Anaerobic digestion of onion waste by means of rumen microorganisms
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Bioh~gical Wastes 25 (1988) 61 -67
Anaerobic Digestion of Onion Waste by Means of Rumen Microorganisms
Henk J. Lubberding, Huub J. Gijzen, Monique Heck & Godfr ied D. Vogels*
Department of Microbiology, Faculty of Science, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Nctherlands
(Received 10 December 1987: accepted 5 January 1988)
Onion waste materials have been screened on anaerohic digestion h v a mixed population o.[rumen microorganisms. Both peels and pulp are degraded with an e~'ciency ?[ ahout 50 70/, at retention times o[ only 6Oh. The degradation ?['onion pulp, rich #z cell soluhh, s. ,gave rise to u relatiz'elv high pro~hwtion ~?[" hutyrate and yah'rate.
In the onion processing industry three important kinds of waste arc produced:industrial waste water, a mixture of peels, small onions and sand. and onion pulp. Essential oils, used for pharmaceutical purposes or in the flavour and fragrance industry, are extracted from onions and the residual pulp is a wet (dry matter about 7-5%) slurry with an offensive odour which cannot be drained off or" fed to cattle. Different attempts to attack this environmental pollution have failed up to now.
Recently, we demonstrated that various organic waste materials of agricultural, industrial and municipal origin can be anaerobically converted into biogas by the action of rumen microorganisms (Gijzen et al., 1987). Application of these microorganisms, specialised in the digestion of fibrous materials, showed a high degradation efficiency not only for (ligno-)
* To whom all correspondence should be addressed.
61 Biological Wastes 0269-7483/88/$03.50 ~ 1988 Elsevier Applied Science Publishers I,Id. England Printed in Great Britain
62 Henk J. Lubherding, lluub J. G(]zen, Monique lteck, Godfried D. Vogels
cellulosic wastes, but also for substrates with a relatively low fibre content such as vegetable auction waste and horticultural waste.
In this study we demonstrate that rumen microorganisms can also be applied in the conversion of onion waste, which contains relatively little fibre and many low molecular weight carbohydrates (Becker & Nehring, 1969), into biogas.
Onion waste, peels and pulp, was produced by Mol Products BV ('s- Gravenpolder, The Netherlands). In addition to the onion waste, alfalfa hay (Medicago sativa), purchased from van Heeswijk (Veghel, The Netherlands), was used as an undefined source of nutrients. The chemical compositions of the waste materials are shown in Table 1.
Fermenter design and experimental conditions
Experiments were carried out in a fermenter as described previously (Gijzen et al., 1986; Gijzen et al., 1987). Digester feed was added once a day except at the day of inoculation, when twice the normal loading rate was administered. In some experiments the substrate was added by continuous pumping. Operating conditions of hydraulic (HRT) and solid retention times (SRT) and of loading rates during the various experiments are listed in Table 2.
TABLE I Chemical Composition of Substrates tbr Anaerobic Digestion
Onion peels a Onion pulp A#a#a
TS (%) 96 7 94 VS (% of TS) 54 93 92 NDF (% of VS) 50 12 49 Cellulose (% of VS) nd b 4 24 Hemicellulose (% of VS) nd 4 14 Lignin (% of VS) nd 4 I 1 Cell solubles (% of VS) 50 88 51 COD (g 02 g i TS) 0-68 1.05 IO8
a The onion peels contain sand and very small onions. h nd = Not determined.
Anaerobic d~e.stion qf onion wa,stc 63
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64 Henk J. Lubberding, Huub J. Gijzen, Monique Heck, Godfried D. Vogels
Neutral detergent fibre (NDF), cellulose, hemicellulose and lignin (permanganate method) were estimated according to Goering & van Soest (1970). Analyses of Total Solids (TS), Volatile Solids (VS) and chemical oxygen demand (COD) were carried out according to Standard Methods (Anon., 1975). The extent of VS degradation was estimated by filtering fermenter samples and washing the residue three times with the same volume of water (40C), after which VS content was analysed. Calculation of VS degradation was similar to that of fibre constituents (Gijzen et al., 1986).
Biogas production was measured daily by means of 10-1itre mariotte flasks containing acidified tap water (approximately 0-02% HCI). Determin- ation of volatile fatty acids (VFA) and ciliate numbers and subdivision of ciliates into different groups were as described previously (Gijzen et al., 1986).
RESULTS AND DISCUSSION
The onion peels contained small onions and sand, which resulted in a high inorganic fraction. By means of the available methods it was not possible to assess the content of cellulose, hemicellulose and lignin in the peel fraction. On account of the relatively high content of CS (cell solubles), onion peels should be regarded as being one of the so-called CS-rich substrates (Gijzen et al., 1987) and digestibility was established on the basis of NDF and VS (Table 3). At both loading rates, digestibility was in the same range as for the CS-rich substrates; VS degradation being about 10% higher than NDF degradation (Gijzen et al., 1987). However, the production of VFA and biogas was significantly lower.
The high amount of low molecular weight sugars in onions (Becker & Nehring, 1969) results in a very high CS content and a low NDF content in onion pulp (Table 1). Due to the fast fermentation of these easily degradable carbohydrates, onion pulp could not be fed at more than 13"9 g (VS basis) a day. The pH in the fermenter decreased from 6"7 to 6'2 within one hour of feeding, but afterwards rose gradually to 6.7 within 4h because of the continuous supply of flesh buffer. If more than 13"9g onion pulp was administered at once, the pH in the culture vessel decreased below the critical value of 6"0. So, at higher loading rates the pulp was supplied continuously together with the buffer; the pulp was kept cold and was changed daily to prevent bacterial contamination in the buffer vessel. At the lower loading rate (13.9 g VS) the pulp was digested efficiently with a high production of VFA and biogas (Table 3); at the higher loading rate (24'0 g)
Anaerobic digestion of onion waste 65
TABLE 3 Digestibility, pH and Fermentation Products During Steady-State Degradation
Substrate pH Degradation VFA production Biogas production (%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
mrnol rnmol mmol mmol NDF I/S litre i day ~ g- l VS litre-I day 1 g- i VS
Onion peels I 7.0 54 63 45.4 4.34 1.7 016 I| 7.0 67 74 45.3 3"05 1-7 0.11
Onion pulp I l l 6.7 81 64 91-4 10'27 4.6 052 IV 6'4 34 53 114.4 9"01 8'3 0'65
Peels + pulp V 6.9 75 65 94"8 10'64 3'0 0'34 V! 6'9 82 60 83.6 9'88 28 0-33
a Specitic production of VFA and biogas per g VS digested.
the degradation efficiency was decreased. A combination of peels and pulp was also degraded efficiently. Onion peels produced only small amounts of VFA and biogas per g VS digested, resembling waste materials rich in lignin (Gijzen et al., 1987). In contrast, onion pulp produced large amounts of VFA and biogas per g VS digested. The difference between peels and pulp in the molar ratio of the VFA produced was remarkable (Table 4). Onion peels produced a small amount of butyrate and no higher VFA and resemble cellulose-rich materials (Gijzen et al., 1987). The degradation of pulp, on the other hand, gave rise to relatively high amounts of butyrate and valerate.
Digestion of the mixture of" peels and pulp resulted in rather high propionate concentrations. These results suggest that, depending on the composition of the substrate fed to the digester, microbial population will change, resulting in different fermentation patterns. High amounts of easily- degradable sugars result in a relatively low acetate concentration to the benefit of butyrate and higher VFA, as was also shown before with vegetable auction waste (Gijzen et al., 1987). A direct correlation between pH and VFA
TABLE 4 Molar Ratio (%) of VFA, Produced During Steady-State Degradation of Onion Waste
Suhstrate Acetate Propionate But)'rate Valerate
Onion peels I 71 24 5 0 II 73 22 5 0
Onion pulp I l l 54 18 21 7 IV 60 16 19 5
Pecls+ pulp V 58 32 10 0 VI 61 31 8 0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66 Henk J. Lubherding, Huuh J. Gij'zen, Monique Heck, Godfried D. Vogels
TABLE 5 Ciliate Numbers and Species Distribution During Steady-State Fermentation of Onion Waste
Substrate Protozoa % Of total (103 ml I I
Diplodinium Epidinium Entodinium Holotrich
Peels 15+3 19+8 28+23 49+24 4+2 Pulp 26+9 2+3 9+5 81 +4 8+6 Peels+pulp 41 _+ 1 4-t-4 12_+6 14_+8 70_+4 Inoculum 413+184 11+6 6+4 68+15 15+_8
distribution, resulting in a higher propionate content at lower pH (Erfle et al., 1982; Gijzen et at., 1988a) could not be observed in this study.
Total ciliate numbers were low and representatives of the Entodin ium group were predominant (Table 5), as was established before with CS-rich waste materials (Gijzen et al., 1987). Remarkably, in all experiments with onion waste, holotrich ciliates could be maintained, albeit in low numbers. The mixture of peels and pulp, either with or without alfalfa, seemed to be a good substrate for the culturing of holotrich ciliates.
In conclusion, rumen microorganisms can be applied successfully to the degradation of onion waste, although the maximal loading rate is lower than for lignocellulosic substrates. Whether the loading rate of waste materials rich in fast-degrading compounds can be increased by optimising the conditions, by keeping the pH constant or by introducing a UASB reactor as a methanogenic phase (Gijzen et al., 1988b), is now under investigation.
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Anaerobic dige.stion o/onion waste 67
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