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Anaerobic membrane bioreactor for the treatment of leachates from Jebel Chakir discharge in Tunisia
Journal of Hazardous Materials 177 (2010) 918–923
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Anaerobic membrane bioreactor for the treatment of leachates from Jebel Chakir discharge in Tunisia Amal Zayen, Sami Mnif, Fathi Aloui, Firas Fki, Slim Loukil, Mohamed Bouaziz, Sami Sayadi ∗ Laboratoire des Bioprocédés, Pôle d’Excellence Régional AUF (PER-LBP), Centre de Biotechnologie de Sfax, BP, 1177, 3018 Sfax, Tunisia
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Article history: Received 22 August 2009 Received in revised form 25 December 2009 Accepted 4 January 2010 Available online 11 January 2010 Keywords: Anaerobic membrane bioreactor Landfill leachate Methanization
a b s t r a c t Landfill leachate (LFL) collected from the controlled discharge of Jebel Chakir in Tunisia was treated without any physical or chemical pretreatment in an anaerobic membrane bioreactor (AnMBR). The organic loading rate (OLR) in the AnMBR was gradually increased from 1 g COD l−1 d−1 to an average of 6.27 g COD l−1 d−1 . At the highest OLR, the biogas production was more than 3 volumes of biogas per volume of the bioreactor. The volatile suspended solids (VSSs) reached a value of approximately 3 g l−1 in the bioreactor. At stable conditions, the treatment efficiency was high with an average COD reduction of 90% and biogas yield of 0.46 l biogas per g COD removed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In Tunisia, 2.2 millions of tonnes of domestic garbage are collected annually. Landfilling is a fundamental step in any waste management strategy. The site of Jebel Chakir is the first controlled discharge in Tunisia, located at 10 km at the Western South of Tunis. It receives 2000 tonnes of domestic and assimilated waste per day coming from great Tunis since 1999. Approximately 68% of disposed waste are of organic matter origin (food waste), while plastic, paper, leather, metals and textile constitute 11%, 10%, 2%, 4% and 2% of the total waste mass, respectively [1]. Wastes are characterized with high humidity between 65% and 70% [1]. Daily, this site generates around 270 m3 of leachate piped to and stocked, without any treatment, in eight collecting basins of total capacity 130,000 m3 [2]. Since Tunisian LFL is typically heavily loaded by organics, ammonia nitrogen and even harmful compounds such as heavy metals [3], these stocked leachates constitute a real threat to the surrounding environment. Stringent regulations would not allow direct discharge of the leachates neither into a water body nor into the sewer system. Therefore, new treatment processes are needed in order to ensure compliance and safe discharge [2].
Abbreviations: AnMBR, anaerobic membrane bioreactor; BOD, biological oxygen demand; COD, chemical oxygen demand; GC–MS, gas chromatography coupled with mass spectroscopy; HRT, hydraulic retention time; IC, inorganic carbon; LFL, landfill leachate; OLR, organic loading rate; TKN, total Kjeldahl nitrogen; TMP, transmembrane pressure; TOC, total organic carbon; TSSs, total suspended solids; VFAs, volatile fatty acids; VSSs, volatile suspended solids. ∗ Corresponding author. Tel.: +216 74 874 452; fax: +216 74 874 452. E-mail address: [email protected] (S. Sayadi). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.004
In order to find the suitable treatment process, LFL needs to be characterized, an issue which seems to be difficult because their compositions and concentrations depend on a variety of factors, such as waste composition, age of landfill site, temperature, moisture content and other seasonal and hydrological factors [4]. The biological treatments attracts much attention because of its success in the degradation of non-stabilized organic matter of young LFL and its efficiency in the removal of organics and toxic compounds [5]. Despite the several advantages of the anaerobic treatment (i.e. production of usable energy, low sludge production) [6], the development and use of anaerobic processes have been limited to some extent, mainly because of one major drawback: the slow-growing methanogenic bacteria for which doubling time can vary broadly from 12 h to 1 week. The necessity of imposing and keeping long biomass retention time has usually been addressed to either hydraulically (i.e. long retention time) or more often by separating biomass and hydraulic retention times (i.e. solid/liquid separation). Following the development of a new generation of more productive and less expensive filtration membranes, a new concept has emerged in water treatment technology: the membrane bioreactor (MBR) treatment process [7]. MBR was an option for sustainable treatment of LFL. It provides generally greater COD removal than conventional systems for less biodegradable feeds [8] Since LFL is an effluent with highly variable physical, chemical and biological properties, it is often difficult to design a suitable treatment system. Thus, the purpose of this study was to characterize samples at different acidification stage from Jebel Chakir discharge and to investigate the long-term performance of a pilot-scale cross-flow ultrafiltration anaerobic membrane
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Table 1 Physicochemical characteristics of the four samples of landfill leachate.
a
Age (year) Number of samples pH EC (mS cm−1 ) COD (mg l−1 ) BOD5 (mg l−1 ) BOD5 /COD TOC (mg l−1 ) IC (mg l−1 ) SS (g l−1 ) VSS (g l−1 ) TKN (g l−1 ) NH4 + (mg l−1 ) NH4 + /TKN VFAs concentration (g l−1 ) Acetic acid Propionic acid Butyric acid Isobutyric acid Valeric acid Total VFAs (g COD l−1 ) Ions and heavy metals (mg l−1 ) Calcium (Ca) Potassium (K) Sodium (Na) Magnesium (Mg) Iron (Fe) Chrome (Cr) Copper (Cu) Nickel (Ni) Lithium (Li) Lead (Pb) Mercury (Hg) a
A
B
C
D
0.5 3 7.31 ± 0.11 46 ± 0.2 85135 ± 542 48750 ± 310 0.57 24800 200 1.97 ± 0.05 1.46 ± 0.03 3176.6 ± 115 2800 ± 95 0.88
1 3 8.03 ± 0.23 58.8 ± 0.3 18113 ± 420 5300 ± 76 0.29 3000 8000 0.53 ± 0.02 0.26 ± 0.01 5425 ± 230 4968.6 ± 190 0.91
3 3 8.26 ± 0.03 35.5 ± 0.1 12000 ± 552 3300 ± 102 0.27 4500 1000 1.259 ± 0.35 0.723 ± 0.11 1281 ± 95 1211 ± 92 0.94
4 3 8.37 ± 0.1 43.6 ± 0.6 7245.28 ± 621 1300 ± 98 0.18 2600 2500 0.68 ± 0.05 0.316 ± 0.03 3464.3 ± 123 3462.2 ± 115 0.99
0.4232 0.5592 0.4841 2.7504 1.3669 9.94
– 0.167 0.1797 0.2007 1.04
975.5 8.02 20 14.37 67.75 55.3 0.935 0.42 1.585 2.505
81.05 10.27 21 14.31 11.315 – 10.9 0.345 0.47 – –
– – –
2.825 12.565 20.4 14.085 5.56 – – – 0.285 – –
– – –
47.08 8.455 21.3 14.295 18.575 – 1.76 – 0.42 – –
Age mentioned is the age of the leachate from the date of its disgorging into the basins.
bioreactor (AnMBR). The impact of OLR on effluent quality was evaluated. 2. Materials and methods 2.1. Landfill leachate
anaerobic bioreactor is constructed of Plexiglass having a working volume of 50 l and coupled with a cross-flow ultrafiltration membrane with 1 m2 area and 100 kDa cut-off. The temperature was maintained constant at 37 ◦ C. The seed sludge was obtained from a full-scale anaerobic wastewater treatment plant. To maintain an acceptable flux, the ultrafiltration membrane was frequently chemically cleaned (almost every 45 days). The
Different samples of leachate were collected from four storage basins (Table 1) during summer time from the controlled discharge of Jebel Chakir. Samples were stored in drums and transported to the laboratory. If not immediately analysed, samples were stored at 4 ◦ C until use. For the treatment in the AnMBR, high quantity of leachate was collected from the first storage basin (with approximately the same characteristics of sample A) and stored at 4 ◦ C to be used for the feed of the reactor. During the anaerobic treatment, the feed solution was diluted to reduce the COD value. This study attempted to start by diluting the feed solution as a first step (during acclimatization) and then to reduce the dilution factor progressively until managing to feed with LFL with minimum dilution. Indeed, during the three operational periods, the COD of the feed solution was increased from 15 to 30 and to 41 g/l. The HRT was kept constant (HRT = 7 days) during all the treatment and the OLR was increased by the decrease of the dilution of the feed solution. 2.2. Experimental apparatus An anaerobic pilot-scale cross-flow ultrafiltration membrane bioreactor was operated for more than 6 months (Fig. 1). The same AnMBR was previously described by Saddoud et al. [12]. The jet flow
Fig. 1. Schematic diagram of the experimental set-up. 1: raw landfill leachate reservoir, 2: peristaltic feed pump, 3: jet flow anaerobic reactor, 4: circulation pump, 5: flow meter, 6: manometer, 7: ultrafiltration membrane, 8: manometer, 9: permeate tank, 10: permeate discharged, 11: permeate recycling, 12: inner tube, 13: nozzle, 14: gas flow meter.
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chemical cleaning was performed using NaOH (pH 11–12), then sodium bisulfate (pH 5–6). Each cleaning step was performed for 1 h at 35 ◦ C and was followed by water cycle to restore a neutral pH. If the flux was not recovered, an additional cycle with citric acid and 0.5% EDTA was performed. 2.3. Analytical methods The pH and the electrical conductivity were determined using a pH meter model Istek-NeoMet and a conductivimeter model CONSORT C 831, respectively. Soluble COD was determined according to Knechtel [9]. Five-d biological oxygen demand (BOD5 ) was determined by the manometric method with a respirometer (BSB-controller Model 620 T (WTW)). Total Kjeldahl nitrogen was determined as described by Kjeldahl [10]. Total suspended solids (TSSs) and volatile suspended solids (VSSs) were determined according to the standard methods [11]. Volatile fatty acids (VFAs) were analysed by a gas chromatograph (SHIMADZU GC-9A) equipped with a flame ionisation detector (SHIMADZU CR 6A). 2.4. GC–MS analysis GC–MS was carried out to identify hydrocarbons and phenols present in LFL. Samples (40 ml) were extracted two times with dichloromethane (DCM). The organic fraction was evaporated, dissolved in equal volume of DCM and then analysed by gas chromatography mass spectrometry. GC–MS was performed with a Hewelett-Packard model 6890N chromatograph apparatus equipped with a capillary Hewelett-Packard HP-5 column (length, 30 m; internal diameter, 250 m; film thickness, 0.25 m). The carrier gas was helium used at a flow rate of 1 ml min−1 . The temperature was first set at 70 ◦ C for 2 min and was increased to 230 ◦ C at 20 ◦ C min−1 , then to 300 ◦ C at 40 ◦ C min−1 and finally set at 300 ◦ C for 10 min. The control of the GC–MS system and the data peak processing were carried out by means of the MSDCHEM Software. 3. Results and discussion 3.1. Jebel Chakir landfill leachate characterization LFL collected from four different storage basins from the Jebel Chakir discharge were characterized (Table 1). 3.1.1. Physicochemical characteristics Physicochemical characteristics of the four samples are mentioned in Table 1. The conductivity was high for all samples (>35 mS cm−1 ). All samples were characterized with high COD values. However, the youngest leachate presents the highest COD (>85 g l−1 ). This value decreases with the age of leachate to reach 7 g l−1 for sample D (CODD < CODC < CODB ). In the literature, there have been reported a considerable variation in the quality of leachate produced from different landfills in the world. In older studies, authors concluded that leachate from young landfill is characterized by high COD, even several thousands of mg l−1 , while in leachate from old landfill COD concentrations are below few hundreds of mg l−1 [13]. Renou et al. concluded that COD values vary from almost 70 g l−1 in young landfills to 100 mg l−1 in old ones [14]. However, the COD of the youngest leachate sampled from the Jebel Chakir discharge exceeded all this values and even for aged leachate, this value remains high. The distribution of total carbon can give a clear idea about the evolution of the quality of leachate within the age (Table 1). In fact,
for the youngest leachate almost 100% of the carbon was organic. This proportion decreased for older leachate. It is commonly known that organics in leachate are characterized by different biodegradability. A measure of biodegradability is BOD5 /COD ratio. Thus, this ratio was 0.57 for the youngest leachate which suggest that a great part of organic matter in this sample was biodegradable. For leachate aged of 4 years, this ratio was only 0.18 which indicate an important proportion of inert matter. Results obtained showed that ammonia constituted the major TKN component. Similar results were reported in the literature. Values up to 2 and 3 g l−1 (as NH3 -N) were found to be common in leachate from ten sanitary landfills in Hong Kong [15]. It is concluded that, apart from organics, ammonia is the principal pollutant in leachate. The concentrations of NH3 -N in leachate are usually found to be high following hydrolysis and fermentation of the protein fraction of biodegradable substrates. 3.1.2. pH, VFA pH values of LFL from Jebel Chakir showed a nearly constant trend with small variation between 7.31 and 8.37. However, the volatile fatty acids (VFAs) concentration varied widely within the leachate age (Table 1). In fact, leachate A was heavily loaded with VFAs, especially isobutyric acid. No VFAs were determined in leachate C and D and this may be attributed to bioconversion. In young leachate, containing large amounts of biodegradable organic matter, a rapid anaerobic fermentation takes place, resulting in volatile fatty acids (VFA) as the main fermentation products. This early phase of a landfill’s lifetime is called the acidogenic phase, and leads to the release of large quantities of free VFAs [16]. As leachate matures, the methanogenic phase occurs. The organic fraction in the leachate becomes dominated by refractory (nonbiodegradable) compounds such as humic substances [17]. 3.1.3. Heavy metals Landfill leachate sampled from Jebel Chakir discharge seems to be highly loaded by ions and heavy metals (Table 1). According to the literature, high metal concentrations are stated only in young landfills (during the acidification stage) because of high degree of metal solubilization as a result of low pH caused by organic acids production. As the landfill age increased, further increase in pH values caused a certain decrease in metal solubility. This results in a drastic fall of the metal concentrations [17]. According to results shown in Table 1, there is a tendency to decrease for most heavy metals. 3.2. Landfill leachate treatment in anaerobic membrane bioreactor 3.2.1. Methanization performance During a period of 30 days, the OLR was increased progressively to reach a value of 1 g COD l−1 d−1 . With a feed solution of 8 g l−1 of COD (CODf ), the COD permeate (CODp ) remained below 0.5 g l−1 (data not shown). During this period, the concentration of the biomass increased in the AnMBR. At the end of the start-up period, VSS reached a value of 0.8 g l−1 . After the start-up period, the OLR was increased in the AnMBR. A value of 2.24 g COD l−1 d−1 was applied until the day 30. The CODp revealed an average of 1 g l−1 (Fig. 2). However, biogas productivity increased rapidly to reach a maximum value of 1.36 l l−1 d−1 and the biogas yield (l biogas per g of CODremoved ), reached an average of 0.45 (Fig. 3). Moreover, the VFAs concentration in the permeate remained relatively low with an average concentration of 0.53 g l−1 which proved that they were metabolized into biogas. Between days 31 and 123, the OLR was increased to 4.66 g COD l−1 d−1 . The bioreactor showed a drastically decrease of its performance. By the day 87, the CODp attained a maximum value
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Fig. 2. Evolution of CODf and CODp during the treatment of LFL in the AnMBR. Fig. 4. VFAs concentration in the permeate of the AnMBR operated at three organic loading rates.
Fig. 5. Evolution of the biomass concentration in the AnMBR.
Fig. 3. Evolution of biogas productivity (a) and biogas yield (b) during methanization of LFL in the AnMBR.
of 5.9 g l−1 (Fig. 2). The COD removal (CODr ) efficiency decreased to 80%. In parallel, the biogas productivity and the biogas yield declined, respectively to 0.42 l l−1 d−1 and 0.07 l biogas per g of CODr (Fig. 3). This perturbation in the bioreactor could be due to VFAs accumulation (Fig. 4). Since the increase of the OLR, the acetic acid concentration increased to reach a maximum value of 2.59 g l−1 in day 79. The pH of the reactor was not a good indicator of its acidification. Indeed, the high alkalinity inside the bioreactor (4350 mg CaCO3 l−1 ) leads to low variations of the pH values. During this perturbation, the reactor pH did not decrease below 7.5 which mean that monitoring volatile acidity more frequently is the only reliable indicator of methanization failure in this case. To allow methanogenic bacteria consuming intermediates and residual COD, the feeding of the AnMBR was stopped for 3 days.
Just after this interruption, the CODp decreased to 4 g l−1 and the biogas yield increased progressively. The same OLR of 4.66 g COD l−1 d−1 was re-applied. Subsequently, the AnMBR showed a better performance: an average CODp of 3.8 g l−1 (Fig. 2), a maximum biogas productivity of 2.5 l l−1 d−1 , a biogas yield with an average of 0.48 (Fig. 3) and finally, low VFA concentrations in the bioreactor and the permeate (Fig. 4) were recorded. Between the days 124 and 152, an OLR of 6.27 g COD l−1 d−1 was applied. During this experimental period, the performance of the AnMBR was stable. The CODp had an average of 3.8 g l−1 . The biogas yield kept a value of 0.5 with a maximum biogas productivity of 3.3 l l−1 d−1 . Table 2 summarizes the process parameters values. Few research studies are related to landfill leachate purification by membrane bioreactors. In addition, the majority of researchers studied LFL treatment on an aerobic MBR and with low COD values for the feed solution [14]. 3.2.2. Biomass growth inside the AnMBR Anaerobic bacteria are known for their slow growth, resulting in a relatively small population size [18]. The ultrafiltration membrane with a cut-off of 100 kDa was capable to retain all the bacteria, avoid their wash-out, and consequently increase its concentration inside the bioreactor. However, the biomass showed a very slow
Table 2 Summary of the AnMBR performance. Operation period
HRT (d)
OLR (g COD l−1 d−1 )
CODf (g l−1 )
CODp (g l−1 )
COD removal (%)
Biogas yield
OLR 1 OLR 2 OLR 3
7 7 7
2.24 ± 0.35 4.66 ± 1.23 6.27 ± 0.78
14.87 ± 0.88 30.8 ± 3.15 41 ± 3.14
1.17 ± 0.21 2.96 ± 1.23 3.77 ± 0.34
92.0 ± 1.3 88.8 ± 6.8 90.7 ± 1.1
0.45 ± 0.08 0.37 ± 0.15 0.48 ± 0.09
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Fig. 6. GC–MS chromatogram of ethylacetate-extractable products. The MS-identified compounds with respect to their retention time are the following: (a) untreated LFL: 4.387: phenol; 4.769: hexanoic acid; 5.587: heptanoic acid; 5.887: cyclohexanecarboxylic acid; 6.075: hexanamide; 6.328: octanoic acid; 6.540: 2-piperidinone; 7.463: benzenepropanoic acid; 7.640: pyridine; 7.810: 1-tetradecene; 7.857: tetradecane; 7.893: 7-methylindole; 9.081: 1-hexadecene; 9.122: hexadecane; 10.210: E-15-heptadecenal; 10.245: octadecane; 10.628: caffeine; 10.898: 1-docosene; 10.969: methyl-3-(3,5-diterbutyl-4-hydroxyphenyl) propionate; 10.022: cyclohexadecane; 11.104: cycloeicosane; 11.798: 7,9-di-tert-butyl-1-oxaspiro[4.5]deca-6,9-diene-2,8-dione. (b) Permeate: 9.081: 1-hexadecene; 9.122: hexadecane; 10.210: E-15-heptadecenal; 10.898: 1-docosene; 11.104: 1-octadecene; 11.798: 7,9-di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione.
growth during the first 5 months of system operation. Indeed, it reached only 3 g l−1 at the end of the third operational period. Fig. 5 shows that the biomass inside the anaerobic digester exhibited frequent fluctuations. Indeed from days 36 to 41, VSS declined to reach a drastic value (0.44 g l−1 ). This decline in the biomass concentration might be due to oxygen entrance inside the bioreactor during the chemical cleaning of the ultrafiltration membrane. Furthermore, LFL exerted a toxic effect on biomass due to its high concentrations in hydrocarbons and heavy metals. 3.2.3. Organics degradation in the AnMBR Fig. 6a shows that LFL was highly loaded with hydrocarbons and phenols. These substances were reported as toxic for biological growth [19]. The challenge of this study was to investigate if adapted anaerobic bacteria were able to degrade such compounds. The GC–MS analysis (Fig. 6b) showed a complete removal of compounds having retention times below 8 min (hexanoic acid, heptanoic acid, cyclohexanecarboxylic acid; octanoic
acid, etc.). For the others (hexadecane, docosene, octadecene, etc.), they seem to be slightly biodegradable by anaerobic bacteria. The same substances are found in the permeate but at lower concentrations. 3.2.4. Filtration performance The ultrafiltration membrane was a key player regarding process efficiency. Indeed, the high molecular-mass substances were retained by this physical barrier. Their residence time was then increased in the bioreactor which resulted in more adapted anaerobic consortia. In order to avoid fouling, the cross-flow velocity was fixed to a relatively high value of 3 m s−1 and the trans-membrane pressure varied from 1 to 2 bar. However, ultrafiltration membrane showed a low flux. AnMBR are reported to be much more fouling than aerobic ones [20]. Fig. 7 shows the permeate flux evolution during two cycles of filtration. After cleaning chemically the ultrafiltration membrane, the flux declined in few days from 8.3 to 2.5 l h−1 m−2 .
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Research and Technology and AUF are gratefully acknowledged. We are grateful to the ANGED, Tunisia for the help for the collect of samples of LFL. References
Fig. 7. Permeate flux evolution during 2 cycles of filtration. The arrow indicates the chemical cleaning of the ultrafiltration membrane.
4. Conclusions The overall results suggest that AnMBR is a promising process to treat landfill leachate with high efficiency in terms of degradation yield and biogas productivity. It achieved a COD removal up to 90%. At high OLR of 6.27 g COD l−1 d−1 , the biogas productivity was more than 3 l l−1 d−1 . At the beginning of each phase, when OLR was increased, there was an increase of VFAs concentration and then a decrease in the removal efficiency but the system recovered shortly and adapted to the new conditions with time. The bacteria showed a slow growth rate, VSS reached only 3 g l−1 after 5 months of system operation. However, hydrocarbons and organics were efficiently degraded. This efficiency was possible by the adaptation of the consortium to the complex nature of LFL as a result of the increase of its residence time due to membrane filtration. The ultrafiltration membrane showed low fluxes and frequent chemical cleaning cycles were necessary to maintain an acceptable flux. Acknowledgment Financial assistance under the project PRF, “Eau” (2004–2007) from the Tunisian Minister for Superior Teaching, Scientific
[1] ANGed: Agence Nationale de la Gestion des Déchets, 2007. [2] C. Tizaoui, L. Bousselmi, L. Mansouri, A. Ghrabi, Landfill leachate treatment with ozone and ozone/hydrogen peroxide systems, J. Hazard. Mater. 140 (2007) 316–324. [3] M. Ellouze, F. Aloui, S. Sayadi, Detoxification of Tunisian landfill leachates by selected fungi, J. Hazard. Mater. 150 (2008) 642–648. [4] R. Connolly, Y. Zhao, G. Sun, S. Allenk, Removal of ammoniacal-nitrogen from an artificial landfill leachate in down flow reed beds, Process. Biochem. 39 (2004) 1971–1976. [5] Y.L. Lin, J.-G. Sah, Nitrogen removal using carroussel-activated sludge treatment process, J. Environ. Sci. Health A37 (2002) 1607–1620. [6] M. Farhadian, M. Borghei, V.V. Umrania, Treatment of beet sugar wastewater by UAFB bioprocess, Bioresour. Technol. 98 (2007) 3080–3083. [7] F. Feki, F. Aloui, M. Feki, S. Sayadi, Electrochemical oxidation post-treatment of landfill leachates treated with membrane bioreactor, Chemosphere 75 (2009) 256–260. [8] J. Zhang, S.I. Padmasiria, M. Fitchc, B. Norddahld, L. Raskina, E. Morgenroth, Influence of cleaning frequency and membrane history on fouling in an anaerobic membrane bioreactor, Desalination 207 (2007) 153–166. [9] R.J. Knechtel, A more economical method for the determination of chemical oxygen demand, J. Water Pollut. Control (1978) 25–29. [10] J. Kjeldahl, A new method for the determination of nitrogen in organic matter, Z. Anal. Chem. 22 (1883) 366. [11] APHA, American Public Health Association, 18th ed., Washington, DC (1992). [12] A. Saddoud, M. Ellouze, A. Dhouib, S. Sayadi, Anaerobic membrane bioreactor treatment of domestic wastewater in Tunisia, Desalination 207 (2007) 205–215. [13] D. Kulikowska, E. Klimiuk, The effect of landfill age on municipal leachate composition, Bioresour. Technol. 99 (2008) 5981–5985. [14] S. Renou, J.G. Givaudan, S. Poulain, F. Dirassouyan, P. Moulin, Landfill leachate treatment: review and opportunity, J. Hazard. Mater. 150 (2008) 468– 493. [15] M.-C. Lo, Characteristics and treatment of leachates from domestic landfills, Environ. Int. 22 (4) (1996) 433–442. [16] U. Welander, T. Henryssow, T. Welander, Nitrification of landfill leachate using suspended-carrier biofilm technology, Water. Res. 31 (9) (1997) 2351– 2355. [17] J. Harsem, Identification of organic compounds in leachate from a waste tip, Water Res. 17 (1983) 699–705. [18] B. Montero, J.L. Garcia-Morales, D. Sales, R. Solera, Evolution of microorganisms in thermophilic–dry anaerobic digestion, Bioresour. Technol. 99 (2008) 3233–3243. [19] Y. Xu, Y. Zhou Yiqi, D. Wang, S. Chen, J. Liu, Z. Wang, Occurrence and removal of organic micropollutants in the treatment of landfill leachate by combined anaerobic—membrane bioreactor technology, J. Environ. Sci. China 20 (2008) 1281–1287. [20] A. Saddoud, S. Sayadi, Application of acidogenic fixed-bed reactor prior to anaerobic membrane bioreactor for sustainable slaughterhouse wastewater treatment, J. Hazard. Mater. 149 (2007) 700–706.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Anaerobic membrane bioreactor for the treatment of leachates from Jebel Chakir discharge in Tunisia Amal Zayen, Sami Mnif, Fathi Aloui, Firas Fki, Slim Loukil, Mohamed Bouaziz, Sami Sayadi ∗ Laboratoire des Bioprocédés, Pôle d’Excellence Régional AUF (PER-LBP), Centre de Biotechnologie de Sfax, BP, 1177, 3018 Sfax, Tunisia
a r t i c l e
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Article history: Received 22 August 2009 Received in revised form 25 December 2009 Accepted 4 January 2010 Available online 11 January 2010 Keywords: Anaerobic membrane bioreactor Landfill leachate Methanization
a b s t r a c t Landfill leachate (LFL) collected from the controlled discharge of Jebel Chakir in Tunisia was treated without any physical or chemical pretreatment in an anaerobic membrane bioreactor (AnMBR). The organic loading rate (OLR) in the AnMBR was gradually increased from 1 g COD l−1 d−1 to an average of 6.27 g COD l−1 d−1 . At the highest OLR, the biogas production was more than 3 volumes of biogas per volume of the bioreactor. The volatile suspended solids (VSSs) reached a value of approximately 3 g l−1 in the bioreactor. At stable conditions, the treatment efficiency was high with an average COD reduction of 90% and biogas yield of 0.46 l biogas per g COD removed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In Tunisia, 2.2 millions of tonnes of domestic garbage are collected annually. Landfilling is a fundamental step in any waste management strategy. The site of Jebel Chakir is the first controlled discharge in Tunisia, located at 10 km at the Western South of Tunis. It receives 2000 tonnes of domestic and assimilated waste per day coming from great Tunis since 1999. Approximately 68% of disposed waste are of organic matter origin (food waste), while plastic, paper, leather, metals and textile constitute 11%, 10%, 2%, 4% and 2% of the total waste mass, respectively [1]. Wastes are characterized with high humidity between 65% and 70% [1]. Daily, this site generates around 270 m3 of leachate piped to and stocked, without any treatment, in eight collecting basins of total capacity 130,000 m3 [2]. Since Tunisian LFL is typically heavily loaded by organics, ammonia nitrogen and even harmful compounds such as heavy metals [3], these stocked leachates constitute a real threat to the surrounding environment. Stringent regulations would not allow direct discharge of the leachates neither into a water body nor into the sewer system. Therefore, new treatment processes are needed in order to ensure compliance and safe discharge [2].
Abbreviations: AnMBR, anaerobic membrane bioreactor; BOD, biological oxygen demand; COD, chemical oxygen demand; GC–MS, gas chromatography coupled with mass spectroscopy; HRT, hydraulic retention time; IC, inorganic carbon; LFL, landfill leachate; OLR, organic loading rate; TKN, total Kjeldahl nitrogen; TMP, transmembrane pressure; TOC, total organic carbon; TSSs, total suspended solids; VFAs, volatile fatty acids; VSSs, volatile suspended solids. ∗ Corresponding author. Tel.: +216 74 874 452; fax: +216 74 874 452. E-mail address: [email protected] (S. Sayadi). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.004
In order to find the suitable treatment process, LFL needs to be characterized, an issue which seems to be difficult because their compositions and concentrations depend on a variety of factors, such as waste composition, age of landfill site, temperature, moisture content and other seasonal and hydrological factors [4]. The biological treatments attracts much attention because of its success in the degradation of non-stabilized organic matter of young LFL and its efficiency in the removal of organics and toxic compounds [5]. Despite the several advantages of the anaerobic treatment (i.e. production of usable energy, low sludge production) [6], the development and use of anaerobic processes have been limited to some extent, mainly because of one major drawback: the slow-growing methanogenic bacteria for which doubling time can vary broadly from 12 h to 1 week. The necessity of imposing and keeping long biomass retention time has usually been addressed to either hydraulically (i.e. long retention time) or more often by separating biomass and hydraulic retention times (i.e. solid/liquid separation). Following the development of a new generation of more productive and less expensive filtration membranes, a new concept has emerged in water treatment technology: the membrane bioreactor (MBR) treatment process [7]. MBR was an option for sustainable treatment of LFL. It provides generally greater COD removal than conventional systems for less biodegradable feeds [8] Since LFL is an effluent with highly variable physical, chemical and biological properties, it is often difficult to design a suitable treatment system. Thus, the purpose of this study was to characterize samples at different acidification stage from Jebel Chakir discharge and to investigate the long-term performance of a pilot-scale cross-flow ultrafiltration anaerobic membrane
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Table 1 Physicochemical characteristics of the four samples of landfill leachate.
a
Age (year) Number of samples pH EC (mS cm−1 ) COD (mg l−1 ) BOD5 (mg l−1 ) BOD5 /COD TOC (mg l−1 ) IC (mg l−1 ) SS (g l−1 ) VSS (g l−1 ) TKN (g l−1 ) NH4 + (mg l−1 ) NH4 + /TKN VFAs concentration (g l−1 ) Acetic acid Propionic acid Butyric acid Isobutyric acid Valeric acid Total VFAs (g COD l−1 ) Ions and heavy metals (mg l−1 ) Calcium (Ca) Potassium (K) Sodium (Na) Magnesium (Mg) Iron (Fe) Chrome (Cr) Copper (Cu) Nickel (Ni) Lithium (Li) Lead (Pb) Mercury (Hg) a
A
B
C
D
0.5 3 7.31 ± 0.11 46 ± 0.2 85135 ± 542 48750 ± 310 0.57 24800 200 1.97 ± 0.05 1.46 ± 0.03 3176.6 ± 115 2800 ± 95 0.88
1 3 8.03 ± 0.23 58.8 ± 0.3 18113 ± 420 5300 ± 76 0.29 3000 8000 0.53 ± 0.02 0.26 ± 0.01 5425 ± 230 4968.6 ± 190 0.91
3 3 8.26 ± 0.03 35.5 ± 0.1 12000 ± 552 3300 ± 102 0.27 4500 1000 1.259 ± 0.35 0.723 ± 0.11 1281 ± 95 1211 ± 92 0.94
4 3 8.37 ± 0.1 43.6 ± 0.6 7245.28 ± 621 1300 ± 98 0.18 2600 2500 0.68 ± 0.05 0.316 ± 0.03 3464.3 ± 123 3462.2 ± 115 0.99
0.4232 0.5592 0.4841 2.7504 1.3669 9.94
– 0.167 0.1797 0.2007 1.04
975.5 8.02 20 14.37 67.75 55.3 0.935 0.42 1.585 2.505
81.05 10.27 21 14.31 11.315 – 10.9 0.345 0.47 – –
– – –
2.825 12.565 20.4 14.085 5.56 – – – 0.285 – –
– – –
47.08 8.455 21.3 14.295 18.575 – 1.76 – 0.42 – –
Age mentioned is the age of the leachate from the date of its disgorging into the basins.
bioreactor (AnMBR). The impact of OLR on effluent quality was evaluated. 2. Materials and methods 2.1. Landfill leachate
anaerobic bioreactor is constructed of Plexiglass having a working volume of 50 l and coupled with a cross-flow ultrafiltration membrane with 1 m2 area and 100 kDa cut-off. The temperature was maintained constant at 37 ◦ C. The seed sludge was obtained from a full-scale anaerobic wastewater treatment plant. To maintain an acceptable flux, the ultrafiltration membrane was frequently chemically cleaned (almost every 45 days). The
Different samples of leachate were collected from four storage basins (Table 1) during summer time from the controlled discharge of Jebel Chakir. Samples were stored in drums and transported to the laboratory. If not immediately analysed, samples were stored at 4 ◦ C until use. For the treatment in the AnMBR, high quantity of leachate was collected from the first storage basin (with approximately the same characteristics of sample A) and stored at 4 ◦ C to be used for the feed of the reactor. During the anaerobic treatment, the feed solution was diluted to reduce the COD value. This study attempted to start by diluting the feed solution as a first step (during acclimatization) and then to reduce the dilution factor progressively until managing to feed with LFL with minimum dilution. Indeed, during the three operational periods, the COD of the feed solution was increased from 15 to 30 and to 41 g/l. The HRT was kept constant (HRT = 7 days) during all the treatment and the OLR was increased by the decrease of the dilution of the feed solution. 2.2. Experimental apparatus An anaerobic pilot-scale cross-flow ultrafiltration membrane bioreactor was operated for more than 6 months (Fig. 1). The same AnMBR was previously described by Saddoud et al. [12]. The jet flow
Fig. 1. Schematic diagram of the experimental set-up. 1: raw landfill leachate reservoir, 2: peristaltic feed pump, 3: jet flow anaerobic reactor, 4: circulation pump, 5: flow meter, 6: manometer, 7: ultrafiltration membrane, 8: manometer, 9: permeate tank, 10: permeate discharged, 11: permeate recycling, 12: inner tube, 13: nozzle, 14: gas flow meter.
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chemical cleaning was performed using NaOH (pH 11–12), then sodium bisulfate (pH 5–6). Each cleaning step was performed for 1 h at 35 ◦ C and was followed by water cycle to restore a neutral pH. If the flux was not recovered, an additional cycle with citric acid and 0.5% EDTA was performed. 2.3. Analytical methods The pH and the electrical conductivity were determined using a pH meter model Istek-NeoMet and a conductivimeter model CONSORT C 831, respectively. Soluble COD was determined according to Knechtel [9]. Five-d biological oxygen demand (BOD5 ) was determined by the manometric method with a respirometer (BSB-controller Model 620 T (WTW)). Total Kjeldahl nitrogen was determined as described by Kjeldahl [10]. Total suspended solids (TSSs) and volatile suspended solids (VSSs) were determined according to the standard methods [11]. Volatile fatty acids (VFAs) were analysed by a gas chromatograph (SHIMADZU GC-9A) equipped with a flame ionisation detector (SHIMADZU CR 6A). 2.4. GC–MS analysis GC–MS was carried out to identify hydrocarbons and phenols present in LFL. Samples (40 ml) were extracted two times with dichloromethane (DCM). The organic fraction was evaporated, dissolved in equal volume of DCM and then analysed by gas chromatography mass spectrometry. GC–MS was performed with a Hewelett-Packard model 6890N chromatograph apparatus equipped with a capillary Hewelett-Packard HP-5 column (length, 30 m; internal diameter, 250 m; film thickness, 0.25 m). The carrier gas was helium used at a flow rate of 1 ml min−1 . The temperature was first set at 70 ◦ C for 2 min and was increased to 230 ◦ C at 20 ◦ C min−1 , then to 300 ◦ C at 40 ◦ C min−1 and finally set at 300 ◦ C for 10 min. The control of the GC–MS system and the data peak processing were carried out by means of the MSDCHEM Software. 3. Results and discussion 3.1. Jebel Chakir landfill leachate characterization LFL collected from four different storage basins from the Jebel Chakir discharge were characterized (Table 1). 3.1.1. Physicochemical characteristics Physicochemical characteristics of the four samples are mentioned in Table 1. The conductivity was high for all samples (>35 mS cm−1 ). All samples were characterized with high COD values. However, the youngest leachate presents the highest COD (>85 g l−1 ). This value decreases with the age of leachate to reach 7 g l−1 for sample D (CODD < CODC < CODB ). In the literature, there have been reported a considerable variation in the quality of leachate produced from different landfills in the world. In older studies, authors concluded that leachate from young landfill is characterized by high COD, even several thousands of mg l−1 , while in leachate from old landfill COD concentrations are below few hundreds of mg l−1 [13]. Renou et al. concluded that COD values vary from almost 70 g l−1 in young landfills to 100 mg l−1 in old ones [14]. However, the COD of the youngest leachate sampled from the Jebel Chakir discharge exceeded all this values and even for aged leachate, this value remains high. The distribution of total carbon can give a clear idea about the evolution of the quality of leachate within the age (Table 1). In fact,
for the youngest leachate almost 100% of the carbon was organic. This proportion decreased for older leachate. It is commonly known that organics in leachate are characterized by different biodegradability. A measure of biodegradability is BOD5 /COD ratio. Thus, this ratio was 0.57 for the youngest leachate which suggest that a great part of organic matter in this sample was biodegradable. For leachate aged of 4 years, this ratio was only 0.18 which indicate an important proportion of inert matter. Results obtained showed that ammonia constituted the major TKN component. Similar results were reported in the literature. Values up to 2 and 3 g l−1 (as NH3 -N) were found to be common in leachate from ten sanitary landfills in Hong Kong [15]. It is concluded that, apart from organics, ammonia is the principal pollutant in leachate. The concentrations of NH3 -N in leachate are usually found to be high following hydrolysis and fermentation of the protein fraction of biodegradable substrates. 3.1.2. pH, VFA pH values of LFL from Jebel Chakir showed a nearly constant trend with small variation between 7.31 and 8.37. However, the volatile fatty acids (VFAs) concentration varied widely within the leachate age (Table 1). In fact, leachate A was heavily loaded with VFAs, especially isobutyric acid. No VFAs were determined in leachate C and D and this may be attributed to bioconversion. In young leachate, containing large amounts of biodegradable organic matter, a rapid anaerobic fermentation takes place, resulting in volatile fatty acids (VFA) as the main fermentation products. This early phase of a landfill’s lifetime is called the acidogenic phase, and leads to the release of large quantities of free VFAs [16]. As leachate matures, the methanogenic phase occurs. The organic fraction in the leachate becomes dominated by refractory (nonbiodegradable) compounds such as humic substances [17]. 3.1.3. Heavy metals Landfill leachate sampled from Jebel Chakir discharge seems to be highly loaded by ions and heavy metals (Table 1). According to the literature, high metal concentrations are stated only in young landfills (during the acidification stage) because of high degree of metal solubilization as a result of low pH caused by organic acids production. As the landfill age increased, further increase in pH values caused a certain decrease in metal solubility. This results in a drastic fall of the metal concentrations [17]. According to results shown in Table 1, there is a tendency to decrease for most heavy metals. 3.2. Landfill leachate treatment in anaerobic membrane bioreactor 3.2.1. Methanization performance During a period of 30 days, the OLR was increased progressively to reach a value of 1 g COD l−1 d−1 . With a feed solution of 8 g l−1 of COD (CODf ), the COD permeate (CODp ) remained below 0.5 g l−1 (data not shown). During this period, the concentration of the biomass increased in the AnMBR. At the end of the start-up period, VSS reached a value of 0.8 g l−1 . After the start-up period, the OLR was increased in the AnMBR. A value of 2.24 g COD l−1 d−1 was applied until the day 30. The CODp revealed an average of 1 g l−1 (Fig. 2). However, biogas productivity increased rapidly to reach a maximum value of 1.36 l l−1 d−1 and the biogas yield (l biogas per g of CODremoved ), reached an average of 0.45 (Fig. 3). Moreover, the VFAs concentration in the permeate remained relatively low with an average concentration of 0.53 g l−1 which proved that they were metabolized into biogas. Between days 31 and 123, the OLR was increased to 4.66 g COD l−1 d−1 . The bioreactor showed a drastically decrease of its performance. By the day 87, the CODp attained a maximum value
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Fig. 2. Evolution of CODf and CODp during the treatment of LFL in the AnMBR. Fig. 4. VFAs concentration in the permeate of the AnMBR operated at three organic loading rates.
Fig. 5. Evolution of the biomass concentration in the AnMBR.
Fig. 3. Evolution of biogas productivity (a) and biogas yield (b) during methanization of LFL in the AnMBR.
of 5.9 g l−1 (Fig. 2). The COD removal (CODr ) efficiency decreased to 80%. In parallel, the biogas productivity and the biogas yield declined, respectively to 0.42 l l−1 d−1 and 0.07 l biogas per g of CODr (Fig. 3). This perturbation in the bioreactor could be due to VFAs accumulation (Fig. 4). Since the increase of the OLR, the acetic acid concentration increased to reach a maximum value of 2.59 g l−1 in day 79. The pH of the reactor was not a good indicator of its acidification. Indeed, the high alkalinity inside the bioreactor (4350 mg CaCO3 l−1 ) leads to low variations of the pH values. During this perturbation, the reactor pH did not decrease below 7.5 which mean that monitoring volatile acidity more frequently is the only reliable indicator of methanization failure in this case. To allow methanogenic bacteria consuming intermediates and residual COD, the feeding of the AnMBR was stopped for 3 days.
Just after this interruption, the CODp decreased to 4 g l−1 and the biogas yield increased progressively. The same OLR of 4.66 g COD l−1 d−1 was re-applied. Subsequently, the AnMBR showed a better performance: an average CODp of 3.8 g l−1 (Fig. 2), a maximum biogas productivity of 2.5 l l−1 d−1 , a biogas yield with an average of 0.48 (Fig. 3) and finally, low VFA concentrations in the bioreactor and the permeate (Fig. 4) were recorded. Between the days 124 and 152, an OLR of 6.27 g COD l−1 d−1 was applied. During this experimental period, the performance of the AnMBR was stable. The CODp had an average of 3.8 g l−1 . The biogas yield kept a value of 0.5 with a maximum biogas productivity of 3.3 l l−1 d−1 . Table 2 summarizes the process parameters values. Few research studies are related to landfill leachate purification by membrane bioreactors. In addition, the majority of researchers studied LFL treatment on an aerobic MBR and with low COD values for the feed solution [14]. 3.2.2. Biomass growth inside the AnMBR Anaerobic bacteria are known for their slow growth, resulting in a relatively small population size [18]. The ultrafiltration membrane with a cut-off of 100 kDa was capable to retain all the bacteria, avoid their wash-out, and consequently increase its concentration inside the bioreactor. However, the biomass showed a very slow
Table 2 Summary of the AnMBR performance. Operation period
HRT (d)
OLR (g COD l−1 d−1 )
CODf (g l−1 )
CODp (g l−1 )
COD removal (%)
Biogas yield
OLR 1 OLR 2 OLR 3
7 7 7
2.24 ± 0.35 4.66 ± 1.23 6.27 ± 0.78
14.87 ± 0.88 30.8 ± 3.15 41 ± 3.14
1.17 ± 0.21 2.96 ± 1.23 3.77 ± 0.34
92.0 ± 1.3 88.8 ± 6.8 90.7 ± 1.1
0.45 ± 0.08 0.37 ± 0.15 0.48 ± 0.09
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Fig. 6. GC–MS chromatogram of ethylacetate-extractable products. The MS-identified compounds with respect to their retention time are the following: (a) untreated LFL: 4.387: phenol; 4.769: hexanoic acid; 5.587: heptanoic acid; 5.887: cyclohexanecarboxylic acid; 6.075: hexanamide; 6.328: octanoic acid; 6.540: 2-piperidinone; 7.463: benzenepropanoic acid; 7.640: pyridine; 7.810: 1-tetradecene; 7.857: tetradecane; 7.893: 7-methylindole; 9.081: 1-hexadecene; 9.122: hexadecane; 10.210: E-15-heptadecenal; 10.245: octadecane; 10.628: caffeine; 10.898: 1-docosene; 10.969: methyl-3-(3,5-diterbutyl-4-hydroxyphenyl) propionate; 10.022: cyclohexadecane; 11.104: cycloeicosane; 11.798: 7,9-di-tert-butyl-1-oxaspiro[4.5]deca-6,9-diene-2,8-dione. (b) Permeate: 9.081: 1-hexadecene; 9.122: hexadecane; 10.210: E-15-heptadecenal; 10.898: 1-docosene; 11.104: 1-octadecene; 11.798: 7,9-di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione.
growth during the first 5 months of system operation. Indeed, it reached only 3 g l−1 at the end of the third operational period. Fig. 5 shows that the biomass inside the anaerobic digester exhibited frequent fluctuations. Indeed from days 36 to 41, VSS declined to reach a drastic value (0.44 g l−1 ). This decline in the biomass concentration might be due to oxygen entrance inside the bioreactor during the chemical cleaning of the ultrafiltration membrane. Furthermore, LFL exerted a toxic effect on biomass due to its high concentrations in hydrocarbons and heavy metals. 3.2.3. Organics degradation in the AnMBR Fig. 6a shows that LFL was highly loaded with hydrocarbons and phenols. These substances were reported as toxic for biological growth [19]. The challenge of this study was to investigate if adapted anaerobic bacteria were able to degrade such compounds. The GC–MS analysis (Fig. 6b) showed a complete removal of compounds having retention times below 8 min (hexanoic acid, heptanoic acid, cyclohexanecarboxylic acid; octanoic
acid, etc.). For the others (hexadecane, docosene, octadecene, etc.), they seem to be slightly biodegradable by anaerobic bacteria. The same substances are found in the permeate but at lower concentrations. 3.2.4. Filtration performance The ultrafiltration membrane was a key player regarding process efficiency. Indeed, the high molecular-mass substances were retained by this physical barrier. Their residence time was then increased in the bioreactor which resulted in more adapted anaerobic consortia. In order to avoid fouling, the cross-flow velocity was fixed to a relatively high value of 3 m s−1 and the trans-membrane pressure varied from 1 to 2 bar. However, ultrafiltration membrane showed a low flux. AnMBR are reported to be much more fouling than aerobic ones [20]. Fig. 7 shows the permeate flux evolution during two cycles of filtration. After cleaning chemically the ultrafiltration membrane, the flux declined in few days from 8.3 to 2.5 l h−1 m−2 .
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Research and Technology and AUF are gratefully acknowledged. We are grateful to the ANGED, Tunisia for the help for the collect of samples of LFL. References
Fig. 7. Permeate flux evolution during 2 cycles of filtration. The arrow indicates the chemical cleaning of the ultrafiltration membrane.
4. Conclusions The overall results suggest that AnMBR is a promising process to treat landfill leachate with high efficiency in terms of degradation yield and biogas productivity. It achieved a COD removal up to 90%. At high OLR of 6.27 g COD l−1 d−1 , the biogas productivity was more than 3 l l−1 d−1 . At the beginning of each phase, when OLR was increased, there was an increase of VFAs concentration and then a decrease in the removal efficiency but the system recovered shortly and adapted to the new conditions with time. The bacteria showed a slow growth rate, VSS reached only 3 g l−1 after 5 months of system operation. However, hydrocarbons and organics were efficiently degraded. This efficiency was possible by the adaptation of the consortium to the complex nature of LFL as a result of the increase of its residence time due to membrane filtration. The ultrafiltration membrane showed low fluxes and frequent chemical cleaning cycles were necessary to maintain an acceptable flux. Acknowledgment Financial assistance under the project PRF, “Eau” (2004–2007) from the Tunisian Minister for Superior Teaching, Scientific
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