Bioresource Technology 46 ( 1993) 227- 231

SOLID-PHASE BIOGAS PRODUCTION WITH GARBAGE OR WATER HYACINTH

H.N. Chanakya, Sushama Borgaonkar, G. Meena & K.S. Jagadish

('entre for the Application of Science and Technology to Rural Areas (ASTRA), Indian Institute of Science, Bangalore - 560 012, India.

(Received 18 December 1992; revised version received 25 March 1993; accepted 6th April 1993)

Abstract Fermentations of market garbage and water hyacinth in laboratory-scale solid-phase fermenters operated in fed batch mode is reported. Solid-phase fermentation was effected by a daily sprinkling of a weekly-fed biomass bed with an aqueous suspension of biodegradative bacteria to initiate and sustain high levels of biogas pro- duction. Gas production rates greater than 0"5 fitres/fitre per day at specific gas yields of 250-300 fitres/kg total solids at residence times" between 150 and 250 days were obtained. Major methanogenic activity appeared to occur in the lower parts of the decomposing bed, there- fore feeding couM be carried out on a once-weekly basis by opening the top of the fermenter and adding the untreated biomass feed, without deleterious effects on the overall gas yields or composition. The compaction of biomass feeds during decomposition permitted the use of high residence times without loss of space economy.

Keywords: Solid-state biogas digestor, urban garbage, water hyacinth, volatile fatty acids.

INTRODUCTION

The floating and stratification problems encoun- tered during biogasification of biomass feeds in con- ventional slurry-based fermenters have necessitated identifying alternative fermenter designs and concepts (Van Braekel, 1980; Hobson et al., 1981; Molnar & Bartha, 1988; Richards et aL, 1991; Lingappa & Lingappa, 1985). Solid-phase fermentation involving liquid recycle with or without a separate methaniser stage avoids most of the common problems (Rijkens, 1981; Colleran et al., 1982; Ghosh 1990; Weiland et al., 1990; Chanakya et al., 1992) and most of these utilise a batch operation• Simple ways of feeding solid biomass and spent feed removal, as well as design simplification for adaptation to small-scale use in India are yet to be investigated. Reducing the frequency and

Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

227

quantity of liquid recycled encourages biogas produc- tion from the decomposing biomass bed and therefore obviates the need for a separate methanisation stage (Anand et al., 1991). An attempt was made in this study to understand the influence of such a method of operation on process stability and potential for scale up to small-scale applications in India. Fresh and dry water hyacinth and urban garbage were fermented in separate digesters without a drastic pretreatment step.

METHODS

Digester design and operation Three laboratory-scale (2-1itre) solid-phase

fermenters fabricated using 100 mm diameter PVC pipes were set up according to Fig. 1 and operated as follows. Electrically operated bellows pumps were used to pump the fermenter liquid (L) from trans- parent liquid-phase reservoirs (0" 5 litre) and sprinkle it over the biomass bed in the fermenters (0-5 litre, twice daily)• The pumps were operated for 20 min twice daily• Following the sprinkling operation, the sprinkled liquid leached through the bed and quickly collected back into L (about 60 min). This leachate was recycled unchanged throughout the experimental period. The gas produced was collected by downward displace- ment of water in calibrated glass cylinders and recorded daily. Sampling ports (So; or SL) were pro- vided to determine the gas composition in the fermenter as well as volatile fatty acid (VFA) content of the liquid phase collected in L. The fermenters were operated at room temperature with a diurnal change in temperature in the range of 21-27°C during the study period (September 1990 to July 1991 ).

Substrates Intact water hyacinth tops (petioles and leaves) and

urban market garbage (Table 1) were used as feed. Water hyacinth was collected from an infested tank nearby, chopped to about 20 mm pieces, and fed to the fresh water hyacinth fermenter. Oven dried (80°C) material was used as dry water hyacinth. Partly dried

228 H.N. Chanakya et al.

INLET PORT

Fig. 1. Laboratory-scale solid phase fermenters used for fermentation of leaf biomass substrates, f, stainless steel filter mesh; S. or SL, sampling port for gas or liquid; v, valve; L, leachate/hquld phase used for spnnkhng.

garbage, collected while it was being dumped at dump sites around Bangalore city, was shredded and dried prior to use. With all the feeds chopping or shredding was carried out only to facilitate feeding through the 20 mm inlet used and it is expected that in larger fermenters even this operation may be avoided.

Feed rates, method of feeding and inoculum The fermenters were started gradually. Spent material (50 g) from an earher experiment was fed initially and 1 g total solids (TS)/week was fed for 2 weeks prior to the start. The fermenters were fed 8 g TS on day 1 and subsequently (from day 21) once weekly with 16 g TS or equivalent (Table 2). Feeding was carried out by pushing the feed through the inlet port and stoppering immediately after the completion of the feeding operation. O2-free N 2 was flushed upwards through the biomass bed during the feeding operation for the first 100 days but subsequently N 2 flushing was abandoned. Feeding was carried out for a period of 278 days without spent material removal.

Effluent slurry from a cattle dung biogas plant was diluted with anoxic water (1:4), stirred well, allowed to

Table 1. Composition and characteristics of feed before and after fermentation

Parameters studied Garbage Fresh water hyacinth Dry water hyacinth

Before After Before After Before After

Total solids (TS) (%) 100 30.86 9.4 18.775 Bulk density

Fresh (g/cm 3) -- 0"76 0" 15 0'86 Dry (g/cm 3) 0-19 0.22 0"01 0' 128

Composition (% TS) Water soluble 30" 10 15"39 21.68 21"77 Hemicellulose 22.17 22.78 33-97 33'53

and proteins Cellulose 14"72 17"60 18"00 11.96 Lignin 33"01 44"23 26-36 32.75

Volatile solids (% TS) 67.22 63'86 83"65 72.23

100 14"25

-- 0"89 0"049 0"13

21"68 19"69 33"97 ND"

18"00 23"03 26"36 ND 83"65 68"69

"ND, not determined.

Table 2. Performance characteristics of solid-phase fermenters on different feeds for a 300 day operation

Parameters studied Garbage Fresh water hyacinth Dry water hyacinth

Total feed (g flesh basis) Total feed (g dry basis) Average feed rate (g TS/litre per day) Average feed rate (g VS/litre per day) Total gas produced (litres) Specific gas production (litres/g TS) Specific gas (litres/g VS added) Average daily gas (total gas/300 days; litres/day) Average gas production rate (average daily gas/

litre digester) Gas production rate (pseudo-steady state,

150-200 days (litres/litre per day))

600 7500 600 600 705 600

1"0 1"175 1-0 0"67 0"98 0"84

161-4 205"1 147"2 0"269 0"291 0"245 0"40 0'348 0"292 0"54 0"68 0"49 0"27 0'34 0"25

0"46 0"47 0"41

Solid-phase anaerobic digestion 229

settle for about 4 h, decanted and filtered through cloth. This inoculum was maintained at 500 ml (L) throughout the study period and any volume lost was replaced with anoxic water.

Physicochemical analyses The composition of the fermenter liquid and gas

were analysed in a gas chromatograph and the feed composition by sequential acid hydrolysis (Chesson, 1978). Other methods adopted conformed to APHA, 1975.

RESULTS AND DISCUSSION

The results of the fermentation monitored for a period of 300 days are presented below.

Changes in feed characteristics The composition and characteristics of the feeds

before and after fermentation are presented in Table 1. The high content of fermentable matter and the low lignin content of water hyacinth suggests a potential for achieving a rapid and high level of biodegradation. The market garbage had a high mineral content (ash) and lignin, suggesting low gas potential. The bulk densities of the substrates fed were low. Such low bulk densities could result in the occurrence of large voids with poor packing and low feed rates. The bulk densities of the spent material were, however, quite high. The fermentation of feed to biogas led to compaction and the spent material thus became easy to handle and transport with little or no dewatering needed. The compaction within the fermenter could also obviate the need for any pretreatment. Table 1 shows that there was an increase in lignin and ash while only marginal changes occurred for the hemicellulose and protein and cellulose fractions.

pH and VFA levels of liquid phase The VFA levels in the fermenter liquids showed a

few peaks in the range of 2-4 g/litre which did not affect the pH in the fermenters with the different sub- strates (Fig. 2). The digester contents thus appeared adequately buffered with alkalinity in all the fermenters in the range of 3-5 g/litre; this did not change signifi- cantly with time.

The total VFA levels were normally low ( < 1 g/litre) indicating that with all the substrates acidogenesis was rate limiting. This is in contrast to the decomposition of leaf biomass where there is generally a problem of VFA accumulation leading to suppression of methano- genesis (ASTRA, 1993, unpublished data). These low acidogenic rates indicate fewer chances of digester 'souring' with these substrates.

The compositions of the VFA in fermenter liquid are presented in Fig. 2. The predominance of acetic and propionic acids suggests that the fermentation of garbage was normal. Peak VFA levels noticed in Fig. 2

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O 50 100 150 200 250 300 Days of operation

Fig. 2. Levels of dominant volatile fatty acid recorded in the liquid phase of the fermenters: - - , acetic; . . . . , prop- ionic; ..... , n-butyric; , total VFA (expressed as g acetic equivalent).

were caused by the accumulation of acetic and propionic acids to a large extent and not due to the branched chain butyric or valeric acids implicated in fermenter failure (Hill & Bolte, 1989). Higher VFA were generally found at concentrations less than 0" lg/ litre during most of the study period. However, transient peaks (1-2 weeks) of branched-chain VFA were always accompanied by peaks of acetic and propionic acids in fresh and dry water hyacinth without seriously affecting gas production (Fig. 3). Such accumulation of VFA is normally implicated in the failure of the hydrogen utilisers and syntropic species (Hill & Bolte, 1989; Beatty & McInerny, 1990; Yang & Tang, 1991; Barredo & Evison, 1991). These transient conditions could not be attributed to any environ- mental or operating conditions and, therefore, were likely to have been caused by alterations in packing conditions caused by the weekly feeding.

Gas production pattern The daily gas production rates obtained for the three

feeds are presented in Fig. 3 and Table 2. The daily gas production in all three digesters reached threshold values of 1 litre/day at 150 days for garbage and dry water hyacinth and 200 days for fresh water hyacinth following which gas productions gradually fell. It was also seen that excessive amounts of feed within the fermenter impeded the ideal flow of sprinkled liquid

230 H.N. Chanakya et al.

through the bed and subsequently leaching of VFA from the bed was not likely to be uniform. From the gas production pattern in Fig. 3, maximum solids retention time (SRT) appears to be between 150 and 200 days for the different feeds corresponding to 125-160 g TS feed per litre retained in the fermenters.

Conversion efficiency and space economy The performance characteristics are summarized in

Table 2. Specific gas yields of 0.27, 0-29 and 0.25 litre/g TS were obtained at 0.27, 0-34 and 0.25 litres gas per litre digester per day for the garbage and fresh and dry water hyacinth feeds, respectively. Gas yields greater than 0"5 litres/litre per day were obtained between 150 and 200 days of operation for all feeds (pseudo-steady-state) and compare well with results from conventional slurry-based fermenters in India (Rajabapaiah et al., 1979; Srivastava et al., 1984; Chanakya, 1985).

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0 2 I i ¢ I I I I I I I I 0 5 50 5 100 125 150 175 200 225 250 275 300

Days of operation

Fig. 3. The average daily biogas production in the fermenters.

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40

20 I I I I

c u 40 u

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2O

F R E S H WH

I I I ~ i

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Fig. 4. Composition of the fermenter gas sampled from the space above the decomposing biomass, o, carbon dioxide; o, methane).

Gas composition The gas composition monitored after stopping of N 2

flushing during feeding (100 day) is presented in Fig. 4. In all the fermenters there was a gradual increase in both methane and carbon dioxide. Air entering the digester during feeding operation as well as that trapped in voids within the feed did not appear to encourage facultative anaerobes leading to increased carbon dioxide content and low methanogenic activity. After about 150 days of operation corresponding to elevated bed heights and very little air introduced while feeding, there was hardly any suppression of methane content. The changes in gas composition monitored on a daily basis after 300 days of operation indicated that combustibility of the gas was likely to be low on the day of feeding (25-35% CH4), but it recovered quickly to normal. This further suggests that these fermenters must always be fed from the top leaving very little headspace so that very little air is permitted to enter the digester during feeding. Under these operating condi- tions a once weekly feeding appeared satisfactory. Fermentation of Jacaranda leaf biomass under similar conditions revealed that over 92% of the gas collected was from the biomass bed and very little gas was pro- duced from the liquid phase L in spite of favourable VFA levels (ASTRA, 1993, unpublished data). Biogas production observed from the biomass bed even in the presence of air accidentally introduced while feeding suggested that methanogenic activity occurred mostly in deeper layers of the decomposing biomass bed.

Optimum sprinkling rates Reduced sprinkling rate and frequency encouraged

biogas production from the bed itself by preventing VFA removal from bed before being converted to biogas (Anand et aL, 1991). However, VFA values in excess of 6-8 g/litre inhibited methanogenesis in the bed (Chanakya et aL, 1992). At a gas production rate of 0"7 litres/litre per day, a maximum of 1 g VFA needs to be carried down to methanogenic sites for conversion to biogas (optimum, 166 htres/m3). Alternatively, optimum sprinkling rate could also be determined from the plot of sprinkling rate (ml/g TS) against the daily gas production (ml gas/g TS held, Fig. 5). For all the feeds the optimum ranged between 2 and 5 ml/g TS held in the fermenter. These two methods of calcula- tion give very similar results for 125 g TS/litre digester. Stopping the daily sprinkling reduced gas production to 30-45%, but this regained the original production within 4-7 days after resuming sprinkling.

These results indicate that reasonable gas produc- tion may be obtained by the above technique. By adopting a simple weekly feeding technique as above and removing spent material from below a semi- continuous operation is possible without complicated fermenter design. By substituting the present sprinkling method by a manual device, scaling-up for small rural applications is feasible.

Solid-phase anaerobic digestion 231

I ~ Daily gas |--spray ML~ 1"s /~ / k - -

O 4

"-2

~00

0 uu~ I "-"

i • Daily gas 0.6, I Sproy MLIg TS _ I00

- ~ ~

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0.4

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Fig. 5. Relation between daily gas production and sprinkling rates. The broken line indicates the visually plotted best fit.

ACKNOWLEDGEMENTS

The authors wish to acknowledge (the late) Pro- fessor K.S. Gopalakrishna for his inspiration, the Ministry of Non-conventional Energy Sources for financial support and Dr Nishath Kowser, ASTRA, for chemical analyses.

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