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Energy saving in the process of bioethanol production from renewable paper mill sludge
Journal Pre-proof Energy Saving in the Process of Bioethanol Production from Renewable Paper Mill Sludge
Tareq Salameh, Muhammad Tawalbeh, Mohammad Al-Shannag, Motasem Saidan, Khalid Bani Melhem, Malek Alkasrawi PII:
S0360-5442(20)30192-4
DOI:
https://doi.org/10.1016/j.energy.2020.117085
Reference:
EGY 117085
To appear in:
Energy
Received Date:
19 June 2019
Accepted Date:
02 February 2020
Please cite this article as: Tareq Salameh, Muhammad Tawalbeh, Mohammad Al-Shannag, Motasem Saidan, Khalid Bani Melhem, Malek Alkasrawi, Energy Saving in the Process of Bioethanol Production from Renewable Paper Mill Sludge, Energy (2020), https://doi.org/10.1016/j. energy.2020.117085
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Energy Saving in the Process of Bioethanol Production from Renewable Paper Mill Sludge Tareq Salameha,b, Muhammad Tawalbeha, Mohammad Al-Shannagc, Motasem Saidanc, Khalid Bani Melhemd, Malek Alkasrawie,* a Sustainable
and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates b Sustainable Energy Development Research Group, Research Institute for Sciences and Engineering, University of Sharjah, PO Box. 27272, Sharjah, United Arab Emirates c Department of Chemical Engineering, The University of Jordan, 11942 Amman, Jordan d Department of Environmental engineering, Hashemite University, Zarqa, Jordan e Department of PS & Chemical Engineering, University of Wisconsin, Stevens Point, USA
*Corresponding author: [email protected]
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Abstract Paper mill sludge (PMS) can be efficiently utilized to produce fuels and chemicals. However, wastewater is usually generated during the de-ashing step of the PMS for fibrous materials recovery. Per process requirements, the wastewater stream must be treated which results in an increase in the overall process production cost. Therefore, this research aims at reusing the wastewater produced during the de-ashing step as a substitute for freshwater addition during the conversion of PMS into ethanol. The advantages of this approach include reducing the amount of wastewater produced and enhancing the overall efficiency of the process. It will contribute to the circular economy of zero waste discharges. The results showed that 30% of the process wastewater can be recycled without affecting the enzymatic hydrolysis and ethanol fermentation. Hence, the amount of wastewater that needs to be treated is reduced by 30% resulting in a cost reduction of 22.5%. The results also showed that wastewater recycling minimized the energy demands in the distillation and evaporation units by 1206 kJ/kg. The energy reduction is due to the increase of metals and total soluble solids in the broth stream after fermentation. This process configuration enhanced the process economy, saved energy and managed waste streams.
Keywords: Process Stream Recycling; Paper Mill Sludge; Ethanol; Energy Reduction, Circular Economy
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1. Introduction In the biorefinery process, multiple technologies for biofuels and chemicals production need to be integrated for energy security. Waste generation from intensive industrial production is an environmental and economic burden. Waste recycling and utilization for energy production leads to zero waste discharge in the circular economy. In the process of fiber recovery from paper mill sludge (PMS), a large amount of wastewater is generated due to the de-ashing process. The wastewater exhibits environmental and economic burdens that must be managed [1,2]. The wastewater from biofuels production increases the energy demand in the life cycle of the bioconversion platform. Energy reduction is the target of all biofuels production processes due to the large variation in the net energy value of biomass-to-biofuels [3]. Energy process integration using the pinch analysis method is very helpful in minimizing the energy loss during chemical engineering processes [4]. However, energy integration technology is limited, as well as the reduction required beyond the energy loss value. There are several process configurations that could be adopted in order to reduce the energy demand [5,6]. These configurations lead to increasing the biofuels concentration prior to the distillation step, as well as the soluble solid prior to the evaporation step [5,7-9]. Therefore, the development and optimization of integrated technologies is a necessity in order to minimize the energy demand in the biorefinery processes [6,10]. In the production of ethanol from PMS, water addition is required at different unit operations, mainly in the enzymatic hydrolysis and fermentation [11]. The fresh water is crucial to run the enzymatic hydrolysis at a certain solids load [12]. The water addition leads to the generation of a huge amount of wastewater that contains water-soluble compounds [5,7-9]. However, the largest generation of wastewater is from the de-ashing step of PMS. This stream contains water-soluble
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inhibitors, chlorine, and minerals resulting from the de-ashing step [13,14]. Chlorine in wastewater originates from chemical industries [15], mainly from the pulp industry after the wood digestion step [16]. Chlorination at low concentration is used worldwide for bacterial disinfection. Nevertheless, the concentration of chlorine in de-ashing wastewater is much higher than that of potable drinking water.
These soluble substances might exhibit potential inhibition to the
saccharification and alcoholic fermentation [17]. The potential inhibition may vary greatly with the nature of PMS and the pulping conditions such as the chemical structure of the raw material and the wood species [18]. The treatment of wastewater is an essential operation in the overall conversion of renewable feedstock to ethanol [19]. Anaerobic digestion and biological platform have been widely used to treat the wastewater and reuse it in various ways [1,20]. The wastewater treatment generated from the bioethanol plants increases the operation and capital expenditures of the entire process. The conversion of PMS to ethanol is one of the renewable processes that can be used to produce biofuels, especially in the autumn season. This conversion process requires energy for the treatment of wastewater production, distillation, and evaporation of liquid biofuel (ethanol) as shown in Fig. 1. The present study investigated the possibilities of recycling the wastewater generated after the de-ashing step of PMS to replace the freshwater addition. The effect of recirculation of the wastewater stream from the de-ashing process on both sugar and ethanol yields to reduce the use of freshwater addition was also investigated. Additionally, the water condensate that is generated after the evaporation step is recycled back to the process in order to close the cycle of the water. This step is very crucial to enhance the circular economy of the zero-waste discharges.
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Fig. 1. The overall process flow diagram of the production of ethanol from PMS with a wastewater recycling configuration (----- recycling, ______ generic processes).
2. Materials and methods 2.1. Paper mills sludge (PMS) description The PMS samples were collected from the central Wisconsin area paper mill, Wisconsin, USA (44°23′12″N 89°49′23″W). The PMS was generated in the Kraft mill pulping process that digests softwood chips (Red Pine) using a mixture of sodium hydroxide and sodium sulfide. The samples were withdrawn from the primary screw press after the first stage of the wastewater treatment. The samples were kept refrigerated at 4°C in a sealed plastic container until further use. The compositional analysis of the PMS was conducted using a method developed by the National Renewable Laboratory (NREL) [21]. Each analysis was performed in three duplicates and the average results are shown in this work. 2.2. De-ashing of PMS The de-ashing process is a step of removing the inorganic and organic materials from the fibrous cellulose. Technically, it is a washing step using water (with dilute acid or base) to clean the fiber from other contaminants. In this study, the de-ashing was performed using washing of 1 M HCl 10 % w/v employed to recover the fiber. The PMS was mixed with 1 M HCl solution and stirred for two hours at 500 rpm. The total washing volume that was used corresponded to 3 L to 1 kg dry PMS. Notably, calcium carbonate is one of the major fillers in the PMS and can be removed as calcium chloride when treated with hydrochloric acid according to the following reaction:
2HCl CaCO 3 CaCl 2 CO 2 H 2 O
(1)
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The acid washed PMS fibers were filtered and washed with distilled water until the pH was neutralized from 1.12 to 5.00 (the optimum pH for cellulolytic enzymes). The washed fiber was kept in a refrigerator at 4oC for further uses. The wastewater generated after washing was used immediately in the enzymatic hydrolysis step. The wastewater after the de-ashing step (DA-WW) was chemically analyzed and characterized. Each analysis was performed in three duplicates and the average results are shown in this work. 2.3.
Characterization and Recycling of de-ashed wastewater
The DA-WW was collected and used immediately after the de-ashing process. The fiber-free ash was collected separately and prepared for later enzymatic hydrolysis and fermentation. In the hydrolysis step, freshwater will be added to the fiber free ash to make the total soluble solid up to 5%. These optimal solid loads guarantee efficient enzymatic hydrolysis on the cellulosic substrate. In the current process configuration, several re-using scenarios are proposed. The first scenario is the base case where there is no wastewater recycling. In the subsequent scenarios, an amount of wastewater from the de-ashing process replaces the freshwater addition in various degrees. The first degree will start with 30%, and the recycling will continue until 100% replacement of freshwater occurs. The effect of degree of wastewater replacement of fresh water on the enzymatic hydrolysis and on fermentation will be studied. The recycling will lead to the accumulation of minerals in the wastewater generated from the distillation step. 2.4.
Enzymatic hydrolysis of sludge
A commercial second-generation cellulase cocktail Cellic®CTec2 provided by Novozymes A/S, Bagsvaerd, Denmark, was used to conduct the experiments of the enzymatic hydrolysis of PMS. The concentration of the protein in Cellic® CTec2 of 73.6 mg/mL was estimated using the Bradford
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Coomassie Blue method according to Adney and Baker [22]. The filter paper units (FPU) activity was estimated to 113.8 FPU/mL using a NREL LAP standard procedure TP-510-42628 [22]. The value of 2.0 mg of reducing sugar as glucose from 50 mg of filter paper (4% conversion) in 60 minutes has been designated as the intercept for calculating cellulase (FPU) by IUPAC. A concentration of 3.4% (w/w) solution was prepared by adding 3.4 g enzyme/100 g dry PMS. A citrate buffer solution was used in order to prepare a suspension of each PMS with a concentration of 5% (dry weight by volume). The specifications of the buffer solution are as follows: 0.05 M and a pH of 5. The total volume was 150 mL in this case. The solutions were poured in 250 mL flasks followed by stirring on a hot plate at a rotation speed of 200 rpm for 48 h and at a temperature of 50°C. After that, samples were taken periodically every three hours to measure the glucose concentration. The final hydrolyzed product was then filtered, and the obtained sugar solution was stored at 4°C and kept until fermentation experiments. 2.5.
Fermentation
The ethanol fermentation experiments were conducted using an Industrial yeast FermPro ™ (purchased from Ferm Solutions Inc., Danville, KY). First, a small amount (1 mL) of frozen yeast was precultured at 37°C for 16 hours in a medium of the following composition: 20 g/L glucose, 20 g/L peptone and 10 g/L yeast extract. The clear PMS sugar solution was then supplied by several nutrients with specific amounts according to Gurram et al. [14]; 20 g/L MgSO4•7H2O, 10 g/L KH2PO4, 4 g/L CaCl2•2H2O, 200 mg/L ZnSO4•7H2O, 20 mg/L Na2MoO4•2H2O, 200 mg/L CoCl2•2H2O, 2 mg/L d-biotin, 5 mg/L p-aminobenzenoic acid, 5 mg/L Nicotinic acid, 5 mg/L calcium pantothenate, 5 mg/L Thiamine-HCl, 10 mg/L Pyridoxine-HCl and 10 mg/L lactoside.
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2.6.
Analytical and Instrumentation Analyses
Glucose, xylose, arabinose, galactose, and mannose concentrations were measured and quantified against a fucose internal standard. The concentration of the polymeric sugar was estimated from the concentration of the corresponding monomeric sugar. This was done using an anhydrous correction of 0.88 (or 132/150) for C-5 sugars (xylose and arabinose) and 0.90 (or 162/180) for C-6 sugar (glucose, galactose, and mannose). The measurements were performed using an Ion Chromatography system (Dionex ICS 3000 Thermo Scientific Waltham, MA, USA) that maintained the temperature at 25oC. The IC system has an electrochemical detector equipped with a gold electrode, along with two guard columns. These two columns are Amino Trap Bio1C and a Carbo Pac PA1 and a 4 × 250 mm Carbopac PA1 column (both from Dionex). The eluents were prepared by adding 3 mM to distilled water. This ultrapure water was partially degassed by employing vacuum and running it at a very low flow rate of 1 mL min-1 through the column. Ethanol analysis was also performed using the Dionex ICS 3000 with an eluent of 100 mM methanesulfonic acid prepared also with ultrapure water. This ultrapure water was also partially degassed by employing vacuum with a running flow rate through the column of 0.18 mL min-1. 2.7 Process description Fig. 1 describes a flowsheet diagram of a conceptual design of a possible process for sugar and ethanol production from PMS. The PMS is washed using a dilute acid, such as HCl or H2SO4 in order to solubilize all substances (mainly minerals) encapsulated in the wood fiber. The clean fiber is transferred to the saccharification step were cellulases and fresh water are added. Freshwater addition is necessary since enzymes operate at a certain substrate concentration [12]. The green sugar produced from enzymatic hydrolysis is transferred to the fermentation reactor. In the proposed process, the sugar concentration should be within the limit of microbial tolerance. Once
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all sugars are converted into ethanol, the fermentation broth enters a stripper column where over 90% of the ethanol is distilled and subsequently fed to the rectifying column to reach higher grade ethanol. The wastewater after ethanol recovery is sent to multiple-effect evaporators in order to reduce the volume of the waste stream. The overall process description of 2,000 dry metric tons of PMS daily for sugar and biofuel production is depicted in Fig 1. The treatment of PMS (mainly washing for fiber recovery) will generate about 6,000 metric tons of DA-WW as the result of removing the fillers and minerals from PMS. The generic process of PMS conversion (without DA-WW recycling) generates a stream of wastewater. This stream is defined as generic wastewater (G-WW) and is produced at a rate of 2,500 metric tons/day. The cleaned fiber, about 1,000 metric tons, would require the addition of 6,200 metric tons of fresh water to reduce the total solids (TS) to 5%. The water addition step is of great importance in order to provide the optimal catalytic conditions for the cellulases activities for cellulose conversion to the economic sugar concentration [23]. The hydrolysate that contains mainly glucose produced in the saccharification process is transferred to the fermentation reactor. Alcoholic fermentation utilizes commercially available yeast for ethanol fermentation. In the base case scenario, this represents the generic scenario; there is no DA-WW recirculation and thereafter it joins the G-WW for further wastewater treatment. The cost calculation of wastewater was based on the chemical oxygen demand content that correspond to US $ 0.11/m3 as a reference cost [24-26]. In the process where multiple effect evaporator is used, energy demand plays an important role in the process economy. One way of reducing the energy demand is by increasing the TS content in the downstream processing of ethanol recovery.
3. Results and discussion 3.1.
Wastewater recycling
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The chemical composition and characterizations of the wastewater effluent from the pulp and paper industry differs drastically from one mill to another according to the pulp process and the singular characteristics of each factory. The chemical composition and characteristics of the wastewater effluent from the pulp and paper industry vary substantially from one mill to another according to the pulping process and the unique characteristics of each mill. The characteristics of the DA-WW stream shown in Fig. 1 are as follows: COD 2,269 mg/L, TS 2,8645 mg/L, Chlorine 35,300 mg/L, Fe 19.5 mg/L, Ca 13,860 mg/L, Mg 146.9 mg/L, Al 55.7 mg/L, P 6.3 mg/L and Na 34.3 mg/L. It is clear that this stream cannot be discharged to the municipal sewage system or directly released into a stream or other body of water. Hence, treatment is required to reduce the level of contaminants to meet the permit limits. For instance, high organic material, heavy metals, and suspended solid contents are considered major pollutants of pulp and paper industry effluents, which is the case in the present study. The DA-WW is ready to replace the freshwater addition in the enzymatic step. It is obvious that the two major components of the stream are chlorine and calcium ions. Chlorine is well known as an inhibitor for bacterial infection in potable water at concentrations of 4 ppm. It is assumed that the present concentration of chlorine will not have a major effect on enzymatic hydrolysis and fermentation once the recycled waste stream replaces the freshwater addition. The COD, 2,269 mg/L from Table 1, was relatively much lower than those for industrial wastewater, e.g. 50,000 mg/L. The impact of recycled DA-WW on the biological activities of the enzymes is anticipated to be less due to lower COD content. In addition, the presence of iron, phosphorous, and magnesium ions at concentration between 34.3-149.9 mg/mL are essential for the alcoholic fermentation as supplemental nutrients. Interestingly, this stream has a considerable amount of TS;
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if recycled back to the process, it will reduce the energy demand in the evaporation unit according to the process description, Fig 1.
Table 1. Setup design for recycling scenarios and possibilities of freshwater replacement as well as pollutants reduction. Data are shown as mean ± standard error of three replicates.
Replacing the fresh water in the saccharification step by DA-WW is a viable option in order to reduce the wastewater stream. In this alternative, it is assumed that the soluble substances do not alter the biological activities for both cellulases and microbial fermentation. This might lead to saving in energy consumption in both the distillation of the fermentation broth and the evaporation of the waste stream [7,8,27,28]. However, progressive DA-WW recirculation leads to the buildup of various substances in the enzymatic and fermentation step. Stenberg et al. [9] showed that stream recirculation increases the concentration of non-volatiles tenfold in various areas of the process. The experimental results obtained by Palmqvist et al. [29] also showed that the nonvolatile substances discharged during steam pretreatment of willow inhibited the growth of microbial fermentation. In the present investigation, it was possible to recycle and replace 30% of the freshwater without affecting either the enzymatic hydrolysis or the fermentation step, in Fig. 2. These results are in agreement with the previous investigation by Alkasrawi et al. [7] where it was possible to recycle the process stream after distillation to a certain extent. However, Alkasrawi et al. [27] showed that it is possible to achieve 100% replacement of freshwater by the collection of evaporation condensate. In the present study, the evaporator condensate was not evaluated, but it was proposed as process configuration in Fig 1.
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Fig. 2. Variation glucose concentration with time without and with wastewater (WW) recycling. Error bars represent mean ± standard error of three replicates.
Interestingly, recycling at a higher degree than 30% reduced the enzymatic yield gradually until glucose yield started to drop at 40% recycling, Fig. 2. A similar pattern of yield decrease was obtained by Alkasrawi et al. [7] when a higher degree of recycling before and after distillation significantly lowered the enzymatic and fermentation mechanism, See Fig. 2. The reason for gradual glucose yield in the hydrolysis step in the study is most likely attributed to the accumulation of various substances that interferes with the catalytic activity of the cellulases. These substances have likely caused uncompetitive enzymatic inhibition that affect the final turnover of the cellulosic fiber. The accumulated chlorine might halt the enzymatic activities in the subsequent recirculation. Although chlorine's value as a disinfectant has been known for a long time, the mode of action and mechanism by which chlorine kills or inactivates microorganisms or enzymes is not clearly understood. It has been postulated that chlorine exists in water as hypochlorite and hypochlorous acid. This mixture in water produces hydrochloric acid, which then decomposes to chlorine, oxygen, and thiol which is immediately oxidized. Since chlorine is electronegative, it can oxidize peptide links and denatures proteins made up of sulfur-containing and aromatic amino acids such as cellulases [30-32]. The so-called "multiple hit" theory of chlorine inactivation of microbial growth suggests that the chemical characteristic of chlorine and its derivatives in aqueous condition dissociates almost all biomolecules: enzymes, nucleic acids, and membrane lipids [33,34]. Though it can be noted note that enzymatic hydrolysis is affecting the enzyme treatment of the biomass and fermentation using microbes, the effect could be directly due
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to the lysis of cellulolytic enzymes and/ or the microbes used for both cellulolytic processes and enzymes involved in the whole fermentation pathways. 3.2.
Kinetic modeling of glucose production
To determine the kinetics of glucose production, including the reaction rate law, different reaction orders were proposed and the integrated linear forms were used to plot the data obtained. The rate law that produces a straight line with a correlation coefficient as high as possible can be considered to represent the experimental data. Various kinetic models are discussed in detail elsewhere [35,36]. These models were investigated and the production rate of glucose in this work can be described using the pseudo first-order kinetic model as shown in the equation below: dC k Ce C dt
; C(0) = C0 = 0 mg/g fiber
(2)
where C is the concentration of glucose in mg/g fiber, t is the operating time in h, k is the pseudo first-order kinetic parameter in h-1, Ce is the equilibrium concentration of glucose as t→. Integration of the above first-order differential equation with C(0) = 0 gives:
C ( t ) C e 1 e kt
(3)
The optimum parameters of both Ce and k at different operating conditions (various levels of wastewater recycling) were obtained by minimizing the sum of square of errors (SSE) between the experimental concentrations, Cexp(t), and the corresponding predicted one from the above equation. Where the SSE is defined as: N
N
SSE Ci ,exp ( t ) Ci (t ) Ci ,exp ( t ) Ce (1 e kt ) i 1
2
2
(4)
i 1
Fig. 3 shows the variations of both experimental and predicted glucose concentrations with time for the cases 0%, 30%, 70%, 100% recycling degree (RD) of wastewater. The legends of the four graphs in the Fig. 3 summarize the optimum values of Ce and k in addition to the value of squared
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correlation coefficient, R2, which is calculated to measure the goodness-of-fit of the pseudo firstorder model. It is clear from the Fig. that the pseudo first-order model can describe the time dependent concentration adequately as R2 approaches unity. Moreover, Fig. 3 clearly shows that the pseudo first-order constant (k) is directly proportional to recycling degree (RD) of wastewater. Nonlinear regression demonstrated that the k parameter can be mathematically represented as function of RD according to a three-parameter power law model with a value of squared correlation coefficient R2 = 0.9862 as shown in Fig. 4.
Fig. 3. Comparison between experimental glucose concentration and predicted ones using pseudo first-order kinetic model for the cases: a) without wastewater recycling, b) 30% wastewater recycling, c) 70% of wastewater recycling, and d) 100% of wastewater recycling.
Fig. 4. Presents a mathematical model of pseudo first-order constant (k) as function of DA-WW recycling degree using three-parameters power law model. The k value of enzymatic reaction when no DA-WW stream was recycled is 0.0469 h-1. However, the k changed to 0.0597 h-1 when 30% of DA-WW was recycled although the final yield was not affected, evident in Fig. 4. This is likely due to minor enzyme noncompetitive inhibition by the recycled TS. It may be postulated that if the enzymatic hydrolysis residence time is prolonged, the same cellulose conversion may be obtained as in the base case (no recycling).
Fig. 4. Mathematical representation of pseudo first-order constant (k) as function of recycling degree using three-parameters power law model.
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3.3.
Energy reduction as a function of DA-WW recirculation
Table 2 present an energy demand and saving in the evaporation unit as a function of DA-WW recycling. The consecutive DA-WW recycling leads to TS accumulation in enzymatic hydrolysis and in the fermentation steps, Table 2. The accumulation of TS in the final streams before the distillation and evaporation processes reduced the heat capacity of that stream, Table 2. This leads to a substantial decrease in the energy demand for both distillation and evaporation steps. The energy used for distillation can reach up to 40% of the cumulative energy charge in bioethanol production [37,38]. Recently, the application of a hybrid process, namely a combination of the distillation with pervaporation, could come to the front due to the relative high permeation rate and selectivity of the pervaporation [39]. At 30% freshwater replacement, yields were unaffected, leading to a reduction in the wastewater treatment (WWT) cost by 22.5%, Table 3. The energy demand was further decreased by around 1206 kJ/kg recycled wastewater as shown in Table 1. This is a very interesting finding and it will further decrease the production cost of ethanol. However, further increase of the recycled wastewater leads to further reduction of the energy demand, which in turn minimizes the sugar yield and consequently, reduces the overall ethanol yield evident from Table 3.
Table 2. Energy reduction in the downstream processing units as a function of DA-WW recycling. Data are shown as mean ± standard error of three replicates.
Table 3. Process cost reduction due to wastewater re-use in the process of bioethanol from PMS.
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Many technologies can be used for the treatment of wastewater that contains metals (Fe, Al, etc.) and TS. These technologies vary from one to another in terms of drying system, mode of the heating process (direct or indirect), type of energy (thermal or electrical) and amount and cost of energy consumption. The amount of recycled wastewater helps in reducing the energy consumption in the distillation and evaporation processes as well as in the WWT process. This can be achieved by increasing the concentration of the TS in the recycled DA-WW used as shown in Table 2. The presence of TS and metals affects the heat transfer mechanism for the evaporation process; this effect helps the water reach the boiling temperature faster than the fresh water. Both TS and metals change the overall heat capacity value of wastewater prior to entering the downstream processing operating units (evaporation and distillations units). This reduction in time is the main reason for saving energy and cost for the conversion of PMS to bioethanol. On the other hand, there is an optimum percentage of using wastewater produced during the conversion process of PMS to ethanol. This percentage was selected without having any effect on the enzymatic hydrolysis and fermentation steps and was cost-effective on both COD and chlorine removal. The cost of both COD and chlorine removal increase with the increasing of recycling degree or freshwater replacement. It is worth mentioning that 30% of wastewater was recycled in this study for the previously mentioned reasons. Table 3 shows the cost required for the conversion of PMS to bioethanol. The cost of WWT decreases as the recycling degree increases, whereas the WWT cost reduction and net cost reduction increase as well as the glucose yield reduction. This reduction in the cost and saving in energy makes the process more sustainable and economically feasible. However, improving energy efficiency could be achieved by applying current energy efficient technologies [39]. For example, a report provided by the German Environment Agency
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[39] discussed various energy efficient commercial technologies with respect to less energy demand for wastewater sludge drying. 3.4.
Impact on processing cost of WWT
The overall treatment cost of WWT in a biofuel production facility would correspond to 2% of the total production cost [40]. Techno-economic analysis is one of the most intensive topics studied so far for bioethanol production from various renewable resources [41,42]. The overall reference cost of total waste water (combination of G-WW and DA-WW) is US $ 0.11/m3 wastewater, although the cost is subject to change with respect to the technologies used and geographical location of the plant [25]. The reference cost was based on published research articles [25,26] as well as current industrial technologies reports [24]. The reference cost is helpful to compare the cost reduction, but it is not intended for the absolute cost calculation. Based on the cost breakdown, the annual costs of such treatment system can be calculated as shown in Table 3. In this study, the annual cost calculation for both G-WW and DA-WW was estimated to be US $ 456,960 according to Table 3. The annual cost for both G-WW and DA-WW was estimated to be US $ 114,240 and US $ 342,720 respectively. Notably, the cost associated with DA-WW raises the processing cost of WWT by three folds. This supports our claim that PMS for biofuel is attractive, however, the generation of DA-WW will affect the overall process cost. As discussed earlier, it was possible to recycle 30% of the DA-WW without impacting the process performance. This means, according to Table 3, a 22.5% reduction of the overall WWT is achieved while a net cost of US $ 103,816 is saved. For future recommendation, other technologies might be implemented to further reduce the processing cost. Ortizet et al. [26] reported that membrane biological reactor (MBR) technology could operate under high concentrations of Mixed liquor suspended solids (MLSS) of 15– 30 g
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MLSS/L. At the highest concentration of MLSS, wastewater could be reduced to 50% or higher levels. For such medium-strength effluents wastewater characteristics, neither chemical nor conventional activated sludge processes are sufficient alone to treat medium-strength effluents down to the permissible discharge limits. Therefore, in the present study, a combination of chemical and biological (extended aeration activated sludge) treatment systems are proposed, and in the present study, 3,000-8,000 m3 per day is the capacity of the treatment plant (chemical and biological treatment systems). For such a combination treatment system of paper effluent wastewater, the costs of treatment depend on the flow rate as shown in Table 3. In the light of this development, future research could focus on a microbial treatment of the non-recycled wastewater and enhance heavy metals recovery in an integrated biorefinery process [43]. This work could be even expanded further by integrating the bio-oil production from microalgae cultivation on the wastewater stream generated from washing the PMS [43, 44]. Another potential effect of process streams reuse and recycle in bioethanol production plants is the corrosion impact inside the pipelines and in the downstream processing equipment of biofuel recovery [45]. The DA-WW recycling in the long-term process operation might affect the maintenances scheduling of the downstream processing equipment and subsequently exhibiting a corrosion effect. The effect on corrosion might be negative or positive based on the chemical content of the recycled stream and the nature of materials used in the construction of equipment and the pipelines. In the recycled DA-WW there are two major elements, chlorine and calcium, that are reported to have an antagonistic effect on corrosion [46,47]. The chlorine is an electronegative element that exhibits pitting corrosion in the presence of oxygen and corrosion rate increases with temperatures [48]. The chlorine is stripped off in the first distillation column and it
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leaves the top of the distillation column as dry gas. The gaseous chlorine does not manifest any corrosion effect unlike the wet chlorine [49]. Owning to distillation, chlorine is stripped off, hence, any potential corrosion in the down streaming process is greatly minimized. In contrast, calcium is known for being effective as a corrosion inhibitor [47,50]. It is believed that the inhibitory effect on the corrosion of calcium offsets the impact of chlorine on corrosion. In addition, the equipment made for ethanol production from lignocellulosic feedstock is made to be corrosion resistant. Davis et al. [51] reported that all conversion operation units of biomass to biofuel are constructed of 304 or 316 stainless steel. These classes of stainless steel are essential to secure the corrosion resistance to caustic solutions, as well as an axenic environment where maintenances are scheduled regularly. Davis et al. [51] reported a techno-economic analysis of bioethanol production and showed that the capital investment of distillation and solids recovery is US $ 22.3 MM meanwhile wastewater treatment would cost around US $ 49.4 MM. This shows that the cost associated with wastewater is two times higher than the downstream processing. This would support the finding of this study that a reduction in wastewater treatment cost is significant and would contribute to the overall process economy. In case the effect is positive, the impact would be on a small increment of capital expenditure of the process equipment and pipelines. By looking at the techno-economic evaluation of cellulosic ethanol reported by Humbird et al. [52] the corrosion prevention cost is associated with the capital investment cost and not with the operation cost. Moreover, Söylev and Richardson [53] reported several corrosion inhibitors that could be applied without the need for manufacturing new materials with high corrosion resistance. 4. Conclusions PMS conversion into biofuels is an attractive approach to manage paper mill waste and to a create commercial value out of it. The main problem associated with this approach is the huge generation
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of wastewater due to the PMS de-ashing. The wastewater generated from the de-ashing process must be treated once the process is scaled up. Consequently, the energy consumption for the entire process would be increased, hence, affecting the final production cost. The analysis showed that recycling this wastewater to replace the freshwater addition is feasible up to 30% without affecting the process performance. Moreover, an energy saving was achieved in the energy requirement of the downstream processing demand of the biofuel. DA-WW recirculation saved about 22.5% of the overall wastewater cost. The future work will be focused on minimizing the DA-WW by adopting new washing techniques that use less water combined with physical removal of the ash from the PMS. Therefore, future work should focus on performing detailed engineering analysis to study the effect of water reuse on the operation stability and durability of the process pipes as well as the other operating units.
Acknowledgments The Authors would like to acknowledge the US Department of Agriculture.
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[38] McAloon AT, F.; Yee, W.; Ibsen, K.; Wooley, R. Determining the cost of producing ethanol from corn starch and lignocellulosic feedstocks. National Renewable Energy Laboratory Report 2000. [39] German Environment Agency wu. Technical Guide on the treatment and recycling techniques for sludge from municipal waste water treatment. 2018. [40] Klein‐Marcuschamer D, Oleskowicz‐Popiel P, Simmons BA, Blanch HW. The challenge of enzyme cost in the production of lignocellulosic biofuels. Biotechnol Bioeng. 2012;109(4):10837. [41] García-Velásquez CA, Cardona CA. Comparison of the biochemical and thermochemical routes for bioenergy production: A techno-economic (TEA), energetic and environmental assessment. Energy. 2019;172:232-42. [42] Khajeeram S, Unrean P. Techno-economic assessment of high-solid simultaneous saccharification and fermentation and economic impacts of yeast consortium and on-site enzyme production technologies. Energy. 2017;122:194-203. [43] Salam KA. Towards sustainable development of microalgal biosorption for treating effluents containing heavy metals. Biofuel Research Journal. 2019;6(2):948-61. [44] Ekendahl S, Bark M, Engelbrektsson J, Karlsson C-A, Niyitegeka D, Strömberg N. Energyefficient outdoor cultivation of oleaginous microalgae at northern latitudes using waste heat and flue gas from a pulp and paper mill. Algal research. 2018;31:138-46.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Figures
Paper Mill Sludge
Washer
Fresh water
Fermentation
Enzymatic hydrolysis
P-1
Evaporation
G-WW
Ethanol
Distillation
P-9
Fig. 1. The overall process flow diagram of the production of ethanol from PMS with a wastewater recycling configuration (----- recycling,
______ generic processes).
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0.8
Dry fiber (mg/g)
0.7 0.6 0.5 0.4 0% DA-WW Recyling 30% DA-WW Recyling
0.3
70% DA-WW Recyling 100% DA-WW Recyling
0.2 0.0
20.0
40.0 time (h)
60.0
80.0
Fig. 2. Variation glucose concentration with time without and with wastewater (WW) recycling. Error bars represent mean ± standard error of three replicates.
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b)
0.8
0.8
0.6
0.6
0.4
C (mg/g fiber)
C (mg/g fiber)
a)
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.6544 mg/g fiber
0.2
0.4
Experimental results Pseudo first-order model: C (t) = Ce(1-e-kt ) Ce =0.6594 mg/g fiber
0.2
k = 0.0597 h-1 R2 = 0.9772
k =0.0469 h-1 R2 = 0.9643 0.0
0.0 0
25
50
75
100
0
25
t (h)
50
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c)
d)
0.8
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C (mg/g fiber)
C (mg/g fiber)
0.6
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.5507 mg/g fiber
0.2
0.25
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.4420 mg/g fiber
k = 0.0695 h-1 R2 = 0.9904
k = 0.0707 h-1 R2 = 0.8741
0.0
0.00 0
25
50
t (h)
75
100
0
25
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t (h)
Fig. 3. Comparison between experimental glucose concentration and predicted ones using pseudo first-order kinetic model for the cases: a) without wastewater recycling, b) 30% wastewater recycling, c) 70% of wastewater recycling, and d) 100% of wastewater recycling.
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0.08
k ( h-1 )
0.06
0.04
Experimental results Power law model: k = 0.0468+0.0025(RD)0.4982 R2 = 0.9862
0.02 0
25
50
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Recycling degree RD (%)
Fig. 4. Mathematical representation of pseudo first-order constant (k) as function of recycling degree using three-parameters power law model.
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Highlights:
Reuse of wastewater in the paper mill sludge conversion. A certain degree of recycling has no impact on the glucose yield. Wastewater recycling reduced the energy demand of the process by 1251 kJ/kg metric tons wastewater. Cost-saving of wastewater treatment by 22.5% when a 30% recycling degree achieved.
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Tables Table 1. Setup design for recycling scenarios and possibilities of freshwater replacement as well as pollutants reduction. Data are shown as mean ± standard error of three replicates. Recycling degree (%) 0 30 70 100
Fresh water replacement (metric t/day) 0 1800 4200 6000
COD (mg/L)
Calcium (mg/L)
Chlorine (mg/L)
4 ± 0.2 680 ± 14 1580 ± 39 2269 ± 68
32 ± 1.1 4158 ± 118 9702 ± 193 13860 ± 337
45 ± 2 12000 ± 360 30000 ± 604 35300 ± 883
Table 2. Energy reduction in the downstream processing units as a function of DA-WW recycling. Data are shown as mean ± standard error of three replicates. DA-WW (%) 0 30 70 100
TS (mg/L) 254 ± 8 8550 ± 214 20050 ± 602 28645 ± 573
Energy Demand (kJ/kg) 2554 1348 1303 1196
Energy Saving (kJ/kg) 0 1206 1251 1358
Table 3. Process cost reduction due to wastewater re-use in the process of bioethanol from PMS. Recycling Treatment cost of WWT cost Glucose yield degree wastewater reduction reduction (%) (US $) (%) (%) 0 456960 0 0 30 354144 22.5 0 70 217056 52.5 20 100 114290 75.0 40 a: Annual cost treatment based on daily processing 2000 metric ton dry PMS
Net cost reduction (US $) 0 102816 239904 342720
Tareq Salameh, Muhammad Tawalbeh, Mohammad Al-Shannag, Motasem Saidan, Khalid Bani Melhem, Malek Alkasrawi PII:
S0360-5442(20)30192-4
DOI:
https://doi.org/10.1016/j.energy.2020.117085
Reference:
EGY 117085
To appear in:
Energy
Received Date:
19 June 2019
Accepted Date:
02 February 2020
Please cite this article as: Tareq Salameh, Muhammad Tawalbeh, Mohammad Al-Shannag, Motasem Saidan, Khalid Bani Melhem, Malek Alkasrawi, Energy Saving in the Process of Bioethanol Production from Renewable Paper Mill Sludge, Energy (2020), https://doi.org/10.1016/j. energy.2020.117085
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Energy Saving in the Process of Bioethanol Production from Renewable Paper Mill Sludge Tareq Salameha,b, Muhammad Tawalbeha, Mohammad Al-Shannagc, Motasem Saidanc, Khalid Bani Melhemd, Malek Alkasrawie,* a Sustainable
and Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates b Sustainable Energy Development Research Group, Research Institute for Sciences and Engineering, University of Sharjah, PO Box. 27272, Sharjah, United Arab Emirates c Department of Chemical Engineering, The University of Jordan, 11942 Amman, Jordan d Department of Environmental engineering, Hashemite University, Zarqa, Jordan e Department of PS & Chemical Engineering, University of Wisconsin, Stevens Point, USA
*Corresponding author: [email protected]
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Abstract Paper mill sludge (PMS) can be efficiently utilized to produce fuels and chemicals. However, wastewater is usually generated during the de-ashing step of the PMS for fibrous materials recovery. Per process requirements, the wastewater stream must be treated which results in an increase in the overall process production cost. Therefore, this research aims at reusing the wastewater produced during the de-ashing step as a substitute for freshwater addition during the conversion of PMS into ethanol. The advantages of this approach include reducing the amount of wastewater produced and enhancing the overall efficiency of the process. It will contribute to the circular economy of zero waste discharges. The results showed that 30% of the process wastewater can be recycled without affecting the enzymatic hydrolysis and ethanol fermentation. Hence, the amount of wastewater that needs to be treated is reduced by 30% resulting in a cost reduction of 22.5%. The results also showed that wastewater recycling minimized the energy demands in the distillation and evaporation units by 1206 kJ/kg. The energy reduction is due to the increase of metals and total soluble solids in the broth stream after fermentation. This process configuration enhanced the process economy, saved energy and managed waste streams.
Keywords: Process Stream Recycling; Paper Mill Sludge; Ethanol; Energy Reduction, Circular Economy
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1. Introduction In the biorefinery process, multiple technologies for biofuels and chemicals production need to be integrated for energy security. Waste generation from intensive industrial production is an environmental and economic burden. Waste recycling and utilization for energy production leads to zero waste discharge in the circular economy. In the process of fiber recovery from paper mill sludge (PMS), a large amount of wastewater is generated due to the de-ashing process. The wastewater exhibits environmental and economic burdens that must be managed [1,2]. The wastewater from biofuels production increases the energy demand in the life cycle of the bioconversion platform. Energy reduction is the target of all biofuels production processes due to the large variation in the net energy value of biomass-to-biofuels [3]. Energy process integration using the pinch analysis method is very helpful in minimizing the energy loss during chemical engineering processes [4]. However, energy integration technology is limited, as well as the reduction required beyond the energy loss value. There are several process configurations that could be adopted in order to reduce the energy demand [5,6]. These configurations lead to increasing the biofuels concentration prior to the distillation step, as well as the soluble solid prior to the evaporation step [5,7-9]. Therefore, the development and optimization of integrated technologies is a necessity in order to minimize the energy demand in the biorefinery processes [6,10]. In the production of ethanol from PMS, water addition is required at different unit operations, mainly in the enzymatic hydrolysis and fermentation [11]. The fresh water is crucial to run the enzymatic hydrolysis at a certain solids load [12]. The water addition leads to the generation of a huge amount of wastewater that contains water-soluble compounds [5,7-9]. However, the largest generation of wastewater is from the de-ashing step of PMS. This stream contains water-soluble
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inhibitors, chlorine, and minerals resulting from the de-ashing step [13,14]. Chlorine in wastewater originates from chemical industries [15], mainly from the pulp industry after the wood digestion step [16]. Chlorination at low concentration is used worldwide for bacterial disinfection. Nevertheless, the concentration of chlorine in de-ashing wastewater is much higher than that of potable drinking water.
These soluble substances might exhibit potential inhibition to the
saccharification and alcoholic fermentation [17]. The potential inhibition may vary greatly with the nature of PMS and the pulping conditions such as the chemical structure of the raw material and the wood species [18]. The treatment of wastewater is an essential operation in the overall conversion of renewable feedstock to ethanol [19]. Anaerobic digestion and biological platform have been widely used to treat the wastewater and reuse it in various ways [1,20]. The wastewater treatment generated from the bioethanol plants increases the operation and capital expenditures of the entire process. The conversion of PMS to ethanol is one of the renewable processes that can be used to produce biofuels, especially in the autumn season. This conversion process requires energy for the treatment of wastewater production, distillation, and evaporation of liquid biofuel (ethanol) as shown in Fig. 1. The present study investigated the possibilities of recycling the wastewater generated after the de-ashing step of PMS to replace the freshwater addition. The effect of recirculation of the wastewater stream from the de-ashing process on both sugar and ethanol yields to reduce the use of freshwater addition was also investigated. Additionally, the water condensate that is generated after the evaporation step is recycled back to the process in order to close the cycle of the water. This step is very crucial to enhance the circular economy of the zero-waste discharges.
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Fig. 1. The overall process flow diagram of the production of ethanol from PMS with a wastewater recycling configuration (----- recycling, ______ generic processes).
2. Materials and methods 2.1. Paper mills sludge (PMS) description The PMS samples were collected from the central Wisconsin area paper mill, Wisconsin, USA (44°23′12″N 89°49′23″W). The PMS was generated in the Kraft mill pulping process that digests softwood chips (Red Pine) using a mixture of sodium hydroxide and sodium sulfide. The samples were withdrawn from the primary screw press after the first stage of the wastewater treatment. The samples were kept refrigerated at 4°C in a sealed plastic container until further use. The compositional analysis of the PMS was conducted using a method developed by the National Renewable Laboratory (NREL) [21]. Each analysis was performed in three duplicates and the average results are shown in this work. 2.2. De-ashing of PMS The de-ashing process is a step of removing the inorganic and organic materials from the fibrous cellulose. Technically, it is a washing step using water (with dilute acid or base) to clean the fiber from other contaminants. In this study, the de-ashing was performed using washing of 1 M HCl 10 % w/v employed to recover the fiber. The PMS was mixed with 1 M HCl solution and stirred for two hours at 500 rpm. The total washing volume that was used corresponded to 3 L to 1 kg dry PMS. Notably, calcium carbonate is one of the major fillers in the PMS and can be removed as calcium chloride when treated with hydrochloric acid according to the following reaction:
2HCl CaCO 3 CaCl 2 CO 2 H 2 O
(1)
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The acid washed PMS fibers were filtered and washed with distilled water until the pH was neutralized from 1.12 to 5.00 (the optimum pH for cellulolytic enzymes). The washed fiber was kept in a refrigerator at 4oC for further uses. The wastewater generated after washing was used immediately in the enzymatic hydrolysis step. The wastewater after the de-ashing step (DA-WW) was chemically analyzed and characterized. Each analysis was performed in three duplicates and the average results are shown in this work. 2.3.
Characterization and Recycling of de-ashed wastewater
The DA-WW was collected and used immediately after the de-ashing process. The fiber-free ash was collected separately and prepared for later enzymatic hydrolysis and fermentation. In the hydrolysis step, freshwater will be added to the fiber free ash to make the total soluble solid up to 5%. These optimal solid loads guarantee efficient enzymatic hydrolysis on the cellulosic substrate. In the current process configuration, several re-using scenarios are proposed. The first scenario is the base case where there is no wastewater recycling. In the subsequent scenarios, an amount of wastewater from the de-ashing process replaces the freshwater addition in various degrees. The first degree will start with 30%, and the recycling will continue until 100% replacement of freshwater occurs. The effect of degree of wastewater replacement of fresh water on the enzymatic hydrolysis and on fermentation will be studied. The recycling will lead to the accumulation of minerals in the wastewater generated from the distillation step. 2.4.
Enzymatic hydrolysis of sludge
A commercial second-generation cellulase cocktail Cellic®CTec2 provided by Novozymes A/S, Bagsvaerd, Denmark, was used to conduct the experiments of the enzymatic hydrolysis of PMS. The concentration of the protein in Cellic® CTec2 of 73.6 mg/mL was estimated using the Bradford
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Coomassie Blue method according to Adney and Baker [22]. The filter paper units (FPU) activity was estimated to 113.8 FPU/mL using a NREL LAP standard procedure TP-510-42628 [22]. The value of 2.0 mg of reducing sugar as glucose from 50 mg of filter paper (4% conversion) in 60 minutes has been designated as the intercept for calculating cellulase (FPU) by IUPAC. A concentration of 3.4% (w/w) solution was prepared by adding 3.4 g enzyme/100 g dry PMS. A citrate buffer solution was used in order to prepare a suspension of each PMS with a concentration of 5% (dry weight by volume). The specifications of the buffer solution are as follows: 0.05 M and a pH of 5. The total volume was 150 mL in this case. The solutions were poured in 250 mL flasks followed by stirring on a hot plate at a rotation speed of 200 rpm for 48 h and at a temperature of 50°C. After that, samples were taken periodically every three hours to measure the glucose concentration. The final hydrolyzed product was then filtered, and the obtained sugar solution was stored at 4°C and kept until fermentation experiments. 2.5.
Fermentation
The ethanol fermentation experiments were conducted using an Industrial yeast FermPro ™ (purchased from Ferm Solutions Inc., Danville, KY). First, a small amount (1 mL) of frozen yeast was precultured at 37°C for 16 hours in a medium of the following composition: 20 g/L glucose, 20 g/L peptone and 10 g/L yeast extract. The clear PMS sugar solution was then supplied by several nutrients with specific amounts according to Gurram et al. [14]; 20 g/L MgSO4•7H2O, 10 g/L KH2PO4, 4 g/L CaCl2•2H2O, 200 mg/L ZnSO4•7H2O, 20 mg/L Na2MoO4•2H2O, 200 mg/L CoCl2•2H2O, 2 mg/L d-biotin, 5 mg/L p-aminobenzenoic acid, 5 mg/L Nicotinic acid, 5 mg/L calcium pantothenate, 5 mg/L Thiamine-HCl, 10 mg/L Pyridoxine-HCl and 10 mg/L lactoside.
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2.6.
Analytical and Instrumentation Analyses
Glucose, xylose, arabinose, galactose, and mannose concentrations were measured and quantified against a fucose internal standard. The concentration of the polymeric sugar was estimated from the concentration of the corresponding monomeric sugar. This was done using an anhydrous correction of 0.88 (or 132/150) for C-5 sugars (xylose and arabinose) and 0.90 (or 162/180) for C-6 sugar (glucose, galactose, and mannose). The measurements were performed using an Ion Chromatography system (Dionex ICS 3000 Thermo Scientific Waltham, MA, USA) that maintained the temperature at 25oC. The IC system has an electrochemical detector equipped with a gold electrode, along with two guard columns. These two columns are Amino Trap Bio1C and a Carbo Pac PA1 and a 4 × 250 mm Carbopac PA1 column (both from Dionex). The eluents were prepared by adding 3 mM to distilled water. This ultrapure water was partially degassed by employing vacuum and running it at a very low flow rate of 1 mL min-1 through the column. Ethanol analysis was also performed using the Dionex ICS 3000 with an eluent of 100 mM methanesulfonic acid prepared also with ultrapure water. This ultrapure water was also partially degassed by employing vacuum with a running flow rate through the column of 0.18 mL min-1. 2.7 Process description Fig. 1 describes a flowsheet diagram of a conceptual design of a possible process for sugar and ethanol production from PMS. The PMS is washed using a dilute acid, such as HCl or H2SO4 in order to solubilize all substances (mainly minerals) encapsulated in the wood fiber. The clean fiber is transferred to the saccharification step were cellulases and fresh water are added. Freshwater addition is necessary since enzymes operate at a certain substrate concentration [12]. The green sugar produced from enzymatic hydrolysis is transferred to the fermentation reactor. In the proposed process, the sugar concentration should be within the limit of microbial tolerance. Once
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all sugars are converted into ethanol, the fermentation broth enters a stripper column where over 90% of the ethanol is distilled and subsequently fed to the rectifying column to reach higher grade ethanol. The wastewater after ethanol recovery is sent to multiple-effect evaporators in order to reduce the volume of the waste stream. The overall process description of 2,000 dry metric tons of PMS daily for sugar and biofuel production is depicted in Fig 1. The treatment of PMS (mainly washing for fiber recovery) will generate about 6,000 metric tons of DA-WW as the result of removing the fillers and minerals from PMS. The generic process of PMS conversion (without DA-WW recycling) generates a stream of wastewater. This stream is defined as generic wastewater (G-WW) and is produced at a rate of 2,500 metric tons/day. The cleaned fiber, about 1,000 metric tons, would require the addition of 6,200 metric tons of fresh water to reduce the total solids (TS) to 5%. The water addition step is of great importance in order to provide the optimal catalytic conditions for the cellulases activities for cellulose conversion to the economic sugar concentration [23]. The hydrolysate that contains mainly glucose produced in the saccharification process is transferred to the fermentation reactor. Alcoholic fermentation utilizes commercially available yeast for ethanol fermentation. In the base case scenario, this represents the generic scenario; there is no DA-WW recirculation and thereafter it joins the G-WW for further wastewater treatment. The cost calculation of wastewater was based on the chemical oxygen demand content that correspond to US $ 0.11/m3 as a reference cost [24-26]. In the process where multiple effect evaporator is used, energy demand plays an important role in the process economy. One way of reducing the energy demand is by increasing the TS content in the downstream processing of ethanol recovery.
3. Results and discussion 3.1.
Wastewater recycling
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The chemical composition and characterizations of the wastewater effluent from the pulp and paper industry differs drastically from one mill to another according to the pulp process and the singular characteristics of each factory. The chemical composition and characteristics of the wastewater effluent from the pulp and paper industry vary substantially from one mill to another according to the pulping process and the unique characteristics of each mill. The characteristics of the DA-WW stream shown in Fig. 1 are as follows: COD 2,269 mg/L, TS 2,8645 mg/L, Chlorine 35,300 mg/L, Fe 19.5 mg/L, Ca 13,860 mg/L, Mg 146.9 mg/L, Al 55.7 mg/L, P 6.3 mg/L and Na 34.3 mg/L. It is clear that this stream cannot be discharged to the municipal sewage system or directly released into a stream or other body of water. Hence, treatment is required to reduce the level of contaminants to meet the permit limits. For instance, high organic material, heavy metals, and suspended solid contents are considered major pollutants of pulp and paper industry effluents, which is the case in the present study. The DA-WW is ready to replace the freshwater addition in the enzymatic step. It is obvious that the two major components of the stream are chlorine and calcium ions. Chlorine is well known as an inhibitor for bacterial infection in potable water at concentrations of 4 ppm. It is assumed that the present concentration of chlorine will not have a major effect on enzymatic hydrolysis and fermentation once the recycled waste stream replaces the freshwater addition. The COD, 2,269 mg/L from Table 1, was relatively much lower than those for industrial wastewater, e.g. 50,000 mg/L. The impact of recycled DA-WW on the biological activities of the enzymes is anticipated to be less due to lower COD content. In addition, the presence of iron, phosphorous, and magnesium ions at concentration between 34.3-149.9 mg/mL are essential for the alcoholic fermentation as supplemental nutrients. Interestingly, this stream has a considerable amount of TS;
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if recycled back to the process, it will reduce the energy demand in the evaporation unit according to the process description, Fig 1.
Table 1. Setup design for recycling scenarios and possibilities of freshwater replacement as well as pollutants reduction. Data are shown as mean ± standard error of three replicates.
Replacing the fresh water in the saccharification step by DA-WW is a viable option in order to reduce the wastewater stream. In this alternative, it is assumed that the soluble substances do not alter the biological activities for both cellulases and microbial fermentation. This might lead to saving in energy consumption in both the distillation of the fermentation broth and the evaporation of the waste stream [7,8,27,28]. However, progressive DA-WW recirculation leads to the buildup of various substances in the enzymatic and fermentation step. Stenberg et al. [9] showed that stream recirculation increases the concentration of non-volatiles tenfold in various areas of the process. The experimental results obtained by Palmqvist et al. [29] also showed that the nonvolatile substances discharged during steam pretreatment of willow inhibited the growth of microbial fermentation. In the present investigation, it was possible to recycle and replace 30% of the freshwater without affecting either the enzymatic hydrolysis or the fermentation step, in Fig. 2. These results are in agreement with the previous investigation by Alkasrawi et al. [7] where it was possible to recycle the process stream after distillation to a certain extent. However, Alkasrawi et al. [27] showed that it is possible to achieve 100% replacement of freshwater by the collection of evaporation condensate. In the present study, the evaporator condensate was not evaluated, but it was proposed as process configuration in Fig 1.
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Fig. 2. Variation glucose concentration with time without and with wastewater (WW) recycling. Error bars represent mean ± standard error of three replicates.
Interestingly, recycling at a higher degree than 30% reduced the enzymatic yield gradually until glucose yield started to drop at 40% recycling, Fig. 2. A similar pattern of yield decrease was obtained by Alkasrawi et al. [7] when a higher degree of recycling before and after distillation significantly lowered the enzymatic and fermentation mechanism, See Fig. 2. The reason for gradual glucose yield in the hydrolysis step in the study is most likely attributed to the accumulation of various substances that interferes with the catalytic activity of the cellulases. These substances have likely caused uncompetitive enzymatic inhibition that affect the final turnover of the cellulosic fiber. The accumulated chlorine might halt the enzymatic activities in the subsequent recirculation. Although chlorine's value as a disinfectant has been known for a long time, the mode of action and mechanism by which chlorine kills or inactivates microorganisms or enzymes is not clearly understood. It has been postulated that chlorine exists in water as hypochlorite and hypochlorous acid. This mixture in water produces hydrochloric acid, which then decomposes to chlorine, oxygen, and thiol which is immediately oxidized. Since chlorine is electronegative, it can oxidize peptide links and denatures proteins made up of sulfur-containing and aromatic amino acids such as cellulases [30-32]. The so-called "multiple hit" theory of chlorine inactivation of microbial growth suggests that the chemical characteristic of chlorine and its derivatives in aqueous condition dissociates almost all biomolecules: enzymes, nucleic acids, and membrane lipids [33,34]. Though it can be noted note that enzymatic hydrolysis is affecting the enzyme treatment of the biomass and fermentation using microbes, the effect could be directly due
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to the lysis of cellulolytic enzymes and/ or the microbes used for both cellulolytic processes and enzymes involved in the whole fermentation pathways. 3.2.
Kinetic modeling of glucose production
To determine the kinetics of glucose production, including the reaction rate law, different reaction orders were proposed and the integrated linear forms were used to plot the data obtained. The rate law that produces a straight line with a correlation coefficient as high as possible can be considered to represent the experimental data. Various kinetic models are discussed in detail elsewhere [35,36]. These models were investigated and the production rate of glucose in this work can be described using the pseudo first-order kinetic model as shown in the equation below: dC k Ce C dt
; C(0) = C0 = 0 mg/g fiber
(2)
where C is the concentration of glucose in mg/g fiber, t is the operating time in h, k is the pseudo first-order kinetic parameter in h-1, Ce is the equilibrium concentration of glucose as t→. Integration of the above first-order differential equation with C(0) = 0 gives:
C ( t ) C e 1 e kt
(3)
The optimum parameters of both Ce and k at different operating conditions (various levels of wastewater recycling) were obtained by minimizing the sum of square of errors (SSE) between the experimental concentrations, Cexp(t), and the corresponding predicted one from the above equation. Where the SSE is defined as: N
N
SSE Ci ,exp ( t ) Ci (t ) Ci ,exp ( t ) Ce (1 e kt ) i 1
2
2
(4)
i 1
Fig. 3 shows the variations of both experimental and predicted glucose concentrations with time for the cases 0%, 30%, 70%, 100% recycling degree (RD) of wastewater. The legends of the four graphs in the Fig. 3 summarize the optimum values of Ce and k in addition to the value of squared
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correlation coefficient, R2, which is calculated to measure the goodness-of-fit of the pseudo firstorder model. It is clear from the Fig. that the pseudo first-order model can describe the time dependent concentration adequately as R2 approaches unity. Moreover, Fig. 3 clearly shows that the pseudo first-order constant (k) is directly proportional to recycling degree (RD) of wastewater. Nonlinear regression demonstrated that the k parameter can be mathematically represented as function of RD according to a three-parameter power law model with a value of squared correlation coefficient R2 = 0.9862 as shown in Fig. 4.
Fig. 3. Comparison between experimental glucose concentration and predicted ones using pseudo first-order kinetic model for the cases: a) without wastewater recycling, b) 30% wastewater recycling, c) 70% of wastewater recycling, and d) 100% of wastewater recycling.
Fig. 4. Presents a mathematical model of pseudo first-order constant (k) as function of DA-WW recycling degree using three-parameters power law model. The k value of enzymatic reaction when no DA-WW stream was recycled is 0.0469 h-1. However, the k changed to 0.0597 h-1 when 30% of DA-WW was recycled although the final yield was not affected, evident in Fig. 4. This is likely due to minor enzyme noncompetitive inhibition by the recycled TS. It may be postulated that if the enzymatic hydrolysis residence time is prolonged, the same cellulose conversion may be obtained as in the base case (no recycling).
Fig. 4. Mathematical representation of pseudo first-order constant (k) as function of recycling degree using three-parameters power law model.
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3.3.
Energy reduction as a function of DA-WW recirculation
Table 2 present an energy demand and saving in the evaporation unit as a function of DA-WW recycling. The consecutive DA-WW recycling leads to TS accumulation in enzymatic hydrolysis and in the fermentation steps, Table 2. The accumulation of TS in the final streams before the distillation and evaporation processes reduced the heat capacity of that stream, Table 2. This leads to a substantial decrease in the energy demand for both distillation and evaporation steps. The energy used for distillation can reach up to 40% of the cumulative energy charge in bioethanol production [37,38]. Recently, the application of a hybrid process, namely a combination of the distillation with pervaporation, could come to the front due to the relative high permeation rate and selectivity of the pervaporation [39]. At 30% freshwater replacement, yields were unaffected, leading to a reduction in the wastewater treatment (WWT) cost by 22.5%, Table 3. The energy demand was further decreased by around 1206 kJ/kg recycled wastewater as shown in Table 1. This is a very interesting finding and it will further decrease the production cost of ethanol. However, further increase of the recycled wastewater leads to further reduction of the energy demand, which in turn minimizes the sugar yield and consequently, reduces the overall ethanol yield evident from Table 3.
Table 2. Energy reduction in the downstream processing units as a function of DA-WW recycling. Data are shown as mean ± standard error of three replicates.
Table 3. Process cost reduction due to wastewater re-use in the process of bioethanol from PMS.
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Many technologies can be used for the treatment of wastewater that contains metals (Fe, Al, etc.) and TS. These technologies vary from one to another in terms of drying system, mode of the heating process (direct or indirect), type of energy (thermal or electrical) and amount and cost of energy consumption. The amount of recycled wastewater helps in reducing the energy consumption in the distillation and evaporation processes as well as in the WWT process. This can be achieved by increasing the concentration of the TS in the recycled DA-WW used as shown in Table 2. The presence of TS and metals affects the heat transfer mechanism for the evaporation process; this effect helps the water reach the boiling temperature faster than the fresh water. Both TS and metals change the overall heat capacity value of wastewater prior to entering the downstream processing operating units (evaporation and distillations units). This reduction in time is the main reason for saving energy and cost for the conversion of PMS to bioethanol. On the other hand, there is an optimum percentage of using wastewater produced during the conversion process of PMS to ethanol. This percentage was selected without having any effect on the enzymatic hydrolysis and fermentation steps and was cost-effective on both COD and chlorine removal. The cost of both COD and chlorine removal increase with the increasing of recycling degree or freshwater replacement. It is worth mentioning that 30% of wastewater was recycled in this study for the previously mentioned reasons. Table 3 shows the cost required for the conversion of PMS to bioethanol. The cost of WWT decreases as the recycling degree increases, whereas the WWT cost reduction and net cost reduction increase as well as the glucose yield reduction. This reduction in the cost and saving in energy makes the process more sustainable and economically feasible. However, improving energy efficiency could be achieved by applying current energy efficient technologies [39]. For example, a report provided by the German Environment Agency
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[39] discussed various energy efficient commercial technologies with respect to less energy demand for wastewater sludge drying. 3.4.
Impact on processing cost of WWT
The overall treatment cost of WWT in a biofuel production facility would correspond to 2% of the total production cost [40]. Techno-economic analysis is one of the most intensive topics studied so far for bioethanol production from various renewable resources [41,42]. The overall reference cost of total waste water (combination of G-WW and DA-WW) is US $ 0.11/m3 wastewater, although the cost is subject to change with respect to the technologies used and geographical location of the plant [25]. The reference cost was based on published research articles [25,26] as well as current industrial technologies reports [24]. The reference cost is helpful to compare the cost reduction, but it is not intended for the absolute cost calculation. Based on the cost breakdown, the annual costs of such treatment system can be calculated as shown in Table 3. In this study, the annual cost calculation for both G-WW and DA-WW was estimated to be US $ 456,960 according to Table 3. The annual cost for both G-WW and DA-WW was estimated to be US $ 114,240 and US $ 342,720 respectively. Notably, the cost associated with DA-WW raises the processing cost of WWT by three folds. This supports our claim that PMS for biofuel is attractive, however, the generation of DA-WW will affect the overall process cost. As discussed earlier, it was possible to recycle 30% of the DA-WW without impacting the process performance. This means, according to Table 3, a 22.5% reduction of the overall WWT is achieved while a net cost of US $ 103,816 is saved. For future recommendation, other technologies might be implemented to further reduce the processing cost. Ortizet et al. [26] reported that membrane biological reactor (MBR) technology could operate under high concentrations of Mixed liquor suspended solids (MLSS) of 15– 30 g
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MLSS/L. At the highest concentration of MLSS, wastewater could be reduced to 50% or higher levels. For such medium-strength effluents wastewater characteristics, neither chemical nor conventional activated sludge processes are sufficient alone to treat medium-strength effluents down to the permissible discharge limits. Therefore, in the present study, a combination of chemical and biological (extended aeration activated sludge) treatment systems are proposed, and in the present study, 3,000-8,000 m3 per day is the capacity of the treatment plant (chemical and biological treatment systems). For such a combination treatment system of paper effluent wastewater, the costs of treatment depend on the flow rate as shown in Table 3. In the light of this development, future research could focus on a microbial treatment of the non-recycled wastewater and enhance heavy metals recovery in an integrated biorefinery process [43]. This work could be even expanded further by integrating the bio-oil production from microalgae cultivation on the wastewater stream generated from washing the PMS [43, 44]. Another potential effect of process streams reuse and recycle in bioethanol production plants is the corrosion impact inside the pipelines and in the downstream processing equipment of biofuel recovery [45]. The DA-WW recycling in the long-term process operation might affect the maintenances scheduling of the downstream processing equipment and subsequently exhibiting a corrosion effect. The effect on corrosion might be negative or positive based on the chemical content of the recycled stream and the nature of materials used in the construction of equipment and the pipelines. In the recycled DA-WW there are two major elements, chlorine and calcium, that are reported to have an antagonistic effect on corrosion [46,47]. The chlorine is an electronegative element that exhibits pitting corrosion in the presence of oxygen and corrosion rate increases with temperatures [48]. The chlorine is stripped off in the first distillation column and it
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leaves the top of the distillation column as dry gas. The gaseous chlorine does not manifest any corrosion effect unlike the wet chlorine [49]. Owning to distillation, chlorine is stripped off, hence, any potential corrosion in the down streaming process is greatly minimized. In contrast, calcium is known for being effective as a corrosion inhibitor [47,50]. It is believed that the inhibitory effect on the corrosion of calcium offsets the impact of chlorine on corrosion. In addition, the equipment made for ethanol production from lignocellulosic feedstock is made to be corrosion resistant. Davis et al. [51] reported that all conversion operation units of biomass to biofuel are constructed of 304 or 316 stainless steel. These classes of stainless steel are essential to secure the corrosion resistance to caustic solutions, as well as an axenic environment where maintenances are scheduled regularly. Davis et al. [51] reported a techno-economic analysis of bioethanol production and showed that the capital investment of distillation and solids recovery is US $ 22.3 MM meanwhile wastewater treatment would cost around US $ 49.4 MM. This shows that the cost associated with wastewater is two times higher than the downstream processing. This would support the finding of this study that a reduction in wastewater treatment cost is significant and would contribute to the overall process economy. In case the effect is positive, the impact would be on a small increment of capital expenditure of the process equipment and pipelines. By looking at the techno-economic evaluation of cellulosic ethanol reported by Humbird et al. [52] the corrosion prevention cost is associated with the capital investment cost and not with the operation cost. Moreover, Söylev and Richardson [53] reported several corrosion inhibitors that could be applied without the need for manufacturing new materials with high corrosion resistance. 4. Conclusions PMS conversion into biofuels is an attractive approach to manage paper mill waste and to a create commercial value out of it. The main problem associated with this approach is the huge generation
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of wastewater due to the PMS de-ashing. The wastewater generated from the de-ashing process must be treated once the process is scaled up. Consequently, the energy consumption for the entire process would be increased, hence, affecting the final production cost. The analysis showed that recycling this wastewater to replace the freshwater addition is feasible up to 30% without affecting the process performance. Moreover, an energy saving was achieved in the energy requirement of the downstream processing demand of the biofuel. DA-WW recirculation saved about 22.5% of the overall wastewater cost. The future work will be focused on minimizing the DA-WW by adopting new washing techniques that use less water combined with physical removal of the ash from the PMS. Therefore, future work should focus on performing detailed engineering analysis to study the effect of water reuse on the operation stability and durability of the process pipes as well as the other operating units.
Acknowledgments The Authors would like to acknowledge the US Department of Agriculture.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Figures
Paper Mill Sludge
Washer
Fresh water
Fermentation
Enzymatic hydrolysis
P-1
Evaporation
G-WW
Ethanol
Distillation
P-9
Fig. 1. The overall process flow diagram of the production of ethanol from PMS with a wastewater recycling configuration (----- recycling,
______ generic processes).
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0.8
Dry fiber (mg/g)
0.7 0.6 0.5 0.4 0% DA-WW Recyling 30% DA-WW Recyling
0.3
70% DA-WW Recyling 100% DA-WW Recyling
0.2 0.0
20.0
40.0 time (h)
60.0
80.0
Fig. 2. Variation glucose concentration with time without and with wastewater (WW) recycling. Error bars represent mean ± standard error of three replicates.
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b)
0.8
0.8
0.6
0.6
0.4
C (mg/g fiber)
C (mg/g fiber)
a)
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.6544 mg/g fiber
0.2
0.4
Experimental results Pseudo first-order model: C (t) = Ce(1-e-kt ) Ce =0.6594 mg/g fiber
0.2
k = 0.0597 h-1 R2 = 0.9772
k =0.0469 h-1 R2 = 0.9643 0.0
0.0 0
25
50
75
100
0
25
t (h)
50
75
100
t (h)
c)
d)
0.8
0.50
0.4
C (mg/g fiber)
C (mg/g fiber)
0.6
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.5507 mg/g fiber
0.2
0.25
Experimental results Pseudo first-order model: C (t)= Ce (1-e-kt ) Ce = 0.4420 mg/g fiber
k = 0.0695 h-1 R2 = 0.9904
k = 0.0707 h-1 R2 = 0.8741
0.0
0.00 0
25
50
t (h)
75
100
0
25
50
75
t (h)
Fig. 3. Comparison between experimental glucose concentration and predicted ones using pseudo first-order kinetic model for the cases: a) without wastewater recycling, b) 30% wastewater recycling, c) 70% of wastewater recycling, and d) 100% of wastewater recycling.
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0.08
k ( h-1 )
0.06
0.04
Experimental results Power law model: k = 0.0468+0.0025(RD)0.4982 R2 = 0.9862
0.02 0
25
50
75
100
Recycling degree RD (%)
Fig. 4. Mathematical representation of pseudo first-order constant (k) as function of recycling degree using three-parameters power law model.
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Highlights:
Reuse of wastewater in the paper mill sludge conversion. A certain degree of recycling has no impact on the glucose yield. Wastewater recycling reduced the energy demand of the process by 1251 kJ/kg metric tons wastewater. Cost-saving of wastewater treatment by 22.5% when a 30% recycling degree achieved.
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Tables Table 1. Setup design for recycling scenarios and possibilities of freshwater replacement as well as pollutants reduction. Data are shown as mean ± standard error of three replicates. Recycling degree (%) 0 30 70 100
Fresh water replacement (metric t/day) 0 1800 4200 6000
COD (mg/L)
Calcium (mg/L)
Chlorine (mg/L)
4 ± 0.2 680 ± 14 1580 ± 39 2269 ± 68
32 ± 1.1 4158 ± 118 9702 ± 193 13860 ± 337
45 ± 2 12000 ± 360 30000 ± 604 35300 ± 883
Table 2. Energy reduction in the downstream processing units as a function of DA-WW recycling. Data are shown as mean ± standard error of three replicates. DA-WW (%) 0 30 70 100
TS (mg/L) 254 ± 8 8550 ± 214 20050 ± 602 28645 ± 573
Energy Demand (kJ/kg) 2554 1348 1303 1196
Energy Saving (kJ/kg) 0 1206 1251 1358
Table 3. Process cost reduction due to wastewater re-use in the process of bioethanol from PMS. Recycling Treatment cost of WWT cost Glucose yield degree wastewater reduction reduction (%) (US $) (%) (%) 0 456960 0 0 30 354144 22.5 0 70 217056 52.5 20 100 114290 75.0 40 a: Annual cost treatment based on daily processing 2000 metric ton dry PMS
Net cost reduction (US $) 0 102816 239904 342720