methane generation from chemically pretreated cellulose by anaerobic fluidized-bed reactors

Biological Wastes 29 (1989) 201-210

Methane Generation from Chemically Pretreated Cellulose by Anaerobic Fluidized-Bed Reactors

Milagro Reig,* Fidel Toldrfi,:~ Gow J. Tsai, N o r m a n B. Jansen & George T. T s a o

Laboratory of Renewable Resources Engineering, Purdue University, West Lafayette, Indiana 47907, USA

(Received 18 November 1988; revised version received 5 January 1989; accepted 9 January 1989)

A B S T R A C T

Alkali cooking of cellulose at 10% (w/v) consistency, performed at 250°C, gives a liquor containing organic acids as main components (lactic, acetic and suceinie acids). A synthetic substrate, representative of this liquor, was used as feed for two anaerobic fluidized-bed reactors operating at 35°C. The effects of hydraulic retention time and influent substrate concentration on substrate consumption andmethane generation were studied. Organic removals of up to 95% and methane generation rates of l'09 litres CH 4 litre~ ~ day- ~ were achieved at a retention time (OR) of 8"3h and at an influent substrate concentration of 1400 mg COD litre- ~. At O R = 3"5 h, conversion decreased to 60%. When feeding the cooked liquor, conversions were slightly lower, around 50% at OR = 4 h and 1300mg COD litre -1 of influent substrate concentration. However, the methane volumetric production rate was as high as 2"5 litres CH 4 litre~ 1 day-~ under the same conditions. The results suggest that alkaline cooking with subsequent methane fermentation offers a viable process for the treatment of cellulosic materials, such as municipal solid waste.

N O T A T I O N

C O D Chemical oxygen demand (mgli t re-1) K Variable-order kinetic cons tant (h- ~ mg I -" litre ~ -")

* Present address: Auto Reig, Passeig del Comtat 38, Cocentaina, Alicante, Spain. :~ To whom correspondence should be addressed at: Instituto de Agroquimica y Tecnologia de Alimentos (CSIC), Jaime Roig 11, Valencia 46010, Spain.

201 Biological Wastes 0269-7483/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

202 Milagro Reig et al.

LA LR litr% n N ?.

s. So T

~V OR

Organic loading rate (mg COD litre; 1 day - ~) Organic removal rate (mg COD litre~ i day- 1) Bed volume (litre) Reaction order Number of experimental points Coefficient of correlation Effluent substrate concentration (mg litre-1) Influent substrate concentration (mg litre-1) Temperature (°C)

Methane volumetric production rate (litre litre; 1 day- 1) Hydraulic retention time (h)

INTRODUCTION

The anaerobic digestion of municipal solid waste (MSW) containing a high proportion of cellulose requires retention times as long as 20-60 days (Clausen & Gaddy, 1984). The conversion achieved is, however, rarely complete; usually only 50-60% of the initial organic material is converted. Further treatment is required (Ghosh, 1984). Very recently, Gijzen et al. (1988) have developed a rumen-derived anaerobic digestion process for the degradation of cellulose which seems promising.

Yang & Chan (1985) developed a novel process for treating MSW based on the chemical solubilization of cellulosics. They tested alkali cooking of cellulose with NaOH and the treatment of the resulting liquor in small anaerobic batch fermenters. Their results showed good possibilities for methane generation although methane was produced at a very slow rate, probably due to inhibition by Na +. However, there are no data available on long-term operation of fermenters which are continuously fed with this liquor.

Anaerobic fluidized-bed reactors have recently been used successfully in the treatment of different industrial wastewaters (Switzenbaum & Jewell, 1980; Boening & Larsen, 1982; Toldrfi et al., 1984, 1987). These fermenters operate at short retention times (hours) and work efficiently (up to 95% conversion) when treating low- to medium-strength wastewaters (Toldrfi et al., 1986), making them attractive from an economic point of view (Jeris, 1983).

In the present study, the liquor resulting from alkali cooking of cellulose and a synthetic solution representative of this liquor were used as feed for two anaerobic fluidized-bed fermenters. The synthetic solution was used to model the system. The alkali cooked liquor was then fed for testing both the

Cellulose fluidized-bed biomethanation 203

model and the feasibility of this process for continuous long-term operation, especially in order to get rapid rates of conversion of MSW with a high methane generation.

METHODS

Installation

Two fluidized-bed reactors, controlled at 35°C, were used. Each one contained 605 g of sand particles (size range: 600-710 #m) which occupied a packed-bed volume of 0.4 litres (41-2 cm height). The reactors were built in methacrylate (76cm height x 3.5cm diameter) and equipped with gas separator (20cm height × 10.9cm diameter) and bed particle sampling device (70cm length x 0.4 cm diameter) as well as feeding and recycling pumps (Fig. 1).

Alkali cooking of cellulose

Sigma s-cellulose, of particle size 60 mesh or less, was used. Alkali cooking of the cellulose was performed according to the method described by Yang & Chan (1985). A 10% (w/v) cellulose suspension (100 ml) containing 6% (w/v)

Fig. !. Scheme of the anaerobic fluidized- bed installation (1) Jacketed reactor; (2) thermometer; (3) recirculation pump; (4) peristaltic pump; (5) feed reservoir; (6) gas liquid separator; (7) syphon discharge; (8) to the gas meter; (9) liquid and bed particle

sampling; and (10) gas sampling.


Ca(OH)2 was cooked at 250°C for 30 min. Ca(OH)2 was used instead of NaOH because of the high solubility of the Na + ion and its inhibitory effect on methanogenic bacteria. The resulting liquor was stored overnight at 2-3°C. No neutralization was required because pH decreased spontaneously from 11 to 7-7.2 during this period. C a 2+ ions in solution, which are potentially inhibitory to methanogenic bacteria, were precipitated as C a C O 3 by bubbling CO 2 through, and separated from the liquor by decanting.

The organic concentration of the liquor was 29-31 g COD litre- 1. The main components were lactic, acetic and succinic acids in a 3:1:1 ratio, respectively. Before feeding the fermenters, cooked liquor was diluted 1/20 with a nutrient solution containing (g litre - 1): yeast extract, 0.1; NH4C1, 0.5; KH2PO4, 0.22; NaHCO 3, 1.0; and Na2S.9H20, 0.1. This solution was prepared fresh daily, de-aerated and kept in anaerobiosis under an atmosphere of nitrogen. The containers and tubing were exhaustively cleaned after being replaced by new ones.

Feed substrate

The composition of the synthetic solution fed to the reactors was (g litre- 1): calcium lactate.5H20, 0.173-1.767; anhydrous sodium acetate, 0.040- 0.440; succinic acid, 0.034-0.370; yeast extract, 0.1; NH4C1, 0.5; KH2PO 4, 0.22; NaHCO3, 1.0 and Na2S.9H20, 0.1. Total COD ranged from 200 to 1700 mg litre- 1. All chemicals were reagent grade and from Sigma (St Louis, MO). They were dissolved in distilled water. Solutions were de-aerated and kept in anaerobic conditions under an atmosphere of nitrogen. The selected ratio of lactic, acetic and succinic acids (3:1:1, respectively) was approximately the same as in the liquor resulting from alkali cooking of cellulose. The nutrient composition was also the same.

Reactor start-up

Sludge collected from a 2000m 3 anaerobic digester operating in the Municipal Wastewater Treatment Plant of West Lafayette (Indiana) was used as inoculum. One litre of this sludge was filtered (0.5 mm mesh size) and fed to each reactor while it was operating with nitrogenated water in standard conditions (T, 35°C; bed expansion, 25%).

Feed substrate (50, 100, 200 and 200 cm a) with the lower concentration (200mg CODlitre -1) was fed to the reactors after 4, 8, 12 and 16 days, respectively. After 21 days, continuous feeding at 8 h retention time was initiated. Steady-state conditions were achieved one month after the commencement of the continuous feeding.


Experiments

Various retention times, from 3-3 to 8"3h, and influent substrate concentrations, 600-1500 mg COD litre-1 for Reactor 1 and 200-700 mg COD litre -1 for Reactor 2, were tested. A minimum of 10 hydraulic retention times (HRT) were allowed to pass after each change of retention time or substrate concentration. Steady-state conditions were assumed when consistent COD values were obtained in samples taken on 3-5 consecutive days. The expanded-bed volume throughout all the experiments was 0"5 litres. Feed and recycle rates were measured daily and controlled. Influent and effluent compositions and concentrations, pH, gas production and composition, and attached biomass were analyzed daily.

Analytical methods

Influent and effluent compositions and product concentrations were analyzed by liquid chromatography using a Waters refractive index detector and an HPX-87H (7.8 mm i.d. x 30cm length) column operated at 65°C. COD and gas compositions were analyzed according to Standard Methods (1980). Biomass attached to the particles, sampled from the middle of the bed, was determined as protein using the Lowry method (Lowry et al., 195 I).

RESULTS AND DISCUSSION

Alkali cooking gave a yield of 85% of the theoretical organic acid production.

Fermentation experiments

Results obtained from both fermenters for long-term operation confirmed the possibility of treating the liquor resulting from alkali cooking of cellulose as Yang & Chang (1985) concluded from a short experience with a laboratory anaerobic filter.

The results showed the possibility of removing 90% of the influent synthetic substrate (expressed as COD) at HRT, O R, of only 6 h (see Fig. 2). This confirms the high efficiency of anaerobic fluidized-bed reactors, achieving 95% COD reduction at 8 h retention time. On the other hand, COD reduction decreased rapidly as retention times decreased below 5 h (for instance, COD reduction was only 60% at 3.5 h).

Figure 3(A) shows the per cent reduction of substrate (expressed as COD) as a function of influent substrate concentration (So) and organic loading


100

50-

or

C3 0 L)

0 Retention time, OR (h)

Fig. 2. Percentage of COD reduction versus HRT at different influent substrate concentrations (rag COD litre-1): (~) 200-500, (~7) 500-750, (I-l) 750-1000, (A) 1000-1250, (C)) 1250-1500, and (0) cooked

liquor.

100

~ 50- g

C3 0

O.

"C

~5oo E

o

250

'E

w

0

~.e'~..~..,

/

V" l J "

_ ..44v , / / /

2ooo 4ooo 6obo 8doo Organic loading rate (mg CODI liter R day)

Fig. 3. Percentage of (A) COD reduction and (Bt effluent substrate concentration versus organic loading-rate at different influent substrate concentrations (mg COD litre- 1): (Q) 200-500, (~) 500-750, (l-l) 750- 1000, (A) 1000-1250, (©) 1250-1500, and

(0) cooked liquor.

Fig. 4.

6000- E

8 ~ 4 0 0 0 . E

~ 2ooo E u 'E

P 0

o o o

o •

o o o • •

° o o o o o o

~,o ° ~ ° ° o °~°

2obo 4obo 6obo aobo Organic loading rate (mg CODI liter R day)

Relationship between organic removal rate and organic loading rate. Feed: (©) synthetic substrate, and (0) cooked liquor.


rate (LA). As expected, substrate reduction decreased as organic loading rate increased. Effluent substrate concentration (Se) also increased with organic loading rate (see Fig. 3(B)).

The rate of substrate removal (LR) versus organic loading rate (LA) is shown in Fig. 4. Even though substrate reduction efficiency decreased with organic load (Fig. 3(A)), the system was able to degrade more substrate at higher organic loading rates (Fig. 4). The relationship between both parameters is:

La = 0"961Lk (1) r = 0"9145 N = 4 1

The attached biomass concentration on the bed remained around 80 mg protein litre-1 in Reactor 1 (S o = 600-1500mg COD litre-1) and 25mg protein litre-1) in Reactor 2 (S o = 200-700 mg COD litre-1).

When feeding the cooked liquor (S O -- 1300 mg COD litre- 1), fermenters were allowed to operate for 21 days before sampling. The COD reductions (around 45-55%) and the organic removal rates (around 4000mg COD litter 1 day-1) were slightly lower than those obtained with the synthetic substrate in similar conditions (see Figs 2 and 4, respectively). Effluent substrate concentration (Se) was around 650 mg COD litre~ 1 (see Fig. 3(B)).

Methane volumetric production rate (7v) is given in Fig. 5(A) as a function of organic loading rate. Methane production rate increases with organic loading rate, rising to 2 litres CH4 litre; 1 day- 1 at 4-5 h retention time and 1250mg COD litre -1 influent substrate concentration. Representing methane production rate versus organic removal rate (Fig. 5(B)) experimental results agree, relatively well, with the line corresponding to theoretical methane production which is 0"35 litres CH 4 g- t COD removed (Chen et al., 1985; Van der Berg et aL, 1985).

In the case of cooked liquor, the methane volumetric production rate was

Fig. 5.

0 00 0

0 0 0

3 I ~ A

=~ ~2 t

i1~ ('-) o c o3 ~ L 0

o 2ooo 4ooo

o

o

o o

B

• 6~y 'S ~ o

o ~°°°'<~ co o

S ° o o

6oo0 8obo o 205o 40oo 6ooo sooo Organic loading rate (rag COD/liter R day) Organic removal rate (rag COD/lffA~r~day)

Methane volumetric production rate versus (A) organic loading rate and (B) organic removal rate. Feed: (©) synthetic substrate, and (Q) cooked liquor.


Fig. 6. Fit of experimental data to a zero-order kinetic model equation: So~S,= 1 + KORS, -1 at 35°C. Feed: (O) synthetic solution, and (0) cooked

liquor.

20,

15

o 10

5

° °

0 0

0 0 0

° o

o.6s o.l'o o.~5 0 R S~ 1 ( h (rag COD/l i ter) -1 )

as high as 2.5 litres CH 4 litreff ~ day-1 at OR = 4h and S o = 1300mg COD litreff 1 (see Fig. 5(A,B))---much better than that obtained with the synthetic substrate. These high rates indicated that MSW could be an adequate and economic substrate for energy generation.

Kinetic model

The experimental results seem to agree with a zero-order kinetic model (see Fig. 6):

S o / S e = 1 + 148.51 OR S~ -~

r = 0"8208 N = 32 (2)

The results of feeding the cooked liquor are also represented in the same figure. They fit very well to this model. This is in accordance with the 'n'- order kinetic model suggested by Toldr/l et al. (1986):

S o / S e = 1 + KORS~ -1 (3)

who worked with similar influent substrate concentrations: 500-1500mg COD litre- t of acetate. The reaction order n approached 0 as the temperature increased up to 35°C, which was the operational temperature used in this study. It also suggests Monod kinetics for the intrinsic reaction in the biofilm.

CONCLUSIONS

High organic removals, up to 95% COD reduction, can be achieved at H R T as low a.s 8h. Process efficiency increases with influent substrate concentration for the same organic loading rate.


Methane production rate increases with organic loading rate and is in accordance with the theoretical methane production (0-35 litres CH4 g-1 COD removed). Up to 2 litres CH4 litreR 1 day-1 were obtained at HRT (OR) = 4.5 h and influent (synthetic) substrate concentration (So)= 1250 mg COD litre- 1. In the case of the cooked liquor, 2.5 litres CH4 litre¢ 1 d a y - 1 were obtained at OR = 4 h and So = 1300 mg COD litre-1.

Results agree with a zero-order kinetic model, indicating Monod kinetics for the intrinsic reaction in the biofilm.

Anaerobic fluidized-bed reactors seem to be an adequate technique for the treatment of liquors resulting from alkali cooking of cellulose and so MSW, offering good perspectives for both organic removal (up to 95%) and methane production (up to 2.5 litres CH4 litreR-1 day-1).

A C K N O W L E D G E M E N T S

The authors wish to thank the Argonne National Laboratory for a contract to support the reported work. A Postdoctoral Fulbright Fellowship to Fidel Toldrfi is also acknowledged.

REFERENCE S

Boening, P. H. & Larsen, V. F. (1982). Anaerobic fluidized bed whey treatment. Biotechnol. Bioeng., 24, 2539-56.

Chen, S. J., Li, C. T. & Shieh, W. K. (1985). Performance evaluation of the anaerobic fluidised bed system: I. Substrate utilisation and gas production. J. Chem. Tech. Biotechnol., 35B, 101-9.

Clausen, E. C. & Gaddy, J. L. (1984). High solids digestion of MSW to methane by anaerobic digestion. In Proc. of the First Syrup. on Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, 15-17 August 1984, Minneapolis, MN, ed. A. A. Antonopoulos. Argonne National Laboratory, Argonne, IL, pp. 283-302.

Ghosh, S. (1984. Gasification of concentrated particulate and solid substrates by biphasic anaerobic digestion. In Proc. of the First Syrup. on Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, 15-17 August 1984, Minneapolis, MN, ed. A. A. Antonopoulos. Argonne National Laboratory, Argonne, IL, pp. 303-20.

Gijzen, H. J., Zwart, K. B., Verhagen, F. J. M. & Vogels, G. D. (1988). High-rate two- phase process for the anaerobic degradation of cellulose employing rumen microorganisms for an efficient acidogenesis. Biotechnol. Bioeng., 31, 418-25.

Jeris, J. S. (1983). Industrial wastewater treatment using anaerobic fluidized bed reactors. Water Sci. Technol., 15, 169-76.

Lowry, O. H., Rosebrough, N. J., Farr, A. L. & Randall, R. J. (1951). Protein measurement with the Folin phenol reagent. J. Biol. Chem., 193, 265-75.

Standard Methods (1980). Standard Methods for the Examination of Water and Wastewaters, 15th edn. Am. Publ. Health Assoc., Washington, DC.


Switzenbaum, M. S. & Jewell, W. J. (1980). Anaerobic attached-film expanded-bed reactor treatment. J. Water Pollut. Control Fed., 52, 1953-65.

Toldrfi, F., Flors, A., Lequerica, J. L. & Vall6s, S. (1984). Anaerobic fluidized bed treatment of slaughterhouse wastewaters. In Proc. of the Third Mediterranean Congress on Chemical Engineering, 19-21 November 1984, Barcelona. Feria de Barcelona, Barcelona, pp. 203-4.

Toldrfi, F., Flors, A., Lequerica, J. L. & Vall6s, S. (1986). Fluidized bed biomethanation of acetic acid. Appl. Microbiol. Biotechnol., 23, 336-41.

Toldrfi, F., Flors, A., Lequerica, J. L. & Vall6s, S. (1987). Fluidized bed biodegradation of" food industry wastewaters. Biol. Wastes, 21, 55-62.

Van der Berg, L., Duff, S. J. & Kennedy, K. J. (1985). Methane production from anaerobic digestion. In Comprehensive Biotechnology, 2nd edn, vol. 4. ed. C. W. Robinson, J. A. Havell & M. Moo Young, Pergamon Press, Oxford, pp. 1051-8.

Yang, S. T. & Chang, M. (1985). Solubilizing cellulosic materials by alkali-cooking for anaerobic methane production. In Annual Reports on Fermentation Processes, vol. 8. ed. G. T. Tsao. Academic Press, New York, pp. 187-209.

methane generation from chemically pretreated cellulose by anaerobic fluidized-bed reactors

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