ELSEVIER PII:SO960-8524(98)00009-l

Biorvsourcr Technology 65 (1098) 97-103 0 1998 Elsevier Science Ltd. All rights reserved

Printed in &eat Britain 0960-X524/98 $19.00

TWO-PHASE ANAEROBIC DIGESTION OF SOLID WASTES BY A MULTIPLE LIQUEFACTION REACTORS PROCESS

J. Raynal, J. P. Delgenks & R. Moletta

Lahoratoire de Biotechnologie de I’Environnement, Institut National de la Recherche Agronomique, Avenue des Etangs, 11100 Narhonne, France

(Received 14 Augu:it 1997; revised version received 8 December 1997; accepted 20 December 1997)

Abstract A new solid-waste treatment process was applied to different vegetable substrates: potato peelings, green salad leaves, green peas mixed with carrots, apple pomace. It involved, at a laboratory scale, several liquefaction digesters, each of them treating one fype of waste, coupled to a central methane @ed-film reactor: The influence of pH, load and hydraulic retention time on the process performances was studied at 35°C. On average, except for apple pomace, hydrolysis yields were high (up to 80%) during the liquefaction step. Likewise, the mixture of the acidogenic eff7uents was degraded in a methanation reactor to an extent of 80%. In a final run with average values near 4 g COD 1.. ’ day-’ for loading rate, and 17days for hydraulic retention time, overall organic matter removal reached 87%. 0 1998 Elsevier Science Ltd. All rights reserved

Key words: solid vegetable wastes, microbial ecosys- tems, liquefaction, acidification, methane.

INTRODUCTION

As a consequence of our way of life, the problem of wastes produced in developed countries is increasing quickly. As an illustration, the whole production of wastes in France has been evaluated at 580 million- tons per year. It includes industrial wastes

(150 million), domestic wastes (30 million), and agricultural wastes (400 million). Inert and hazard- eous wastes from industrial activities follow reuse or complete elimination pathways. Wastes from agriculture and vegetable food production are usually recycled through animal feeding, energy production or agricultural fertilizer. Nevertheless, large amounts coming from urban markets or gener- ated by the variations in the production (surplus products, seasonal activity) are, at the moment, carried to landfill sites or disposed of into waste-pits

97

with or without burying, generating smell and water pollution. From the year 2002, European regulation will reserve landfill sites for final wastes. If energy revalorization by combustion could take in a great part of the organic fraction of solid wastes, biological processes could also constitute an alterna- tive way of valorization: especially anaerobic diges- tion which leads, after polymer hydrolysis, to volatile fatty acids (VFA) during a liquefaction phase. Earlier reports show that a valorization of the organic fraction of solid wastes can be achieved as VFA production at useful1 conversion rates (Brummeler et al., 1991; Viturtia et al., 1992; D’Addario et al., 1993). This carbon source can be used in wastewaters treatment plants for methane production or to improve denitrification and dephos- phatation processes (Toerien et al., 1990; Isaacs and Henze, 1995). However, the diversity of organic solid wastes, related to their origin, composition and production period, needs the planning of a specific treatment for every kind of waste. So initially, the liquefaction stage of each substrate could be performed in separate digesters in order to get the best liquefaction yield and VFA conversion by a specialized microbial ecosystem. Following this, the effluent mixture from liquefaction digesters could be directed towards a denitrification, dephosphatation or methanization plant.

The aim of this work was to determine the feasability and performances of a two-stage fermen- tation process applied to the degradation of vegetable solid wastes including multiple liquefac- tion reactors and a central methanizer.

METHODS

Vegetable substrates Four types of agro-industrial wastes were used. They were from the potato processing industry (peeling wastes), a salads-producing factory (salad leaves), a vegetable cannery (green peas and carrots mixture)

98 J. Raynal et al.

Table 1. Substrate characteristics

Wastes Potato peelings Salad leaves Peas-carrots Apple pomace

Total solids (g kg-‘) 119.2 79.4 179.4 384.0 Volatile solids (g kp-‘) 105.5 72.1 171.0 365.0 Total COD (g kg- ) 126 57.8 185 370 Particulate COD (g kg- ‘) 80.6 39.3 123.9 244,2 Total suspended matter (g kg-‘) 80 39 145 305 Cellulose (g kg-‘) 12.9 13.5 16.1 ND

and apple-juice production (apple pomace). Their characteristics are given in Table 1.

Start-up of the fermentation All the hydrolytic reactors were inoculated with an anaerobic ecosystem obtained from a methanation pond treating winery effluents. First, reactors were fed with a synthetic sugar medium, at a loading rate of 05 g COD 1-l day-‘, for 5 days at 35°C. The load was then increased to 1 g COD 1-i day-’ by the addition of solid waste, and it remained at this level for 5 days before the start of the experimental step named Run 1.

The methanation reactor was first fed with a synthetic mixture of acetic, propionic, and butyric acids, at 1 g COD 1-l day-’ for 1 month. Then the reactor was fed with the mixture of the liquid phases from the first-stage reactors.

Operation timing The whole experiment was carried out over 15 months in three runs:

Run 1: start up and pH adjustment. Run 2: first load increase. Run 3: second load increase.

Experimental set-up and operation The process included several satellite acidogenic reactors, each of them treating one type of waste, coupled to a central methanation reactor.

The hydrolysis-acidification step was carried out in 2.5-l glass reactors, with pH and temperature regulation. The temperature was controlled at 35°C by circulation of thermostated water through a jacket. The pH was monitored by automatic addition of 5N NaOH. The reactors were stirred by an inox propeller and operated according to the sequential batch reactor mode (daily SBR cycle: 20 h of mixing, 4 h of decantation) in order to avoid loss of micro- bial sludge. Every 2 days a volume of supernatant digested medium was pumped off and immediatly replaced by an equal feeding volume of crude solid waste, except for the apple pomace digester where decantation was not effective, and so stirring was continuous. So, for this digester only, the output pumped off was the mixed content. Reactors were loaded by hand through a funnel because of the pasty consistency of the substrates. For all the digesters, the output was filtered using a O-5-mm

iron sieve during the first experiment only (Run 1). During both of the following experiments (Runs 2 and 3) the sieve became rapidly clogged, filtering was therefore replaced by centrifugation. In all cases, filtered or centrifuged solid residue was recycled into the acidogenic reactor but was not counted as part of the loading rate. Then liquid phases were mixed together to feed the methanation reactor.

The methane fermentation step was performed at 35°C in a 10-l fixed film reactor packed with plastic media in the form of corrugated rings (Bories et al., 1982). This type of methane reactor is now widely used in the food and agricultural industries (Raynal and Bories, 1987; Bories et al., 1988). The reactor operated in the upflow mode with a 300 ml/h contin- uous recirculation flow. Six times per day, feed medium was injected into the recirculation tubing by a timer peristaltic pump. Output was kept to dilute input substrates of acidogenic reactors.

Analyses Total solids (TS), total volatile solids (TVS), total suspended solids (TSS), pH, and chemical oxygen demand (COD) were determined according to the Standard Methods (American Public Health Associ- ation, 1992). The particulate COD (pCOD) was COD provided by TSS and equal to the difference between crude sample COD or total COD (tCOD) and centrifuged sample COD or dissolved COD (dCOD).

Total organic carbon (TOC) was measured by catalytic oxidation on a TOC analyser.

Volatile fatty acids (VFA) were identified by semi-capillary-gas chromatography (Chrompack CP 9000) with a FID detection.

Biogas was measured on the methanizer only with an accumulating volumeter, and analysed by gas chromatography (Shimadzu GC8A) with a catha- rometer detector.

Analyses were performed at least once a week.

Fermentation parameters Hydrolysis (or liquefaction) rate was evaluated from the particulate COD removal measured in the digester versus time and expressed in g pCOD 1-l day-‘.

Hydrolysis yield (%) equalled the difference between the input particulate COD and the

Two-phase anaerobic digestion of solid wastes 99

remaining particulate COD in the digester, over the input particulate COD ( x 100).

Acidification yield (%) was calculated from the total VFA (expressed in mg COD 1-l day-‘) in the digester and total COD of crude input.

Pollutant removal yield (%) was measured from the total COD removal. It equalled the degradation of total organic matter (expressed in mg COD 1-l day-‘) by the whole process during a steady-state period.

RESULTS

Hydrolysis and acidification stage The first experiment (Run 1) was performed to study the influence of pH on hydrolysis yield at a constant loading rate of 4 g COD 1-l day-‘, obtained by substrate dilution. At the first tested pH value of 65 hydrolysis was not favoured in the four hydrolytic digesters because of the poor inhibition of methane fermentation: volatile fatty acids were rapidly degraded and the pH increased. Then, the inhibition of methanogenesis was achieved by lowering the pH to 5.50. ‘The aim of the second experiment (Run 2) was to check the effect of a first load increase on performances of the digesters, keeping the same pH and retention time, by reducing substrate dilution. The next experiment (Run 3) consisted in increasing the load once more by further reducing substrate dilution. But the main problem was the difficulty in mixing and pumping the digest medium, above all for potatoes and salad wastes. So it was necessary to dilute both substrates

with the methanized effluent and to decrease reten- tion time simultaneously. For the green peas and carrots mixture, pumping remained possible after increasing concentration by reducing substrate dilution. So, for this last sample only, retention time modification was not necessary.

Experimental data obtained at steady-state condi- tions are presented in Table 2.

Potato peelings substrate In the digester treating the potato peelings, Run 1 started at pH 5.50 and at a load of 4 g COD 1-l day-‘. Methane fermentation was inhibited and volatile fatty acids accumulated to a maximum value of 11.47 g I- ‘. Acetate was the most important one (about 40%). However, biogas composition indicated 23% of methane and more than 75% of carbon dioxide as mean values, which showed a low methanogenic activity. Hydrolysis rate was 1.62 g pCOD I-’ day-’ and hydrolysis yield reached 77.7% of the input total COD (Table 2). During Run 2, enhancing the loading rate by 60% (4-6.4 g COD l- ’ day-‘) generated an increase in VFA concentration of 79% (10.95-19.57 g I-‘), in hydrolysis yield of 13% (77.7-87.7%) and in acidifi- cation rate of 11% (40-44.4%). Hydrolysis rate reached 3.5 g pCOD I-’ day-‘. A large part (38.1%) of the COD was removed (Table 2). The next step (Run 3) with doubled loading rate (8.7 instead of 4 g COD for Run 1) and reduced retention time, allowed production of a high concentration of VFA (21 g I- ‘) with a stable hydrolysis yield which remained about 85%, but with an increasing hydro- lysis rate which reached 4.49 g pCOD I-_ ’ d -~ ‘.

Table 2. Average values of potato peelings, salad leaves, green peas-carrots, apple pomace, liquefaction stage

Apple Potato peelings Salad waste Green peas-carrots p0mace

Run 1 Run 2 Run 3 Run 1 Run 2 Run 3 Run 1 Run 2 Run 3 Run 1

Substrate dilution I:3 Retention time (days) 10.4 Loading rate (g total 4

CODI ‘day ‘) PH 5.5 Separation Decantation

and sieve

I:2 1:2 1:2 IO.4 7.7 7 6.4 X.7 4

2:3

5.7

2:3 15 I:3 I:2 I:10 4% 9 9 9 9 X.6 4 7.2 10 4

550 Centrifuge

5.5 Centrifuge

5.50 Decantation

and sieve

5w Centrifuge

540 Centrifuge

530 Decantation

and sieve

5~50 5.50 6 Centrifuge Centrifuge Centrifuge

Total VFA (g I ‘) I WJS 19.57 21.2 7.89 I6 16.3 11.21 23.4 23.34 x.25 Acetic acid (g I ‘) 4.27 654 7.62 4.75 6.2 7.09 5.1 IO.63 12.84 4.5 I Propionic acid (g I ‘) 1.92 3.88 3.31 1.82 3.47 3.99 1.41 4.87 5.33 2.26 Butyric acid (g I ‘) 2.93 5.60 7.18 0.78 3.89 3.32 3.19 4.66 3.19 0.96 Valerie acid (g I ‘) I.84 3.55 3.08 0.53 2.45 2.17 152 3.27 1.98 0.49

TSS (g I ‘) Total COD (mg I I) Dissolved COD

(mgl ‘) TOC(mgl ‘)

49.26 55 58300 X3541 18891 30 347

7653 15 739

88.5 33.8 71 75.X 22.52 43 x1.1 26.3 1 116770 37991 88 203 94 890 27 634 x3 103 I29 305 36305 44516 10951 30713 32 168 16276 4X 997 h I 92s 17 181

18004 4131 12358 12X31 I9 600 25 306

Hydrolysis yield (%) Acidification yield

(S) Total COD removal

(%)

77.7 x7.7 40 44.4

- 38.1

85.2 67.02 50.21 36

30.5

87.2 5X.3

20

91.7 62.9

x4.7 so.7

16.7 23.7

X8.5 34. I

34. I

21.7 29.7

100 .I. Raynal et al.

However, concentration of solids in the reactor became high (88.5 g 1-l of TSS)

Salad leaves substrate. In the digester treating the salad leaves, reducing the pH from 6.5 to 5.5 increased the hydrolysis efficiency and partially inhibited methane fermenta- tion. The average rate was 1.10 g pCOD 1-l day-’

liquefaction and the yield remained

around 67% (Table 2). Total VFA remained below 8gl-‘. In order to stop methanisation and to improve liquefaction, the pH was regulated at 5.00. After this operation, the concentration of volatile fatty acids reached 9.1 g 1-l. In the next experiment (Run 2), when the loading was increased to 42%, VFA concentration was doubled, hydrolysis rate increased to 3.14 g pCOD 1-l day-‘: an increase of 30% for hydrolysis yield and 62% for acidification yield was obtained (Table 2). So, this digester showed the best efficiency at the end of the run. This could have been explained by an increase of hydrolytic activity after several months of operation. During the next run (Run 3), increasing the loading rate by reducing retention time improved hydrolysis and acidification yields a little (Table 2). Hydrolysis rate reached 5 g pCOD 1-l day-‘, but with a high TSS content in the digester (about 76gl-‘). VFA concentration remained at the same level (about 16 g l-‘), as in the potato peelings treating digester.

Green peas-carrots substrate With green peas-carrots canning wastes, reducing the pH to 5.5 decreased the methanogenic activity, and this was accompanied by an increase in the concentration of acetic acid (5.1 g l-l), and an improved acidification. But the hydrolysis yield remained below 70% and the hydrolysis rate was calculated as 1.8 g pCOD 1-l day-’ (Table 2). During Run 2, an increase in the loading rate of 80% (4-7.2 g COD I-’ day-‘) resulted in a doubling of VFA concentration, and enhancements of 28% for hydrolysis yield and 14% for acidification yield. The hydrolysis rate increased to 3.76 g pCOD 1-l day-‘. This indicated a good adaptation of the microbial ecosystem. Then, during the last run (Run 3), the loading rate was increased, for the second time, from 7.2 to 10 g pCOD 1-l day-’ (+39%), without reducing retention time. The hydrolysis yield increased slightly to 88.5% and the liquefaction rate became 5.56 g pCOD 1-l day-‘, while the concentration of solids in the digester increased from 43 to 81 g 1-l. On the contrary, acidification yield decreased to 34%. This could have been related to an inhibition of activities of the acidogenic ecosystem by the high concentration of acetic acid in the medium.

Apple pomace substrate. After more than 4 months of operation with a 4-g COD 1-l day-’ loading rate, the results showed

--o-*potatoas *salads

10

0 0 2

Loading4 rate (g tsc*lMd) 8 10

Fig. 1. Hydrolysis yields of vegetable solid wastes versus loading rate.

very low values in hydrolysis yield (around 22%) and hydrolysis rate (O-47 g pCOD 1-l day-‘) (Table 2). The relative high concentrations of both simple sugars and cellulose/hemicellulose content of this type of solid waste did not favour the development of hydrolysing bacteria and could explain the observed results. Furthermore decreasing the pH from 6.00 to 5.50 did not improve hydrolytic perfor- mances. So, operations on the apple pomace digester were stopped at the end of Run 1.

Effect of the increase in organic loading rate on the performances of the acidogenic reactors Results of hydrolysis yields versus loading rate (Fig. 1) showed that the best yields were obtained with the highest loads. However, maximal yields appeared at around 6 g of COD I-’ day-‘. At this value, the liquefaction yield was stabilized between 80 and 90% (Fig. 1). On the contrary, the relation between hydrolysis rates and loading rates appeared to be linear for the three substrates (Fig. 2). As for acidification (Fig. 3), the best yields were from potatoes and salad wastes, at between 8.5 and 9 g of COD 1-l day-‘. On the contrary, the acidification

.!I 2

8 2 1 u

ZO 0 2 4 6 8 10

Loading rate (g tCODlld)

Fig. 2. Hydrolysis rates of vegetable solid wastes versus loading rate.

Two-phase anaerobic digestion of solid wastes 101

0 2 4 6 6 10

Loading rate (g tCODM) Fig. 3. Acidification yields of vegetable solid wastes versus

loading rate.

yield for green peas-carrots reached its highest value at around a loading rate of 7 g of COD 1-l day-‘, but decreased from 51 to 34% at around 10 g of COD 1-l day-‘. A partial inhibition of the acidogenic phase could have been caused by such organic loading rates.

All digesters showed better efficiency during both final runs. In spite of a load increase, hydrolysis yields remained stable between 85 and 92%, but the VFA concentration did not increase over 23 g I-‘. This means that, under such culture conditions, hydrolysis reactions can take place but VFA produc- tion slows and it seems difficult to get more than 25 g I-’ of VFA. The high concentrations of hydro- lytic microorganisms and VFA could inhibit the acidogenic activity. But acidogenic activity could continue inside the methane reactor after filtering or centrifugation of the hydrolysis-digester contents. In agreement with Sans et al. (1995) the VFA profile showed that butyric acid concentrations were quite high for digester contents recycled by centrifugation. During the liquefaction phase, the production of carbon dioxide and small quantities of methane and hydrogen resulted from COD degradation. However, the latter was more important in the potato and the green peas-carrots mixture than in the salad wastes because, of their greater content of starch which was more easily hydrolysed.

Methanation stage The methanation experiment on the fixed-film digester was carried out over 450 days by treating the whole output-liquid-phase of the acidogenic reactors. The average performances of the methano- genie digester versus load were determined with regard to COD removal and biogas recovery. The results are given in Table 3. The loading rate was increased slightly from 1 to 3 g COD I-’ day-’ and the retention times did not change very much, being between 12 and 10 days. Global COD removal was above 80% in spite of the substrate diversity and the

Table3. Average performances of the methanogenic reactor during Run 3

Load (g tCOD I-’ day-‘) Retention time (days)

F&‘t tCOD (mg 1-l) Output tCOD (mg 1-l) Biogas productivity (1 per 1 of reactor per day) Biogas production (1 per 1 of influent) Biogas yield (1 per g of input COD) Average methane content (%) Methane yield (1 of CH, per g of input COD) COD removal (%)

2.61

:ps 37 330 7306 1.30 19.8 050 69

0.35 80.43

composition of the feed, which contained about 18 g 1-l of VFA (48% of the dCOD) and other less biodegradable components (52% of the dCOD). Analysis of output showed that VFA could consti- tuate up to 70% of dissolved COD. So, the pollutant removal rate could still be improved by recycling the effluent in the reactor. Indeed, VFA removal reached 77.3% during the last run and even 89.1% for butyrate which was the best degraded acid. The major VFA was acetate although propionate concentration increased quickly after every load increase. Methane content of the biogas was high (Table 3) because of a high carbon dioxide dissolu- tion into bicarbonate form at pH 7.9, which enhanced the buffering capacity of the reactor. The response of biogas production to load variations was very fast over the whole experiment and stopping or reducing feeding did not modify reactor efficiency. Methane yield was near the stoechiometric value of the sugar methanization reaction.

Process performance Degradation yield of organic matter by the whole process (17 1 of digest total volume) was established with regard to total COD removal and biogas recovery, during a steady state period of each run. Results are reported in Table 4. These results showed that global degradation yields remained

Table 4. Mean depuration yield of the whole digestion process

Total COD load (g tCOD) Retention time (days) Loading rate (g tCOD I- ’

day- )

Run 1 Run 2 Run 3

1350 1680 1890 20 20 1.8 2.8 :::

tCOD removal (first stage) 30.0170 33.35% 36.2% tCOD removal (second 89.4% 78.8% 80.4%

stage) tCOD removal (whole 92.6% 85.9% 87.5%

process) Biogas production (1) 327 455 557 Biogas yield (1 per g of input 0.24 0.27 0.29

tCOD)

102 J. Raynal et al.

above 85%, in spite of a load increase from 28 to 3.7 g pCOD 1-i day-’ (+32%). A significant COD removal (30% at Run 1) was achieved during the liquefaction step (Table 4). Besides carbon dioxide, a small production of methane and some hydrogen was detected in the liquefaction reactors but was unquantified, the digesters being not completely gas tight. This contribution of the acidogenic step to the global COD removal increased with loading rate (36% at Run 3) thanks to an acclimation of the acidogenic ecosystem. Overall, biogas yield was low owing to losses during the liquefaction-acidification step.

DISCUSSION

The results obtained by the two-phase digestion process are in agreement with other work where different kinds of vegetable substrates have been subjected to the hydrolysis-acidification reaction: single substrates, time-sequenced substrates or substrate mixtures (Verrier et al., 1987; Viturtia et al., 1989; Knol et al., 1978). One of them (Knol et al., 1978) indicated that anaerobic degradation of several species of fruit and vegetables wastes varied between 40 and 75%. In a similar study (Kalia et al., 1992), vegetable wastes were subjected indepen- dently and in succession, to anaerobic digestion. They showed that digestion efficiency could be enhanced or slowed down according to the nature of the waste. This means that the microbial ecosystem needs time to adapt itself to the new substrate and the efficiency of the hydrolysis reactor will depend on its specialisation and will improve with time. For these reasons, the multiple-liquefaction digester process is particularly adapted to treat successions of different substrates. Monophasic digesters were previously developed and their performances compared to the two-stage processes. The latter seem to be better suited to several waste sources and loads and to the fermentative conditions of the different microbial flora, as reported by Ghosh and Klass (1978) about cellulose and sewage sludge digestions, Nagori and Rao (1992) working on green banana stems, and White et al. (1989) on apple pomace and wastewaters. These authors generally found that phase-separated digesters may offer the best choice for high efficiency, concerning both depuration rates and energy recovery. It has also been shown that the time required for a two-phase system to return to its initial performance after a loading shock is only half that required for the monophasic system (Dinopoulo and Lester, 1989). Actually, the first step of the process provides optimal conditions to the single methane reactor which can be fed by sources of VFA from different origins over all the year. The acidification stage serves as a buffer system. Such a process makes the

management easier of a wide range of wastes on a single plant.

The overall results of the two-stage process suggest that anaerobic digestion is available to treat vegetable solid wastes with a high hydrolytic yield. This efficiency was allowed by recycling total suspended solids into the first-stage digesters and by the adaptation of each ecosystem to its own substrate. Actually the main limiting parameter for liquefaction is the physical consistency of input substrates, which must be diluted, and the high concentration of solids of digesters which decreases decantation efficiency and requires mechanical clari- fication. On a pilot- or industrial-scale, the process could be improved by using a first stage of digesters which would operate according to the SBR mode using natural decantation in order to avoid sieve or centrifugation operations. Substrates would have to be diluted by the effluent in order to get sufficient water content to allow decantation. The second stage could be a completely-mixed methane reactor in which the end of liquefaction coupled with a major methane fermentation stage would take place at the same time. Thanks to its flexibility, the described two-phase digestion process could be enlarged to cope with the treatment on the same site of several organic solid-waste sources, like the organic fraction of municipal and industrial solid wastes, agricultural wastes, urban wastes like the primary or/and sewage sludges.

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