Bioresource Technology 170 (2014) 100–107
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Application of hydrothermal treatment to affect the fermentability of Pinus radiata pulp mill effluent sludge John Andrews a,⇑, Anne-Marie Smit a, Suren Wijeyekoon a, Ben McDonald a, Saeid Baroutian a,b, Daniel Gapes a a b
Scion. Te Papa Tipu Innovation Park, 49 Sala Street, Rotorua 3046, New Zealand Department of Chemical & Material Engineering, Faculty of Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
h i g h l i g h t s Significant removal of lignin and hemicellulose components. Reduction in the concentration of extractive compounds, such as resin acids, >99%. Some resistance to degradation of the cellulose component. No demonstrable increase in biological VFA or methane after pre-treatment.
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Article history: Received 28 April 2014 Received in revised form 8 July 2014 Accepted 9 July 2014 Available online 16 July 2014 Keywords: Wet oxidation Lignocellulose Biochemical acidogenic potential assay Resin extractives Waste minimisation
a b s t r a c t A hybrid technique incorporating a wet oxidation stage and secondary fermentation step was used to process Pinus radiata pulp mill effluent sludge. The effect of hydrothermal oxidation at high temperature and pressure on the hydrolysis of constituents of the waste stream was studied. Biochemical acidogenic potential assays were conducted to assess acid production resulting from anaerobic hydrolysis of the wet oxidised hydrolysate under acidogenic conditions. Significant degradation of the lignin, hemicellulose, suspended solids, carbohydrates and extractives were observed with wet oxidation. In contrast, cellulose showed resistance to degradation under the experimental conditions. Extensive degradation of biologically inhibitory compounds by wet oxidation did not show a beneficial impact on the acidogenic or methanogenic potential compared to untreated samples. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Current commercial pulp and paper manufacturing processes involve the use of large amounts of water, from which substantial wastewater volumes are generated. This wastewater contains a number of dissolved and suspended solids. The waste solids from effluent treatments can be utilised for energy generation, recycled beneficially to land or, as is often the case, simply sent to landfill. There is a need to maximise the beneficial use of this biomaterial and find more sustainable long-term solutions to landfill. Resin extractives are one component of pulp-mill effluent. Their hydrophobic nature results in their becoming associated with the fibre and biomass fractions of pulp-mill waste. They are seen to inhibit biological action, making the solids potentially toxic and less amenable to biological treatment technologies (Kostamo and ⇑ Corresponding author. Tel.: +64 7 3435605. E-mail address: [email protected] (J. Andrews). http://dx.doi.org/10.1016/j.biortech.2014.07.037 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Kukkonen, 2003; Makris and Banerjee, 2002). Methane yields become negatively affected, resulting in little uptake of anaerobic digestion as a process to treat pulp-mill effluent solids (Kennedy et al., 1992; Lettinga et al., 1991; Sierra-Alvarez et al., 1994; Vidal and Diez, 2005; Puhakka et al., 1992; Turick et al., 1991). A number of pre-treatments can be used to reduce the concentration of inhibitory resin extractives in order to improve anaerobic degradability. Such treatments include ozonation (Pokhrel and Viraraghavan, 2004), thermal or thermochemical (caustic) degradation (Elliott and Mahmood, 2007; Wood et al., 2009), treatment with strong alkali with ultrasound (Park et al., 2012), wet oxidation (Laari et al., 1999; Verenich et al., 2004), hydrolysis using enzymes from fungi (Hodgson et al., 1998) and separation methods, such as ion exchange chromatography and nanofiltration (Ciputra et al., 2010). Addition of activated carbon to effluent can assist with filtration as well as adsorbing certain organic compounds. It is proposed in this study that the selective removal of resin extractives will enhance the anaerobic biodegradability of pulp
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mill effluent solids. The objective is to use wet oxidation as a pre-treatment, to quantify the extent of degradation of resin acids and phenolics, and to assess the biodegradability of the treated residues under anaerobic conditions. 2. Methods 2.1. Sample preparation Biological sludge was obtained from the wastewater treatment works of the pulp mill at Karioi, New Zealand, which is owned by Winstone Pulp International’s (WPI). This mill processes Pinus radiata wood. This sludge was derived from the treatment of a lignocellulosic effluent and consisted of a mixture of waste-activated sludge (WAS), process-dissolved air flotation (DAF) sludge and primary effluent DAF sludge. The sludge was dewatered at the mill to give a residue containing approximately 35% dry solids. Upon collection, this sludge was frozen until needed for further processing. When required, a sample was taken and resuspended by diluting with tap water to give a concentration of 3% total suspended solids (TSS). 2.2. Wet oxidation of Pinus radiata pulp mill effluent solids Wet oxidation was conducted using 150 mL samples of the 3% TSS slurry. The WO reactions were undertaken in a Parr high-pressure reactor (Parr Instrument Company, Model 4540, total volume 600 mL) equipped with a stirrer and heating jacket (Fig. 1). The
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apparatus was set up so that the biomass was conveyed to the Parr reactor only when it had reached the desired temperature. This was done in order to avoid a slow start-up heating stage, during which unintended reactions may take place. The process involved first adding 150 mL of tap water to the Parr reactor, charging it with 20 bar of pure oxygen (BOC NZ Ltd. – zero grade), stirring at 500 rpm and heating to 220 °C. A 190 mL stainless-steel thermal reactor (Berghof, Germany) was connected to the Parr reactor to which the 150 mL lignocellulosic sample was added. Nitrogen gas (BOC NZ Ltd. – zero grade) was added to the Berghof reactor at a pressure 10 bar greater than that present in the Parr reactor at temperature, and the pressure differential was used to convey material. The sample was maintained at 220 °C for a set time period before being transferred to a container stored in an ice bath to rapidly cool the sample. A portion of the sample was immediately frozen and stored ( 20 °C) prior to being analysed for resin extractives and carbohydrates. Another portion of the sample was stored in a chiller (4 °C) before being processed within 24 h for analysis of total suspended solids, volatile suspended solids (VSS), dissolved organic carbon (DOC), volatile fatty acids (VFA) and soluble chemical oxygen demand (sCOD). Wet oxidation treatments were carried out for 5 min (T5), 20 min (T20) and 60 min (T60). Separate runs were undertaken using different aliquots of starting material. Each run was carried out in triplicate. The time immediately prior to commencing the wet oxidation process for each experimental run was designated as T0 which represents the untreated raw material.
Fig. 1. Wet oxidation reactor configuration. CW = cold water.
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2.3. Performance of wet oxidation liquor to anaerobic conditions The effect of anaerobic fermentation on the wet oxidised samples was assessed in terms of VFA production, using a biochemical acidogenic potential (BAP) assay. This was an adaptation of the biochemical methane potential (BMP) methodology (ASTM, 2008). Specifically for this work, an anaerobic acidogenic inoculum was used. Single representative samples for each timepoint (T0, T5, T20 and T60 min), were prepared by mixing duplicates together from the WO stage. For each timepoint, 10 mL (12.5 g VSS/L) of sample, 10 mL (12.5 g VSS/L) of inoculum, medium, Na2S and water as defined in the standard (ASTM, 2008) were added to separate 160 mL serum bottles. Nickel (NiCl26H2O) at 0.005 g/L was included in the medium as a microelement (Speece et al., 1983). Serum bottles were flushed with nitrogen, sealed, crimped and placed on a shaker-incubator at 36 °C. The following controls were prepared: a blank sample containing only medium; a negative control consisting of inoculum and medium; a positive control containing inoculum, medium and cellulose (1000 mg/L, Whatman microgranular cellulose powder CC31). A sacrificial sampling regime was used as a non-invasive means to measure degradation rates of solids, VFA production and changes to DOC and sCOD concentration over time. This meant that for each condition (including the blank and controls) 10 samples were prepared which were then sacrificed for analysis on discrete days, reducing the number of samples from 10 to one by the 10th day of sampling. Each of these sacrificial samples and the controls were initially prepared in triplicate. The BAP assay was run for a duration of 27 days, which is shorter than 50–100 days typically used in BMP assays. This was used as the target was enhanced acid production, rather than methane production. Gas was produced in the sealed serum bottles as a consequence of the fermentation. 2.4. Analytical procedures 2.4.1. Solids analysis Total suspended solids and volatile suspended solids were determined using standard methods for the examination of water and wastewater (APHA, 1998). The values obtained were used to calculate relevant yields of products. Soluble chemical oxygen demand was determined using according to a standard method (APHA, 1998). 2.4.2. Chemical analysis 2.4.2.1. Lignocellulosic extractives and volatile fatty acids (VFAs). Lignocellulosic extractive concentrations and VFAs were determined before and after wet oxidation. In addition, the VFA concentrations were determined before and after fermentation. A 5 mL sample was taken and shaken with 5 mL dichloromethane. A 1 mL sample of the organic layer was dried with anhydrous sodium sulphate and transferred to a GC vial. An injection standard (50 lL 9,10-dibromoanthracene) and pyridine (50 lL) were added to the vial with derivatization reagents N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) and 1% trimethylchlorosilane (TCMS). The vial was heated to 70 °C (1 h) prior to analysis. The analysis of extractive compounds and VFAs was performed on an Agilent 6890N gas chromatograph with a CTC CombiPal auto-sampler connected to an Agilent 5973N mass selective detector (MSD). A high performance Ultra-2 column (50 m 0.200 mm 0.33 lm) was used with helium as the carrier gas. The temperature was increased from 60 °C to 300 °C during the analysis. 2.4.2.2. Neutral carbohydrate analysis. The types and amounts of neutral monosaccharides present in the liquid phase of each sample were determined directly on filtered solutions. Individual
monosaccharides were separated using a high performance anion exchange chromatograph (Dionex IC3000) fitted with a pulsed amperometric detector and a PA10 column. The eluent was 2 mM KOH. Identification of monosaccharides was undertaken by comparison of retention times with standards containing arabinose, xylose, galactose, glucose, and mannose. Fucose was added as an internal standard to enable the amount of each monosaccharide to be quantified. A separate aliquot of each sample was hydrolysed using H2SO4 (final concentration 4%) at 15psi (103 kPa, 121 °C) for 60 min. These samples were analysed as above to determine the amount and composition of total soluble carbohydrates present. 2.4.2.3. Carbon and nitrogen analysis. Dissolved organic carbon (DOC) was determined using a High TOC II analyser (Elementar, Hanau, Germany). Total Kjeldahl nitrogen (TKN), dissolved Kjeldahl nitrogen (DKN) and ammoniacal nitrogen (NH4-N) were analysed using a modified Dumas method (Rayment and Lyons, 2011) on a LECO (Laboratory Equipment Corporation) CNS-2000 analyzer (St Joseph, MI). The amount of each of these components was calculated according to standard methods for water and wastewater (APHA, 1998) with a detection limit set at 0.2 mg/L in each case. Total organically bound nitrogen (TON) was calculated as the difference between TKN and NH4-N. 2.4.3. Gas analysis Pressure was monitored using an in-house built electrical device consisting of a reader coupled to a pressure sensor (Sensor Technics). Once the pressure reached a level of 6 kPa, the composition of the gas was analysed using gas chromatography. Samples were collected approximately every 3 days. Methane, carbon dioxide, and hydrogen concentrations in the head space were determined using a gas chromatograph (GC) (Carle Instruments, Inc., GC 8700, USA) with a thermal conductivity detector and helium as the carrier gas (22 mL/min, 54 °C). 2.5. Solid state NMR spectroscopy Solid state 13C NMR spectroscopy was used to generate spectra of lignocellulose from freeze-dried non-WO and WO samples. All 13 C NMR spectra generated had a Gaussian line broadening of 25 Hz applied prior to Fourier transformation. Spectra were calibrated so that the cellulose interior C-4 peak was assigned a value of 89.3 ppm (Wikberg and Liisa Maunu, 2004; Liitiä et al., 2003), previously established relative to polydimethylsilane at 1.96 ppm, and in turn measured relative to tetramethylsilane at 0 ppm. The samples were spun at 5 kHz in a 4 mm Bruker SB magic-angle spinning probe, for 13C NMR at 50.3 MHz using a Bruker 200 DRX spectrometer. For standard cross-polarisation (CP) experiments, each 1.5 s pulse delay was followed by a proton preparation pulse of duration tp = 4.6 ls, a 1 ms contact time and a 30 ms acquisition time. The proton transmitter power was increased to a value corresponding to a 90° pulse width of 2.8 ls for proton decoupled during 13C data acquisition. 2.6. Statistical analysis Data were analysed using Statgraphics Centurion XVI (Statpoint Technologies Inc.) and Minitab 15Ò Statistical Software. Multiple variable analysis using Pearson product-moment correlations was carried out with 95% confidence intervals to determine linear relationships between pairs of variables. A one-way analysis of variance (ANOVA) was used to test for differences between variables for each variable measured (p < 0.05). Multiple range tests were performed using Fisher’s Least Squares Significant Difference
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Fig. 2. Compositional changes determined in Pinus radiata pulp mill effluent solids’ samples with hydrothermal treatment (A) percentage TSS and VSS, (B) sCOD and DOC, (C) TKN, DKN, TON, NH4-N, (D) VFA concentrations.
(LSD) method to determine which means differed significantly from each other at the 95% confidence level. 3. Results and discussion 3.1. Hydrothermal-wet oxidation The WO of pulp mill effluent solids released high levels of lignocellulosic-derived residues from the original untreated biomass. Temporal changes through WO and subsequent anaerobic fermentation are discussed below. 3.1.1. Change in TSS, VSS and carbon availability A 78% destruction of TSS and VSS had occurred after 5 min which had increased to 88% destruction of both TSS and VSS by twenty minutes (Fig. 2a). The oxidative hydrolysis resulted in solubilisation of particulate organics, leading to a significant increase in sCOD and DOC after 5 min (Fig. 2b). This result is comparable with that previously achieved under similar conditions with municipal biosolids (Strong et al., 2011). As oxidation reactions in the liquid phase progressed, DOC was observed to decline from a
maximum, as evidenced from the T5 to T60 sample data. The purity of the DOC changed from 12% acetic acid at T5 to 48% acetic acid at T60. 3.1.2. Nitrogen Changes in TKN, DKN, TON and NH4-N with reaction time are presented in Fig. 2c. Statistical analysis showed no significant differences existed between concentrations of TKN at each time point. The DKN concentration doubled after 5 min of treatment but showed no significant change subsequently. This result demonstrated that significant solubilisation of particulate TKN occurred rapidly in WO. Notably, the amount of total organic nitrogen was negatively correlated with prolonged oxidative treatment whereas the amount of NH4-N was positively correlated, with a significant difference in concentrations between T0 and T60. These data suggest that prolonged wet oxidation treatment ultimately converts more dissolved organic nitrogen from the sludge to NH4-N. 3.1.3. Volatile fatty acids (VFAs) Acetic acid was the dominant VFA, increasing during WO with 0.8 g/L at T5 and greater than 2 g/L by T60 (Fig. 2d). Acetic acid is a key product from the oxidation of a range of organic
Fig. 3. Concentration of neutral carbohydrates in Pinus radiata pulp mill effluent solids following hydrothermal treatment.
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Table 1 Characteristic changes in Pinus radiata pulp mill effluent solids extractives (mg/L) following hydrothermal treatment. Type
Compound
T0 mina
T5 minb,c
T20 minb,c
T60 minb,c
Monoterpenes
Alpha-pinene Alpha-terpineol Beta-pinene Borneol Camphor Fenchone Terpinen-4-ol Total monoterpenes
10.6 ± 1.3 1.16 ± 0.11 13.6 ± 1.5 0.04 ± 0.01 0.45 ± 0.01 0.17 ± 0.02 0.42 ± 0.05 26.5 ± 2.8
n.d n.d n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d n.d n.d
Phenolics
Acetovanillone Eugenol Ferulic acid Guaiacol Pinosylvin, mono methyl ether Vanillin Vanillic acid Total phenolics
0.03 ± 0.03 n.d 0.31 ± 0.03 n.d 36.23 ± 0.91 0.17 ± 0.04 n.d 36.7 ± 1.0
0.62 ± 0.08 n.d n.d 0.10 ± 0.03 0.15 ± 0.07 7.5 ± 1.8 0.45 ± 0.32 8.8 ± 2.2
0.12 ± 0.01 n.d n.d n.d n.d 0.51 ± 0.07 0.01 ± 0.01 0.64 ± 0.09
n.d n.d n.d n.d n.d 0.10 ± 0.03 n.d 0.10 ± 0.03
Fatty acids
Decanoic acid (F10:0) Docosanoic acid (F22:0) Dodecanoic acid (F12:0) Eicosanoic acid (F20:0) Elaidic acid (F18:1) Linoleic acid (F18:2) Linolenic acid (F18:3) Margaric acid (F17:0) Oleic acid (F18:1) Palmitic acid (F16:0) Palmitoleic acid (F16:1) Stearic acid (F18:0) Tetradecanoic acid (F14:0) Tetracosanoic acid (F24:0) Total fatty acids
0.05 ± 0.05 16.77 ± 0.72 0.17 ± 0.01 8.49 ± 0.28 21.44 ± 0.55 389 ± 26 56.0 ± 3.6 16.57 ± 0.95 368 ± 15 42.12 ± 0.04 5.24 ± 0.33 10.72 ± 0.38 3.69 ± 0.25 2.58 ± 0.35 941 ± 35
0.23 ± 0.05 n.d 0.06 ± 0.03 0.10 ± 0.10 n.d 4.3 ± 2.2 n.d 0.30 ± 0.15 6.1 ± 2.6 1.83 ± 0.36 n.d 0.64 ± 0.10 0.28 ± 0.16 n.d 13.8 ± 4.8
n.d n.d 0.07 ± 0.07 n.d n.d 0.53 ± 0.53 n.d 0.06 ± 0.06 2.2 ± 1.1 1.89 ± 0.79 n.d 0.38 ± 0.25 0.27 ± 0.27 n.d 5.4 ± 1.0
n.d n.d 0.01 ± 0.01 n.d n.d 0.59 ± 0.59 n.d 0.04 ± 0.04 1.85 ± 0.93 2.9 ± 1.6 n.d 0.40 ± 0.27 0.07 ± 0.07 n.d 5.84 ± 0.15
Resin acid neutrals
Dehydroabietin Methyldehydroabietin Total resin acid neutrals
0.28 ± 0.04 0.39 ± 0.06 0.67 ± 0.10
n.d n.d n.d
n.d n.d n.d
n.d n.d n.d
Resin acids
Abietic acid Dehydroabietic acid Isopimarenic acid Isopimaric acid Levopimaric acid Neoabietic acid Palustric acid Pimaric acid Sandaracopimaric acid Seco-1-dehydroabietic acid Seco-2-dehydroabietic acid 7-Oxodehydroabietic acid Total resin acids
4711 ± 299 229 ± 7.0 0.89 ± 0.89 131.4 ± 7.9 258 ± 15 750 ± 37 531 ± 38 238 ± 12 44.2 ± 2.5 70.9 ± 3.5 63.5 ± 2.9 8.7 ± 3.0 7038 ± 426
61 ± 18 6.4 ± 1.8 n.d 1.57 ± 0.48 n.d 5.0 ± 2.6 3.4 ± 2.0 3.6 ± 1.0 0.34 ± 0.19 1.19 ± 0.31 0.72 ± 0.42 9.5 ± 6.4 92 ± 22
29.2 ± 8.3 3.29 ± 0.71 n.d 0.59 ± 0.30 n.d 1.91 ± 0.98 1.57 ± 0.46 1.81 ± 0.40 0.12 ± 0.12 0.19 ± 0.19 0.18 ± 0.18 0.32 ± 0.32 39.18 ± 11.72
25.3 ± 7.9 3.37 ± 0.60 n.d 0.55 ± 0.30 0.42 ± 0.42 1.1 ± 1.1 1.60 ± 0.64 1.62 ± 0.39 0.09 ± 0.09 0.19 ± 0.19 0.15 ± 0.15 n.d 34.3 ± 11.4
Phytosterols
Campesterol Cholesterol Sitostanol Sitosterol Stigmasterol Total phytosterols
1.13 ± 0.13 0.37 ± 0.37 3.72 ± 0.18 28.5 ± 1.3 n.d 33.7 ± 2.0
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
8077 ± 392
115 ± 26
45 ± 13
40 ± 11
Total extractives
n.d. = not detected, method detection limit is 0.01 lg/L, standard error of the means are given. a Denotes raw feed samples with no added oxidant, i.e. before initiation of wet oxidation designated time zero (T0). b Denotes samples subjected to wet oxidation treatment for 5 min (T5 min), 20 min (T20 min) and 60 min (T60 min) with added oxygen at 20 bar. c Denotes all compounds were normalised to raw feed initial biomass and reported here as mg/L.
compounds. Under the temperatures used in the current study, acetic acid degradation is kinetically limited (Shende and Levec, 1999). The degradation of sugars is also likely to be a source of acetic acid. 3.1.4. Solid state NMR spectroscopy Solid-state 13C NMR spectra of non-WO (T0) and WO (T5) samples were compared. Both spectra are dominated by signals characteristic of cellulose. The signals were assigned to the six carbon atoms of a glucosyl structural unit as follows (Liitiä et al., 2003):
C-1 (105 ppm), C-2,3,5 (73 and 75 ppm), C-4 (84 and 89 ppm), C-6 (64 and 66 ppm). Signals from hemicellulose were mostly obscured by signals from cellulose, although a signal at 102 ppm was assigned to C-1 of mannosyl structural units in glucomannan (Liitiä et al., 2003). Signals characteristic of lignin are labelled ‘‘L’’ (Wikberg and Liisa Maunu, 2004): a signal at 56 ppm was assigned to methoxyl groups, and a band of signals between 140 and 155 ppm showed a profile characteristic of C-3 and C-4 of the aromatic rings in the guaiacyl lignin of a softwood. A band of signals between 110 and 140 ppm was consistent with C-1,2,5 of the
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aromatic rings of lignin, but was considered less diagnostic because of contributing signals assigned to proteins and resin acids. Signals at 18, 24, 38, 48 and 187 ppm were assigned to resin acids, including abietic acid (Lambert et al., 1999). A signal at 173 ppm was assigned to the amide carbon of amino acid structural units in proteins (Calucci et al., 2003). Whilst proteins also contribute signals in the range 10–50 ppm, assigned to the sidechains of amino acids, that range was obscured by signals assigned to resin acids. The signals assigned to resin acids were barely detectable in the NMR spectrum of the sample subjected to 5 min of WO treatment. The signals assigned to lignin and hemicellulose were decreased in strength, indicating partial solubilisation by wet oxidation, and a new signal at 177 ppm was assigned to carboxylic acid functional groups (ACO2H) in partly-oxidised structures remaining associated with the insoluble matter. Signals assigned to cellulose and protein persisted in the sample subjected to 5 min of WO. 3.1.5. Neutral carbohydrates In non-WO (T0) samples, arabinose, galactose, glucose, xylose and mannose were exclusively present in polymeric forms. Within 5 min of wet oxidation (T5) the monomeric saccharides predominated for arabinose, galactose, xylose and mannose, and the concentrations had decreased (Fig. 3). The monomers had themselves degraded by 20 min. Glucose polymers showed greater resistance to oxidation. Polymeric forms of glucose remained throughout the 60 min hydrothermal treatment with reducing concentrations over time. The glucose monomer increased in concentration by T5 but had itself been degraded considerably by T60. The hemicellulose fraction degrades readily in hydrothermal conditions but the cellulose is much more resistant to degradation (Garrote et al., 1999). 3.1.6. Lignocellulose-derived extractives The concentrations of lignocellulose-derived extractives from the solubilized hydrolysate are presented in Table 1. Total extractives concentrations decreased to 0.1 g/L at T5 from 8 g/L at T0. The different groups of extractives detected and quantified are discussed below. Monoterpenes. The total monoterpene concentrations determined at T0 were 26.5 mg/L, dominated by alpha- and beta-pinene. By the T5 timepoint monoterpenes were totally degraded. Phenolics. From a suite of 15 phenols that were screened, the main phenols present in the raw feed were pinosylvin monomethyl ether (PSMME), vanillin and vanillin derivatives. Low concentrations of guaiacol (0.10 mg/L), acetovanillone (0.62 mg/L) and vanillic acid (0.45 mg/L) formed by T5 but had substantially degraded by T20. The stilbene, PSMME, was the dominant phenolic at T0 (36 mg/ L), but had degraded substantially by T5. By the T5 timepoint, the WO treatment used had degraded 36.7 mg phenolics per g sCOD released. Unlike most of the other extractive compounds, phenolics such as vanillin, actually increased in concentration by T5. Vanillin was subsequently degraded with further wet oxidation. Fatty Acids. Fatty acids were reduced from an initial concentration of 941 mg/L to concentrations of 13.8 mg/L following 5 min of WO and 5 mg/L by 20 min. Linoleic acid and its precursor, oleic acid, were the dominant fatty acids present in the T0 samples (389.3 mg/L and 367.7 mg/L respectively). Degradation of these compounds occurred rapidly as levels were just 4.2 mg/L and 6.1 mg/L respectively by T5. Oleic acid (at concentrations above 30 mg/L) can inhibit acetate degradation and aceticlastic methanogenesis (Lalman and Bagley, 2001) so degradation of this compound should be beneficial in subsequent anaerobic fermentation. Resin acids. Resin acids contributed 87% of the extractives in this study and were present in the raw feed at a concentration of 7 g/ L, degrading massively to 0.09 g/L by T5. Abietic acid was the
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dominant resin acid and dehydroabietic acid, which is known to be particularly recalcitrant to degradation, was severely depleted from 229 mg/L at T0 to 6.4 mg/L at T5. Degradation of resin acids by WO is a promising result in overcoming inhibitory effects on anaerobic digestion. Phytosterols. Of the four phytosterols detected by GC/MS, sitosterol was present at the highest concentration of 28.5 mg/L at T0. All phytosterols were totally degraded following 5 min of WO, and is consistent with the extent of degradation observed in previous research (Baroutian et al., 2013). 3.1.7. Biochemical acidogenic potential assay (BAP) The BAP assay results for acetic acid and gas production are presented in Fig. 4 and Table 2. Anaerobic fermentation of wet oxidised lignocellulosic residues investigated in this study produced relatively low amounts of methane (300 °C and a pressure of at least 25 MPa are required for degradation of cellulose due to its crystalline structure. The lower temperature and pressure used in the current study would be ineffective at hydrolysis and conversion of the cellulose to glucose and may explain the persistence of cellulose as a major component of WO lignocellulose in this study, as determined by NMR. Hydrolysis of cellulose can therefore be considered a rate-limiting step in the biodegradability of lignocellulose under the WO conditions used here. The presence of a significant proportion of undegraded biomass is important for bacterial degradation since cellulose that is not converted to glucose, is not in a form that can be metabolized by bacteria. Extractive compounds initially bound in lignin, and dominated by resin acids, were reduced by over 99% through wet oxidation. Vanillin, however, did initially increase in concentration. Vanillin is a widely known reaction product occurring from the WO of lignin (Araújo et al., 2010). PSMME was the dominant phenolic in the feed T0 sample. PSMME is toxic to aquatic life with long lasting effects, so its effective degradation is beneficial. This work had proposed that through the elimination of potentially biologically inhibitory compounds such as resin acids, anaerobic fermentation and methanogenic activity would be noticeably enhanced. The results from the BAP assay showed a reduction over time in the acetic acid concentrations present in the T0 and T60 samples, combined with similar cumulative yields of methane. Given the T0 sample was not inhibitory to methane production and yields were similar to the highly treated T60 sample, a correlation between reduction in inhibitory compounds and methanogenic activity was not observed in this study. Despite extensive resin acid degradation in the T5 and T20 samples, little cumulative methane was observed. End-products, such as furan, were not characterised in this study. Furan can be directly produced by thermal decomposition of pentose-containing materials and cellulosic solids as found in pine. Further work would be recommended to examine the contribution of the cellulose fraction as well as the pentose-containing sugars of the hemicellulose to end-products such as furans.
4. Conclusions Wet oxidation resulted in substantial degradation of lignocellulosic residues with 77% destruction of TSS, predominantly occurring within 5 min. The removal of lignin and hemicellulose was observed along with extensive destruction of hemicellulose sugars and resin acids and production of acetic acid. Acetic acid indicates potential for resource recovery coupled with destruction of toxic compounds and inhibitors of anaerobic digestion. Despite the extensive degradation of inhibitory compounds through wet
oxidation there was not a measurable increase in VFA or methane production. There was an increase in acetic acid and hydrogen production in those anaerobic reactors which lacked methanogenic activity. The solid-state 13C NMR spectra of P. radiata pulp mill effluent solids are provided in the supplementary material in the electronic annex. Acknowledgements This work was supported by MFI from the Foundation for Research, Science & Technology (NZ) to Scion. We thank Stefan Hill and Roger Newman for assistance with NMR analysis, Laura Raymond for providing freeze-dried biomass for NMR analysis, Murray Robinson for extractives analysis, Ruth Falshaw for editorial assistance and Michael Weber for experimental data collection. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014.07. 037. References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th Edition, APHA American Public Health Association. Araújo, J.D.P., Grande, C.A., Rodrigues, A.E., 2010. Vanillin production from lignin oxidation in a batch reactor. Chem. Eng. Res. Des. 88, 1024–1032. ASTM, 2008. E2170-01 Standard Test Method for Determining Anaerobic Biodegradation Potential of Organic Chemicals under Methanogenic Conditions. ASTM International, West Conshohocken, PA. Baroutian, S., Robinson, M., Smit, A.M., Wijeyekoon, S., Gapes, D., 2013. Transformation and removal of wood extractives from pulp mill sludge using wet oxidation and thermal hydrolysis. Bioresour. Technol. 146, 294–300. Calucci, L., Forte, C., Galleschi, L., Geppi, M., Ghiringhelli, S., 2003. 13C and 1H solid state NMR investigation of hydration effects on gluten dynamics. Int. J. Biol. Macromol. 32, 179–189. Ciputra, S., Antony, A., Phillips, R., Richardson, D., Leslie, G., 2010. Comparison of treatment options for removal of recalcitrant dissolved organic matter from paper mill effluent. Chemosphere 81, 86–91. Elliott, A., Mahmood, T., 2007. Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues. Water Res. 41, 4273–4286. Garrote, G., Domínguez, H., Parajó, J.C., 1999. Hydrothermal processing of lignocellulosic materials. Holz als Roh- und Werkstoff 57, 191–202. Hodgson, J., Laugero, C., Leduc, R., Asther, M., Guiot, S.R., 1998. Fungal pretreatment by Phanerochaete chrysosporium to reduce the inhibition of methanogenesis by dehydroabietic acid. Appl. Microbiol. Biotechnol. 49, 538–544. Kennedy, K.J., McCarthy, P.J., Droste, R.L., 1992. Batch and continuous anaerobic toxicity of resin acids from chemithermomechanical pulp wastewater. J. Ferment. Bioeng. 73, 206–212. Kostamo, A., Kukkonen, J.V.K., 2003. Removal of resin acids and sterols from pulp mill effluents by activated sludge treatment. Water Res. 37, 2813–2820. Laari, A., Korhonen, S., Tuhkanen, T., Verenich, S., Kallas, I., 1999. Ozonation and wet oxidation in the treatment of thermomechanical pulp (TMP) circulation waters. Water Sci. Technol. 40, 51–58. Lalman, J.A., Bagley, D.M., 2001. Anaerobic degradation and methanogenic inhibitory effects of oleic and stearic acids. Water Res. 35, 2975–2983. Lambert, J.B., Shawl, C.E., Poinar Jr, G.O., Santiago-Blay, J.A., 1999. Classification of modern resins by solid state nuclear magnetic resonance spectroscopy. Bioorg. Chem. 27, 409–433. Lettinga, G., Field, J.A., Sierra-Alvarez, R., Van Lier, J.B., Rintala, J., 1991. Future perspectives for the anaerobic treatment of forest industry wastewaters. Water Sci. Technol. 24, 91–102. Liitiä, T., Maunu, S., Hortling, B., Tamminen, T., Pekkala, O., Varhimo, A., 2003. Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods. Cellulose 10, 307–316.
J. Andrews et al. / Bioresource Technology 170 (2014) 100–107 Makris, S.P., Banerjee, S., 2002. Fate of resin acids in pulp mill secondary treatment systems. Water Res. 36, 2878–2882. Park, N.D., Helle, S.S., Thring, R.W., 2012. Combined alkaline and ultrasound pretreatment of thickened pulp mill waste activated sludge for improved anaerobic digestion. Biomass Bioenergy 46, 750–756. Pokhrel, D., Viraraghavan, T., 2004. Treatment of pulp and paper mill wastewater—a review. Sci. Total Environ. 333, 37–58. Puhakka, J.A., Alavakeri, M., Shieh, W.K., 1992. Anaerobic treatment of kraft pulpmill waste activated-sludge: gas production and solids reduction. Bioresour. Technol. 39, 61–68. Rayment, G.E., Lyons, D.J., 2011. Soil Chemical Methods: Australasia. CSIRO Publishing. Shende, R.V., Levec, J., 1999. Wet oxidation kinetics of refractory low molecular mass carboxylic acids. Ind. Eng. Chem. Res. 38, 3830–3837. Sierra-Alvarez, R., Field, J.A., Kortekaas, S., Lettinga, G. 1994. Overview of the anaerobic toxicity caused by organic forest industry wastewater pollutants. Proc. 4th IAWQ Symp. Forest industry wastewaters. Tampere, Finland (1993). Ook: Water Sci. Technol. 29 (1994) 353–363.
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Speece, R.E., Parkin, G.F., Gallagher, D., 1983. Nickel stimulation of anaerobic digestion. Water Res. 17, 677–683. Strong, P.J., McDonald, B., Gapes, D.J., 2011. Combined thermochemical and fermentative destruction of municipal biosolids: a comparison between thermal hydrolysis and wet oxidative pre-treatment. Bioresour. Technol. 102, 5520–5527. Turick, C.E., Peck, M.W., Chynoweth, D.P., Jerger, D.E., White, E.H., Zsuffa, L., Andy Kenney, W., 1991. Methane fermentation of woody biomass. Bioresour. Technol. 37, 141–147. Verenich, S., Garcia Molina, V., Kallas, J., 2004. Lipophilic wood extractives abatement from TMP circulation waters by wet oxidation. Adv. Environ. Res. 8, 293–301. Vidal, G., Diez, M.C., 2005. Methanogenic toxicity and continuous anaerobic treatment of wood processing effluents. J. Environ. Manage. 74, 317–325. Wikberg, H., Liisa Maunu, S., 2004. Characterisation of thermally modified hard- and softwoods by 13C CPMAS NMR. Carbohydr. Polym. 58, 461–466. Wood, N., Tran, H., Master, E., 2009. Pretreatment of pulp mill secondary sludge for high-rate anaerobic conversion to biogas. Bioresour. Technol. 100, 5729–5735.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Application of hydrothermal treatment to affect the fermentability of Pinus radiata pulp mill effluent sludge John Andrews a,⇑, Anne-Marie Smit a, Suren Wijeyekoon a, Ben McDonald a, Saeid Baroutian a,b, Daniel Gapes a a b
Scion. Te Papa Tipu Innovation Park, 49 Sala Street, Rotorua 3046, New Zealand Department of Chemical & Material Engineering, Faculty of Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
h i g h l i g h t s Significant removal of lignin and hemicellulose components. Reduction in the concentration of extractive compounds, such as resin acids, >99%. Some resistance to degradation of the cellulose component. No demonstrable increase in biological VFA or methane after pre-treatment.
a r t i c l e
i n f o
Article history: Received 28 April 2014 Received in revised form 8 July 2014 Accepted 9 July 2014 Available online 16 July 2014 Keywords: Wet oxidation Lignocellulose Biochemical acidogenic potential assay Resin extractives Waste minimisation
a b s t r a c t A hybrid technique incorporating a wet oxidation stage and secondary fermentation step was used to process Pinus radiata pulp mill effluent sludge. The effect of hydrothermal oxidation at high temperature and pressure on the hydrolysis of constituents of the waste stream was studied. Biochemical acidogenic potential assays were conducted to assess acid production resulting from anaerobic hydrolysis of the wet oxidised hydrolysate under acidogenic conditions. Significant degradation of the lignin, hemicellulose, suspended solids, carbohydrates and extractives were observed with wet oxidation. In contrast, cellulose showed resistance to degradation under the experimental conditions. Extensive degradation of biologically inhibitory compounds by wet oxidation did not show a beneficial impact on the acidogenic or methanogenic potential compared to untreated samples. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Current commercial pulp and paper manufacturing processes involve the use of large amounts of water, from which substantial wastewater volumes are generated. This wastewater contains a number of dissolved and suspended solids. The waste solids from effluent treatments can be utilised for energy generation, recycled beneficially to land or, as is often the case, simply sent to landfill. There is a need to maximise the beneficial use of this biomaterial and find more sustainable long-term solutions to landfill. Resin extractives are one component of pulp-mill effluent. Their hydrophobic nature results in their becoming associated with the fibre and biomass fractions of pulp-mill waste. They are seen to inhibit biological action, making the solids potentially toxic and less amenable to biological treatment technologies (Kostamo and ⇑ Corresponding author. Tel.: +64 7 3435605. E-mail address: [email protected] (J. Andrews). http://dx.doi.org/10.1016/j.biortech.2014.07.037 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Kukkonen, 2003; Makris and Banerjee, 2002). Methane yields become negatively affected, resulting in little uptake of anaerobic digestion as a process to treat pulp-mill effluent solids (Kennedy et al., 1992; Lettinga et al., 1991; Sierra-Alvarez et al., 1994; Vidal and Diez, 2005; Puhakka et al., 1992; Turick et al., 1991). A number of pre-treatments can be used to reduce the concentration of inhibitory resin extractives in order to improve anaerobic degradability. Such treatments include ozonation (Pokhrel and Viraraghavan, 2004), thermal or thermochemical (caustic) degradation (Elliott and Mahmood, 2007; Wood et al., 2009), treatment with strong alkali with ultrasound (Park et al., 2012), wet oxidation (Laari et al., 1999; Verenich et al., 2004), hydrolysis using enzymes from fungi (Hodgson et al., 1998) and separation methods, such as ion exchange chromatography and nanofiltration (Ciputra et al., 2010). Addition of activated carbon to effluent can assist with filtration as well as adsorbing certain organic compounds. It is proposed in this study that the selective removal of resin extractives will enhance the anaerobic biodegradability of pulp
J. Andrews et al. / Bioresource Technology 170 (2014) 100–107
mill effluent solids. The objective is to use wet oxidation as a pre-treatment, to quantify the extent of degradation of resin acids and phenolics, and to assess the biodegradability of the treated residues under anaerobic conditions. 2. Methods 2.1. Sample preparation Biological sludge was obtained from the wastewater treatment works of the pulp mill at Karioi, New Zealand, which is owned by Winstone Pulp International’s (WPI). This mill processes Pinus radiata wood. This sludge was derived from the treatment of a lignocellulosic effluent and consisted of a mixture of waste-activated sludge (WAS), process-dissolved air flotation (DAF) sludge and primary effluent DAF sludge. The sludge was dewatered at the mill to give a residue containing approximately 35% dry solids. Upon collection, this sludge was frozen until needed for further processing. When required, a sample was taken and resuspended by diluting with tap water to give a concentration of 3% total suspended solids (TSS). 2.2. Wet oxidation of Pinus radiata pulp mill effluent solids Wet oxidation was conducted using 150 mL samples of the 3% TSS slurry. The WO reactions were undertaken in a Parr high-pressure reactor (Parr Instrument Company, Model 4540, total volume 600 mL) equipped with a stirrer and heating jacket (Fig. 1). The
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apparatus was set up so that the biomass was conveyed to the Parr reactor only when it had reached the desired temperature. This was done in order to avoid a slow start-up heating stage, during which unintended reactions may take place. The process involved first adding 150 mL of tap water to the Parr reactor, charging it with 20 bar of pure oxygen (BOC NZ Ltd. – zero grade), stirring at 500 rpm and heating to 220 °C. A 190 mL stainless-steel thermal reactor (Berghof, Germany) was connected to the Parr reactor to which the 150 mL lignocellulosic sample was added. Nitrogen gas (BOC NZ Ltd. – zero grade) was added to the Berghof reactor at a pressure 10 bar greater than that present in the Parr reactor at temperature, and the pressure differential was used to convey material. The sample was maintained at 220 °C for a set time period before being transferred to a container stored in an ice bath to rapidly cool the sample. A portion of the sample was immediately frozen and stored ( 20 °C) prior to being analysed for resin extractives and carbohydrates. Another portion of the sample was stored in a chiller (4 °C) before being processed within 24 h for analysis of total suspended solids, volatile suspended solids (VSS), dissolved organic carbon (DOC), volatile fatty acids (VFA) and soluble chemical oxygen demand (sCOD). Wet oxidation treatments were carried out for 5 min (T5), 20 min (T20) and 60 min (T60). Separate runs were undertaken using different aliquots of starting material. Each run was carried out in triplicate. The time immediately prior to commencing the wet oxidation process for each experimental run was designated as T0 which represents the untreated raw material.
Fig. 1. Wet oxidation reactor configuration. CW = cold water.
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2.3. Performance of wet oxidation liquor to anaerobic conditions The effect of anaerobic fermentation on the wet oxidised samples was assessed in terms of VFA production, using a biochemical acidogenic potential (BAP) assay. This was an adaptation of the biochemical methane potential (BMP) methodology (ASTM, 2008). Specifically for this work, an anaerobic acidogenic inoculum was used. Single representative samples for each timepoint (T0, T5, T20 and T60 min), were prepared by mixing duplicates together from the WO stage. For each timepoint, 10 mL (12.5 g VSS/L) of sample, 10 mL (12.5 g VSS/L) of inoculum, medium, Na2S and water as defined in the standard (ASTM, 2008) were added to separate 160 mL serum bottles. Nickel (NiCl26H2O) at 0.005 g/L was included in the medium as a microelement (Speece et al., 1983). Serum bottles were flushed with nitrogen, sealed, crimped and placed on a shaker-incubator at 36 °C. The following controls were prepared: a blank sample containing only medium; a negative control consisting of inoculum and medium; a positive control containing inoculum, medium and cellulose (1000 mg/L, Whatman microgranular cellulose powder CC31). A sacrificial sampling regime was used as a non-invasive means to measure degradation rates of solids, VFA production and changes to DOC and sCOD concentration over time. This meant that for each condition (including the blank and controls) 10 samples were prepared which were then sacrificed for analysis on discrete days, reducing the number of samples from 10 to one by the 10th day of sampling. Each of these sacrificial samples and the controls were initially prepared in triplicate. The BAP assay was run for a duration of 27 days, which is shorter than 50–100 days typically used in BMP assays. This was used as the target was enhanced acid production, rather than methane production. Gas was produced in the sealed serum bottles as a consequence of the fermentation. 2.4. Analytical procedures 2.4.1. Solids analysis Total suspended solids and volatile suspended solids were determined using standard methods for the examination of water and wastewater (APHA, 1998). The values obtained were used to calculate relevant yields of products. Soluble chemical oxygen demand was determined using according to a standard method (APHA, 1998). 2.4.2. Chemical analysis 2.4.2.1. Lignocellulosic extractives and volatile fatty acids (VFAs). Lignocellulosic extractive concentrations and VFAs were determined before and after wet oxidation. In addition, the VFA concentrations were determined before and after fermentation. A 5 mL sample was taken and shaken with 5 mL dichloromethane. A 1 mL sample of the organic layer was dried with anhydrous sodium sulphate and transferred to a GC vial. An injection standard (50 lL 9,10-dibromoanthracene) and pyridine (50 lL) were added to the vial with derivatization reagents N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) and 1% trimethylchlorosilane (TCMS). The vial was heated to 70 °C (1 h) prior to analysis. The analysis of extractive compounds and VFAs was performed on an Agilent 6890N gas chromatograph with a CTC CombiPal auto-sampler connected to an Agilent 5973N mass selective detector (MSD). A high performance Ultra-2 column (50 m 0.200 mm 0.33 lm) was used with helium as the carrier gas. The temperature was increased from 60 °C to 300 °C during the analysis. 2.4.2.2. Neutral carbohydrate analysis. The types and amounts of neutral monosaccharides present in the liquid phase of each sample were determined directly on filtered solutions. Individual
monosaccharides were separated using a high performance anion exchange chromatograph (Dionex IC3000) fitted with a pulsed amperometric detector and a PA10 column. The eluent was 2 mM KOH. Identification of monosaccharides was undertaken by comparison of retention times with standards containing arabinose, xylose, galactose, glucose, and mannose. Fucose was added as an internal standard to enable the amount of each monosaccharide to be quantified. A separate aliquot of each sample was hydrolysed using H2SO4 (final concentration 4%) at 15psi (103 kPa, 121 °C) for 60 min. These samples were analysed as above to determine the amount and composition of total soluble carbohydrates present. 2.4.2.3. Carbon and nitrogen analysis. Dissolved organic carbon (DOC) was determined using a High TOC II analyser (Elementar, Hanau, Germany). Total Kjeldahl nitrogen (TKN), dissolved Kjeldahl nitrogen (DKN) and ammoniacal nitrogen (NH4-N) were analysed using a modified Dumas method (Rayment and Lyons, 2011) on a LECO (Laboratory Equipment Corporation) CNS-2000 analyzer (St Joseph, MI). The amount of each of these components was calculated according to standard methods for water and wastewater (APHA, 1998) with a detection limit set at 0.2 mg/L in each case. Total organically bound nitrogen (TON) was calculated as the difference between TKN and NH4-N. 2.4.3. Gas analysis Pressure was monitored using an in-house built electrical device consisting of a reader coupled to a pressure sensor (Sensor Technics). Once the pressure reached a level of 6 kPa, the composition of the gas was analysed using gas chromatography. Samples were collected approximately every 3 days. Methane, carbon dioxide, and hydrogen concentrations in the head space were determined using a gas chromatograph (GC) (Carle Instruments, Inc., GC 8700, USA) with a thermal conductivity detector and helium as the carrier gas (22 mL/min, 54 °C). 2.5. Solid state NMR spectroscopy Solid state 13C NMR spectroscopy was used to generate spectra of lignocellulose from freeze-dried non-WO and WO samples. All 13 C NMR spectra generated had a Gaussian line broadening of 25 Hz applied prior to Fourier transformation. Spectra were calibrated so that the cellulose interior C-4 peak was assigned a value of 89.3 ppm (Wikberg and Liisa Maunu, 2004; Liitiä et al., 2003), previously established relative to polydimethylsilane at 1.96 ppm, and in turn measured relative to tetramethylsilane at 0 ppm. The samples were spun at 5 kHz in a 4 mm Bruker SB magic-angle spinning probe, for 13C NMR at 50.3 MHz using a Bruker 200 DRX spectrometer. For standard cross-polarisation (CP) experiments, each 1.5 s pulse delay was followed by a proton preparation pulse of duration tp = 4.6 ls, a 1 ms contact time and a 30 ms acquisition time. The proton transmitter power was increased to a value corresponding to a 90° pulse width of 2.8 ls for proton decoupled during 13C data acquisition. 2.6. Statistical analysis Data were analysed using Statgraphics Centurion XVI (Statpoint Technologies Inc.) and Minitab 15Ò Statistical Software. Multiple variable analysis using Pearson product-moment correlations was carried out with 95% confidence intervals to determine linear relationships between pairs of variables. A one-way analysis of variance (ANOVA) was used to test for differences between variables for each variable measured (p < 0.05). Multiple range tests were performed using Fisher’s Least Squares Significant Difference
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Fig. 2. Compositional changes determined in Pinus radiata pulp mill effluent solids’ samples with hydrothermal treatment (A) percentage TSS and VSS, (B) sCOD and DOC, (C) TKN, DKN, TON, NH4-N, (D) VFA concentrations.
(LSD) method to determine which means differed significantly from each other at the 95% confidence level. 3. Results and discussion 3.1. Hydrothermal-wet oxidation The WO of pulp mill effluent solids released high levels of lignocellulosic-derived residues from the original untreated biomass. Temporal changes through WO and subsequent anaerobic fermentation are discussed below. 3.1.1. Change in TSS, VSS and carbon availability A 78% destruction of TSS and VSS had occurred after 5 min which had increased to 88% destruction of both TSS and VSS by twenty minutes (Fig. 2a). The oxidative hydrolysis resulted in solubilisation of particulate organics, leading to a significant increase in sCOD and DOC after 5 min (Fig. 2b). This result is comparable with that previously achieved under similar conditions with municipal biosolids (Strong et al., 2011). As oxidation reactions in the liquid phase progressed, DOC was observed to decline from a
maximum, as evidenced from the T5 to T60 sample data. The purity of the DOC changed from 12% acetic acid at T5 to 48% acetic acid at T60. 3.1.2. Nitrogen Changes in TKN, DKN, TON and NH4-N with reaction time are presented in Fig. 2c. Statistical analysis showed no significant differences existed between concentrations of TKN at each time point. The DKN concentration doubled after 5 min of treatment but showed no significant change subsequently. This result demonstrated that significant solubilisation of particulate TKN occurred rapidly in WO. Notably, the amount of total organic nitrogen was negatively correlated with prolonged oxidative treatment whereas the amount of NH4-N was positively correlated, with a significant difference in concentrations between T0 and T60. These data suggest that prolonged wet oxidation treatment ultimately converts more dissolved organic nitrogen from the sludge to NH4-N. 3.1.3. Volatile fatty acids (VFAs) Acetic acid was the dominant VFA, increasing during WO with 0.8 g/L at T5 and greater than 2 g/L by T60 (Fig. 2d). Acetic acid is a key product from the oxidation of a range of organic
Fig. 3. Concentration of neutral carbohydrates in Pinus radiata pulp mill effluent solids following hydrothermal treatment.
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Table 1 Characteristic changes in Pinus radiata pulp mill effluent solids extractives (mg/L) following hydrothermal treatment. Type
Compound
T0 mina
T5 minb,c
T20 minb,c
T60 minb,c
Monoterpenes
Alpha-pinene Alpha-terpineol Beta-pinene Borneol Camphor Fenchone Terpinen-4-ol Total monoterpenes
10.6 ± 1.3 1.16 ± 0.11 13.6 ± 1.5 0.04 ± 0.01 0.45 ± 0.01 0.17 ± 0.02 0.42 ± 0.05 26.5 ± 2.8
n.d n.d n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d n.d n.d
Phenolics
Acetovanillone Eugenol Ferulic acid Guaiacol Pinosylvin, mono methyl ether Vanillin Vanillic acid Total phenolics
0.03 ± 0.03 n.d 0.31 ± 0.03 n.d 36.23 ± 0.91 0.17 ± 0.04 n.d 36.7 ± 1.0
0.62 ± 0.08 n.d n.d 0.10 ± 0.03 0.15 ± 0.07 7.5 ± 1.8 0.45 ± 0.32 8.8 ± 2.2
0.12 ± 0.01 n.d n.d n.d n.d 0.51 ± 0.07 0.01 ± 0.01 0.64 ± 0.09
n.d n.d n.d n.d n.d 0.10 ± 0.03 n.d 0.10 ± 0.03
Fatty acids
Decanoic acid (F10:0) Docosanoic acid (F22:0) Dodecanoic acid (F12:0) Eicosanoic acid (F20:0) Elaidic acid (F18:1) Linoleic acid (F18:2) Linolenic acid (F18:3) Margaric acid (F17:0) Oleic acid (F18:1) Palmitic acid (F16:0) Palmitoleic acid (F16:1) Stearic acid (F18:0) Tetradecanoic acid (F14:0) Tetracosanoic acid (F24:0) Total fatty acids
0.05 ± 0.05 16.77 ± 0.72 0.17 ± 0.01 8.49 ± 0.28 21.44 ± 0.55 389 ± 26 56.0 ± 3.6 16.57 ± 0.95 368 ± 15 42.12 ± 0.04 5.24 ± 0.33 10.72 ± 0.38 3.69 ± 0.25 2.58 ± 0.35 941 ± 35
0.23 ± 0.05 n.d 0.06 ± 0.03 0.10 ± 0.10 n.d 4.3 ± 2.2 n.d 0.30 ± 0.15 6.1 ± 2.6 1.83 ± 0.36 n.d 0.64 ± 0.10 0.28 ± 0.16 n.d 13.8 ± 4.8
n.d n.d 0.07 ± 0.07 n.d n.d 0.53 ± 0.53 n.d 0.06 ± 0.06 2.2 ± 1.1 1.89 ± 0.79 n.d 0.38 ± 0.25 0.27 ± 0.27 n.d 5.4 ± 1.0
n.d n.d 0.01 ± 0.01 n.d n.d 0.59 ± 0.59 n.d 0.04 ± 0.04 1.85 ± 0.93 2.9 ± 1.6 n.d 0.40 ± 0.27 0.07 ± 0.07 n.d 5.84 ± 0.15
Resin acid neutrals
Dehydroabietin Methyldehydroabietin Total resin acid neutrals
0.28 ± 0.04 0.39 ± 0.06 0.67 ± 0.10
n.d n.d n.d
n.d n.d n.d
n.d n.d n.d
Resin acids
Abietic acid Dehydroabietic acid Isopimarenic acid Isopimaric acid Levopimaric acid Neoabietic acid Palustric acid Pimaric acid Sandaracopimaric acid Seco-1-dehydroabietic acid Seco-2-dehydroabietic acid 7-Oxodehydroabietic acid Total resin acids
4711 ± 299 229 ± 7.0 0.89 ± 0.89 131.4 ± 7.9 258 ± 15 750 ± 37 531 ± 38 238 ± 12 44.2 ± 2.5 70.9 ± 3.5 63.5 ± 2.9 8.7 ± 3.0 7038 ± 426
61 ± 18 6.4 ± 1.8 n.d 1.57 ± 0.48 n.d 5.0 ± 2.6 3.4 ± 2.0 3.6 ± 1.0 0.34 ± 0.19 1.19 ± 0.31 0.72 ± 0.42 9.5 ± 6.4 92 ± 22
29.2 ± 8.3 3.29 ± 0.71 n.d 0.59 ± 0.30 n.d 1.91 ± 0.98 1.57 ± 0.46 1.81 ± 0.40 0.12 ± 0.12 0.19 ± 0.19 0.18 ± 0.18 0.32 ± 0.32 39.18 ± 11.72
25.3 ± 7.9 3.37 ± 0.60 n.d 0.55 ± 0.30 0.42 ± 0.42 1.1 ± 1.1 1.60 ± 0.64 1.62 ± 0.39 0.09 ± 0.09 0.19 ± 0.19 0.15 ± 0.15 n.d 34.3 ± 11.4
Phytosterols
Campesterol Cholesterol Sitostanol Sitosterol Stigmasterol Total phytosterols
1.13 ± 0.13 0.37 ± 0.37 3.72 ± 0.18 28.5 ± 1.3 n.d 33.7 ± 2.0
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
n.d n.d n.d n.d n.d n.d
8077 ± 392
115 ± 26
45 ± 13
40 ± 11
Total extractives
n.d. = not detected, method detection limit is 0.01 lg/L, standard error of the means are given. a Denotes raw feed samples with no added oxidant, i.e. before initiation of wet oxidation designated time zero (T0). b Denotes samples subjected to wet oxidation treatment for 5 min (T5 min), 20 min (T20 min) and 60 min (T60 min) with added oxygen at 20 bar. c Denotes all compounds were normalised to raw feed initial biomass and reported here as mg/L.
compounds. Under the temperatures used in the current study, acetic acid degradation is kinetically limited (Shende and Levec, 1999). The degradation of sugars is also likely to be a source of acetic acid. 3.1.4. Solid state NMR spectroscopy Solid-state 13C NMR spectra of non-WO (T0) and WO (T5) samples were compared. Both spectra are dominated by signals characteristic of cellulose. The signals were assigned to the six carbon atoms of a glucosyl structural unit as follows (Liitiä et al., 2003):
C-1 (105 ppm), C-2,3,5 (73 and 75 ppm), C-4 (84 and 89 ppm), C-6 (64 and 66 ppm). Signals from hemicellulose were mostly obscured by signals from cellulose, although a signal at 102 ppm was assigned to C-1 of mannosyl structural units in glucomannan (Liitiä et al., 2003). Signals characteristic of lignin are labelled ‘‘L’’ (Wikberg and Liisa Maunu, 2004): a signal at 56 ppm was assigned to methoxyl groups, and a band of signals between 140 and 155 ppm showed a profile characteristic of C-3 and C-4 of the aromatic rings in the guaiacyl lignin of a softwood. A band of signals between 110 and 140 ppm was consistent with C-1,2,5 of the
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aromatic rings of lignin, but was considered less diagnostic because of contributing signals assigned to proteins and resin acids. Signals at 18, 24, 38, 48 and 187 ppm were assigned to resin acids, including abietic acid (Lambert et al., 1999). A signal at 173 ppm was assigned to the amide carbon of amino acid structural units in proteins (Calucci et al., 2003). Whilst proteins also contribute signals in the range 10–50 ppm, assigned to the sidechains of amino acids, that range was obscured by signals assigned to resin acids. The signals assigned to resin acids were barely detectable in the NMR spectrum of the sample subjected to 5 min of WO treatment. The signals assigned to lignin and hemicellulose were decreased in strength, indicating partial solubilisation by wet oxidation, and a new signal at 177 ppm was assigned to carboxylic acid functional groups (ACO2H) in partly-oxidised structures remaining associated with the insoluble matter. Signals assigned to cellulose and protein persisted in the sample subjected to 5 min of WO. 3.1.5. Neutral carbohydrates In non-WO (T0) samples, arabinose, galactose, glucose, xylose and mannose were exclusively present in polymeric forms. Within 5 min of wet oxidation (T5) the monomeric saccharides predominated for arabinose, galactose, xylose and mannose, and the concentrations had decreased (Fig. 3). The monomers had themselves degraded by 20 min. Glucose polymers showed greater resistance to oxidation. Polymeric forms of glucose remained throughout the 60 min hydrothermal treatment with reducing concentrations over time. The glucose monomer increased in concentration by T5 but had itself been degraded considerably by T60. The hemicellulose fraction degrades readily in hydrothermal conditions but the cellulose is much more resistant to degradation (Garrote et al., 1999). 3.1.6. Lignocellulose-derived extractives The concentrations of lignocellulose-derived extractives from the solubilized hydrolysate are presented in Table 1. Total extractives concentrations decreased to 0.1 g/L at T5 from 8 g/L at T0. The different groups of extractives detected and quantified are discussed below. Monoterpenes. The total monoterpene concentrations determined at T0 were 26.5 mg/L, dominated by alpha- and beta-pinene. By the T5 timepoint monoterpenes were totally degraded. Phenolics. From a suite of 15 phenols that were screened, the main phenols present in the raw feed were pinosylvin monomethyl ether (PSMME), vanillin and vanillin derivatives. Low concentrations of guaiacol (0.10 mg/L), acetovanillone (0.62 mg/L) and vanillic acid (0.45 mg/L) formed by T5 but had substantially degraded by T20. The stilbene, PSMME, was the dominant phenolic at T0 (36 mg/ L), but had degraded substantially by T5. By the T5 timepoint, the WO treatment used had degraded 36.7 mg phenolics per g sCOD released. Unlike most of the other extractive compounds, phenolics such as vanillin, actually increased in concentration by T5. Vanillin was subsequently degraded with further wet oxidation. Fatty Acids. Fatty acids were reduced from an initial concentration of 941 mg/L to concentrations of 13.8 mg/L following 5 min of WO and 5 mg/L by 20 min. Linoleic acid and its precursor, oleic acid, were the dominant fatty acids present in the T0 samples (389.3 mg/L and 367.7 mg/L respectively). Degradation of these compounds occurred rapidly as levels were just 4.2 mg/L and 6.1 mg/L respectively by T5. Oleic acid (at concentrations above 30 mg/L) can inhibit acetate degradation and aceticlastic methanogenesis (Lalman and Bagley, 2001) so degradation of this compound should be beneficial in subsequent anaerobic fermentation. Resin acids. Resin acids contributed 87% of the extractives in this study and were present in the raw feed at a concentration of 7 g/ L, degrading massively to 0.09 g/L by T5. Abietic acid was the
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dominant resin acid and dehydroabietic acid, which is known to be particularly recalcitrant to degradation, was severely depleted from 229 mg/L at T0 to 6.4 mg/L at T5. Degradation of resin acids by WO is a promising result in overcoming inhibitory effects on anaerobic digestion. Phytosterols. Of the four phytosterols detected by GC/MS, sitosterol was present at the highest concentration of 28.5 mg/L at T0. All phytosterols were totally degraded following 5 min of WO, and is consistent with the extent of degradation observed in previous research (Baroutian et al., 2013). 3.1.7. Biochemical acidogenic potential assay (BAP) The BAP assay results for acetic acid and gas production are presented in Fig. 4 and Table 2. Anaerobic fermentation of wet oxidised lignocellulosic residues investigated in this study produced relatively low amounts of methane (300 °C and a pressure of at least 25 MPa are required for degradation of cellulose due to its crystalline structure. The lower temperature and pressure used in the current study would be ineffective at hydrolysis and conversion of the cellulose to glucose and may explain the persistence of cellulose as a major component of WO lignocellulose in this study, as determined by NMR. Hydrolysis of cellulose can therefore be considered a rate-limiting step in the biodegradability of lignocellulose under the WO conditions used here. The presence of a significant proportion of undegraded biomass is important for bacterial degradation since cellulose that is not converted to glucose, is not in a form that can be metabolized by bacteria. Extractive compounds initially bound in lignin, and dominated by resin acids, were reduced by over 99% through wet oxidation. Vanillin, however, did initially increase in concentration. Vanillin is a widely known reaction product occurring from the WO of lignin (Araújo et al., 2010). PSMME was the dominant phenolic in the feed T0 sample. PSMME is toxic to aquatic life with long lasting effects, so its effective degradation is beneficial. This work had proposed that through the elimination of potentially biologically inhibitory compounds such as resin acids, anaerobic fermentation and methanogenic activity would be noticeably enhanced. The results from the BAP assay showed a reduction over time in the acetic acid concentrations present in the T0 and T60 samples, combined with similar cumulative yields of methane. Given the T0 sample was not inhibitory to methane production and yields were similar to the highly treated T60 sample, a correlation between reduction in inhibitory compounds and methanogenic activity was not observed in this study. Despite extensive resin acid degradation in the T5 and T20 samples, little cumulative methane was observed. End-products, such as furan, were not characterised in this study. Furan can be directly produced by thermal decomposition of pentose-containing materials and cellulosic solids as found in pine. Further work would be recommended to examine the contribution of the cellulose fraction as well as the pentose-containing sugars of the hemicellulose to end-products such as furans.
4. Conclusions Wet oxidation resulted in substantial degradation of lignocellulosic residues with 77% destruction of TSS, predominantly occurring within 5 min. The removal of lignin and hemicellulose was observed along with extensive destruction of hemicellulose sugars and resin acids and production of acetic acid. Acetic acid indicates potential for resource recovery coupled with destruction of toxic compounds and inhibitors of anaerobic digestion. Despite the extensive degradation of inhibitory compounds through wet
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