treatment of coffee waste by slurry-state anaerobic digestion

JOURNAL OF I~ERMENTATION AND BIOF.n6t~EEaJ~O Vol. 73, No. 5,390-395. 1992

Treatment of Coffee Waste by Slurry-State Anaerobic Digestion KENJI KIDA,* IKBAL, AND YORIKAZU SONODA

Department of Applied Chemistry, Faculty of Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto City, Kumamoto 860, Japan

Received 24 August 1991/Accepted 11 March 1992

Slurries containing 2 0 ~ (w/v) coffee waste solids were treated anaerobically in one- and two-phase thermophilic methane fermentation systems (53°C) with or without pH control. In one-phase methane fermentation using a roller bottle reactor, the maximum gas evolution rate of 0.87 l / l . d was achieved during treatment for 91 d. However, this one-phase methane fermentation did not yield reproducible data. In a two-phase methane fermentation system consisting of a completely stirred tank reactor type (CSTR-type) liquefaction reactor without pH control and an anaerobic fluidized bed type gasification reactor, three-repetitions of treatment were conducted. Each treatment was very stable and the average gas evolution rate per volume of the gasification reactor was about 2.41/ l .d. Two-repetitions of treatment were then done while controlling pH in the liquefaction at more than 6. The average gas evolution rate per volume of gasification reactor was found to have increased to 10.2 l / l . d, a value which corresponded to 0.84 l / l . d per total volume, including the liquefaction reactor. It was observed that treatment in a two-phase methane fermentation could be repeated in a stable fashion even in the closed system without discharging anything but the coffee waste residues.

Methane fermentation has attracted attention both as an energy-producing and as an energy-saving process for the treatment of wastewater.

Recently, novel bioreactors, such as UASB (upflow anaerobic sludge blanket) (l, 2) and AFBR (anaerobic fluidized-bed reactor) (3, 4), and UAFP (upflow anaerobic filter process) (5, 6) and two-phase methane fermentation (7, 8) have been developed to improve the reaction rate and for the treatment of wastewater with concentrations of organic matter less than 3,000 mg BOD/I (low-strength wastewater).

Using an AFBR, we have been studying ways to improve the reaction rate of anaerobic treatment and to develop it as a general process for the treatment of low-strength wastewater (9, I0), such as domestic sewage, as well as for wastewater with high concentrations of organic matter (high-strength wastewater) (l 1-13) greater than 10,000 mg BOD//.

Large quantities of waste slurries containing 5-15% solids (w/v) are discharged from animal production and food processing facilities. Meanwhile, the amount of energy available from the solid waste of crop residues is said to be ten times that from animal waste.

Solid waste from animal production has been treated at concentrations of solids below 7% (w/v) (14), and Wujcik and Jewell (15) investigated the thermophilic anaerobic digestion of straw mixed with dairy cow manure (total solids, 25%). Ghosh et al. (Proc. of 1983 International Gas Research Conference, London, June, 330-339, 1983) have proposed the solid-bed anaerobic digestion of municipal solid waste, and Baere et al. (Proc. of 3rd Inter- national Symposium on Materials and Energy from Refuse, Antwerten, 6.1-6.4, 1986) developed a dry anaerobic composting process for garbage (total solids, 30- 35%). Furthermore, the treatments of solid waste slurry, pig manure (16), and garbage mixed with sewage sludge (Ishida, M., Odawara, Y., Geyo, T., and Okum, A . H . , Proc. of recycling Berlin '79., In K. J. Thome-Kozmiensky

* Corresponding author.

(ed.), Freitag Verlag, Berlin, 797-802, 1979) have been investigated. However, methane fermentation at high levels (total solids > 10%) has not been tested in a full-scale plant.

During the course of this research and as part of an effort to develop a treatment system for high-solid waste material as a general process, we investigated the anaerobic fermentation of coffee bean residues after extraction with hot water (hereafter, called coffee waste). Calzada et al. (17-19) investigated methane fermentation using coffee pulp (skin and mesocarp of coffee berries) juice generated when coffee beans are separated from coffee berries. However, methane fermentation of coffee waste has not been examined. We investigated the possibility of anaerobic treatment with slurries that contained 20% (w/v) coffee waste solids. First, we examined one-phase methane fermentation in a roller bottle reactor (RB reactor), then examined two-phase methane fermentation consisting of liquefaction (CSTR type) and gasification (AFBR type).

MATERIALS AND METHODS

Coffee waste Coffee waste was provided by Minami Kyushu Coca Cola Bottling (Kumamoto). The compo- nents of the waste were organic matter (98.5%), protein (13.1%) and ash (1.5%); the particle diameter of the unground coffee waste was 0.25-1.0 mm~ and the moisture content was 65.5%.

Seeding sludge Thermophilic sludge provided by Oriental Yeast Co. (Tokyo) has been cultured anaerobically in synthetic wastewater at an organic loading rate of 3-5 g/l .d for several years by the draw-and-fill method (after settling and withdrawal of the culture broth, fresh synthetic wastewater equivalent to the volume withdrawn was placed in the digester) in our laboratory, hereafter called sludge. The sludge was used as seeding sludge. The synthetic wastewater (TOC, 18,800mg//) contains (g//): glucose, 35; corn steep liquor, 35; K2HPO4, 3; KH2PO4, 2; (NH4)2CO3.H20 , 5; Na2CO3, 3; FeC13.6H20, 1.

Support medium Cristobalite (particle diameter, 0.1-0.3mm~; provided by Nittetsu Mining Co. Ltd.,

390

VOL. 73, 1992 TREATMENT OF COFFEE-WASTE 391

Gas holder TrI'--IHi--- Roller bottle

53 *C

FIG. I. tion.

Incubator Laboratory equipment for one-phase methane fermenta-

Tokyo), which was superior to other media examined (20), was used as a support medium for microbial adhesion in the AFBR.

One-phase methane fermentation Figure 1 shows a schematic diagram of the experimental apparatus (roller culture apparatus; Wheaton, Tokyo). A glass RB reactor (Pyrex Roller Bottle; 100mm outside diameter and 290 mm height; Iwaki Glass Co., Tokyo), with a working volume of 800ml, was used as the one-phase methane fermentation reactor. Unground or ground coffee waste (20%, w/v) and 24.8 g (wet weight) of the sludge were added to the reactor, following which the air within the reactor was replaced with N2 gas. During treatment, the temperature and rotation speeds were maintained at 53°C and 1.5 rpm, respectively. The ground coffee waste was prepared by grounding with a Micro Grinder (Micro Tech. 21 Co., Fukuoka).

Two-phase methane fermentation Figure 2 shows a

schematic diagram of the two-phase methane fermentation. A CSTR-type liquefaction reactor (model LF-60; Labotec, Tokyo) with a working volume of 5.01 was used as the liquefaction reactor, and the temperature was controlled at 53°C. Unground coffee waste (1,000g) and the sludge (500ml) were added into the reactor, then the reactor was filled with tap water to the working volume. During the experimental period, the agitation speed was maintained at 200 rpm.

The gasification reactor was an anaerobic fluidized bed reactor (AFB reactor) with a working volume of 0.451, in which the reactor was composed of a settling portion and a fluidization portion (30 mm in diameter and 500 mm in height). The temperature was maintained at 53°C by the circulation of water through a water jacket, and the liquid in the reactor was circulated at a rate of 7.6 l /h by a roller pump P-3 (RP-30; Tokyo Rika Co., Tokyo) to fluidize the support medium. The evolved gas entered a wet-gas holder and was analyzed once a day. Ninety grams of cristobalite were added to the reactor, the reactor filled with the sludge to 450 ml, and then the liquid in the reactor was circulated overnight by a roller pump P-3. Ten-fold diluted synthetic wastewater was then continuously fed into the bottom of the reactor at a rate of 480 ml/d (which corresponds to an organic loading rate of 2 g/l. d) via a peristaltic pump P-2 (SJ-1211H; Atto, Tokyo) for 11 d. The ten-fold diluted wastewater was then replaced by two-fold diluted wastewater. Cultivation was continued until the organic loading rate reached 7 g/l. d.

After cultivation, liquefied material was pumped into the separator by a pump P-1 to separate the solid particles from the liquid, following which the liquid was fed into the bottom of the gasification reactor by a pump P-2. If the pH in the AFB reactor dropped to less than 7, the pump P-2 which connected to a pH controller (HB-96K2; Denki Kagaku Keiki Co. Ltd., Tokyo) stopped automati- cally. When the pH was greater than 7, total organic carbon (TOC) and the volatile fatty acids (VFA) loading rate in the gasification reactor were properly determined by the flow rate and working time of pump P-2. The pH in the liquefaction reactor was controlled with a pH controller.

Analytical methods The concentration of in_fluent organic matter and the ash content of the coffee waste

Separator

I r - - -

I I I

' i i I

I

"~"Thermostated .: wat=r

NaOH P 4 P-1 P-2 p-3 Liquefact ion Gasif icat ion

FIG. 2. Laboratory equipment for two-phase methane fermentation.

392 KIDA ET AL. J. FERMENT. B]OENO.,

were calculated from the ignition loss by a method for testing industrial wastewater (JIS K 0102-1986) (21). The crude protein content was then calculated from the nitrogen content obtained by the Kjeldahl method (21), which was multiplied by 6.25.

The conductivity and concentrations of total organic carbon (TOC) and volatile fatty acids (VFA) were analyzed in the supernatant after centrifugation at 10,600 x g for 10 rain. Soluble TOC was analyzed with a TOC autoana- lyzer (TOC-500; Shimadzu, Kyoto) (21). VFA were analyzed by a post-label method in which the absorbance of volatile compounds that reacted with a bromothymol blue solution was measured at 450 nm (870-UV detector; Japan Spectroscopic Co. Ltd., Tokyo) after separation of VFA from the wastewater by HPLC (high-performance liquid chromatography; 880-PU, 860-CO; Japan Spectroscopic Co. Ltd.) on a column designed for the analysis of VFA (Shim-pack SCR-101 H; Shimadzu, Kyoto).

Conductivity was measured with a conductivity meter (AOL-40; DKK Co., Tokyo).

The methane content of the biogas was measured by TCD (thermal conductivity detector) gas chromatography (KOR-2G; GL Sciences Co., Tokyo) on a pre-packed column (Poropak Q; GL Sciences Co., Tokyo).

RESULTS AND DISCUSSION

One-phase thermophi l i c methane fermentat ion As shown in Fig. 3 for ground coffee waste, the gas evolution rate declined after 10 d. The pH of the liquid in the reactor was 5.6 after 12 d as a result of the accumulation of VFA. The pH was then controlled at 7 by the addition of NaHCO3. The gas evolution rate did not increase. The integrated volume of gas for the first 32 d was no more than 21 !/l . It was likely that the rapid increase in levels of VFA inhibited the gasification reaction. In the case of unground coffee waste the maximum integrated volume of gas during the first 32 d reached 48.6 I l l at nearly a constant rate. After that, the gas evolution rate decreased, while the

A ".. 80

"8

m ¢13

,~ so Q

~ 4o

3o o

i I i I I I ! I 0 O0 10 20 30 40 50 60 70 80 90 1 0

Time (d)

FIG. 3. Effect of particle size of coffee waste on gas evolution in one-phase methane fermentation. Arrows indicate addition of NaHCO3 to control pH at 7. Symbols: O, unground coffee waste; • , ground coffee waste.

integrated volume of gas and the average gas evolution rate during 91 d fermentation reached 79 .4 / / /and 0.87 l / / . d, respectively. The maximum gas evolution rate of 3.5///. d in 19 d fermentation was also obtained in one-phase fermentation. However, stable values could not be obtained even when using unground coffee waste in spite of adjust- ing the pH, decreasing the initial concentrations of coffee waste and increasing the sludge (data not shown). From these results, in one-phase methane fermentation and espe- cially in the early stage of fermentation, it was very di~cult to obtain a suitable VFA loading rate for the growth of methane-forming bacteria because the initial acidification rate would vary depending on the particle size of the coffee waste and initial pH, while the gasification rate would depend on the concentration and activity of the methane forming bacteria.

T w o - p h a s e thermophi l i c methane fermentat ion In the anaerobic fermentation of solid materials it may be very important to maintain a good balance between the rates of liquefaction and gasification. A divided system for the liquefaction and gasification reactions may result in more stable operation than a single reactor because an inde- pendent gasification reactor could be operated at the controlled VFA loading rate. Therefore, two-phase methane fermentation was carried out.

Figure 4 shows the time course of the gasification process when the pH in the liquefaction reactor was not controlled. The integrated volumes of gas per working volume of the gasification reactor during 30 d and 57 d were 71 and 135 Ill , respectively. At the end of the experiment, 5 / of slurry were filtered through a screen (1.41-mm mesh; no. 12) and 2.3 / of filtrate were obtained. This brown filtrate had a high TOC concentration of about 5,400 mg/l, and required further treatment prior to discharge. Therefore, this filtrate was reused as the water for the treatment of fresh, raw coffee waste in the next batch. A 2.3 I portion of filtrate was added to 1,000 g of fresh, raw coffee waste, and the liquefaction reactor was filled to 5 1 with tap water. The slurry was treated in the liquefaction reactor for 1 d, following which the effluent was supplied to the gasification reactor after settling. Thus, liquefaction of the second and third samples of fresh coffee waste and the gasification were repeated. After the second repeat of treatment,

. A poo I. ,8o ~80! S ; - ~ is0 e- 6 0 - > o

" W 120 o m

~; O - o

N so

"O

2o

x x R x

xX X~X X X'~'XxX/XX~XR xXXXxx / XX XXX XXXX

X

oO9

o o

I I I I I | I I I 5 10 15 20 25 30 35 40 45 50 55 60

Time (d)

FIG. 4. Time course of gas evolution in two-phase methane fermentation without pH control in the liquefaction phase. Sym- bols: - - , gas evolved in Ist hatch; O, gas evolved in 2nd hatch; - -, gas evolved in 3rd hatch; ×, methane content in 2nd hatch.


5,000 O~

E 4,000 P .9 ~ 3,ooo ¢-

~ 2,ooo i - 0

,~ 1,ooo U .

oo , , , I . . . . I . . . . I . . . . I , , , , I , , ~ ' , , I . . . . I , , , L~ I

5 10 15 20 25 30 35 40 Time (d)

FIG. 5. Changes in volatile fatty acids (VFA) concentration in liquefaction and gasification in two-phase methane fermentation without pH control in the liquefaction phase. Symbols: A, liquefaction; A, gasification.

the liquefied slurry was filtered through a screen (0.25- mm mesh; no. 60). The gas evolution in each treatment test was almost the same as that shown in Fig. 4. It was found that repeated treatments could be performed stably without any discharges, except for coffee waste residues. During the treatment, the average gas evolution rate per volume of the gasification reactor was about 2.4 l/l.d, which corresponded to 0.201/l.d per total volume, including the l iquefaction reactor. This value is only 23% of that of the maximum gas evolution rate achieved in the one- phase methane fermentation, although the methane content was high (60-70~) , compared with that obtained in the one-phase fermentat ion (45-50%).

To examine the cause of the lower gas evolution rate, the time courses of VFA concentration in the second liquefaction and gasification reactors were investigated. As shown in Fig. 5, the VFA concentration in the liquefaction reactor decreased gradually to 1,000 mg/I on the 30th d so that the VFA concentrations in both reactors became virtually the same. From this result, it appears that the liquefaction rate was lower than the gasification rate.

The effect of pH in the l iquefaction was investigated in order to improve the liquefaction rate. As shown in Fig. 6, the VFA product ion rate during the first 5 d was 0.78 g / l . d under a controlled pH of 6. This rate was 2.5-fold

12,000

A 10,000

E 8,000 c" o ~ 6,ooo

i 4,000 O

,,oo0 L

0

FIG. 6. ing liquefaction with and without pH control. o , pH 6; zx, without pH control.

I I I I I I I I I I I t 2 0 2 4 6 8 10 1

Tlme (d )

Changes in volatile fatty acids (VFA) concentration dur- Symbols: Q, pH 7;

~8o ~ 3 o o F

~60o ~ 2so~ -xx-xx-xxxxx~xxxxxx-x-x

"E • o40 ~ > 200 1 0 0"0'0"00"00

20 ~ I "°'° " • '~ "B 1so 0.0 "0

~; o ~ ~ I 0 "0"0"0" "~ 100

.-.- - - I o "°'°'°~° . . . . . . . " " " o l ' - U , ~ - , " , " r T - " , , , . . . . , . . . . , . . . . i

0 5 10 15 20 25

Time (d) FIG. 7. Comparison of gas evolution with and without pH con-

trol in the liquefaction phase in two-phase methane fermentation. Symbols: o , gas evolved with pH control; - - , gas evolved without pH control; x, methane content in cases with pH control.

higher than that without pH control. Moreover, the VFA product ion rate during the first 5 d at a controlled pH of 7 increased to 1.4 g/I. d, however, VFA were not pro- duced. This indicates that the l iquefaction or acidification process seems to be inhibited by VFA concentrations of more than 10,000 mg/I.

On the basis of the results mentioned above and the alkali consumption for the pH control, the l iquefaction process was carried out at pH 6.

Two-phase methane fermentation at a controlled pH Treatment of a slurry by two-phase methane fermentation, with the pH of the l iquefaction process controlled at 6, was carried out. As shown in Fig. 7, the integrated volume of gas per working volume of the gasification reactor reached 245 1/l during 24d. During this period, the average gas evolution rate was 10.21/I. d, and it was 4-fold higher than that without pH control. This rate was calculated as 0.84 Ill.d, including the volume of the liquefaction reactor, and was almost the same as the maximum gas evolution rate in the one-phase methane fermentat ion. However, the methane content in the biogas was 6 0 - 7 0 ~ , the same as that without pH control, and it was higher than that in the one-phase methane fermentat ion (45- 50~o). The time course of gas evolution during treatment of the second batch of coffee waste was also very similar. It was found that five repetitions were possible, including without pH control (data not shown).

In order to confirm the possibility of treatment in a closed system, the pH and conductivity in both the liquefaction and gasification reactors at a controlled p H were investigated. Figure 8 shows the time courses of the first and second treatments. When the effluent o f the gasification reactor was recycled to the l iquefaction reactor, the pH of the liquid gradually increased, and it exceed- ed 6 on the 12th d and 8th d during the first and second treatments, respectively. Even if the pH in the l iquefaction reactor was controlled, a lot of alkali was not consumed. In fact, an alkali consumption of 2.6 g/l liquid in the first t reatment decreased to 2.2 g/l in the second treatment. No extra alkali was consumed as a result of the repeti t ion of treatment. Moreover, the conductivity increased immedi- ately after starting, but then gradually decreased during the first 10 d. A conductivity of 4.5 to 4.8 mS/e ra (at 250C) was maintained during the repeti t ion of t reatment. Low

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

8

~ 71

~ 6 I

5

4 0

~" 8

o~ 7

4 " 0 C o 3 0 0

(a) (b)

1 1 1 1 1 _ 1 _ 1 i i i i i / i i i i i \ 1 1 8 F ~ I i i i . . . , . 1 1 1 l . , , . l _ i i I _ 1 i _ i _ i i

,o OOo o/_~.,,~o>O o o o O ~ O o o o o / ° ° ~ 71 o o o o o / O o ~ O - - O ° : . o o . o : . . . . . . . . . . . . .

4 I . . . . I~ , , , I , i i I 11 i 1 , 1 , , , l l , , , I . . . . I . . . . I . . . . I , , , , I 5 10 15 20 25

Tlme (d)

/ % J°7

0 5 10 15 20 25 Time (d)

~" 8

o~ 7i E

_~s,- _>

4 - i

"O

, , , I . . . . l , , , , I . . . . I , ~ = ~ 1 8 3 , , , I . . . . I . . . . I . . . . I , , ~ l l 5 10 15 20 25 o 5 10 15 20 25

Time (d) Time (d)

FIG. 8. Changes in pH and conductivity during 25-d operation of liquefaction phase in the 1st batch (a) and the 2nd batch (b). Symbols: uefaction; i , conductivity in gasification.

tO_O/ ~O" ~ O O

the two-phase methane fermentation system with pH control in the o , pH in liquefaction; • , pH in gasification; <>, conductivity in liq-

conductivity results from the low consumption of alkali and the low ash content (about 1.5%) of coffee waste. The maximum conductivity of the effluent of both the liquefaction and the gasification processes during five repetitions, including without pH control, was 7.4 mS/cm (at 25°C). This value was quite low when compared with the conductivity of 13.0 mS/cm of the synthetic wastewater that was used in the cultivation of thermophilic sludge. It appears that further repetitions may be possible in such a two- phase methane fermentation system.

In conclusion, reproducible data could not be obtained in one-phase methane fermentation. However, in two- phase methane fermentation, five treatment repetitions with and without pH control were carried out without any problems. The closed system could be operated stably with no discharges, except for coffee waste residues. The closed system may be practical because of the low ash content (about 1.5°/~) of coffee waste.

ACKNOWLEDGMENT

The authors wish to thank H. Kobayashi and N. Hayashida for their excellent technical assistance.

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from coffee pulp juice: one- and two-phase systems. Agric. Wastes, 9, 217-230 (1984).

19. Calzada, J. F., Rolz, C., de Arriola, M. C., Castaneda, H. O., and Godoy, J. E.: Biogas production from coffee pulp juice using packed reactors: scale-up experiments. MIRCEN J., 2, 489-492 (1986).

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Support media for microbial adhesion in an anaerobic fluidized- bed reactor. J. Ferment. Bioeng., 69, 354-359 (1990).

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treatment of coffee waste by slurry-state anaerobic digestion

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