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Municipal sewage sludge to biodiesel by simultaneous extraction and conversion of lipids
Energy Conversion and Management 103 (2015) 111–118
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Municipal sewage sludge to biodiesel by simultaneous extraction and conversion of lipids J.A. Melero ⇑, R. Sánchez-Vázquez, I.A. Vasiliadou, F. Martínez Castillejo, L.F. Bautista, J. Iglesias, G. Morales, R. Molina School of Experimental Sciences and Technology (ESCET), Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles, E28933 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 16 March 2015 Accepted 16 June 2015
Keywords: Waste water sewage sludge In-situ treatment Biodiesel Heterogeneous catalyst Zr-SBA-15
a b s t r a c t Two different approaches have been investigated for the production of biodiesel from glycerides and free fatty acids (FFAs) extracted from sewage sludge. The first one is a two-step process consisting of organic solvent extraction followed by acid-catalyzed esterification/transesterification of the isolated oil fraction. The second one is a one-step direct transformation consisting of the simultaneous extraction and conversion of the lipid fraction contained in the sewage sludge. In both alternatives, a heterogeneous acid Zr-SBA-15 catalyst has been used. In the two-step extraction–reaction process, conversion close to 90% of the saponifiable fraction (including FFAs and glycerides) were achieved. Remarkably, the catalyst provided such high conversion in the presence of high amounts of unsaponifiable matter. Furthermore, the catalyst kept its activity in successive catalytic runs in presence of this low-quality lipid fraction. In the one-step direct conversion of the dried sludge, the overall weight FAME yield, based on the initial mass of dried sewage sludge, was around 15.5 wt% for primary and 10.0 wt% for secondary sludge. In contrast, this FAME yield was lower than 6 wt% for two-step process when processing primary sludge (being negligible for the secondary sludge). Finally, the results of this work proof the high potential of Zr-SBA-15 as catalyst for the production of biodiesel from a low quality oleaginous feedstock such as municipal sewage sludge. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important challenging in the conventional biodiesel industry is how to overcome the high costs associated to conventional oleaginous feedstock and eliminate the competition between biofuels and food industries for the same raw material [1–3]. One convenient way to simultaneously overcome both drawbacks is using an inedible, residual and hence inexpensive-oleaginous raw material. Therefore, important efforts are currently being applied in biodiesel production research aiming to find new raw materials fulfilling both requirements. One of the potential candidates is the municipal waste water treatment sewage sludge, which is gaining attraction around the world as a lipid feedstock for biodiesel production. Recent research has indicated that the sewage sludge contains significant quantity of free lipids (mono-, di- and triglycerides, phospholipids and fatty acids), as well as high concentrations of microorganisms. The microorganisms can suppose a significant source of oils, since their cell ⇑ Corresponding author. Tel.: +34 914887399. E-mail address: [email protected] (J.A. Melero). http://dx.doi.org/10.1016/j.enconman.2015.06.045 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
membranes are lipid-rich and include phospholipids, steroids and fatty acids, mostly in the range of C10–C18 [4,5]. Waste water treatment plants continuously produce huge amounts of sewage sludge. More than 20 million ton of dry sludge are produced worldwide every year. This figure is expected to increase in the near future due to the increasing urbanization and industrialization [6]. The management of this waste currently presents difficult environmental challenges. Its incineration results in emissions containing dioxins and metals [7]. In the same way, the use of the sludge for the production of compost and fertilizers is restricted in many countries by the presence of metals [8] and residual pharmaceuticals [9]. Therefore, there is a need to identify sustainable solutions for the utilization of this residue. In this context, the exploitation of sewage sludge for biodiesel production is a promising alternative that would also account as waste valorisation [10], solving at the same time energy and environmental concerns. Municipal sewage sludge thus appears as an alternative non-food feedstock offering a significant potential that could help to overcome the competition between biofuel and food industries. However, this feedstock usually contains, together with relevant amounts of lipids and free fatty acids, unsaponifiable matter such
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as hydrocarbons, carotenes, tocopherols, sterols [7]. This matter could disrupt the activity of catalysts in biodiesel production [1]. Additionally, water can represent more than 50 wt% of the total weight of sewage sludge, compromising the overall yield of biodiesel as a consequence of esterification/hydrolysis equilibrium [11]. Biodiesel production from sewage sludge has received considerable attention during the last years, resulting in the development and application of different methods for extraction and esterifica tion/transesterification of the lipid fraction of such waste feedstock. Nonetheless, there are still few studies focused on the use of heterogeneous acid catalysts, which involve several important technical benefits as compared to their homogeneous counterparts, to drive the production of fatty acid methyl esters (FAME) from sewage sludge (cheaper separation processes; reduced water effluent load, capital and energy costs; the catalyst can be reused; there would be no neutralization products, so a higher grade of glycerol is produced; there would be fewer inputs and less wastes). Nevertheless, the commercial introduction of these catalysts need still important advances to impact in a positive way in the biodiesel synthesis technologies (increasing of the stability of acid sites avoiding their leaching; increasing of thermal stability; enhancement of the mass transfer avoiding diffusional limitations; milder operation conditions and increasing of the resistance to water and other impurities). Pokoo-Aikins et al. [11] studied the extraction of lipids from sewage sludge using different organic solvents, obtaining lipids yields close to 25 wt%. Pastore et al. [12] proposed a two-step production of FAME from municipal waste water sludge using n-hexane in acidic ambient followed by methanolysis with sulfuric acid allowing FAME yields between 12 and 22 wt%. In the same way, Huynh et al. [13] reported the in-situ production of FAME, from untreated wet activated sludge under subcritical water and methanol conditions with sulfuric acid. Additionally, Mondala et al. [10] investigated the feasibility of using homogeneous acid catalysts to produce biodiesel from primary and secondary sewage sludge by in-situ transesterification process, with sulfuric acid and hexane to improve the solubility in the reaction mixture; obtaining FAME yields close to 15 wt% from primary sludge and 3 wt% from secondary sludge. The in-situ approach is gaining great interest since it eliminates the need of extraction with organic solvents and the subsequent separation of lipids and fatty acids from the extraction solvent prior to the esterification/transesterification reaction [14–16]. In this context, the aim of the present study has been to evaluate the catalytic behavior of the acid catalyst Zr-SBA-15 in the production of biodiesel from waste water treatment sewage sludge. In previous works [17–19], Zr-SBA-15 catalyst showed high activity as well as very good stability when treating low-grade oleaginous feedstocks containing significant amounts of free fatty acids (FFAs), water, alkaline metals and unsaponifiable matter. Therefore, the present work seeks to validate the promising properties of this catalyst in the production of biodiesel from sewage sludge. In order to perform this evaluation, primary and secondary sludge from a waste water treatment plant located at Universidad Rey Juan Carlos were explored as renewable sources of saponifiable lipids for biodiesel. Furthermore, the two above commented approaches for the production of biodiesel from sewage sludge, i.e. separated extraction–reaction and in-situ processes, have been thoroughly assessed in the present study. 2. Materials and methods 2.1. Chemicals Methanol (synthesis grade, Scharlab) and n-hexane (purity 96%, Scharlab) were used as received for the extraction and reaction
assays. Tetraethylorthosilicate (TEOS, Aldrich), Pluronic P123 (Aldrich) and zirconocene dichloride (Aldrich) were used for the preparation of the catalyst Zr-SBA-15 [17]. 2.2. Sludge sampling and preparation Sewage sludge samples used in this study were collected from the pilot-scale waste water treatment plant (WWTP) located at Universidad Rey Juan Carlos at Móstoles, Madrid, Spain. As shown in Fig. 1, the WWTP has two main sludge streams, as usual in waste water treatment facilities employing an activated sludge process. The primary sludge, which is stored in the primary tank (sampling point 1), is a combination of floating grease and solids collected at the top of the flotation tank. After the primary treatment, the effluent is directed to the activated sludge system for the removal of soluble organic contaminants, denitrification and nitrification. The activated sludge collected in the secondary sedimentation tank, and stored in the secondary tank (sampling point 2), is composed mainly of microbial cells and suspended solids produced during the aerobic biological treatment of waste water. Finally, both primary and secondary sludge are collected and further treated and stabilized by anaerobic digestion. The digested sludge is finally dewatered by centrifugation and disposed. The hydraulic retention time (HRT) of the WWTP, calculated from the flow and the volume of the treatment tanks, varies between 3 and 4 h in a season-dependent manner. Some of the operational parameters of the pilot-scale WWTP measured during the monitoring period (2012–2013) are given in Table 1. Sludge samples tested in the present work were collected at sampling points 1 & 2, for primary and secondary sewage sludge, respectively. Thereafter, they were concentrated by simple settling and, following the work by Mondala et al. [10], the supernatant was discarded and the wet sludge were centrifuged at 3000 rpm for 20 min. The dewatered samples were then dried in an oven at 80 °C. Finally, dried sludge were crushed into a fine powder (with particle size ranging from 0.5 to 1.0 mm) in order to form a sufficiently homogenized suspension during the extraction/transesteri fication process. 2.3. Extraction assays n-hexane or methanol are the extraction solvents used in this work. In this stage, the influence of the extraction time (2.5 and 4 h) and sewage sludge to solvent ratio (10 g of dried sewage sludge to 100 mL and 150 mL of solvent) was assessed. The resultant suspension was then filtered with nylon-membrane filters (0.45 lm) and the solvent was removed to yield the crude oleaginous residue. The collected crude oil was quantified and characterized by means of: (i) acid value (UNE EN ISO 660:2000), (ii) fatty acid profile (UNE EN ISO 5508:1996 & 5509:2000), (iii) unsaponifiable matter content following the method described by Plank and Lorbeer [20], and (iv) glycerides and glycerol content (UNE EN ISO 14105:2003). 2.4. Synthesis of Zr-SBA-15 Zr-SBA-15 material was prepared according to a method previously described in literature [17]. In a typical synthesis, triblock copolymer P123, used as structure directing agent, was dissolved in hydrochloric acid (0.67 N) at room temperature. Upon dissolution, zirconocene dichloride, used as zirconium source, was added and the resultant suspension was stirred for 3 h and heated at 40 °C. Afterwards, TEOS was added to the synthesis medium and vigorous stirring was kept for 20 h at 40 °C. The resultant white suspension was hydrothermally aged (130 °C) for 24 h, the solid was recovered by filtration and air-dried overnight. Surfactant
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Fig. 1. Scheme of the pilot-scale WWTP of Rey Juan Carlos University.
Table 1 Parameters characterization of the pilot-scale WWTP over a two-year period.
a b c d e
Parameter
CODa (mg/L)
Ammoniumb (mg/L)
Nitratesc (mg/L)
TSSd (mg/L)
pHe
Influent Influent Effluent Effluent Effluent
345.9 ± 234.9 416.1 ± 275.8 515.9 ± 190.0 196.1 ± 187.2 71.1 ± 58.2
121.8 ± 38.5 101.3 ± 36.2 98.9 ± 32.2 24.4 ± 12.9 2.0 ± 5.1
267.8 ± 84.3 148.1 ± 94.9 145.6 ± 79.7 123.6 ± 93.5 101.2 ± 60.3
0.88 ± 0.17 0.28 ± 0.09 0.23 ± 0.10 0.02 ± 0.01 0.01 ± 0.00
6.8 ± 0.3 7.3 ± 0.2 7.3 ± 0.5 7.2 ± 0.5 6.8 ± 0.6
pilot-plant primary treatment Primary treatment/influent RBC RBC/influent disinfection pilot-plant
Chemical oxygen demand was determined using a dichromate-reflux colorimetric method. Ammonium (NH+4) was measured using an electrode equipped in Metrohm781 calibrated using standard solution of NH4Cl. Nitrates (NO 3 ) were measured using a pH-meter system of Maxidirect (Lovibong). Total suspended solids, the suspensions were passed through pre-weighed membrane filters (0.45 lm pore size) and then dried at 105 °C. pH was measured using a pH/ion-meter (Metrohm781, Herisau/Switzerland).
removal was performed by controlled calcination in air at 450 °C for 5 h under static conditions.
was determined by ammonia TPD in a Micromeritics 2910 unit fitted with a TCD detector. More details about the physico-chemical properties of this kind of catalyst can be found in literature [17].
2.5. Catalyst characterization and properties The most relevant physicochemical properties for the Zr-SBA-15 catalyst are summarized in Table 2. XRD experiments were recorded on a Philips X´Pert diffractometer using the Cu Ka line in the 2h angle range of 0.6–5.0° with a step size of 0.02° for low angle analysis, and a step size of 0.04° in the 2h angle range of 5.0–50.0° for high angle analysis. Nitrogen adsorption isotherms were carried out at 77 K using a Micromeritics TRISTAR 3000 unit (outgassing conditions with nitrogen: a heating rate of 5 °C min1 up to 90 °C kept for 30 min and thereafter increase up to 120 °C and kept for 720 min). Both analyses evidenced high mesoscopic ordering and high surface area (531 m2/g) with a narrow pore size distribution around 13 nm (size enough to avoid steric hindrances to the mass transfer of relatively bulky substrates such as tri-glycerides with and average molecular size of ca. 4 nm). Zr-SBA-15 catalyst contains 9.7 wt% of zirconium determined by Inductively Coupled Plasma-Optical Emission Spectroscopy using a Varian Vista AX CCD Simultaneous unit. The amount of acid sites
Table 2 Physicochemical properties of the Zr-SBA-15 material.
a
Sample
Zra (% wt)
H+b (meq/g)
SBETc (m2/g)
Vpd (cm3/g)
Dpe (Å)
a0f (Å)
Zr-SBA-15
9.7
0.28
531
1.41
128
136
Metal loading calculated by means of ICP–OES. Acid loading calculated by NH3 temperature programmed desorption analysis. Specific surface area calculated by the B.E.T. method. d Total pore volume recorded at p/p0 = 0.985. e Mean pore size calculated as the maximum of the B.J.H. pore sizes distribution applying the K.J.S. correction. p f Unit cell size calculated as 2/( 3d100), where d100 is the Bragg’s lattice parameter. b
c
2.6. Catalytic tests: two-steps extraction–reaction and in-situ method FAME production tests were accomplished in a 25 mL stainless-steel autoclave (Autoclave Engineers) fitted with temperature controller, mechanical stirrer and pressure transducer for monitoring the reaction conditions. The tests were performed taking into account the reaction conditions previously optimized for the use of Zr-SBA-15 catalyst in biodiesel production from vegetable oils under autogenous pressure (temperature: 209 °C; stirring speed: 2000 rpm; 50:1 methanol to saponifiable matter molar ratio; 12.5 wt% catalyst based on lipids mass) [17]. For the two-steps extraction–reaction method, in a typical assay five grams of the extracted crude oil were mixed together with methanol (50:1 methanol to saponifiable matter molar ratio) and the catalyst (12.5 wt% catalyst based on lipids mass) in the reaction vessel. The system was hermetically closed and the temperature (209 °C) and stirring conditions (2000 rpm) were set up. The reaction was allowed to proceed for 3 or 6 h, before cooling down in an ice bath. The reaction suspension was then filtered using a nylon filter to recover the catalyst for reuse. For the in-situ method, the same temperature reaction conditions were used (209 °C), but directly using dry sludge (2.5 g) which was added to the reactor and mixed with methanol and catalyst. In order to make a direct comparison, the ratio of dried sludge to methanol is similar to that used for the extraction step in the two-step extraction–reaction method, that mean 0.10 g dried sludge/mL methanol. A lower methanol quantity (0.25 g dried sludge/mL methanol) was also studied. The catalyst amount was 12.5 wt% based on the amount of lipids potentially extracted in the dried sewage sludge determined in the extraction analysis. Likewise, the effect of the reaction time (3 h and 6 h) was investigated.
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The yield of the transformation of fatty acid alkyl chains into fatty acid methyl esters was calculated from 1H NMR measurements following the method described by Wahlen et al. [21]. Catalyst reusability was assessed for the two-step and in-situ procedures. For the two-step process, the catalyst after reaction was recovered by filtration and calcined at 450 °C in air during 5 h to remove organic adsorbed compounds on the catalyst surface. In the in-situ procedure, the residual solid was recovered by filtration and calcined at 450 °C in air during 5 h to remove all the organic matter and to obtain the spent catalyst. Afterward, the recovered catalyst was tested again under identical reaction conditions.
Table 3 Total extracted lipids (expressed as wt% based on the starting dry sludge) and their composition in terms of unsaponifiable matter, glycerides and free fatty acids obtained from primary and secondary sludge (10 g) by two-step extraction–reaction with methanol or n-hexane (100 mL per 10 g of dried sludge). Extraction time: 2.5 h.
3. Results and discussion
extraction yield. It is noteworthy the high content of the unsaponifiable matter in the extracted lipid fraction (50 wt%). The presence of such impurities in the sewage sludge makes the composition of the extracted oil more complex for producing biodiesel using heterogeneous acid catalysts. Additionally, in the range of study the amount of extracted lipids seems to be independent on the time and volume of solvent used in experiments A to C. Therefore, the extraction conditions used in the rest of the experiments were fixed at 100 mL of solvent, 10 g of dried sludge and refluxing for 2.5 h. In view of the results, methanol was selected for subsequent experiments due to the high extraction yields as well as it acts as dual esterification/transesterification agent in the acid-catalyzed production of biodiesel.
3.1. Lipids extraction and characterization The first aim of the present work was to select the best solvent for the lipids extraction as well as to assess the composition of extracted lipid fraction, identifying the compounds which could exert a more relevant effect in the catalytic performance of Zr-SBA-15 catalyst. 3.1.1. Lipids extracted from primary sewage sludge Fig. 2 shows the amount of extracted crude oil from primary sludge, obtained under different extraction conditions by refluxing with n-hexane or methanol. The values represent the average of the results obtained from three different replicas of the extraction process. As shown, methanol achieved a significantly greater lipids extraction yield (expressed as wt% based on the starting dry sludge) as compared to n-hexane under each assayed extraction conditions. Table 3 summarizes the amount of total extracted lipids, together with their composition in terms of unsaponifiable matter, glycerides and FFA. The results exhibited a strong difference in the FFA and glycerides fractions, which might be due to the different polarity of the used solvents. Thus, methanol is able to extract a higher fraction of the more polar FFAs, whereas n-hexane appears as a better extraction agent for the non-polar glycerides. It must be noted that the lipids content of the primary sewage sludge are expected to contain a higher FFA content because of their aqueous origin, which promotes the hydrolysis of glycerides into free fatty acids. Hence the use of methanol allows an enhancement of the
16
Extraction Yield (wt%)
Secondary sewage sludge
Primary sewage sludge
14 12
Sludge type
Solvent
Total lipids Lipid fraction composition (wt%) (wt%) Glycerides FFA Unsaponifiable matter
Primary sludge n-Hexane 7.4 Primary sludge Methanol 13.6 Secondary sludge Methanol 2.1
36.5 5.2 62.3
17.0 46.5 43.0 51.8 20.7 16.9
3.1.2. Lipids extracted from secondary sewage sludge Table 3 and Fig. 2 illustrate the comparison between primary and secondary sludge. Although the amount of extracted lipids from primary sludge was much higher than that from secondary sludge, the latter gave in relative terms a greater proportion of saponifiable matter (glycerides plus FFA) in the extracted lipids (83.0% vs. 48.2%) when using methanol as extraction solvent. On the other hand, n-hexane was not effective for the extraction of secondary sludge, leading to negligible yields. Differences can be attributed to the nature in the original feedstocks, both in terms of composition and form in which the lipids are present. Secondary sludge is mostly composed of microbial cells and the extracted lipids come from phospholipids in the cell membranes of such microorganisms [4]. Due to their nature, such lipids exhibits low extractability through a conventional method like the one herein used because they first need to be released from their bacteria membrane [22]. On the other hand, primary sludge consists of a combination of floating grease and solids, which are much more amenable to be recovered by solvent washing [23]. For the same reason, the fraction of FFAs is lower in these lipids than in those from primary sludge, as such lipids have been protected from the aqueous hydrolytic environment.
10 8 6 4 2 0
Methanol n-Hexane
A
B
C
D
Fig. 2. Extraction yield (wt% based on dry sludge) from primary and secondary sludge (10 g), obtained under different extraction conditions with n-hexane or methanol. (A) Primary sludge 2.5 h 100 mL of solvent, (B) primary sludge 4 h 100 mL of solvent, (C) primary sludge 4 h 150 mL of solvent and (D) secondary sludge 2.5 h 100 mL of solvent.
3.1.3. Fatty acid profile of extracted lipids The fatty acid profiles of the extracted lipids from primary and secondary sludge are shown in Fig. 3. It was found that both of the extracted lipid mainly contain palmitic acid (C16:0), stearic acid (C18:0) and oleic acid (C18:1), but there are significant differences in the relative ratios. Thus, there is a larger amount of unsaturated fatty acids in the secondary sludge than in the primary one, linoleic acid (C18:2) (10 mol%), linolenic acid (C18:3) (7 mol%), eicosaenoic acid (C20:1) (7 mol%) and eicosapentaenoic acid (C20:5) (7 mol%). In consequence, the biodiesel produced from secondary sewage sludge will have better cold flow properties [12]. The melting point of biodiesel product strongly depends on chain length and degree of unsaturation, therefore the biodiesel produced from primary sludge will have unfavourable cold temperature behavior [24]. These results are in agreement with those reported by Mondala
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% FFA % Gly
Primary sludge Secondary sludge
FAMEYield (%)
(% molar composition)
50
40
30
20
10
80
80
60
60
40
40
20
20
0
14 C
C
:0 16 :0 C 18 :0 C 18 :1 C 18 :2 C 18 :3 C 20 :0 C 20 :1 C 20 :2 C 20 :4 C 20 :5 C 22 :0 C 22 :1 C 24 :0
0
Fig. 3. Fatty acid profiles of extracted lipids from primary and secondary sludge.
100
A
B
Molar composition (%)
100
60
0
C
Fig. 4. FAME yield and molar composition of unreacted glycerides (Gly) and free fatty acids (FFA) obtained from saponifiable extracted fraction from primary sewage sludge achieved by Zr-SBA-15 catalyst. (A) Reaction product after 3 h with fresh catalyst; (B) reaction product after 6 h with fresh catalyst; and (C) reaction product after 3 h with used and regenerated catalyst. Reaction conditions: 209 °C, 2000 rpm, 50:1 methanol to saponifiable matter molar ratio, 12.5 wt% catalyst based on lipids mass.
et al. [10], Olkiewicz et al. [23], Huynh et al. [13] and Siddiquee and Rohani [7].
3.2.1. Biodiesel production by the two-steps extraction–reaction method The starting oleaginous material used as feedstock in the reaction step of this method came from the extraction step of primary sludge using the previously selected extraction conditions. Fig. 4 shows the FAME yield and the composition of unreacted glycerides and FFAs obtained from saponifiable extracted fraction, through methanolysis of primary sewage sludge extracted lipid. Two experiments were conducted: using a fresh sample of catalyst (Fig. 4, experiment A) and reusing the catalyst sample in a second consecutive run with intermediate regeneration by thermal treatment at 450 °C in air (Fig. 4, experiment C). The catalytic activity displayed by Zr-SBA-15 was outstanding in both a first and a second use (Fig. 4, experiments A and C), converting almost 90% of the saponifiable extracted fraction (free fatty acids and glycerides) into FAME after 3 h of reaction. Although a small amount of FFA and glycerides remained unconverted (Fig. 4, experiment A), the final FAME yield was almost the same achieved after 6 h of reaction (Fig. 4, experiment B). 3.2.2. Biodiesel production by the in-situ method Primary sludge was also used to study the efficiency of the in-situ method (simultaneous extraction and FAME production in the presence of methanol). In this case, dried primary sludge was used directly as reaction substrate with no pre-treatment. As in the conventional two-step extraction–reaction method, the performance of the Zr-SBA-15 catalyst and the capability of its reuse were tested. Three sets of catalytic experiments were conducted in order to: (i) compare the conventional two-steps extraction–reaction
100
100
% FFA % Gly
80
80
60
60
40
40
20
20
0
Molar composition (%)
Both conventional two-steps extraction–reaction and in-situ methods were applied to the acid-catalyzed production of biodiesel over Zr-SBA-15 catalyst. Conventional two-steps extraction–reaction method was only used for primary sludge, since the extraction yield from secondary sludge was too low to be practically exploited as biodiesel source (as above mentioned). On the other hand, the in-situ approach was applied to both types of sewage sludge.
FAMEYield (%)
3.2. Transformation of sewage sludge into biodiesel
method with the in-situ method; (ii) decrease the amount of required methanol and (iii) study the effect of reaction time. Fig. 5 shows the FAME yield and the composition of unreacted glycerides and FFAs obtained from saponifiable extracted fraction achieved by Zr-SBA-15 catalyst in the in-situ method. Zr-SBA-15 provided again very good catalytic activity in terms of transesterifi cation/esterification to give FAME from the FFA and glycerides fractions, regardless of the presence of a large portion of non-extractable solid residue in the reaction medium. Thus, 92% of the extractable saponifiable fraction in the primary sewage sludge was converted into FAME after 3 h (Fig. 5, experiment A). Note that this reaction was conducted using a ratio of dried sludge to methanol similar to that used for the methanol extraction step in the conventional two-step extraction–reaction method (0.1 g dried sludge/mL methanol) in order to make a direct comparison. On the other hand, a higher ratio (i.e. less alcohol) of sewage sludge
0 Blank
A
B
C
D
E
Fig. 5. FAME yield and molar composition of unreacted glycerides (Gly) and free fatty acids (FFA) obtained from saponifiable extracted fraction by Zr-SBA-15 catalyst in the in-situ transformation of primary sewage sludge. (Blank) Reaction product after 3 h without catalyst and dry sludge to methanol ratio of 0.25 g/mL. (A) Reaction product after 3 h with fresh catalyst and a dry sludge to methanol ratio of 0.1 g/mL; (B) reaction product after 3 h with fresh catalyst and a dry sludge to methanol ratio of 0.25 g/mL; (C) reaction product after 6 h with fresh catalyst an a dry sludge to methanol ratio of 0.25 g/mL; (D) reaction product after 3 h with recovered and regenerated catalyst an a dry sludge to methanol ratio of 0.25 g/mL; and (E) reaction product after 3 h with fresh catalyst and wet sludge (0.25 g sludge in dry-basis/mL methanol); reaction conditions: 209 °C, 2000 rpm, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
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to solvent (0.25 g/mL) was studied in order to evaluate the effect on the in-situ production of FAME. Far from negatively affecting the FAME yield, this experiment resulted in a slight increase in the production of FAME, totalizing 95.0 wt% of the extractable saponifiable fraction (Fig. 5, experiment B), which can be ascribed to a better contact between catalyst and saponifiable substrates in a less diluted system. After reaction, the catalyst was recovered as described in Experimental Section and used again under the same reaction conditions. The reusability of the catalyst was thus confirmed using the ratio of 0.25 g dried sewage sludge/mL methanol, leading to a similar FAME yield (95.0%) as compared with the first use (Fig. 5, experiment D vs. Fig. 5, experiment B). Finally, in order to further study the activity of the catalyst, reaction time was increased up to 6 h. The results showed a slight increase in the amount of FAME produced, though too small to compensate the longer time (Fig. 5, experiment C). A blank run (no catalyst) has also been performed in the same conditions in order to determine the extension of thermally-driven reactions (Fig. 5). This blank gave 46.9% yield to FAME of the extractable saponifiable fraction, which can be mainly attributed to the thermal esterification of free fatty acids present in the sludge [25]. Since the tolerance of the catalyst to the presence of water is a key point for the potential exploitation of sewage sludge as biodiesel feedstock, and bearing in mind that this kind of waste material usually contain high water loadings, our study has been completed by treating a sample of wet primary sludge (72 wt% water content) by the in-situ procedure. In a remarked result, the FAME production remained very similar to that obtained with dried sludge even though the ratio of dry sludge to methanol is higher (Fig. 5, experiment E). The relative low acid strength of the active sites of Zr-SBA-15 catalyst do not promote the hydrolysis of formed methyl esters in this highly aqueous medium and hence keeping high the yield to FAME [17]. These results demonstrate that, due to the high water tolerance of the Zr-SBA-15 catalyst, the drying pre-treatment of the primary sludge could be avoided. In such case, there would be fewer inputs and less waste outputs associated with the treatment of the raw material, reducing the overall production costs. Regarding the in-situ transformation of secondary sewage sludge, the method was performed using the optimal operating conditions resulting from the in-situ treatment of primary sludge (209 °C, 0.25 g dried sludge/mL methanol, 12.5 wt% catalyst based on lipids mass potentially extracted in the dried sewage sludge, 2000 rpm). Fig. 6 depicts the results based on dried sludge mass, showing that a mass FAME yield of ca. 10% is obtained (almost 85% of the saponifiable fraction) after 3 h, keeping the activity in two consecutive reactions runs with intermediate catalytic thermal regeneration (not shown). Though this value is a bit lower than that achieved with primary sludge, probably as a result of lower extractability of lipids in these samples, it is indeed much higher than that reported in literature using a similar secondary sewage sludge [10]. This data confirms that Zr-SBA-15 is an excellent catalyst to convert such a complex substrate into biodiesel. This result is quite remarkable since it implies that, under the used reaction conditions, the lipids from the microorganisms can be simultaneously extracted and esterified/transesterified into FAME by the Zr-SBA-15 catalyst. 3.2.3. Product distribution: comparison of the transformation methods Fig. 7 illustrates a comparison of the products distribution, based on dried sludge mass, obtained applying both treatments, the two-steps extraction–reaction (Fig. 7A) and the in-situ methods (Fig. 7B), both of them starting from primary sludge. Solid residue (non-extractable and non-transformable under the reaction conditions) has also been included in order to have a better idea of the efficiency of each method. The achieved yield to FAME was clearly
100
Mass composition (wt%)
116
Solid Residue FAME Glycerides FFA Unsaponifiable Matter
80
60
40
15.5 wt. % 20
10.0 wt. % 0
Primary
Secondary
Fig. 6. Mass composition of the reaction product achieved by Zr-SBA-15 catalyst in the in-situ transformation of primary and secondary sewage sludge based on dried sludge mass. Reaction conditions: 209 °C, 3 h, 2000 rpm, 0.25 g dried sludge/mL methanol, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
higher in the one-step in-situ treatment (15.5% vs. 5.8%). Furthermore, the overall extracted fraction, comprising FAME, glycerides, FFA and unsaponifiable matter, is likewise larger for the in-situ method (34.9% vs. 13.5%). This can be explained by the different temperature used in each treatment, refluxing methanol, approx. 65 °C, in the two-step extraction–reaction method versus 209 °C in the in-situ transformation. Moreover, the low temperature used in the drying stage of the two-step extraction–reaction method hinders the extraction of saponifiable matter from the sludge [22]. Therefore, these results demonstrate that the in-situ treatment is a more efficient approach in terms of FAME yield for the catalytic production of biodiesel from primary sewage sludge, potentially leading to a more cost-efficient and environmentally sustainable process, in as much as the lipids extraction and transformation into FAME are performed at the same process. It must be pointed out that although a final purification step at the end of the process is necessary to remove the solid residue and unsaponifiable matter [12], also in the case of the two-step extraction–reaction method, wherein there are no solid residues in the reaction product, a purification process is needed to remove the remaining unsaponifiable matter. 3.3. Biodiesel refining and characterization Aside of the excellent catalytic results shown above, a crucial issue for the future use of sewage sludge as a substrate for the production of FAME is the quality of the biodiesel resulting therefrom. Such biodiesel should meet the main requirements established by, e.g., the European quality Standard UNE-EN 14214. As shown in the previous sections, the reaction product, either from primary or secondary sludge, is not only composed by FAME but also unreacted glycerides and FFA, and a remarkable unsaponifiable fraction which could comprise wax esters, steroids, terpenoids and pharmaceutical chemicals. Therefore, a purification treatment is mandatory to achieve the standardized FAME purity (>96 wt%). In a preliminary attempt aiming to explore the feasibility of such a purification treatment, we performed a simple aqueous washing followed by a drying step on the biodiesel obtained from the in-situ treatment of the primary sludge. Thus, the reaction product was washed with distilled water in a volume ratio of water to reaction product of 2 to 1, and subsequently dried in an oven at 40 °C under low vacuum (0.5 bar) for 12 h. After this simple washing–drying treatment, the unsaponifiable matter decreases from 44% to 8%,
J.A. Melero et al. / Energy Conversion and Management 103 (2015) 111–118
A
Unsaponifiable Maer 7.0 %
FFA 0.62% Glycerides 0.1% FAME 5.8%
Solid residue 86.4%
117
two-step extraction–reaction method in terms of FAME yield and potentially leading to a more cost-efficient and environmentally sustainable process. This in-situ approach avoids the use of organic solvents for lipids extraction and allows the reduction of treatment steps. On the other hand, Zr-SBA-15 showed a high catalytic performance in the in-situ processing of wet sludge and hence the energetic requirements for the pre-treatment of the primary sludge might be minimized. Additionally, an increase of the FAME content from 46 wt% to 80 wt% resulted after a simple washing–drying treatment of the reaction product coming from the in-situ treatment of primary sludge. The catalytic results of this work proof the high potential of Zr-SBA-15 as catalyst for the production of biodiesel from sewage sludge. Acknowledgements Financial support from the Spanish Science and Innovation Ministry through the project CTQ2008-01396 and from the Regional Government of Madrid through the project S2009-ENE1743 is gratefully acknowledged. The Spanish government is kindly acknowledged by the award of a FPI grant to RS-V. I.A. Vasiliadou thanks the Marie Curie program (action FP7-PEOPLE-2010-IEF-273654) for a post-doctoral fellowship.
Two-step process Unsaponifiable Maer 14.0%
B
FFA 1.9% Glycerides 3.5%
FAME 15.5% Solid residue 65.1%
In-situ process Fig. 7. Mass products distribution based on dried sludge mass achieved by: (A) two-step extraction–reaction method for primary sewage sludge; (B) in-situ method for primary sewage sludge (0.25 g sludge/mL methanol). Reaction conditions: 209 °C, 3 h, 2000 rpm, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
the FFA content slightly decreases from 6% to 5% and the glycerides content remains the same (6%), resulting in an increase of the FAME content from 46 wt% to 80 wt%. Probably, a more rigorous purification treatment would improve the FAME content, getting closer to the objective of >96 wt%. Another alternative to achieve the commercial standards would be by blending with high-purity biodiesel from a different source. Anyway, these results show the potential of producing biodiesel from sewage sludge following a process heterogeneously catalyzed by Zr-SBA-15. 4. Conclusions The results show that the catalytic activity displayed by Zr-SBA-15 was outstanding in both the two steps extraction–reaction method and the in-situ method, converting more than 90% of the saponifiable extracted oil fraction (free fatty acids and glycerides) into FAME regardless of the presence of large amounts of solid residue and unsaponifiable matter in the raw-material. Likewise, the in-situ treatment is more efficient than the
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2015. 06.045. References [1] Morales G, Bautista LF, Melero JA, Iglesias J, Sánchez-Vázquez R. Low-grade oils and fats: effect of several impurities on biodiesel production over sulfonic acid heterogeneous catalysts. Bioresour Technol 2011;102:9571–8. [2] Chen Y, Xiao B, Chang j, Fu Y, Lv P, Wang X. Synthesis of biodiesel from waste cooking oil using immobilized lipase in fixed bed reactor. Energy Convers Manage 2009;50:668–73. [3] Mahesh Sneha E, Ramanathan Anand, Meera KM, Begum S, Narayanan Anantharaman. Biodiesel production from waste cooking oil using KBr impregnated CaO as catalyst. Energy Convers Manage 2015;91:442–50. [4] Karbo DM. Biodiesel production from municipal sewage sludge. Energy Fuels 2010;24:2791–4. [5] Olkiewicz M, Fortuny A, Stüber F, Fabregat A, Font J, Bengoa C. Effects of pretreatment on the lipid extraction and biodiesel production from WWTP sludge. Fuel 2015;141:250–7. [6] Iglesias J, Morales G. Biodiesel from waste oils and fats: methods and catalysis. In: Luque R, Melero JA, editors. Advances in biodiesel preparation. Second generation processes and technologies. Cambridge: Woodhead Publisher Ld.; 2012. p. 154–78. [7] Siddiquee MN, Rohani S. Lipid extraction and biodiesel production from municipal sewage sluges: a review. Renew Sustain Energy Rev 2011;15:1067–73. [8] Smith SR. A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge. Environ Int 2009;35:142–56. [9] Motoyama M, Nakawaga S, Tanoues R, Sato Y, Nomiyama K, Shinohara R. Residues of pharmaceutical products in recycled organic manure produced from sewage sludge and solid waste from livestock and relationship to their fermentation level. Chemosphere 2011;84:432–8. [10] Mondala A, Liang K, Toghiani H, Hernandez R, French T. Biodiesel production by in situ transesterification of municipal primary and secondary sludges. Bioresour Technol 2009;100:1203–10. [11] Pokoo-Aikins G, Heath A, Mentzer RA, Mannan MS, Rogers WJ, El-Halwagi MM. A multi-criteria approach to screening alternatives for converting sewage sludge to biodiesel. J Loss Prev Proc Ind 2010;23:412–20. [12] Pastore C, Lopez A, Lotito V, Mascolo G. Biodiesel from dewatered wastewater sludge: a two-step process for a more advantageous production. Chemosphere 2013;92:667–73. [13] Huynh LH, Nguyen PLT, Ho QP, Ju Y. Catalyst free fatty acid methyl ester production from wet activated sludge under subcritical water and methanol conditions. Bioresour Technol 2012;123:112–6. [14] Choi OK, Song JS, Cha DK, Lee JW. Biodiesel production from wet municipal sludge: evaluation of in situ transesterification using xylene as a cosolvent. Bioresour Technol 2014;166:51–6.
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[15] Gunuawan F, Kurniawan A, Gunuawan I, Ju Y-H, Ayucitra A, Soetaredjo FE, et al. Synthesis of biodiesel from vegetable oils wastewater sludge by in-situ subcritical methanol transesterification: process evaluation and optimization. Biomass Bioenergy 2015;69:28–38. [16] Sangaletti-Gerhard N, Cea M, Risco V, Navia R. In situ biodiesel production from greasy sewage sludge using acid and enzymatic catalysts. Bioresour Technol 2015;179:63–70. [17] Melero JA, Bautista LF, Iglesias J, Morales G, Sánchez-Vázquez R. Zr-SBA-15 acid catalyst: optimization of the synthesis and reaction conditions for biodiesel production from low-grade oils and fats. Catal Today 2012;195:45–53. [18] Iglesias J, Melero JA, Bautista LF, Morales G, Sanchez-Vazquez, Andreola MT, et al. Zr-SBA-15 as an efficient acid catalyst for FAME production from crude palm oil. Catal Today 2011;167:46–55. [19] Iglesias J, Melero JA, Bautista LF, Morales G, Sanchez-Vazquez. Continuous production of biodiesel from low grade feedstock in presence of Zr-SBA-15: catalyst performance and resistance against deactivation. Catal Today 2014;234:174–81.
[20] Plank C, Lorbeer E. Minor components in vegetable oil methyl esters: sterols in rape seed oil methyl ester. Fat Sci Technol 1994;96(10):379–86. [21] Wahlen BD, Barney BM, Seefeldt LC. Synthesis of biodiesel from mixed feedstocks and longer chain alcohols using an acid-catalyzed method. Energy Fuel 2008;22:4223–8. [22] Olkiewicz M, Caporgno MP, Fortuny A, Stüber F, Fabregat A, Font J, et al. Direct liquid–liquid extraction of lipid from municipal sewage sludge for biodiesel production. Fuel Process Technol 2014;128:331–8. [23] Olkiewicz M, Fortuny A, Stüber F, Fabregat A, Font J, Bengoa Ch. Evaluation of different sludges from WWTP as a potential source for biodiesel production. Procedia Eng 2012;42:634–43. [24] Carrero A, Pérez A. Advances in biodiesel quality control, characterization and standards developments. In: Luque R, Melero JA, editors. Advances in biodiesel preparation. Second generation processes and technologies, vol. 5. Cambridge: Woodhead Publisher Ld.; 2012. p. 91–130. [25] Pasias SA, Barakos NK, Papayannakos NG. Catalytic effect of free fatty acids on cotton seed oil thermal transesterification. Ind Eng Chem Res 2009;48:4266–73.
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Municipal sewage sludge to biodiesel by simultaneous extraction and conversion of lipids J.A. Melero ⇑, R. Sánchez-Vázquez, I.A. Vasiliadou, F. Martínez Castillejo, L.F. Bautista, J. Iglesias, G. Morales, R. Molina School of Experimental Sciences and Technology (ESCET), Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles, E28933 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 16 March 2015 Accepted 16 June 2015
Keywords: Waste water sewage sludge In-situ treatment Biodiesel Heterogeneous catalyst Zr-SBA-15
a b s t r a c t Two different approaches have been investigated for the production of biodiesel from glycerides and free fatty acids (FFAs) extracted from sewage sludge. The first one is a two-step process consisting of organic solvent extraction followed by acid-catalyzed esterification/transesterification of the isolated oil fraction. The second one is a one-step direct transformation consisting of the simultaneous extraction and conversion of the lipid fraction contained in the sewage sludge. In both alternatives, a heterogeneous acid Zr-SBA-15 catalyst has been used. In the two-step extraction–reaction process, conversion close to 90% of the saponifiable fraction (including FFAs and glycerides) were achieved. Remarkably, the catalyst provided such high conversion in the presence of high amounts of unsaponifiable matter. Furthermore, the catalyst kept its activity in successive catalytic runs in presence of this low-quality lipid fraction. In the one-step direct conversion of the dried sludge, the overall weight FAME yield, based on the initial mass of dried sewage sludge, was around 15.5 wt% for primary and 10.0 wt% for secondary sludge. In contrast, this FAME yield was lower than 6 wt% for two-step process when processing primary sludge (being negligible for the secondary sludge). Finally, the results of this work proof the high potential of Zr-SBA-15 as catalyst for the production of biodiesel from a low quality oleaginous feedstock such as municipal sewage sludge. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important challenging in the conventional biodiesel industry is how to overcome the high costs associated to conventional oleaginous feedstock and eliminate the competition between biofuels and food industries for the same raw material [1–3]. One convenient way to simultaneously overcome both drawbacks is using an inedible, residual and hence inexpensive-oleaginous raw material. Therefore, important efforts are currently being applied in biodiesel production research aiming to find new raw materials fulfilling both requirements. One of the potential candidates is the municipal waste water treatment sewage sludge, which is gaining attraction around the world as a lipid feedstock for biodiesel production. Recent research has indicated that the sewage sludge contains significant quantity of free lipids (mono-, di- and triglycerides, phospholipids and fatty acids), as well as high concentrations of microorganisms. The microorganisms can suppose a significant source of oils, since their cell ⇑ Corresponding author. Tel.: +34 914887399. E-mail address: [email protected] (J.A. Melero). http://dx.doi.org/10.1016/j.enconman.2015.06.045 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
membranes are lipid-rich and include phospholipids, steroids and fatty acids, mostly in the range of C10–C18 [4,5]. Waste water treatment plants continuously produce huge amounts of sewage sludge. More than 20 million ton of dry sludge are produced worldwide every year. This figure is expected to increase in the near future due to the increasing urbanization and industrialization [6]. The management of this waste currently presents difficult environmental challenges. Its incineration results in emissions containing dioxins and metals [7]. In the same way, the use of the sludge for the production of compost and fertilizers is restricted in many countries by the presence of metals [8] and residual pharmaceuticals [9]. Therefore, there is a need to identify sustainable solutions for the utilization of this residue. In this context, the exploitation of sewage sludge for biodiesel production is a promising alternative that would also account as waste valorisation [10], solving at the same time energy and environmental concerns. Municipal sewage sludge thus appears as an alternative non-food feedstock offering a significant potential that could help to overcome the competition between biofuel and food industries. However, this feedstock usually contains, together with relevant amounts of lipids and free fatty acids, unsaponifiable matter such
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as hydrocarbons, carotenes, tocopherols, sterols [7]. This matter could disrupt the activity of catalysts in biodiesel production [1]. Additionally, water can represent more than 50 wt% of the total weight of sewage sludge, compromising the overall yield of biodiesel as a consequence of esterification/hydrolysis equilibrium [11]. Biodiesel production from sewage sludge has received considerable attention during the last years, resulting in the development and application of different methods for extraction and esterifica tion/transesterification of the lipid fraction of such waste feedstock. Nonetheless, there are still few studies focused on the use of heterogeneous acid catalysts, which involve several important technical benefits as compared to their homogeneous counterparts, to drive the production of fatty acid methyl esters (FAME) from sewage sludge (cheaper separation processes; reduced water effluent load, capital and energy costs; the catalyst can be reused; there would be no neutralization products, so a higher grade of glycerol is produced; there would be fewer inputs and less wastes). Nevertheless, the commercial introduction of these catalysts need still important advances to impact in a positive way in the biodiesel synthesis technologies (increasing of the stability of acid sites avoiding their leaching; increasing of thermal stability; enhancement of the mass transfer avoiding diffusional limitations; milder operation conditions and increasing of the resistance to water and other impurities). Pokoo-Aikins et al. [11] studied the extraction of lipids from sewage sludge using different organic solvents, obtaining lipids yields close to 25 wt%. Pastore et al. [12] proposed a two-step production of FAME from municipal waste water sludge using n-hexane in acidic ambient followed by methanolysis with sulfuric acid allowing FAME yields between 12 and 22 wt%. In the same way, Huynh et al. [13] reported the in-situ production of FAME, from untreated wet activated sludge under subcritical water and methanol conditions with sulfuric acid. Additionally, Mondala et al. [10] investigated the feasibility of using homogeneous acid catalysts to produce biodiesel from primary and secondary sewage sludge by in-situ transesterification process, with sulfuric acid and hexane to improve the solubility in the reaction mixture; obtaining FAME yields close to 15 wt% from primary sludge and 3 wt% from secondary sludge. The in-situ approach is gaining great interest since it eliminates the need of extraction with organic solvents and the subsequent separation of lipids and fatty acids from the extraction solvent prior to the esterification/transesterification reaction [14–16]. In this context, the aim of the present study has been to evaluate the catalytic behavior of the acid catalyst Zr-SBA-15 in the production of biodiesel from waste water treatment sewage sludge. In previous works [17–19], Zr-SBA-15 catalyst showed high activity as well as very good stability when treating low-grade oleaginous feedstocks containing significant amounts of free fatty acids (FFAs), water, alkaline metals and unsaponifiable matter. Therefore, the present work seeks to validate the promising properties of this catalyst in the production of biodiesel from sewage sludge. In order to perform this evaluation, primary and secondary sludge from a waste water treatment plant located at Universidad Rey Juan Carlos were explored as renewable sources of saponifiable lipids for biodiesel. Furthermore, the two above commented approaches for the production of biodiesel from sewage sludge, i.e. separated extraction–reaction and in-situ processes, have been thoroughly assessed in the present study. 2. Materials and methods 2.1. Chemicals Methanol (synthesis grade, Scharlab) and n-hexane (purity 96%, Scharlab) were used as received for the extraction and reaction
assays. Tetraethylorthosilicate (TEOS, Aldrich), Pluronic P123 (Aldrich) and zirconocene dichloride (Aldrich) were used for the preparation of the catalyst Zr-SBA-15 [17]. 2.2. Sludge sampling and preparation Sewage sludge samples used in this study were collected from the pilot-scale waste water treatment plant (WWTP) located at Universidad Rey Juan Carlos at Móstoles, Madrid, Spain. As shown in Fig. 1, the WWTP has two main sludge streams, as usual in waste water treatment facilities employing an activated sludge process. The primary sludge, which is stored in the primary tank (sampling point 1), is a combination of floating grease and solids collected at the top of the flotation tank. After the primary treatment, the effluent is directed to the activated sludge system for the removal of soluble organic contaminants, denitrification and nitrification. The activated sludge collected in the secondary sedimentation tank, and stored in the secondary tank (sampling point 2), is composed mainly of microbial cells and suspended solids produced during the aerobic biological treatment of waste water. Finally, both primary and secondary sludge are collected and further treated and stabilized by anaerobic digestion. The digested sludge is finally dewatered by centrifugation and disposed. The hydraulic retention time (HRT) of the WWTP, calculated from the flow and the volume of the treatment tanks, varies between 3 and 4 h in a season-dependent manner. Some of the operational parameters of the pilot-scale WWTP measured during the monitoring period (2012–2013) are given in Table 1. Sludge samples tested in the present work were collected at sampling points 1 & 2, for primary and secondary sewage sludge, respectively. Thereafter, they were concentrated by simple settling and, following the work by Mondala et al. [10], the supernatant was discarded and the wet sludge were centrifuged at 3000 rpm for 20 min. The dewatered samples were then dried in an oven at 80 °C. Finally, dried sludge were crushed into a fine powder (with particle size ranging from 0.5 to 1.0 mm) in order to form a sufficiently homogenized suspension during the extraction/transesteri fication process. 2.3. Extraction assays n-hexane or methanol are the extraction solvents used in this work. In this stage, the influence of the extraction time (2.5 and 4 h) and sewage sludge to solvent ratio (10 g of dried sewage sludge to 100 mL and 150 mL of solvent) was assessed. The resultant suspension was then filtered with nylon-membrane filters (0.45 lm) and the solvent was removed to yield the crude oleaginous residue. The collected crude oil was quantified and characterized by means of: (i) acid value (UNE EN ISO 660:2000), (ii) fatty acid profile (UNE EN ISO 5508:1996 & 5509:2000), (iii) unsaponifiable matter content following the method described by Plank and Lorbeer [20], and (iv) glycerides and glycerol content (UNE EN ISO 14105:2003). 2.4. Synthesis of Zr-SBA-15 Zr-SBA-15 material was prepared according to a method previously described in literature [17]. In a typical synthesis, triblock copolymer P123, used as structure directing agent, was dissolved in hydrochloric acid (0.67 N) at room temperature. Upon dissolution, zirconocene dichloride, used as zirconium source, was added and the resultant suspension was stirred for 3 h and heated at 40 °C. Afterwards, TEOS was added to the synthesis medium and vigorous stirring was kept for 20 h at 40 °C. The resultant white suspension was hydrothermally aged (130 °C) for 24 h, the solid was recovered by filtration and air-dried overnight. Surfactant
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Fig. 1. Scheme of the pilot-scale WWTP of Rey Juan Carlos University.
Table 1 Parameters characterization of the pilot-scale WWTP over a two-year period.
a b c d e
Parameter
CODa (mg/L)
Ammoniumb (mg/L)
Nitratesc (mg/L)
TSSd (mg/L)
pHe
Influent Influent Effluent Effluent Effluent
345.9 ± 234.9 416.1 ± 275.8 515.9 ± 190.0 196.1 ± 187.2 71.1 ± 58.2
121.8 ± 38.5 101.3 ± 36.2 98.9 ± 32.2 24.4 ± 12.9 2.0 ± 5.1
267.8 ± 84.3 148.1 ± 94.9 145.6 ± 79.7 123.6 ± 93.5 101.2 ± 60.3
0.88 ± 0.17 0.28 ± 0.09 0.23 ± 0.10 0.02 ± 0.01 0.01 ± 0.00
6.8 ± 0.3 7.3 ± 0.2 7.3 ± 0.5 7.2 ± 0.5 6.8 ± 0.6
pilot-plant primary treatment Primary treatment/influent RBC RBC/influent disinfection pilot-plant
Chemical oxygen demand was determined using a dichromate-reflux colorimetric method. Ammonium (NH+4) was measured using an electrode equipped in Metrohm781 calibrated using standard solution of NH4Cl. Nitrates (NO 3 ) were measured using a pH-meter system of Maxidirect (Lovibong). Total suspended solids, the suspensions were passed through pre-weighed membrane filters (0.45 lm pore size) and then dried at 105 °C. pH was measured using a pH/ion-meter (Metrohm781, Herisau/Switzerland).
removal was performed by controlled calcination in air at 450 °C for 5 h under static conditions.
was determined by ammonia TPD in a Micromeritics 2910 unit fitted with a TCD detector. More details about the physico-chemical properties of this kind of catalyst can be found in literature [17].
2.5. Catalyst characterization and properties The most relevant physicochemical properties for the Zr-SBA-15 catalyst are summarized in Table 2. XRD experiments were recorded on a Philips X´Pert diffractometer using the Cu Ka line in the 2h angle range of 0.6–5.0° with a step size of 0.02° for low angle analysis, and a step size of 0.04° in the 2h angle range of 5.0–50.0° for high angle analysis. Nitrogen adsorption isotherms were carried out at 77 K using a Micromeritics TRISTAR 3000 unit (outgassing conditions with nitrogen: a heating rate of 5 °C min1 up to 90 °C kept for 30 min and thereafter increase up to 120 °C and kept for 720 min). Both analyses evidenced high mesoscopic ordering and high surface area (531 m2/g) with a narrow pore size distribution around 13 nm (size enough to avoid steric hindrances to the mass transfer of relatively bulky substrates such as tri-glycerides with and average molecular size of ca. 4 nm). Zr-SBA-15 catalyst contains 9.7 wt% of zirconium determined by Inductively Coupled Plasma-Optical Emission Spectroscopy using a Varian Vista AX CCD Simultaneous unit. The amount of acid sites
Table 2 Physicochemical properties of the Zr-SBA-15 material.
a
Sample
Zra (% wt)
H+b (meq/g)
SBETc (m2/g)
Vpd (cm3/g)
Dpe (Å)
a0f (Å)
Zr-SBA-15
9.7
0.28
531
1.41
128
136
Metal loading calculated by means of ICP–OES. Acid loading calculated by NH3 temperature programmed desorption analysis. Specific surface area calculated by the B.E.T. method. d Total pore volume recorded at p/p0 = 0.985. e Mean pore size calculated as the maximum of the B.J.H. pore sizes distribution applying the K.J.S. correction. p f Unit cell size calculated as 2/( 3d100), where d100 is the Bragg’s lattice parameter. b
c
2.6. Catalytic tests: two-steps extraction–reaction and in-situ method FAME production tests were accomplished in a 25 mL stainless-steel autoclave (Autoclave Engineers) fitted with temperature controller, mechanical stirrer and pressure transducer for monitoring the reaction conditions. The tests were performed taking into account the reaction conditions previously optimized for the use of Zr-SBA-15 catalyst in biodiesel production from vegetable oils under autogenous pressure (temperature: 209 °C; stirring speed: 2000 rpm; 50:1 methanol to saponifiable matter molar ratio; 12.5 wt% catalyst based on lipids mass) [17]. For the two-steps extraction–reaction method, in a typical assay five grams of the extracted crude oil were mixed together with methanol (50:1 methanol to saponifiable matter molar ratio) and the catalyst (12.5 wt% catalyst based on lipids mass) in the reaction vessel. The system was hermetically closed and the temperature (209 °C) and stirring conditions (2000 rpm) were set up. The reaction was allowed to proceed for 3 or 6 h, before cooling down in an ice bath. The reaction suspension was then filtered using a nylon filter to recover the catalyst for reuse. For the in-situ method, the same temperature reaction conditions were used (209 °C), but directly using dry sludge (2.5 g) which was added to the reactor and mixed with methanol and catalyst. In order to make a direct comparison, the ratio of dried sludge to methanol is similar to that used for the extraction step in the two-step extraction–reaction method, that mean 0.10 g dried sludge/mL methanol. A lower methanol quantity (0.25 g dried sludge/mL methanol) was also studied. The catalyst amount was 12.5 wt% based on the amount of lipids potentially extracted in the dried sewage sludge determined in the extraction analysis. Likewise, the effect of the reaction time (3 h and 6 h) was investigated.
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The yield of the transformation of fatty acid alkyl chains into fatty acid methyl esters was calculated from 1H NMR measurements following the method described by Wahlen et al. [21]. Catalyst reusability was assessed for the two-step and in-situ procedures. For the two-step process, the catalyst after reaction was recovered by filtration and calcined at 450 °C in air during 5 h to remove organic adsorbed compounds on the catalyst surface. In the in-situ procedure, the residual solid was recovered by filtration and calcined at 450 °C in air during 5 h to remove all the organic matter and to obtain the spent catalyst. Afterward, the recovered catalyst was tested again under identical reaction conditions.
Table 3 Total extracted lipids (expressed as wt% based on the starting dry sludge) and their composition in terms of unsaponifiable matter, glycerides and free fatty acids obtained from primary and secondary sludge (10 g) by two-step extraction–reaction with methanol or n-hexane (100 mL per 10 g of dried sludge). Extraction time: 2.5 h.
3. Results and discussion
extraction yield. It is noteworthy the high content of the unsaponifiable matter in the extracted lipid fraction (50 wt%). The presence of such impurities in the sewage sludge makes the composition of the extracted oil more complex for producing biodiesel using heterogeneous acid catalysts. Additionally, in the range of study the amount of extracted lipids seems to be independent on the time and volume of solvent used in experiments A to C. Therefore, the extraction conditions used in the rest of the experiments were fixed at 100 mL of solvent, 10 g of dried sludge and refluxing for 2.5 h. In view of the results, methanol was selected for subsequent experiments due to the high extraction yields as well as it acts as dual esterification/transesterification agent in the acid-catalyzed production of biodiesel.
3.1. Lipids extraction and characterization The first aim of the present work was to select the best solvent for the lipids extraction as well as to assess the composition of extracted lipid fraction, identifying the compounds which could exert a more relevant effect in the catalytic performance of Zr-SBA-15 catalyst. 3.1.1. Lipids extracted from primary sewage sludge Fig. 2 shows the amount of extracted crude oil from primary sludge, obtained under different extraction conditions by refluxing with n-hexane or methanol. The values represent the average of the results obtained from three different replicas of the extraction process. As shown, methanol achieved a significantly greater lipids extraction yield (expressed as wt% based on the starting dry sludge) as compared to n-hexane under each assayed extraction conditions. Table 3 summarizes the amount of total extracted lipids, together with their composition in terms of unsaponifiable matter, glycerides and FFA. The results exhibited a strong difference in the FFA and glycerides fractions, which might be due to the different polarity of the used solvents. Thus, methanol is able to extract a higher fraction of the more polar FFAs, whereas n-hexane appears as a better extraction agent for the non-polar glycerides. It must be noted that the lipids content of the primary sewage sludge are expected to contain a higher FFA content because of their aqueous origin, which promotes the hydrolysis of glycerides into free fatty acids. Hence the use of methanol allows an enhancement of the
16
Extraction Yield (wt%)
Secondary sewage sludge
Primary sewage sludge
14 12
Sludge type
Solvent
Total lipids Lipid fraction composition (wt%) (wt%) Glycerides FFA Unsaponifiable matter
Primary sludge n-Hexane 7.4 Primary sludge Methanol 13.6 Secondary sludge Methanol 2.1
36.5 5.2 62.3
17.0 46.5 43.0 51.8 20.7 16.9
3.1.2. Lipids extracted from secondary sewage sludge Table 3 and Fig. 2 illustrate the comparison between primary and secondary sludge. Although the amount of extracted lipids from primary sludge was much higher than that from secondary sludge, the latter gave in relative terms a greater proportion of saponifiable matter (glycerides plus FFA) in the extracted lipids (83.0% vs. 48.2%) when using methanol as extraction solvent. On the other hand, n-hexane was not effective for the extraction of secondary sludge, leading to negligible yields. Differences can be attributed to the nature in the original feedstocks, both in terms of composition and form in which the lipids are present. Secondary sludge is mostly composed of microbial cells and the extracted lipids come from phospholipids in the cell membranes of such microorganisms [4]. Due to their nature, such lipids exhibits low extractability through a conventional method like the one herein used because they first need to be released from their bacteria membrane [22]. On the other hand, primary sludge consists of a combination of floating grease and solids, which are much more amenable to be recovered by solvent washing [23]. For the same reason, the fraction of FFAs is lower in these lipids than in those from primary sludge, as such lipids have been protected from the aqueous hydrolytic environment.
10 8 6 4 2 0
Methanol n-Hexane
A
B
C
D
Fig. 2. Extraction yield (wt% based on dry sludge) from primary and secondary sludge (10 g), obtained under different extraction conditions with n-hexane or methanol. (A) Primary sludge 2.5 h 100 mL of solvent, (B) primary sludge 4 h 100 mL of solvent, (C) primary sludge 4 h 150 mL of solvent and (D) secondary sludge 2.5 h 100 mL of solvent.
3.1.3. Fatty acid profile of extracted lipids The fatty acid profiles of the extracted lipids from primary and secondary sludge are shown in Fig. 3. It was found that both of the extracted lipid mainly contain palmitic acid (C16:0), stearic acid (C18:0) and oleic acid (C18:1), but there are significant differences in the relative ratios. Thus, there is a larger amount of unsaturated fatty acids in the secondary sludge than in the primary one, linoleic acid (C18:2) (10 mol%), linolenic acid (C18:3) (7 mol%), eicosaenoic acid (C20:1) (7 mol%) and eicosapentaenoic acid (C20:5) (7 mol%). In consequence, the biodiesel produced from secondary sewage sludge will have better cold flow properties [12]. The melting point of biodiesel product strongly depends on chain length and degree of unsaturation, therefore the biodiesel produced from primary sludge will have unfavourable cold temperature behavior [24]. These results are in agreement with those reported by Mondala
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% FFA % Gly
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(% molar composition)
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:0 16 :0 C 18 :0 C 18 :1 C 18 :2 C 18 :3 C 20 :0 C 20 :1 C 20 :2 C 20 :4 C 20 :5 C 22 :0 C 22 :1 C 24 :0
0
Fig. 3. Fatty acid profiles of extracted lipids from primary and secondary sludge.
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B
Molar composition (%)
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Fig. 4. FAME yield and molar composition of unreacted glycerides (Gly) and free fatty acids (FFA) obtained from saponifiable extracted fraction from primary sewage sludge achieved by Zr-SBA-15 catalyst. (A) Reaction product after 3 h with fresh catalyst; (B) reaction product after 6 h with fresh catalyst; and (C) reaction product after 3 h with used and regenerated catalyst. Reaction conditions: 209 °C, 2000 rpm, 50:1 methanol to saponifiable matter molar ratio, 12.5 wt% catalyst based on lipids mass.
et al. [10], Olkiewicz et al. [23], Huynh et al. [13] and Siddiquee and Rohani [7].
3.2.1. Biodiesel production by the two-steps extraction–reaction method The starting oleaginous material used as feedstock in the reaction step of this method came from the extraction step of primary sludge using the previously selected extraction conditions. Fig. 4 shows the FAME yield and the composition of unreacted glycerides and FFAs obtained from saponifiable extracted fraction, through methanolysis of primary sewage sludge extracted lipid. Two experiments were conducted: using a fresh sample of catalyst (Fig. 4, experiment A) and reusing the catalyst sample in a second consecutive run with intermediate regeneration by thermal treatment at 450 °C in air (Fig. 4, experiment C). The catalytic activity displayed by Zr-SBA-15 was outstanding in both a first and a second use (Fig. 4, experiments A and C), converting almost 90% of the saponifiable extracted fraction (free fatty acids and glycerides) into FAME after 3 h of reaction. Although a small amount of FFA and glycerides remained unconverted (Fig. 4, experiment A), the final FAME yield was almost the same achieved after 6 h of reaction (Fig. 4, experiment B). 3.2.2. Biodiesel production by the in-situ method Primary sludge was also used to study the efficiency of the in-situ method (simultaneous extraction and FAME production in the presence of methanol). In this case, dried primary sludge was used directly as reaction substrate with no pre-treatment. As in the conventional two-step extraction–reaction method, the performance of the Zr-SBA-15 catalyst and the capability of its reuse were tested. Three sets of catalytic experiments were conducted in order to: (i) compare the conventional two-steps extraction–reaction
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Both conventional two-steps extraction–reaction and in-situ methods were applied to the acid-catalyzed production of biodiesel over Zr-SBA-15 catalyst. Conventional two-steps extraction–reaction method was only used for primary sludge, since the extraction yield from secondary sludge was too low to be practically exploited as biodiesel source (as above mentioned). On the other hand, the in-situ approach was applied to both types of sewage sludge.
FAMEYield (%)
3.2. Transformation of sewage sludge into biodiesel
method with the in-situ method; (ii) decrease the amount of required methanol and (iii) study the effect of reaction time. Fig. 5 shows the FAME yield and the composition of unreacted glycerides and FFAs obtained from saponifiable extracted fraction achieved by Zr-SBA-15 catalyst in the in-situ method. Zr-SBA-15 provided again very good catalytic activity in terms of transesterifi cation/esterification to give FAME from the FFA and glycerides fractions, regardless of the presence of a large portion of non-extractable solid residue in the reaction medium. Thus, 92% of the extractable saponifiable fraction in the primary sewage sludge was converted into FAME after 3 h (Fig. 5, experiment A). Note that this reaction was conducted using a ratio of dried sludge to methanol similar to that used for the methanol extraction step in the conventional two-step extraction–reaction method (0.1 g dried sludge/mL methanol) in order to make a direct comparison. On the other hand, a higher ratio (i.e. less alcohol) of sewage sludge
0 Blank
A
B
C
D
E
Fig. 5. FAME yield and molar composition of unreacted glycerides (Gly) and free fatty acids (FFA) obtained from saponifiable extracted fraction by Zr-SBA-15 catalyst in the in-situ transformation of primary sewage sludge. (Blank) Reaction product after 3 h without catalyst and dry sludge to methanol ratio of 0.25 g/mL. (A) Reaction product after 3 h with fresh catalyst and a dry sludge to methanol ratio of 0.1 g/mL; (B) reaction product after 3 h with fresh catalyst and a dry sludge to methanol ratio of 0.25 g/mL; (C) reaction product after 6 h with fresh catalyst an a dry sludge to methanol ratio of 0.25 g/mL; (D) reaction product after 3 h with recovered and regenerated catalyst an a dry sludge to methanol ratio of 0.25 g/mL; and (E) reaction product after 3 h with fresh catalyst and wet sludge (0.25 g sludge in dry-basis/mL methanol); reaction conditions: 209 °C, 2000 rpm, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
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to solvent (0.25 g/mL) was studied in order to evaluate the effect on the in-situ production of FAME. Far from negatively affecting the FAME yield, this experiment resulted in a slight increase in the production of FAME, totalizing 95.0 wt% of the extractable saponifiable fraction (Fig. 5, experiment B), which can be ascribed to a better contact between catalyst and saponifiable substrates in a less diluted system. After reaction, the catalyst was recovered as described in Experimental Section and used again under the same reaction conditions. The reusability of the catalyst was thus confirmed using the ratio of 0.25 g dried sewage sludge/mL methanol, leading to a similar FAME yield (95.0%) as compared with the first use (Fig. 5, experiment D vs. Fig. 5, experiment B). Finally, in order to further study the activity of the catalyst, reaction time was increased up to 6 h. The results showed a slight increase in the amount of FAME produced, though too small to compensate the longer time (Fig. 5, experiment C). A blank run (no catalyst) has also been performed in the same conditions in order to determine the extension of thermally-driven reactions (Fig. 5). This blank gave 46.9% yield to FAME of the extractable saponifiable fraction, which can be mainly attributed to the thermal esterification of free fatty acids present in the sludge [25]. Since the tolerance of the catalyst to the presence of water is a key point for the potential exploitation of sewage sludge as biodiesel feedstock, and bearing in mind that this kind of waste material usually contain high water loadings, our study has been completed by treating a sample of wet primary sludge (72 wt% water content) by the in-situ procedure. In a remarked result, the FAME production remained very similar to that obtained with dried sludge even though the ratio of dry sludge to methanol is higher (Fig. 5, experiment E). The relative low acid strength of the active sites of Zr-SBA-15 catalyst do not promote the hydrolysis of formed methyl esters in this highly aqueous medium and hence keeping high the yield to FAME [17]. These results demonstrate that, due to the high water tolerance of the Zr-SBA-15 catalyst, the drying pre-treatment of the primary sludge could be avoided. In such case, there would be fewer inputs and less waste outputs associated with the treatment of the raw material, reducing the overall production costs. Regarding the in-situ transformation of secondary sewage sludge, the method was performed using the optimal operating conditions resulting from the in-situ treatment of primary sludge (209 °C, 0.25 g dried sludge/mL methanol, 12.5 wt% catalyst based on lipids mass potentially extracted in the dried sewage sludge, 2000 rpm). Fig. 6 depicts the results based on dried sludge mass, showing that a mass FAME yield of ca. 10% is obtained (almost 85% of the saponifiable fraction) after 3 h, keeping the activity in two consecutive reactions runs with intermediate catalytic thermal regeneration (not shown). Though this value is a bit lower than that achieved with primary sludge, probably as a result of lower extractability of lipids in these samples, it is indeed much higher than that reported in literature using a similar secondary sewage sludge [10]. This data confirms that Zr-SBA-15 is an excellent catalyst to convert such a complex substrate into biodiesel. This result is quite remarkable since it implies that, under the used reaction conditions, the lipids from the microorganisms can be simultaneously extracted and esterified/transesterified into FAME by the Zr-SBA-15 catalyst. 3.2.3. Product distribution: comparison of the transformation methods Fig. 7 illustrates a comparison of the products distribution, based on dried sludge mass, obtained applying both treatments, the two-steps extraction–reaction (Fig. 7A) and the in-situ methods (Fig. 7B), both of them starting from primary sludge. Solid residue (non-extractable and non-transformable under the reaction conditions) has also been included in order to have a better idea of the efficiency of each method. The achieved yield to FAME was clearly
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Solid Residue FAME Glycerides FFA Unsaponifiable Matter
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Fig. 6. Mass composition of the reaction product achieved by Zr-SBA-15 catalyst in the in-situ transformation of primary and secondary sewage sludge based on dried sludge mass. Reaction conditions: 209 °C, 3 h, 2000 rpm, 0.25 g dried sludge/mL methanol, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
higher in the one-step in-situ treatment (15.5% vs. 5.8%). Furthermore, the overall extracted fraction, comprising FAME, glycerides, FFA and unsaponifiable matter, is likewise larger for the in-situ method (34.9% vs. 13.5%). This can be explained by the different temperature used in each treatment, refluxing methanol, approx. 65 °C, in the two-step extraction–reaction method versus 209 °C in the in-situ transformation. Moreover, the low temperature used in the drying stage of the two-step extraction–reaction method hinders the extraction of saponifiable matter from the sludge [22]. Therefore, these results demonstrate that the in-situ treatment is a more efficient approach in terms of FAME yield for the catalytic production of biodiesel from primary sewage sludge, potentially leading to a more cost-efficient and environmentally sustainable process, in as much as the lipids extraction and transformation into FAME are performed at the same process. It must be pointed out that although a final purification step at the end of the process is necessary to remove the solid residue and unsaponifiable matter [12], also in the case of the two-step extraction–reaction method, wherein there are no solid residues in the reaction product, a purification process is needed to remove the remaining unsaponifiable matter. 3.3. Biodiesel refining and characterization Aside of the excellent catalytic results shown above, a crucial issue for the future use of sewage sludge as a substrate for the production of FAME is the quality of the biodiesel resulting therefrom. Such biodiesel should meet the main requirements established by, e.g., the European quality Standard UNE-EN 14214. As shown in the previous sections, the reaction product, either from primary or secondary sludge, is not only composed by FAME but also unreacted glycerides and FFA, and a remarkable unsaponifiable fraction which could comprise wax esters, steroids, terpenoids and pharmaceutical chemicals. Therefore, a purification treatment is mandatory to achieve the standardized FAME purity (>96 wt%). In a preliminary attempt aiming to explore the feasibility of such a purification treatment, we performed a simple aqueous washing followed by a drying step on the biodiesel obtained from the in-situ treatment of the primary sludge. Thus, the reaction product was washed with distilled water in a volume ratio of water to reaction product of 2 to 1, and subsequently dried in an oven at 40 °C under low vacuum (0.5 bar) for 12 h. After this simple washing–drying treatment, the unsaponifiable matter decreases from 44% to 8%,
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A
Unsaponifiable Maer 7.0 %
FFA 0.62% Glycerides 0.1% FAME 5.8%
Solid residue 86.4%
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two-step extraction–reaction method in terms of FAME yield and potentially leading to a more cost-efficient and environmentally sustainable process. This in-situ approach avoids the use of organic solvents for lipids extraction and allows the reduction of treatment steps. On the other hand, Zr-SBA-15 showed a high catalytic performance in the in-situ processing of wet sludge and hence the energetic requirements for the pre-treatment of the primary sludge might be minimized. Additionally, an increase of the FAME content from 46 wt% to 80 wt% resulted after a simple washing–drying treatment of the reaction product coming from the in-situ treatment of primary sludge. The catalytic results of this work proof the high potential of Zr-SBA-15 as catalyst for the production of biodiesel from sewage sludge. Acknowledgements Financial support from the Spanish Science and Innovation Ministry through the project CTQ2008-01396 and from the Regional Government of Madrid through the project S2009-ENE1743 is gratefully acknowledged. The Spanish government is kindly acknowledged by the award of a FPI grant to RS-V. I.A. Vasiliadou thanks the Marie Curie program (action FP7-PEOPLE-2010-IEF-273654) for a post-doctoral fellowship.
Two-step process Unsaponifiable Maer 14.0%
B
FFA 1.9% Glycerides 3.5%
FAME 15.5% Solid residue 65.1%
In-situ process Fig. 7. Mass products distribution based on dried sludge mass achieved by: (A) two-step extraction–reaction method for primary sewage sludge; (B) in-situ method for primary sewage sludge (0.25 g sludge/mL methanol). Reaction conditions: 209 °C, 3 h, 2000 rpm, 12.5 wt% catalyst based on lipids mass potentially extracted in the dry sewage sludge.
the FFA content slightly decreases from 6% to 5% and the glycerides content remains the same (6%), resulting in an increase of the FAME content from 46 wt% to 80 wt%. Probably, a more rigorous purification treatment would improve the FAME content, getting closer to the objective of >96 wt%. Another alternative to achieve the commercial standards would be by blending with high-purity biodiesel from a different source. Anyway, these results show the potential of producing biodiesel from sewage sludge following a process heterogeneously catalyzed by Zr-SBA-15. 4. Conclusions The results show that the catalytic activity displayed by Zr-SBA-15 was outstanding in both the two steps extraction–reaction method and the in-situ method, converting more than 90% of the saponifiable extracted oil fraction (free fatty acids and glycerides) into FAME regardless of the presence of large amounts of solid residue and unsaponifiable matter in the raw-material. Likewise, the in-situ treatment is more efficient than the
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