Article pubs.acs.org/IECR
Process for Treating Spent Liquor of the TMP Process with BiomassBased Fly Ash Farshad Oveissi and Pedram Fatehi* Chemical Engineering Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada ABSTRACT: The spent liquor (SL) of the thermomechanical pulping (TMP) process has a high effluent load. In this work, biomass-based fly ash was used for adsorbing lignin of SL and thus for reducing the COD and turbidity of SL prior to sending SL to a wastewater treatment system. By treating SL with fly ash in a one-stage adsorption process under the conditions of a fly ash/ SL ratio of 55 mg/g and 3 h at 30 °C, 67 mg/g of lignin was adsorbed on fly ash at room temperature. Adjusting the pH of SL had an insignificant influence on the lignin isolation from the system but affected the turbidity of SL. The lignin removal was 68% in a two-stage fly ash adsorption process, while the COD and turbidity removal were 70% and 94%, respectively. On the basis of the results, a process for reducing the effluent load of SL using fly ash was proposed.
1. INTRODUCTION The pulp and paper industry is one of the main industries contributing to the gross domestic products (GDP) of Canada and the United States.1,2 However, the pulp and paper industry is currently struggling financially due to strong competition from countries with low labor costs. One strategy to reduce the production costs, and thus to increase the economic benefits of the pulp and paper industry, is to utilize their wasted materials more effectively. The amount of wastewater generated in the pulp and paper industry was estimated to be one-half of all waste effluents released to surface water in Canada.3 Recently, the capital cost for a lignocellulosic-based wastewater plant with a hydraulic load of 2.15 MMgal/d was estimated to be $49.4 million, and the annual chemical cost for this plant was predicted to be $2.83 million.4 In the thermomechanical pulping (TMP) process, wood chips are pretreated with steam, which extracts some organic materials including lignin from wood and dissolves it in pressate (i.e., the spent liquor (SL)) of this process. This extract is sent to a wastewater treatment plant in order to remove the suspended solids and dissolved organic materials prior to its discharge. The treatment of SL in the wastewater system may not be an efficient way of utilizing its organic compounds. Lignin has a heating value of 27 MJ/kg, which is worth $99 per oven dry metric ton.2 Possessing such a high heating value would make lignin of SL an alternative fuel. It was stated that the main source of the chemical oxidation demand (COD) of SL is dissolved lignin and its derivatives.5−7 In this regard, the COD reduction from lignocellulosic-based wastewater effluents was the subject of several research projects.8−11 It was claimed that 90% of COD from SL was removed at a hydraulic retention time of 21 h in two stages of anaerobic reactors.12 Although biological methods are efficient in removing COD, the treated wastewater has color as not all of lignocelluloses will decompose by biological treatments.10 It was reported that ultrafiltration was effective in removing COD from the effluent of cane molasses.13,14 Although some of these attempts resulted in more than 90% COD removal, membrane processes may not be economical for treating the effluent of © 2015 American Chemical Society
TMP processes due to the very dilute nature of the pressate.15To improve the COD removal from a TMP spent liquor, the codigestion of lignocelluloses with glucose using thermophilic acidogens was suggested in anaerobic reactors.9 The main disadvantage of this process is the decomposition and thus wasting of the dissolved lignocelluloses. In other words, the biological treatment improved the COD removal from wastewater at the expense of decomposing lignocelluloses. Coagulation with metal salts and polymers (mostly anionic) was proposed to improve the removal of lignocelluloses and COD from SL. In one study, the aerobic fermentation of effluent of alkaline peroxide mechanical pulping (APMP) with Aspergillus niger showed 30% COD reduction via adding 1 g/L of alum, as a coagulant, and 2 mg/L of cationic polyacrylamide (CPAM), as a flocculant.8 In a similar study, 90% of COD from the secondary treatment of a wastewater effluent was removed by adding 4.5 mg/L of aluminum sulfate and 2 mg/L of CPAM.11 Although coagulation and flocculation treatments are more effective than biological processes for removing lignocelluloses and COD, they generate a considerable amount of sludge and their operating cost may be significant. Acidification was used for isolating lignin from black liquor in the past.16,17 However, this method might not be economical for dilute liquors such as various pulping spent liquors.15 Adsorption is regarded as a fast, selective, and economical method for lignin removal from spent liquors. In one study, a two-stage adsorption process (using activated carbon with a dosage of 1 g of activated carbon per 90 g of SL) reduced the lignin, COD, and turbidity of SL of a TMP process by 60%, 32%, and 39%, respectively.15 Fly ash is produced in boilers by burning wood residuals, bark, or coal; thus, it is an underutilized byproduct of boilers in the pulping industry. Biomass-based fly ash can be used as an adsorbent. In the literature, the utilization of coal-based fly ash Received: Revised: Accepted: Published: 7301
April 18, 2015 June 30, 2015 July 2, 2015 July 2, 2015 DOI: 10.1021/acs.iecr.5b01473 Ind. Eng. Chem. Res. 2015, 54, 7301−7308
Article
Industrial & Engineering Chemistry Research
Figure 1. Process alternatives: (a) one-stage adsorption, (b) one stage with pre-pH adjustment, (c) one-stage adsorption with post-pH adjustment, (d) two-stage adsorption, and (e) two-stage adsorption with post-pH adjustment.
for 24 h prior to use. The pressate, which is the spent liquor (SL), of a thermomechanical pulping (TMP) process was received from the same mill and used as received. H2SO4 and NaOH (analytical grades) were received from Sigma-Aldrich Co. 2.2. Elemental Analysis. The metal content of fly ash was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a CETAC ASX-510 Auto Sampler (Canada). ICP-AES analysis was conducted via a Varian Vista Pro CCD (Canada) according to the method established in the literature.20 Elemental (ultimate) analysis was performed using a Vario EL cube instrument (Germany) according to the procedure described in the literature.21 2.3. Surface Area, Charge Density, and Zero-Point Charge Analyses. The BET surface area of fly ash was determined using a NOVA-2200e Autosorb under N 2 atmosphere according to a previously established method.20 The charge density of fly ash was determined by using a Mütek PCD04 charge detector as previously described.15,22 The point of zero charge (PZC) of fly ash samples was measured according to the solid addition method.23 Initially deionized water was boiled at 100 °C for 10 min and then cooled to room temperature. This treatment aimed to remove CO2 and O2 from water. The treated water was then used in preparing solutions of 0.03 M NaCl, 1 M NaOH, and 0.1 M HCl. Subsequently, 0.12, 0.24, and 0.36 g of fly ash samples were added to 20 mL of 0.03 M NaCl solution. The samples were then sealed and kept for 20 h to reach equilibrium. The point of
for adsorption of organic compounds (i.e., phenols) from wastewater effluents was discussed.18It was stated that up to 90% of lignin was removed by treating with 50 g/L of biomassbased fly ash from a bleaching effluent, i.e., not a TMP spent liquor.19 In this work, the application of biomass-based fly ash as an adsorbent for the organic compounds of a TMP spent liquor is studied for the first time. In this work, operating conditions (treatment time and dosage of fly ash) for removing lignin, COD, and turbidity were optimized. Subsequently, the impact of a two-stage process on treating SL was studied under various conditions. On the basis of the results, an integrated process was proposed for not only removing lignin from SL (and thus for decreasing the load of TMP spent liquor) but also using treated fly ash as a fuel source in the biomass boiler in an effort to better utilize biomass resources. The main novelties of this work are the analysis on the application of biomass-based fly ash in the spent liquor of TMP process and the development of a process for applying fly ash in the TMP spent liquor.
2. MATERIALS AND METHODS 2.1. Materials. Fly ash was collected from a bark boiler of a pulp mill in Northern Ontario, Canada. A combination of generated sludge of wastewater, sawdust, and bark from softwood and hardwood species is used as feed for the aforementioned boiler. All fly ash samples were grinded with a laboratory grinder so that more than 95 wt % of particles was smaller than 1 mm in diameter (passing through No. 18 US sieve series). All fly ash samples were then oven dried at 105 °C 7302
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and the average values were reported. The chemical oxygen demand (COD) of SL samples before and after fly ash treatment was measured as previously described, and the average values of three repetitions were reported in this work.24 The assay was treated with a COD CR2200 WTW reactor based on dichromate digestion. 2.8. Heating Value and LOI Analyses. A high heating value of fly ash samples was measured by a PARR 6200 oxygen bomb calorimeter, according to ASTM E711-87. Loss of ignition (LOI) for washed and unwashed fly ash samples was determined by combusting samples via a thermogravimetric analyzer (TGA)-i1000 series (Instrument specialist Inc., USA) under air (30 mL/min) at 550 °C according to the method described in the literature.27 The ratio of weight loss of fly ash via burning at 550 °C (i.e., the difference between the weight of fly ash at 105 °C and 550 °C) to the weight of fly ash at 105 °C was reported as the loss of ignition (LOI).27
zero charge was determined by titrating samples against 0.1 M HCl. 2.4. Single Stage Adsorption Process. In one set of experiments, different amounts of fly ash were added to 45 g of SL samples in 125 mL Erlenmeyer flasks. Then, all flasks were sealed and incubated in a Boekel water bath shaker at 30 °C and 100 rpm for 3 h. This set of experiments helped optimize the dosage of fly ash in SL (i.e., the dosage that induced the maximum removal of lignin, COD, and turbidity). On the basis of these results, the adsorption process was investigated at various time intervals and 100 rpm. In this set of experiments, control samples were prepared under the same conditions as stated above but without fly ash. The temperature of these experiments was fixed at 30 °C, as an earlier study showed that the adsorption of lignin on activated carbon was the maximum at 30 °C.15 The stirring rate was chosen based on previous studies on SL and prehydrolysis liquor.19,20,24−26 However, more studies on the effect of stirring rate should be considered in order to investigate if the adsorption/coagulation process of lignocelluloses on fly ash were possible under different conditions. The treated SLs were centrifuged at 1000 rpm for 10 min using a Survall ST16 centrifuge. The filtrates were collected for lignin, COD, and turbidity analyses. To satisfy statistical consistency, all tests were repeated three times, and the average of three repetitions was reported in this study. The error bars in all figures account for standard deviations of each triplicate. Alternatively, fly ash was washed with deionized water (incubated at 30 °C, 100 rpm for 24 h) and then dried. Different amounts of washed fly ash were added to SL, and samples were centrifuged at 1000 rpm for 10 min. This set of experiments was conducted to investigate the effect of fly ash metal ions on lignin, COD, and turbidity removal. 2.5. Removal Alternatives. To find the maximum removal of lignin, COD, and turbidity from SL, various processes were studied as depicted in Figure 1. In option A, 2.5 g of fly ash was added to 45 g of SL and then shaken at 100 rpm and room temperature for 3 h. In option B, the pH of SL samples was set to 5.3 (i.e., pH of original SL) after adding fly ash but before incubation (shaking at 100 rpm for 3 h). In option C, the pH of the SL was adjusted with sulfuric acid after adsorption and separation. In option D, the SL that was already treated with fly ash was retreated with fresh fly ash under the same optimal conditions in order to further reduce the organic materials from the SL without any pH adjustment. In option E, the two-stage adsorption stages were performed with the pH adjustment step after the final adsorption step. The filtrates of these processes were analyzed and compared with original SL. 2.6. Lignin Analysis. The lignin content of all solutions was determined by UV−vis spectrophotometry, Genesys 10S, at a wavelength of 205 nm according to TAPPI UM 250.21 Calibration curves were generated, and the average of three testing results was reported. To confirm that there is no interaction between fly ash and water, 2.5 g of fly ash was added to 45 g of deionized water and incubated overnight at 30 °C and 100 rpm (i.e., control sample). After separation, the filtrate was collected and analyzed in order to confirm that there was no interference from fly ash in lignin analysis using UV−vis spectrophotometry at a wavelength of 205 nm. 2.7. Turbidity and Chemical Oxygen Demand (COD) Analyses. The turbidity of SL samples was assessed before and after the adsorption experiments using a Hach 2100AN turbidity meter.25 This procedure was repeated three times,
3. RESULTS AND DISCUSSION The spent liquor of a TMP process contained 4.5 g/L lignin, 0.7 g/L hemicelluloses, 5311 mg/L COD, and 486 NTU. In the past, it was reported that hemicelluloses were marginally removed from SL via adsorption on different adsorbents,15,25 and the hemicelluloses had a low heating value.2 Therefore, the main focus of this work was on lignin, COD, and turbidity removal, but hemicellulose analysis was excluded from this work. 3.1. Ash Characterization. Table 1 shows the properties of unwashed and washed fly ash. Evidently, fly ash contained 30 Table 1. Elemental Analysis of Fly Ash element
unwashed fly ash (wt %)
washed fly ash (wt %)
calcium potassium magnesium aluminum sodium iron oxygen nitrogen
14.60 4.06 1.96 0.99 0.93 0.89 26.33 0.14
14.51 0.70 2.01 1.03 0.38 0.84 28.91 0.14
element
unwashed fly ash (wt %)
washed fly ash (wt %)
phosphorus manganese zinc silicon sulfur carbon hydrogen
0.87 0.33 0.19 0.08 4.60 34.60 1.59
0.92 0.38 0.21 0.13 2.44 36.72 1.79
wt % metals, such as calcium, potassium, magnesium, and aluminum, which may create fly ash as a potential coagulant for an effluent treatment. These metals were also reported as the most common constituents of fly ash in the literature.27 By washing fly ash, potassium, sodium, and sulfur contents of fly ash decreased by 3.36%, 0.55%, and 2.16% (based on fly ash mass), respectively. However, the carbon and oxygen contents increased by 2.12% and 2.58% (based on fly ash mass), respectively. It should be stated that the results in Table 1 may not show all elements present in fly ash samples due to the detection limit of the instruments/experimental procedure, which might have introduced about a 7−8% error in the results (the summation of elements is about 91−92 wt % in Table 1). Table 2 lists the surface area and charge density of unwashed and washed fly ash samples. As can be seen, unwashed fly ash had a 35 μeq/g cationic charge density, while washed fly ash had a 17.2 μeq/g cationic charge density. The anionic charge density of fly ash was negligible before and after washing. Also, the point of zero charge (PZC) of fly ash sample was 7303
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would be needed to investigate the morphology and heating values of biomass-based fly ash. 3.2. Adsorption on Unwashed Fly Ash. Figure 2 shows the changes in the lignin, COD, turbidity, and pH of SL after treating with fly ash as a function of the weight ratio of fly ash to SL. It is evident that as fly ash content increased, the lignin removal from the SL increased. The increase in lignin removal was due to the adsorption of lignin on fly ash.18 At a dosage of 55 mg/g fly ash/SL, lignin removal reached the maximum amount (53%), which corresponded to a lignin adsorption of 67 mg/g on fly ash. In another study, 67 mg/g of phenolic compounds of effluent containing phenol, 3-chlorophenol, and 2,4-dichlorophenol was adsorbed on fly ash generated by a power generator.33 In another study, by adding 100 mg/L of fly ash generated by a steam boiler to 50 g/L of bleaching effluent (stirred at 200 rpm for 6 h), 5 mg/g of lignin was adsorbed on fly ash.6 It is also apparent in Figure 2 that the COD level of SL decreased by increasing fly ash ratio. As can be observed, at a 55 mg/g fly ash/SL ratio, almost 50% (4728 ppm) of COD was removed. It was claimed that lignin-related substances significantly contributed to the COD content of pulping effluent.5,7 Hence, a decrease in COD could be attributed to the reduction in lignin content of SL. As can be seen, by increasing the dosage of fly ash, the turbidity of SL significantly decreased and reached a plateau of 220 NTU (89% turbidity removal). The decrease in turbidity removal can be attributed to two phenomena of adsorption and coagulation: (1) Lignin concentration in SL decreased as it was adsorbed on fly ash (i.e., adsorption); (2) It was claimed that lignin of pulping effluent had carboxylate groups, which implies that lignin might have an anionic charge density.36,37 Thus, lignin and other anionic components of SL would be
Table 2. Properties of Fly Ash anionic charge density (μeq/g) cationic charge density (μeq/g) pH of PZC BET surface area (m2/g) loss of ignition (LOI) (%)
unwashed fly ash
washed fly ash
0.0 35.0 9.4 63.70 48.15
1.8 17.1 7.7 90.20 44.24
determined before and after the washing treatment. As can be seen in Table 2, the washing treatment resulted in dropping the PZC from a pH of 9.4 to a pH of 7.7. A decrease in the cationic charge density and PZC of fly ash through washing might be due to the decrease in the metal components of fly ash, such as potassium and sodium. The surface area for unwashed and washed fly ash was determined to be 63.72 and 90.2 m2/g, respectively. It can be understood from the results that washing either removed the large metal components from fly ash or opened the structure of fly ash (i.e., improved the porosity of fly ash). In the literature, it was claimed that fly ash, obtained from the Obra thermal power station, had a surface area of 4.87 m2/g.28 In another study, the surface area of fly ash received from the Poplar River power station operated by the Saskatchewan Power Cooperation was 1.5−1.7 m2/g.29 Fly ash is produced from different sources and via altered processes. This will significantly affect the properties (e.g., surface area) of the generated fly ash,30,31 and this was probably the reason for the different surface areas of fly ash of this study and those reported in the literautre.33,34 Table 2 also indicates the LOI for both fly ashes at 550 °C. As can be seen, the LOI for washed and unwashed fly ash was 44.24% and 48.15%, respectively. The lower LOI of unwashed fly ash might be due to its lower carbon content.27 More studies
Figure 2. Effect of dosage of fly ash (unwashed, washed, and washed fly ash with post-pH adjustment) to SL ratio on removal of lignin, COD, turbidity, and pH of SL (conducted via adding 1 g of unwashed or washed fly ash to 45 g of SL at 30 °C, 100 rpm for 3 h). 7304
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Industrial & Engineering Chemistry Research neutralized by fly ash’s metals (such as Ca2+, Al3+, Fe2+, and Fe3+). The hydrolysis of metals and subsequent precipitation of metal hydroxides and other metal−lignocellulosic compounds would contribute to the decrease in the turbidity of SL (i.e., coagulation).37,38 Additionally, the pH of samples increased with adding fly ash (Figure 2). It was discussed that most fly ashes are alkaline due to earth metal compounds.18 Increasing the pH of the SL may be due to the hydrolysis of fly ash constituents (mainly metals) in SL.39 The solubility of some technical lignins is pH sensitive (e.g., kraft lignin).40 However, it was previously shown that the lignin of TMP spent liquor was not sensitive to pH and was soluble at a low pH of 4.15 Lignin has anionic charge density40 that can interact with cationic charges of fly ash through electrostatic charge interactions. The main mechanism for the isolation of lignin from SL is adsorption of lignin on the surface of fly ash and the coagulation of lignin with metal compounds (cationic particles) of fly ash in SL. In another attempt, two common adsorption models of Langmuir and Freundlich were applied to experimental data for washed and unwashed fly ashes in Figure 2. The description and assumptions of each model were extensively discussed in previous studies.15,24,34,35 The R2 values for unwashed fly ash data were 0.71 and 0.63 in Langmuir and Freundlich models, while these values for washed fly ash data were 0.69 and 0.66 in Langmuir and Freundlich equations, respectively. Due to the low values of regression in all cases, we can conclude that the models could not predict the results. As the adsorption of lignocelluloses on activated carbon was investigated and simulated with acceptable regression values, the low regression values in this work may suggest that adsorption was not the only factor affecting the removal. The existence of coagulation along with adsorption would most probably be the main reason for invalidity of the aforementioned models (models can only predict adsorption).34,35 Further studies on fundamentals of coagulation/adsorption are necessary to prove this hypothesis. 3.3. Adsorption on Washed Fly Ash. Figure 2 also shows lignin, turbidity, and COD removal as a function of the dosage of washed fly ash (mg/g) with and without pH adjustment. As can be seen, washing fly ash insignificantly affected the removal of lignin. In this case, an increase in the surface area of fly ash through washing (Table 2) compensated for the decrease in cationic charge density of fly ash. In other words, the overall adsorption might have been increased, while the overall coagulation might have been decreased in treating SL with washed fly ash compared with unwashed fly ash, which implies that washing fly ash had an inconsiderable effect on the adsorption of lignin. The COD and turbidity analyses (Figure 2) showed that the treatment with washed fly ash had less COD and turbidity reductions compared to the treatment with unwashed fly ash. As explained earlier, by washing fly ash, the metal components of fly ash decreased, and thus, its coagulating performance was reduced. The reduction in coagulating performance of fly ash would reduce its affinity in removing colloidal components from effluents. It should be highlighted that the difference in end pH of SL between unwashed and washed fly ash is due to the reduction in metal ions (mainly sodium and potassium) and thus alkalinity of fly ash through the washing process. The results in Figure 2 also suggest that the pH adjustment of the effluent after fly ash treatment did not improve the overall removal of lignin, turbidity, and COD from SL. Determining
details on the adsorption and coagulation processes involved in the removal of organic compounds from SL via fly ash treatment is currently under investigation by the same group. 3.4. Kinetics of Adsorption. Figure 3 shows the impact of time of unwashed fly ash treatment on lignin, COD, and
Figure 3. Effect of treatment time on the adsorption of lignin on fly ash and removal COD and turbidity from SL (conducted at a fly ash/ SL ratio of 55 mg/g, 30 °C, and 100 rpm).
turbidity contents of SL samples. It is observable that lignin reached a saturation level of 67 mg/g adsorption in 3 h. However, COD and turbidity reached the plateau in 45 and 90 min, respectively. These results are in agreement with an earlier study on the adsorption of lignin from SL on various adsorbents.15,19,41,42 In the literature it was claimed that the maximum adsorption of calcium lignosulfonate (34.2 mg/g) onto coal-based fly ash was obtained in 2 h under conditions of 30 °C and 150 rpm.43 These results may imply that the coagulation of metals with components of SL was a fast process, while the adsorption of lignin on fly ash was a slower process in the overall removal of lignin, COD, and turbidity analyses. In order to investigate the adsorption kinetics of lignin on fly ash, pseudo-first-order (eq 1), pseudo-second-order (eq 2), and Elovich (eq 3) models were employed15,44,45 ln(qe − qt ) = ln(qe) − kt
(1)
t 1 1 = + t qt qe k′qe2
(2)
qt =
1 1 ln(ab) + ln(t ) b b
(3)
where qt and qe (mg/g) are related to adsorption amount at time t (min) and equilibrium, k (1/min) and k′ (g/(mg·min)) are kinetic constants, and a (mg/(g·min)) and 1/b (mg/g) correspond to initial adsorption rate and available active sites, respectively.15,44,45 Experimental data in Figure 3 were fitted into eqs 1−3. Subsequently, the model parameters were determined and are listed in Table 3. As can be seen, the pseudo-second-order model provided the best correlation (R2 = 7305
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Industrial & Engineering Chemistry Research Table 3. Kinetic Parameters of Adsorption by Fitting the Experimental Data of Figure 3 pseudo-first-order model
pseudo-second-order model
Elovich’s equation
component
k (1/min)
R2
k′ (g/mg.min)
R2
1/b (mg/g)
R2
lignin
3.2 × 10−4
0.658
3.3 × 10−4
0.999
7.13
0.847
adjusting the pH after the adsorption stage, but the turbidity was unfavorably affected. An earlier study showed that two-stage adsorption was a more efficient option than one-stage adsorption for lignin removal from SL (lignin removal increased from 45% to 60%).15 The results of option D depicted that after two stages of adsorption, lignin, COD ,and turbidity removal was 66%, 68%, and 94%, respectively. In this case, the lignin adsorption on fly ash corresponded to 67 and 17 mg/g in the first and second stages, respectively. In option E, the two-stage adsorption was followed by a neutralization step, which led to 68% of lignin, 70% of COD, and 94% of turbidity reductions. 3.6. High Heating Value of Treated Fly Ash. Figure 4 shows the high heating value of the fly ash treated with SL in a
0.999) with experimental data. On the basis of these results, it can be concluded that chemical sorption was the ratecontrolling step.44,45 This might be due to the sharing and exchanging of electrons between cationic fly ash and anionic lignin.15,40 The same mechanism was observed in previous studies on the adsorption of lignin on activated carbon.15,44 3.5. Process Modification. As illustrated in the Methods and Materials, five alternatives were assessed under the optimized conditions (3 h and dosage of 55 mg/g of fly ash/ SL), and the experimental data are listed in Table 4. The results Table 4. Concentration of Lignin, COD, Turbidity, and pH of SL under Different Process Optionsa option control A B C D E
lignin concentration, g/L 7.05 3.29 3.32 3.06 2.37 2.28
± ± ± ± ± ±
0.12 0.09 0.08 0.11 0.14 0.16
COD, mg/L 9456 4840 6085 4508 2985 2873
± ± ± ± ± ±
510 246 489 341 342 358
turbidity, NTU 2060 221 620 301 126 121
± ± ± ± ± ±
82 90 73 65 13 18
end pH 5.3 12.1 5.3 5.3 12.3 7.1
± ± ± ± ± ±
0.1 0.1 0.0 0.1 0.2 0.1
a Conducted under the optimal conditions of 55 mg/g unwashed fly ash/SL, 3 h, and 30 °C.
showed that a single-stage adsorption resulted in 53% of lignin, 49% of COD, and 89% of turbidity removal from SL (option A). However, the pH of the sample increased to 12.1, which is unfavorable for subsequent anaerobic wastewater treatment, as it is mostly performed at 7−7.5 pH.7 Therefore, pH was adjusted before incubation in option B, and the results are listed in Table 4. As can be seen, the pH adjustment before incubation (option B) caused 53% lignin removal, but the turbidity and COD were less reduced compared with option A. This analysis indirectly implies that lignin removal was somehow independent of the pH of the process (adsorption was independent), but the removal of other compounds from SL (via coagulation) was pH dependent. In the literature, it was reported that the metal−lignocellulosic compounds were more effectively formed under alkaline pH, which indirectly confirms the dependency of coagulation with pH.46 Option C considers the scenario that the SL is treated with fly ash, and its pH is adjusted afterward so that the subsequent biological process can be conducted on the treated SL. In option C, the addition of acid after adsorption slightly improved the overall removal of lignin and COD but not that of turbidity. The small decrease in lignin content of SL is attributed to the adsorption of more lignin on fly ash, which resulted in a further COD reduction. However, the addition of acid affects the overall ionic strength of the SL. Under acidic condition, the hydrogen ion will replace the metal ion on the metal−lignocellulosic compounds. The solubility of a hydrogen-based compound might be higher than that of metal-based compounds, which resulted in its dissolution in SL after readjusting the pH to 5.3.47 It should be highlighted that slightly more lignin and COD removal was obtained via
Figure 4. High heating values of lignocellulose-treated fly ash as a function of lignocellulose adsorption on unwashed fly ash.
single adsorption step (option A). Generally, the higher adsorption of lignin on fly ash was obtained when the ratio of fly ash to SL in Figure 2 was lower. As can be seen, an increase in the adsorption of lignin on fly ash escalated the heating value of biomass-based fly ash. This would indicate that the lignocellulosic-treated fly ash could be introduced as a source of energy in the boiler. 3.7. Process for Utilizing Fly Ash in SL. On the basis of the results in Table 3, a process was proposed in Figure 5 to treat the SL of a TMP process with fly ash. In this process, the SL of TMP would be treated via a two-step adsorption stage. Then, the treated fly ash will be separated from the filtrate in each adsorption stage, combined together, and sent to a mechanical press to increase its dryness. Once the water content of the treated fly ash is reduced, it will be sent to the bark boiler for combustion. It will be recycled to the adsorption stages after combustion. To avoid accumulation of fly ash in the system, part of the fly ash would be purged from the boiler. For the spent liquors with higher concentrations of sugars and lignin, a combination of ultrafiltration and the proposed process may be an appropriate option to (1) remove lignin compounds and (2) produce value-added products, e.g., ethanol from the sugars remaining in the spent liquors after adsorption.13 The lignocellulosic materials attached to fly ash will ideally generate a net heat in the boiler; hence, it will help the overall economy 7306
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Figure 5. Proposed process for SL of TMP (FA is fly ash in this figure).
of the mill. On the basis of the experimental data in Figure 3, it can be concluded that by treating 1 ton of SL with 10 kg of fly ash at 30 °C for 3 h, 12% of lignin, 15% of COD, and 40% of turbidity were removed from SL, and the treated fly ash had a high heating value of 10.5 MJ/kg (increased from 4 MJ/kg on untreated fly ash) (Figure 4). By treating 1 ton of SL with 55 kg of fly ash at 30 °C for 3 h, 53% of lignin, 49% of COD, and 89% of turbidity were removed from SL, but the treated fly ash had a high heating value of 6.1 MJ/kg. Consequently, not only is the boiler integrated into the process (and lignocelluloses will be more effectively utilized) but also the load to TMP spent liquor (pressate) of this process will be reduced significantly. This is important as the current anaerobic, aerobic, and polymers in wastewater treatment systems are expensive, and this process will reduce their needs. The developed process is environmentally friendly, simple, and well integrated into the existing facilities of pulping processes. In addition, the developed process works for any integrated mill that has a bark/biomass boiler. Alternatively, if a biomass boiler is not available, biomass-based fly ash can be obtained from other places and used in the developed adsorption process, but the treated biomass-based fly ash cannot be further processed on site. It should be stated that washing fly ash did not significantly improve the lignin and COD removal; hence, no pretreatment would be required for this process at large scales. A detailed analysis is needed to evaluate the economical aspect of the suggested process. In this regard, a comparison between the suggested process and other alternatives13,48 would be needed.
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[email protected], Tel: 807-343-8697, Fax: 807346-7943. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support of this work was provided by the NSERC Discovery grant program of the Government of Canada. Also, the Canada Research Chair and Canadian Foundation for Innovation programs are acknowledged for their support. The authors also thank Ms. Germaine Cave for her help in this work.
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REFERENCES
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4. CONCLUSIONS The adsorption of lignin on fly ash was insensitive to pH, but the coagulation of constituents of SL with fly ash components was pH sensitive. Under optimal conditions (fly ash/SL ratio of 55 mg/g for 3h), 53% of lignin, 49% of COD, and 89% of turbidity were removed. The two-stage adsorption process had 68%, 70%, and 94% lignin, COD, and turbidity removal, respectively. The adsorption process with fly ash can be fully integrated into the wastewater systems in which (1) organic compounds in the SL are better utilized and (2) the load to the wastewater is reduced. 7307
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