Anaerobic digestion of tomato processing waste: Effect of alkaline pretreatment

Journal of Environmental Management 163 (2015) 49e52

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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

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Anaerobic digestion of tomato processing waste: Effect of alkaline pretreatment  a, *, Rosa Greco a, Alexandros Evangelou b, Dimitrios Komilis b Paolo S. Calabro a  Mediterranea di Reggio Calabria, Via Graziella loc. Feo di Vito, Department of Civil, Energy Environmental and Materials Engineering, Universita 89122 Reggio Calabria, Italy b Department of Environmental Engineering, Democritus University of Thrace, Xanthi 671 00, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2015 Received in revised form 18 May 2015 Accepted 30 July 2015 Available online xxx

The objective of the work was to assess the effect of mild alkaline pretreatment on the anaerobic biodegradability of tomato processing waste (TPW). Experiments were carried out in duplicate BMP bottles using a pretreatment contact time of 4 and 24 h and a 1% and 5% NaOH dosage. The cumulative methane production during a 30 d period was recorded and modelled. The alkaline pretreatment did not significantly affect methane production in any of the treatments in comparison to the control. The average methane production for all runs was 320 NmL/gVS. Based on first order kinetic modelling, the alkaline pretreatment was found to slow down the rate of methanogenesis, mainly in the two reactors with the highest NaOH dosage. The biodegradability of the substrates ranged from 0.75 to 0.82 and from 0.66 to 0.72 based on two different approaches. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Alkaline pretreatment Anaerobic digestion Biochemical methane potential Kinetics Tomato waste

1. Introduction Tomato (Solanum lycopersicum) is the second most produced vegetable in the world, with a generation of 162 Mt in 2012 (FAOSTAT, 2014); main producers are China, USA, India, Turkey, Egypt, Italy, Iran and Spain; about 50 Mt are grown for processing industry that yields to the production of various by-products (pomace, skins and seeds) representing 4e13% of the whole tomato weight (Ventura et al., 2009; del Valle et al., 2007) or around 3 to 6 Mt/year. Tomato by-products can be further exploited according to the biorefinery concept, since they contain large amounts of high added-value compounds such as carotenoids mainly in the form of lycopene and b-carotene that are authorized natural food pigments (Riggi and Avola, 2008). However tomato processing waste (TPW) is often dumped near productions sites or landfilled causing liquid emissions to soil, odour problems and, due to uncontrolled anaerobic fermentation, methane emissions in the atmosphere (Encinar et al., 2008). TPW can be also used as animal feed, but they are usually ensiled or dried before being fed to ru et al., 2013), this minants, poultry and other livestock (Heuze management option is however not fully satisfactory due to the low

* Corresponding author.  ). E-mail address: [email protected] (P.S. Calabro http://dx.doi.org/10.1016/j.jenvman.2015.07.061 0301-4797/© 2015 Elsevier Ltd. All rights reserved.

nutritional value of TPW and high transportation costs. The use of agricultural wastes and residues for energy production is a very promising management option since its social, environmental and economic sustainability is fully demonstrated (Schievano et al., 2009; Valentine et al., 2012; Rahman et al., 2014). The use of tomato waste for biomethane production through anaerobic digestion has been also proposed with reported yields that range from 199 to 384 NmL CH4/gVS (Gunaseelan, 2004; Ward lez-Gonza lez and Cuadros, et al., 2008; Dinuccio et al., 2010; Gonza 2013). TPW is acidic (pH around 4.5) and is relatively rich in protein and fat. Fibre content is high (crude fibre is in the 33e57% DM range), while lignin content has been reported to be around 4% on a DM basis (Dinuccio et al., 2010). The acidic pH of TPW is a potential limitation for its use as a substrate for anaerobic digestion. Therefore, the either the codigestion with other substrates or, if this latter is not feasible for technical-economic reasons, a neutralisation pretreatment could be considered beneficial (Bouallagui et al., 2009). Actually, alkaline pretreatment (with NaOH, KOH, lime, ammonia, and urea) has been widely studied in the past before the anaerobic digestion of various other organic substrates and has been efficient in neutralizing pH, in altering the structure of lignin, in solubilizing hemicellulose and in increasing the accessibility of cellulose by the anaerobic consortium (Montgomery and

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Bochmann, 2014). However, the obtained results with NaOH pretreatment have been quite variable in the literature even when similar substrates were compared (McIntosh and Vancov, 2010; Sills and Gossett, 2011; Monlau et al., 2012; Sambusiti et al., 2013a,b). No information exists in the literature on the effect of alkaline pretreatment on the anaerobic digestion of TPW. The objective of this work was to assess if a mild alkaline pretreatment carried out at 20
C, with a duration of either 4 and 24 h and with a limited NaOH dosage (1% or 5%) could be beneficial to the anaerobic biodegradability of TPW. Biochemical methane potential (BMP) experiments were carried out in duplicate and obtained results were modelled with first order kinetics to study the effect of alkaline treatment on methane generation rates.

2. Materials and methods 2.1. Sample characterization Approximately 2 L of TPW were obtained from a small plant located in the province of Reggio Calabria (Italy) that processes tomatoes to obtain tomato puree. The received sample was distributed in 250 mL glass digestion flasks, closed with caps. In each flask, the TPW samples were soaked in NaOH solutions at different dosages (1 and 5 gNaOH/100 gTS) with a total solid concentration of about 120 gTS/L. The flasks were maintained at 20
C for either 4 or 24 h in an incubator, according to the experimental design of Table 1. Note that sample (a) is the TPW as received without any addition of water, while sample (A) is the control obtained after adding the same amount of water that had been added to the other samples. Total solids (TS), volatile solids (VS), pH and COD (for the pretreated samples) were determined according to standard procedures (APHA et al., 2005). Ultimate analysis (C, H, N, S) of the control and of the pretreated samples was performed with an elemental analyzer (Thermo-Electron, USA, model: EA-1110, CHNSeO) according to Komilis et al. (2012). Biochemical methane potential (BMP) tests were performed in duplicate under mesophilic conditions (35 ± 0.5
C). Tests were performed using a custom made method that is a modification of that described in Schievano et al. (2008). The method employs WTW OxiTop® AN6 (Germany) 1.1 L bottles that were placed in a thermostatic cabinet at 35 ± 0.5
C and were equipped with manometric OxiTop® heads. The contents (substrate and inoculum) of each bottle were mixed by a magnetic stirrer throughout the 30 d period. When pressure inside the bottle reached the maximum allowable level (350 hPa) biogas was slowly transferred into a second OxiTop® bottle containing 0.3 L of a 3 M NaOH solution using a 100 mL syringe (Schievano et al., 2008). Through a side opening of the second bottle, a tube allowed to transfer biogas by the syringe, whilst a valve placed in the tube was used to open or close the tube that was immersed into the alkaline solution. The pressure increase in the second bottle was converted, using the ideal gas law, to moles of CH4 generated. Those methane measurements were performed 6 to

8 times during each experiment with the goal to obtain an average CH4 content. Biogas measurements in the first bottle were calculated via the ideal gas law using the pressure readings that were logged every 2 h during the 30 d experimental period. The CH4 production was then calculated via the biogas production profile and an average CH4 content. All samples were inoculated with an anaerobic inoculum that was taken from the second stage of a digester the operated under mesophilic conditions and had been fed on agro-residuals (cattle manure, chicken manure, agriculture residuals and other industrial residues coming from the transformation of agriculture products such as olive and oranges). Immediately after sampling, inoculum was sieved (<1 mm) to remove large fibrous materials (e.g. straw) and was then kept under endogenous anaerobic conditions at 35
C for about 10 days to reduce non-specific biogas generation. The inoculum had a solid content of 49.0 gTS/L and 34.4 gVS/L. Each batch was prepared by mixing 50 mL of inoculum with 50 mL of distilled water, then, immediately at the end of the pretreatment period, substrate was added by keeping an inoculum to substrate ratio (ISR, on a VS basis) at around 2.5. Around 0.75 gTS of substrate were placed in each BMP bottle. The net specific biochemical methane production (BMP) was calculated as follows:

VCH4;s  VCH4;blank Net BMP ¼ VSs $Vs


(1)

where: BMP in NmL CH4/g VS; VCH4 is the 30 d gross methane production from all treatments (substrate þ inoculum) in NmL CH4; VCH4,blank is the 30 d methane production of the inoculum in NmL CH4; VSs is the concentration of volatile solids from the feedstock in the bottle at the beginning of the test (g VS/L) and VS is the liquid volume (L) in the BMP bottle. The biodegradability (B) of the substrates was calculated according to Equation (2) (Sambusiti et al., 2013a):

BMP B ¼ COD VS $350

(2)

where: BMP is the net volume of methane produced (NmL/g VS) after 30 d, 350 is the theoretical volume (mL) of methane produced during the degradation of 1 g of COD and COD/VS is in units of g/g. The anaerobic digestion process was assumed to follow first order kinetics (Angelidaki et al., 2009; Sambusiti et al., 2013a,b). The first order kinetic constant was estimated by fitting experimental data using the following first order equation:

  BMPðtÞ ¼ BMPt/∞ $ 1  expkh $t

(3)

where: BMP (t) is the cumulative methane yield at time t, BMPt/∞ is the ultimate methane yield of the substrate, kh is the first order kinetic constant (d1) and t is the time (d). Based on the above, the individual cumulative methane measurements collected during the experimental period were fitted to Equation (3) to calculate kh (d1)

Table 1 Characteristics of untreated and treated TPW. Sample

NaOH dosage [gNaOH/100gDM]

Incubation time [h]

TS [% WM]

VS [%DM]

pH achieved after pretreatment

COD [mg/gDM]

aa Aa B C D

0 0 5 1 5

0 24 4 24 24

16.5 10.9 13.2 10.9 12.4

94.6 92.1 90.9 90.3 89.8

4.3 4.6 9.6 4.5 7.1

N.A. 1180 1000 1035 1030

a

Controls in which no NaOH was added. Results from reactor A were used in subsequent calculations to calculate net BMP.

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and BMPt/∞. The regression modelling was done with an Excel® spreadsheet using the least squares method after applying the Solver® optimization routine.

3. Results and discussion Fig. 1 shows the specific methane production of each treatment and for each replicate separately. The fitted equation is based on the aforementioned first order kinetic model (R2 was always higher than 0.94 in all fittings). It is noted that this first order kinetic profile, with a very short methane generation lag phase, has been also observed with other acidic organic wastes studied with BMP experiments, such as citrus cs et al., 2012). ANOVA showed that at p < 0.01, the wastes (Forga COD of the starting material was not influenced by the pretreatment; therefore, in the following calculations (e.g. biodegradability) the average COD value (1060 mg/g DM) was used. This result was somewhat confirmed by a visual inspection carried out by SEM (Phenom ProX) which actually did not show evidence of any clear effect of alkaline pretreatment on the structure of the substrate. Table 2 presents the ultimate analysis of all substrates (before and after pretreatment), the calculated theoretical empirical formula as well as the theoretical methane production per substrate based on the widely used Buswell formula. As shown, only the total carbon contents of substrates A (control) and B differed significantly (at p < 0.05). Still, this was a marginal difference, whilst all other substrates had statistically similar carbon contents. Table 3 summarizes the experimental and modelling results. According to Table 3, the methane yield was comparable to that reported in the scientific literature for other organic wastes (Gunaseelan, 2004; Ward et al., 2008; Dinuccio et al., 2010; lez and Cuadros, 2013). Moreover the BMP reGonz alez-Gonza sults, in a similar manner to the characterization results, clearly demonstrated that the alkaline pretreatment did not influence significantly BMPexp, BMPt/∞ and biodegradability. Most likely, this was due to the fact that the substrate was already sufficiently

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accessible to biomass, which can be explained by the already low lignin content of TPW. Therefore, despite the acidic initial pH of raw TPW (around 4.5), no differences in terms of methane yield were observed after 30 days among any of the treatments. The average BMPexp from all treatments (including the control) was 320 NmL/ gVS. Interestingly, however, a reduction in the rate of methane production (see kinetic constants of Table 3) was observed for all treated samples compared to the control. In particular, reactors B and D (both with the 5% NaOH dosage) resulted in the lowest kinetic constants, which can be explained by either an accumulation of salts (Montgomery and Bochmann, 2014) or the highly alkaline pH (9.6) of the substrate in the case of reactor B. It is well known that alkaline pHs can lead to increased ammonia concentrations that can inhibit methanogenesis (Montgomery and Bochmann, 2014). In fact, the time for reaching 75% of the experimental final BMP increased from about 10 days (untreated sample) to about 15 days (sample treated with 5% NaOH for 24 h). According to Equation (2), the biodegradability of all substrates ranged from 0.75 to 0.82 (see Table 3). Similarly, using the kinetic modelling and the calculations of the theoretical methane potential of all samples (based on the Buswell formula and on the ultimate analyses results), it was found that the biodegradable fraction of the TPW under anaerobic conditions ranged from 0.66 to 0.72 (see Table 3). Thus, both approaches give comparable results with regard to the biodegradable fraction of TPW under anaerobic conditions.

4. Conclusions Alkaline pretreatment of TPW did not affect methane yield in any of the treatments in comparison to the control. The average net methane production of all treatments (including the control) was 320 NmL CH4/g VS. On the other hand, alkaline pretreatment reduced the rate of methane generation, compared to the control, probably due to the high pH achieved (reactor B) or due to a likely accumulation of salts. The biodegradability of the substrates ranged from 0.75 to 0.82, based on Equation (2), and from 0.66 to 0.72

Fig. 1. Actual measurements (the dotted values shown are from 2 replicate reactors per treatment) and fitting of the modelled methane production (solid lines) for: A) untreated TPW; B) 5% NaOH e 4 h TPW; C) 1% NaOH e 24 h TPW and D) 5% NaOH e 24 h TPW.

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Table 2 Ultimate analysis of samples and theoretical methane generation according to the Buswell formula. Sample

NaOH dosage [gNaOH/100gDM]

Incubation time [h]

C (%DM)

N (%DM)

H (%DM)

Oa (%DM)

Empirical formula

A B C D

0 5 1 5

24 4 24 24

43.4a 39.3b 40.4ab 39.6ab

2.56a 1.96b 2.48a 2.13ab

8.44a 8.48a 7.84a 8.58a

37.7 41.1 39.6 39.5

C20H46O13N C23H61O18N C19H44O14N C22H56O16N

Means on the same column that share the same letter indicate statistical similarity at p < 0.05 based on Tukey's test with n ¼ 5. a Oxygen content was indirectly calculated by the difference of the sum of C, H, N, S from the VS content. No sulphur was detected in any of the samples.

Table 3 BMP results and calculated kinetic coefficients. Substrate

BMPexp [NmL/gVS]a

Untreated (A) 5% NaOH e 4 h (B) 1% NaOH e 24 h (C) 5% NaOH e 24 h (D)

330 305 335 324

± ± ± ±

10 1 20 29

Diff.b [%]

Biodegr.c [e]

Theor. CH4 yieldd [NmL/g VS]

BMPt/∞ [NmL/gVS]e

kh [d1]e

Biodegr. fraction in anaerobic conditionsf [e]

e 7.6 þ1.5 1.8

0.82 0.75 0.82 0.78

536 493 491 511

353 344 354 358

0.110 0.076 0.097 0.075

0.66 0.70 0.72 0.70

a

Methane yield after 30 days of incubation based on duplicate runs (mean ± STD). Deviation between the BMP of each run and the BMP of the control (untreated sample). c Biodegradation as defined in Equation (2). d Calculated theoretically by the Buswell formula (CcHhOoNnSs þ 1/4(4c  h  2o þ 3n þ 2s)H2O / 1/8(4c þ h  2o  3n  2s)CH4 þ 1/8(4c  h þ 2o þ 3n þ 2s) CO2 þ nNH3 þ sH2S), according to the ultimate analysis results of Table 2 assuming that all organic matter is degradable. e BMPt/∞ and kh are calculated via the kinetic modelling. f BMPt/∞ expressed as a fraction of the theoretical methane yield that was calculated by the Buswell formula. It indicates the biodegradable fraction of the volatile solids of the TPW under anaerobic conditions. b

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