Provided for non-commercial research and educational use only. Not for reproduction or distribution or commercial use. This article was originally published by IWA Publishing. IWA Publishing recognizes the retention of the right by the author(s) to photocopy or make single electronic copies of the paper for their own personal use, including for their own classroom use, or the personal use of colleagues, provided the copies are not offered for sale and are not distributed in a systematic way outside of their employing institution. Please note that you are not permitted to post the IWA Publishing PDF version of your paper on your own website or your institution’s website or repository. Please direct any queries regarding use or permissions to wst@iwap.co.uk Q IWA Publishing 2009 Water Science & Technology—WST | 60.1 | 2009 9 Influence of temperature on the hydrolysis, acidogenesis and methanogenesis in mesophilic anaerobic digestion: parameter identification and modeling application A. Donoso-Bravo, C. Retamal, M. Carballa, G. Ruiz-Filippi and R. Chamy ABSTRACT The effect of temperature on the kinetic parameters involved in the main reactions of the A. Donoso-Bravo (corresponding author) C. Retamal M. Carballa G. Ruiz-Filippi R. Chamy School of Biochemical Engineering, Pontificia Universidad Católica de Valparaı́so, General Cruz 34, Valparaı́so, Chile E-mail: andres.donoso.b@mail.ucv.cl anaerobic digestion process was studied. Batch tests with starch, glucose and acetic acid as substrates for hydrolysis, acidogenesis and methanogenesis, respectively, were performed in a temperature range between 15 and 458C. First order kinetics was assumed to determine the hydrolysis rate constant, while Monod and Haldane kinetics were considered for acidogenesis and methanogenesis, respectively. The results obtained showed that the anaerobic process is strongly influenced by temperature, with acidogenesis exerting the highest effect. The Cardinal Temperature Model 1 with an inflection point (CTM1) fitted properly the experimental data in the whole temperature range, except for the maximum degradation rate of acidogenesis. A simple case-study assessing the effect of temperature on an anaerobic CSTR performance indicated that with relatively simple substrates, like starch, the limiting reaction would change depending on M. Carballa Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, Rúa Lope Gómez de Marzoa s/n, 15782 Santiago de Compostela Spain temperature. However, when more complex substrates are used (e.g. sewage sludge), the hydrolysis might become more quickly into the limiting step. Key words | acidogenesis, anaerobic digestion, hydrolysis, modeling, temperature INTRODUCTION Nowadays, anaerobic digestion might be considered as a the methanization process. On the contrary, when treating consolidated technology with more than 2200 high-rate complex organic matter or anaerobic treatment is carried reactors implemented worldwide, treating different types of out at low temperature, hydrolysis is often considered as the wastes and wastewaters coming from different sectors, such limiting step (Vavilin et al. 2008). as agro-food industry, beverage, alcohol distillery and pulp and paper industries (van Lier 2008). The temperature at which the anaerobic digestion occurs can significantly affect the conversion, kinetics, However, there are a number of factors which affect stability, effluent quality, and consequently, the methane the anaerobic digestion process, including the substrate yield of the process (Sanchez et al. 2001). Microorganisms characteristics, the reactor configuration, the operational are generally divided into three thermal groups, psycro- parameters, such as hydraulic retention time (HRT), solids philes, mesophiles and thermophiles, with optimum retention time (SRT) and organic loading rate (OLR), temperatures below 208C, 25 –408C and higher than 458C, and the environmental factors like temperature and pH respectively (van Lier et al. 1997), and it has been (Banerjee et al. 1998). demonstrated that the anaerobic degradation rate of As a complex multi-step process, the overall kinetics of organic matter increases with temperature when psychro- waste utilization during anaerobic treatment is governed by philic, the kinetics of the slowest step, which often corresponds to compared (Sanchez et al. 2001). However, anaerobic doi: 10.2166/wst.2009.316 mesophilic and thermophilic conditions are 10 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling Water Science & Technology—WST | 60.1 | 2009 digestion has been traditionally performed in mesophilic extract were added according to Field et al. (1988). Starch range (35 – 378C) despite the temperature of certain waste- (1.0 –3.0 g/L), waters might be either warmer or cooler. Treating these (0.1 –25.0 g/L) were used for hydrolysis, acidogenesis and glucose (0.1 –1.0 g/L) and acetic acid wastewaters at their natural temperatures would often be methanogenesis assays, respectively. Several temperatures beneficial because of the reduced resources and costs were tested for the different stages, i.e. hydrolysis (12, 22, (Kettunen & Rintala 1997). 30, 37 and 458C), acidogenesis (12, 22, 30, 37, 42 and 458C) The influence of temperature has been extensively and methanogenesis (12, 25, 37 and 458C). Sodium studied on the rate-limiting methanogenic phase at bicarbonate was used as buffer at a concentration of both mesophilic and thermophilic conditions (Hegde & 1 g/gCODadded (Soto et al. 1993) in the hydrolysis assays, Pullammanappallil 2007). However, little attention has been while paid on the effect of temperature on hydrolysis and phosphate was used at concentrations ranging from acidogenesis. Hydrolysis rate constants are highly depen- 0.007 – 0.070 M and 0.009 M, respectively (Retamal 2008). for acidogenesis and methanogenesis assays, dent on temperature since hydrolysis is a biochemical The temperature of the experiments was maintained by reaction catalyzed by enzymes, which are very thermally using a thermostatic water bath. The bottles were inocu- sensitive (Sanders et al. 2000). Moreover, a better under- lated with anaerobic biomass coming from a mesophilic standing of temperature effects on acidogenesis can result in sewage sludge digester at a final concentration of the improvement of digester stability due to physical 1.03 ^ 0.14 gVSS/L. Different substrate concentrations separation of phases, the increase in the concentration of were tested in duplicate or triplicate at each temperature soluble organics and the optimization of biological nutrient and the substrate consumption over time, pH and sus- removal processes (Banerjee et al. 1998). In addition, few pended solids concentration at the end of the experiments studies have been carried out on the effect of temperature were the parameters monitored. on the overall process of anaerobic digestion. Banik et al. For acidogenesis and methanogenesis, a biomass lineal (1998) studied the effect of temperature on the overall range study was performed at 378C with glucose (0.09 g/L) kinetic parameters of the anaerobic biomass but just up to and acetic acid (0.12 g/L), respectively. For acidogenesis, 258C. Since temperature variations may not have the same three biomass concentrations, 0.20, 0.54 and 1.07 g VSS/L, effect on the different stages of anaerobic digestion were used and the volumetric consumption rates of glucose (hydrolysis, were acidogenesis and methanogenesis), more calculated. The glucose consumption rate at knowledge is required to identify and select corrective 0.54 g VSS/L was double than the value obtained at actions for temperature disturbances on anaerobic reactors. 0.20 g VSS/L, while no differences were observed between The objective of this study was to determine the kinetic 0.54 and 1.07 g VSS/L. Hence, a biomass concentration of parameters that characterize hydrolysis, acidogenesis and 0.54 g/L was selected. For methanogenesis, two biomass methanogenic stages at different temperatures in order to concentrations, 0.32 and 0.65 g VSS/L, were used and the develop a mathematical model describing the influence of volumetric consumption rates of acetic acid were 9.6·1023 temperature on each single stage. and 1.9·1022 g/L·d, respectively. Thus, a biomass concentration of 0.4 g VSS/L was used. MATERIALS AND METHODS Determination of kinetic parameters Experimental set-up Kinetic equations Batch experiments were run in glass serum bottles with First order kinetics was considered for the hydrolysis a total liquid volume of 200, 250 and 100 mL for of particulate organic matter (Equation 1), while Monod- hydrolysis, assays, type (Equation 2) and Haldane-type (Equation 3) kinetics respectively. Macronutrients, micronutrients and yeast were assumed for acidogenesis and methanogenesis acidogenesis and methanogenesis 11 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling (Bernard et al. 2001). Water Science & Technology—WST | 60.1 | 2009 Finally, a nonlinear optimization by least squares vh ¼ 2kh ·Sh ð1Þ S1 v1 ¼ vmax 1 KS1 þ S S2 v2 ¼ vmax 2 S2 KS2 þ S2 þ K21 ð2Þ ð3Þ procedure is applied to calculate the unknown parameters by minimizing a cost function (Equation 6), which measures the difference between the experimental measurements and the corresponding simulated value (the values obtained with the linearization method are used as initial values in the simulation process). where vh is the hydrolysis reaction rate (g/L·d), Sh is the hydrolysis substrate concentration (g/L), kh is the hydroly21 sis rate constant (d ), v1 is the acidogenesis reaction rate JðcÞ ¼ min N X t¼1
2 vm ðtÞ 2 vðt; cÞ jc0 ¼cL ð6Þ (g/gVSS·d), S1 is the acidogenesis substrate concentration (g/L), vmax1 is the maximum degradation rate for acidogen- where vm is the velocity consumption obtained for esis (g/gVSS·d), KS1 is the half saturation constant for measurements, v is the corresponding simulated velocity acidogenesis (g/L), v2 is the methanogenesis reaction rate and N is the number of measurements. (g/gVSS·d), S2 is the methanogenesis substrate concentration (g/L), vmax2 is the maximum degradation rate for methanogenesis (g/gVSS·d), KS2 is the half saturation Models of temperature effect constant for methanogenesis (g/L) and KI is the inhibition The influence of temperature on the hydrolysis constant rate constant associated with substrate S2. has been often described by the Arrhenius model. However, this model is limited by a maximum value of temperature above which the temperature effect can not be further Parameters estimation evaluated. In contrast, the Cardinal Temperature Model 1 The hydrolysis rate constant is obtained from Equation (1), with an inflection point (CTM1) proposed by Rosso et al. by representing ln S0 h/Sh versus time (slope). The determi- (1993) is able to model all temperatures tested (Equation 7). b ¼ bopt ðT 2 T max ÞðT 2 T min Þ2   ðT opt 2 T min Þ ðT opt 2 T min Þ·ðT 2 T opt Þ 2 ðT opt 2 T max Þ·ðT opt þ T min 2 2TÞ nation of the kinetic parameters for acidogenesis and ð7Þ where Tmin, Topt and Tmax were the minimum, optimum methanogenesis was carried out from the initial consumption and maximum temperatures, respectively (8C), and bopt is rate (Retamal 2008) by using the linearization Lineweaver- the optimum value of the kinetic parameter under study. Burke method (Equations (4) and (5)). Since the initial All these parameters were obtained from a nonlinear concentration of biomass may exert an important effect optimization by least squares procedure as described on the biodegradation kinetics (Urra et al. 2008), these previously. Figure 1 summarizes all the calculations assays were performed within the biomass lineal range. procedure. 1 KS1 1 1 ¼ · þ v1 Vm1 S1 Vm1 ð4Þ Analytical methods Starch concentrations were determined as the difference 1 KS2 1 1 S2 ¼ þ · þ v2 Vm2 S2 Vm2 Vm2 ·KI between the total sugar concentrations (Dubois et al. 1956) ð5Þ and the reducing sugar concentrations, i.e. glucose concentrations (Miller 1959). pH, Volatile Suspended Solids (VSS) where cL ¼ ðKS1 ; KS2 ; KI ; Vm1 ; Vm2 Þ are the unknown and Volatile Fatty Acids (VFA) were determined according parameters. to standard methods (APHA 1995). 12 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling Figure 1 | Water Science & Technology—WST | 60.1 | 2009 Calculation procedure for the determination of the kinetic parameters and the evaluation of temperature influence. Case-study: acidogenesis. (A) Initial rates calculation; (B) kinetic-profile construction; (C) model fit; (D) temperature influence; and, (E) temperature-model fit. RESULTS AND DISCUSSION to describe the effect of temperature over the whole temperature range tested. The parameters of CTM1 are Hydrolysis rate constants as a function shown in Table 1. It can be observed that the optimum of temperature modeled temperature for hydrolysis (40.38C) is lower than Figure 2a shows the hydrolysis rate constants of starch at the different temperatures tested. It can be observed that the hydrolysis rate constants increased with temperature up to the value obtained in the experiments (378C), while the optimum modeled hydrolysis rate constant (22.8 d21) is higher than the experimental value (21.1 d21). an optimum value of 21.1 ^ 2.2 d21 at 378C. However, a The values obtained for the hydrolysis constant in this lower value was obtained at 458C (7.9 ^ 1.7 d21), which is study were largely greater than other values reported in probably due to the fact that this temperature is in between literature (Siegrist et al. 2002; Vavilin et al. 2008). This fact is the optimum values for mesophilic and thermophilic probably explained by the use of starch as substrate, which conditions. In the temperature range of 12 to 378C, the is one of the most readily hydrolysable substrates in contrast hydrolysis rate constants fitted well the Arrhenius equation to the more complex substrates, such as sludge or lipids, and a value of 72 kJ/mol for the activation energy was used in other literature studies. With regard to the calculated, which is a typical value for enzymatic kinetics hydrolysis constant dependency on temperature, Siegrist under anaerobic conditions (Veeken & Hamelers 1999). et al. (2002) also found that this parameter is strongly On the contrary, the CTM1 model is more appropriate influenced by temperature. 13 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling Figure 2 | Water Science & Technology—WST | 60.1 | 2009 (a) Hydrolysis rate constants in function of temperature using starch as substrate; (b) Maximum degradation rates of acidogenesis in function of temperature using glucose as substrate; (c) Maximum degradation rates of methanogenesis using acetic as substrate; (d) Influence of temperature on half saturation constant in acidogenesis stage; (e) Influence of temperature on half saturation constant in methanogenic stage; (f) Influence of temperature inhibition constant in methanogenesis stage. B, experimental data; —, CTM1 fit. A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling 14 Table 1 | Water Science & Technology—WST | 60.1 | 2009 CTM1 parameters for optimum degradation rates during hydrolysis, acidogenesis and methanogenesis using starch, glucose and acetic acid as substrates, respectively Tmin (8C) Parameter Topt (8C) Tmax (8C) kH opt (d21) Vm1opt (g/gVSS·d) Vm2 opt (g/gVSS·d) KIopt (g/L) Hydrolysis kh 4.2 40.3 45.5 22.8 – – – Acidogenesis Vm1 28.9 37.0 45.2 – 33.6 – – Methanogenesis Vm2 11.1 34.1 46.3 – – 0.72 – KI 11.9 34.3 46.2 – – – 6.45 Influence of temperature on acidogenesis and temperature of an anaerobic reactor, a 58C decrease would methanogenesis result in 50 and 10% slower kinetics of acidogenesis and hydrolysis, Maximum degradation rates respectively, with almost no effect on methanogenesis. Figure 2b shows the influence of temperature on the maximum degradation rate during acidogenesis. Whereas there was almost no difference between 12 and 308C, a Affinity and inhibition constant significant increase was observed at 378C with again a sharp Figure 2b and Figure 2d show the influence of temperature decrease above 408C. In this case, only the temperature on glucose (acidogenesis) and acetic acid (methanogenesis) range between 30 and 458C was considered for CTM1 affinity constants. Similar values were obtained by Siegrist modeling, which showed that the optimum temperature for et al. (2002), but these authors also found an increase of acidogenesis is lower than that for hydrolysis (Table 1). these parameters at thermophilic conditions (558C), while Figure 2c shows the influence of temperature on the in the present study, no significant variations were observed maximum degradation rate during methanogenesis and the for affinity constant of glucose in the temperature range CTM1 fit over the whole temperature range tested. It can be tested. This finding was also obtained by Banik et al. (1998) observed that the maximum degradation rate during for the global affinity constant of anaerobic population. methanogenesis was negligible below 158C and it increased However, the half saturation constant of acetic acid progressively with temperature up to 30– 358C, with lower decreased from 4.25 g/L at 258C to 1.5 g/L at 458C. values again at higher temperatures. CTM1 model indicates The assay carried out at 128C did not produce methane that the optimum temperature for methanogenesis (34.18C) during the 3 months of experiment, thus not being possible is lower than for hydrolysis and acidogenesis (Table 1). the calculation of this parameter at this low temperature. Overall, the three reactions diminished their activity as Concerning the inhibition constant of acetic acid (Figure 2f), the temperature decreased, as also observed by Banik et al. the influence of temperature was similar to that on the (1998). However, the impact of temperature decrease is maximum degradation rate (Figure 2e) and it could be different for each stage. Assuming 378C as the operational modeled by CTM1 (Table 1). Figure 3 | Profiles of the initial reaction rate at different substrates concentrations at 378C for. acidogenesis (a) and methanogesis (b). 15 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling Figure 4 | Water Science & Technology—WST | 60.1 | 2009 Steady state COD consumption rate during (W) hydrolysis, (A) acidogenesis and (B) methanogenesis between 15 and 458C. (a) starch-wastewater (b) sludge. Dynamic behavior Figure 3 shows the dynamic behavior of the consumption rate at different concentration of substrate for acidogenesis (Figure 3a) and methanogesis (Figure 3b) at one temperature. It can be observed that the reaction behaves according to a Monod and Haldane model. retention time (HRT) of 20 d and six temperatures (15, 25, 30, 35, 40 and 458C) were assumed for the simulation. For each temperature, the kinetic parameters were calculated from the results obtained in the previous experiments in the case of starch-wastewater. For the anaerobic sludge treatment, a hydrolysis constant value of 0.016 d21 (which is three orders of magnitude lower than that of starch) determined by Retamal (2008) at 378C was used, Case-study: continuous stirred tank reactor (CSTR) assuming a CTM1 behavior for the same range of A two-population and three-reaction model (Donoso-Bravo temperature. In both cases, a 90% of particulate organic 2008) adapted from the two-reaction model developed by fraction was assumed. Bernard et al. (2001) was used to evaluate the influence of The results of the variables were obtained at steady state temperature on the performance of an anaerobic CSTR. and they were considered as the initial conditions of the A starch-wastewater containing 5.0 g COD/L and sludge system for the next run. The system was run in total for from an activated sludge plant were considered. A hydraulic 500 days, and the results are shown in Figure 4. Figure 5 | Non-degraded COD accumulation during hydrolysis, acidogenesis and methanogenesis at different temperatures. (a) starch-wastewater (b) sludge. 16 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling In general terms, it can be noted that the methanogenic reaction rate is the lowest for the temperature range studied Water Science & Technology—WST | 60.1 | 2009 complex substrates are used (e.g. sewage sludge), the hydrolysis was clearly the limiting step. and for the kinetic parameters determined. However, with sludge (Figure 4b), the hydrolysis and the methanogenesis rates are closer than with starch, which shows that the ACKNOWLEDGEMENTS nature of the substrate has a significant influence on the overall process. In order to evaluate the limiting step at different This work was funded by 1060220 and 1090482 projects from Fondecyt and ALEGAS from PUCV, Chile. temperatures, a model was used to determine the nondegraded COD in each phase (Figure 5). Figure 5a shows that, for starch-wastewater, an important accumulation of particulate COD would occur below 308C, whereas at higher temperatures the methanogenic reaction would turn into in the limiting step. For sludge (Figure 5b), hydrolysis remains as the limiting step for all studied temperatures. That is the reason why the increasing use of sludgepretreatment methods prior to anaerobic digestion to diminish the particulate organic fraction of the sludge, and thus increasing the overall rate of the anaerobic digestion. CONCLUSIONS Knowledge about the effect of temperature on the kinetic parameters of the main anaerobic reactions represents an important progress in the anaerobic digestion field. This work shows that the anaerobic process is strongly influenced by temperature, with acidogenesis showing the highest effect. Therefore, it can be concluded that this stage plays a key role in the stability of the overall anaerobic process, since a small change in the operational temperature (around 58C), results in a decrease of 50% in the acidogenesis reaction rate, affecting consequently the methanogenesis stage. Moreover, CTM1 modeled properly the influence of temperature on the different kinetic parameters involved in anaerobic process in the temperature range tested, except for the maximum degradation rate of acidogenesis, which could be only modeled between 30 and 458C. The simple case-study to asses the effect of temperature on an anaerobic CSTR performance indicated that with relatively simple substrates, like starch, the limiting reaction would change depending on temperature. However, when more REFERENCES APHA, AWWA, WPCP 1995 Standard Methods for the Examination of Water and Wastewater, 19th edition. APHA, AWWA, WPCP, Washington, DC. Banerjee, A., Elefsiniotis, P. & Tuhtar, D. 1998 Effect of HRT and temperature on the acidogenesis of primary municipal sludge and industrial wastewater. Water Sci. Technol. 38, 417 –423. Banik, G. C., Viraraghavan, T. & Dague, R. R. 1998 Low temperature effects on anaerobic microbial kinetic parameters. Environ. Technol. 19(5), 503– 512. Bernard, O., Hadj-Sadok, Z., Dochain, D., Genovesi, A. & Steyer, J. P. 2001 Dynamical model development and parameter identification for an anaerobic wastewater treatment process. Biotechnol. Bioeng. 75(4), 424 –438. Donoso-Bravo, A. 2008 Operation and modeling of Anaerobic Sequencing Batch Reactors (ASBR) in the Treatment of Complex Effluents (Spanish). PhD Thesis, Biochemical Engineering School. Pontificia Universidad Catolica de Valparaiso, Chile. Dubois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A. & Smith, F. 1956 Colorimetric method for determination of sugars and related susbstances. Anal. Chem. 28(3), 350 –356. Field, J., Sierra, R. & Lettinga, G. 1988 Ensayos Anaerobios. Valladolid, España, pp. 52 –79. Hegde, G. & Pullammanappallil, P. 2007 Comparison of thermophilic and mesophilic one-stage, batch, high-solids anaerobic digestion. Environ. Technol. 28, 361– 369. Kettunen, R. H. & Rintala, J. A. 1997 The effect of low temperature (5 –298C) and adaptation on the methanogenic activity of biomass. Appl. Microbiol. Biotechnol. 48(4), 570 –576. Miller, G. L. 1959 Use of dinitrosalicylic acid reagent for determination of reducing sugar. Anal. Chem. 31, 426. Retamal, C. 2008 Estudio del efecto de la temperatura sobre las etapas de hidrólisis, acidogenesis y metanogénesis de la digestión anaerobia. Master Thesis Biochemical Engineering School. Pontificia Universidad Catolica de Valparaiso, Chile. Rosso, L., Lobry, J. R. & Flandrois, J. P. 1993 An unexpected correlation between cardinal temperatures of microbial-growth highlighted by a new model. J. Theor. Biol. 162(4), 447– 463. Sanchez, E., Borja, R., Weiland, P., Travieso, L. & Martin, A. 2001 Effect of substrate concentration and temperature on the anaerobic digestion of piggery waste in a tropical climate. Process Biochem. 37(5), 483 –489. 17 A. Donoso-Bravo et al. | Mesophilic anaerobic digestion: parameter identification and modelling Sanders, W. T. M., Geerink, M., Zeeman, G. & Lettinga, G. 2000 Anaerobic hydrolysis kinetics of particulate substrates. Water Sci. Technol. 41(3), 17 – 24. Siegrist, H., Vogt, D., Garcia-Heras, J. & Gujer, W. 2002 Mathematical model for meso- and thermophilic anaerobic sewage sludge digestion. Environ. Sci. Technol. 36(5), 1113 –1123. Soto, M., Mendez, R. & Lema, J. M. 1993 Methanogenic and nonmethanogenic activity tests—theoretical basis and experimental set-up. Water Res. 27(8), 1361 – 1376. Urra, J., Poirrier, P., Segovia, J., Lesty, Y. & Chamy, R. 2008 Analysis of the methodology to determine anaerobic toxicity: evaluation of main compounds present in wastewater treatment plants (WWTPs). Water Sci. Technol. 57(6), 857– 862. Water Science & Technology—WST | 60.1 | 2009 van Lier, J. B., Rebac, S. & Lettinga, G. 1997 High-rate anaerobic wastewater treatment under psychrophilic and thermophilic conditions. Water Sci. Technol. 35(10), 199 –206. van Lier, J. 2008 High-rate anaerobic wastewater treatment: diversifying from end-of-the-pipe treatment to resourceoriented conversion techniques. Water Sci. Technol. 57(8), 1137 –1148. Vavilin, V. A., Fernandez, B., Palatsi, J. & Flotats, X. 2008 Hydrolysis kinetics in anaerobic degradation of particulate organic material: an overview. Waste Manage. 28(6), 939– 951. Veeken, A. & Hamelers, B. 1999 Effect of temperature on hydrolysis rates of selected biowaste components. Bioresour. Technol. 69(3), 249–254.