Anaerobic digestion of tannery wastes
Post on 21-Jun-2016
Agricultural Wastes 4 (1982) 241 243
Anaerobic Digestion of Tannery Wastes
The purpose of this short communication is to draw attention to a field which has not yet received due notice--the potential for recovering energy from tannery wastes through anaerobic digestion. The anaerobic treatment of these wastes has been said to be more of a problem than the digestion of other organic wastes (von Herfeld & Pauckner, 1961), so we have carried out a preliminary investigation of the susceptibility of fleshings to digestion.
METHODS AND RESULTS
Fleshings are tannery wastes resulting from the treatment of hides and skins: salted hides are soaked in an aqueous solution of surfactants, dehaired with lime, soda and sodium hydrosulphide and then mechanically fleshed. In this study we employed calf fleshings (from the Sebastiano Bocciardo E C. SpA tannery, Genoa, Italy). The fleshings were minced and repeatedly rinsed with water to partly remove the above- mentioned chemicals. After this treatment, fleshings (26-3 ~o Total Solids (TS), 23.7 ~ Volatile Solids (VS)) were diluted with tap water to a concentration of 5 oj~, VS. Eight hundred grammes of dilute fleshings were introduced into a laboratory digester, the pH was adjusted to 7 by the addition of HzSO 4 and 20 ml ofinoculum (swine dung) were added to initiate fermentation. The inoculation is necessary because the chemical treatment of hides affects their microbial population.
The batch digester was a 1-1itre glass bottle in a heated water bath; the temperature was maintained at 35 -+ 1 C. The biogas was collected in a glass tube containing saturated sodium chloride solution ; this measuring tube was connected with a levelling bottle. To withdraw the biogas, the bottle was raised to apply a positive pressure on the system.
241 Agricultural Wastes 0141-4607/82/0004-0241/$02.75 Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
242 F. CENNI, G. DONDO, F. TOMBETTI
"T ~" 40(
300. Z g ~200, Q 0 100. o. U)
. - . " ,,.. . . . . ~b
. . . , .
Fig. 1. Daily gas production from 800 g of dilute fleshings (5 ~o VS) at 35 C.
The gas production was recorded daily with the aspirator bottle and measuring tube at equal levels. Gas samples were analysed using a Porapak Q column in a Carlo Erba Fractovap 4200 gas chromatograph with a thermal conductivity detector, helium as carrier gas and an oven temperature of 50C.
The volatile fatty acids (VFAs) produced during anaerobic digestion were analysed using a glass column (15 ~o SP-1220/1 ~o H3PO4 on 100/120 Chromosorb W AW) in the same gas chromatograph, with helium as the carrier gas; the column temperature was programmed from 100C to 145 C. VFAs were extracted with ethyl ether and the extract was treated with anhydrous NazSO 4 to remove dissolved water from the ether (VPI, 1977).
The daily biogas production during the test (134 days) is shown in Fig. 1 : the total production of biogas from 800 g of dilute fleshings (5 ~o VS) was 33 litres. At the end of the test the concentration of digested material was 0.9 ~o VS. The gas composition during the test was almost constant (an average of 74 ~o CH4, 20 ~o CO2)-
The proportions of volatile fatty acids in two different situations are shown in Table 1. The first column shows VFAs determined when the gas production rate was
TABLE 1 PROPORTIONS OF VOLATILE FATTY ACIDS IN FERMENTING FLESHINGS
Normal anaerobic Incomplete anaerobic fermentation* (%) fermentationt (%)
Acetic 30.8 66"8 Propionic 51.8 5.7 Iso-butyric 7.5 7.2 n-Butyric 0.6 14.1 Iso-valeric 8-6 5.2 n-Valeric 0.6 0.9
* At time of maximum gas production, Fig. 1. Acid concentration at end of active digestion, 4000 mg litre-~; in another digestion acid concentration at the time of maximum gas production was 7500 mg litre-~. t A digestion that did not produce biogas; acid concentration 15000mglitre 1.
ANAEROBIC DIGESTION OF TANNERY WASTES 243
at a maximum whilst the second column shows VFAs from a digester where the fermentation of dilute fleshings did not properly start (production of fatty acids but absence of methane). The fleshings do digest, the main problem is the slow rate of gas production. The fleshings were inoculated with swine dung and probably the bacterial population required some time to grow and to adopt to the new substrate. In further tests a more rapid initiation of gas production occurred when the digester was inoculated with fermenting fleshings from an active digestion. The anaerobic fermentation may also be partly inhibited by toxic substances used in the tannery treatment and/or might be slowed down by an unsuitable carbon nitrogen ratio in the fleshings. This study is the preliminary phase of work and investigations on these problems are continuing.
The authors are most grateful to Mrs Emma Frassine for expert technical assistance.
VON HERFELD, H. & PAUCKNER, W. (1961). Verwertungsm6glichkeiten von Leimleder. Leder- und Hdutemarkt (January, 1961), l-11.
VP1 (1977). Anaerobe laboratory manual(4th edn). Virginia Polytechnic Institute and State University, Blacksburg
F. CENNI, G. DONDO & F. TOMBETTI, Research Laboratory, Eneco SpA, Genoa, Italy