BMP in all of the mixtures at mesophilic digestion (see Fig. 2). The tendency was that similar synergetic effect was not seen in thermophilic digestion. Microalgae B had significant higher
BMP than dried microalgae C in both mesophilic and thermophilic digestion. The pH was measured at the end of the trial period in study 2 and varied between 6.91 and 7.03. The neutral pH in all the mixtures indicates that stable conditions were reached by the end of the test in all the studied mixtures. The theoretical BMP was calculated according to the methods described in Section 2.4. The results were compared with the measured BMP at both mesophilic and thermophilic conditions. The highest BMP (80% of the theoretical methane potential) was found in the mixture no. 12. The control substance had a BMP of 408 ± 77 Ncm3 CH4 g VS1 in mesophilic conditions (mix. no. 18) and 378 ± 70 Ncm3 CH4 g VS1 in thermophilic conditions (mix. no. 28). These results were more than 70% of the theoretical methane potential, and thus validated the test. The study showed that co-digestion with microalgae in certain ratios increased biogas production in mesophilic digestion. However, the highest production of methane for the different mixtures was from different ratios of microalgae together with the sewage sludge. It indicated that for different substrates different ratios should be chosen in order to obtain maximal biogas production. In the study with microalgae B a synergetic effect in mesophilic conditions could also be seen. This is different from the results presented by Wang et al. (2013b). In their study co-digestion of sludge and microalgae did not produce more methane than sludge alone. However Wang et al. (2013b), used waste activated sludge (WAS) as a co-substrate to microalgae, while present study used a more representative mixed sludge from a municipal WWTP together with the microalgae. Further in the study of Wang et al. (2013b) just duplicate samples were used in the anaerobic batch test and no statistical analysis were made while triplicates and statistical analysis were made in present study. One explanation for the enhanced biogas production in mesophilic conditions could be an optimized C/N –ratio (C/N = 20–25/ 1) according to Yen and Brune (2007). In present study both the undigested sewage sludge E and the microalgae B had a C/N-ratio of 9.4/1 and 9.3/1 (calculated from Table 2), respectively, which were much lower than the mentioned optimized C/N-ratio. Another theory could be addition of minerals (micronutrients) introduced with the addition of microalgae. Micronutrients have been shown by Karlsson et al. (2012) to improve the performance of the anaerobic process during start-up and early operation. This study also showed that co-digestion of microalgae with undigested sewage sludge had no tendencies to increase biogas production and no synergetic effects from the substrate mixture was found in thermophilic conditions. This agrees with the results made by Samson and Leduyt (1986) who concluded that mesophilic conditions are more preferable for anaerobic digestion of microalgae. The reason for this might be due to that Scenedesmus sp. and C. vulgaris often contain high percentage (50–60%) of proteins. Degradation of proteins releases ammonium which in
208
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
higher temperature (thermophilic conditions) will be converted, to a higher extent, into ammonia. This substance can be toxic to methanogenic bacteria which might explain why there was a lower biogas production in thermophilic conditions. Scenedesmus sp. and C. vulgaris were found in both microalgae A and B. The study made by Frigon et al. (2013) showed that pure strains of different types of Scenedesmus and C. vulgaris produce methane from 258 ± 7 to 410 ± 6 and from 263 ± 3 to 361 ± 11 Ncm3 CH4 g VS1 in 37 °C, respectively. BMP of microalgae A alone were not analyzed, however, microalgae B produced 367 ± 4 Ncm3 CH4 g VS1, which is within the range of the previous study. The microalgae C, which was dried and was dominated by Scenedesmus had given BMP values that was lower than results reported by Frigon et al. (2013) for Scenedesmus but similar with results reported by Mussgnug et al. (2010) for Scenedesmus obliquus (177.9 Ncm3 CH4 g VS1). The low BMP in microalgae C is probably due to that S. obliquus is dominant in microalgae C, or due to the drying process as reported by Mussgnug et al. (2010). The lower amount of lipids in microalgae C compared with microalgae B (see Table 2) could also be part of the explanation why the BMP was lower for microalgae C. 3.3. Degradation rate and kinetic modeling The accumulated methane production curves for the tested microalgae B and C co-digested with undigested sewage sludge E are presented in Fig. 3a–d.
Methane production curves are divided into three stages: lag phase, decomposition phase and flattening phase (Carlsson and Schnürer, 2011). The lag phase is the time from the start of the experiment to the start of methane production. A long lag phase indicates that the material contains high levels of material that is not easily hydrolyzed. All samples except for the pure dry microalgae C had a short lag phase (Fig. 3 a–d), which indicated that microorganisms adapted easily to the conditions and utilized the substrate efficiently. However, digestion of dried microalgae C on its own had a longer lag phase in both mesophilic and thermophilic conditions. These results contradicts the findings made by Wang et al. (2013b) where the lag phase in the batch test of the microalgae slurry as sole feed were 20 days. The decomposition phase followed a similar linear pattern in all the samples, indicating that the substrates were homogenous with no persistent particles as mentioned by Carlsson and Schnürer (2011). The results of the determination of the constants in the kinetic model (Gompertz equation) are shown in Table 3. The Gompertz equation fitted the data well for most of the studied cases and the correlation coefficient, R2, was in the same range as it was shown in previous studies fitting BMP data to the equation (Kavitha et al., 2014; Parameswaran and Rittmann, 2012; Syaichurrozi et al., 2013; Prajapati et al., 2014; Zhu et al., 2009) except for mixture no. 17 and 27 where the R2 value was below 0.96. Mixture no. 17 and 27 are the two mixtures with 100% dried algae (algae C). It can be seen in Fig. 3b and d that the three stages 450.00
450.00
400.00
400.00
350.00 [Ncm3 CH4 g VS-1]
[Ncm3 CH4 g VS-1]
350.00 300.00 250.00 200.00 150.00
9
300.00 250.00 200.00 150.00
10 100.00
100.00
11
12 50.00
50.00
13
0.00
0.00 0
5
10
15
20
25
(a)
30 35 Days
40
45
50
55
0
60
15
20
25
30
35
40
45
60
Days
450.00 400.00
400.00
350.00 [Ncm3 CH4 g VS-1]
350.00 [Ncm3 CH4 g VS-1]
10
(b)
450.00
300.00 250.00 200.00 19 20 21 22 23
150.00 100.00 50.00
300.00 250.00 200.00 150.00 100.00 19 25 27
50.00
0.00
0.00 0
(c)
5
9 14 15 16 17 50 55
5
10
15
20
25
30 35 Days
40
45
50
55
60
0
(d)
5
10
15
20
25
30 35 Days
40
24 26 45
50
55
60
Fig. 3. Methane potential per gram VS for sewage sludge E and its co-substrates with 0%, 12%, 25%, 37%, 100% microalgae for algae substrate B at mesophilic condition (a), algae substrate C at mesophilic condition (b), algae substrate B at thermophilic condition (c), algae substrate C at thermophilic condition (d).
209
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210 Table 3 Constants of the Gompertz equation determined from the experimental data. Mix. comp. no.
Temp. (°C)
Micro-algae (%)
k (d)
BGP (Ncm3 gVS1)
Rm (Ncm3 gVS1 d1)
R2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
37 37 37 37 55 55 55 55 37 37 37 37 37 37 37 37 37 37 55 55 55 55 55 55 55 55 55 55
0 12 25 37 0 12 25 37 0 12 25 37 100 12 25 37 100 0 0 12 25 37 100 12 25 37 100 0
2.1 2.1 3.2 7.6 8.3 8.3 7.3 8.6 3.8 3.9 4.6 4.8 5.1 2.3 2.9 2.8 7.8 10.7 3.7 3.1 3.3 2.5 3.1 4.1 3.4 2.0 1.5 13.7
299 307 256 206 159 113 109 81 332 344 359 409 368 387 349 325 180 408 366 390 339 323 319 324 299 278 141 379
21 25 32 25 15 13.5 13.1 8.5 32 27 30 33 26 26 27 25 11 33 39 44 39 34 31 35 29 27 19 23
0.97 0.98 0.96 0.98 0.98 0.99 0.99 0.99 0.97 0.98 0.98 0.98 0.97 0.97 0.97 0.96 0.94 0.97 0.98 0.98 0.98 0.98 0.97 0.97 0.97 0.97 0.87 0.99
methane production curves normally can be divided into (lag phase, decomposition phase and flattening phase) are not so distinct for those mixtures. 4. Conclusions Microalgae improved the BMP of undigested sewage sludge significantly, particularly in mesophilic conditions. In contrary, the addition of algae affect the BMP negatively in thermophilic conditions. Wet microalgae B, cultivated from samples from lake Mälaren in December 2012, showed the far highest measured BMP in the study when added to sludge in 37/63% ratio. When microalgae were digested alone the short lag-phase indicated that microorganisms in the inocula adapted easily to the conditions and utilized the substrate efficiently. Altogether, this argues for further research on the advantages of using microalgae as a co-substrate in mesophilic anaerobic digestion. Acknowledgements This research has been performed in close collaboration with the end user of the studied system, as a co-production study within the framework of the ACWA-project and the VA-kluster Mälardalen. Most significantly, Knowledge Foundation, Vinnova, SVU, Läckeby Water, Mälarenergi, The Swedish Energy Agency and Processum Biorefinery Initiative should be thanked for their funding, knowledge and development contributions in the study. References Angelidaki, I., Alves, M., Bolzonella, D., Borzacconi, L., Campos, J.L., Guwy, A.J., Kalyuzhnyi, S., Jenicek, P., van Lier, J.B., 2009. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Sci. Technol. 59, 927–934. APHA, 1995. Standard method for the examination of water and wastewater. American Public Health Association, New York. Axelsson, M., Gentili, F., 2014. A single-step method for rapid extraction of total lipids from green microalgae. PLos One 9, 6.
Brune, D.E., Lundquist, T.J., Benemann, J.R., 2009. Microalgal biomass for greenhouse gas reductions: potential for replacement of fossil fuels and animal feeds. J. Environ. Eng., 1136–1144. Carlsson, M., Schnürer, A., 2011. RAPPORT U2011:12 – Handbok metanpotential Avfall Sverige utveckling. In Swedish. Dererie, D.Y., Trobro, S., Momeni, M.H., Hansson, H., Blomqvist, J., Passoth, V., Schnürer, A., Sandgren, M., Ståhlberg, J., 2011. Improved bio-energy yields via sequential ethanol fermentation and biogas digestion of steam exploded oat straw. Bioresour. Technol. 102, 4449–4455. Doria, E., Longoni, P., Scibilia, L., Iazzi, N., Cella, R., Nielsen, E., 2012. Isolation and characterization of a Scenedesmus acutus strain to be used for bioremediation of urban wastewater. J. Appl. Phycol. 24, 375–383. Ficara, E., Uslenghi, A., Basilico, D., Mezzanotte, V., 2014. Growth of microalgal biomass on supernatant from biosolid dewatering. Water Sci. Technol. 69, 896– 902. Formagini, E.L., Marques, F.R., Serejo, M.L., Paulo, P.L., Boncz, M.A., 2014. The use of microalgae and their culture medium for biogas production in an integrated cycle. Water Sci. Technol. 69, 941–946. Frigon, J.-C., Matteau-Lebrun, F., Hamani Abdou, R., McGinn, P.J., O’Leary, S.J.B., Guiot, S.R., 2013. Screening microalgae strains for their productivity in methane following anaerobic digestion. Appl. Energy 108, 100–107. Karlsson, A., Einarsson, P., Schnürer, A., Sundberg, C., Ejlertsson, J., Svensson, B.H., 2012. Impact of trace element addition on degradation efficiency of volatile fatty acids, oleic acid and phenyl acetate and on microbial populations in a biogas digester. J. Biosci. Bioeng. 114, 446–452. Kavitha, S., Jayashree, C., Adish Kumar, S., Tae Yeom, I., Rajesh Banu, J., 2014. The enhancement of anaerobic biodegradability of waste activated sludge by surfactant mediated biological pretreatment. Bioresour. Technol. 168, 159–166. Khalid, A., Arshad, M., Anjum, M., Mahmood, T., Dawson, L., 2011. The anaerobic digestion of solid organic waste. Waste Manage. 31, 1737–1744. Krustok, I., Nehrenheim, E., Odlare, M., Liu, X., Li, S., 2013. Cultivation of indigenous algae for increased biogas production. International Conference on Appl. Energy, Preotria, South Africa 1–4 July 2013. Larsdotter, K., 2006. Wastewater treatment with microalgae – a literature review. VATTEN 62, 31–38. Ma, X., Zhou, W., Fu, Z., Cheng, Y., Min, M., Liu, Y., Zhang, Y., Chen, P., Ruan, R., 2014. Effect of wastewater-borne bacteria on algal growth and nutrients removal in wastewater-based algae cultivation system. Bioresour. Technol. 167, 8–13. Mata-Alvarez, J., Dosta, J., Macé, S., Astals, S., 2011. Codigestion of solid wastes: a review of its uses and perspectives including modeling. Crit. Rev. Biotechnol. 31, 99–111. Mussgnug, J.H., Klassen, V., Schlüter, A., Kruse, O., 2010. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotech. 150, 51–56. NMKL, 1989. Fat. Determination according to SBR in meat and meat products, Vol. 131, Nordic Commitee on Food Analysis. Odlare, M., Nehrenheim, E., Ribé, V., Thorin, E., Gavare, M., Grube, M., 2011. Cultivation of algae with indigenous species – potentials for regional biofuel production. Appl. Energy 88, 3280–3285.
210
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Parameswaran, P., Rittmann, B.E., 2012. Feasibility of anaerobic co-digestion of pig waste and paper sludge. Bioresour. Technol. 124, 163–168. Prajapati, S.K., Kumar, P., Malik, A., Vijay, V.K., 2014. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: a closed loop bioenergy generation process. Bioresour. Technol. 158, 174–180. Rusten, B., Sahu, A., 2011. Microalgae growth for nutrient recovery from sludge liquor and production of renewable bioenergy. Water Sci. Technol. 64, 1195– 1201. Salo-väänänen, P.P., Koivistoinen, P.E., 1996. Determination of protein in foods: comparison of net protein and crude protein (N 6.25) values. Food Chem. 57, 27–31. Samson, R., Leduyt, A., 1986. Detailed study of anaerobic digestion of Spirulina maxima algal biomass. Biotechnol. Bioeng. 28, 1014–1023. Schwede, S., Kowalczyk, A., Gerber, M., Span, R., 2013. Anaerobic co-digestion of the marine microalga Nannochloropsis salina with energy crops. Bioresour. Technol. 148, 428–435. Su, Y., Mennerich, A., Urban, B., 2011. Municipal wastewater treatment and biomass accumulation with a wastewater-born and settleable algal-bacterial culture. Water Res. 45, 3351–3358.
Syaichurrozi, I., Budiyono, Sumardiono, S., 2013. Predicting kinetic model of biogas production and biodegradability organic materials: biogas production from vinasse at variation of COD/N ratio. Bioresour. Technol. 149, 390–397. Tchobanoglous, G., Burton, L.F., 2002. Wastewater Engineering: Treatment and Reuse, fourth ed. McGraw & Hill. Wang, H., Hu, Z., Xiao, B., Cheng, Q., Li, F., 2013a. Ammonium nitrogen removal in batch cultures treating digested piggery wastewater with microalgae Oedogonium sp.. Water Sci. Technol. 68, 269–275. Wang, M., Sahu, K.A., Björn, R., Chul, P., 2013b. Anaerobic co-digestion of microalgae Chlorella sp. and waste activated sludge. Bioresour. Technol. 142, 585–590. Wiley, P.E., Campell, J.E., McKuin, B., 2011. Production of biodiesel and biogas from algae: a review of process train options. Water Environ. Res. 82, 326–338. Yen, H., Brune, D., 2007. Anaerobic co-digestion of algal sludge and waste paper to produce methane. Bioresour. Technol. 98, 130–134. Zhu, B., Gikas, P., Zhang, R., Lord, J., Jenkins, B., Li, X., 2009. Characteristics and biogas production potential of municipal solid wastes pretreated with a rotary drum reactor. Bioresour. Technol. 100, 1122–1129.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment Jesper Olsson a,⇑, Xin Mei Feng b, Johnny Ascue b, Francesco G. Gentili c, M.A. Shabiimam d, Emma Nehrenheim a, Eva Thorin a a
The School of Business, Society and Engineering, Mälardalen University, Box 883, SE-721 23 Västerås, Sweden JTI – Swedish Institute of Agricultural and Environmental Engineering, Box 7033, SE-750 07 Uppsala, Sweden Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden d Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microalgae were co-digested with a
sewage sludge mixture in batch studies. Highest CH4-yield, of 408 ± 16 Ncm3 g VS1 was reached in 37 °C with 37% microalgae. In 55 °C the same increase in methane yield with microalgae added could not be seen. The microalgae seems to be easily degraded with a short lag phase in the batches.
a r t i c l e
i n f o
Article history: Received 8 June 2014 Received in revised form 14 August 2014 Accepted 16 August 2014 Available online 23 August 2014 Keywords: Biogas production Co-digestion Microalgae Sewage sludge
Microalgae unit
0 % microalgae, 100 % sludge mixture 12 % microalgae, 88 % sludge mixture
Primary sludge
Waste activated sludge
37 % microalgae, 63 % sludge mixture
Methane yield (CH4 g VS-1) from batch test
100 % microalgae, 0 % sludge mixture Anaerobic digestion - thermophilic conditions
a b s t r a c t In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic codigestion of microalgae and undigested sewage sludge did not give the same synergy. Ó 2014 Published by Elsevier Ltd.
Anaerobic digestion of sewage sludge from municipal wastewater treatment is a common stabilization method for biosolids. The combustible biogas produced in the process is considered to be a ⇑ Corresponding author. Tel.: +46 21 10 70 35; fax: +46 21 10 14 80.
http://dx.doi.org/10.1016/j.biortech.2014.08.069 0960-8524/Ó 2014 Published by Elsevier Ltd.
Anaerobic digestion - mesophilic conditions
25 % microalgae, 75 % sludge mixture
1. Introduction
E-mail address: [email protected] (J. Olsson).
Methane yield (CH4 g VS-1) from batch test
renewable energy source thereby making the expansion of biogas production systems an important contributor to the global conversion from fossil fuels to renewable energy systems (Tchobanoglous and Burton, 2002). The demand for biogas continues to grow and it is therefore important for municipal wastewater treatment plants (WWTP) to maximize biogas production. One way to do this is by using microalgae cultivated with wastewater as a co-substrate in the digestion process. Microalgae can be cultivated in a treatment
204
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
step in photo bioreactors or microalgae ponds in the final polishing wastewater treatment in the WWTP (Larsdotter, 2006) or as a treatment for nutrient rich side streams like reject water from sludge dewatering (Ficara et al., 2014; Rusten and Sahu, 2011). Simultaneous nutrient recovery, water treatment and biomass production are thus possible. Successful cultivation of microalgae in wastewater has been demonstrated in recent studies (Odlare et al., 2011; Su et al., 2011). Rusten and Sahu (2011), Ficara et al. (2014) and Ma et al. (2014) have evaluated the process of cultivation of microalgae from reject water, and have found that microalgae remove nitrogen and phosphorous from the reject water, consequently reducing the impact of returning side streams to the main stream of the treatment plant. Microalgae have also been used to successfully reduce nutrients in nutrient rich digested piggery wastewater (Wang et al., 2013a). According to Wang et al. (2013b), waste activated sludge (WAS) can be a suitable co-substrate with microalgae. Co-digesting a microalgae mix containing Chlorella sp. with varying amounts of waste activated sludge (WAS), 59–96% in mass of VS-content, the biogas yield is improved which is 73–79% larger than the gas yield from pure algae digestion sets. Krustok et al. (2013) have shown in an anaerobic batch experiment with municipal food waste and harvested microalgae cultivated in lake water that the biogas production is improved after addition of microalgae. The experiment has been carried out with fermentation bottles where 0%, 12%, 25% and 37% of the food waste is replaced with harvested microalgae. During the first 25 days of fermentation the replacement of 12% food waste with microalgae has given the highest biogas production rate. A possible explanation for the synergetic effect of co-digesting microalgae with other substrates is an optimization of the C/N-ratio. Since anaerobic digestion is inhibited when using microalgae alone as a substrate due to the high protein content in the algae biomass (Brune et al., 2009; Wiley et al., 2011) other carbon rich substrates can enhance the C/N-ratio. The use of co-digestion of substrates to stabilize the digestion process by improving the C/N-ratio has been suggested by several authors (Brune et al., 2009; Khalid et al., 2011; Mata-Alvarez et al., 2011). Yen and Brune (2007) have reported increased gas yields from co-digestion of microalgae with waste paper from recycling bins by adjusting the low C/N-ratio in microalgae. The results have shown that an optimum C/N-ratio for co-digestion of algal sludge and wastepaper is in the range of 20/1–25/1. The anaerobic co-digestion between corn silage and the marine microalga Nannochloropsis salina have also shown a better process stability and enhanced biogas yields by microalgae addition (Schwede et al., 2013). This positive influence in co-digestion according to Schwede et al. (2013) can be explained by an optimized C/N-ratio but also enhanced alkalinity and addition of important trace elements for the digestion. The enhanced alkalinity that microalgae bring to the co-digestion has also been mentioned by Formagini et al. (2014). In this study vinasse from ethanol production was co-digested with microalgae. The alkalinity of the algae suspension is higher but not enough to keep the pH –level stabilized to a neutral level together with the vinasse (Formagini et al., 2014). Although there have been recent development in the field of co-digestion of microalgae with other substrates, there is still a lack of knowledge regarding co-digestion of microalgae with a representative mix of sewage sludge for a municipal WWTP in both thermophilic- and mesophilic conditions. This would be important knowledge for further studies and development of systems for using microalgae as a co-substrate together with traditional wastewater treatment and anaerobic digestion in full scale applications. Earlier studies in the area of co-digestion between sewage sludge and microalgae have only focused on co-digesting the microalgae and the biological waste activated sludge (WAS). However, the
experiments have only been made in mesophilic conditions (Ficara et al., 2014; Wang et al., 2013b). This study was designed to investigate cultivation of indigenous microalgae from a nearby lake and in municipal wastewater and using it as a co-substrate in anaerobic digestion with undigested sewage sludge in various proportions. The methane potentials of the different mixtures were evaluated in anaerobic batch tests in mesophilic and thermophilic conditions. The aim of the study was to investigate whether co-digestion of microalgae and sewage sludge were more efficient for biogas production than digestion of the sewage sludge alone. 2. Methods 2.1. Substrates and inocula To get a broad approach in the study three different microalgae cultures were used as a co-substrate to the undigested sewage sludge. The inoculums used in the BMP-experiments were all taken from digesters at waste water treatment plants so that the micro culture were already adapted to the undigested sewage sludge. 2.1.1. Microalgae A – wet Microalgae A were cultivated in a water sample from Lake Mälaren taken in mid-June 2012. Cultivation began on the day of the water sampling, without any prior preservation or storage step. Batch cultivation was set up in two 120 dm3 glass aquariums each containing 10.5 dm3 lake water and 21.5 dm3 tap water. A modified version of the nutrient mix Jaworski’s medium (3.5 dm3) as described by Odlare et al. (2011) was added to each aquarium in order to ensure sufficient growth of microalgae. The aquariums were placed in a room with constant light. Light intensity at the beginning of the cultivation period was measured at 7000 lux (100 lmol photons m2 s1). The microalgae culture were all stored at 2 °C until the start of the anaerobic batch tests. 2.1.2. Microalgae B – wet Microalgae B were cultivated in a water sample taken in midDecember 2012 at the same place in Lake Mälaren. The batch cultivation setup was the same as for microalgae A. 2.1.3. Microalgae C – dry A third microalgae culture was cultivated in municipal wastewater in a photo bioreactor placed on the roof of the power plant in Umeå, northern Sweden (63°52). This culture was a mixture of natural green freshwater algal species and was grown in a 650 dm3 open natural light photo bioreactor for 5 days in August 2012. The photo bioreactor was constructed following the open ponds principle, and water flow was generated by a mechanical device (paddles). A metal supporting structure held the photo bioreactor above the roof surface. The reactor was made from thin fiberglass in order to allow light penetration on all surfaces. The municipal wastewater influent was collected at the local wastewater treatment plant (Umeva, Umeå) and transported once a week to the power plant station. A 1 m3 tank was used for transportation and for partial settling of the influent (Axelsson and Gentili, 2014). Treated flue gases from the local combined heat and power plant (Umeå Energi, Umeå), which burns municipal and partly industrial solid wastes were pumped from the smokestack and bubbled into the algae culture through a ceramic tubular gas diffuser (Cole-Parmer, USA) at approximately 3 dm3/min. The bubbling was stopped at night. The length of the night varied from 4 h 45 min in the beginning of August to 8 h 40 min at the end of the same month (Axelsson and Gentili, 2014).
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
2.1.4. Undigested sewage sludge The substrate to be co-digested with the microalgae in the anaerobic batch tests were undigested sewage sludge collected from the municipal WWTP in Västerås, central Sweden. The process configuration at the WWTP consists of: 1. Mechanical treatment with screens, sand grit and presedimentation 2. Biological treatment with an activated sludge process. In the biological treatment a poly-electrolyte is also added between the aeration and the sedimentation to lower the amount of outgoing phosphorous. The undigested sewage sludge was a representative mixture of primary sludge from the pre-sedimentation and a poly-electrolyte treated waste activated sludge from the biological treatment. The sludge sample was taken directly after the gravimetric thickening step of the mixed sludge and transported in 25 dm3 containers. It was then stored at +2 °C prior to the experiments. Undigested sewage sludge D was taken in mid-June 2012 and undigested sewage sludge E was taken in mid-December 2012, prior to the respective experiment. 2.1.5. Inocula for anaerobic digestion The different inocula used in the experiments were: F – Mesophilic digester at the municipal WWTP in Västerås from 2012. G – Mesophilic digester at the municipal WWTP in Västerås from 2013. H – Thermophilic pilot digester at the municipal WWTP in Uppsala from 2013. In order to ensure degradation of the remaining easily degradable organic matter and to remove dissolved methane, the inocula were stored with an anaerobic headspace for 10 days prior to the start of each experiment. Mesophilic inoculum were incubated at 37 °C and the thermophilic inoculum was incubated at 55 °C according to the method described by Angelidaki et al. (2009). Anaerobic digestion batch tests using a substrate with known theoretical methane potential were used to evaluate the activity of the inocula. In this study cellulose were used with a theoretical methane potential of 415 Ncm3/g VS. Inocula are not suitable to use if the yield is less than 70% of the substrate’s theoretical potential. This value is based on the combined experience of the reference group and authors in the unique study of Carlsson and Schnürer (2011).
205
to 6 g dm3) were added to flasks and the volume was made up to 0.07 dm3 with tap water. For the mesophilic and thermophilic experiment with microalgae B and C 0.5 dm3 conical flasks were used. In this case the working volume in each flask was 0.398 dm3, the added substrate was equivalent to 1.4 g VS (3.5 g VS dm3) and added inoculum G and H was equivalent to 2.8 g VS (7.0 g VS dm3), respectively. Different substrate mixtures were prepared for both the mesophilic and the thermophilic experiments by replacing undigested sludge with the cultivated microalgae. The algae concentrations were chosen based on the previous study made by Krustok et al. (2013). All substrate mixtures and blanks (inoculum only) were run in triplicate and incubated on a rotary shaker at 130 rpm (model: Orbital shaker 4535/4536) for 35 days incubations time in the experiments with microalgae A and for 57 days incubation time in the experiments with microalgae B and C. All the substrate mixtures are presented in Table 1. 2.3. Analytical procedure Gas production was determined by measuring pressure in the flasks using a pressure gauge (model: GMH 3111) equipped with a pressure sensor (GMSD 2BR, 1000 to 2000 mbar), while a gas sample was taken for methane content analysis. Pressure was normalized to take into account the volume of gas under standard conditions (Ncm3) i.e. at atmospheric pressure and at 0 °C. The frequency of pressure measurements and sampling depended on the biogas production rate. Methane content was analyzed by gas chromatography (PerkinElmer Arnel Clarus 500; column: 7’’ HayeSep N 60/80, 1/8’’ SF; FID Detector 250 °C, carrier gas: helium, flow 31 cm3/min, injector temperature: 60 °C; injection using Headspace sampler Turbo Matrix 110). Methane production was estimated based on the total biogas pressure, the gas volume, methane content and the gas temperature. The estimated methane production from the inoculum was subtracted from the total methane production. Methane production was calculated relative to the amount of VS added to each bottle under standard conditions (Ncm3). The calculated BMP were determined by combining the measured BMP-results from the anaerobic batch test on pure algae and on pure sewage sludge (mix. no. 9, 13, 17, 19, 23 and 27) (see Table 1). Confidence intervals (p = 0.05) were calculated using Microsoft Excel to indicate statistically significant differences between treatments. 2.4. Estimation of the theoretical BMP
2.1.6. TS- and VS-content in the substrates and inocula The content of total solids (TS) and volatile solids (VS) in the microalgae slurries, the undigested sewage sludge and the inocula were determined using standard techniques (APHA, 1995). 2.2. Anaerobic batch test Anaerobic batch experiments were conducted to determine the methane potential of different mixtures of microalgae cultures and undigested sewage sludge according to the protocol described by Dererie et al. (2011) with substrate:inoculum ratio 1:2 based on VS. The mesophilic experiment (37 °C) with microalgae A and undigested sewage sludge was performed in 1 dm3 conical flasks. Flasks were filled with substrate equivalent to 2.1 g VS (at 3 g VS dm3 loading rate) and inoculum F equivalent to 4.2 g VS (6 g VS dm3) and the total volume was made up to 0.7 dm3 with tap water. For the thermophilic experiment (55 °C) with microalgae A anaerobic digestion was performed in 0.1 dm3 conical bottles. Substrate equivalent to 0.21 g VS (corresponding to 3 g dm3) and inoculum F equivalent to 0.42 g VS (corresponding
The composition of a substrate influences the biogas production and the methane content of the biogas. The important parameters of the substrate are TS, VS and the nutrient composition. The organic fraction of the substrate can be divided into lipids, carbohydrates and protein. Approximate methane yield for lipids are 1.0 m3 kg VS1, for proteins 0.53 m3 kg VS1 and for carbohydrates 0.38 m3 kg VS1 (Carlsson and Schnürer, 2011). In order to estimate the theoretical methane potential of the microalgae and the sewage sludge, the substrates were analyzed for lipids, protein and carbohydrates. The amount of lipids was determined by SBR analysis (Schmid-Bondzynski-Ratslaff) according to standard method No. 131 from the Nordic Committee of Food Analysis (NMKL, 1989). The Kjeldahl method for nitrogen analysis was used to determine the protein content. The nitrogen content was multiplied by 6.25, which is the conversion factor for protein in food samples (Salo-väänänen and Koivistoinen, 1996). Ammonium content was determined using a standard technique (APHA, 1995). Carbohydrates could then be calculated according to Eq. (1).
206
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Table 1 Description of substrate mixtures and control substance in the anaerobic batch experiments. Mix. comp. no.
Temp. (°C)
Micro-algae A (%)
Micro-algae B (%)
Micro-algae C (%)
Sewage sludge D (%)
Sewage sludge E (%)
Control subst. (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
37 37 37 37 55 55 55 55 37 37 37 37 37 37 37 37 37 37 55 55 55 55 55 55 55 55 55 55
– 12 25 37 – 12 25 37 – – – – – – – – – – – – – – – – – – – –
– – – – – – – – – 12 25 37 100 – – – – – – 12 25 37 100 – – – – –
– – – – – – – – – – – – – 12 25 37 100 – – – – – – 12 25 37 100 –
100 88 75 63 100 88 75 63 – – – – – – – – – – – – – – – – – – – –
– – – – – – – – 100 88 75 63 – 88 75 63 – – 100 88 75 63 – 88 75 63 – –
– – – – – – – – – – – – – – – – – 100 – – – – – – – – – 100
Carbohydrate content ½W% ¼ 100 H2 O content ½W% Inorganic matter ½W% lipids ½W% protein ½W% NHþ4 ½W% 3
ð1Þ 1
Theoretical methane potentials (Ncm CH4 g VS) were then calculated from the lipids, protein and carbohydrates content and their methane potentials. 2.5. Kinetic model of biogas production
through a 100 lm filter to remove extracellular water and then dried at 70 °C for 24 h. The dominant algae belonging to the genus Scenedesmus were identified morphologically using light microscopy. Algae belonging to the genus Scenedesmus grow well in municipal wastewater according to Doria et al. (2012). Since all microalgae substrates in the batch tests contained Scenedesmus it is possible to have similar mixtures of microalgae and sewage sludge in anaerobic digestion in possible future full-scale applications. 3.2. Co-digestion of microalgae with undigested sewage sludge
The biogas production was modelled using the modified Gompertz equation (Zhu et al., 2009) (Eq. (2)).
BG ¼ BGP expðexp ½Rm e=BGP ðk tÞ þ 1Þ 3
ð2Þ 1
BGP is the biogas yield potential (Ncm g VS ), Rm = maximal daily biogas yield (Ncm3 g VS1 d1), k = bacteria growth lag time (d), e = mathematical constant (2.718), t = digestion time (d), BG = cumulative biogas yield (Ncm3 g VS1). The constants k, BGP and Rm were determined from the experimental data using the MS Excel Solver Toolpak. 3. Results and discussion 3.1. Characteristics of microalgae Microalgae A were harvested from the water surface of the aquariums after 20 days of cultivation. Scenedesmus and Chlorella vulgaris were identified in the microalgae mixture by microscopic examination. As microalgae B had a slower growth it was harvested after 55 days of cultivation. Scenedesmus and C. vulgaris were also identified in these samples by microscopic examination. The microalgae B were harvested by centrifugation at 5000g for 15 min. Microalgae C were harvested hourly by sedimentation in a semi continuous regime. Once harvested the algae paste was filtered
The TS and VS of substrates and inocula are presented in Table 2. The chemical composition of microalgae B, microalgae C and undigested sewage sludge E are also presented in the same table. Fig. 1 shows the results from the anaerobic batch experiment after 35 days with mixture no. 1–8 (see Table 1). The highest measured BMP was reached with mesophilic digestion of 12% microalgae A and 88% sewage sludge (mixture no. 2) with approximately 3% higher BMP compared to flasks with sludge alone. The same effect could not be seen in the results from the experiment in thermophilic conditions. In this study the inoculum for 55 °C was the same as the one for the mesophilic conditions, therefore it might not be adapted to 55 °C which can explain the low BMP in thermophilic conditions. The results from the anaerobic batch experiment with mixture no. 9–17 and 19–27 were presented in Fig. 2. The highest measured BMP in mesophilic conditions was reached in the mixture no. 12 with 63% undigested sewage sludge E and 37% microalgae B. The BMP in this sample was 408 ± 16 Ncm3 CH4 g VS1, 23% higher than the BMP from 100% undigested sewage sludge E (mix. no. 9). This difference was statistically significant. Samples with other substrate ratios in the same temperature had also tendencies to have higher methane levels than 100% undigested sewage sludge, but the differences were not statistically significant. Samples incubated in thermophilic conditions did not show the same tendencies to have an increase in BMP with addition of
207
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Table 2 The content of TS and VS in the microalgae slurries, the undigested sewage sludge and the inocula (average values from duplicates) and the chemical composition of microalgae B and C, and sewage sludge E. Substrates
TS (%)
VS (%)
VS/TS (%)
Microalgae A Microalgae B Microalgae C Sew. sludge D Sew. sludge E
6.05 4.30 90.0 6.75 3.50
3.09 3.01 58.5 3.66 2.70
51 70 65 54 77
2.70 3.10 6.40
1.46 1.80 3.44
54 58 53
Inocula Mes. digested sludge F Mes. digested sludge G Thermo. digested sludge H
Protein (% of TS)
Carbo-hydrates (% of TS)
C-tot (g kg1)
N-tot(g kg1)
7.36 2.99
25.95 25.91
36.49 30.88
17 306
1.8 39
11.34
25.28
43.47
12
1.3
Lipids (% of TS)
350 Measured BMP (mesophilic conditions)
300 Measured BMP (thermophilic conditions)
[Ncm3 CH4 g VS-1]
250 200 150 100 50 0 1, 5
2, 6 3, 7 [Mixture number]
4, 8
Fig. 1. Ncm3 CH4 g VS1 for co-digestion of microalgae A and undigested sewage sludge D, mixture no. 1–8 (±confidence interval, p = 0.05).
700
Theoretical BMP Measured BMP (mesophilic conditions) Measured BMP (thermophilic conditions) Calculated BMP (mesophilic conditions) Calculated BMP (thermophilic conditions)
[Ncm3 CH4 g VS-1]
600 500 400 300 200 100 0 9, 19
10, 20 11, 21 12, 22 13, 23 14, 24 15, 25 16, 26 17, 27 [Mixture number]
Fig. 2. BMP-comparison between different mixture no. with algae B and C (±confidence interval, p = 0.05).
microalgae B or C to the undigested sewage sludge. The mixture no. 20, with 88% undigested sewage sludge E and 12% microalgae B, had the highest BMP. The BMP in these samples measured 388 ± 83 Ncm3 CH4 g VS1, 7% higher than in flasks containing 100% undigested sewage sludge E (mix. no. 19), but this difference was not statistically significant. Comparison of the methane production by co-digestion of algae and undigested sewage sludge indicated that the microalgae and sewage sludge in the co-substrates had a synergetic effect in mesophilic conditions. The measured BMP was higher than the calculated BMP in all of the mixtures at mesophilic digestion (see Fig. 2). The tendency was that similar synergetic effect was not seen in thermophilic digestion. Microalgae B had significant higher
BMP than dried microalgae C in both mesophilic and thermophilic digestion. The pH was measured at the end of the trial period in study 2 and varied between 6.91 and 7.03. The neutral pH in all the mixtures indicates that stable conditions were reached by the end of the test in all the studied mixtures. The theoretical BMP was calculated according to the methods described in Section 2.4. The results were compared with the measured BMP at both mesophilic and thermophilic conditions. The highest BMP (80% of the theoretical methane potential) was found in the mixture no. 12. The control substance had a BMP of 408 ± 77 Ncm3 CH4 g VS1 in mesophilic conditions (mix. no. 18) and 378 ± 70 Ncm3 CH4 g VS1 in thermophilic conditions (mix. no. 28). These results were more than 70% of the theoretical methane potential, and thus validated the test. The study showed that co-digestion with microalgae in certain ratios increased biogas production in mesophilic digestion. However, the highest production of methane for the different mixtures was from different ratios of microalgae together with the sewage sludge. It indicated that for different substrates different ratios should be chosen in order to obtain maximal biogas production. In the study with microalgae B a synergetic effect in mesophilic conditions could also be seen. This is different from the results presented by Wang et al. (2013b). In their study co-digestion of sludge and microalgae did not produce more methane than sludge alone. However Wang et al. (2013b), used waste activated sludge (WAS) as a co-substrate to microalgae, while present study used a more representative mixed sludge from a municipal WWTP together with the microalgae. Further in the study of Wang et al. (2013b) just duplicate samples were used in the anaerobic batch test and no statistical analysis were made while triplicates and statistical analysis were made in present study. One explanation for the enhanced biogas production in mesophilic conditions could be an optimized C/N –ratio (C/N = 20–25/ 1) according to Yen and Brune (2007). In present study both the undigested sewage sludge E and the microalgae B had a C/N-ratio of 9.4/1 and 9.3/1 (calculated from Table 2), respectively, which were much lower than the mentioned optimized C/N-ratio. Another theory could be addition of minerals (micronutrients) introduced with the addition of microalgae. Micronutrients have been shown by Karlsson et al. (2012) to improve the performance of the anaerobic process during start-up and early operation. This study also showed that co-digestion of microalgae with undigested sewage sludge had no tendencies to increase biogas production and no synergetic effects from the substrate mixture was found in thermophilic conditions. This agrees with the results made by Samson and Leduyt (1986) who concluded that mesophilic conditions are more preferable for anaerobic digestion of microalgae. The reason for this might be due to that Scenedesmus sp. and C. vulgaris often contain high percentage (50–60%) of proteins. Degradation of proteins releases ammonium which in
208
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
higher temperature (thermophilic conditions) will be converted, to a higher extent, into ammonia. This substance can be toxic to methanogenic bacteria which might explain why there was a lower biogas production in thermophilic conditions. Scenedesmus sp. and C. vulgaris were found in both microalgae A and B. The study made by Frigon et al. (2013) showed that pure strains of different types of Scenedesmus and C. vulgaris produce methane from 258 ± 7 to 410 ± 6 and from 263 ± 3 to 361 ± 11 Ncm3 CH4 g VS1 in 37 °C, respectively. BMP of microalgae A alone were not analyzed, however, microalgae B produced 367 ± 4 Ncm3 CH4 g VS1, which is within the range of the previous study. The microalgae C, which was dried and was dominated by Scenedesmus had given BMP values that was lower than results reported by Frigon et al. (2013) for Scenedesmus but similar with results reported by Mussgnug et al. (2010) for Scenedesmus obliquus (177.9 Ncm3 CH4 g VS1). The low BMP in microalgae C is probably due to that S. obliquus is dominant in microalgae C, or due to the drying process as reported by Mussgnug et al. (2010). The lower amount of lipids in microalgae C compared with microalgae B (see Table 2) could also be part of the explanation why the BMP was lower for microalgae C. 3.3. Degradation rate and kinetic modeling The accumulated methane production curves for the tested microalgae B and C co-digested with undigested sewage sludge E are presented in Fig. 3a–d.
Methane production curves are divided into three stages: lag phase, decomposition phase and flattening phase (Carlsson and Schnürer, 2011). The lag phase is the time from the start of the experiment to the start of methane production. A long lag phase indicates that the material contains high levels of material that is not easily hydrolyzed. All samples except for the pure dry microalgae C had a short lag phase (Fig. 3 a–d), which indicated that microorganisms adapted easily to the conditions and utilized the substrate efficiently. However, digestion of dried microalgae C on its own had a longer lag phase in both mesophilic and thermophilic conditions. These results contradicts the findings made by Wang et al. (2013b) where the lag phase in the batch test of the microalgae slurry as sole feed were 20 days. The decomposition phase followed a similar linear pattern in all the samples, indicating that the substrates were homogenous with no persistent particles as mentioned by Carlsson and Schnürer (2011). The results of the determination of the constants in the kinetic model (Gompertz equation) are shown in Table 3. The Gompertz equation fitted the data well for most of the studied cases and the correlation coefficient, R2, was in the same range as it was shown in previous studies fitting BMP data to the equation (Kavitha et al., 2014; Parameswaran and Rittmann, 2012; Syaichurrozi et al., 2013; Prajapati et al., 2014; Zhu et al., 2009) except for mixture no. 17 and 27 where the R2 value was below 0.96. Mixture no. 17 and 27 are the two mixtures with 100% dried algae (algae C). It can be seen in Fig. 3b and d that the three stages 450.00
450.00
400.00
400.00
350.00 [Ncm3 CH4 g VS-1]
[Ncm3 CH4 g VS-1]
350.00 300.00 250.00 200.00 150.00
9
300.00 250.00 200.00 150.00
10 100.00
100.00
11
12 50.00
50.00
13
0.00
0.00 0
5
10
15
20
25
(a)
30 35 Days
40
45
50
55
0
60
15
20
25
30
35
40
45
60
Days
450.00 400.00
400.00
350.00 [Ncm3 CH4 g VS-1]
350.00 [Ncm3 CH4 g VS-1]
10
(b)
450.00
300.00 250.00 200.00 19 20 21 22 23
150.00 100.00 50.00
300.00 250.00 200.00 150.00 100.00 19 25 27
50.00
0.00
0.00 0
(c)
5
9 14 15 16 17 50 55
5
10
15
20
25
30 35 Days
40
45
50
55
60
0
(d)
5
10
15
20
25
30 35 Days
40
24 26 45
50
55
60
Fig. 3. Methane potential per gram VS for sewage sludge E and its co-substrates with 0%, 12%, 25%, 37%, 100% microalgae for algae substrate B at mesophilic condition (a), algae substrate C at mesophilic condition (b), algae substrate B at thermophilic condition (c), algae substrate C at thermophilic condition (d).
209
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210 Table 3 Constants of the Gompertz equation determined from the experimental data. Mix. comp. no.
Temp. (°C)
Micro-algae (%)
k (d)
BGP (Ncm3 gVS1)
Rm (Ncm3 gVS1 d1)
R2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
37 37 37 37 55 55 55 55 37 37 37 37 37 37 37 37 37 37 55 55 55 55 55 55 55 55 55 55
0 12 25 37 0 12 25 37 0 12 25 37 100 12 25 37 100 0 0 12 25 37 100 12 25 37 100 0
2.1 2.1 3.2 7.6 8.3 8.3 7.3 8.6 3.8 3.9 4.6 4.8 5.1 2.3 2.9 2.8 7.8 10.7 3.7 3.1 3.3 2.5 3.1 4.1 3.4 2.0 1.5 13.7
299 307 256 206 159 113 109 81 332 344 359 409 368 387 349 325 180 408 366 390 339 323 319 324 299 278 141 379
21 25 32 25 15 13.5 13.1 8.5 32 27 30 33 26 26 27 25 11 33 39 44 39 34 31 35 29 27 19 23
0.97 0.98 0.96 0.98 0.98 0.99 0.99 0.99 0.97 0.98 0.98 0.98 0.97 0.97 0.97 0.96 0.94 0.97 0.98 0.98 0.98 0.98 0.97 0.97 0.97 0.97 0.87 0.99
methane production curves normally can be divided into (lag phase, decomposition phase and flattening phase) are not so distinct for those mixtures. 4. Conclusions Microalgae improved the BMP of undigested sewage sludge significantly, particularly in mesophilic conditions. In contrary, the addition of algae affect the BMP negatively in thermophilic conditions. Wet microalgae B, cultivated from samples from lake Mälaren in December 2012, showed the far highest measured BMP in the study when added to sludge in 37/63% ratio. When microalgae were digested alone the short lag-phase indicated that microorganisms in the inocula adapted easily to the conditions and utilized the substrate efficiently. Altogether, this argues for further research on the advantages of using microalgae as a co-substrate in mesophilic anaerobic digestion. Acknowledgements This research has been performed in close collaboration with the end user of the studied system, as a co-production study within the framework of the ACWA-project and the VA-kluster Mälardalen. Most significantly, Knowledge Foundation, Vinnova, SVU, Läckeby Water, Mälarenergi, The Swedish Energy Agency and Processum Biorefinery Initiative should be thanked for their funding, knowledge and development contributions in the study. References Angelidaki, I., Alves, M., Bolzonella, D., Borzacconi, L., Campos, J.L., Guwy, A.J., Kalyuzhnyi, S., Jenicek, P., van Lier, J.B., 2009. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Sci. Technol. 59, 927–934. APHA, 1995. Standard method for the examination of water and wastewater. American Public Health Association, New York. Axelsson, M., Gentili, F., 2014. A single-step method for rapid extraction of total lipids from green microalgae. PLos One 9, 6.
Brune, D.E., Lundquist, T.J., Benemann, J.R., 2009. Microalgal biomass for greenhouse gas reductions: potential for replacement of fossil fuels and animal feeds. J. Environ. Eng., 1136–1144. Carlsson, M., Schnürer, A., 2011. RAPPORT U2011:12 – Handbok metanpotential Avfall Sverige utveckling. In Swedish. Dererie, D.Y., Trobro, S., Momeni, M.H., Hansson, H., Blomqvist, J., Passoth, V., Schnürer, A., Sandgren, M., Ståhlberg, J., 2011. Improved bio-energy yields via sequential ethanol fermentation and biogas digestion of steam exploded oat straw. Bioresour. Technol. 102, 4449–4455. Doria, E., Longoni, P., Scibilia, L., Iazzi, N., Cella, R., Nielsen, E., 2012. Isolation and characterization of a Scenedesmus acutus strain to be used for bioremediation of urban wastewater. J. Appl. Phycol. 24, 375–383. Ficara, E., Uslenghi, A., Basilico, D., Mezzanotte, V., 2014. Growth of microalgal biomass on supernatant from biosolid dewatering. Water Sci. Technol. 69, 896– 902. Formagini, E.L., Marques, F.R., Serejo, M.L., Paulo, P.L., Boncz, M.A., 2014. The use of microalgae and their culture medium for biogas production in an integrated cycle. Water Sci. Technol. 69, 941–946. Frigon, J.-C., Matteau-Lebrun, F., Hamani Abdou, R., McGinn, P.J., O’Leary, S.J.B., Guiot, S.R., 2013. Screening microalgae strains for their productivity in methane following anaerobic digestion. Appl. Energy 108, 100–107. Karlsson, A., Einarsson, P., Schnürer, A., Sundberg, C., Ejlertsson, J., Svensson, B.H., 2012. Impact of trace element addition on degradation efficiency of volatile fatty acids, oleic acid and phenyl acetate and on microbial populations in a biogas digester. J. Biosci. Bioeng. 114, 446–452. Kavitha, S., Jayashree, C., Adish Kumar, S., Tae Yeom, I., Rajesh Banu, J., 2014. The enhancement of anaerobic biodegradability of waste activated sludge by surfactant mediated biological pretreatment. Bioresour. Technol. 168, 159–166. Khalid, A., Arshad, M., Anjum, M., Mahmood, T., Dawson, L., 2011. The anaerobic digestion of solid organic waste. Waste Manage. 31, 1737–1744. Krustok, I., Nehrenheim, E., Odlare, M., Liu, X., Li, S., 2013. Cultivation of indigenous algae for increased biogas production. International Conference on Appl. Energy, Preotria, South Africa 1–4 July 2013. Larsdotter, K., 2006. Wastewater treatment with microalgae – a literature review. VATTEN 62, 31–38. Ma, X., Zhou, W., Fu, Z., Cheng, Y., Min, M., Liu, Y., Zhang, Y., Chen, P., Ruan, R., 2014. Effect of wastewater-borne bacteria on algal growth and nutrients removal in wastewater-based algae cultivation system. Bioresour. Technol. 167, 8–13. Mata-Alvarez, J., Dosta, J., Macé, S., Astals, S., 2011. Codigestion of solid wastes: a review of its uses and perspectives including modeling. Crit. Rev. Biotechnol. 31, 99–111. Mussgnug, J.H., Klassen, V., Schlüter, A., Kruse, O., 2010. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotech. 150, 51–56. NMKL, 1989. Fat. Determination according to SBR in meat and meat products, Vol. 131, Nordic Commitee on Food Analysis. Odlare, M., Nehrenheim, E., Ribé, V., Thorin, E., Gavare, M., Grube, M., 2011. Cultivation of algae with indigenous species – potentials for regional biofuel production. Appl. Energy 88, 3280–3285.
210
J. Olsson et al. / Bioresource Technology 171 (2014) 203–210
Parameswaran, P., Rittmann, B.E., 2012. Feasibility of anaerobic co-digestion of pig waste and paper sludge. Bioresour. Technol. 124, 163–168. Prajapati, S.K., Kumar, P., Malik, A., Vijay, V.K., 2014. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: a closed loop bioenergy generation process. Bioresour. Technol. 158, 174–180. Rusten, B., Sahu, A., 2011. Microalgae growth for nutrient recovery from sludge liquor and production of renewable bioenergy. Water Sci. Technol. 64, 1195– 1201. Salo-väänänen, P.P., Koivistoinen, P.E., 1996. Determination of protein in foods: comparison of net protein and crude protein (N 6.25) values. Food Chem. 57, 27–31. Samson, R., Leduyt, A., 1986. Detailed study of anaerobic digestion of Spirulina maxima algal biomass. Biotechnol. Bioeng. 28, 1014–1023. Schwede, S., Kowalczyk, A., Gerber, M., Span, R., 2013. Anaerobic co-digestion of the marine microalga Nannochloropsis salina with energy crops. Bioresour. Technol. 148, 428–435. Su, Y., Mennerich, A., Urban, B., 2011. Municipal wastewater treatment and biomass accumulation with a wastewater-born and settleable algal-bacterial culture. Water Res. 45, 3351–3358.
Syaichurrozi, I., Budiyono, Sumardiono, S., 2013. Predicting kinetic model of biogas production and biodegradability organic materials: biogas production from vinasse at variation of COD/N ratio. Bioresour. Technol. 149, 390–397. Tchobanoglous, G., Burton, L.F., 2002. Wastewater Engineering: Treatment and Reuse, fourth ed. McGraw & Hill. Wang, H., Hu, Z., Xiao, B., Cheng, Q., Li, F., 2013a. Ammonium nitrogen removal in batch cultures treating digested piggery wastewater with microalgae Oedogonium sp.. Water Sci. Technol. 68, 269–275. Wang, M., Sahu, K.A., Björn, R., Chul, P., 2013b. Anaerobic co-digestion of microalgae Chlorella sp. and waste activated sludge. Bioresour. Technol. 142, 585–590. Wiley, P.E., Campell, J.E., McKuin, B., 2011. Production of biodiesel and biogas from algae: a review of process train options. Water Environ. Res. 82, 326–338. Yen, H., Brune, D., 2007. Anaerobic co-digestion of algal sludge and waste paper to produce methane. Bioresour. Technol. 98, 130–134. Zhu, B., Gikas, P., Zhang, R., Lord, J., Jenkins, B., Li, X., 2009. Characteristics and biogas production potential of municipal solid wastes pretreated with a rotary drum reactor. Bioresour. Technol. 100, 1122–1129.
