anaerobic digestion of wastes containing pyrolignitic acids

Biological Wastes 34 (1990) 203-214

Anaerobic Digestion of Wastes Containing Pyrolignitic Acids

V. Andreoni

lstituto di Microbiologia ed Industrie Agrarie, Universitfi di Torino, Via Giuria 15, 10100 Torino, Italy

P. Bonfanti

Istituto di Produzione Vegetale, Universitfi di Udine, P.le Kolbe 4, 33100 Udine, Italy

D. Daffonchio, C. Sorlini & M. Villa

Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Sezione di Microbiologia Agraria, Alimentare, Ecologica, Universitfi degli studi di Milano,

Via Celoria 2, 20133, Milano, Italy

(Received 24 April 1990; revised version received 2 June 1990; accepted 6 June 1990)

ABSTRACT

Small quantities of residues from wood pyrolysis (pyrolignitic acids) added to swine slurry were digested in two laboratory anaerobic, fixed-bed, upflow digesters, filled with wood-chips or PVC as support media.

The two digesters showed about the same efficiency when treating swine slurry containing pyrolignitic acids up to 6"5% (v/v). With a 10% (v/v) concentration, COD removal efficiency, specific biogas production, pH of the effluent, utilized as process-efficiency parameters, decreased remarkably for both plants. However, the digester with wood-chips showed a stronger resistance to the presence of pyrolignitic acids.

INTRODUCTION

Liquid effluents from pyrolisis of wood residues (pyrolignitic acids) are known to contain several compounds originating from the breakdown of lignin (Table 1) (Yasuhara & Sugiura, 1987).

203 Biological Wastes 0269-7483/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

204 V. Andreoni et al.

TABLE 1 Analyses of the Liquid Fraction Containing Pyrolignitic Acids

Relative density at 15°C 1"034 pH 4-6 Total acidity 57"25 mg KOH/g Residue at 120°C 7"9 % w/w Residue at 650°C 0"7 % w/w TOC 65 000.0 mg/litre COD 133 000.0 mg/litre Total N 0.35 % w/w P 0.27 % on dry residue at

650°C Methanol Traces Acetic acid 2"8 % w/w Phenols 0.2 % w/w Cresols 0.2 % w/w

The use of anaerobic treatment for highly concentrated wastewaters is growing since the absence of oxygenation, the small amount of sludge generated and the production ofbiogas are all advantages over conventional aerobic processes (Colleran et al., 1982; Kennedy & Van den Berg, 1982; Singh et al., 1982; Hobson et al., 1983; Oleszkiewikz, 1983; Tesch et al., 1983).

The aim of this work was to apply anaerobic digestion to the treatment of wastewaters rich in phenols and cresols (Boari et aL, 1984; Carrieri et al., 1986; Vogel & Winter, 1988). These compounds are potentially toxic and inhibit the development of microorganisms in both aerobic and anaerobic conditions (Koike et al., 1979; Fedorak & Hrudey, 1984).

In order to adjust pH and to dilute the solutions containing pyrolignitic acids to a concentration of phenolic compounds not potentially inhibiting to anaerobic microorganisms, wastewaters were mixed with swine slurry.

METHODS

Reactor configuration

Two 15 litre laboratory anaerobic, fixed-bed, upflow digesters, consisting of cylindrical reactors made of plexiglass (Sorlini et al., 1990), filled with wood- chips or PVC as support media, were used (Table 2) (Murray & Van den Berg, 1981).

The feedstock was delivered from the storage tank to the digesters by two timer-controlled peristaltic pumps, that loaded intermittently (30 s loading alternated to 60s pause). Retention time was two days. The plants were

Digestion of waste containing pyrolignitic acids

TABLE 2 The Main Characteristics of the Packing Materials Used

205

Wood-chips P VC

Specific weight (kg/m 3) 250 35 Useful volume (%) 63 96 Specific surface (m2/m 3) 538 132 Mean diameter (mm) - - 50 Shape Chip Empty open sphere

installed in a controlled environment and the filters were connected to gas- holders (floating-bell) at 30°C and 130 Pa to collect and meter the biogas output.

Feedstock

The digesters had been previously utilized for digestion of swine slurry for some years. At the start of the experimentation the digesters were fed with swine slurry for 30 days.

A first cycle of experiments was performed using, as feedstock, a mixture of a solution of pyrolignitic acids at 2"5% (v/v) and diluted swine slurry. Diluted swine slurry (TS = 0"8-1"4%) was from a swine breeding farm where water is widely used for the cleaning of the building yards. This phase of acclimatization to the new waste lasted about 18 days; then the concentration ofpyrolignitic acids was increased to 4% (v/v) and, after 7 days, to 10% (v/v). A second cycle of treatment was performed using progressively increasing concentrations of pyrolignitic acids (4%; 5%; 6.5%; 10%; v/v). Finally, in order to confirm the results, a third cycle of feeding with the same concentrations of pyrolignitic acids, as in the second cycle, was performed.

Analyses

Performances of the digesters were measured by determining biogas production, pH, COD of the feed and the effluent. COD determinations were carried out according to Standard Methods (American Public Health Association, 1975).

Methane and CO2 composition of the biogas was determined by gas chromatography using a Carlo Erba gas chromatograph (model GT 200) equipped with an SSMM column (length, 2m; internal diameter, 5 mm), packed with FVT GT; N2 as carrier gas at 35 ml/min and a flame ionization detector (H2, 0.7 atm, and air, 1-2 atm); temperature 50°C.


Microbiological analyses

Microbiological analyses of sediments and supernatant effluents of digesters were performed five times in 17 days during the third cycle of experiments, when the pyrolignitic acids concentration was 10% (v/v).

Sampling was performed both from the bottom (sediment) and the top (supernatant effluent) of the digesters; samples were collected in sterile, nitrogen-purged plastic bags, stored at 4°C and analysed within 2 h.

Counts of the most important microbial groups involved in the methanogenesis were determined in duplicate-set test tubes, by the Most Probable Number (MPN) technique (Harrigan & McCance, 1976). The bacterial groups determined and the media utilized were: anaerobic heterotrophic bacteria, Todd Hewitt Broth medium; anaerobic, cellulolytic, bacteria, liquid medium according to Mann (Mann, 1968); acidogenic bacteria (peptone-glucose-fermenting), peptone broth with 1% glucose and 0" 1% litmus; methanogenic bacteria, Todd Hewitt Broth medium with 0.2 % acetate and 0.2% formate under H2"CO 2 (80:20).

Tenfold serial dilutions in sterile Ringer's solution and inoculations in liquid media were carried out in an anaerobic glove cabinet (Anaerobic System--Mod. 1028 Forma Scientific, USA; atmosphere N2:H2"CO2, 85:10:5). The first three microbial groups were incubated at 37°C in Gas Pack System for 4, 21 and 4 days, respectively.

The presence of anaerobic heterotrophic bacteria was assessed by turbidity; that of peptone-glucose-fermenting bacteria by colour change of litmus; that of cellulolytic bacteria by pitting and, finally, by disintegration of the filter paper strips suspended in the culture medium. For the methanogenic bacteria count, the initial suspension was inoculated directly into vials, containing pre-reduced culture medium and fitted with butyl- rubber, air-tight stopcocks (Alltech Associates, Inc., Deerfield, Illinois). After inoculation, the sealed test tubes were removed from the glove cabinet, the atmosphere was replaced with H 2:CO 2 (80:20) and the vials were incubated at 37°C for 30 days. At the end of the incubation period, the headspace gas of each tube was analysed by gas chromatography (Sorlini et al., 1983). The presence ofmethanogenic bacteria was shown by presence of methane in the biogas.

RESULTS AND DISCUSSION

The influence of pyrolignitic acids on the performance of the digesters was assessed by varying the concentration of pyrolignitic acids added to the swine slurry. For this purpose three cycles of tests were carried out. In the

Digestion of waste containing pyrolignitic acids 207

initial phase of the first cycle, when pyrolignitic acids were up to 4 % (v/v), the digesters did not show remarkable differences in COD removal efficiency, specific biogas production (CH 4:75%) and pH of the effluent, from when treating swine slurry. Increasing the concentration to 10% (v/v), a pH decrease of the influent and a deterioration of the process efficiency were observed in both digesters (Fig. 1).

During the second experimental cycle the digesters also showed the same

S O L U T I O N (%)

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D A Y S

Fig. 1. Performance of the digesters during the first cycle (O, wood-chips packing; A , PVC packing). A: Biogas production, - ; C O D r e m o v e d , - - - - . B: C O D concentration, ; loading rate, - - - - . C: pH. , , Influent" solution of pyrolignitic acids; Q, A , effluents. Dashed

lines apply to right-hand axis.

208 K Andreoni et al.

S O L U T I O N (%)

Fig. 2.

I A ,',, / / . . ' - ~ \ _ -18o

u O.al- , , / ,.. ,e'~ /

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Performance o f the digesters during the second cycle. For symbols see Fig. 1.

capacity in degrading the waste containing pyrolignitic acids at concentrations ranging from 4-6.5% (v/v): in fact COD removals were about 60-70%. At a 10% concentration of pyrolignitic acids, COD removal drastically decreased (Fig. 2).

Finally, Fig. 3 shows performances of the digesters during the third cycle of experiments. Operating with organic loads that increased from 3 kg COD/m a day, with increasing concentrations of pyrolignitic acids, COD removal efficiency dropped from 70% to 55 % in the digester with PVC and from 75 to 45 % in the digester with wood-chips (up to about day 34). Specific biogas production was in the range of0.10-O.06 m a biogas/kg COD added in


S O L U T I O N ( % )

o A -ao o o.~,~

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Fig. 3. Performance of the digesters during the third cycle. For symbols see Fig. 1.

the digester filled with PVC (CH4: 76-64%) and 0.15-0.10m 3 biogas/kg COD added in that filled with wood-chips (CH4:82-65 %). Plant operations were not affected by pH variations: while the pH of the feedstock varied from 5.65-6.65, the pH of the effluent ranged from 7.04-7.65 for both digesters.

However, after 4 days of feeding with a 10% (v/v) concentration of pyrolignitic acids, a different behaviour of the two digesters was observed. COD removal and specific biogas production dropped near to zero in the digester with PVC, whereas in that with wood-chips the COD removal efficiency decreased to 25% and specific biogas production to 0-06m 3 biogas/kg COD added (CH4:53 %).


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T I M E ( d a y s )

Fig. 4. MPN counts of anaerobic heterotrophic (AH), Cellulolytic (C), acidogenic (A), and methanogenic (M) bacteria, at the top and at the bottom of the two digesters fed with swine slurries containing 10% (v/v) pyrolignitic acids. The MPN counts at t o (time = zero) were determined in the effluents when waste was supplemented with 6"5 % (v/v) pyrolignitic acids.

The 95% confidence limits are also shown.

A decrease of pH also occurred; in the digester filled with PVC pH (5.5) was lower than in that with wood-chips, where pH first decreased near to 6 and then went up again to about 7; in PVC, the pH did not go up again. Table 3 indicates the performances of the filters during the final 17 days of the third cycle. Subsequently, however, while the digester with wood-chips seemed to return to the same conditions observed in the initial phase of the third cycle of experimentation, as evidenced by COD removal, specific biogas product ion and effluent's pH, the digester with PVC did not.

The determinations of microbial groups involved in methanogenesis, performed on consecutive days during the third cycle, when digesters were

Digestion of waste containing pyrolignitic aCids 211

TABLE 3 Average Performance of the Digesters During the 17 Days of Feeding

with Swine Slurry Containing 10% (v/v) of Pyrolignitic Acids

Support matrix

Wood-chips P VC

Volumetric loading rate (kg COD m3/day) 10-6 9-2

Volumetric CH4 productivity (m 3 CH4 m3/day) 0.584).44 0.324)'05

Surface loading rate ° (kg COD m2/day) 0.019 0.069

Surface CH4 productivity ~ (m 3 CH4 m2/day) 0.8-1.1 x l0 -3 0.3-2.4 x l0 -3

CH 4 in biogas (%) 64 53

COD removal efficiency (%) 45-25 54-5

CH 4 yield (m 3 CH 4 kg/CODaa.) 0"0594)'041 0"0364)'001

° Values calculated on the range of single support element.

fed with a 10% (v/v) concentration ofpyrolignitic acids in the liquid fraction, showed that a higher number of microorganisms did not adhere to the supports in the digesters.

The counts relative to the four microbial groups determined (t 0, t 3, t~, tl 0, tl 7) are reported in Fig. 4.

Submitting the results obtained to statistical analysis of the variance, it was found that anaerobic heterotrophic, cellulolytic and acidogenic bacteria counts, determined in the course of five determinations, at the top of the digester with wood-chips, were significantly higher than those present at the top of the digester with PVC, while the counts of the same microbial groups at the bot tom of both digesters did not show significant differences.

The numbers of methanogenic bacteria in both digesters were not significantly different, when considered over the five determinations; but observing their behaviour at 17 days, it was noticed that they rapidly decreased at the bot tom as well as at the top of both digesters, beginning during the first days of treatment with waste containing 10% (v/v) pyrolignitic acids. The decrease was more drastic at the bot tom than at the top. However, at the bot tom of the digester with wood-chips, methanogenic bacteria increased after 17 days, going back to values initially observed, while in the digester with PVC they continued decreasing, in accordance


with the COD removals. The behaviour of the methanogens is in accord with data obtained in previous work, where methanogenic bacteria showed a higher sensitivity to phenolic acids than anaerobic heterotrophic, cellulolytic and acidogenic bacteria (Sorlini et al., 1986). The decrease of methanogenic bacteria corresponded to a smaller production of methane in both digesters.

The results suggest that under the present experimental conditions, swine slurry containing pyrolignitic acids may be anaerobically treated, with biogas production, even if process efficiency is decreased by increasing the concentration of such acids.

Regarding organic polluting removal, both digesters showed about the same efficiency when treating swine slurries containing pyrolignitic acids up to 6-5 % (v/v). At a concentration of 10% (v/v), the digester with wood-chips seemed to be more resistant to a higher content of pyrolignitic acids than the digester with PVC. In fact, after the initial phase of stress the plant seemed to adapt to this new concentration of pyrolignitic acids. This is evidenced by reactivation of the process in the digester with wood-chips (pH of the effluent, COD removal efficiency, and biogas production), and by the increase of methanogenic bacteria.

The different behaviour of the two plants might be because the wood- chips, as supports, create more favourable conditions for microbial methanogenic consortia, in particular for the actual methanogenic bacteria. In the PVC digester, higher acidity was accompanied by a decrease of pH that would help to lower the methanogen number and inhibit their activity.

The better process stability and efficiency in the digester with wood-chips may be attributed, not to attached microflora composition (in both plants the support matrices are hydrophilic and allow adhesion of the same hydrophilic population of microorganisms (Verrier et al., 1988)), but to the attached biomass quantity per volume unit, that is higher in the digester with wood-chips with the greater specific surface (Sorlini et al., 1990). Moreover, we can suppose that process efficiency is determined by, as well as the attached microflora, the high number of microorganisms found in the circulating fraction (Dahab & Young, 1983).

In conclusion, a fixed-bed upflow digester with wood-chips can be considered a suitable plant to treat wastes with a high organic concentration and containing toxic substances.

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anaerobic digestion of wastes containing pyrolignitic acids

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