- Email: [email protected]
Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry
Waste Management xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry Steven Nartker a,⇑, Michelle Ammerman a, Jennifer Aurandt a, Michael Stogsdil a, Olivia Hayden b, Chad Antle b a b
Kettering University, 1700 University Avenue, Flint, MI 48504, USA Swedish Biogas International, 1300 Bluff Street, Flint, MI 48504, USA
a r t i c l e
i n f o
Article history: Received 3 June 2014 Accepted 19 August 2014 Available online xxxx Keywords: Glycerol Anaerobic digestion Sewage sludge Biogas Co-digestion
a b s t r a c t In an effort to convert waste streams to energy in a green process, glycerol from biodiesel manufacturing has been used to increase the gas production and methane content of biogas within a mesophilic anaerobic co-digestion process using primary sewage sludge. Glycerol was systematically added to the primary digester from 0% to 60% of the organic loading rate (OLR). The optimum glycerol loading range was from 25% to 60% OLR. This resulted in an 82–280% improvement in specific gas production. Following the feeding schedule described, the digesters remained balanced and healthy until inhibition was achieved at 70% glycerol OLR. This suggests that high glycerol loadings are possible if slow additions are upheld in order to allow the bacterial community to adjust properly. Waste water treatment plant operators with anaerobic digesters can use the data to increase loadings and boost biogas production to enhance energy conversion. This process provides a safe, environmentally friendly method to convert a typical waste stream to an energy stream of biogas. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion (AD) is a multistage process in which microorganisms breakdown biodegradable material in the absence of oxygen. The AD process starts with hydrolysis of the input materials. During this phase, bacteria convert insoluble organic polymers such as carbohydrates to soluble derivatives. Then acidogenic bacteria convert the soluble sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids. Next acetogenic bacteria create acetic acid, ammonia, carbon dioxide and hydrogen from the fermentation products of the previous step. Finally methanogens convert the products of acidogenesis and acetogenesis to methane (Grady Jr et al., 2011). AD is a practical method for degrading and stabilizing primary sewage sludge prior to its disposal (Fountoulakis et al., 2010). In order for a sewage AD facility to operate more economically and efficiently, the production of biogas must increase, which can be achieved through a co-digestion process. Anaerobic co-digestion (AcoD) consists of a mixture of two or more substrates with complementary ⇑ Corresponding author. Tel.: +1 810 249 4041. E-mail addresses: [email protected] (S. Nartker), mammerman@kettering. edu (M. Ammerman), [email protected] (J. Aurandt), [email protected] (M. Stogsdil), [email protected] (O. Hayden), Chad.Antle@ swedishbiogas.com (C. Antle).
characteristics, and has proven to be a reliable option for increasing methane yield (Holm-Nielsen et al., 2008). When sewage sludge is combined with highly concentrated organic co-substrates, such as food waste, agricultural waste or crude glycerol from the biodiesel industry, biogas output and organic matter removal can be improved without sacrificing reactor stability or health (Astals et al., 2011). The AD process is a green method that can sustainably convert waste to energy. The main by-product of biodiesel production is crude glycerol, which is about 10% by weight of the starting materials. Crude glycerol is a mixture of glycerol, alcohol, water, salts, heavy metals, free fatty acids, unreacted mono-, di- and tri-glycerides and methyl esters (Hu et al., 2012). Co-digestion of glycerol with sewage sludge is a promising solution, since a renewable source of energy is obtained from the treatment. Several successful studies, in batch and continuously stirred reactor experiments, have been published with reference to the benefits of the addition of glycerol to enhance the AD of agro-wastes (Amon et al., 2006; Kacprzak et al., 2009), cattle manure (Chen et al., 2008; Robra et al., 2010) fruit and vegetable wastes (Astals et al., 2011; Bouallagui, 2003) organic fraction of municipal solid waste – OFMSW (Fountoulakis and Manios, 2009), pig manure (Alvarez et al., 2010; Amon et al., 2006; Astals et al., 2011; Galí et al., 2009), sewage sludge (Fountoulakis et al., 2010), mixture of pig manure and OFMSW (Schievano et al.,
http://dx.doi.org/10.1016/j.wasman.2014.08.017 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
2
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
2009), mixture of olive mill and slaughterhouse wastewaters (Fountoulakis and Manios, 2009) and mixture of manure and organic industrial wastes (Holm-Nielsen et al., 2009). The amounts are variable and depend on the quality of the feed material and the chemical process used to obtain the biodiesel (Pagliaro et al., 2008; Robra et al., 2010). In certain markets, crude glycerol can be sold depending on purity and availability (Johnson and Taconi, 2007). In other markets, the glycerol has to be disposed of as waste due to market saturation; excessive treatment or refining costs (Pachauri and He, 2006); and lack of direct uses (Pagliaro et al., 2008). The utilization of crude glycerol in AD could benefit the crude glycerol producers as well as the AD operators. Using glycerol as the co-digestate in municipal waste digesters has shown significant increases in biogas production (Fountoulakis et al., 2010). However in order to maintain a stable digestion process, the amount of glycerol added needs to be limited to a certain concentration level. Recommendations for glycerol loading remain low, and vary from 0.05 wt% to 1 wt% to avoid the risk of organic overloading (Fountoulakis et al., 2010; Holm-Nielsen et al., 2008). The purpose of this study was to evaluate the use of soybean derived crude glycerol as a co-substrate in order to increase biogas production of an anaerobic sewage sludge digester. The effect of glycerol addition on the methane yield was determined in continuously stirred tank reactor experiments. Glycerol load was slowly increased in the reactors providing the bacterial community in the AD time to acclimate to the new food source. From these experiments, the optimum and maximum concentration of glycerol that can be added to the AD for co-digestion was estimated based on reactor stability and health parameters, which are monitored by analysis of volatile fatty acids (VFAs), pH, and alkalinity. The TS/VS of the digestate was continuously monitored to ensure that OLR levels were maintained. 2. Materials 2.1. Substrates The samples consisted of crude glycerol and primary sewage sludge (primary). The crude glycerol was derived from local biodiesel manufacturing that used soybean oil as its primary source of raw materials. This crude glycerol contained glycerol, fatty acids, methanol, salts and water. The primary sewage sludge, which is untreated municipal waste and digestate (seed), the material remaining after the anaerobic digestion of the waste, were obtained from the Water Pollution Control Authority in Flint, Michigan. The substrates were initially characterized by determining total and volatile solids (TS/VS) as well as pH. Total solids (TS) were determined by drying at 105 °C for 24 h, and then volatile solids (VS) were found by placing those samples in a 550 °C for 1 h in accordance with APHA standard method 2540 (APHA, 2005). The pH was measured using a Hach SensION3 pH meter. The methanol content was determined by thermal gravimetric analysis and the glycerol content by using GC. These results for the BMP samples are shown in Table 1. 2.2. Biomethane potential An AMPTS Biomethane Potential (BMP) Test system (Bioprocess Control) was used to initially characterize glycerol and sewage Table 1 Substrate testing. Sample
TS (%)
VS (% of TS)
pH
Methanol (wt%)
Glycerol (wt%)
Digestate Glycerol
4.85 78.24
61.68 95.03
7.1 10.4
– 5.05
– 46.5
sludge co-digestion. A test series of three replicates was carried out in sealed glass bottles (500 ml) for the crude glycerol co-digested with digestate, which was the liquid material leftover from the anaerobic treatment of sewage sludge at the waste water treatment plant (WWTP), and was compared to a reference series of digestate alone in three replicates. All bottles were loaded with a 2:1 VS (w/w) digestate to substrate ratio to a total of 400 g and placed in a 37 °C water bath throughout the experiment. A 3M NaOH solution was used for scrubbing out all CO2 from the produced biogas. Once the gas passed through the NaOH scrubbers, the remaining methane entered a calibrated flow cell which recorded the gas production in real time by sending data to the AMPTS software. Prior to the start of the analysis, the system is flushed with nitrogen. The BMP reactors were operated for 30 days. From these experiments the methane production from the glycerol waste and digestate from the WWTP was obtained. 2.3. Continuously Stirred Tank Reactors Two Continuously Stirred Tank Reactors (CSTRs) were set up; a control CSTR was run using primary as the only feed and this was compared to a co-digestion CSTR that was run using primary and increasing amounts of glycerol. Reactors were run on a 32 day hydraulic retention time (HRT) determined by Eq. (1) where V is the reactor volume in ml and Q is the daily input flow in ml/day.
HRT ¼ V=Q
ð1Þ
Reactors were maintained by feeding 125 ml of new substrate daily and removing digestate to keep the operating volume at 4 L. The test CSTR began with 1 g of glycerol (with 124 g of primary) and was increased to 10 g, with one additional gram being added every 3–4 weeks. The feeding schedule shown in Table 2 was followed. The stepwise increase in glycerol addition was done to allow sufficient time to buffer changes in primary, and to allow the microorganisms to adjust to new feed stock. Each new batch of primary required TS/VS analysis to determine the OLR. OLR was determined using Eq. (2), where M is total daily mass input flow in kilograms, TS and VS are percentages based on substrate and V is reactor volume in cubic meters.
OLR ¼ ðM TS VSÞ=V
ð2Þ
The glycerol content was increased from 19% to 69% of the total OLR. The digesters were run until the glycerol loading became too large (70 wt% of the total OLR), and the reactor could no longer sustain gas production. This was done to stress test the digester to determine maximum glycerol loading. The gas production from the glycerol waste and primary sludge were compared throughout the test period. The reactors were maintained at 37 °C throughout the experiment. The reactors were operated for approximately 9 months. 2.4. Analyses Biogas that was produced in the CSTRs flowed through calibrated Ritter wet test meters that continuously measured the biogas production. The amount of gas produced was recorded daily to understand the impact of crude glycerol on the co-digestion. Weekly gas analysis was performed to ascertain the methane content in the biogas, which was expected to vary with crude glycerol loading and primary composition. Due to variations in primary composition, the primary was subtracted from primary and glycerol to understand the contributions of glycerol alone as shown in Fig. 2. Gas analysis was done using a Perkin Elmer Clarus 600 Gas Chromatograph equipped with Perkin Elmer Elite-PLOT Q column and a Thermal Conductivity Detector. Reactor health is defined by key variables that determine stable operating
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
3
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx Table 2 Feeding schedule for the CSTRs.
Table 3 CSTR health parameters during the experiment and the final analysis.
Glycerol load (grams)
Total OLR (kg VS/m3 d)
Glycerol in OLR (%)
Primary in OLR (%)
Days
1 2 3 4 5 6 7 8 9 10
0.98 1.26 1.70 1.67 1.90 1.88 2.09 2.54 2.42 2.88
19 30 33 45 49 59 64 61 69 65
81 70 67 55 51 41 36 39 31 35
36 19 35 22 18 25 27 28 26 8
Primary
Primary + glycerol
Overall range (excluding final analysis) pH range during experiment Average pH Alkalinity range during experiment (g/L) Alkalinity average (g/L)
7.24–7.55 7.34 (±0.08) 1.9–4.2 3.0 (± 0.5)
7.12–7.53 7.29 (±0.25) 1.7–3.6 2.7 (±1.2)
Final analysis pH Alkalinity (g/L) OLR % OLR from glycerol
7.3 2.6 1.11 0
5.98 1.5 2.88 65
of glycerol co-digestion with digestate, as compared to digestate alone. The hydraulic retention time for co-digestion was determined to be 32 days using Eq. (1) and when the glycerol was loaded at 33% of the total OLR of the reactor, no inhibitory effects were evident. The maximum Biological Methane Potential (BMP) for glycerol co-digested with digestate samples was 766 ± 42 ml/gVS as shown in Fig. 1. The digestate alone reached a maximum methane production of 112 ± 14 ml/gVS. The average difference between the two samples was 608 ml/gVS, which is 7 times as much as the gas production of digestate alone. This indicates that glycerol adds significant methane production when co-digested with digestate, and does not show short term toxicity effects when loaded at 33% of the total OLR. Crude glycerol co-digestion with various substrates has been reported to increase gas output in BMP and CSTR tests (Astals et al., 2011; Heaven et al., 2011).
conditions as shown in Table 3. Reactor health was monitored through weekly analysis of volatile fatty acids (VFAs), pH, alkalinity, and TS/VS on the digestate. The pH was tested using a calibrated Hach SensION 3 pH meter, alkalinity analysis was performed using Hach digital titrator, and VFAs were analyzed using the Perkin Elmer Clarus 600 Gas Chromatograph equipped with a Perkin Elmer Elite-FFAP column with a 5 m guard column and a flame ionization detector (FID). 3. Results and discussion 3.1. Increased biogas production – BMP BMP analysis was initially used to determine hydraulic retention time (HRT), potential inhibitory effects, and biogas potential
3.2. Increased gas output – CSTRs After the BMP tests were performed; analyses of the glycerol co-digestion systems were moved to 4 L continuously stirred tank reactors (CSTRs). Co-digestion in the CSTRs was done using primary sewage sludge as the substrate material. The CSTRs allowed for continuous gas output monitoring using wet test meters and gas composition analysis by collecting samples in Tedlar bags weekly and performing GC analysis of the biogas. The CSTR also allows for variation of glycerol and primary sludge loading. This data along with digestate and co-digestate monitoring, determined the range of maximum benefit for addition of glycerol to the system. The addition of increasing amounts of glycerol resulted in a corresponding increase in gas production (Fig. 2). The specific gas production increased from 0.35 to 0.92 m3/kg VS d. Fig. 2
Fig. 1. Average BMP results for glycerol and primary sludge without glycerol.
Primary
Specific Gas Producon (m3/kg VS*d)
Primary + Glycerol
1
Glycerol Alone (E-C)
0.9 0.8
m3/kg VS*d
0.7 0.6 0.5 0.4 0.3 0.2
10
9
8
7
6
5
4
3
2
1
0.5
0
0
0.1
Grams of Glycerol Fig. 2. Increased specific gas production using glycerol as a co-digestion substrate.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
4
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
Fig. 3. Increased methane content of primary with indicated amounts of glycerol versus primary sludge.
Biogas Producon Increases with Glycerol Load 3.5
9000 8000
3
7000 6000 2
5000 4000
1.5
3000
Gas Producon (ml/day)
Total OLR (kg VS/m3d)
2.5
Primary in OLR Glycerol in OLR average Gas Flow
1 2000 0.5
0
1000
1
2
3
4
5
6
7
8
9
10
0
Glycerol Loading (grams) Fig. 4. Gas production increases as the percentage of glycerol in the total OLR increases.
shows that beginning with the addition of 2 g of glycerol, or 30% of the OLR as described in Table 2, addition of glycerol to the digester produces more biogas than primary alone; which would increase the operating efficiency of the digester and plant by providing a higher flow of gas. The maximum gain in gas production, the difference between the primary and the co-digested glycerol and primary, was observed in the 4–9 g of glycerol loading range representing 45– 69% of OLR. The overall maximum benefit of glycerol addition was observed at 7 g of glycerol loading (64% glycerol in OLR as shown in Table 2) due to enhanced methane content as compared to primary alone. This maximum benefit is found by considering the balance between the amounts of gas produced (Fig. 2), the methane content of the produced gas (Fig. 3) and the amount of glycerol required to obtain this enrichment. The methane content was variable, but after the startup instabilities, there was an upward trend in biogas content from 5 g of glycerol loading to 10 g as shown in Fig. 4. It is important to note that variations in primary sludge, a product of weather conditions, TS/VS and other WWTP issues were significant and showed an effect on the gas production throughout the experiment. The maximum benefit range was determined to be between 25% glycerol OLR to 60%.
Glycerol load was varied from 0% to 70% of the total OLR, but the concentration was dependent upon the primary sewage sludge, which showed inconsistency. This altered the actual amounts of organics available for the digestion process. Fig. 4 shows the increasing glycerol loading and general primary load reducing trend. It is also evident in this figure that the increased gas production rate is due to the increased glycerol loading. After loading at 10 g of glycerol (70% OLR), the reactor became unstable and did not produce biogas due to propionic acid accumulation which became inhibitory. 4. Conclusions Waste glycerol has been used to increase the gas production and methane content of biogas within an anaerobic co-digestion process using primary sewage sludge. The increase in biogas and consequently energy production is significant, with a 25% OLR glycerol load; the biogas production increased 82% as compared to a digester operating on primary sewage sludge alone. This data is meant to give operators at a WWTP confidence in co-digestion with glycerol as long as the optimized range of 25% OLR to 60% OLR is maintained. Without co-digestion of glycerol, a typical
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
WWTP digesting primary sludge could install a modest engine generator set. With the addition of glycerol in the optimized range of 25% OLR to 50% OLR, the WWTP would be able to produce approximately twice the electrical power using 25% glycerol loading and four times the kW rating using 50% loading. Acknowledgments This research was financially supported by the Michigan Soybean Promotion Committee. Special acknowledgement to Chad Antle at Swedish Biogas International LLC for continuous support. References Alvarez, J.A., Otero, L., Lema, J.M., 2010. A methodology for optimising feed composition for anaerobic co-digestion of agro-industrial wastes. Bioresour. Technol. 101 (4), 1153–1158. http://dx.doi.org/10.1016/j.biortech.2009.09.061. Amon, T., Amon, B., Kryvoruchko, V., Bodiroza, V., Pötsch, E., Zollitsch, W., 2006. Optimising methane yield from anaerobic digestion of manure: effects of dairy systems and of glycerine supplementation. Int. Congr. Ser. 1293, 217–220. http://dx.doi.org/10.1016/j.ics.2006.03.007. APHA, 2005. Standard methods for the examination of water & wastewater: centennial edition. In: Eaton, A.D. (Ed.), Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association. Astals, S., Ariso, M., Galí, A., Mata-Alvarez, J., 2011. Co-digestion of pig manure and glycerine: experimental and modelling study. J. Environ. Manage. 92 (4), 1091– 1096. http://dx.doi.org/10.1016/j.jenvman.2010.11.014. Bouallagui, H., 2003. Mesophilic biogas production from fruit and vegetable waste in a tubular digester. Bioresour. Technol. 86 (1), 85–89. http://dx.doi.org/ 10.1016/S0960-8524(02)00097-4. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99 (10), 4044–4064. http://dx.doi.org/10.1016/ j.biortech.2007.01.057. Fountoulakis, M.S., Manios, T., 2009. Enhanced methane and hydrogen production from municipal solid waste and agro-industrial by-products co-digested with crude glycerol. Bioresour. Technol. 100 (12), 3043–3047. http://dx.doi.org/ 10.1016/j.biortech.2009.01.016.
5
Fountoulakis, M.S., Petousi, I., Manios, T., 2010. Co-digestion of sewage sludge with glycerol to boost biogas production. Waste Manage. 30 (10), 1849–1853. http:// dx.doi.org/10.1016/j.wasman.2010.04.011. Galí, A., Benabdallah, T., Astals, S., Mata-Alvarez, J., 2009. Modified version of ADM1 model for agro-waste application. Bioresour. Technol. 100 (11), 2783–2790. http://dx.doi.org/10.1016/j.biortech.2008.12.052. Grady Jr, C.P.L., Daigger, G.T., Love, N.G., Filipe, C.D.M., 2011. Biological Wastewater Treatment 63, 632. Heaven, S., Salter, A.M., Banks, C.J., 2011. Integration of on-farm biodiesel production with anaerobic digestion to maximise energy yield and greenhouse gas savings from process and farm residues. Bioresour. Technol. 102 (17), 7784–7793. http://dx.doi.org/10.1016/j.biortech.2011.06.029. Holm-Nielsen, J.B., Lomborg, C.J., Oleskowicz-Popiel, P., Esbensen, K.H., 2008. Online near infrared monitoring of glycerol-boosted anaerobic digestion processes: evaluation of process analytical technologies. Biotechnol. Bioeng. 99 (2), 302–313. http://dx.doi.org/10.1002/bit.21571. Holm-Nielsen, J.B., Al Seadi, T., Oleskowicz-Popiel, P., 2009. The future of anaerobic digestion and biogas utilization. Bioresour. Technol. 100 (22), 5478–5484. http://dx.doi.org/10.1016/j.biortech.2008.12.046. Hu, S., Luo, X., Wan, C., Li, Y., 2012. Characterization of crude glycerol from biodiesel plants. J. Agri. Food Chem. 60 (23), 5915–5921. http://dx.doi.org/10.1021/ jf3008629. Johnson, D.T., Taconi, K.a., 2007. The Glycerin Glut: options for the value-added conversion of crude glycerol resulting from biodiesel production. Environ. Prog. 26 (4), 338–348. http://dx.doi.org/10.1002/ep. Kacprzak, A., Krzystek, L., Ledakowicz, S., 2009. Co-digestion of agricultural and industrial wastes. Chem. Pap. 64 (2), 127–131. http://dx.doi.org/10.2478/ s11696-009-0108-5. Pachauri, N., He, B., 2006. Value-added utilization of crude glycerol from biodiesel production: a survey of current research activities. Written for Presentation at the 2006 ASABE Annual International Meeting Sponsored by ASABE Glycerol Utilization for Specific Products, 0300(06). Pagliaro, M., Rossi, M., Clark, J.H, Kraus, G.A, 2008. Future of glycerol. The Royal Society of Chemistry, pp. P001–P127. http://dx.doi.org/10.1039/ 9781847558305. Robra, S., Serpa da Cruz, R., de Oliveira, A.M., Neto, J.A.A., Santos, J.V., 2010. Generation of biogas using crude glycerin from biodiesel production as a supplement to cattle slurry. Biomass Bioenergy 34 (9), 1330–1335. http:// dx.doi.org/10.1016/j.biombioe.2010.04.021. Schievano, A., D’Imporzano, G., Adani, F., 2009. Substituting energy crops with organic wastes and agro-industrial residues for biogas production. J. Environ. Manage. 90 (8), 2537–2541. http://dx.doi.org/10.1016/j.jenvman.2009.01.013.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry Steven Nartker a,⇑, Michelle Ammerman a, Jennifer Aurandt a, Michael Stogsdil a, Olivia Hayden b, Chad Antle b a b
Kettering University, 1700 University Avenue, Flint, MI 48504, USA Swedish Biogas International, 1300 Bluff Street, Flint, MI 48504, USA
a r t i c l e
i n f o
Article history: Received 3 June 2014 Accepted 19 August 2014 Available online xxxx Keywords: Glycerol Anaerobic digestion Sewage sludge Biogas Co-digestion
a b s t r a c t In an effort to convert waste streams to energy in a green process, glycerol from biodiesel manufacturing has been used to increase the gas production and methane content of biogas within a mesophilic anaerobic co-digestion process using primary sewage sludge. Glycerol was systematically added to the primary digester from 0% to 60% of the organic loading rate (OLR). The optimum glycerol loading range was from 25% to 60% OLR. This resulted in an 82–280% improvement in specific gas production. Following the feeding schedule described, the digesters remained balanced and healthy until inhibition was achieved at 70% glycerol OLR. This suggests that high glycerol loadings are possible if slow additions are upheld in order to allow the bacterial community to adjust properly. Waste water treatment plant operators with anaerobic digesters can use the data to increase loadings and boost biogas production to enhance energy conversion. This process provides a safe, environmentally friendly method to convert a typical waste stream to an energy stream of biogas. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion (AD) is a multistage process in which microorganisms breakdown biodegradable material in the absence of oxygen. The AD process starts with hydrolysis of the input materials. During this phase, bacteria convert insoluble organic polymers such as carbohydrates to soluble derivatives. Then acidogenic bacteria convert the soluble sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids. Next acetogenic bacteria create acetic acid, ammonia, carbon dioxide and hydrogen from the fermentation products of the previous step. Finally methanogens convert the products of acidogenesis and acetogenesis to methane (Grady Jr et al., 2011). AD is a practical method for degrading and stabilizing primary sewage sludge prior to its disposal (Fountoulakis et al., 2010). In order for a sewage AD facility to operate more economically and efficiently, the production of biogas must increase, which can be achieved through a co-digestion process. Anaerobic co-digestion (AcoD) consists of a mixture of two or more substrates with complementary ⇑ Corresponding author. Tel.: +1 810 249 4041. E-mail addresses: [email protected] (S. Nartker), mammerman@kettering. edu (M. Ammerman), [email protected] (J. Aurandt), [email protected] (M. Stogsdil), [email protected] (O. Hayden), Chad.Antle@ swedishbiogas.com (C. Antle).
characteristics, and has proven to be a reliable option for increasing methane yield (Holm-Nielsen et al., 2008). When sewage sludge is combined with highly concentrated organic co-substrates, such as food waste, agricultural waste or crude glycerol from the biodiesel industry, biogas output and organic matter removal can be improved without sacrificing reactor stability or health (Astals et al., 2011). The AD process is a green method that can sustainably convert waste to energy. The main by-product of biodiesel production is crude glycerol, which is about 10% by weight of the starting materials. Crude glycerol is a mixture of glycerol, alcohol, water, salts, heavy metals, free fatty acids, unreacted mono-, di- and tri-glycerides and methyl esters (Hu et al., 2012). Co-digestion of glycerol with sewage sludge is a promising solution, since a renewable source of energy is obtained from the treatment. Several successful studies, in batch and continuously stirred reactor experiments, have been published with reference to the benefits of the addition of glycerol to enhance the AD of agro-wastes (Amon et al., 2006; Kacprzak et al., 2009), cattle manure (Chen et al., 2008; Robra et al., 2010) fruit and vegetable wastes (Astals et al., 2011; Bouallagui, 2003) organic fraction of municipal solid waste – OFMSW (Fountoulakis and Manios, 2009), pig manure (Alvarez et al., 2010; Amon et al., 2006; Astals et al., 2011; Galí et al., 2009), sewage sludge (Fountoulakis et al., 2010), mixture of pig manure and OFMSW (Schievano et al.,
http://dx.doi.org/10.1016/j.wasman.2014.08.017 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
2
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
2009), mixture of olive mill and slaughterhouse wastewaters (Fountoulakis and Manios, 2009) and mixture of manure and organic industrial wastes (Holm-Nielsen et al., 2009). The amounts are variable and depend on the quality of the feed material and the chemical process used to obtain the biodiesel (Pagliaro et al., 2008; Robra et al., 2010). In certain markets, crude glycerol can be sold depending on purity and availability (Johnson and Taconi, 2007). In other markets, the glycerol has to be disposed of as waste due to market saturation; excessive treatment or refining costs (Pachauri and He, 2006); and lack of direct uses (Pagliaro et al., 2008). The utilization of crude glycerol in AD could benefit the crude glycerol producers as well as the AD operators. Using glycerol as the co-digestate in municipal waste digesters has shown significant increases in biogas production (Fountoulakis et al., 2010). However in order to maintain a stable digestion process, the amount of glycerol added needs to be limited to a certain concentration level. Recommendations for glycerol loading remain low, and vary from 0.05 wt% to 1 wt% to avoid the risk of organic overloading (Fountoulakis et al., 2010; Holm-Nielsen et al., 2008). The purpose of this study was to evaluate the use of soybean derived crude glycerol as a co-substrate in order to increase biogas production of an anaerobic sewage sludge digester. The effect of glycerol addition on the methane yield was determined in continuously stirred tank reactor experiments. Glycerol load was slowly increased in the reactors providing the bacterial community in the AD time to acclimate to the new food source. From these experiments, the optimum and maximum concentration of glycerol that can be added to the AD for co-digestion was estimated based on reactor stability and health parameters, which are monitored by analysis of volatile fatty acids (VFAs), pH, and alkalinity. The TS/VS of the digestate was continuously monitored to ensure that OLR levels were maintained. 2. Materials 2.1. Substrates The samples consisted of crude glycerol and primary sewage sludge (primary). The crude glycerol was derived from local biodiesel manufacturing that used soybean oil as its primary source of raw materials. This crude glycerol contained glycerol, fatty acids, methanol, salts and water. The primary sewage sludge, which is untreated municipal waste and digestate (seed), the material remaining after the anaerobic digestion of the waste, were obtained from the Water Pollution Control Authority in Flint, Michigan. The substrates were initially characterized by determining total and volatile solids (TS/VS) as well as pH. Total solids (TS) were determined by drying at 105 °C for 24 h, and then volatile solids (VS) were found by placing those samples in a 550 °C for 1 h in accordance with APHA standard method 2540 (APHA, 2005). The pH was measured using a Hach SensION3 pH meter. The methanol content was determined by thermal gravimetric analysis and the glycerol content by using GC. These results for the BMP samples are shown in Table 1. 2.2. Biomethane potential An AMPTS Biomethane Potential (BMP) Test system (Bioprocess Control) was used to initially characterize glycerol and sewage Table 1 Substrate testing. Sample
TS (%)
VS (% of TS)
pH
Methanol (wt%)
Glycerol (wt%)
Digestate Glycerol
4.85 78.24
61.68 95.03
7.1 10.4
– 5.05
– 46.5
sludge co-digestion. A test series of three replicates was carried out in sealed glass bottles (500 ml) for the crude glycerol co-digested with digestate, which was the liquid material leftover from the anaerobic treatment of sewage sludge at the waste water treatment plant (WWTP), and was compared to a reference series of digestate alone in three replicates. All bottles were loaded with a 2:1 VS (w/w) digestate to substrate ratio to a total of 400 g and placed in a 37 °C water bath throughout the experiment. A 3M NaOH solution was used for scrubbing out all CO2 from the produced biogas. Once the gas passed through the NaOH scrubbers, the remaining methane entered a calibrated flow cell which recorded the gas production in real time by sending data to the AMPTS software. Prior to the start of the analysis, the system is flushed with nitrogen. The BMP reactors were operated for 30 days. From these experiments the methane production from the glycerol waste and digestate from the WWTP was obtained. 2.3. Continuously Stirred Tank Reactors Two Continuously Stirred Tank Reactors (CSTRs) were set up; a control CSTR was run using primary as the only feed and this was compared to a co-digestion CSTR that was run using primary and increasing amounts of glycerol. Reactors were run on a 32 day hydraulic retention time (HRT) determined by Eq. (1) where V is the reactor volume in ml and Q is the daily input flow in ml/day.
HRT ¼ V=Q
ð1Þ
Reactors were maintained by feeding 125 ml of new substrate daily and removing digestate to keep the operating volume at 4 L. The test CSTR began with 1 g of glycerol (with 124 g of primary) and was increased to 10 g, with one additional gram being added every 3–4 weeks. The feeding schedule shown in Table 2 was followed. The stepwise increase in glycerol addition was done to allow sufficient time to buffer changes in primary, and to allow the microorganisms to adjust to new feed stock. Each new batch of primary required TS/VS analysis to determine the OLR. OLR was determined using Eq. (2), where M is total daily mass input flow in kilograms, TS and VS are percentages based on substrate and V is reactor volume in cubic meters.
OLR ¼ ðM TS VSÞ=V
ð2Þ
The glycerol content was increased from 19% to 69% of the total OLR. The digesters were run until the glycerol loading became too large (70 wt% of the total OLR), and the reactor could no longer sustain gas production. This was done to stress test the digester to determine maximum glycerol loading. The gas production from the glycerol waste and primary sludge were compared throughout the test period. The reactors were maintained at 37 °C throughout the experiment. The reactors were operated for approximately 9 months. 2.4. Analyses Biogas that was produced in the CSTRs flowed through calibrated Ritter wet test meters that continuously measured the biogas production. The amount of gas produced was recorded daily to understand the impact of crude glycerol on the co-digestion. Weekly gas analysis was performed to ascertain the methane content in the biogas, which was expected to vary with crude glycerol loading and primary composition. Due to variations in primary composition, the primary was subtracted from primary and glycerol to understand the contributions of glycerol alone as shown in Fig. 2. Gas analysis was done using a Perkin Elmer Clarus 600 Gas Chromatograph equipped with Perkin Elmer Elite-PLOT Q column and a Thermal Conductivity Detector. Reactor health is defined by key variables that determine stable operating
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
3
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx Table 2 Feeding schedule for the CSTRs.
Table 3 CSTR health parameters during the experiment and the final analysis.
Glycerol load (grams)
Total OLR (kg VS/m3 d)
Glycerol in OLR (%)
Primary in OLR (%)
Days
1 2 3 4 5 6 7 8 9 10
0.98 1.26 1.70 1.67 1.90 1.88 2.09 2.54 2.42 2.88
19 30 33 45 49 59 64 61 69 65
81 70 67 55 51 41 36 39 31 35
36 19 35 22 18 25 27 28 26 8
Primary
Primary + glycerol
Overall range (excluding final analysis) pH range during experiment Average pH Alkalinity range during experiment (g/L) Alkalinity average (g/L)
7.24–7.55 7.34 (±0.08) 1.9–4.2 3.0 (± 0.5)
7.12–7.53 7.29 (±0.25) 1.7–3.6 2.7 (±1.2)
Final analysis pH Alkalinity (g/L) OLR % OLR from glycerol
7.3 2.6 1.11 0
5.98 1.5 2.88 65
of glycerol co-digestion with digestate, as compared to digestate alone. The hydraulic retention time for co-digestion was determined to be 32 days using Eq. (1) and when the glycerol was loaded at 33% of the total OLR of the reactor, no inhibitory effects were evident. The maximum Biological Methane Potential (BMP) for glycerol co-digested with digestate samples was 766 ± 42 ml/gVS as shown in Fig. 1. The digestate alone reached a maximum methane production of 112 ± 14 ml/gVS. The average difference between the two samples was 608 ml/gVS, which is 7 times as much as the gas production of digestate alone. This indicates that glycerol adds significant methane production when co-digested with digestate, and does not show short term toxicity effects when loaded at 33% of the total OLR. Crude glycerol co-digestion with various substrates has been reported to increase gas output in BMP and CSTR tests (Astals et al., 2011; Heaven et al., 2011).
conditions as shown in Table 3. Reactor health was monitored through weekly analysis of volatile fatty acids (VFAs), pH, alkalinity, and TS/VS on the digestate. The pH was tested using a calibrated Hach SensION 3 pH meter, alkalinity analysis was performed using Hach digital titrator, and VFAs were analyzed using the Perkin Elmer Clarus 600 Gas Chromatograph equipped with a Perkin Elmer Elite-FFAP column with a 5 m guard column and a flame ionization detector (FID). 3. Results and discussion 3.1. Increased biogas production – BMP BMP analysis was initially used to determine hydraulic retention time (HRT), potential inhibitory effects, and biogas potential
3.2. Increased gas output – CSTRs After the BMP tests were performed; analyses of the glycerol co-digestion systems were moved to 4 L continuously stirred tank reactors (CSTRs). Co-digestion in the CSTRs was done using primary sewage sludge as the substrate material. The CSTRs allowed for continuous gas output monitoring using wet test meters and gas composition analysis by collecting samples in Tedlar bags weekly and performing GC analysis of the biogas. The CSTR also allows for variation of glycerol and primary sludge loading. This data along with digestate and co-digestate monitoring, determined the range of maximum benefit for addition of glycerol to the system. The addition of increasing amounts of glycerol resulted in a corresponding increase in gas production (Fig. 2). The specific gas production increased from 0.35 to 0.92 m3/kg VS d. Fig. 2
Fig. 1. Average BMP results for glycerol and primary sludge without glycerol.
Primary
Specific Gas Producon (m3/kg VS*d)
Primary + Glycerol
1
Glycerol Alone (E-C)
0.9 0.8
m3/kg VS*d
0.7 0.6 0.5 0.4 0.3 0.2
10
9
8
7
6
5
4
3
2
1
0.5
0
0
0.1
Grams of Glycerol Fig. 2. Increased specific gas production using glycerol as a co-digestion substrate.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
4
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
Fig. 3. Increased methane content of primary with indicated amounts of glycerol versus primary sludge.
Biogas Producon Increases with Glycerol Load 3.5
9000 8000
3
7000 6000 2
5000 4000
1.5
3000
Gas Producon (ml/day)
Total OLR (kg VS/m3d)
2.5
Primary in OLR Glycerol in OLR average Gas Flow
1 2000 0.5
0
1000
1
2
3
4
5
6
7
8
9
10
0
Glycerol Loading (grams) Fig. 4. Gas production increases as the percentage of glycerol in the total OLR increases.
shows that beginning with the addition of 2 g of glycerol, or 30% of the OLR as described in Table 2, addition of glycerol to the digester produces more biogas than primary alone; which would increase the operating efficiency of the digester and plant by providing a higher flow of gas. The maximum gain in gas production, the difference between the primary and the co-digested glycerol and primary, was observed in the 4–9 g of glycerol loading range representing 45– 69% of OLR. The overall maximum benefit of glycerol addition was observed at 7 g of glycerol loading (64% glycerol in OLR as shown in Table 2) due to enhanced methane content as compared to primary alone. This maximum benefit is found by considering the balance between the amounts of gas produced (Fig. 2), the methane content of the produced gas (Fig. 3) and the amount of glycerol required to obtain this enrichment. The methane content was variable, but after the startup instabilities, there was an upward trend in biogas content from 5 g of glycerol loading to 10 g as shown in Fig. 4. It is important to note that variations in primary sludge, a product of weather conditions, TS/VS and other WWTP issues were significant and showed an effect on the gas production throughout the experiment. The maximum benefit range was determined to be between 25% glycerol OLR to 60%.
Glycerol load was varied from 0% to 70% of the total OLR, but the concentration was dependent upon the primary sewage sludge, which showed inconsistency. This altered the actual amounts of organics available for the digestion process. Fig. 4 shows the increasing glycerol loading and general primary load reducing trend. It is also evident in this figure that the increased gas production rate is due to the increased glycerol loading. After loading at 10 g of glycerol (70% OLR), the reactor became unstable and did not produce biogas due to propionic acid accumulation which became inhibitory. 4. Conclusions Waste glycerol has been used to increase the gas production and methane content of biogas within an anaerobic co-digestion process using primary sewage sludge. The increase in biogas and consequently energy production is significant, with a 25% OLR glycerol load; the biogas production increased 82% as compared to a digester operating on primary sewage sludge alone. This data is meant to give operators at a WWTP confidence in co-digestion with glycerol as long as the optimized range of 25% OLR to 60% OLR is maintained. Without co-digestion of glycerol, a typical
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017
S. Nartker et al. / Waste Management xxx (2014) xxx–xxx
WWTP digesting primary sludge could install a modest engine generator set. With the addition of glycerol in the optimized range of 25% OLR to 50% OLR, the WWTP would be able to produce approximately twice the electrical power using 25% glycerol loading and four times the kW rating using 50% loading. Acknowledgments This research was financially supported by the Michigan Soybean Promotion Committee. Special acknowledgement to Chad Antle at Swedish Biogas International LLC for continuous support. References Alvarez, J.A., Otero, L., Lema, J.M., 2010. A methodology for optimising feed composition for anaerobic co-digestion of agro-industrial wastes. Bioresour. Technol. 101 (4), 1153–1158. http://dx.doi.org/10.1016/j.biortech.2009.09.061. Amon, T., Amon, B., Kryvoruchko, V., Bodiroza, V., Pötsch, E., Zollitsch, W., 2006. Optimising methane yield from anaerobic digestion of manure: effects of dairy systems and of glycerine supplementation. Int. Congr. Ser. 1293, 217–220. http://dx.doi.org/10.1016/j.ics.2006.03.007. APHA, 2005. Standard methods for the examination of water & wastewater: centennial edition. In: Eaton, A.D. (Ed.), Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association. Astals, S., Ariso, M., Galí, A., Mata-Alvarez, J., 2011. Co-digestion of pig manure and glycerine: experimental and modelling study. J. Environ. Manage. 92 (4), 1091– 1096. http://dx.doi.org/10.1016/j.jenvman.2010.11.014. Bouallagui, H., 2003. Mesophilic biogas production from fruit and vegetable waste in a tubular digester. Bioresour. Technol. 86 (1), 85–89. http://dx.doi.org/ 10.1016/S0960-8524(02)00097-4. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99 (10), 4044–4064. http://dx.doi.org/10.1016/ j.biortech.2007.01.057. Fountoulakis, M.S., Manios, T., 2009. Enhanced methane and hydrogen production from municipal solid waste and agro-industrial by-products co-digested with crude glycerol. Bioresour. Technol. 100 (12), 3043–3047. http://dx.doi.org/ 10.1016/j.biortech.2009.01.016.
5
Fountoulakis, M.S., Petousi, I., Manios, T., 2010. Co-digestion of sewage sludge with glycerol to boost biogas production. Waste Manage. 30 (10), 1849–1853. http:// dx.doi.org/10.1016/j.wasman.2010.04.011. Galí, A., Benabdallah, T., Astals, S., Mata-Alvarez, J., 2009. Modified version of ADM1 model for agro-waste application. Bioresour. Technol. 100 (11), 2783–2790. http://dx.doi.org/10.1016/j.biortech.2008.12.052. Grady Jr, C.P.L., Daigger, G.T., Love, N.G., Filipe, C.D.M., 2011. Biological Wastewater Treatment 63, 632. Heaven, S., Salter, A.M., Banks, C.J., 2011. Integration of on-farm biodiesel production with anaerobic digestion to maximise energy yield and greenhouse gas savings from process and farm residues. Bioresour. Technol. 102 (17), 7784–7793. http://dx.doi.org/10.1016/j.biortech.2011.06.029. Holm-Nielsen, J.B., Lomborg, C.J., Oleskowicz-Popiel, P., Esbensen, K.H., 2008. Online near infrared monitoring of glycerol-boosted anaerobic digestion processes: evaluation of process analytical technologies. Biotechnol. Bioeng. 99 (2), 302–313. http://dx.doi.org/10.1002/bit.21571. Holm-Nielsen, J.B., Al Seadi, T., Oleskowicz-Popiel, P., 2009. The future of anaerobic digestion and biogas utilization. Bioresour. Technol. 100 (22), 5478–5484. http://dx.doi.org/10.1016/j.biortech.2008.12.046. Hu, S., Luo, X., Wan, C., Li, Y., 2012. Characterization of crude glycerol from biodiesel plants. J. Agri. Food Chem. 60 (23), 5915–5921. http://dx.doi.org/10.1021/ jf3008629. Johnson, D.T., Taconi, K.a., 2007. The Glycerin Glut: options for the value-added conversion of crude glycerol resulting from biodiesel production. Environ. Prog. 26 (4), 338–348. http://dx.doi.org/10.1002/ep. Kacprzak, A., Krzystek, L., Ledakowicz, S., 2009. Co-digestion of agricultural and industrial wastes. Chem. Pap. 64 (2), 127–131. http://dx.doi.org/10.2478/ s11696-009-0108-5. Pachauri, N., He, B., 2006. Value-added utilization of crude glycerol from biodiesel production: a survey of current research activities. Written for Presentation at the 2006 ASABE Annual International Meeting Sponsored by ASABE Glycerol Utilization for Specific Products, 0300(06). Pagliaro, M., Rossi, M., Clark, J.H, Kraus, G.A, 2008. Future of glycerol. The Royal Society of Chemistry, pp. P001–P127. http://dx.doi.org/10.1039/ 9781847558305. Robra, S., Serpa da Cruz, R., de Oliveira, A.M., Neto, J.A.A., Santos, J.V., 2010. Generation of biogas using crude glycerin from biodiesel production as a supplement to cattle slurry. Biomass Bioenergy 34 (9), 1330–1335. http:// dx.doi.org/10.1016/j.biombioe.2010.04.021. Schievano, A., D’Imporzano, G., Adani, F., 2009. Substituting energy crops with organic wastes and agro-industrial residues for biogas production. J. Environ. Manage. 90 (8), 2537–2541. http://dx.doi.org/10.1016/j.jenvman.2009.01.013.
Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017