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Anaerobic digestion of olive mill wastewaters in fully mixed reactors and in fixed film reactors
Process
Biochemisrry
27 (1992) 37 42
Anaerobic Digestion of Olive Mill Wastewaters in Fully Mixed Reactors and in Fixed Film Reactors M. Hamdi,
C. Festino
BIOMAGAZ-Laboratoire
& C. Aubart
de Recherches sur les Fermentations, Aspach la Bas, 68700 Cernay, France
(Received 20 September 1990; revised version received 3 July 1991; accepted 25 July 1991)
Calcium hydroxide precipitates phenolic compounds and long chain fatty acids toxic to methanogenic bacteria, and improves the total alkalinity; it is therefore better for the adjustment of the pH of olive mill wastewaters (OMW) than sodium hydroxide. The fur/y mixed reactors are adequate for volatile fatty acid production from OMW, although an anaerobic jilter is advisable for methanogenesis. The anaerobic filter filled with a clay media matrix which is less porous and has a larger spectfic area than PVC, gives a quick start-up and a satisfactory performance.
means of limiting the toxicity of inhibitory compounds.lO~ll This paper describes experiments carried out at BIOMAGAZ when three important problems in the anaerobic digestion of OMW were investigated in laboratory scale reactors:
INTRODUCTION Reports on the anaerobic digestion of olive mill wastewaters (OMW) date to the beginning of this decade1 when biologicaP3 and chemicaP5 treatments were little known or expensive. The advantages of anaerobic biological processes related to energy and saving and low sludge production. Many anaerobic processes such as anaerobic contact,‘*’ UASB’ and anaerobic filters’ have been applied to diluted OMW treatment but problems arose due to the toxicity and biodegradability of this effluent and the acidification of reactors.’ The start-up time of anaerobic filters fed on OMW is appreciably shorter than suspended growth reactors such as those involved in the anaerobic contact process and the UASB.’ In addition, the use of anaerobic fixed film reactors was found to be a
(1) A comparison between Ca(OH), and NaOH on the batch anaerobic digestion of OMW. (2) Anaerobic treatment of OMW in fully mixed reactors at two pH values. (3) Influence of the geometry of media matrix on the start-up and performance of the anaerobic filters.
MATERIALS
Reactors The laboratory digesters described in Figure 1.
Corresponding author: Centre de Biotechnologie de Sfax, BPW, 3038 Sfax. Tunisia. T&phone : (04) 74110; Telex: 40982.
37 Process
Biochemistry
0032.9592/92/$5.00
0
AND METHODS
1992 Elsevier Science Publishers
Ltd,
England.
used in this study are
38
M. Hamdi. C. Festino, C. Aubart Table
1. Chemical
characteristics
of olive mill wastewaters.
PH COD (g dm-“) TS (g dm-“) VS (g dm-3) TSS (g dm “) TA (g dm-‘) VFA (g dm-3) Oil (g dm-“) Reducing sugars (g dmm3) Total Kjeldahl nitrogen (g dmm3) Ammoniacal nitrogen (g dme3) K (g dmm3)
Ca (g dmm3) Na (g dm-3) Mg (g dm-9 NO, (g dm-‘)
Fig. 1. Schematic diagram of the reactors: (A) fully mixed fermenter; (B) fully mixed fermenter; (C) anaerobic filter fikd with clay; (D) anaerobic filter filled with PVC; (G) gasometer; (EF) effluent ; (IN) influent; (PM) pump.
The two fully mixed reactors were constructed of PVC tube 36 cm long and 80 cm internal diameter’” of effective volume 2 dm3. These reactors were placed in a bath held at 35 “C and stirred every 30 s for a period of 2 min, at a speed of 50 rpm. The anaerobic filter column was constructed of a 50 cm long, 8 cm internal diameter plexiglass glass inner tube and had an effective volume of 2 dm”. The inner tube was enclosed in a jacket through which hot water was circulated to maintain the temperature of the filter at 35 “C. A distributer perforated plate was located at the bottom and acted as a feed line. Each reactor was packed with a different medium. One contained corrugated modular plastic blocks having a porosity greater than 95 % and a specific area of 98 m2 rnm3. The corrugated panels were cross-stacked at 60” angles and welded at the contact points. A second reactor contained granular clay (Attapulgite), with a 5 mm diameter. The porosity was 40% and the unit surface area was 125 m2 g-l. Olive mill wastewater, inocula and experimental procedure The anaerobic treatment was operated on OMW provided from the traditional mill and press processes (Southern France). Because of the seasonal production and the instability of the waste,
5.3 1 73.6 48 43.8 7.5 0.55 0.088
1.2 6.8 0.08 0.02
2.6 0.18 0.28 O-084 0.038
200 dm3 of wastewaters was stored at 4 “C and used (raw or diluted) to prepare the feed for experiments. The chemical characteristics of this wastewater are given in Table 1. Further information about these wastewaters can be found in Refs 13 and 14. The COD of feed OMW in the fully mixed reactors was about 50 g dme3. OMW was supplemented with urea at 6 g dmp3. Two parallel reactors (A and B) were used and fed with OMW at pH 7.2 (adjusted by Ca(OH),) and 5.5, respectively. Since the hydraulic retention time in both cases was 20 days, the fully mixed reactors, were fed once a day with 50 ml of OMW, at the same time (volumetric loading rate, 2.5 g COD 1-l d-l). The COD of feed OMW for the anaerobic filters was varied from 10 to 30 g dma3 by dilution. OMW was supplemented with urea (COD: N, 50: 1) and Ca(OH), sufficient to adjust the pH to 7. The feed from the reservoir was pumped into these anaerobic filters at a rate of 10 cm3 min-‘. These pumps were operated for 10 min every 8 h to provide an average hydraulic retention time of 7 days in the reactors under a loading rate between 1 and 5 g of COD dm-” d-l. Problems of start-up were avoided by diluting the waste and increasing the concentration of available nitrogen by addition of urea.* Since mixing of the different sludges improved the start-up of anaerobic fixed film reactors,‘” all reactors were inoculated with the same anaerobic mixed sludge from two activated sludge digestors; the first treating domestic wastewaters, the second treating pig manure. Gas production was recorded daily and the composition of the gas determined once a week. The input and output of OMW were analyzed once a week on a sample taken at random from the week’s inputs and outputs.
Anaerobic
digestion
Analytical methods The determination of chemical oxygen demand (COD) was performed according to standard methods.16 COD was determined on unfiltered and/or filtered samples. Total alkalinity (TA) was determined potentiometrically (end-point pH = 4) and total and volatile solid (TS and VS) were obtained with unfiltered samples. Volatile suspended solids (VSS) were obtained by filtering the samples on GF/C Whatman glass filters drying overnight at 105 “C and weighing before and after three hours in a muffle furnace at 600 “C. Gas flow rates were measured by liquid displacement as described elsewhere or by using a gasometer (Figure 1).12 Gas samples (methane and carbon dioxide concentrations) were taken with a syringe from the headspace of the gasometer and analyzed using a gas chromatograph equipped with a catharometer detector. For the analysis of volatile fatty acids (VFA), liquid samples were centrifuged at 3000 r min-’ for 10 min, acidified by addition of 1% of 50% H,PO, and analyzed with a PYE UNICAM gas chromatograph equipped with a flame ionization detector. The column used was 80 cm stainless steel and packed with 4% H,PO, on Porapak Q (80-100 mesh). Nitrogen was used as carrier gas at 28 cm3 min-’ with hydrogen and air flows of 25 and 30 cm3 min-‘, respectively. The oven, injector and detector temperature was 200 “C.
RESULTS
AND DISCUSSION
Comparison between the effect of Ca(OH), and NaOH on the batch anaerobic digestion of OMW Bottles were filled according to Table 2, the first bottle served as control, while the second and third bottles were used to analyze the effect of the addition of NaOH and Ca(OH), used respectively to adjust the pH of OMW. Less methane was produced in the batch where the pH was adjusted by NaOH than where Ca(OH), was employed (Figure 2). Phenolic acids present in OMW have an adverse effect on methane production even at low concentrations.17 The main long chain fatty acids (LCFA) contained in OMW are unsaturated with oleic acid being the most abundant (65 %). Unsaturated LCFA are more efficient inhibitors of the microbial
c$ olive
mill
39
wastewaters
Table 2. Composition
of batch anaerobic digestion
Batch (400 cm”)
Subsrrare (12 m.u)
1
s
0
2 3
s S
100 100
OMW (cm?
Inocula (cm”)
Water (cm”)
100 100 100
300 200 200
S: 5 mM of acetate; 5 tItM of formate and 2 mM of methanol. (COD : N) was increased by urea addition to an average ratio of 50 in all batches. OMW : 18.2 g 1-l of COD. Inoculum: pig manure (VSS: 17.5 g 1-l) and domestic sludge (VSS: 14-7 g 1-l).
o.55
0
4
2 t-TIME
6
(day)
Fig. 2. Cumulative methane formation with OMW when the pH was adjusted by Ca(OH), (A); or NaOH (0) and control (I)-
formation of methane from acetate than saturated LCFA. The toxicity level of oleic acid on Methanosarcina sp. was 2.4 mM. The toxicity of a mixture of LCFA can be enhanced significantly by synergism between individual LCFA ;18 indeed, it was reported that the rate of methane production decreased when the concentration of OMW increased because of the inhibition of the methanogenic bacteria.lg The positive effect of Ca(OH), compared to that of NaOH can be explained by the effect of calcium on the polymerization and precipitation of the phenolic compoundszO and in the precipitation of LCFA.*l.** The addition of Ca(OH), also improves the total alkalinity (TA).23 Anaerobic treatment of OMW in fully mixed reactors The results involving biogas production, VFA accumulation, percentage of methane and COD removal as a function of time are reported in Figure 3. It can be seen that reactor A is closer to being a
M. Hamdi, C. Festino, C. Aubart
40
80-
16 12 8 4 0 1001
2
I
5
80-
2
60-
g
40-
D p1
20-
”
0
1
8o60 40
20 1 01 10
5
15
,__
0
10
5
15
I
80 60 40 20
i
0-l 0
I 5
10 t-Time
15
(week)
Fig. 3. Biogas production (1-l d-l), % methane, WA concentrations and COD removal, plotted against time for the mixed fermenters A (m) and B (0).
methanogenic reactor, while reactor B is closer to an acidogenic reactor. Thus, biogas production, methane per cent and COD removal are better in reactor A. COD removal in both reactors is of only 40% on average because of the bioconversion of organic matter to VFA. Indeed, VFA concentrations in reactors A and B were, on average, 7 and 13 eq acetate dmm3, respectively. Two reasons may be considered for the high production rate of VFA : the increase in agitation speed which may have increased the growth rate of acidogenic bacteria by improving mass transfer, and the methanogenic bacteria may be inhibited most by phenolic compounds and LCFA contained in OMW especially under agitation. I9 The pH in reactors A and B was stabilized at 5.5 and 5.3, respectively. The fully mixed fermenter can therefore be adequate for VFA production (acidogenesis) from OMW (pH 5.5) at
O+T--T-x t-Time
(Week)
Fig. 4. Volumetric loading rates, biogas production (1-l d-l), % methane and COD removal, plotted against time for anaerobic filters C (I) and D(O).
high concentrations. Acidified OMW with acidogenesis was slightly less toxic to methanogenic bacteria than unmodified OMW.ls Influence of the geometry of media matrix Two anaerobic filters (C and D) operated in parallel and under the same conditions (concentration of OMW and volumetric loading rate) were used to investigate the influence of the geometry of media matrix on the anaerobic digestion of OMW. The mean volumetric loading rates maintained during the experiment and results involving biogas production, percentage of methane and COD removal as a function of time are reported in Figure 4. During weeks 1-9, the production of biogas increased (OalM.3 dm3 dm-’ d-l) with time in reactor C because of the accumulation of VS. The
Anaerobic Table 3. Performance mill wastewaters
data of anaerobic
digestion
filters fed with olive
VolW?le (0
Package used
Load (g Cod 1-l d-‘)
Efficiency (% COD)
21 300 10 10 11
Polyurethane Polyurethane Prisms cubes Cyl. plugs T30 Cyl. plugs TR30 Plastic Clay
2-80 8.00 2-50 2.50 3.00
83 87 60 55 65
26 26 9 9 9
3.00
60
9
2.70 4.40
65 75
II 2 2
Reference
(This work) (This work)
weak porosity of clay appears to trap VSS more effectively than PVC. The use of the anaerobic fixed film reactors has been shown to limit the toxicity of inhibitor compounds ; l1 indeed, the immobilization of methanogenic bacteria decreases the toxicity of phenolic compounds.” The growth and the accumulation of methanogenic bacteria in the digestor increase the % methane. The biogas production increased (to 45 dm3 dmm3d-l) on increasing the volumetric loading rate (9 and 13 weeks) while the COD removal was maintained at an average of 80%. In reactor D where VSS accumulation was weak, the production of biogas was maintained at O-15 dm3 dmm3d-l. The rate of start-up of fixed film reactors depended strongly on the fraction of the VSS lost in the effluent (wash-out).25 When the volumetric loading rate was increased (9 weeks) the performance was not improved ; in fact, there was some deterioration in biogas production, % methane and COD removal. The yields (m” methane kg-’ of COD removed) were not satisfactory in comparison to theoretical values (0.35 ms methane kg-’ of COD removed) but in spite of the weak methane production, the COD removal was on average 65 % because of the upflow process behaviour. Under all volumetric loading rates, the VFA concentration was on average of O-5 g 111 in both fermenters and the pH of their effluents remained between 75 and 7-8. The yields obtained in both reactors were weak and on average of O-15 m3 methane kg-l of COD removed because of the toxic effect of OMW on methanogenic bacteria and the COD fraction removed by physical retention in the fixed bed reactor. Although the COD was reduced by 80% (reactor C) the darkly coloured polyphenols were not removed. 4
of olive
mill
wastewaters
41
These results show that clay is better than plastic media for a quick start-up and better performance of anaerobic filters treating OMW. The difference between the performances of the different anaerobic filters fed by OMW is due especially to the texture and structure of sludge fixed on different packing media (Table 3). The choice of packing depends on the OMW concentration and the contents of TSS. In certain circumstances, the OMW destined for anaerobic filters must be filtered to lower TSS concentration which can reach 70 g 11.i3 CONCLUSIONS Ca(OH), was more useful than NaOH for the adjustment of the pH of OMW. This may be due to the precipitation of phenolic compounds and LCFA which are toxic to methanogenic bacteria. A fully mixed fermenter can be used to carry out the acidogenesis step of OMW degradation. The anaerobic filter filled with media similar to clay (in porosity and specific area) can be a useful reactor for anaerobic digestion of OMW with a quick start-up. REFERENCES 1. Anderson, G., Donnelly, T. & Rippon, G. M. In Actas I. Congreso National de Quimica. Vigo, 1977, p. 549. 2. Fiestas Ros de Ursinos, J. A. Grussa y Aceites, 17 (1966) 41. 3. Fiestas Ros de Ursinos, J. A. Grussa y Aceites. 28 (1977) 2113. 4. Arpino, A. & Carola, A. Riv. It&. Sostanze Grosse, 55 (1978) 24. 5. Carola, A., Arpino, A. & Lanzani, A. Riv. Ital. Sostanze Grasse, 52, (1975) 335. 6. Antonacci, R., Brunetti, A., Rozzi, A. & Santori, M. Ingegneria Sunitara, 6 (1981) 257. I. Fiestas Ros de Ursinos, J. A., Navarro, G. R., Gamero, R., Leon Gabello, R., Garcia Buendia, A. J. & Mastrrojuan Saez de Jauragui, G. M. Grassa y AceiteJ, 33 (1982) 265. 8. Boari, G., Brunetti, A., Passino, R. & Rozzi, A. Agricultural Wastes, 10 (1984) 161. 9. Rozzi, A., Passino, R. & Limoni, M. Process Biochemistry, 29 (1989) 1988. 10. Khan, K. A., Suidan, M. T. & Cross, W. H. Journal WPCF, 53 (1981) 1519. 11. Parkin, G. F. & Speece, R. E. Wat. Sci. Techn., 15 (1983) 261. 12. Aubart, C. These Docteur Ingenieur, INPL, Nancy, 1982. 13. Balice, V., Boari, G., Cera, 0. & Abbaticchio, P. Inguinamento, 7-8 (1982) 49. 14. Fiestas Ros de Ursinos. J. A. Proc. Int. Svmo. on Olive Byproducts Valorisation, FAO. Monastir, 198f. 15. Salkinoja-Salomen, M. S., Nyns, E. J., Sutton, P. M., Van Den Berg, L.& Wheatly, A. D. Bat. Sci. Techn., 15 (1983) 345. EPB
27
42
M. Hamdi,
C. Festino, C. Aubart
16. American Public Health Association, Standard Methods .for the Examination of Water and Wastewater (14th Edn). American Public Health Association, 1987. 17. Andreoni, V., Ferrari, A., Ranali, G. & Sorlini, C. Proc. Inter. Symp. on Olive Byproducts Valorisation, FAO, 5-7 March 1985, Spain. 18. Koster, I. W. 8c Kramer, A. Appl. Environ. Microbial., 53 (1980) 403. 19. Hamdi, M. Bioresource TeehnoJ., 36 (1991) 173. 20. Field, J. A. & Lettinga, G. Wat. Sci. Tech., 24 (1991) 127. 21. Tschocke, C. Proc. Ink Symp. on Olive Oil Mill Wastewaters Treatment, COI-FAO-APROL, l&17 November 1989, Leece, Italy.
22. Roy, 23. 24. 25.
26.
F., Albagnac, G. & Semain, E. Appl. Environ. Microbial. 49 (1985) 702. Brown, G. J., Lin, K. C., Landine, R. C., Locci, A. A. & Viraraghavan, T. J. J. Env. Eng. Div., 100 (1980) 837. Dwyer, D. F., Krumme, M. L., Boyd, S. A. & Tiedje, J. M. AppJ. Environ. Microbial., 47 (1986) 850. Van Den Berg, L., Lentz, C. P. & Armostrong, D. W. Proc. 6th Int. Fermentation Symp. July 20-25 1980, London, Canada. Rigoni-Stern, S., Rismondo, R., Szpyrkowicz, L. & Zilio Grandi F. Proc. 5th Int. Symp. on Anaerobic Digestion. Bologna, Italy, 22-26 May, ed. A. Tilche & A. Rozzi. 1988, p. 561.
Biochemisrry
27 (1992) 37 42
Anaerobic Digestion of Olive Mill Wastewaters in Fully Mixed Reactors and in Fixed Film Reactors M. Hamdi,
C. Festino
BIOMAGAZ-Laboratoire
& C. Aubart
de Recherches sur les Fermentations, Aspach la Bas, 68700 Cernay, France
(Received 20 September 1990; revised version received 3 July 1991; accepted 25 July 1991)
Calcium hydroxide precipitates phenolic compounds and long chain fatty acids toxic to methanogenic bacteria, and improves the total alkalinity; it is therefore better for the adjustment of the pH of olive mill wastewaters (OMW) than sodium hydroxide. The fur/y mixed reactors are adequate for volatile fatty acid production from OMW, although an anaerobic jilter is advisable for methanogenesis. The anaerobic filter filled with a clay media matrix which is less porous and has a larger spectfic area than PVC, gives a quick start-up and a satisfactory performance.
means of limiting the toxicity of inhibitory compounds.lO~ll This paper describes experiments carried out at BIOMAGAZ when three important problems in the anaerobic digestion of OMW were investigated in laboratory scale reactors:
INTRODUCTION Reports on the anaerobic digestion of olive mill wastewaters (OMW) date to the beginning of this decade1 when biologicaP3 and chemicaP5 treatments were little known or expensive. The advantages of anaerobic biological processes related to energy and saving and low sludge production. Many anaerobic processes such as anaerobic contact,‘*’ UASB’ and anaerobic filters’ have been applied to diluted OMW treatment but problems arose due to the toxicity and biodegradability of this effluent and the acidification of reactors.’ The start-up time of anaerobic filters fed on OMW is appreciably shorter than suspended growth reactors such as those involved in the anaerobic contact process and the UASB.’ In addition, the use of anaerobic fixed film reactors was found to be a
(1) A comparison between Ca(OH), and NaOH on the batch anaerobic digestion of OMW. (2) Anaerobic treatment of OMW in fully mixed reactors at two pH values. (3) Influence of the geometry of media matrix on the start-up and performance of the anaerobic filters.
MATERIALS
Reactors The laboratory digesters described in Figure 1.
Corresponding author: Centre de Biotechnologie de Sfax, BPW, 3038 Sfax. Tunisia. T&phone : (04) 74110; Telex: 40982.
37 Process
Biochemistry
0032.9592/92/$5.00
0
AND METHODS
1992 Elsevier Science Publishers
Ltd,
England.
used in this study are
38
M. Hamdi. C. Festino, C. Aubart Table
1. Chemical
characteristics
of olive mill wastewaters.
PH COD (g dm-“) TS (g dm-“) VS (g dm-3) TSS (g dm “) TA (g dm-‘) VFA (g dm-3) Oil (g dm-“) Reducing sugars (g dmm3) Total Kjeldahl nitrogen (g dmm3) Ammoniacal nitrogen (g dme3) K (g dmm3)
Ca (g dmm3) Na (g dm-3) Mg (g dm-9 NO, (g dm-‘)
Fig. 1. Schematic diagram of the reactors: (A) fully mixed fermenter; (B) fully mixed fermenter; (C) anaerobic filter fikd with clay; (D) anaerobic filter filled with PVC; (G) gasometer; (EF) effluent ; (IN) influent; (PM) pump.
The two fully mixed reactors were constructed of PVC tube 36 cm long and 80 cm internal diameter’” of effective volume 2 dm3. These reactors were placed in a bath held at 35 “C and stirred every 30 s for a period of 2 min, at a speed of 50 rpm. The anaerobic filter column was constructed of a 50 cm long, 8 cm internal diameter plexiglass glass inner tube and had an effective volume of 2 dm”. The inner tube was enclosed in a jacket through which hot water was circulated to maintain the temperature of the filter at 35 “C. A distributer perforated plate was located at the bottom and acted as a feed line. Each reactor was packed with a different medium. One contained corrugated modular plastic blocks having a porosity greater than 95 % and a specific area of 98 m2 rnm3. The corrugated panels were cross-stacked at 60” angles and welded at the contact points. A second reactor contained granular clay (Attapulgite), with a 5 mm diameter. The porosity was 40% and the unit surface area was 125 m2 g-l. Olive mill wastewater, inocula and experimental procedure The anaerobic treatment was operated on OMW provided from the traditional mill and press processes (Southern France). Because of the seasonal production and the instability of the waste,
5.3 1 73.6 48 43.8 7.5 0.55 0.088
1.2 6.8 0.08 0.02
2.6 0.18 0.28 O-084 0.038
200 dm3 of wastewaters was stored at 4 “C and used (raw or diluted) to prepare the feed for experiments. The chemical characteristics of this wastewater are given in Table 1. Further information about these wastewaters can be found in Refs 13 and 14. The COD of feed OMW in the fully mixed reactors was about 50 g dme3. OMW was supplemented with urea at 6 g dmp3. Two parallel reactors (A and B) were used and fed with OMW at pH 7.2 (adjusted by Ca(OH),) and 5.5, respectively. Since the hydraulic retention time in both cases was 20 days, the fully mixed reactors, were fed once a day with 50 ml of OMW, at the same time (volumetric loading rate, 2.5 g COD 1-l d-l). The COD of feed OMW for the anaerobic filters was varied from 10 to 30 g dma3 by dilution. OMW was supplemented with urea (COD: N, 50: 1) and Ca(OH), sufficient to adjust the pH to 7. The feed from the reservoir was pumped into these anaerobic filters at a rate of 10 cm3 min-‘. These pumps were operated for 10 min every 8 h to provide an average hydraulic retention time of 7 days in the reactors under a loading rate between 1 and 5 g of COD dm-” d-l. Problems of start-up were avoided by diluting the waste and increasing the concentration of available nitrogen by addition of urea.* Since mixing of the different sludges improved the start-up of anaerobic fixed film reactors,‘” all reactors were inoculated with the same anaerobic mixed sludge from two activated sludge digestors; the first treating domestic wastewaters, the second treating pig manure. Gas production was recorded daily and the composition of the gas determined once a week. The input and output of OMW were analyzed once a week on a sample taken at random from the week’s inputs and outputs.
Anaerobic
digestion
Analytical methods The determination of chemical oxygen demand (COD) was performed according to standard methods.16 COD was determined on unfiltered and/or filtered samples. Total alkalinity (TA) was determined potentiometrically (end-point pH = 4) and total and volatile solid (TS and VS) were obtained with unfiltered samples. Volatile suspended solids (VSS) were obtained by filtering the samples on GF/C Whatman glass filters drying overnight at 105 “C and weighing before and after three hours in a muffle furnace at 600 “C. Gas flow rates were measured by liquid displacement as described elsewhere or by using a gasometer (Figure 1).12 Gas samples (methane and carbon dioxide concentrations) were taken with a syringe from the headspace of the gasometer and analyzed using a gas chromatograph equipped with a catharometer detector. For the analysis of volatile fatty acids (VFA), liquid samples were centrifuged at 3000 r min-’ for 10 min, acidified by addition of 1% of 50% H,PO, and analyzed with a PYE UNICAM gas chromatograph equipped with a flame ionization detector. The column used was 80 cm stainless steel and packed with 4% H,PO, on Porapak Q (80-100 mesh). Nitrogen was used as carrier gas at 28 cm3 min-’ with hydrogen and air flows of 25 and 30 cm3 min-‘, respectively. The oven, injector and detector temperature was 200 “C.
RESULTS
AND DISCUSSION
Comparison between the effect of Ca(OH), and NaOH on the batch anaerobic digestion of OMW Bottles were filled according to Table 2, the first bottle served as control, while the second and third bottles were used to analyze the effect of the addition of NaOH and Ca(OH), used respectively to adjust the pH of OMW. Less methane was produced in the batch where the pH was adjusted by NaOH than where Ca(OH), was employed (Figure 2). Phenolic acids present in OMW have an adverse effect on methane production even at low concentrations.17 The main long chain fatty acids (LCFA) contained in OMW are unsaturated with oleic acid being the most abundant (65 %). Unsaturated LCFA are more efficient inhibitors of the microbial
c$ olive
mill
39
wastewaters
Table 2. Composition
of batch anaerobic digestion
Batch (400 cm”)
Subsrrare (12 m.u)
1
s
0
2 3
s S
100 100
OMW (cm?
Inocula (cm”)
Water (cm”)
100 100 100
300 200 200
S: 5 mM of acetate; 5 tItM of formate and 2 mM of methanol. (COD : N) was increased by urea addition to an average ratio of 50 in all batches. OMW : 18.2 g 1-l of COD. Inoculum: pig manure (VSS: 17.5 g 1-l) and domestic sludge (VSS: 14-7 g 1-l).
o.55
0
4
2 t-TIME
6
(day)
Fig. 2. Cumulative methane formation with OMW when the pH was adjusted by Ca(OH), (A); or NaOH (0) and control (I)-
formation of methane from acetate than saturated LCFA. The toxicity level of oleic acid on Methanosarcina sp. was 2.4 mM. The toxicity of a mixture of LCFA can be enhanced significantly by synergism between individual LCFA ;18 indeed, it was reported that the rate of methane production decreased when the concentration of OMW increased because of the inhibition of the methanogenic bacteria.lg The positive effect of Ca(OH), compared to that of NaOH can be explained by the effect of calcium on the polymerization and precipitation of the phenolic compoundszO and in the precipitation of LCFA.*l.** The addition of Ca(OH), also improves the total alkalinity (TA).23 Anaerobic treatment of OMW in fully mixed reactors The results involving biogas production, VFA accumulation, percentage of methane and COD removal as a function of time are reported in Figure 3. It can be seen that reactor A is closer to being a
M. Hamdi, C. Festino, C. Aubart
40
80-
16 12 8 4 0 1001
2
I
5
80-
2
60-
g
40-
D p1
20-
”
0
1
8o60 40
20 1 01 10
5
15
,__
0
10
5
15
I
80 60 40 20
i
0-l 0
I 5
10 t-Time
15
(week)
Fig. 3. Biogas production (1-l d-l), % methane, WA concentrations and COD removal, plotted against time for the mixed fermenters A (m) and B (0).
methanogenic reactor, while reactor B is closer to an acidogenic reactor. Thus, biogas production, methane per cent and COD removal are better in reactor A. COD removal in both reactors is of only 40% on average because of the bioconversion of organic matter to VFA. Indeed, VFA concentrations in reactors A and B were, on average, 7 and 13 eq acetate dmm3, respectively. Two reasons may be considered for the high production rate of VFA : the increase in agitation speed which may have increased the growth rate of acidogenic bacteria by improving mass transfer, and the methanogenic bacteria may be inhibited most by phenolic compounds and LCFA contained in OMW especially under agitation. I9 The pH in reactors A and B was stabilized at 5.5 and 5.3, respectively. The fully mixed fermenter can therefore be adequate for VFA production (acidogenesis) from OMW (pH 5.5) at
O+T--T-x t-Time
(Week)
Fig. 4. Volumetric loading rates, biogas production (1-l d-l), % methane and COD removal, plotted against time for anaerobic filters C (I) and D(O).
high concentrations. Acidified OMW with acidogenesis was slightly less toxic to methanogenic bacteria than unmodified OMW.ls Influence of the geometry of media matrix Two anaerobic filters (C and D) operated in parallel and under the same conditions (concentration of OMW and volumetric loading rate) were used to investigate the influence of the geometry of media matrix on the anaerobic digestion of OMW. The mean volumetric loading rates maintained during the experiment and results involving biogas production, percentage of methane and COD removal as a function of time are reported in Figure 4. During weeks 1-9, the production of biogas increased (OalM.3 dm3 dm-’ d-l) with time in reactor C because of the accumulation of VS. The
Anaerobic Table 3. Performance mill wastewaters
data of anaerobic
digestion
filters fed with olive
VolW?le (0
Package used
Load (g Cod 1-l d-‘)
Efficiency (% COD)
21 300 10 10 11
Polyurethane Polyurethane Prisms cubes Cyl. plugs T30 Cyl. plugs TR30 Plastic Clay
2-80 8.00 2-50 2.50 3.00
83 87 60 55 65
26 26 9 9 9
3.00
60
9
2.70 4.40
65 75
II 2 2
Reference
(This work) (This work)
weak porosity of clay appears to trap VSS more effectively than PVC. The use of the anaerobic fixed film reactors has been shown to limit the toxicity of inhibitor compounds ; l1 indeed, the immobilization of methanogenic bacteria decreases the toxicity of phenolic compounds.” The growth and the accumulation of methanogenic bacteria in the digestor increase the % methane. The biogas production increased (to 45 dm3 dmm3d-l) on increasing the volumetric loading rate (9 and 13 weeks) while the COD removal was maintained at an average of 80%. In reactor D where VSS accumulation was weak, the production of biogas was maintained at O-15 dm3 dmm3d-l. The rate of start-up of fixed film reactors depended strongly on the fraction of the VSS lost in the effluent (wash-out).25 When the volumetric loading rate was increased (9 weeks) the performance was not improved ; in fact, there was some deterioration in biogas production, % methane and COD removal. The yields (m” methane kg-’ of COD removed) were not satisfactory in comparison to theoretical values (0.35 ms methane kg-’ of COD removed) but in spite of the weak methane production, the COD removal was on average 65 % because of the upflow process behaviour. Under all volumetric loading rates, the VFA concentration was on average of O-5 g 111 in both fermenters and the pH of their effluents remained between 75 and 7-8. The yields obtained in both reactors were weak and on average of O-15 m3 methane kg-l of COD removed because of the toxic effect of OMW on methanogenic bacteria and the COD fraction removed by physical retention in the fixed bed reactor. Although the COD was reduced by 80% (reactor C) the darkly coloured polyphenols were not removed. 4
of olive
mill
wastewaters
41
These results show that clay is better than plastic media for a quick start-up and better performance of anaerobic filters treating OMW. The difference between the performances of the different anaerobic filters fed by OMW is due especially to the texture and structure of sludge fixed on different packing media (Table 3). The choice of packing depends on the OMW concentration and the contents of TSS. In certain circumstances, the OMW destined for anaerobic filters must be filtered to lower TSS concentration which can reach 70 g 11.i3 CONCLUSIONS Ca(OH), was more useful than NaOH for the adjustment of the pH of OMW. This may be due to the precipitation of phenolic compounds and LCFA which are toxic to methanogenic bacteria. A fully mixed fermenter can be used to carry out the acidogenesis step of OMW degradation. The anaerobic filter filled with media similar to clay (in porosity and specific area) can be a useful reactor for anaerobic digestion of OMW with a quick start-up. REFERENCES 1. Anderson, G., Donnelly, T. & Rippon, G. M. In Actas I. Congreso National de Quimica. Vigo, 1977, p. 549. 2. Fiestas Ros de Ursinos, J. A. Grussa y Aceites, 17 (1966) 41. 3. Fiestas Ros de Ursinos, J. A. Grussa y Aceites. 28 (1977) 2113. 4. Arpino, A. & Carola, A. Riv. It&. Sostanze Grosse, 55 (1978) 24. 5. Carola, A., Arpino, A. & Lanzani, A. Riv. Ital. Sostanze Grasse, 52, (1975) 335. 6. Antonacci, R., Brunetti, A., Rozzi, A. & Santori, M. Ingegneria Sunitara, 6 (1981) 257. I. Fiestas Ros de Ursinos, J. A., Navarro, G. R., Gamero, R., Leon Gabello, R., Garcia Buendia, A. J. & Mastrrojuan Saez de Jauragui, G. M. Grassa y AceiteJ, 33 (1982) 265. 8. Boari, G., Brunetti, A., Passino, R. & Rozzi, A. Agricultural Wastes, 10 (1984) 161. 9. Rozzi, A., Passino, R. & Limoni, M. Process Biochemistry, 29 (1989) 1988. 10. Khan, K. A., Suidan, M. T. & Cross, W. H. Journal WPCF, 53 (1981) 1519. 11. Parkin, G. F. & Speece, R. E. Wat. Sci. Techn., 15 (1983) 261. 12. Aubart, C. These Docteur Ingenieur, INPL, Nancy, 1982. 13. Balice, V., Boari, G., Cera, 0. & Abbaticchio, P. Inguinamento, 7-8 (1982) 49. 14. Fiestas Ros de Ursinos. J. A. Proc. Int. Svmo. on Olive Byproducts Valorisation, FAO. Monastir, 198f. 15. Salkinoja-Salomen, M. S., Nyns, E. J., Sutton, P. M., Van Den Berg, L.& Wheatly, A. D. Bat. Sci. Techn., 15 (1983) 345. EPB
27
42
M. Hamdi,
C. Festino, C. Aubart
16. American Public Health Association, Standard Methods .for the Examination of Water and Wastewater (14th Edn). American Public Health Association, 1987. 17. Andreoni, V., Ferrari, A., Ranali, G. & Sorlini, C. Proc. Inter. Symp. on Olive Byproducts Valorisation, FAO, 5-7 March 1985, Spain. 18. Koster, I. W. 8c Kramer, A. Appl. Environ. Microbial., 53 (1980) 403. 19. Hamdi, M. Bioresource TeehnoJ., 36 (1991) 173. 20. Field, J. A. & Lettinga, G. Wat. Sci. Tech., 24 (1991) 127. 21. Tschocke, C. Proc. Ink Symp. on Olive Oil Mill Wastewaters Treatment, COI-FAO-APROL, l&17 November 1989, Leece, Italy.
22. Roy, 23. 24. 25.
26.
F., Albagnac, G. & Semain, E. Appl. Environ. Microbial. 49 (1985) 702. Brown, G. J., Lin, K. C., Landine, R. C., Locci, A. A. & Viraraghavan, T. J. J. Env. Eng. Div., 100 (1980) 837. Dwyer, D. F., Krumme, M. L., Boyd, S. A. & Tiedje, J. M. AppJ. Environ. Microbial., 47 (1986) 850. Van Den Berg, L., Lentz, C. P. & Armostrong, D. W. Proc. 6th Int. Fermentation Symp. July 20-25 1980, London, Canada. Rigoni-Stern, S., Rismondo, R., Szpyrkowicz, L. & Zilio Grandi F. Proc. 5th Int. Symp. on Anaerobic Digestion. Bologna, Italy, 22-26 May, ed. A. Tilche & A. Rozzi. 1988, p. 561.