JOURNAL OF FERMENTATION AND BIOENGINEERING Vol. 77, No. 3, 335-338. 1994

Anaerobic Digestion of Coffee Waste by Two-Phase Methane Fermentation with Slurry-State Liquefaction

KENJI KIDA, 1. IKBAL, l MOTOWO TESHIMA, 1 YORIKAZU SONODA, 1 AND KOUHEI TANEMURA 2

Department o f Applied Chemistry, Faculty o f Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto City, Kumamoto 860,1 and Yatsushiro National College of Technology, Hirayama

Shin-machi 2627, Yatsushiro 866, 2 Japan

Received 7 July 1993/Accepted 22 November 1993

Anaerobic digestion of components of coffee waste was investigated. When the coffee waste was treated by a liquefaction process only, the degradation efficiency was 42%, taking into account the soluble materials that adhered to the surface of the waste. However, in a two-phase anaerobic digestion system consisting of liquefac- tion and gasification processes, the degradation efficiency increased to 7 0 ~ . The components of coffee waste, namely, materials that could be eluted by a mixture of alcohol and benzene (hereafter called alcohol-benzene soluble materials), holocellulose and lignin, were degraded by 9 1 ~ , 7 0 ~ and 4 5 ~ , respectively. The gas yield was 451 ml /g degraded coffee waste and 2 8 . 2 ~ of the carbon in the waste was converted to biogas. The methane content in the gas evolved from the gasification reactor was high, being 66°~ (v/v).

Methane fermentation processes have attracted atten- tion as methods for the energy recovery from organic waste, in particular from waste water with high concen- trations of organic matter (1) and slurry-state organic wastes, which were reported by Ghosh, S. et al. (Proc. Int. Gas Research Conf. London, p. 330-339, 1983), de Baere, L. et al. (Proc. 3rd Int. Syrup. on Materials and Energy from Refuse, Antwerpen, 6.1-6.4, 1986) and Kida et al. (2).

The amount of solid organic residues emitted from the agro-industry has been increasing annually. However, the possibilities of utilizating these residues as foodstuffs for animals are limited, while dumping in landfills is becoming increasingly difficult as the regulations of the waste authorities are made stricter (Weiland, P., Proc. Int. Symp. on Anaerobic Digestion of Solid Waste, Venice, Italy, p. 193-199, 1992).

With a view to addressing this problem, we have been studying the slurry-state anaerobic digestion of suspen- sions that contain 20% (w/v) coffee waste in one-phase and two-phase systems (2). To improve the gas evolution rate, we have also investigated the optimum ratio of the volumes of the liquefaction and gasification reactors in the two-phase system. As a result, a maximum gas evolu- tion rate of 1.43 l / l .d was achieved (3).

Recently, Wellinger, A. et al. (Proc. Int. Symp. on Anaerobic Digestion of Solid Waste, Venice, Italy, p. 29-42, 1992) investigated the pilot-scale thermophilic anaerobic digestion of mixtures of fruit, yard and vegeta- ble waste (total solids, 15-40%), and they reported that the gas evolution yield was 370 ml/g of volatile solids. Ahring, B.K. et al. (Proc. Int. Syrup. on Anaerobic Digestion of Solid Waste, Venice, Italy, p. 203-208, 1992) investigated the large-scale thermophilic anaerobic digestion of mixtures of source-sorted household solid waste together with manure and organic industrial waste, and Edelmann, W. et al. (Proc. Int. Symp. on Anaero- bic Digestion of Solid Waste, Venice, Italy, p. 225-240, 1992) proposed a combination of composting and anaer-

* Corresponding author.

obic digestion for treatment of mixtures of organic indus- trial and municipal wastes.

However, the degradation efficiency of solid waste and the degraded components have not yet been investigated in detail. In the course of our research, in order to exam- ine the degradation efficiency of coffee waste and its com- ponents, we carried out the following experiments. First, we investigated the quantity of soluble materials that adhered to the surface of the coffee waste, and then the washed coffee waste was treated by a liquefaction process. Finally, in order to increase the degradation efficiency, we conducted an experiment by two-phase methane fermentation with slurry-state liquefaction. Our results provide potentially useful findings relevant to the degradation of components of solid waste, viz. coffee waste.

Materials The coffee waste was provided by Minami Kyushu Coca Cola Bottling (Kumamoto) (2). The organic matter and ash content of the dry matter were 98.5% and 1.5%, respectively. The components of the organic matter, i.e. the levels of alcohol-benzene solu- ble materials, holocellulose and lignin in the coffee waste, were 22.1%, 52.1% and 25.8%, respectively, and the di- ameter of the particles was 0.25-1.0 mm~. The analytical methods for organic matter, ash, total organic carbon (TOC), volatile fatty acids (VFA) and methane content of the evolved gas have been described previously (2). The levels of alcohol-benzene soluble materials, holocel- lulose and lignin in the coffee waste were analyzed by methods that are used for testing pulpwood (4). The CHN (carbon, hydrogen and nitrogen) contents in coffee waste were analyzed by a CHN coder MT-3 (Yanako, Tokyo).

The thermophilic sludge was provided by Oriental Yeast Co. (Tokyo) (2). This sludge has been cultured an- aerobically with synthetic wastewater for several years in our laboratory, and it was used as seeding sludge.

Liquefaction process A CSTR-type (completely stirred tank reactor-type) reactor (Jar Fermentor MBF, Eyela Tokyo Rikakikai Co. Ltd., Tokyo) with a working volume of 2 l was used as the liquefaction reactor. An

335

336 KIDA ET AL. J. FERMENT. BIOENG.,

TABLE 1. Material balance of fresh-raw coffee waste treated by liquefaction or two-phase methane fermentation with

slurry-state liquefaction

Liquefaction process Two-phase Washing-Liquefaction process

Fresh, raw coffee waste 400 g -- 400 g+45.8 ga Fresh, raw coffee waste 284 g -- --

after two-washings Residual coffee waste -- 233 g 134.7 g Degradation efficiency of -- 18o//00 --

washed coffee waste Total degradation efficiency 42%0 70%o Degradation of each component

of fresh, raw coffee waste alcohol-benzene soluble materials-- 75~ 91 °/0 holocellulose - - 27~o 700/00 lignin - - 24~ 45%0

a This value (45.8 g) was the amount of insoluble materials contain- ed in the filtrate from the liquefied material of the first treatment. Coffee waste washed twice with tap water was treated by liquefaction, while fresh, raw coffee waste was treated by two-phase methane fermentation.

aliquot of 0.2 l (10°Joo, v/v) of sludge and 400 g (20°Jo, w/v) of washed coffee waste were added to the reactor and then the reactor was filled with tap water to the working volume. During the experimental period, the pH, temperature and agitation speed were maintained at 6, 53°C and 300 rpm, respectively. The pH in the reactor was controlled automatically by the feeding of l N NaOH solution via a peristaltic pump (pump P-4, SJ- 1211L; Atto, Tokyo), which was turned on or off by a pH controller (HB-96K2; Denki Kagaku Keiki Co. Ltd., Tokyo).

To examine the liquefaction efficiency of the coffee waste, at the end of the treatment the liquefied slurry was filtered through a screen (1.00-mm mesh; no. 16). Then the residue of the liquefied slurry and 0.2 ! of the sludge were added back to the reactor, and the reactor was filled with tap water to a working volume of 2 l. The liquefaction treatment was then repeated. Five sequential liquefactions were performed in the same manner.

Two-phase methane fermentation The equipment used was the same as shown in our previous paper (3). After the slurry of coffee waste (20%, w/v) had been treated in the liquefaction reactor (working volume, 2/) for 3 d, the liquefaction reactor was connected in series to a gasification reactor, in which methanogens were being cultivated at a TOC volumetric loading rate of 5 g / l - d , as follows: ninety grams of cristobalite were added to the reactor (working volume, 0.45/), the reactor was filled with the sludge to 0.45 l, and then methano- gens were cultivated in the same manner (3) until the or- ganic loading rate reached from 2 to 5 g/l .d.

Liquefied material was pumped into the separator by a peristaltic pump (pump P- l , SJ-1211H; Atto, Tokyo) for the separation of solid particles from the liquid, and then 0.85 l/d of the liquid were fed continuously into the bot tom of the gasification reactor by another peristaltic pump (pump P-2, SJ-1211H; Atto, Tokyo). The over- flow of the gasification reactor was returned automati- cally to the liquefaction reactor. To achieve the maximum volumetric loading rate of organic matter in the gasifica- t ion reactor, the flow rate of liquefied material was con- trolled by the pump P-2. However, if the pH in the gasification reactor dropped below 7, the pump P-2,

which was connected to a pH controller, automatically ceased operation.

At the end of the first treatment, 2 l of slurry in the liquefaction reactor were filtered through a screen (1.00- mm mesh; no. 16), and the filtrate was reused as the water for the preparation of a suspension of fresh, raw coffee waste in the second batch (repeated treatment).

Degradation of coffee waste by the liquefaction proc- ess It was found in the preliminary experiment that soluble materials adhered to the surface of the coffee waste. As shown in Table 1, the amount of soluble mate- rials (116.4g) was estimated to be about 2 9 ~ from the reduction in weight when 400g of fresh, raw coffee waste was washed twice with tap water.

The washed coffee waste was treated anaerobically by several repetitions of the liquefaction procedure to inves- tigate the degradation efficiency. Figure 1 shows the changes of degradation of the washed coffee waste by the liquefaction process. The decrease in VSS concentra- t ion and the increase in TOC concentrat ion demonstrate the liquefaction efficiency, and the increase in VFA con- centration demonstrates the acidification efficiency. After the first liquefaction, the TOC and VFA concentrations increased to 8,600 and 4,000 mg// , respectively, and the VSS concentrat ion decreased to 120.1 g/l. However, the increases in the TOC and VFA concentrations, as well as the decrease in the VSS concentration, declined with the repetition of treatment. During the fifth liquefaction (for 8 d) the TOC, VFA and VSS concentrations in the liquid were unchanged. From these results, it was calculated, as shown in Table 1, that the extent of degradation of the washed coffee waste was about 18~ . The total liquefac- tion efficiency, taking into account the soluble materials eluted by the two washings was 42°Joo. With respect to the degradation of each component of fresh-raw coffee waste, the amounts of alcohol-benzene soluble materials, holocellulose and lignin were 75o//00, 27~o and 24°Joo, respectively.

Degradation of coffee waste by two-phase methane fer- mentat ion with slurry-state liquefaction When the VFA concentration in the liquefied material had in- creased to 5 ,030mg// , after the first 3 d of operation (Fig. 2), the liquefaction reactor was connected to the gasification reactor in series, as described in Methods, and the liquefied material was fed to the gasification reac- tor at a VFA volumetric loading rate of 10g / / .d , which was the maximum loading rate achieved in our previous paper (3). After the eleventh day of treatment, the VFA concentrat ion in the liquefied material decreased rapidly, as shown in Fig. 2. In order to maintain the VFA volu- metric loading rate in the gasification reactor at a con- stant value of 10g / l . d , the feeding rate of the liquefied material to the gasification reactor was increased in step- wise fashion. Figure 2 also shows the time course of the gas evolution per total volume of the reactors, which in- cluded the liquefaction reactor, for the first and second treatments. In the first treatment, the average gas evolu- t ion rate during the first 47 d of operation was 1.25 l~ l -d. This value was a little lower than that reported previously (1.43 l/1.d) (3). At the end of the first treat- ment, 2 ! of slurry was filtered through a screen (1.00- mm mesh; no. 16) and 1.1 l of filtrate were obtained. This filtrate was reused as the water for making the suspension of fresh, raw coffee waste for the next batch. The 1.1 l of filtrate were added to 400g of fresh, raw coffee waste, and the liquefaction reactor was filled to

VoL. 77, 1994 NOTES 337

200 ~ ~ 1 0 F Run1

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0 4 8 12 Time (d)

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Run 2 Run 3 Run 4

D~~ D~ D~•

. , . , . . . . . . . . . . . , . . . . . 0 4 8 12 0 4 8 12 0 4 8 12

Time (d) Time (d) Time (d)

Degradation of washed coffee waste by slurry-state liquefaction.

Run 5

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2 l with tap water; the t reatment was then repeated. As shown in Fig. 2, the gas evolution rate was almost the same as that in the first t reatment . Af ter 50 d of oper- at ion, little gas was evolved f rom either reactor. This was because the concentrat ions of V F A in both reactors decreased to undetectable levels (data not shown).

Figure 3 shows the balance of insoluble materials and carbon during the second treatment . The coffee waste and the filtrate contained 445.8 g of insoluble material , and the residual insoluble materials weighed 134.7 g after the second t reatment . F rom these values, the degrada- t ion efficiency o f the coffee waste was calculated to be 70% (as shown in Table 1) and the gas yield was 451 ml /g degraded solid materials. The methane content of the gas evolved f rom the gasification and the liquefac- t ion reactors was 66% and 450/00 (v/v), respectively, and 83% of the total gas was evolved f rom the gasification reactor. If 311.1 g of coffee waste were degraded during the first 50 d of opera t ion , as deduced f rom Fig. 2, the degradat ion rate of the coffee waste per total volume of

both reactors (2.45/) was 2 . 5 4 g / I . d . The analytical results for the C H N of each type of mater ia l indicated that 28.2% of the carbon in the supplied coffee waste and in the filtrate were converted to biogas and 6.1% of the carbon was consumed for the prol i fera t ion of methanogens. The amount of carbon in the methano- gens in the gasification reactor was calculated, based on the assumption that the suppor t medium, cristobali te, did not contain any carbon. However , unaccounted loss o f carbon was high, being about 2 3 . 5 ~ , perhaps due to defects in the equipment.

Table 1 also shows the degradat ion of each compo- nent o f fresh, raw coffee waste after the second treat- ment. The degradat ion efficiency of alcohol-benzene solu- ble materials, holocellulose and lignin were 9 1 ~ , 70°/00 and 45°/00, respectively.

In conclusion, when the coffee waste was treated only by l iquefaction, the degradat ion efficiency was 42%. However , in the two-phase anaerobic digestion the degra- da t ion efficiency increased to 70%. Moreover , 45% of

5 "6 "" 50 > - ;

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0==30 S o >

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10 20 30 40 50 60 70

Time (d)

FIG. 2. Time courses of changes in VFA concentration and total evolution of gas per total working volume of the reactors during repeated treatments by two-phase methane fermentation with slurry-state liquefaction. Symbols: ©, total gas evolved during the first treatment; [], total gas evolved during the second treatment; A, concentration of VFA in the liquefaction reactor during the first treatment; A, concentration of VFA in the gasification reactor during the first treatment.

338 KIDA ET AL. J. FERmeNT. BIOENG.,

Coffee waste, 400 g (209.8 g)

Filtrate from the first treatment Insoluble meterials, 45.8 g (22.6 g) Soluble materials, 1.1 1 (13.18 g)

Liquefaction 23.4 ! (10.01 g)

! Biogaa, 140.2 1

Process :

Gasification 116.8 ! ( 59.34 g)

Liquefaction reactor, 2.0 1

Gasification reactor, 0.45 1

- -m- Residue Insoluble materials, 134.7 g (69.04 g) Soluble materials, 1.37 l (18.8 g)

- - I~ Grown cells (15.06 g)

~ 1 ~ Unidentified (57.61 g)

¢ Sampling (15.72 g)

FIG. 3. Balance of insoluble materials and carbon during anaerobic digestion by two-phase methane fermentation with slurry-state liquefac- tion. The numbers in brackets show the amounts of carbon.

the lignin in the coffee waste was degraded even though lignin is said to be difficult for microorganisms to de- grade.

REFERENCES

1. Kids, K., Ikbal, Sonoda, Y., Kawase, M., and Nomura, T.: Influence of mineral nutrients on high performance during anaerobic treatment of wastewater from a beer brewery. J. Fer- ment. Bioeng., 72, 54--57 (1991).

2. Kida, K., Ikbal, and Sonoda, Y.: Treatment of coffee waste by

3.

4.

slurry-state anaerobic digestion. J. Fermem. Bioeng., 73, 390- 395 (1992). Ikbal, Klda, K., and Sonoda, Y.: Liquefaction and gasification during anaerobic digestion of coffee waste by two-phase methane fermentation with slurry-state liquefaction. J. Fer- ment. Bioeng., 77, 85-89 (1994). Japanese Industrial Standards Commit t ee : Testing Methods for Lignin in Wood for Pulp, JIS P 8008-1976, Testing Methods for Alcohol-Benzene Solubility of Pulpwood, JIS P 8010-1976, Testing Methods for Holocellulose in Pulpwood, JIS P 8012- 1976, Japanese Standards Association, Tokyo (1976).