biochemical methane potential and solid state anaerobic digestion of korean food wastes

ELSEVIER 0 9 6 0 - 8 5 2 4 ( 9 5 ) 0 0 0 3 1 - 3

Bioresource Technolo~ $2 (1995) 245-253 O 1995 Elsevier Science Limited

Printed in Great Britain. All rights reserved O960-8524/95/$9.50

BIOCHEMICAL METHANE POTENTIAL AND SOLID STATE ANAEROBIC DIGESTION OF KOREAN FOOD WASTES

Jae Kyoung Cho & Soon Chul Park

Biomass Research Team, Korea Institute of Energy Research, 71-2, Jang-dong, Yusung-gt6 Taejon 305-343, Korea

&

Ho Nam Chang*

Bioprocess Engineering Research Center and Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-L Kusung-don~ Yusung-gu, Taejon 305-701, Korea

(Received 20 October 1994; revised version received 21 February 1995; accepted 2 March 1995)

Abstract In order to develop an anaerobic digestion process for Korean food wastes containing 15-30% total solids (TS) the biochemical methane potentials (BMP) of their components and mixture (mixed food waste, MFW) were measured. The methane yields of cooked meat, cellulose (as control), boiled rice, fresh cabbage and MFW were 482, 356, 294, 277 and 472 ml CH4/ gVS added and anaerobic biodegradabilities based on the stoichiometric methane yield were 0.82, 0.92, O" 72, 0"73 and 0"86, respectively.

In a one-phase methane reactor system, VFAs were accumulated so excessively as to inhibit subsequent methane fermentation. Hence, the MFW with 20% TS was digested in solid-bed, two-phase systems of I l and 8 1 methane reactors. In the 8 1 methane reactor system, also, the effect of effluent recirculation to the solid-bed was studied. No nutrients, buffer or seed inoculum were added in the 5 l solid-bed. In all cases, 87-90% of TS in the solid-bed was reduced and approximately 90% of the initial bed volatile solid was converted to biogas. The treatment periods for the complete digestion of waste depended mostly upon the performances of the methane fermenter, especially on hydraulic retention time (HRT) and loading rates. The methane yield was around 405-415 ml/gVS reduced during entire periods. This value is about 86-88% of the BMP test data (the ultimate methane yield) with the same sample.

Key words: Mixed food waste, biochemical methane potential, solid-state anaerobic digestion, two-phase, solid-bed, leaehate.

*Author to whom correspondence should be addressed. 245

INTRODUCTION

Almost all the food wastes, which constitute about 30% of total municipal solid waste (MSW), in Korea are cmTently landfilled. Existing landfill sites are nearly exhausted and new landfill sites are hardly available because of the shortage of utilizable land and the objection of residents near the proposed landfill sites. Incineration is not suitable because of the high moisture content and low heating value of MSW, especially those of food wastes. For Korean food wastes, solid-state anaerobic digestion can be considered as an excellent alternative of volume reduction and energy recovery. Since the food waste contain high soluble organics, they are converted rapidly to volatile fatty acids (WAs) at an early stage of digestion. A drastic pH drop will inhibit the initiation of methane fermentation with no sufficient buffering capacity (Kang & Jewell, 1990).

In order to reduce the inhibition of methane fermentation by organic adds, Ghosh (1983) proposed the concept of biphasic fermentation with high solid feeds using a solid-bed for acid fermentation and subsequent methane fermentation. Refuse-derived cellulosics (Ghosh et al., 1983), the putrescible fraction of MSW rejected from the production of RDF (Anderson & Saw, 1992), slurries containing 20% (w/v) coffee-waste solids (Kenji et al., 1994), market garbage (Chanakya et al., 1993a) and so forth were treated using a two-phase anaerobic digestion system. Weiland et al. (1992) treated the different agro-industrial residues in pilot scale using two-stage operation.

In this study, a traditional Korean food called Bibimbab was taken as a sample for BMP test and, subsequently, for two-phase anaerobic digestion.

246 J. K. Cho, S. C. Park, H. N. Chang

Actual food wastes are not homogeneous in their day by day compositions, hence it is very difficult to obtain duplicate and representative samples from actual food wastes. Bibimbab is a Korean food based on boiled rice mixed with vegetables, meat and eggs and can simulate Korean food wastes relatively well. The BMPs of the MFW (actually Bibimbab) and its components were measured to compare extents and rates of their conversion to methane. The BMP data were used for the estimation of biodegradabilities of the typical Korean food components. Meanwhile, the main aim of the work was to develop a process for energy recovery from solid-state anaerobic digestion of food waste discharged at 15-30% TS. In order to achieve this objective, digestion tests with various initial organic loadings in a single reactor were carried out to investigate the inhibition effect. Furthermore, MFW was digested in a two-phase anaerobic digestion system since methanogenic reac- tion of MFW having higher than 5% (w/v) TS was found to be difficult without buffer addition and dilution rate control during the BMP and preliminary digestion tests. The MFW was hydrolyzed in an acidogenic reactor at 20% TS and was subsequently digested in a partially-packed methanogenic reactor to produce methane as a final product.

METHODS

MFW samples Components of the MFW are shown in Table 1. All samples were ground in a pulverizer (Wiley mill, Thomas Scientific, USA) and freeze-dried (Lab- conco corp. Cat. No. 7753500, USA) for the BMP test. Prior to the BMP test and digestion experiments samples were analyzed for their volatile solid (VS) contents and elemental composition. The feed (MFW) for two-phase anaerobic digestion was adjusted to 20% TS without any pretreatment.

BMP and preliminary digestion tests The BMP test was conducted using the method of Owen et al. (1979). Duplicate bottles were assayed for BMP using a sample concentration of 2 g VS/1. Each assay was accompanied with ~¢-cellulose pow- der and blank controls containing only inoculated

Table 1. Components of MFW used in BMP test and two- phase anaerobic digestion

Moisture Composition (%) content (%) (based on dry material)

Boiled rice 65 Cooked meat 47 Fried egg 78 Fresh cabbage 95 Bean sprouts 80 Spinach 92 MFW 74

73-0 6-4 9.0 1.3 8-0 2.3

100.0

medium. Inoculum was prepared with anaerobic- digestion sludge from Taejon sanitary treatment plant, whose amount was 20% (v/v) of the total inoculated medium.

A preliminary digestion test at a higher sample loading (4, 10 and 50 g VS/1) was conducted using the same method for the BMP test but with a different initial sample concentration.

Two-phase anaerobic digestion A schematic diagram of the two-phase anaerobic digestion system is shown in Fig. 1, in which the solid-bed reactor for acid fermentation is connected to the methane fermenter (upflow blanket filter) in series. Both reactors were made of acrylic columns. The solid-bed reactor was installed with a distributor at the top to uniformly sprinkle the effluent from the methane reactor. Wire mesh with 1-mm open- ings supported by an acrylic perforated plate (3-mm holes) was installed at 5 cm from the bottom to separate large particles from liquefied leachate.

The upper half of the methane reactor was packed with support media (polyethylene biolette, Sunkyung Engineering and Construction Ltd, Seoul, Korea), and the remaining lower part of the column was left empty so that it could serve as a sludge bed (Choi et al., 1989; Guiot et al., 1985). Both reactors were operated in a constant temperature chamber maintained at 37°C. The methane reactor was inoculated with anaerobic digestion sludge from Taejon sanitary treatment plant.

MFW was initially loaded in a solid-bed at 20% TS and digested in a batch mode. The leachate produced in the solid-bed was introduced at the bottom of the methane reactor by a tubing pump (Master- flex, 7554-60, Cole-Parmer, USA). The effluent overflowing from the methane reactor was recirculated to the solid-bed reactor to provide buffering capacity.

Experiments were performed in three modes, as shown in Table 2, to examine the performances of the reactor system.

Gas Mater

Packing Port Material . ,

Solid-bed UBF

1 Gas Meter

Fig. 1. Schematic diagram of the two-phase anaerobic digestion system.

BMP and anaerobic digestion of food wastes

Table 2. Operating methods of two-phase anaerobic digestion

247

Reactor system Loading rate to methane fermenter

Exp. 1:5 I solid-bed + 1 I methane fermenter Stepwise increase Exp. 2:5 I solid-bed + 8 1 methane fermenter Stepwise increase Exp. 3:5 I solid-bed + 8 1 methane fermenter Constant (7-9 kg COD/m 3 d)

Analytical procedures Flow rate, pH and temperature were monitored and gas volume was measured continuously using a wet C H O N VS (% TS) gas meter (Shinagawa, W-NK-0.5B, Japan). The composition of the gas produced was analyzed by Boiled rice 40-9 7-5 50-0 1.6 99 gas chromatography (GC, HP-5890A, USA) using a Cooked meat 53.4 8.4 27-7 10.5 97 porapaq Q column (6 ft, 80/100 mesh) and a TCD Fresh cabbage 39.9 5-4 50.6 4.1 84

MFW 51.2 7.8 37.8 3.2 95 detector. The individual volatile fatty acid was deter-

Table 3. Elemental compositions and VS contents of samples

mined on a GC fitted with FID using a FFAP capillary column (30 m x 0.2 mm x 0"33 m; Hewlett Packard, USA) and quantified using iso-butanol as ~ s00 the internal standard. The pH of the samples was ~ 450 measured by a pH meter (Perkin-Elmer, 28C, USA) '~

400 and the pH in the reactors was monitored using a pH controller (Cole-Parmer, 5997-30, USA). COD ~ 3so

(soluble COD), TS and VS of samples were deter- mined by standard methods (ADHA, AWWA & WPCF, 1985). Elemental composition of the samples was analyzed by elemental analyzer (Perkin- Elmer, 240C, USA).

RESULTS AND DISCUSSION

BMP and preliminary digestion tests Table 3 shows the VS content and the elemental composition of the MFW and its components. The C/N ratio of MFW is estimated to be similar to that of typical Korean food wastes (Koo et al., 1992). Thus, the MFW used in this study represents the typical Korean food wastes. All components except for the fresh cabbage showed a high VS content.

The stoichiometric methane yields calculated from the elemental composition data shown in Table 3 were 408, 587, 382, 546, 374 ml/gVS for boiled rice, cooked meat, fresh cabbage, MFW and cellulose, respectively.

A temporal plot of the cumulative methane yields, corrected for their inoculum methane yields, of the MFW and its several components, along with the cellulose control, is shown in Fig. 2. The data obtained in this study show, on the whole, high values in comparison with the previous methane yields data summarized in Table 4. As pointed out by Chynoweth et al. (1993), the highest BMP values obtained for wastes and vegetable oil might be attributed to the high H/C ratio of these fat-rich samples and their high degree of biodegradability. Meanwhile, when these BMP results are compared with the stoichiometric methane yields, the anaerobic biodegradability of each sample is calculated to be 0.72, 0.82, 0.73, 0.86 and 0-95, respectively.

300

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Fig. 2. Cumulative methane production of various samples.

Accordingly, the anaerobic digestion of MFW having high methane potential and biodegradability is estimated to be an attractive approach for the reduction of its volume with methane recovery.

Figure 3 shows the BMP test data according to the initial loading of MFW. The cumulative methane production showed all around the 470-475 ml/gVS added; however, the rates of methane production differ significantly according to the organic loading. This means that the methane fermentation is inhibited significantly owing to the build-up of VFAs at a higher initial loading (a lower inoculum to feed, I/F, ratio). With 10 g VS/I, the inhibition of methane production is due to the low pH resulting from the VFAs produced rapidly in the initial stage of the digestion. However, as time elapsed, the methane formers became acclimated by the buffer capacity compensation, due to the degradation of protein contained in the substrate. Ammonia is an end product when protein is degraded anaerobically, and, therefore, ammonia buffers volatile acids by

248 J. K. Cho, S. C. Park, H. N.. Chang

Table 4. Comparison of ultimate methane yields

Sample Ultimate methane yield Reference (m 3 kg VS added)

Boiled rice 0.294 This study Cooked meat 0.482 Fresh cabbage 0.277 Cellulose 0.356 MFW 0.472

MSW 0.186-0.222 Yard waste 0.134-0.209 Paper 0.084-0.349 Food packaging 0.318-0.349

Sorghum cultivars 0.28-0.4

Ligno-cellulosics 0.02-0.27 Fresh water aquatics 0.07-0.43 Forage and grasses 0-07-0.41 Root and tuber 0.19-0-43 Marine 0.08-0.38 Crop residue 0.08-0.53

RDF 0.26-0.3

Vegetable oil 0.94 Primary sludge 0.59 Food waste 0.54 Eucalyptus 0.014 Pine 0.059 Bamboo 0.016 Avicel cellulose 0.37

Owen & Chynoweth (1992)

Jerger & Chynoweth (1987)

Shiralipour & Smith (1984)

Chen et al. (1990)

Chynoweth et al. (1993)

.~. 5OO

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m 400

350

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100

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Time(days)

90

8O

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Fig. 3. Cumulative methane production from MFW and methane content of produced gas with the initial loading. Filled symbol, cumulative methane yield; hollow symbol,

methane content.

maintaining a high level of bicarbonate (Georgacakis et al., 1982). Chynoweth (1993) tried to increase the I/F ratio in order to obtain the maximum rate of methane production by avoiding imbalances resulting from the presence of higher concentrations of VFAs in the assays with the lowest I/F ratio.

The effects of organic loading and initial production of VFAs were further compared between 10

and 50 g VS/1 loading by continuous monitoring of the pH and total (C2-C6) VFA change. With 10 g VS/1 loading, as shown in Fig. 4, pH declined in early stage, and then recovered gradually with the initiation of methane production. On the contrary, with 50 g VS/1 loading, the early decline of pH did not recover, owing to the insufficient buffer capacity. As a result, the methane production did not start in 50 days. The acid-consuming methanogen species are more seriously inhibited by the declining pH than the acid-producing species, resulting in an irre- versible acidification of digester liquor and shut-down of the digestion process, as pointed out by Powell and Archer (1989). In consequence, at 50 g VS/1 loading, the acid production, as well as the methane production, could not continue because of the excessive acidification of the reactor. The VFA concentration increased up to 20 g/l in 3 days, and then remained steady during 50 days of incubation. This saturation of VFA concentration at 20 g/1 was also reported by some researchers (De Baere, 1985; Ghosh, 1984). At this condition, VFAs were produced or consumed no longer without buffer addition or removal of VFAS.

However, buffer addition is not practicable and needs careful control of the quantities applied, since cation toxicity is also caused by excessive addition (George et al., 1989). Therefore, in order to digest the MFW properly, which contains such a high concentration of soluble organics, VFAs produced should be fed to a separate reactor which maintains

25, 10 20

b'l 20 15 ~ * T I - ~. T --';- ";" 86 i

,~ 10 4

0 10 20 ,:30 40 T i m e ( d a y s )

Fig. 4. The variation in pH and total VFA concentration with initial loading in a single reactor. Filled symbol, 50 g

VS/I; hollow symbol, 10 g VS/I.

2

' 0 50 60

good conditions of methane fermentation using a two-phase anaerobic digestion system, and the effluent of the methane reactor recirculated to provide buffer capacity in the acidification reactor.

Two-phase anaerobic digestion Experiment 1: exploratory study As described previously in this paper, the two-phase anaerobic digestion system consisted of a 5 1 (working volume: 41) solid bed for hydrolysis and acidogenesis and a 11 (working volume: 0.741) methane reactor (upflow blanket filter). The solid- bed was filled with the MFW having C/N ratio 16: 1. The leachate percolated from the solid bed was introduced to the methane reactor at 10 ml/day and the flow rate was increased to 700 ml/day by day 120 (the final day of experiment 1).

The methane reactor inoculated with the digester sludge of Taejon wastewater treatment plant, and acclimated with the leachate of MFW for 15 days, was supplied with the acid-rich leachate from the solid bed. Leachate was introduced into the methane reactor twice a day using a peristaltic pump, since it was impossible to supply such a small volume of leachate continuously during 24 hours. Right after the introduction of leachate the methane content of the produced gas decreased temporarily below 30% (v/v), due to the carbon dioxide dissolved in the leachate. The organic loading rate to the methane reactor was increased by stepwise increments of the leachate flow rate, according to the COD concentration of leachate.

Figure 5 shows the change of pH, COD and VFA concentrations of leachate from the solid bed for the whole period of the experiments. The COD and VFA concentration data dearly indicated that MFW liquefaction and acidification occurred early and immediately after digestion started with this two-

COD

"•m15

10

20 40 60 80 100 120

BMP and anaerobic digestion of food wastes 249

Time(days)

10

8 6O

u 4

20 2

J0

Fig. 5. VFAs, COD and pH of leachate produced from solid bed (exp. 1). A, Acetic; P, propionic; B, butyric; V,

valeric; C, caproic acid; TVFA, total volatile fatty acid.

phase system. The steady value of the data to around day 60 indicates the occurrence of inhibition; which indicates that acetate, propionate, and butyrate concentrations higher than 8, 6'5 and 3.5 g/ 1, respectively, could be inhibitory to acidogenic conversion (Ghosh, 1990). Conversely, the decline of COD and VFA concentrations with continued operation suggested that certain easily-biodegradable substrate fractions of MFW were exhausted after day 60. A clear phase separation was obtained until day 80, when the total VFA levels exceeded 6 g/l. Chanakya et al. (1993b) suggest that p H < 5 and a VFA level>6 g/1 need to be maintained in the recycled liquid of acidogenic digester for ideal phase separation.

The examination of individual VFA concentrations gives an interesting point. It shows that even-numbered acids are abundant in comparison with odd-numbered acids. Also, it can be seen that the acetic acid concentration is predominantly higher than those of others up until day 70, however, and thereafter the concentration of butyric acid increases with time and remains relatively constant during the final stage of experiments because the feed was mainly composed of easily-degradable starch. According to the experimental results by Noike et al. (1985), the major volatile fatty acids produced in anaerobic digestion of starch feed were even-numbered acids, namely acetic acid and butyric acid; butyric acid was the most abundant fermentation product in a low SRT but was not detected in the cellulose fermentation. The direct reasons for rapid decline of COD and VFA concentrations and pH increase are the exhaustion of biodegradable substrates and the dilution of the solid bed by the recirculated liquid from the methane reactor.

Figure 6 shows the organic loading rate (OLR) of the methane reactor in conjunction with the feed flow rate (equal to recirculating flow rate to the solid bed) and HRT. The OLR was increased approximately stepwise by controlling HRT. Meth-


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Fig. 7. Methane production rate, effluent COD and COD removal efficiency in methane reactor (exp. 1).

Fig. 6. Flow rate and organic loading rate in methane reactor (exp. 1).

ane production rate, COD removal efficiency and the effluent COD in the methane fermenter during the experiments are shown in Fig. 7. Methane production rate increased approximately in proportion to the OLR up to 8 g COD/1.d (until day 100) and, hence, methane yield was maintained around 310-360 ml CH4/g COD at all OLRs. The COD removal efficiency was maintained higher than 90% until day 100 and thereafter declined to around 70%. The COD of methane reactor effluent was maintained below 3-4 g/l, at all times except for the period of the highest OLR (after day 100). The pH of the effluent remained high at around 8, due to low C/N ratio-induced ammonia at all times (Kay- hanian et al., 1994). Hence the recirculation of the effluent to the solid bed was believed to provide buffering capacity to the solid bed and, subsequently, enhance acid production during operation (Cecchi et al., 1990; Ghosh, 1985). The abrupt low- ering of COD removal efficiency and simultaneous uprising of the effluent COD concentration at the highest OLRs (12-15 g COD/I.d) are due to the recirculation of the refractory materials remaining in the solid bed. Therefore, it is believed that the maximum possible OLR with the methane reactor used is around 8 g COD/I.d at a HRT around 3.4 days. Accumulated total methane production was approximately 280 1 during 120 days of batch operation with 750 g of the initial 750 g of the initial VS added in the solid bed. In contrast, the TS and VS contents of digested residue were measured to be 90 and 80 g, respectively, at day 120. Overall methane yield with the two-phase anaerobic digestion system is, therefore, 0.42 I/gVS reduced in the reactor. Ninety percent of both TS (800 g) and VS (750 g) initially loaded were reduced by the reactor system in 120 days.

If we consider the BMP of the MFW sample (see above, 0.472 1 CH4/gVS added) as the ultimate methane yield, and comparing the observed methane yield with the BMP test yield, it can be noted that approximately 90% of the ultimate methane yield was obtained from the reduced VS. The loss of methane yield, approximately 10% based upon the VS actually reduced, might be due to the loss of methane produced in the solid bed, since a small volume of methane contained in the solid bed exhaust gas was neglected in the experiments. How- ever, methane produced in the solid bed (after the pH of the leachate in the solid bed exceeded 6) was measured and included in the methane yield calcula- tion.

Experiment 2: degradation test in an 81 methane reactor It is evident that the methane reactor capacity (working volume: 0.74 1) in experiment 1 is small in comparison with the solid bed capacity, and so the COD concentration (or VFAs concentration) in the solid bed remained constant for a long time. In experiment 1, the increase of recirculating flow-rate was actually limited because of the methane reactor capacity. Therefore, in this second experiment the nominal capacity of the methane reactor was increased from 1 to 8 1 (working volume: 7"3 1) to enhance the methane production capacities of the whole system, and consequently accelerate the decomposition of the MFW in the solid bed. The solid bed was charged with 4 kg (800 g TS) of the same MFW utilized in experiment 1. The flow rate of recirculation was gradually increased in a similar way to experiment 1.

The increase of recirculation flow rate, the corresponding HRT, OLRs and methane production rate in the methane reactor of experiment 2 is shown in Fig. 8. With those conditions of operation, the pH, COD and VFA concentration of the solid-bed leachate are varied as shown in Fig. 9. Examinations of


Fig. 8.

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Flow rate, organic loading rate and methane production rate in methane reactor (exp. 2).

20 10

70

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pH 50

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Fig. 9. VFAs, COD and pH of leachate produced from solid bed (exp. 2). A, Acetic; P, propionic; B, butyric; V,

valeric; C, caproic acid; TVFA, total volatile fatty acid.

Fig. 9 show that the period of maximum COD and VFA concentration is less than 10 days, and, subsequently, the period of quasi-complete exhaustion of the substrate is also reduced significantly to within 33 days (about one-third of experiment 1). After 33 days of operation, the TS and VS content of the solid bed residue were measured to be 98 and 83 g, respectively, which amounted to 88% of the TS reduction and 89% of the initial VS conversion to biogas.

Total methane production was 275 1 during 33 days of operation, and hence the overall methane yield based upon the VS reduced is estimated to be 0-41 1 CH4/gVS reduced. The results as a whole showed almost the same aspects, except for the shorter periods of digestion in comparison with experiment 1.

Experiment 3: the effects of recirculation In experiment 3, with the same reactors in series as with experiment 2, the maximum possible OLRs of

20 10 1 7 0

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0 ~ 6 ' 1 I I I - - I 0 0 3 9 12 15 18

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Fig. ]O. VFAs, COD and pH of leachate produced from solid bed (exp. 3). A, Acetic; P, propionic; B, butyric; V,

valcric; C, caproic acid; qA'FA, total volatile fatty acid.

0 i i J t I

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Fig. 11. Flow rate, organic loading rate and methane production rate in methane reactor (exp. 3).

the methane reactor were maintained to examine the effects of increased recirculation in the solid bed. Actually, it was difficult to maintain the OLRs as constant because of the variations of COD and VFA concentration of the effluent from the solid bed. The variations of COD, VFA concentrations and pH in the solid bed are shown in Fig. 10.

The control of HRT and the corresponding flow- rate changes to maintain OLRs at around 8 g COD/1.d are shown in Fig. 11. The OLRs were actually maintained at 7-9 g COD/I.d, except for the starting and the dosing day, by adjusting the recirculation flow-rate according to the COD concentration of the solid-bed leachate. With the recirculation flow-rate at 1.2 l/d initially, the total VFA concentration of leachate remained around 10 g/l. In comparison with the initial VFA concentrations in the previous experiments, which were 15 g/1 (Figs 5


Table 5. Comparison of performance obtained in this process and that in the SEBAC process

Parameter This process SEBAC process

Exp. 1 Exp. 2 Exp. 3 42-day run 21-day run

Fermentation periods (days) 120 33 17 Fermentation temperature (°C) 37 37 37 BMP (m 3 kg/VS) 0-472 0.472 0.472 Loading (kg VS/ma/d) 1.04 1.75 3.39 Methane yield (m3/kg VS added) 0-373 0.367 0.360 Methane rate (vvd) 0.39 0-64 1.22 VS reduction (%) 90.0 89.2 89.8 Conversion eft. (%) 79 78 76

42 21 55 55 0-2 0.2 3.2 6-4 0.19 0.16 0.61 1.02

49.7 36.0 95 80

and 9), the acid concentration of this experiment was lowered significantly. This, of course, is due to the excessive recirculation of digested liquid. Even though the recirculated liquid supplied sufficient buffering capacity for acid production in the solid bed, at the same time it diluted the acid produced from the solid bed too early. In this experiment, the acid production rate is a limiting step when con- sidering the whole system. Conversely, because of the buffering capacity provided by the recirculating fluid, the pH of the leachate increased rapidly around day 11 of the experiments, and consequently methane production took place in the solid bed significantly after the VFA level fell below 5 g/l. Therefore, it appears that strictly-separated acido- genie and methanogenic fermentation cannot be maintained at the final stage (Chanakya et al., 1993b). The degradation rate in this experiment was accelerated by recycling the bed leachate through the methane-phase filter at high rates, and consequently the conversion of the feed was completed in a much shorter duration than in previous experiments (Ghosh, 1984).

At day 17 the TS and VS content of the solid bed residue was reduced to 102 and 78 g, respectively. Hence, 87% of TS reduction and 90% of VS conversion to biogas were achieved within 17 days. Furthermore, 270 1 of methane gas was produced during the period, and the methane yield was recor- ded to be 0.40 1 CHdgVS reduced.

Operation and performance data for three experiments are summarized in comparison with those of the Sequential Batch Anaerobic Composting (SEBAC) process using processed MSW (O'Keefes et al., 1993) in Table 5. As shown in the SEBAC process, the conversion efficiency and the associated methane yield in this process were slightly lower at a higher loading. On the whole, the conversion effi- ciencies (vs BMP values) of 76-79% in these experiments show slightly lower values relative to the SEBAC process. Lower yields could be attributed to the lower fermentation temperature in this process (Ghosh, 1984).

CONCLUSIONS

The following conclusions may be drawn based on the observed results:

(1) The methane yield and anaerobic biodegradability of Korean MFW are high: 472 ml CHJgVS added and 86%, respectively.

(2) To perform solid-state anaerobic digestion of Korean food wastes discharged at 15-30% TS successfully, the VFAs produced rapidly at the initial stage of fermentation need to be con- trolled using a two-phase digestion method.

(3) Clear phase separation was obtained in two- phase anaerobic digestion with a MFW sample when the total VFA levels exceeded 6 g/l (pH <6). After the VFA levels fell below 5-6 g/1 (pH>6) strictly-separated acidogenic and methanogenic fermentation could not be maintained.

(4) In the leach-bed two-phase anaerobic digestion experiment on wastes, the degradation rate depended on recycle flow rate and the methane digester HRT.

ACKNOWLEDGEMENTS

This work was made possible by funding from the Ministry of Trade, Industry and Energy of Korea. The authors are grateful to J. P. Lee for the excellent technical support.

REFERENCES

Anderson, G. K. & Saw, C. B. (1992). Leach-bed two- phase anaerobic digestion of MSW. Proc. Symp. on Anaerobic Digestion of Solid Waste, Venice, 14-17 April 1992, pp. 171-9.

Cecchi, F., Marcomini, A., Pavan, P., Fazzini, G. & Meta- Alvarez, J. (1990). Anaerobic digestion of municipal solid waste. Biocycle, 31, 42-3.

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biochemical methane potential and solid state anaerobic digestion of korean food wastes

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