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Hydrogen production in a high rate fluidised bed anaerobic digester
~ Pergamon PII: S0043-1354(96)00235-7
Wat. Res. Vol. 31, No. 6, pp. 1291-1298,1997 © 1997ElsevierScienceLtd All rights reserved.Printedin Great Britain 0043-1354/97$17.00+ 0.00
H Y D R O G E N PRODUCTION IN A HIGH RATE FLUIDISED BED ANAEROBIC DIGESTER A. J. GUWY I, F. R. HAWKES j*, D. L. HAWKES 2 and A. G. ROZZI 3 ~School of Applied Sciences, 2Department of Mechanical and Manufacturing Engineering, University of Glamorgan, Pontypridd, Mid-Glamorgan CF37 1DL, U.K. and 3politecnico di Milano, DIIR, Sezione Ambientale, Piazza L da Vinci 32, 20133 Milan, Italy (First received November 1995; accepted in revised form July 1996) Abstract--F|iogas hydrogen content varies with the operational conditions of an anaerobic digester, and may be a useful control parameter. Experiments were carried out on a laboratory scale fluidised bed anaerobic digester, fed with a synthetic baker's yeast wastewater. Step overloads produced a sharp peak in biogas hydrogen level measured on-line, e.g. an increase of loading rate from 40 to 63 kgCOD m -3 day -~ increased hydrogen concentration from 290 to 640 ppm within 3 h. However, switching from an older, partly acidified batch to a fresh batch of feed at constant COD gave a marked increase in lhe biogas hydrogen content from 200 to 800 ppm. In these studies propionic acid and biogas hydrogen concentrations were not linearly correlated. According to the present results, biogas hydrogen content is unsuitable as a stand-alone control variable for anaerobic digestion, but its rapid response and ease of on-line measurement supports its use in digester control along with other liquid phase parameters to be measured on-line. © 1997 Elsevier Science Ltd. Key words--hydrogen, VFA, bicarbonate alkalinity, fluidised bed, anaerobic digester
INTRODUCTION Hydrogen transfer The transfer of hydrogen plays a vital part in the anaerobic process and the term "interspecies H2 transfer" describes the phenomenon in which the presence of one or more H~-consuming species significantly alters the metabolism of a fermentative organism towards production of more oxidised end products (Mclnerney et al., 1980). The hydrogen transfer system results in greater availability of energy for acidogens and acetogens, increased substrate utilisation, increased growth of all participants and the displacement of unfavourable reaction equilibria (Harper and Pohland, 1986). According to Dornseiffer et al. (1995) interspecies formate transfer is not a significant alternative to intcrspecies hydrogen transfer. The interspecies hydrogen transfer system was thought to occur between the syntropic bacterial associations through a hydrogen pool in the liquid phase. However, according to Conrad et al. (1985) the large hydrogen turnover rates and the small size of the hydrogen pool suggested that as much as 95% of the hydrogen produced never entered a common hydrogen pool. Hydrogen is not very soluble; 0.017 cm 3 hydrogen gas dissolves in 1 cm 3 of water at 1 bar and 37°C. This would explain the importance of the formation of micro-environments, biofilms in *Author to whom al)icorrespondence should be addressed.
anaerobic filters or granules in UASB reactors, because the close association of the different bacterial groups, which transfer hydrogen directly from one to another, increases their overall efficiency. Hydrogen concentration Free energy changes of the main anaerobic degradation reactions strongly depend on hydrogen partial pressure Pn2. Therefore several investigators assumed the P.2 in the off gas to be a good indicator of digester performance, but this only holds assuming equilibrium conditions between the liquid and gas phases. Otherwise the free energy depends on the concentration of dissolved hydrogen in the liquid phase only. As it is relatively difficult and expensive to determine both the P~2 and the H2 concentration in the liquid, only a few investigators measured both variables. In the majority of published studies H2 was measured in the gas after H2S scrubbing using a polarograph, calculating H2 concentration in the liquid from equilibrium considerations. In the following PH: is given as ppm which roughly corresponds to 10-6 bar or 10 pascal. In a healthy, stable anaerobic digester, very low partical pressures of hydrogen usually occur in the biogas. Collins and Paskins (1987) found that Ps~ in 20 U.K. sludge digesters operating with hydraulic retention times (HRT) of 8-40 days had P,2 which varied from 15-199 ppm. Hickey and Switzenbaum (1991), operating digesters on waste activated sludge at a 10 day HRT, observed that P.2 varied in response
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to overloads. Hickey et al. (1987, 1989) showed that the concentration of hydrogen in the biogas of a digester operating on activated sludge responded rapidly to pulse additions of toxicants such as chloroform and heavy metals. Kidby and Nedwell (1991) found P.2 values around 30 ppm from sludge digesters operating at a 20 day HRT. Operation on an 8 day HRT led to failure, but PH2 showed little change until after failure had occurred. They concluded that use of gaseous hydrogen gives no early warning of overloads in sludge digesters, because of the high fraction of refractory compounds in the feed. They suggest it may be better exploited with reactors treating more readily biodegradable wastes. PH2 variations in high rate anaerobic digesters in non-steady state operation have also been reported. Mathiot et al. (1992) using a fluidised bed reactor operating on wine distillery wastewater at a volumetric loading rate (By) of 12.8 kg COD m -3 day-' identified PH2 as a very important disturbance indicator. Although P,2 responds quickly to changes in the substrate quantity and composition, increases in PH2 do not necessarily imply process inhibition. Mosey and Fernandez (1989), operating a 20 day HRT once-through laboratory digester on reconstituted skimmed milk, showed that more than a tenfold change in PH~ (10-120 ppm) could occur without significant changes in bacterial performance. Thus in practice both the absolute value and the rate of change of P.~, which indicate incoming inhibition of anaerobic digestion, seem to be quite variable and therefore difficult to use in process control. However, the residence time of hydrogen in the reactor is very low, so variations in the P.2 are rapid, and occur in response to a variety of disturbances. Hence it has often been proposed along with another parameter, e.g. pH (Moletta, 1989), as an instability indicator to monitor changes in digester input. Dissolved hydrogen measurements are not yet routine. Sensor types include membrane inlet mass spectrometry, an amperometric dissolved hydrogen probe or a fuel cell detector (Whitmore and Lloyd, 1986; Strong and Cord-Ruwisch, 1995; Pauss et al., 1990, respectively). Liquor phase measurements are in principle more representative of the microbial environment, and there is controversy about the correlation between dissolved hydrogen concentration and P.2 in anaerobic systems. Pauss and co-workers (1990, 1993) reported that equilibrium dissolved hydrogen concentrations calculated from P.~ were up to 100 times less than the actual dissolved hydrogen concentrations. The transfer of hydrogen from the liquid into the gas phase meets with large physico-chemical resistance (Robinson and Tiedje, 1982; Pauss et al., 1990). Pauss and Guiot (1993) conclude that the deduction of the dissolved hydrogen value from the P ~ is questionable unless the particular operation studied can increase drasti-
cally the interfacial specific area between gaseous and liquid phases. This paper investigates PH2 and its relevance as a control parameter, reporting on experiments in which the influent to a fluidised bed anaerobic digester was varied both in quantity and quality. MATERIALS AND METHODS
The anaerobic digester system, operating at 37°C, consisted of two perspex 7 1fluidisedbed reactors connected in parallel (11 I total liquid volume) using a Siran~ sintered glass carrier (Schott Glaswerke, Germany). A 1031 EHEIM centrifugal pump (Monside Ltd, Letchworth, Herts, U.K.) was used to recyclethe liquor contents at an up-flowvelocity of approximately 0.55mmin-'. The influent was a simulated baker's yeast wastewater (COD approx. 12,000mg021-t) giving a steady state By = 2733 kg COD m 3day-~ and HRT = 10.2-8.7 h. The wastewater was made up batch-wise in the feed tank, which was cooled to 13-15°C. The Pco: and PH2 were monitored on-line using an ADC monitor type SBG100-002-15290 (ADC Ltd, Hodderson, U.K.) and a GMI Exhaled Hydrogen Monitor (Gas Measurement Instruments Ltd, Renfrew, Scotland), respectively. Routine calibration of the GMI monitor with standard gases was performed at least once per month, with little adjustment needed. To avoid corrosion of these instruments and interferences in the H2 measurements, H2S was removed from the biogas by passing through a saturated solution of CuSO4. The biogas flow rate was measured with a low flow on-line gas meter (Guwy et al., 1995) and recorded every 6 min with the data acquisition system. The pH was measured by a Kent EIL9142 meter using a Ingold Xerolyte electrode (type HA405-DXK-S8/ 120). A bicarbonate alkalinity (BA) monitor (Hawkes et al., 1993; Guwy et al., 1994) was used on-line in the effluent stream. Two process control systems were used during the overload experiments. In both cases BA concentration was continually monitored in the emuent and maintained at a value equal or higher than a set-point by addition of bicarbonate solution. The operating principles of the two control systems were an on-off controller (set-point: 2700 mg 1-' CaCO3 + 100 mg 1-' CaCO3) and a neural network (Wilcox et al., 1995). Soluble COD was determined using standard methods (APHA, 1989). Levels of CH4 and CO2 in the biogas and of individual VFA in the effluentwere determined by GC (Peck et al., 1986), the biogas composition measurements being used to check the on-line CO2 analysis. Ammonium and phosphorus levels were determined using standard methods described by APHA (1989) and Cleseri et al. (1989), respectively. RESULTS AND DISCUSSION The reactor was operated for 1 yr, routinely giving 75% COD removal. Typical steady state conditions were: influent feed COD 12,170 mg COD -~, effluent COD 2940 mg COD- ', total phosphorus (influent) 186 mg 1-' and NH4-N (effluent) 930 mg 1-'. N o n - s t e a d y state operation
In an experiment carried out after a 2 week shut-down period, By to the reactor was increased step-wise, raising the flow rate of a new batch of feed but keeping the COD constant (Fig. 1). At point A ( t = 0 ) , By changed from 0 to
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1 2 . 3 k g C O D m - 3 d a y - J ; at point B ( t = 5 5 h ) , Bv changed from 12.3 to 24.8 kg COD m -3day-l; at point C (t = 160 h), to 30.8 kg COD m -3 day-J; and at D (t = 335 h), to 38.5 k g C O D m -3day ~. At t = 379 h, Bv returned to 30.8 kg COD m -3 day -~ because of foaming. It can be seen from Fig. 1 that biogas flow increased approximately proportionally to each increase of By, while PH~ showed no such linear relationship,, but gave very distinct peaks just after the changes in feed quantity and quality.
Changes due to feed pre-acidification An increase in PH~ was observed in steady state operation at constant loading rate every time a new batch of feed was used. Since the COD of each batch of feed was not significantly different, the change was thought to be related to fermentation products building up in the feed tank. When industrial process productions stop periodically a similar situation may arise where wastewater in a balance tank acidifies during storage. To investigate this further, an experiment was carried out over 6 days during which only one batch of feed was used. The reactor was operated with a B~ = 27.1 kg COD m -3 day -~ and a HRT = 10.2 h. Changes in the feed tank contents (VFA, COD, pH and temperature) and in the reactor receiving this feed (BA, Pco~, PH: and gas flow) were monitored. Hot water was used to solubilise some of the feed ingredients. The initial high temperature of 24°C encouraged the growth of fermentative bacteria naturally present in the batch which was then kept at 14°C. The pH gradually increased from 4.87 at the make-up of new feed to pH 5.55 after 137 h. A pH drop occurs in effluents with carbohydrates as main
pollutants, due to acidification. However breakdown of protein in this feed releases excess ammonia which neutralises the VFA and increases the pH. COD in the feed tank decreased slightly, by less than 10%. Fig. 2 shows the increase in VFA in the feed tank during the 6 day experiment, together with the corresponding PM2 in the reactor biogas. In the fresh feed a very small fraction of its components is hydrolysed. Hydrolysis and associated fermentation activity were thus higher in the anaerobic digester, causing increased P,: and % CO2 in the biogas. These variables reached a maximum within 24h after introduction of the new feed (Fig. 3), while the maximum VFA concentration in the feed tank was reached after 48 h approximately (Fig. 2). As the feed entering the reactor gradually fermented, the PH2 and % CO2 decreased (Fig. 3). This would suggest some link between P.. in the biogas and dissolved hydrogen concentrations, as presumably the fall in PH2 reflected the decreased VFA fermentation rate in the reactor. These conditions would be thermodynamically more favourable for the conversion of propionic to acetic acid. During the first part of the experiment, the feed being unfermented, the more oxidised substrate produced a biogas with a higher % CO2. This could also be due to a higher concentration of VFA which partly destroyed the BA. Later, because of fermentation in the feed and consequent release of CO2 directly from the feed tank, the more reduced substrate was degraded within the reactor, giving a biogas richer in CH4. The reactor performance, as indicated by gas production, did not appear to be affected by the significant increase of PH2 (2501500 ppm as seen in Fig. 3).
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maintained close to 2500 mg CaCO31-~ by an on-off controller, the pH falling by 0.1 only briefly. Feed used at time 0 was already pre-acidified. At time 29 h fresh feed was used. The effect of feed change on P.2, presumably due to the increased fermentation reactions occurring in the reactor, is shown in Fig. 4. As in the previous test, new feed resulted in a P.2 peak with the maximum 19 h after changing the feed. The absolute P.2 increases in Figs 3 and 4 differ (from 220 to 1500 ppm and from 200 to 800 ppm, respectively), although the lag to
Organic overloads
Two organic overloads, each 4 h long, were performed during an 85 h period by increasing the feed flow rate. By was increased from 27.1 to 57.6 kg COD m -3 day -1 on the first occasion and then from 27.1 to 64.5 kg COD m-3 day -~ on the second. In overload 1, BA fell to 2000 mg CaCO31- ~, the pH falling by 0.2 to pH 6.9 for the last 2 h of the shock, returning to 7.1 within 3 h of the end of the overload. During the second overload the BA was
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Time ( h o u r s ) Fig. 4. Biogas and hydrogen during two overloads and a feed change. reach the maximum is similar. In neither case did effluent COD vary significantly. Feed pH increased from 4.86 to 5.45, ,;imilarly to the previous test, while feed COD did not show the slow decline found in the former experiment. In overload 1, the PH2 increased from 200 to 300 ppm (Fig. 4). The corresponding changes in VFA concentration are given in Fig. 5 and show that propionate increased by 280%, from 250 to 700 mg 1-~, compared to a 174% increase in acetic acid. At the end of overload 1, the biogas hydrogen concentration rapidly decreased and propionic acid concentration decreased rapidly compared to that of
acetic acid, which was slower to return to its initial level. When the new batch of feed was used (t = 29 h), the propionic acid concentration increased from 150 to 300 mg !-~. The propionic concentration remained around this value until the start of the second overload, while the PH2 increased from 200 to 800 ppm. It can be seen from Fig. 4 that although the two overloads and the consequent rises in biogas production were comparable, the related PH2 maxima were very different, being, respectively, 300 and 1450ppm. This five-fold difference seems to be related to the change in feed quality at hour 29,
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Fig. 6. BA and PH2 during organic overload with BA neural network control.
which gave the increase of PH2 seen from that time on, the effect of the second overload being superimposed on this. However during the second overload the maximum propionic acid concentration was 900 ppm, only 200 ppm (or 30%) higher than in overload 1, while the acetic acid concentration was actually lower than in overload 1. Thus two similar overloads gave very different responses in terms of maximum gaseous H2, propionic and acetic acid levels. For the last two parameters, the ratios of the maximum values were actually reversed on going from overload 1 to overload 2. This could be explained by the fact that at the higher PH2 of
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overload 2 the free ~¢nergy of acetogenesis from propionate becomes positive (Thauer et al., 1977), while acetate keeps being consumed by acetoclasts. From the above it can be deduced that the relationship between PH2 and the concentration of propionate is much more complex than previously assumed and is also difficult to use. A similar overload experiment was conducted keeping the BA >/2000 mg CaCO31-~ by a neural network-based control system. The steady state Bv = 40 kg COD m -3 day -~ was increased during the 4 h overload to 63 kg COD m -3 day -~. A single batch of feed (12,000 mg COD 1-~) was used. Both PH2 and
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Fig. 7. Individual volatile fatty acid production during overload with neural network control•
Hydrogen production in anaerobic digester V F A levels responded rapidly to the increase in By (see Figs 6 and 7). However PH2 dropped to its steady state value in approximately 3 h from the end of the overload. It took somewhat longer for the acetic acid concentration to return to its initial concentration, and about twice as long as hydrogen for propionic acid to regain its initial level. Again, the dependence of propionate levels on PH2 seems to be unclear. The results of these experiments show that whilst PH, is a good instability indicator, because of the complex dynamics of hydrogen in an anaerobic system and its dependency on the waste type and condition, it is unsuitable as 'a stand-alone control variable. As Switzenbaum et al. (1990) point out, changes in PH2 rapidly indicate short-term events but may not show a concurrent variation in digester performance. Hydrogen seems to be unrelated to stress exerted on tile acetogenic methanogens, which is monitored by acetate and biogas CO levels. Powell and Archer (1989) and Moletta (1989) suggested that anaerobic process control based on PH2 monitoring should include the determination of a liquor phase parameter such as pH, bicarbonate or V F A concentration. The results obtained here during steady state periods and subsequent overloads would support this proposal. CONCLUSIONS Step overloads produced a sharp PH2 peak, but similar overloads did not give rise to the same absolute levels of biogas hydrogen. In these studies a clear relationship between levels of digester propionic acid and biogas hydrogen was not found. Hydrogen also signals changes which are not associated with organic overload instability, such as changes in feed quality. On-line PH2 measurement showed considerable fluctuations depending on the degree of acidification of the feed, although overall performance of the anaerobic digester did not appear to be affected. The use of H2 ilevels in the biogas as a control parameter is questionable especially if the pre-acidification of the feed to the digester is variable, but its rapid response ar.Ld ease of on-line measurement support its use in digester control along with other liquid phase parameters which can be measured on-line. authors are grateful for the assistance of Mr J Langton and Mr Rene Hoogstraate in the experimental work, to Prof. C Wandrey and Dr A Aivasidis of KFA, Julich, Germany for the supply of Siran~, initial culture and cyclone chambers, and to the European Commission for funds to conduct part of this work under project EV5V-CT92-0233. Acknowledgements---The
REFERENCES
APHA (American Public Health Association) (1989) Standard Methods for the Examination of Water and Wastewater. New York.
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Archer D. B. (1983) The microbiological basis for process control in methanogenic fermentation of soluble wastes. Enz. Microb. Technol. 5, 162-170. Cleseri L. S., Greenberg A. E. and Rhodes-Trussell R. (1989) Standard Methods for the Examination of Water and Wastewater. American Public Health Association, New York. Collins L. J. and Paskins A. R. (1987) Measurement of trace concentrations of hydrogen in biogas from anaerobic digesters using an exhaled hydrogen monitor. Wat. Res. 21(12), 1567-1572. Conrad R., Phelps T. J. and Zeikus J. G. (1985) Gas metabolism evidence in support of the juxtaposition of hydrogen-producing and methanogenic bacteria in sewage sludge and lake sediments. Appl. Environ. Microbiol. 50, 595-601. Dornseiffer P., Meyer B. and Heinzel E. (1995) Modelling of anaerobic formate kinetics in mixed biofilm culture using dynamic membrane mass spectrometric measurement. Biotechnol. Bioeng. 45, 219-228. Guwy A. J., Hawkes D. L., Hawkes F. R. and Rozzi A. G. (1994) Characterisation of a prototype industrial on-line analyser for bicarbonate/carbonate monitoring. Biotechnol. Bioeng. 44, 1325-1330. Guwy A. J., Hawkes D. L. and Hawkes F. R. (1995) On-line low flow high-precision gas metering systems. Wat. Res. 29(3), 977-979. Harper S. R. and Pohland F. G. (1986) Recent developments in hydrogen management during anaerobic biological wastewater treatment. Biotechnol Bioeng. 28, 585-602. Hawkes F. R., Guwy A. J., Rozzi A. G. and Hawkes D. L. (1993) A new instrument for on-line measurement of bicarbonate alkalinity. Wat. Res. 27(I), 167-170. Hickey R. F. and Switzenbaum M. S. (1991) The response and utility of hydrogen and carbon monoxide as process indicators of anaerobic digesters subject to organic and hydraulic overloads. J. WPCF 63(2), 129-140. Hickey R. F., Vanderielen J. and Switzenbaum M. S. (1987) The effects of organic toxicants on methane production and hydrogen gas levels during the anaerobic digestion of waste activated sludge. Wat. Res. 21, 1417-1427. Hickey R. F., Vanderielen J. and Switzenbaum M. S. (1989) The effect of heavy metals on the methane production and hydrogen and carbon monoxide during batch anaerobic sludge digestion. Wat. Res. 23, 207-218. Kidby D. W. and Nedwell D. B. (1991) An investigation into the suitability of biogas hydrogen concentration as a performance monitor for anaerobic sewage sludge digesters. Wat. Res. 25(8), 1007-1012. Mathiot S., Escoflier Y., Ehlinger F., Couderc J. P., Leyris J. P. and Moletta R. (1992) Control parameter variations in an anaerobic fluidised bed reactor subject to organic shockloads. Wat. Sci. Technol. 25(7), 93-101. McInerney M. J., Bryant M. P. and Stafford D. A. (1980), pp. 91-98. In Anaerobic Digestion (Edited by Stafford D. A., Wheatley B. I. and Hughes D. F.). Applied Science Publishers Ltd, U.K. Moletta R. (1989) Comparaison de la reponse de differents parametres ~t une surcharge organique d'un digester anaerobie. Environ. Technol. Lett. 10, 173-184. Mosey F. E. and Fernandex X. A. (1989) Patterns of hydrogen in biogas from the anaerobic digestion of milk-sugars. Wat. Sci. Technol. 21(4/5), 187-196. Pauss R. and Guiot S. (1993) Hydrogen monitoring in anaerobic sludge bed reactors at various hydraulic regimes and loading rates. Wat. Environ. Res. 65, 276-280. Pauss R., Samson R. and Guiot S. (1990) Continuous measurement of dissolved H2 in an anaerobic reactor using a new hydrogen fuel cell detector. BiotechnoL Bioeng. 35, 492-501. Peck M. W., Skilton J. M., Hawkes F. R. and Hawkes
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D. L. (1986) Effects of temperature shock treatments on the stability of anaerobic digesters operated on separated cattle slurry. War. Res. 20(4), 453~,62. Powell G. E. and Archer D. B. (1989) On-line titration method for monitoring buffer capacity and total volatile fatty acid level in anaerobic digesters. Biotechnol. Bioeng. 33, 57(~601. Robinson J. A. and Tiedje J. M. (1982) Kinetics of hydrogen consumption by rumen fluid, anaerobic sludge, and sediment. Appl. Environ. Microbiol. 41, 545-548. Strong G. E. and Cord-Ruwisch R. (1995) An in situ dissolved-hydrogen probe for monitoring anaerobic digesters under overload conditions. Biotechnol. Bioeng. 45, 63-68.
Switzenbaum M. S., Giraldo-Gomez E. and Hickey R. F. (1990) Monitoring of anaerobic methane fermentation process. Enz. Microb. Technol. 12, 722-730. Thauer R. K., Jungermann K. and Decker K. (1977) Energy conversion in chemotrophic anaerobic bacteria. Bacteriol. Rev. 41, 100-180. Whitmore T. N. and Lloyd D. (1986) Mass spectrometric control of the thermophilic anaerobic digestion process based on levels of dissolved hydrogen. Biotechnol. Lett. 8, 203-208. Wilcox S. J., Hawkes D. L., Hawkes F. R. and Guwy A. J. (1995) A neural network, based on bicarbonate monitoring, to control anaerobic digestion. War. Res. 29(6), 1465-1470.
Wat. Res. Vol. 31, No. 6, pp. 1291-1298,1997 © 1997ElsevierScienceLtd All rights reserved.Printedin Great Britain 0043-1354/97$17.00+ 0.00
H Y D R O G E N PRODUCTION IN A HIGH RATE FLUIDISED BED ANAEROBIC DIGESTER A. J. GUWY I, F. R. HAWKES j*, D. L. HAWKES 2 and A. G. ROZZI 3 ~School of Applied Sciences, 2Department of Mechanical and Manufacturing Engineering, University of Glamorgan, Pontypridd, Mid-Glamorgan CF37 1DL, U.K. and 3politecnico di Milano, DIIR, Sezione Ambientale, Piazza L da Vinci 32, 20133 Milan, Italy (First received November 1995; accepted in revised form July 1996) Abstract--F|iogas hydrogen content varies with the operational conditions of an anaerobic digester, and may be a useful control parameter. Experiments were carried out on a laboratory scale fluidised bed anaerobic digester, fed with a synthetic baker's yeast wastewater. Step overloads produced a sharp peak in biogas hydrogen level measured on-line, e.g. an increase of loading rate from 40 to 63 kgCOD m -3 day -~ increased hydrogen concentration from 290 to 640 ppm within 3 h. However, switching from an older, partly acidified batch to a fresh batch of feed at constant COD gave a marked increase in lhe biogas hydrogen content from 200 to 800 ppm. In these studies propionic acid and biogas hydrogen concentrations were not linearly correlated. According to the present results, biogas hydrogen content is unsuitable as a stand-alone control variable for anaerobic digestion, but its rapid response and ease of on-line measurement supports its use in digester control along with other liquid phase parameters to be measured on-line. © 1997 Elsevier Science Ltd. Key words--hydrogen, VFA, bicarbonate alkalinity, fluidised bed, anaerobic digester
INTRODUCTION Hydrogen transfer The transfer of hydrogen plays a vital part in the anaerobic process and the term "interspecies H2 transfer" describes the phenomenon in which the presence of one or more H~-consuming species significantly alters the metabolism of a fermentative organism towards production of more oxidised end products (Mclnerney et al., 1980). The hydrogen transfer system results in greater availability of energy for acidogens and acetogens, increased substrate utilisation, increased growth of all participants and the displacement of unfavourable reaction equilibria (Harper and Pohland, 1986). According to Dornseiffer et al. (1995) interspecies formate transfer is not a significant alternative to intcrspecies hydrogen transfer. The interspecies hydrogen transfer system was thought to occur between the syntropic bacterial associations through a hydrogen pool in the liquid phase. However, according to Conrad et al. (1985) the large hydrogen turnover rates and the small size of the hydrogen pool suggested that as much as 95% of the hydrogen produced never entered a common hydrogen pool. Hydrogen is not very soluble; 0.017 cm 3 hydrogen gas dissolves in 1 cm 3 of water at 1 bar and 37°C. This would explain the importance of the formation of micro-environments, biofilms in *Author to whom al)icorrespondence should be addressed.
anaerobic filters or granules in UASB reactors, because the close association of the different bacterial groups, which transfer hydrogen directly from one to another, increases their overall efficiency. Hydrogen concentration Free energy changes of the main anaerobic degradation reactions strongly depend on hydrogen partial pressure Pn2. Therefore several investigators assumed the P.2 in the off gas to be a good indicator of digester performance, but this only holds assuming equilibrium conditions between the liquid and gas phases. Otherwise the free energy depends on the concentration of dissolved hydrogen in the liquid phase only. As it is relatively difficult and expensive to determine both the P~2 and the H2 concentration in the liquid, only a few investigators measured both variables. In the majority of published studies H2 was measured in the gas after H2S scrubbing using a polarograph, calculating H2 concentration in the liquid from equilibrium considerations. In the following PH: is given as ppm which roughly corresponds to 10-6 bar or 10 pascal. In a healthy, stable anaerobic digester, very low partical pressures of hydrogen usually occur in the biogas. Collins and Paskins (1987) found that Ps~ in 20 U.K. sludge digesters operating with hydraulic retention times (HRT) of 8-40 days had P,2 which varied from 15-199 ppm. Hickey and Switzenbaum (1991), operating digesters on waste activated sludge at a 10 day HRT, observed that P.2 varied in response
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to overloads. Hickey et al. (1987, 1989) showed that the concentration of hydrogen in the biogas of a digester operating on activated sludge responded rapidly to pulse additions of toxicants such as chloroform and heavy metals. Kidby and Nedwell (1991) found P.2 values around 30 ppm from sludge digesters operating at a 20 day HRT. Operation on an 8 day HRT led to failure, but PH2 showed little change until after failure had occurred. They concluded that use of gaseous hydrogen gives no early warning of overloads in sludge digesters, because of the high fraction of refractory compounds in the feed. They suggest it may be better exploited with reactors treating more readily biodegradable wastes. PH2 variations in high rate anaerobic digesters in non-steady state operation have also been reported. Mathiot et al. (1992) using a fluidised bed reactor operating on wine distillery wastewater at a volumetric loading rate (By) of 12.8 kg COD m -3 day-' identified PH2 as a very important disturbance indicator. Although P,2 responds quickly to changes in the substrate quantity and composition, increases in PH2 do not necessarily imply process inhibition. Mosey and Fernandez (1989), operating a 20 day HRT once-through laboratory digester on reconstituted skimmed milk, showed that more than a tenfold change in PH~ (10-120 ppm) could occur without significant changes in bacterial performance. Thus in practice both the absolute value and the rate of change of P.~, which indicate incoming inhibition of anaerobic digestion, seem to be quite variable and therefore difficult to use in process control. However, the residence time of hydrogen in the reactor is very low, so variations in the P.2 are rapid, and occur in response to a variety of disturbances. Hence it has often been proposed along with another parameter, e.g. pH (Moletta, 1989), as an instability indicator to monitor changes in digester input. Dissolved hydrogen measurements are not yet routine. Sensor types include membrane inlet mass spectrometry, an amperometric dissolved hydrogen probe or a fuel cell detector (Whitmore and Lloyd, 1986; Strong and Cord-Ruwisch, 1995; Pauss et al., 1990, respectively). Liquor phase measurements are in principle more representative of the microbial environment, and there is controversy about the correlation between dissolved hydrogen concentration and P.2 in anaerobic systems. Pauss and co-workers (1990, 1993) reported that equilibrium dissolved hydrogen concentrations calculated from P.~ were up to 100 times less than the actual dissolved hydrogen concentrations. The transfer of hydrogen from the liquid into the gas phase meets with large physico-chemical resistance (Robinson and Tiedje, 1982; Pauss et al., 1990). Pauss and Guiot (1993) conclude that the deduction of the dissolved hydrogen value from the P ~ is questionable unless the particular operation studied can increase drasti-
cally the interfacial specific area between gaseous and liquid phases. This paper investigates PH2 and its relevance as a control parameter, reporting on experiments in which the influent to a fluidised bed anaerobic digester was varied both in quantity and quality. MATERIALS AND METHODS
The anaerobic digester system, operating at 37°C, consisted of two perspex 7 1fluidisedbed reactors connected in parallel (11 I total liquid volume) using a Siran~ sintered glass carrier (Schott Glaswerke, Germany). A 1031 EHEIM centrifugal pump (Monside Ltd, Letchworth, Herts, U.K.) was used to recyclethe liquor contents at an up-flowvelocity of approximately 0.55mmin-'. The influent was a simulated baker's yeast wastewater (COD approx. 12,000mg021-t) giving a steady state By = 2733 kg COD m 3day-~ and HRT = 10.2-8.7 h. The wastewater was made up batch-wise in the feed tank, which was cooled to 13-15°C. The Pco: and PH2 were monitored on-line using an ADC monitor type SBG100-002-15290 (ADC Ltd, Hodderson, U.K.) and a GMI Exhaled Hydrogen Monitor (Gas Measurement Instruments Ltd, Renfrew, Scotland), respectively. Routine calibration of the GMI monitor with standard gases was performed at least once per month, with little adjustment needed. To avoid corrosion of these instruments and interferences in the H2 measurements, H2S was removed from the biogas by passing through a saturated solution of CuSO4. The biogas flow rate was measured with a low flow on-line gas meter (Guwy et al., 1995) and recorded every 6 min with the data acquisition system. The pH was measured by a Kent EIL9142 meter using a Ingold Xerolyte electrode (type HA405-DXK-S8/ 120). A bicarbonate alkalinity (BA) monitor (Hawkes et al., 1993; Guwy et al., 1994) was used on-line in the effluent stream. Two process control systems were used during the overload experiments. In both cases BA concentration was continually monitored in the emuent and maintained at a value equal or higher than a set-point by addition of bicarbonate solution. The operating principles of the two control systems were an on-off controller (set-point: 2700 mg 1-' CaCO3 + 100 mg 1-' CaCO3) and a neural network (Wilcox et al., 1995). Soluble COD was determined using standard methods (APHA, 1989). Levels of CH4 and CO2 in the biogas and of individual VFA in the effluentwere determined by GC (Peck et al., 1986), the biogas composition measurements being used to check the on-line CO2 analysis. Ammonium and phosphorus levels were determined using standard methods described by APHA (1989) and Cleseri et al. (1989), respectively. RESULTS AND DISCUSSION The reactor was operated for 1 yr, routinely giving 75% COD removal. Typical steady state conditions were: influent feed COD 12,170 mg COD -~, effluent COD 2940 mg COD- ', total phosphorus (influent) 186 mg 1-' and NH4-N (effluent) 930 mg 1-'. N o n - s t e a d y state operation
In an experiment carried out after a 2 week shut-down period, By to the reactor was increased step-wise, raising the flow rate of a new batch of feed but keeping the COD constant (Fig. 1). At point A ( t = 0 ) , By changed from 0 to
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1 2 . 3 k g C O D m - 3 d a y - J ; at point B ( t = 5 5 h ) , Bv changed from 12.3 to 24.8 kg COD m -3day-l; at point C (t = 160 h), to 30.8 kg COD m -3 day-J; and at D (t = 335 h), to 38.5 k g C O D m -3day ~. At t = 379 h, Bv returned to 30.8 kg COD m -3 day -~ because of foaming. It can be seen from Fig. 1 that biogas flow increased approximately proportionally to each increase of By, while PH~ showed no such linear relationship,, but gave very distinct peaks just after the changes in feed quantity and quality.
Changes due to feed pre-acidification An increase in PH~ was observed in steady state operation at constant loading rate every time a new batch of feed was used. Since the COD of each batch of feed was not significantly different, the change was thought to be related to fermentation products building up in the feed tank. When industrial process productions stop periodically a similar situation may arise where wastewater in a balance tank acidifies during storage. To investigate this further, an experiment was carried out over 6 days during which only one batch of feed was used. The reactor was operated with a B~ = 27.1 kg COD m -3 day -~ and a HRT = 10.2 h. Changes in the feed tank contents (VFA, COD, pH and temperature) and in the reactor receiving this feed (BA, Pco~, PH: and gas flow) were monitored. Hot water was used to solubilise some of the feed ingredients. The initial high temperature of 24°C encouraged the growth of fermentative bacteria naturally present in the batch which was then kept at 14°C. The pH gradually increased from 4.87 at the make-up of new feed to pH 5.55 after 137 h. A pH drop occurs in effluents with carbohydrates as main
pollutants, due to acidification. However breakdown of protein in this feed releases excess ammonia which neutralises the VFA and increases the pH. COD in the feed tank decreased slightly, by less than 10%. Fig. 2 shows the increase in VFA in the feed tank during the 6 day experiment, together with the corresponding PM2 in the reactor biogas. In the fresh feed a very small fraction of its components is hydrolysed. Hydrolysis and associated fermentation activity were thus higher in the anaerobic digester, causing increased P,: and % CO2 in the biogas. These variables reached a maximum within 24h after introduction of the new feed (Fig. 3), while the maximum VFA concentration in the feed tank was reached after 48 h approximately (Fig. 2). As the feed entering the reactor gradually fermented, the PH2 and % CO2 decreased (Fig. 3). This would suggest some link between P.. in the biogas and dissolved hydrogen concentrations, as presumably the fall in PH2 reflected the decreased VFA fermentation rate in the reactor. These conditions would be thermodynamically more favourable for the conversion of propionic to acetic acid. During the first part of the experiment, the feed being unfermented, the more oxidised substrate produced a biogas with a higher % CO2. This could also be due to a higher concentration of VFA which partly destroyed the BA. Later, because of fermentation in the feed and consequent release of CO2 directly from the feed tank, the more reduced substrate was degraded within the reactor, giving a biogas richer in CH4. The reactor performance, as indicated by gas production, did not appear to be affected by the significant increase of PH2 (2501500 ppm as seen in Fig. 3).
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maintained close to 2500 mg CaCO31-~ by an on-off controller, the pH falling by 0.1 only briefly. Feed used at time 0 was already pre-acidified. At time 29 h fresh feed was used. The effect of feed change on P.2, presumably due to the increased fermentation reactions occurring in the reactor, is shown in Fig. 4. As in the previous test, new feed resulted in a P.2 peak with the maximum 19 h after changing the feed. The absolute P.2 increases in Figs 3 and 4 differ (from 220 to 1500 ppm and from 200 to 800 ppm, respectively), although the lag to
Organic overloads
Two organic overloads, each 4 h long, were performed during an 85 h period by increasing the feed flow rate. By was increased from 27.1 to 57.6 kg COD m -3 day -1 on the first occasion and then from 27.1 to 64.5 kg COD m-3 day -~ on the second. In overload 1, BA fell to 2000 mg CaCO31- ~, the pH falling by 0.2 to pH 6.9 for the last 2 h of the shock, returning to 7.1 within 3 h of the end of the overload. During the second overload the BA was
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Time ( h o u r s ) Fig. 4. Biogas and hydrogen during two overloads and a feed change. reach the maximum is similar. In neither case did effluent COD vary significantly. Feed pH increased from 4.86 to 5.45, ,;imilarly to the previous test, while feed COD did not show the slow decline found in the former experiment. In overload 1, the PH2 increased from 200 to 300 ppm (Fig. 4). The corresponding changes in VFA concentration are given in Fig. 5 and show that propionate increased by 280%, from 250 to 700 mg 1-~, compared to a 174% increase in acetic acid. At the end of overload 1, the biogas hydrogen concentration rapidly decreased and propionic acid concentration decreased rapidly compared to that of
acetic acid, which was slower to return to its initial level. When the new batch of feed was used (t = 29 h), the propionic acid concentration increased from 150 to 300 mg !-~. The propionic concentration remained around this value until the start of the second overload, while the PH2 increased from 200 to 800 ppm. It can be seen from Fig. 4 that although the two overloads and the consequent rises in biogas production were comparable, the related PH2 maxima were very different, being, respectively, 300 and 1450ppm. This five-fold difference seems to be related to the change in feed quality at hour 29,
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which gave the increase of PH2 seen from that time on, the effect of the second overload being superimposed on this. However during the second overload the maximum propionic acid concentration was 900 ppm, only 200 ppm (or 30%) higher than in overload 1, while the acetic acid concentration was actually lower than in overload 1. Thus two similar overloads gave very different responses in terms of maximum gaseous H2, propionic and acetic acid levels. For the last two parameters, the ratios of the maximum values were actually reversed on going from overload 1 to overload 2. This could be explained by the fact that at the higher PH2 of
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overload 2 the free ~¢nergy of acetogenesis from propionate becomes positive (Thauer et al., 1977), while acetate keeps being consumed by acetoclasts. From the above it can be deduced that the relationship between PH2 and the concentration of propionate is much more complex than previously assumed and is also difficult to use. A similar overload experiment was conducted keeping the BA >/2000 mg CaCO31-~ by a neural network-based control system. The steady state Bv = 40 kg COD m -3 day -~ was increased during the 4 h overload to 63 kg COD m -3 day -~. A single batch of feed (12,000 mg COD 1-~) was used. Both PH2 and
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Hydrogen production in anaerobic digester V F A levels responded rapidly to the increase in By (see Figs 6 and 7). However PH2 dropped to its steady state value in approximately 3 h from the end of the overload. It took somewhat longer for the acetic acid concentration to return to its initial concentration, and about twice as long as hydrogen for propionic acid to regain its initial level. Again, the dependence of propionate levels on PH2 seems to be unclear. The results of these experiments show that whilst PH, is a good instability indicator, because of the complex dynamics of hydrogen in an anaerobic system and its dependency on the waste type and condition, it is unsuitable as 'a stand-alone control variable. As Switzenbaum et al. (1990) point out, changes in PH2 rapidly indicate short-term events but may not show a concurrent variation in digester performance. Hydrogen seems to be unrelated to stress exerted on tile acetogenic methanogens, which is monitored by acetate and biogas CO levels. Powell and Archer (1989) and Moletta (1989) suggested that anaerobic process control based on PH2 monitoring should include the determination of a liquor phase parameter such as pH, bicarbonate or V F A concentration. The results obtained here during steady state periods and subsequent overloads would support this proposal. CONCLUSIONS Step overloads produced a sharp PH2 peak, but similar overloads did not give rise to the same absolute levels of biogas hydrogen. In these studies a clear relationship between levels of digester propionic acid and biogas hydrogen was not found. Hydrogen also signals changes which are not associated with organic overload instability, such as changes in feed quality. On-line PH2 measurement showed considerable fluctuations depending on the degree of acidification of the feed, although overall performance of the anaerobic digester did not appear to be affected. The use of H2 ilevels in the biogas as a control parameter is questionable especially if the pre-acidification of the feed to the digester is variable, but its rapid response ar.Ld ease of on-line measurement support its use in digester control along with other liquid phase parameters which can be measured on-line. authors are grateful for the assistance of Mr J Langton and Mr Rene Hoogstraate in the experimental work, to Prof. C Wandrey and Dr A Aivasidis of KFA, Julich, Germany for the supply of Siran~, initial culture and cyclone chambers, and to the European Commission for funds to conduct part of this work under project EV5V-CT92-0233. Acknowledgements---The
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D. L. (1986) Effects of temperature shock treatments on the stability of anaerobic digesters operated on separated cattle slurry. War. Res. 20(4), 453~,62. Powell G. E. and Archer D. B. (1989) On-line titration method for monitoring buffer capacity and total volatile fatty acid level in anaerobic digesters. Biotechnol. Bioeng. 33, 57(~601. Robinson J. A. and Tiedje J. M. (1982) Kinetics of hydrogen consumption by rumen fluid, anaerobic sludge, and sediment. Appl. Environ. Microbiol. 41, 545-548. Strong G. E. and Cord-Ruwisch R. (1995) An in situ dissolved-hydrogen probe for monitoring anaerobic digesters under overload conditions. Biotechnol. Bioeng. 45, 63-68.
Switzenbaum M. S., Giraldo-Gomez E. and Hickey R. F. (1990) Monitoring of anaerobic methane fermentation process. Enz. Microb. Technol. 12, 722-730. Thauer R. K., Jungermann K. and Decker K. (1977) Energy conversion in chemotrophic anaerobic bacteria. Bacteriol. Rev. 41, 100-180. Whitmore T. N. and Lloyd D. (1986) Mass spectrometric control of the thermophilic anaerobic digestion process based on levels of dissolved hydrogen. Biotechnol. Lett. 8, 203-208. Wilcox S. J., Hawkes D. L., Hawkes F. R. and Guwy A. J. (1995) A neural network, based on bicarbonate monitoring, to control anaerobic digestion. War. Res. 29(6), 1465-1470.