anaerobic digestion of wastes containing pyrolignitic acids
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Biological Wastes 34 (1990) 203-214
Anaerobic Digestion of Wastes Containing Pyrolignitic Acids
lstituto di Microbiologia ed Industrie Agrarie, Universitfi di Torino, Via Giuria 15, 10100 Torino, Italy
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)
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.
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 15C 1"034 pH 4-6 Total acidity 57"25 mg KOH/g Residue at 120C 7"9 % w/w Residue at 650C 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
650C 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.
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
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 30C and 130 Pa to collect and meter the biogas output.
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.
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 50C.
206 V. Andreoni et al.
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 4C 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 37C 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 37C 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
_1_ 4 ~ L ~ 10 . ,.-. 2.5 11 '1o
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0 5 10 15 20 25 30
Fig. 1. Performance of the digesters during the first cycle (O, wood-chips packing; A , PVC packing). A: Biogas production, - ; COD removed, - - - - . B: COD 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.
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a 1o r~~- -~- - - * . . . . ; . -~" // e
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0 10 20 30 40
Performance of the digesters during the second cycle. For sy