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Co-digestion of sewage sludge with glycerol to boost biogas production
Waste Management 30 (2010) 1849–1853
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Co-digestion of sewage sludge with glycerol to boost biogas production M.S. Fountoulakis *, I. Petousi, T. Manios School of Agricultural Technology, Technological Educational Institute of Crete, Heraklion, Greece
a r t i c l e
i n f o
Article history: Received 15 June 2009 Accepted 8 April 2010 Available online 29 April 2010
a b s t r a c t The feasibility of adding crude glycerol from the biodiesel industry to the anaerobic digesters treating sewage sludge in wastewater treatment plants was studied in both batch and continuous experiments at 35 °C. Glycerol addition can boost biogas yields, if it does not exceed a limiting 1% (v/v) concentration in the feed. Any further increase of glycerol causes a high imbalance in the anaerobic digestion process. The reactor treating the sewage sludge produced 1106 ± 36 ml CH4/d before the addition of glycerol and 2353 ± 94 ml CH4/d after the addition of glycerol (1% v/v in the feed). The extra glycerol-COD added to the feed did not have a negative effect on reactor performance, but seemed to increase the active biomass (volatile solids) concentration in the system. Batch kinetic experiments showed that the maximum specific utilization rate (lmax) and the saturation constant (KS) of glycerol were 0.149 ± 0.015 h1 and 0.276 ± 0.095 g/l, respectively. Comparing the estimated values with the kinetics constants for propionate reported in the literature, it can be concluded that glycerol uptake is not the rate-limiting step during the process. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Typical sewage sludge consists of primary sludge separated from wastewater during pre-settling, and biological excess sludge from the activated sludge system. Anaerobic digestion is an appropriate technique for the degradation and stabilisation of sludges before their final disposal. In recent years, much attention has been focused on the improvement of digester biogas production, in order to upgrade their role in stabilizing the sludge and also to produce a feasible bioenergy power plant. An interesting option for improving methane yields is co-digestion. This process is well known, especially in Denmark, resulting in much higher methane yields when food waste and similar types of organic waste were combined with cow and pig slurries at biogas plants (Nielsen et al., 2002). In recent years there have been many successful efforts for the co-digestion of sewage sludge with several other substrates, such as the source-sorted organic fraction of municipal solid waste (Del Borghi et al., 1999; Sosnowski et al., 2003; Go´mez et al., 2006), confectionery waste (Lafitte-Trouque´ and Forster, 2000), sludges from the pulp and paper industry (Einola et al., 2001), coffee waste (Neves et al., 2006) and grease-trap sludge from meat processing plants (Davidsson et al., 2008; Luostarinen et al., 2009). From literature on biogas plants, it emerges that high biogas production is positively correlated with the addition of high concentrate organic by-products. Glycerol is an organic, readilydigestible substance which can be easily stored over a long period. * Corresponding author. Tel.: +30 2810 379456; fax: +30 2810 379477. E-mail address: [email protected] (M.S. Fountoulakis). 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.04.011
These advantages make glycerol an ideal co-substrate for the anaerobic digestion process. In the past, the biodiesel industry considered glycerol a desirable co-product that could contribute to the economic viability of biodiesel production; nowadays, glycerol is often regarded as a waste stream with an associated disposal cost. Recent experiments with co-digestion, applying glycerol to mixtures of slaughterhouse wastewater, municipal solid waste, olivemill wastewater, pig manure, maize silage and rapeseed meal, have shown a significant increase in the methane yield. However, in order to maintain a stable digestion process the amount of glycerol added had a limiting concentration level (Amon et al., 2006; Holm-Nielsen et al., 2007; Fountoulakis and Manios, 2009). These results demonstrate that glycerol can be applied advantageously but a strict control strategy is necessary to regulate the amount added, to avoid the risk of organic overloading. The main objective of this work was to evaluate the use of glycerol as a co-substrate, in order to boost biogas production during the anaerobic treatment of sewage sludge. The effect of glycerol supplementation on methane yield was examined in continuous experiments, and the glycerol limiting concentration in the feed for a stable digestion process was estimated. There was also an attempt to investigate the kinetic removal of glycerol during anaerobic digestion. 2. Materials and methods 2.1. Feedstock Sewage sludge was mixed primary and secondary sludge originating from the municipal sewage treatment plant of the city of
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M.S. Fountoulakis et al. / Waste Management 30 (2010) 1849–1853
Heraklion (population 180,000), Greece. The mixed sludge was stored frozen at 4 °C until use. The characteristics of the sludge are summarized in Table 1. Crude glycerol was obtained from a biodiesel production company (AGROINVEST S.A.) in Central Greece, producing biodiesel mainly from soybean oil, sunflower oil, and rapeseed oil (Table 2). 2.2. Experimental procedure 2.2.1. Continuous experiments Two series of continuous experiments were carried out in order to investigate: (a) the limiting concentration of glycerol in the feed and (b) the methane production of the glycerol-supplemented sludge during anaerobic digestion. Firstly, three digesters with a working volume of 1 l were constructed using glass flasks. The digesters were sealed with rubber stoppers containing an influent/effluent port to allow injection of wastes. A water bath was used to maintain the temperature of the digesters at 35 °C. The flasks were connected to PVC tubes filled with water acidified to pH 3. Biogas was collected by displacement of water. The reactors were operated in a draw-and-fill mode (on a daily basis) with a retention time of 24 days. Initially, the reactors were inoculated with anaerobic sludge originating from the municipal STP of the city of Heraklion. The feed in the reactors was sewage sludge: as sole substrate (R1), supplemented with 1% (v/v) glycerol (R2), and supplemented with 3% (v/v) glycerol (R3). The digesters were operated using this feed for 60 days. In a second experiment, a 4 l single-step anaerobic reactor (CSTR) with 3 l working volume was used. Initially, the reactor was again inoculated with anaerobic sludge originating from the municipal STP of Heraklion and contained 19.6 g/l TSS, 10.8 g/l VSS and 17.5 g/l COD. The reactor was fed eight times a day (every 3 h) with a total feeding volume of 120–130 ml/d, resulting in a hydraulic retention time (HRT) of 23–25 d. The mixed liquid from the reactor was stirred periodically for 15 min, twice an hour. The temperature was maintained at 35 °C via water bath through water jackets surrounding the reactors. The initial feed was sewage sludge and the bioreactor was operated using this feed for 33 days. Glycerol was then added to the feed so that the reactor was fed continuously with sewage sludge containing 1% glycerol. 2.2.2. Batch experiments Biomass was transferred from the glycerol-supplemented CSTR under anaerobic conditions to 160-ml serum bottle reactors, in three different amounts: 90, 45 and 15 ml. An appropriate solution of glycerol-tap water was added to the serum bottles via syringe up to a final volume of 100 ml. The final glycerol concentration was 3 g/l in each case. The serum bottles were sealed immediately using rubber septa and aluminum crimp caps. Samples (of 1 ml) from the mixed liquor were taken at regular intervals to determine the glycerol concentration, while samples (100 ll) from the headspace were take to determine the methane content. The volume of the gas produced was measured via displacement of a syringe piston (Owen et al., 1979). The tests were run in duplicate.
Table 1 Main characteristics of sewage sludge used in the experiments. Parameters
Sewage sludge
pH TS (g/l) VS (g/l) Total COD (g/l) Soluble COD (g/l) TN (mg/l) TP (mg/l)
6.8 ± 0.2 35.4 ± 3.1 26.1 ± 2.8 35.2 ± 2.4 1.9 ± 0.3 1042 ± 157 845 ± 58
Table 2 Main characteristics of crude glycerol. Parameters
Crude glycerol
pH EC (lS/cm) Density (kg/l) Ash (%) TN (mg/l) TP (mg/l)
5.0 ± 0.1 4.2 ± 0.3 1.25 ± 0.1 2.8 ± 0.1 372 ± 21 9.6 ± 1.3
2.3. Analytical methods The pH was measured by an electrode (Crison, GLP 21), while total (TS) and volatile (VS) solids, total and soluble chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were determined according to standard methods (APHA, 1995). Determination of glycerol was carried out using the method developed by Mantzouridou et al. (2008), briefly described as follows: after removal of biomass by centrifugation, glycerol was directly determined in the media, after diluting the media with ethanol (50:50, v/v) and oxidizing glycerol to formaldehyde using periodate anions. The latter then reacts with acetylacetone in the presence of ammonium acetate to give a quantifiable derivative that absorbs strongly at 410 nm. Gas samples were collected in gas-tight syringes and transferred to the gas chromatograph by sealing the needle with a butyl rubber stopper. Twenty microliters were injected into a gas chromatograph (Agilent 6890N GC System) for methane analysis. A thermal conductivity detector and a capillary column (GS Carbonplot, 30 m 0.32 mm, 3 lm) were used. The column was operated isothermally at 80 °C and the detector port was operated at 150 °C. Helium was used as the carrier gas at a flow rate of 15 ml/min.
3. Results and discussion 3.1. Continuous experiments 3.1.1. Determination of limiting concentration of glycerol As shown in Fig. 1, a significant decrease of biogas production in the digester fed with sewage sludge supplemented with 3% crude glycerol was observed after 20 days of operation. This occurred due to low pH and the build-up of volatile fatty acids (VFA). On the other hand, primary sludge containing 1% crude glycerol showed the highest biogas production rate, estimated at 1253 ± 163 ml/d. This is strongly supported by a previous work by Holm-Nielsen et al. (2007), who found that with low concentrations (<5 g/l) of glycerol, VFA and individual fatty acids showed no sign of organic loading; however, when the glycerol content is increased, there are clear tendencies of organic overloading. Amon et al. (2006) found that the amount of glycerol used for co-digestion with a mixture of maize (silage and corn) and pig manure could be as high as 6%. Liu et al. (2008) found that initial C/N ratio played an important role in the acidification efficiency of sewage sludge. The feedstock used by Amon et al. (2006) had a lower C/ N ratio than that used in this work, which may explain their higher limiting concentrations of glycerol. 3.1.2. Determination of methane yield Fig. 2 illustrates the monitoring profiles of methane, pH and VS in the reactor fed with sewage sludge. The pH was approximately stable between 6.8 and 7.4. In stable conditions, methane gas production without and with glycerol addition was 1106 ± 36 and 2353 ± 94 ml/d, respectively. Therefore, the addition of glycerol enhanced methane production by approximately 1247 ml/d.
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glycerol 1%
blank
glycerol 3%
1800
Biogas (ml/d)
1500 1200 900 600 300 0 8.0
0
10
20
0
10
20
30
40
50
60
30
40
50
60
pH (mg/l)
7.5 7.0 6.5 6.0 5.5 5.0
COD (mg/l)
5000 4000 3000 2000 1000 0
Time (d) Fig. 1. Biogas, pH and COD profiles from the semi-continuous digestion of sewage sludge and glycerol.
CH4
pH
VS
8.5
2500
30 8.0
7.5
pH
20
1500
VS (g/l)
CH4 (ml/d)
2000
7.0
1000 10 500
6.5
addition of glycerol 0
0 0
10
20
30
40
50
60
70
6.0
80
Time (d) Fig. 2. Profile of methane production, pH and VS during anaerobic digestion processing of sewage sludge.
The theoretical amount of methane produced per gram of glycerol can be calculated using the Buswell formula (Buswell and Neave, 1930):
C3 H5 ðOHÞ3 ! 1:75CH4 þ 1:25CO2 þ 0:5H2 O and the ideal gas law:
ð1Þ
PV ¼ nRT
ð2Þ
where P is the absolute pressure (atm); V is the volume of gas (L); n is the number of moles of gas; R is the gas constant (0.082 l atm K1 mol1); and T is the temperature in K. The density of glycerol used in this work was 1.25 kg/l and therefore the theoretical methane production from digestion of
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M.S. Fountoulakis et al. / Waste Management 30 (2010) 1849–1853
glycerol in the 3 l CSTR reactor was estimated at 751 ml/d. Therefore, the observed methane production exceeded this theoretical value. The supplementation of the feed with glycerol resulted in a higher organic loading rate; this did not have a negative effect on reactor performance. Similar results were observed in a previous work examining the effect of glycerol on the anaerobic codigestion of the organic fraction of municipal solid waste and agricultural by-products (Fountoulakis and Manios, 2009). VS concentration increased from 17.9 ± 0.8 g/l without glycerol addition to 23.7 ± 0.7 with glycerol addition. It is suggested that the extra organic carbon source (glycerol addition) enhanced the growth of active biomass which increased the total amount of volatile solids. This is supported by previous work examining the effect of glycerol on anaerobic co-digestion with potato processing wastewater in a UASB reactor (Ma et al., 2007). They found a difference of 3 g VS/l in the UASB reactor after supplementation with glycerol. The enhanced methane production beyond theoretical value in this work might be attributed to enhanced active biomass. These extra microorganisms degrade more sewage sludge as substrate resulting to overall higher methane production. 3.2. Batch experiments Glycerol biodegradation rate was assumed to follow Monod kinetics, lmax K SGLY X, where lmax is the maximum specific þGLY glycerol utilization rate, Ks is the saturation constant, GLY is the concentration of glycerol and X is the concentration of total biomass. Results (Fig. 3) indicated that glycerol uptake could be expressed well with Monod kinetics. The maximum specific utilization rate (lmax) and the saturation constant (Ks) were estimated as 0.149 ± 0.015 h1 and 0.276 ± 0.095 g/l. To our knowledge there are no other reports of glycerol kinetic removal in the anaerobic digestion process. Angelidaki et al. (1998) assumed that glycerol biodegradation to propionate took place instantly, as an integral part of lipid hydrolysis. Furthermore, Pavlostathis and Giraldo-Gomez (1991) reviewed degradation rate constants for various lipids and found values to range between 0.04 and 0.92 d1 and maximum specific utilization rates for several long chain fatty acids (LCFA) between 0.085 and 0.55 d1. The results of this work support the assumption of Angelidaki et al. (1998), since the estimated lmax value for glycerol is significantly higher than the kinetic values for lipids and LCFA estimated in the past. In the same study, Angelidaki et al. (1998) reported a maximum specific utilization rate of 0.49 d1 for propionate. Comparing this
Glycerol concentration (g/l)
3.0
, ,
, ,
experimental data monod kinetic
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
50
Time (h) Fig. 3. Glycerol uptake in batch experiments using initial concentration of total biomass: (h) 22.3 g/l, (s) 11.15 g/l and (4) 3.71 g/l.
value with the value estimated for glycerol in this work (0.149 h1), it may be concluded that glycerol uptake is not the rate-limiting step during the process. It is suggested that the extra addition of glycerol in the feedstock (above 1% v/v) resulted in an accumulation of propionate in the reactors and, finally, in the instability of the system.
4. Conclusions In the present study, the feasibility of anaerobic co-digestion of sewage sludge and high-yield glycerol was investigated at 35 °C. It can be concluded that crude glycerol addition at 1% v/v increased CH4 production in the reactor above the expected theoretical value, as it was totally digested and furthermore enhanced the growth of active biomass in the system. On the other hand, for a stable digestion process the amount of glycerol in the feed should not exceed 1%. Kinetic trials have shown that glycerol biodegradation takes place significantly faster than propionate biodegradation, and it is therefore suggested that the glycerol overload in the reactors increased propionate concentration. Crude glycerol, an easily-stored residue of the biodiesel industry, can be beneficial to wastewater treatment plants as it increases methane production significantly. However, the feed should be regulated carefully in order to prevent overloading conditions. Acknowledgements This research was financially supported by the Greek Scholarship Institution (IKY). The authors would like to thank AGROINVEST S.A. for supplying the crude glycerol. References Amon, Th., 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. Angelidaki, I., Ellegaard, L., Ahring, B.K., 1998. A comprehensive model of anaerobic bioconversion of complex substrates to biogas. Biotechnol. Bioeng. 63, 363–372. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington DC, USA. Buswell, E.G., Neave, S.L., 1930. Laboratory studies of sludge digestion. Illinois Division of State Water Survey, Bulletin No. 30. Davidsson, A., Lövstedt, C., la Cour Jansen, J., Gruvberger, C., Aspegren, H., 2008. Co-digestion of grease trap sludge and sewage sludge. Waste Manage. 28, 986–992. Del Borghi, A., Converti, A., Palazzi, E., Del Borghi, M., 1999. Hydrolysis and thermophilic anaerobic digestion of sewage sludge and organic fraction of municipal solid waste. Bioprocess Eng. 20, 553–560. Einola, J.K., Luostarinen, S.A., Salminen, E.A., Rintala, J.A., 2001. Screening for an optimal combination of municipal and industrial wastes and sludges for anaerobic co-digestion. In: Proceedings of the 9th World Congress, Anaerobic Digestion 2001, Anaerobic Conversion for Sustainability 1, 357–362. Fountoulakis, M., 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, 3043–3047. Go´mez, X., Mora´n, A., Cuetos, M.J., Sa´nchez, M.E., 2006. The production of hydrogen by dark fermentation of municipal solid wastes and slaughterhouse waste: a two-phase process. J. Power Sources 157, 727–732. Holm-Nielsen, J.B., Lomborg, C.J., Oleskowicz-Popiel, P., Esbensen, K.H., 2007. Online near infrared monitoring of glycerol-boosted anaerobic digestion processes: evaluation of process analytical technologies. Biotechnol. Bioeng. 99, 302–313. Lafitte-Trouque´, S., Forster, C.F., 2000. Dual anaerobic co-digestion of sewage sludge and confectionery waste. Bioresour. Technol. 71, 77–82. Liu, X., Liu, H., Chen, Y., Du, G., Chen, J., 2008. Effects of organic matter and initial carbon-nitrogen ratio on the bioconversion of volatile fatty acids from sewage sludge. J. Chem. Technol. Biotechnol. 83, 1049–1055. Luostarinen, S., Luste, S., Sillanpää, M., 2009. Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant. Bioresour. Technol. 100, 79–85. Ma, J., Van Wambeke, M., Carballa, M., Verstraete, W., 2007. Improvement of the anaerobic treatment of potato processing wastewater in a UASB reactor by codigestion with glycerol. Biotechnol. Lett. 30, 861–867.
M.S. Fountoulakis et al. / Waste Management 30 (2010) 1849–1853 Mantzouridou, F., Naziri, E., Tsimidou, M.Z., 2008. Industrial glycerol as a supplementary carbon source in the production of b-carotene by Blakeslea trispora. J. Agric. Food Chem. 56, 2668–2675. Neves, L., Oliveira, R., Alves, M.M., 2006. Anaerobic co-digestion of coffee waste and sewage sludge. Waste Manage. 26, 176–181. Nielsen, L.H., Hjort-Gregersen, K., Thygesen, P., Christensen, J., 2002. Socioeconomic Analysis Centralized Biogas Plants. Report no. 136. Danish Ministry of Food Agriculture and Fisheries.
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Owen, W.F., Stuckey, D.C., Healy, J.B., Young, L.Y., McCarty, P.L., 1979. Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Res. 13, 485–492. Pavlostathis, S.G., Giraldo-Gomez, E., 1991. Kinetics of anaerobic treatment: a critical review. Crit. Rev. Envron. Control 21, 411–490. Sosnowski, P., Wieczorek, A., Ledakowicz, S., 2003. Anaerobic co-digestion of sewage sludge and organic fraction of municipal solid wastes. Adv. Environ. Res. 7, 609–616.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Co-digestion of sewage sludge with glycerol to boost biogas production M.S. Fountoulakis *, I. Petousi, T. Manios School of Agricultural Technology, Technological Educational Institute of Crete, Heraklion, Greece
a r t i c l e
i n f o
Article history: Received 15 June 2009 Accepted 8 April 2010 Available online 29 April 2010
a b s t r a c t The feasibility of adding crude glycerol from the biodiesel industry to the anaerobic digesters treating sewage sludge in wastewater treatment plants was studied in both batch and continuous experiments at 35 °C. Glycerol addition can boost biogas yields, if it does not exceed a limiting 1% (v/v) concentration in the feed. Any further increase of glycerol causes a high imbalance in the anaerobic digestion process. The reactor treating the sewage sludge produced 1106 ± 36 ml CH4/d before the addition of glycerol and 2353 ± 94 ml CH4/d after the addition of glycerol (1% v/v in the feed). The extra glycerol-COD added to the feed did not have a negative effect on reactor performance, but seemed to increase the active biomass (volatile solids) concentration in the system. Batch kinetic experiments showed that the maximum specific utilization rate (lmax) and the saturation constant (KS) of glycerol were 0.149 ± 0.015 h1 and 0.276 ± 0.095 g/l, respectively. Comparing the estimated values with the kinetics constants for propionate reported in the literature, it can be concluded that glycerol uptake is not the rate-limiting step during the process. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Typical sewage sludge consists of primary sludge separated from wastewater during pre-settling, and biological excess sludge from the activated sludge system. Anaerobic digestion is an appropriate technique for the degradation and stabilisation of sludges before their final disposal. In recent years, much attention has been focused on the improvement of digester biogas production, in order to upgrade their role in stabilizing the sludge and also to produce a feasible bioenergy power plant. An interesting option for improving methane yields is co-digestion. This process is well known, especially in Denmark, resulting in much higher methane yields when food waste and similar types of organic waste were combined with cow and pig slurries at biogas plants (Nielsen et al., 2002). In recent years there have been many successful efforts for the co-digestion of sewage sludge with several other substrates, such as the source-sorted organic fraction of municipal solid waste (Del Borghi et al., 1999; Sosnowski et al., 2003; Go´mez et al., 2006), confectionery waste (Lafitte-Trouque´ and Forster, 2000), sludges from the pulp and paper industry (Einola et al., 2001), coffee waste (Neves et al., 2006) and grease-trap sludge from meat processing plants (Davidsson et al., 2008; Luostarinen et al., 2009). From literature on biogas plants, it emerges that high biogas production is positively correlated with the addition of high concentrate organic by-products. Glycerol is an organic, readilydigestible substance which can be easily stored over a long period. * Corresponding author. Tel.: +30 2810 379456; fax: +30 2810 379477. E-mail address: [email protected] (M.S. Fountoulakis). 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.04.011
These advantages make glycerol an ideal co-substrate for the anaerobic digestion process. In the past, the biodiesel industry considered glycerol a desirable co-product that could contribute to the economic viability of biodiesel production; nowadays, glycerol is often regarded as a waste stream with an associated disposal cost. Recent experiments with co-digestion, applying glycerol to mixtures of slaughterhouse wastewater, municipal solid waste, olivemill wastewater, pig manure, maize silage and rapeseed meal, have shown a significant increase in the methane yield. However, in order to maintain a stable digestion process the amount of glycerol added had a limiting concentration level (Amon et al., 2006; Holm-Nielsen et al., 2007; Fountoulakis and Manios, 2009). These results demonstrate that glycerol can be applied advantageously but a strict control strategy is necessary to regulate the amount added, to avoid the risk of organic overloading. The main objective of this work was to evaluate the use of glycerol as a co-substrate, in order to boost biogas production during the anaerobic treatment of sewage sludge. The effect of glycerol supplementation on methane yield was examined in continuous experiments, and the glycerol limiting concentration in the feed for a stable digestion process was estimated. There was also an attempt to investigate the kinetic removal of glycerol during anaerobic digestion. 2. Materials and methods 2.1. Feedstock Sewage sludge was mixed primary and secondary sludge originating from the municipal sewage treatment plant of the city of
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M.S. Fountoulakis et al. / Waste Management 30 (2010) 1849–1853
Heraklion (population 180,000), Greece. The mixed sludge was stored frozen at 4 °C until use. The characteristics of the sludge are summarized in Table 1. Crude glycerol was obtained from a biodiesel production company (AGROINVEST S.A.) in Central Greece, producing biodiesel mainly from soybean oil, sunflower oil, and rapeseed oil (Table 2). 2.2. Experimental procedure 2.2.1. Continuous experiments Two series of continuous experiments were carried out in order to investigate: (a) the limiting concentration of glycerol in the feed and (b) the methane production of the glycerol-supplemented sludge during anaerobic digestion. Firstly, three digesters with a working volume of 1 l were constructed using glass flasks. The digesters were sealed with rubber stoppers containing an influent/effluent port to allow injection of wastes. A water bath was used to maintain the temperature of the digesters at 35 °C. The flasks were connected to PVC tubes filled with water acidified to pH 3. Biogas was collected by displacement of water. The reactors were operated in a draw-and-fill mode (on a daily basis) with a retention time of 24 days. Initially, the reactors were inoculated with anaerobic sludge originating from the municipal STP of the city of Heraklion. The feed in the reactors was sewage sludge: as sole substrate (R1), supplemented with 1% (v/v) glycerol (R2), and supplemented with 3% (v/v) glycerol (R3). The digesters were operated using this feed for 60 days. In a second experiment, a 4 l single-step anaerobic reactor (CSTR) with 3 l working volume was used. Initially, the reactor was again inoculated with anaerobic sludge originating from the municipal STP of Heraklion and contained 19.6 g/l TSS, 10.8 g/l VSS and 17.5 g/l COD. The reactor was fed eight times a day (every 3 h) with a total feeding volume of 120–130 ml/d, resulting in a hydraulic retention time (HRT) of 23–25 d. The mixed liquid from the reactor was stirred periodically for 15 min, twice an hour. The temperature was maintained at 35 °C via water bath through water jackets surrounding the reactors. The initial feed was sewage sludge and the bioreactor was operated using this feed for 33 days. Glycerol was then added to the feed so that the reactor was fed continuously with sewage sludge containing 1% glycerol. 2.2.2. Batch experiments Biomass was transferred from the glycerol-supplemented CSTR under anaerobic conditions to 160-ml serum bottle reactors, in three different amounts: 90, 45 and 15 ml. An appropriate solution of glycerol-tap water was added to the serum bottles via syringe up to a final volume of 100 ml. The final glycerol concentration was 3 g/l in each case. The serum bottles were sealed immediately using rubber septa and aluminum crimp caps. Samples (of 1 ml) from the mixed liquor were taken at regular intervals to determine the glycerol concentration, while samples (100 ll) from the headspace were take to determine the methane content. The volume of the gas produced was measured via displacement of a syringe piston (Owen et al., 1979). The tests were run in duplicate.
Table 1 Main characteristics of sewage sludge used in the experiments. Parameters
Sewage sludge
pH TS (g/l) VS (g/l) Total COD (g/l) Soluble COD (g/l) TN (mg/l) TP (mg/l)
6.8 ± 0.2 35.4 ± 3.1 26.1 ± 2.8 35.2 ± 2.4 1.9 ± 0.3 1042 ± 157 845 ± 58
Table 2 Main characteristics of crude glycerol. Parameters
Crude glycerol
pH EC (lS/cm) Density (kg/l) Ash (%) TN (mg/l) TP (mg/l)
5.0 ± 0.1 4.2 ± 0.3 1.25 ± 0.1 2.8 ± 0.1 372 ± 21 9.6 ± 1.3
2.3. Analytical methods The pH was measured by an electrode (Crison, GLP 21), while total (TS) and volatile (VS) solids, total and soluble chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were determined according to standard methods (APHA, 1995). Determination of glycerol was carried out using the method developed by Mantzouridou et al. (2008), briefly described as follows: after removal of biomass by centrifugation, glycerol was directly determined in the media, after diluting the media with ethanol (50:50, v/v) and oxidizing glycerol to formaldehyde using periodate anions. The latter then reacts with acetylacetone in the presence of ammonium acetate to give a quantifiable derivative that absorbs strongly at 410 nm. Gas samples were collected in gas-tight syringes and transferred to the gas chromatograph by sealing the needle with a butyl rubber stopper. Twenty microliters were injected into a gas chromatograph (Agilent 6890N GC System) for methane analysis. A thermal conductivity detector and a capillary column (GS Carbonplot, 30 m 0.32 mm, 3 lm) were used. The column was operated isothermally at 80 °C and the detector port was operated at 150 °C. Helium was used as the carrier gas at a flow rate of 15 ml/min.
3. Results and discussion 3.1. Continuous experiments 3.1.1. Determination of limiting concentration of glycerol As shown in Fig. 1, a significant decrease of biogas production in the digester fed with sewage sludge supplemented with 3% crude glycerol was observed after 20 days of operation. This occurred due to low pH and the build-up of volatile fatty acids (VFA). On the other hand, primary sludge containing 1% crude glycerol showed the highest biogas production rate, estimated at 1253 ± 163 ml/d. This is strongly supported by a previous work by Holm-Nielsen et al. (2007), who found that with low concentrations (<5 g/l) of glycerol, VFA and individual fatty acids showed no sign of organic loading; however, when the glycerol content is increased, there are clear tendencies of organic overloading. Amon et al. (2006) found that the amount of glycerol used for co-digestion with a mixture of maize (silage and corn) and pig manure could be as high as 6%. Liu et al. (2008) found that initial C/N ratio played an important role in the acidification efficiency of sewage sludge. The feedstock used by Amon et al. (2006) had a lower C/ N ratio than that used in this work, which may explain their higher limiting concentrations of glycerol. 3.1.2. Determination of methane yield Fig. 2 illustrates the monitoring profiles of methane, pH and VS in the reactor fed with sewage sludge. The pH was approximately stable between 6.8 and 7.4. In stable conditions, methane gas production without and with glycerol addition was 1106 ± 36 and 2353 ± 94 ml/d, respectively. Therefore, the addition of glycerol enhanced methane production by approximately 1247 ml/d.
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M.S. Fountoulakis et al. / Waste Management 30 (2010) 1849–1853
glycerol 1%
blank
glycerol 3%
1800
Biogas (ml/d)
1500 1200 900 600 300 0 8.0
0
10
20
0
10
20
30
40
50
60
30
40
50
60
pH (mg/l)
7.5 7.0 6.5 6.0 5.5 5.0
COD (mg/l)
5000 4000 3000 2000 1000 0
Time (d) Fig. 1. Biogas, pH and COD profiles from the semi-continuous digestion of sewage sludge and glycerol.
CH4
pH
VS
8.5
2500
30 8.0
7.5
pH
20
1500
VS (g/l)
CH4 (ml/d)
2000
7.0
1000 10 500
6.5
addition of glycerol 0
0 0
10
20
30
40
50
60
70
6.0
80
Time (d) Fig. 2. Profile of methane production, pH and VS during anaerobic digestion processing of sewage sludge.
The theoretical amount of methane produced per gram of glycerol can be calculated using the Buswell formula (Buswell and Neave, 1930):
C3 H5 ðOHÞ3 ! 1:75CH4 þ 1:25CO2 þ 0:5H2 O and the ideal gas law:
ð1Þ
PV ¼ nRT
ð2Þ
where P is the absolute pressure (atm); V is the volume of gas (L); n is the number of moles of gas; R is the gas constant (0.082 l atm K1 mol1); and T is the temperature in K. The density of glycerol used in this work was 1.25 kg/l and therefore the theoretical methane production from digestion of
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glycerol in the 3 l CSTR reactor was estimated at 751 ml/d. Therefore, the observed methane production exceeded this theoretical value. The supplementation of the feed with glycerol resulted in a higher organic loading rate; this did not have a negative effect on reactor performance. Similar results were observed in a previous work examining the effect of glycerol on the anaerobic codigestion of the organic fraction of municipal solid waste and agricultural by-products (Fountoulakis and Manios, 2009). VS concentration increased from 17.9 ± 0.8 g/l without glycerol addition to 23.7 ± 0.7 with glycerol addition. It is suggested that the extra organic carbon source (glycerol addition) enhanced the growth of active biomass which increased the total amount of volatile solids. This is supported by previous work examining the effect of glycerol on anaerobic co-digestion with potato processing wastewater in a UASB reactor (Ma et al., 2007). They found a difference of 3 g VS/l in the UASB reactor after supplementation with glycerol. The enhanced methane production beyond theoretical value in this work might be attributed to enhanced active biomass. These extra microorganisms degrade more sewage sludge as substrate resulting to overall higher methane production. 3.2. Batch experiments Glycerol biodegradation rate was assumed to follow Monod kinetics, lmax K SGLY X, where lmax is the maximum specific þGLY glycerol utilization rate, Ks is the saturation constant, GLY is the concentration of glycerol and X is the concentration of total biomass. Results (Fig. 3) indicated that glycerol uptake could be expressed well with Monod kinetics. The maximum specific utilization rate (lmax) and the saturation constant (Ks) were estimated as 0.149 ± 0.015 h1 and 0.276 ± 0.095 g/l. To our knowledge there are no other reports of glycerol kinetic removal in the anaerobic digestion process. Angelidaki et al. (1998) assumed that glycerol biodegradation to propionate took place instantly, as an integral part of lipid hydrolysis. Furthermore, Pavlostathis and Giraldo-Gomez (1991) reviewed degradation rate constants for various lipids and found values to range between 0.04 and 0.92 d1 and maximum specific utilization rates for several long chain fatty acids (LCFA) between 0.085 and 0.55 d1. The results of this work support the assumption of Angelidaki et al. (1998), since the estimated lmax value for glycerol is significantly higher than the kinetic values for lipids and LCFA estimated in the past. In the same study, Angelidaki et al. (1998) reported a maximum specific utilization rate of 0.49 d1 for propionate. Comparing this
Glycerol concentration (g/l)
3.0
, ,
, ,
experimental data monod kinetic
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
50
Time (h) Fig. 3. Glycerol uptake in batch experiments using initial concentration of total biomass: (h) 22.3 g/l, (s) 11.15 g/l and (4) 3.71 g/l.
value with the value estimated for glycerol in this work (0.149 h1), it may be concluded that glycerol uptake is not the rate-limiting step during the process. It is suggested that the extra addition of glycerol in the feedstock (above 1% v/v) resulted in an accumulation of propionate in the reactors and, finally, in the instability of the system.
4. Conclusions In the present study, the feasibility of anaerobic co-digestion of sewage sludge and high-yield glycerol was investigated at 35 °C. It can be concluded that crude glycerol addition at 1% v/v increased CH4 production in the reactor above the expected theoretical value, as it was totally digested and furthermore enhanced the growth of active biomass in the system. On the other hand, for a stable digestion process the amount of glycerol in the feed should not exceed 1%. Kinetic trials have shown that glycerol biodegradation takes place significantly faster than propionate biodegradation, and it is therefore suggested that the glycerol overload in the reactors increased propionate concentration. Crude glycerol, an easily-stored residue of the biodiesel industry, can be beneficial to wastewater treatment plants as it increases methane production significantly. However, the feed should be regulated carefully in order to prevent overloading conditions. Acknowledgements This research was financially supported by the Greek Scholarship Institution (IKY). The authors would like to thank AGROINVEST S.A. for supplying the crude glycerol. References Amon, Th., 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. Angelidaki, I., Ellegaard, L., Ahring, B.K., 1998. A comprehensive model of anaerobic bioconversion of complex substrates to biogas. Biotechnol. Bioeng. 63, 363–372. APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington DC, USA. Buswell, E.G., Neave, S.L., 1930. Laboratory studies of sludge digestion. Illinois Division of State Water Survey, Bulletin No. 30. Davidsson, A., Lövstedt, C., la Cour Jansen, J., Gruvberger, C., Aspegren, H., 2008. Co-digestion of grease trap sludge and sewage sludge. Waste Manage. 28, 986–992. Del Borghi, A., Converti, A., Palazzi, E., Del Borghi, M., 1999. Hydrolysis and thermophilic anaerobic digestion of sewage sludge and organic fraction of municipal solid waste. Bioprocess Eng. 20, 553–560. Einola, J.K., Luostarinen, S.A., Salminen, E.A., Rintala, J.A., 2001. Screening for an optimal combination of municipal and industrial wastes and sludges for anaerobic co-digestion. In: Proceedings of the 9th World Congress, Anaerobic Digestion 2001, Anaerobic Conversion for Sustainability 1, 357–362. Fountoulakis, M., 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, 3043–3047. Go´mez, X., Mora´n, A., Cuetos, M.J., Sa´nchez, M.E., 2006. The production of hydrogen by dark fermentation of municipal solid wastes and slaughterhouse waste: a two-phase process. J. Power Sources 157, 727–732. Holm-Nielsen, J.B., Lomborg, C.J., Oleskowicz-Popiel, P., Esbensen, K.H., 2007. Online near infrared monitoring of glycerol-boosted anaerobic digestion processes: evaluation of process analytical technologies. Biotechnol. Bioeng. 99, 302–313. Lafitte-Trouque´, S., Forster, C.F., 2000. Dual anaerobic co-digestion of sewage sludge and confectionery waste. Bioresour. Technol. 71, 77–82. Liu, X., Liu, H., Chen, Y., Du, G., Chen, J., 2008. Effects of organic matter and initial carbon-nitrogen ratio on the bioconversion of volatile fatty acids from sewage sludge. J. Chem. Technol. Biotechnol. 83, 1049–1055. Luostarinen, S., Luste, S., Sillanpää, M., 2009. Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant. Bioresour. Technol. 100, 79–85. Ma, J., Van Wambeke, M., Carballa, M., Verstraete, W., 2007. Improvement of the anaerobic treatment of potato processing wastewater in a UASB reactor by codigestion with glycerol. Biotechnol. Lett. 30, 861–867.
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