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Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20
Influence of the food to microorganisms (F/M) ratioand temperature on batch anaerobic digestionprocesses with and without zeolite additionS. Montalvo a , P. Gonzalez a , C. Mena a , L. Guerrero b & R. Borja ca Departamento de Ingeniería Química , Universidad de Santiago de Chile , Chileb Departamento de Ingeniería Química y Ambiental , Universidad Técnica Federico SantaMaría , Valparaíso , Chilec Instituto de la Grasa (CSIC) , Sevilla , SpainPublished online: 04 Jul 2012.
To cite this article: S. Montalvo , P. Gonzalez , C. Mena , L. Guerrero & R. Borja (2012) Influence of the food tomicroorganisms (F/M) ratio and temperature on batch anaerobic digestion processes with and without zeolite addition,Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 47:12,1785-1794
To link to this article: http://dx.doi.org/10.1080/10934529.2012.689235
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Journal of Environmental Science and Health, Part A (2012) 47, 1785–1794Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2012.689235
Influence of the food to microorganisms (F/M) ratioand temperature on batch anaerobic digestion processeswith and without zeolite addition
S. MONTALVO1, P. GONZALEZ1, C. MENA1, L. GUERRERO2 and R. BORJA3
1Departamento de Ingenierıa Quımica, Universidad de Santiago de Chile, Chile2Departamento de Ingenierıa Quımica y Ambiental, Universidad Tecnica Federico Santa Marıa, Valparaıso, Chile3Instituto de la Grasa (CSIC), Sevilla, Spain
The main objective of this work was to evaluate the influence of the food to microorganisms (F/M) ratio and temperature on batchanaerobic digestion processes carried out with and without zeolite addition as a microbial carrier. Three laboratory-scale experimentalruns were conducted using a synthetic substrate with a COD:N:P ratio of 500:5:1. The first run (I) was conducted at a constanttemperature of 27◦C, increasing the F/M ratio from 0.21 to 0.40 (g COD/g VSS). During the second run (II) the temperature andthe F/M ratio increased from 27◦C to 37◦C and from 0.21 to 0.40, respectively. Finally, in the third experimental run (III) the F/Mratio achieved high values (1.92 and 1.30) either by varying the substrate concentration at a constant biomass concentration or byincreasing the biomass concentration at a constant substrate concentration. Higher biomass growth rate, COD removal and methaneproduction were found in the reactors with zeolite, especially at the highest F/M assayed during the first run. The highest ammoniumremovals were also achieved at the highest F/M ratio (0.40) in the reactors with zeolite. Within the range studied (25◦C–37◦C) inthe reactors with zeolite operating at 37◦C, the second run demonstrated the low influence of temperature on substrate consumptionand ammonia removal, with 93% and 70% of COD and ammonia removal efficiencies, respectively. The third run corroborated theresults previously obtained and fit the experimental results to simple kinetic models, the Monod model being the most adequate forpredicting the behavior of the systems studied. The maximum specific microorganism growth rate (µmax) values for the reactors withzeolite were almost twice as high as those obtained for the reactors without zeolite for similar F/M ratios.
Keywords: Anaerobic processes, zeolite, kinetics, food/microrganisms ratio, temperature.
Introduction
The anaerobic treatment of medium and high-strengthwastewaters with a high biodegradable content has a num-ber of advantages:[1] quite a high degree of purificationwith high organic load feeds can be achieved, low nutri-ent requirements are sufficient, only small quantities ofexcess sludge are usually produced and, finally, a renewablecombustible biogas is generated. The production of biogasenables the process to generate or recover energy insteadof just saving it. This can significantly reduce operationalcosts compared with the high energy-consuming aerobicprocess.[2,3]
However, one of the greatest problems in the anaero-bic processing of wastewaters is the loss of biomass in
Address correspondence to R. Borja, Instituto de la Grasa(CSIC), Avda. Padre Garcıa Tejero, 4. 41012-Sevilla, Spain;E-mail: [email protected] February 24, 2012.
systems with high hydraulic loading rates. To solve thisproblem, reactors have been designed with supports thatfix the biomass and result in high loading densities andlow hydraulic retention times.[1–4] With the increase of pop-ulation density on the given support, there is a greaterchange in cross-feeding, co-metabolism and interspecieshydrogen and proton transfer, which may further stimulatethe growth of microcolonies. The use of a porous supportsuch as zeolite enables the anaerobic reactor to retain highbiomass concentrations and thereby operate at significantlyreduced hydraulic retention times.[1,2,4,5]
The structure and physical properties of natural zeolite[channel and pore cavities, minimum diameter of pores inthe range of 3 to 10 Angstroms, average surface area of24.9 m2/g, low bulk density, high exchange (CEC) andadsorption capacities] make it ideal for use in anaerobicdigestion processes for wastewater treatments.[6,7]
Other researchers have demonstrated the suitabilityof the use of zeolite as microbial immobilization sup-port in batch anaerobic processes operating at mesophilic
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temperatures.[8–11] Although anaerobic digestion processesin batch mode are not frequently applied at full-scale, theyare useful at laboratory-scale because they can be per-formed quickly with simple and inexpensive equipment andare helpful in assessing the extent to which a material canbe digested, providing relevant information related to themethane yield coefficient and kinetics of the process. Clayminerals such as zeolites and other surface-active materialshave been reported as influencing the microbial and enzy-matic transformation of a variety of substances, includingammonium, sulfur, carbohydrates, proteinaceous materialsand phenolic compounds.[8–11]
In addition, according to previous results, zeolite hasbeen found to be a successful microbial support in the batchmesophilic anaerobic digestion of different substrates, dueto the following characteristics: (i) its high capacity forimmobilization of microorganisms; (ii) its capacity for im-proving the ammonia/ammonium ion equilibrium; (iii) itsability to reduce the ammonia and ammonium ion in solu-tion.[10,11]
On the other hand, a disadvantage of the anaerobic pro-cess is its slow start-up stage, prior to achieving the designparameters operating at steady-state conditions.[12–16]Avery practical and useful tool for accelerating the start-up in anaerobic processes is to initiate the study with abatch experiment. Although this step may be sped up withthe use of zeolite,[13] it is important to consider that thesuitability of this procedure will be influenced by the food-microorganism (F/M) ratio, which will in turn determinethe different microorganisms-zeolite and substrate-zeoliteratios. In addition, it has recently been reported that theF/M ratio also influences the specific methanogenic activ-ity (SMA) test, this ratio being the most influential parame-ter for the SMA test.[17,18] Other studies have demonstratedthat F/M ratios in the range of 0.5 to 1.0 were suitablein batch anaerobic tests of different food wastes such assoup processing plants, cafeteria wastes, etc. operating atmesophilic temperature (35◦C) for 28 days.[19]
However, other similar wastes such as fish and grease trapwastes took longer to digest completely at these same F/Mratios, generating higher biomass yields than the previouslymentioned wastes.[19] Optimum F/M ratios in the rangeof 0.57 to 0.68 were reported in batch bioassay tests ofsynthetically prepared complex wastewaters, achieving adecrease in the methane potential and sludge activities atincreasing F/M ratios as well as a decrease by 81% in thekinetic constant for methane production when the F/Mratio increased to 0.9.[20] Low F/M ratios (0.5) were alsoused in batch anaerobic assays of mixtures of glycerin anddairy manure containing 60% glycerin (mixture 1) and 45%glycerin (mixture 2) on the VS basis.[21] Methane yields of0.31 and 0.22 L/g VS were achieved for mixtures 1 and 2,respectively.
In this context and taking the information obtained fromthe literature review into account, the aim of this work wasto assess the influence of the F/M ratio and temperature
on batch anaerobic digestion tests carried out with andwithout zeolite addition. For this purpose, a synthetic sub-strate with sucrose as a carbon source and a COD:N:Pratio of 500:5:1 was used, evaluating organic matter andammonium removals as well as methane production underdifferent operational conditions. Finally, different simplekinetic models were used to obtain the kinetic parametersof the processes and to compare the different conditionstested.
Materials and methods
Characteristics of the synthetic wastewater used
A synthetic wastewater with a COD:N:P ratio of 500:5:1was used as substrate. Sucrose was used as a carbon source,ammonia sulfate was used as a source of sulfate and nitro-gen, while potassium phosphate was used as a phosphorussource. A minimum sulfate concentration of 80 mg/L wasused in the experiments with the aim of simulating the char-acteristics of wastewaters derived from the manufacturingof pulp and paper.
Experimental procedure
Three experimental runs (I, II and III) were carried outto assess the influence of the F/M ratio and temperatureon batch anaerobic processes, which were conducted withand without zeolite addition. All experiments were per-formed in batch laboratory-scale reactors of 280 mL vol-ume, which were operated in triplicate. Each reactor wasinoculated with 70 mL of an anaerobic sludge, which wasobtained from a full-scale anaerobic digester treating wasteactivated sludge. During the experiments the contents of thereactors were mixed with magnetic stirrers at 120 rpm.
The reactors were hermetically closed and the methanegas produced was measured by the displacement of a 3 MNaOH solution in order to remove the CO2 and measureonly the methane gas produced in the processes. The char-acteristics of the zeolite used are summarized in Table 1.
Table 1. Chemical and mineralogical compositions of the zeoliteused.
Chemical composition Mineralogical composition(%) (%)
SiO2 66.62 Clinoptilolite 35Al2O3 12.17 Mordenite 15Fe2O3 2.08 Montmorillolite 30CaO 3.19 Others∗ 20MgO 0.77Na2O 1.53K2O 1.20Ignition Waste 11.02
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Table 2. Operating conditions of experimental run I.
Experiment no. Range COD (mg/L) VSS (mg/L) F/M SO2−4 (mg/L)
I-1 Low 800 3700 0.21 80I-2 Medium 1100 3700 0.29 110I-3 High 1500 3700 0.40 150
The diameter of the zeolite particles used in all experimentswas 1 mm.
In the first experimental run (I), three different assayswere conducted with F/M ratios of 0.21, 0.29 and 0.40 (gCOD/g VSS). The temperature of the process was main-tained constant at 27◦C as this is the mean outlet tempera-ture of the wastewater from the manufacturing of pulp andpaper. Table 2 summarizes the main operational conditionsused in run I.
During the second experimental run (II) and with theaim of assessing the influence of temperature, the latterwas kept constant at 27◦C and 37◦C during the first (II-1) and second (II-2) assays, respectively, whereas it wasuncontrolled (ambient temperature: 25◦C–30◦C) during thethird assay (II-3). The F/M ratio also increased from 0.21to 0.29 and 0.40 (g COD/g VSS) for assays II-1, II-2 and II-3, respectively. The amount of zeolite added to the reactorswas constant (3 g) as well as the initial sulfate content(200 mg/L). The main operational conditions used in thesecond experimental run (II) are shown in Table 3.
Finally, during the third experimental run (III), two setsof assays were conducted. The first set of experiments (III-1, III-2 and III-3) increased the F/M ratio from 0.24 to1.91 (by increasing the COD concentration from 500 to4000 mg/L and maintaining the biomass concentrationconstant at 2100 mg VSS/L). The second set of experi-ments (III-4 and III-5) saw a decrease in the F/M ratiofrom 1.33 to 0.77 (by increasing the biomass concentra-tion from 1500 to 2600 mg VSS/L and keeping the CODconcentration constant at 2000 mg/L). The temperaturewas kept constant at 37◦C throughout the run. Other op-erational conditions used in this third experimental runare summarized in Table 4. Different simple kinetic mod-els were applied to obtain the kinetic parameters from theexperimental data achieved in this third experimental run.
Chemical analyses
All chemical analyses were carried out according to therecommendations of the Standard Methods for the Ex-
amination of Water and Wastewater.[22] Specifically, COD,VSS and sulfate were analyzed according to the StandardMethod numbers 5220 C, 2540 E and 4500 A, respectively.Ammonium nitrogen was measured using a selective elec-trode (Hanna Instrument HI4101). Finally, pH was ana-lyzed with a pH-meter (Crison, model Basic 20).
Results and discussion
Experimental run I
Figure 1 shows the variation of the biomass concentration(g VSS/L) with time for the three F/M ratios assayed (0.21,0.29 and 0.40 g COD/g VSS) in the reactors with and with-out zeolite addition. As can be seen, a lag phase of about5 days can be observed in the reactors, both with and with-out support medium, although the duration of this phasewas 10 days for the reactors without zeolite at the lowestF/M ratio assayed. Figure 1 also demonstrates that themicroorganism specific growth rates for the reactors withzeolite were always higher than those for reactors withoutzeolite, independently of the F/M ratio considered.
A similar trend in the variation of biomass concentra-tion was reported by Jia et al. in batch experiments forthree anaerobic sludges, each of which had been enrichedat 35◦C and pH 6.9–7.3 for more than 40 experiments usingpropionate, butyrate and glucose, individually, as the solesubstrate.[23] They also stated that extracellular polymer(ECP) production was dependent on the growth phase ofmicroorganisms. Specifically, it increased at the beginningof all batches when the microorganisms were in the prolific-growth phase and for the higher F/M ratio studied. Later,ECP production gradually returned to its initial levels whenthe microorganisms were in the declined-growth phase, asthe substrate became depleted.[23]
The variation of the COD concentration with time forboth reactors with and without zeolite is plotted in Figure 2.COD removal efficiencies of 40% and 76% were achievedfor the lowest F/M ratio studied in the reactors without and
Table 3. Operating conditions of experimental run II.
Experiment no. Temperature (◦C) COD (mg/L)VSS
(mg/L) F/M Zeolite (g) SO2−4 (mg/L)
II-1 27 800 3700 0.21 3 200II-2 37 1100 3700 0.29 3 200II-3 25–30 1500 3700 0.40 3 200
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Table 4. Operating conditions of experimental run III.
Variation of the substrate concentration (COD)
Experiment No. COD (mg/L) VSS (mg/L) F/M Zeolite (g) N (mg/L) P (mg/L)
III-1 500 2100 0.24 0.5 6.2 1.2III-2 2000 2100 0.96 2.0 25 5III-3 4000 2100 1.91 4.0 50 10
Variation of the biomass concentrationIII-4 2000 1500 1.33 2 25 5III-4 2000 2600 0.77 2 25 5
with zeolite respectively, after the first 5 days of operation,despite this period coinciding with the lag-phase. Thesehigh degradation percentages, can be attributed to the easybiodegradability of the substrate used, mainly composed ofnutrients and sucrose as organic matter source. In general,it can be observed that the percentages of COD removalsincreased at higher F/M ratios, which can be attributedto the higher amounts of organic matter available for themicroorganisms at the greater F/M ratios assayed when noinhibition processes are observed.
Fig. 1. Variation of the biomass concentration (mg VSS/L) withtime for the three F/M ratios assayed in the reactors with andwithout zeolite addition (♦: F/M ratio = 0.21; �: F/M ratio =0.29; �: F/M ratio = 0.40) (color figure available online).
However, it is noteworthy that for the highest F/M stud-ied, the difference in the final COD removal efficiency be-tween the reactors with and without zeolite was only 1.6%(93.6% and 92.0%, respectively), whereas this differencewas 3% and 2.6% for the F/M ratios of 0.21 and 0.29 gCOD/g VSS, respectively. This demonstrated the lowercontribution of the zeolite to organic matter removal and,therefore, its secondary role at the higher F/M ratios stud-ied when no inhibition was observed.
Fig. 2. Variation of the substrate concentration (mg COD/L) withtime for the three F/M ratios assayed in the reactors with andwithout zeolite addition (♦: F/M ratio = 0.21; �: F/M ratio =0.29; �: F/M ratio = 0.40) (color figure available online).
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Fig. 3. Variation of the methane production (mL) with time forthe three F/M ratios assayed in the reactors with and withoutzeolite addition (♦: F/M ratio = 0.21; �: F/M ratio = 0.29; �:F/M ratio = 0.40) (color figure available online).
As a result of the increased COD removals obtained inthe reactors with zeolite, higher methane productions werealso achieved when zeolite was added for the three F/Mratios studied (Fig. 3). In all cases, methane production im-proved at increasing F/M ratios. However, for the highestF/M ratio assayed, the final difference in methane produc-tion was only 30 mL, corresponding to greater productionin the reactors with zeolite added. Higher methane pro-ductions were also reported at higher F/M ratios in batchanaerobic digestion experiments of waste activated sludgepreviously treated with ultrasound to increase its digestibil-ity.[24]
In the same way, an F/M ratio value (0.5) slightly higherthan the highest F/M ratio studied in the present work(0.4) was also reported as an optimum ratio for achievingelevated methane yields during batch anaerobic digestionof glycerin, dairy manure and mixtures of both wastes after14 days of operation.[21] On the other hand, Figure 3 alsoshows that for the lowest F/M ratio (0.21) studied, a periodof low or almost no methane production was observedbetween the 5th and 10th days of digestion, after which aconsiderable increase in methane production was detected.
A similar trend was reported by Chen and Hashimoto[25],where a period of inactivity (with virtually no methane
Fig. 4. Variation of the sulphate concentration (ppm) with timefor the three F/M ratios assayed in the reactors with and withoutzeolite addition (♦: F/M ratio = 0.21; �: F/M ratio = 0.29; �:F/M ratio = 0.40) (color figure available online).
production) was observed between two periods of activemethane production during batch anaerobic digestion ex-periments conducted to determine the effects of the inocu-lum:substrate ratio (inverse of the F/M ratio) and initialpH medium on methane production from glucose. Theseresearchers stated that the generation of methane increasedwhen the F/M ratio rose from 0.02 to 0.2[25], which also oc-curs in the present work when the F/M ratio increases from0.21 to 0.40 g COD/g VSS.
The variation in the concentration of sulfate with time forthe three F/M ratios studied in the reactors with and with-out zeolite is illustrated in Figure 4. In the presence of zeo-lite, the formation of bacterial agglomerates increases andthis favors the proliferation of Methanogen Archaea whencompeting with the sulfate-reducing bacteria (SRB), result-ing in higher sulfate concentrations in the reactors.[26,27]
However, for the highest F/M ratio assayed (0.40), the dif-ference in sulfate concentration at the end of the processbetween the reactors with and without zeolite addition wasonly 4%. Once again, this corroborates the fact that thecontribution of zeolite to the overall process is less for thehighest F/M ratio.
Figure 5 depicts the evolution of the concentration ofammonium with time both for the reactors with and
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Fig. 5. Variation of the ammonia concentration (mg/L) with timefor the three F/M ratios assayed in the reactors with and withoutzeolite addition (♦: F/M ratio = 0.21; �: F/M ratio = 0.29; �:F/M ratio = 0.40) (color figure available online).
without zeolite addition at the three F/M ratios used. Thereis a consumption of ammonium during the first 10 days,due to the necessity of this compound for the microor-ganisms growth and metabolism, observing, afterwards,an accumulation of this chemical specie, which was dueto the hydrolysis of the organic nitrogen contained in thesubstrate. However, inhibitory ammonium concentrationsfor anaerobic microorganisms were never achieved in anycase.[28,29]
It can also be observed that the accumulation of am-monium is much higher in the reactors without zeolite.For instance, for the lowest F/M assayed, the final am-monium concentration achieved in the reactors withoutzeolite (480 mg/L) was higher than that obtained in thereactors with zeolite (380 mg/L). This can be explained bythe physical properties and chemical composition of thezeolite (clinoptilolite) which has an elevated ionic exchangecapacity and adsorption properties, which allow high am-monium removals to be achieved.[30,31] It is noteworthy thatfor the highest F/M assayed in the reactors with zeolite,a continuous decrease in the concentration of ammonium
Fig. 6. Variation of the biomass concentration (mg VSS/L) withtime for the three temperatures assayed in the reactors with andwithout zeolite addition (♦ = non-controlled temperature; �:temperature = 27◦C; �: temperature = 37◦C) (color figure avail-able online).
was observed up to the end of the digestion time, which wasdue to the presence of higher amounts of organic matterand more elevated biomass concentrations.
Experimental run II
Figures 6, 7 and 8 illustrate the variation of biomass (gVSS/L), COD and ammonium concentrations with timerespectively, for the three temperatures assayed (uncon-trolled: 25◦C–30◦C; 27◦C and 37◦C) in the reactors withand without zeolite addition. In general, it was observedthat the behavior of the anaerobic processes carried outwith and without zeolite addition were very similar to thosedescribed in experimental run I. This experimental run fo-cused more specifically on the evaluation of the influenceof temperature on process performance.
Once again, it is observed that the performance of theanaerobic processes gives better results when zeolite wasadded to the reactors. For instance, higher biomass con-centrations were always achieved when zeolite was addedto the reactors (Fig. 6) achieving values close to 14,000 mg
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Fig. 7. Variation of the substrate concentration (mg COD/L)with time for the three temperatures assayed in the reactors withand without zeolite addition (♦ = non-controlled temperature;�: temperature = 27◦C; �: temperature = 37◦C) (color figureavailable online).
VSS/L after the 8th day of digestion at temperatures of27◦C and 37◦C.
Higher biomass growth was detected at 37◦C, althoughsomewhat lower biomass concentration values were notedat the end of the anaerobic process after 14 days of diges-tion independently of the temperature used. To avoid thisproblem, which was specifically observed when the reac-tors operated in continuous mode at very low hydraulicretention times (HRT), some authors have reported the useof bioaugmentation to maintain consistent COD removalsdue to an inability to retain and grow biomass.[16] In thislight, Saravanane et al. achieved a maximum COD reduc-tion of 88.5% in the anaerobic digestion of pharmaceuticaleffluent (COD: 14,000–18,000 mg/L) in a fluidized bed re-actor operating at HRTs of 3–12 h through the periodicaladdition of acclimated biomass every 2 days with 30–73 gof VSS from an off-line enricher reactor.[16]
Figure 7 reveals that lower COD concentrations werealways achieved in the reactors with zeolite addition at atemperature of 37◦C, especially during the first 5 days of di-
Fig. 8. Variation of the ammonia concentration (mg/L) with timefor the three temperatures assayed in the reactors with and with-out zeolite addition (♦ = non-controlled temperature; �: tem-perature = 27◦C; �: temperature = 37◦C) (color figure availableonline).
gestion, achieving maximum COD removals of 93% at theend of the process. A small influence of the operating tem-perature on COD removal during the last days of digestionwas observed. In the same way, similar results were achievedduring batch anaerobic digestion processes of selected foodwastes (i.e., derived from a soup processing plant, cafete-ria, commercial kitchen, etc.) under both mesophilic (35◦C± 2◦C) and thermophilic (50◦C ± 2◦C) temperatures after28-day digestion time.[32] These authors also reported thatmore than 80% of the total biogas generated was producedwithin the first 5 days of digestion.
The evolution of ammonium concentration with time forthe reactors with and without zeolite addition at the threetemperatures assayed is shown in Figure 8. Lower ammo-nium concentrations were always observed in the reactorswith zeolite, independently of the temperature of the test.A 70% ammonia removal was reached at a temperature of37◦C in the reactors with zeolite after 8 days of digestion.No influence or at least very little influence of temperatureon ammonium removal was observed.
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Table 5. Kinetic models that best fit the data for the experiments with zeolite addition.
Biomass at the end of the Model for the DeterminationExperiment F/M ratio experiment, VSS (mg/L) best fit Kinetic parameters∗ coefficient (R2)
III – 1 0.24 3800 Contois µmax = 0.040 ± 0.003 d−1 0.970td = 16.4 ± 0.8 dB = 0.47 ± 0.04
III – 2 0.96 4100 Monod µmax = 0.130 ± 0.03 d−1 0.994td = 5.1 ± 0.1 dKs = 3700 ± 110 mg/L
III-3 1.92 5900 Contois µmax = 0.070 ± 0.002 d−1 0.989td = 9.2 ± 0.3 dB = 1.5 ± 0.1
III-4 1.30 2500 Contois µmax = 0.52 ± 0.04 d−1 0.959td = 1.30 ± 0.09 dB = 5.5 ± 0.4
III-5 0.78 3700 Monod µmax = 0.14 ± 0.03 d−1 0.991td = 4.8 ± 0.1 dKs = 1400 ± 40 mg/L
∗Kinetic parameters with their standard deviations (p < 0.05).
Experimental run III
Tables 5 and 6 show the results obtained after adjustingthe data of experimental run III to different kinetic models,which can be linearized. The following kinetic models wereused:
Monod[33] : µ = µmaxS/(Ks + S) (1)
Monod with inhibition[34]:µ = µmaxS/(Ks + S + S2/KI )(2)
Grau[35] : µ = µmaxS/S0 (3)Contois[36] : µ = µmaxS/(BX + S) (4)
where: µ and µmax are the specific microorganisms growthrate and maximum specific microorganisms growth rate(time−1), respectively; S is the substrate concentration ororganic matter content in the medium (mass/volume); Ksis the Monod saturation constant (mass/volume); KI isan inhibition constant (mass/volume); S0 is the influentsubstrate concentration (mass/volume); B is a kinetic co-efficient that represents the relationship between substrateand microorganism concentrations. Finally, the duplicationtime (td) was also determined in all cases studied.
In general, a good fit of the experimental data of runIII to the Monod and Contois models was observed.The inhibition kinetic model assayed (Eq. 2) and the
Table 6. Kinetic models that best fit the data for the experiments without zeolite addition.
Biomass at the end of the Model for the DeterminationExperiment F/M ratio experiment, VSS (mg /L) best fit Kinetic parameters∗ coefficient (R2)
III – 1 0.24 2400 Contois µmax = 0.020 ± 0.002 d−1 0.962td = 31.7 ± 1.9 dB = 0.32 ± 0.03
III – 2 0.96 2500 Monod µmax = 0.070 ± 0.006 d−1 0.926td = 9.6 ± 0.8 dKs = 2800 ± 250 mg/L
III-3 1.92 2500 Contois µmax = 0.05 d−1 0.763∗∗td = 11.7 dB = 0.88
III-4 1.30 2200 Monod µmax = 0.110 ± 0.006 d−1 0.966td = 6.3 ± 0.4 dKs = 2600 ±150 mg/L
III-5 0.78 3300 Monod µmax = 0.029 ± 0.003 d−1 0.961td = 23.9 ± 1.7 dKs = 1100 ± 80 mg/L
∗Kinetic parameters with their standard deviations (p < 0.05).∗∗Non-adequate correlation, although it was the best fit achieved.
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multicomponent substrate Grau model (Eq. 3) were inade-quate for two reasons: first, no inhibition phenomena wereobserved in this last experimental run, such as also occurredin experimental runs I and II, and second the characteris-tics of the synthetic substrate used in this study were simple.Inhibition phenomena were not detected by either the am-monium or the sulfate generated in the anaerobic processes.The Grau model was found to be more applicable in thecase of more complex substrates with multi-components,for which a dependence of the influent substrate concen-tration on the kinetics of the process was observed.[35]
In general, an increase in the µmax values at increasedF/M ratios within the range studied in this experimentalrun was observed in the reactors both with and without ze-olite addition. The µmax values for the reactors with zeolitewere almost twice as high as those obtained for the reac-tors without zeolite for similar F/M ratios. By contrast,the duplication times, td, were always lower for the reactorswith zeolite, which once again demonstrates the suitabilityof the use of this microorganism carrier in batch anaerobicdigestion processes.
For substrates containing complex compounds andCOD particulate (CODP), F/M ratios in the range of 0.57to 0.68 were reported as optimum from a kinetic point ofview.[20] In this case, when the CODP/VSS ratio increasedfrom 0.1 to 0.9, the kinetic constant for methane generationwas reduced by 81%.[20]
Finally, the high determination coefficients obtainedfrom the adjustment of the experimental data to the Monodmodel and the low standard deviations (less than 5% inmost cases) of the kinetic parameters derived from thismodel (Tables 5 and 6) demonstrate the suitability of the useof the Monod kinetic model and suggests that it describesthe batch anaerobic digestion processes assayed very accu-rately and that the kinetic parameters obtained representthe activity of the different microorganism types effectingthe anaerobic digestion of this substrate at the differentF/M ratios tested.
Conclusions
For the F/M ratios tested, it was found that the use of nat-ural zeolite as biomass carrier increased the efficiency ofthe anaerobic digestion process promoting further growthof biomass and a greater removal of organic matter. As aresult, more methane was produced which lead to a generalincrease in the efficiency of the process with a rise in theF/M ratio. However, the contribution of zeolite to gener-ating methane was less favored by increasing the F/M ra-tio. Maximum levels of methane obtained were lower thanthose expected theoretically due to the presence of sulfatein the synthetic wastewater studied, which brought abouta competition between the archaea methanogens and thesulfate-reducing bacteria, with the latter being thermody-namically favored.
In the experiments with different ranges of temperature,again the contribution of zeolite in the production of bio-gas, generating a greater amount of methane in the pres-ence of this material was demonstrated. The best resultswere achieved at the highest temperature studied (37◦C)with the first days of the process showing the most markedimprovement.
As this wastewater contains relatively simple substrates,the kinetics of the process fitted very well to the simpleMonod model. Finally, the results obtained lead to theconclusion that the addition of natural zeolites to anaero-bic processes can significantly help to accelerate the startupstage of these processes, prior to their initiation in contin-uous mode.
Acknowledgments
The authors wish to express their gratitude to FONDE-CYT project no. 1090414 (Chile) and the University ofSantiago de Chile (project 091111MM) for providing fi-nancial support.
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