Two-stage anaerobic digestion for the treatment of cellulosic wastes

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<ul><li><p>The Chemrcal Engzneenng Journal, 33 (1986) Bl - BlO Bl </p><p>Two-Stage Anaerobic Digestion for the Treatment of Cellulosic Wastes </p><p>P GIRARD*, J M SCHARER and M MOO-YOUNG </p><p>Department of Chemzcal Engmeerrng, Unwerslty of Waterloo, Waterloo, Ontano N2L 3Gl (Canada) </p><p>(Received January 10, 1985) </p><p>ABSTRACT </p><p>The feaslballty of two-stage anaerobic dlgestlon systems for methane genemtlon from celluloslc wastes was studied The microflora comprised a heterogeneous bac- tenal culture at mesophlhc temperature (39 C) In the first stage, optimum organic acid production from cellulose concentrations ranging from 0 5% to 2 3% cellulose feed (w/v basis) occurred at carbon to nitrogen ratios of 4 to 1 High loading mtes favoured butyrlc aczdproductlon, while at low loading rates (0 5 g I- day-) acetic and proptonlc acids were predominant </p><p>Both fuced film and suspension cultures were utilized to study methanogenesls from organic acid muctures (second stage) Acetate utdlzatlon by methanogens followed a Monod-type kinetic model, the maximum specific growth rate p,,, and the half satum- tlon constant K, were found to be 0 17 day- and 8 46 X 1 OA3 M acetate respectively Proplonate and bu tyrate u tlhzatlon were mhlblted by acetate The mhlbltlon resembled classical competltwe mhlbltlon patterns The mechanlstlc models derived from the exper- imental data were applicable for acid matures m both downflow fmed film and suspensions cultures </p><p>1 INTRODUCTION </p><p>Anaeroblc digestion of organic matter has become very attractive as a means of stablllzmg highly concentrated wastes (m- dustnal and agncultural residues) The </p><p>*Present address Biotechnology Research Institute, NatIonal Research Council of Canada, 750 Bel-Air Street, Montreal, Quebec, H4C 2K3, Canada </p><p>0300-9467/86/$3 50 </p><p>pnmary advantages of the anaerobic process include a higher degree of digestion of hgno- celluloslc matenals at high loading rate [l], reduction of pollution m terms of COD or BOD, production of energy m the form of methane gas and recovery of residues as fertlhzer Currently, anaerobic digestion 1s viewed as a three-stage process [ 21. The first stage mvolves acldogenlc bacteria which hydrolyse and ferment carbohydrates, protems and lipids to alcohols, volatile fatty acids, Hz and COZ. The second stage mvolves aceto- geruc bacteria which produce acetate, CO* and Hz from the alcohols and higher fatty acids Finally, m the thu-d stage, methano- gemc bactena utlhze the products of the previous stages, mamly acetate, CO* and Hz, to produce CH4 and CO* Owing to the different nutrltlonal and envu-onmental requirements of the microbial groups, stage separation seems to be a rational approach to optlmlzmg the process [3]. The two stages of the anaerobic process mclude an acldo- gemc and a methanogemc stage. </p><p>Previous anaerobic reactor designs were based on emplrlcal models [4 - 61 which correlate the experimental data on gas production with some space-loading variable These models do not take mto account the mechanism of the anaerobic process and, therefore, cannot predict gas production outside the realm of experunental conditions or digester falure. Mathematical models representing steady-state condltlons [7 - 91 are more fundamental than emplncal models smce they attempt to explam methane production from some mtermedlates present during the fermentation However, anaerobic reactors are seldom at steady state For this reason, the dynamic characterlzatlon of the process seems the best approach m order to understand, quantify and predict methane </p><p>0 Elsevler Sequola/Prmted in The Netherlands </p></li><li><p>B2 </p><p>production The dynamic models of Andrews and Graef [lo] and the modification by Hill and Barth [ 111, however, have only considered acetate as a malor mtermediate for methane production. This approach does not take mto account the different kmetics mvolved m the utihzation of higher fatty acids such as propionic and butync acids These higher volatile fatty acids are not used directly for methane production, but are degraded by acetogemc bacteria m syn- trophic association with methanogens [ 21. </p><p>The research described here focussed on both acid production and methane generation from cellulose. Volatile fatty acid production and acid distribution were assessed as a func- tion of the cellulose loading rate and carbon- to-nitrogen ratio m suspension cultures. Both downflow fixed-film and stirred bioreactors were used to study the kmetics of acid utmzation and methanogenesis. </p><p>2 MATERIALS AND METHODS </p><p>2 1 Orgamc acid production m suspension culture </p><p>The experimental studies were performed m both 500 ml bioreactors and 30 1 digester vessels. The 30 1 apparatus, shown m Fig. 1, consisted of an upright glass cyhnder 23 cm m diameter equipped with a conical-bottomed recirculation pump, temperature controller and wet test meter Cellulose (cr-floe) and urea were used as the primary carbonaceous substrate and the nitrogen source respective- ly. The concentrations of other organic and morgamc constituents were based on levels found m normal swine manure [ 121. The moculum comprised a mixed culture of bacteria from a fed-batch system adapted to the cellulose substrate The fermentations were carried out at 39 C. The pH was ad- Justed to an u-&amp;ml value of 6.0 with HCl. These studies included both batch and fed- batch fermentations with dsuly addition of fresh medium. </p><p>2 2 Organrc aced utakataon and methane generation </p><p>Kmetic studies on acid utilization and methanogenesls were performed m 500 ml bioreactors (suspension culture) and fixed- film reactors. The fixed-film reactors, shown </p><p>&amp;d. </p><p>.51609.05 pk.S= Plus* </p><p>Fig 1 Schematics of the two-stage anaeroblc dlges- tlon apparatus </p><p>m Fig 1, consisted of glass columns (length, 85 cm, diameter, 6.5 cm) packed randomly with ceramic Raschig nngs of diameter 1.25 cm to a height of 62 cm The hquid level was mamtamed 10 cm above the film support and the surface area to volume ratio (243 cm2 me3) was near optimum as reported by Van den Berg and Kennedy [13] The reactors were operated m a downflow manner at a recuculation rate of 6 hquid volumes h- Inocula for both the suspension and fixed-film reactors were denved from a 20 1 methanogenic fermenter, fed continuously with a fatty acids, urea and salts solution The reactor temperature was mamtamed at 39 + 1 C! The pH was controlled at the desired level with an automatic pH controller. Approximately 4 months were required to establish a satisfactory biofilm of methano- genie bacteria m the fixed-film reactors. </p><p>Analytical techniques included cellulose determmations by a modified Updegraff [14] method, ammoma measurements by specific ion electrode, carbon dioxide evolu- tion by adsorption m 0.1 N BaOH solution [ 151, particulate orgamc mtrogen by the KJelfoss method, organic acids and biogas analysis by gas chromatography [ 71 </p><p>3 RESULTS AND DISCUSSION </p><p>3 1 Orgamc acd productwn Fermentations of cellulose to orgamc acids </p><p>were performed at carbon to mtrogen [C]/[N] ratios rangmg from 4 to 24. In Table 1, the product yields, expressed m gramme- equivalents of acetic acid per gramme of </p></li><li><p>B3 </p><p>TABLE 1 </p><p>Effect of carbon to mtrogen rat.10 on volatile acid yield </p><p>Znztzal cellulose C/N ratzo Fermentatzon Cellulose Fznal pH Yzeld content (%) tz me (days) utzlzzatzon (%) (g equw acetic acid (g cellulose)-) </p><p>05 4/l 5 82 1 61 0 57 0 55 24/l 5 87 3 58 0 47 10 4/l 7 93 5 61 0 75 10 24/l 7 79 8 55 0 32 23 411 11 92 8 62 0 87 23 24/l 11 68 4 50 0 45 </p><p>TABLE 2 </p><p>Effect of cellulose loading rate on cellulose uthzatlon and acid dlstrlbutlon </p><p>Loadzng rate Retentzon tzme g 1-l day- (days) </p><p>05 5 10 5 23 5 23 11 </p><p>Aczd dzstrzbutzon </p><p>Acetate Propzonate (W (M) </p><p>0 0250 0 0026 0 0410 0 0183 0 0480 0 0196 0 1030 0 0330 </p><p>Butymte (M) </p><p>0 0014 0 0028 0 0026 0 0267 </p><p>Cellulose utzlzzatzon (%) </p><p>82 1 72 6 27 4 92 2 </p><p>cellulose added, are compared at low and high (24) [C ] / [ N] ratios at various cellulose concentrations m the fermentation broth At low cellulose content m the medium, both the cellulose utlhzatlon efflclency and the product yield were essentially independently of the [C]/[N] ratio At high cellulose con- centratlon, however, both the cellulose utlh- zatlon efflclency and the product yield de- chned slgmflcantly as the [C] /[N] ratio mcreased. This declme IS beheved to be due to the depressed pH condltlons expenenced mth higher [C] /[ N] ratios Durmg the fermentation, the pnmary mtrogen source, urea, was readily hydrolyzed to ammonia and carbon dioxide. At high [Cl/ [N] ratios coupled with high cellulose levels, the am- momum ion product was msufflclent to poise the pH (z e , neutrahze acids) m the optimum range for cellulose hydrolysis and acid gener- ation. </p><p>The effect of cellulose loadmg rate on acid production and acid dlstnbutlon at a [C]/[N] ratio of 4 1s shown m Table 2 At a constant hydraulic retention time (5 days), the cellulose utlllzatlon efflclency dechned with mcreasmg loading rate The mcrease m the retention tnne from 5 to 11 days, however, was beneficial with regard to </p><p>the cellulose utlhzatlon efficiency. As the loadmg rate increased, butyrlc and higher carbon fatty acids (Valerie, caprolc acids) became progressively more promment m the fermentor effluent </p><p>3 2 Orgamc acid utduatlon and methane generation </p><p>Kinetic studies were performed with synthetic acid mixtures as well as acids produced from various carbohydrates. On the basis of these studies, a set of dlfferentlal equations was denved to model acetate, proplonate and butyrate utlhzatlon m both suspension cultures and fured film reactors. </p><p>Acetate 1s the major, but by no means the only mtermedlate produced from the degradation of celluloslc materials.. However, most of the previous studies involved either acetate only m ennchment cultures or higher acids were expressed as acetate equvalents. Prehmmary results mdlcated substantial dlf- ferences between the kmetlcs of acetate and other fatty acid utlhzatlon. </p><p>3 3 Kmetzcs of acetate utlllzatzon Using Methanosarcma stram 227, Smith </p><p>and Mah [16] have shown that the rate of acetate utlhzatlon can be expressed by a Monod model as follows. </p></li><li><p>B4 </p><p>0 024 </p><p>-PredIcted 0 Expenment 1 l Experiment 2 </p><p>0 1 2 3 4 5 </p><p>Time (days) </p><p>Fig 2 Observed and predicted acetate utlllzatlon m free suspensions T= 39 C!, mltlal pH 7 uncon- trolled </p><p>dA ---pmax AX _= dt &amp;a + A)Y, </p><p>(1) </p><p>The concentration of methane bactena can be related to acetate concentration through the growth yield [3] m the followmg manner </p><p>dA -~maxA~(A,- A) + -WY,1 _=- dt Wea + A) </p><p>(2) </p><p>The mam advantage of eqn. (2) 1s that the biomass production 1s replaced by a more readily measurable terms, z e acetate concen- tration The expenments reported were performed with acetate as the pnmary carbon source for methanogenesls with both stirred suspended cultures and fixed-film reactors. The maximum specific growth rate pmax and the half-saturation constant K,, were evaluated for this data by numencally mte- gratmg eqn (2) and comparmg it with the experunental data by non-linear regression The parameters obtamed m this manner were 0.17 day- for /.L,,, and 8.46 X 10e3 M for &amp;a Virtually identical parameter values </p><p>032 t 02 </p><p>02 </p><p>02 </p><p>01 1 OExperlmental </p><p>0 0 25 0 50 0 75 I D 1 25 15 Time (days) </p><p>Fig 3 Observed and predwted acetate utlhzatlon m the fixed film bed reactor T = 39 C, pH 7 un- controlled </p><p>were obtamed m suspension cultures and fured-film reactors The parameter values of the two systems differed only with respect to the active biomass levels The fixed-flm reactors contamed approximately 4 tnnes more biomass than the free suspensions on a unit volume basis. This was expected, since the support retamed more biomass than the conventional reactor The expen- mental data are compared with the snnula- tlons m Fig 2 for the suspension cultures and m Fig. 3 for the fured-hhn reactor 1rutm.l acetate concentrations m these expenments were approxnnately 2 g 1-l No inhibition by acetate was observed at concentrations m excess of 10 g 1-l </p><p>In Table 3, the kinetic parameters obtamed m this study are compared with published data The generally hgher values reported for E.c,,, m suspension cultures may be due to several factors It has been observed by Lawrence and McCarty [9] that acetate utlhzmg bactena adhere to solid surfaces (wall growth) and if neglected m the calcu- lations, could result m apparently high specific growth rate and acid utlhzatlon, particularly m contmuous culture With the </p></li><li><p>B5 </p><p>TABLE 3 </p><p>Comparison of reported maximum speclflc growth rate prnax and half-saturation constant K,, for acetate utlhza- tlon m anaeroblc dIgesters </p><p>T (C) Anax (day-) Ksa (M) Type of drgestzon PH Reference </p><p>35 0 360 3 88 x 10-3 Swine manure 75 Smith et al [24 ] 2 stages </p><p>35 0 356 2 57 x 10-3 Enrichment culture Near neutrahty Lawrence [ 231 </p><p>37 0 45 5 x 10-3 Enrichment culture - Smith and Mah [ 161 (smgle orgamsm) </p><p>37 - 10 x 10-J Digestion sludge 68 Ghosh and Pohland [ 81 (2 stages) </p><p>38 0 400 3 33 x 10-5 Simulation - Andrews and Graef [lo] </p><p>39 0 170 8 46 x 1O-3 Enrichment culture 7 00 This study </p><p>0 0025 </p><p>0 </p><p>0 0 0025 0 005 0 0075 0 010 0 0125 0 015 </p><p>Concentration of Proplonate (M) </p><p>Elg 4 Effect of acetate concentration on the rate of proplonate utlllzatlon m the fixed film bed reactor </p><p>exception of data given by Andrews and terns These higher volatile fatty acids, how- Graef [lo], the reported half-saturation ever, are not directly utilized by methanogens constants range from 2.5 X 10m3 M to 10d2 Rather, they are degraded by acetogenic M acetate. In addition to acetate, propionate bacteria to acetate and other products in and butyrate are also present m significant syntrophic association with methano- concentrations m anaerobic digestion sys- gens </p></li><li><p>B6 </p><p>3 4 Kmetm of proptonate and butyrate u t&amp;a tlon </p><p>The expenmental results have shown that the rate of utlllzatlon of proplonate and butyrate are more complex than the rate of acetate uttizatlon. In Fig 4, the rate of proplonate utlhzatlon 1s plotted at various acetate concentrations. These results were obtamed m the fixed f&amp;n bloreactor at controlled pH values of 7 0. At acetate concentrations, as low as 0 01 M, the rate of proplonate utlllzatlon m 0 005 proplonate solutions declined more than two-fold m comparison to acetate-free solu- tions Acetate concentrations of 0 04 M resulted m approximately 90% reduction of proplonate utilization rates. Accordmg to a model dlscrlmmatlon study [ 171, the effect of acetate on proplonate degradation could best be expressed by a competltlve mhlbltlon model with acetate A as an m- hlbltor. </p><p>ClP P -= dt -k* K, + P + A/k, (3) </p><p>To examine the kmetlcs of multiple acids utllzatlon, the flow of fresh medium to contmuously operating digesters was mter- rupted and the effluent recycled at a hquld reclrculatlon rate of 6 liquid volumes h- Figure 5 gives typical results of acids utlh- zation in acetatepropionate solutions. At high acetate concentrations, the rate of pro- plonate d=appearance 1s relatively low m comparison with that of prop...</p></li></ul>