Characteristics of Pollution from Waste Liquor of Poplar Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping
Zong-zheng Yang,a,* Jie Zhang,b Meng-yao Zhang,b Ming-yue Wang,b and Jing-guo Cao a,*
The molecular weight distribution and composition of organic pollutants from chemi-mechanical pulping waste liquor was studied using ultra-filtration, Fourier transform infrared spectra, and gas chromatography-mass spectrometry. The concentration of dissolved chemical organic compounds was approximately 1.81 × 104 mg·L-1. In addition, the wastewater had poor biodegradability. The chemical oxygen demand (COD) was primarily composed of small molecular organic pollutants, and the larger pollutants in the waste liquor were its primary source of color. The data indicated that there were chromophoric and auxochrome groups in the waste liquor. The waste liquor contained 38 types of organic pollutants, which were aromatic compounds, including phenols, ketones, alcohols, and fatty acids.
Contact information: a: Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, Tianjin, China; b: College of Marine and Environment, Tianjin University of Science and Technology, Tianjin 300457, Tianjin, China;
* Corresponding author: email@example.com; firstname.lastname@example.org
The high pollution and energy consumption of the pulp and paper industry has led to a variety of problems, including water and raw material shortages. Poplar is a raw material most commonly used in the pulp and paper industry. This wood is characterized by its rapid growth, high cellulose content, high degree of whiteness, and easy softening. It has been widely used in the production of chemical and mechanical pulp (Hou et al. 2014).
Poplar chemi-mechanical pulping is a typically high-yield pulping process, which can be up to 95% efficient. The main pollutants in the wastewater are from lignin and its derivatives, fine cellulose, hemicellulose, polysaccharides, and other organic compounds that are dissolved during the pulping process. In addition, the waste liquor contains significant amounts of biological toxins, such as resin acid, phenol, toluene, and aldehyde (Chandra and Singh 2012), which causes considerable damage to the receiving waters (Zhang et al. 2012). Pulping waste liquid belongs to the organic waste liquid in high concentration. The organic waste liquid has a high concentration of organic matter that is produced and discharged as effluent. Therefore, the residual contaminant concentration is high, and the waste liquor does not reach a quality of water suitable for discharge to the environment after traditional processing techniques (Liu et al. 2011). However, the efficiency of the waste liquor treatment is related to the type of organic compounds. Therefore, distributing organic materials with different molecular weights in waste liquor is useful for selecting a treatment process and studying the degradation mechanisms of organic compounds (Leiviska et al. 2009). Therefore, studying the pollution characteristics of pulping waste liquor is meaningful.
At present, conventional treatment methods for treatment of pulping wastewater include two sides: a physical-chemical method and a biological method (Hubbe et al. 2016). As the two methods both have some deficiencies, the two approaches are often employed in combination to deal with waste liquid in practical applications. In previous studies, modern analysis and testing techniques have been commonly employed to detect and identify toxic organic pollutants in waste water. The molecular weight distribution of the contaminants in alkaline-peroxide mechanical pulping (APMP) waste liquor and the composition of small organic pollutants were later examined using the ultra-filtration separation method (Liu and Hu 2007) and gas chromatography-mass spectrometry (GC-MS), respectively. The features of the pollution and the characteristic pollutants in the bleaching effluent of southern hardwood chemical pulp have been investigated using data obtained from infra-red (IR), ultraviolet (UV), and GC-MS (Lei et al. 2012). These technologies provide a foundation for the study of non-biodegradable pulping waste liquors. Because poplar pulping waste liquor has complex components, a bright color, and high chemical oxygen demand, research on the detection of its organic pollutants is less frequently reported.
The various molecular weight distributions of the contaminants and organic pollutants and the functional structure of pre-conditioning refiner chemical alkaline peroxide mechanical pulping (P-RC APMP) waste liquid were analyzed by ultra-filtration, Fourier transform infrared spectroscopy (FTIR), and GC-MS. The pollution characteristics of poplar APMP P-RC pulping waste liquor were analyzed, and the characteristics of pollutants and the difficult degradation of pollutants were identified. The present work attempts to identify the primary organic contaminants to provide a theoretical basis for treating contaminants and the biodegradation mechanisms of waste liquor from poplar P-RC APMP. In order to prepare for the subsequent use of pulping waste liquid as the acclimation function of the substrate to acclimate the functional microflora, making preparation by using the pulping waste liquid which has been treated by degradation of fungus to realize the resource utilization of the pulping waste liquid.
Samples were collected during the preimpregnation process from poplar P-RC APMP in a paper factory in Yanzhou, Shandong.
The waste liquor was filtered with a pore size of 0.45 μm before the trials.
Molecular weight distribution of the pollutants in the pulping waste liquor
Ultra-filtration is a mature technology in membrane separation technology, and its separation mechanism is considered as a physical screening process. The UF membrane is an organic membrane that allows the passage of water, inorganic salts, and small molecules. In this study, the molecular weight distribution of pollutants in waste liquid was investigated by using the MSC cup ultrafiltration device produced by American Millipore company. The ultra-filtration (UF) membrane was made from polyethersulfone (PES). According to the manufacturer (Millipore, Boston, MA, USA), the membranes exhibited cut-off values of 5 kDa, 10 kDa, and 25 kDa. The characteristics of the different filters and membranes are shown in Table 1. The membranes were pre-treated by soaking in ethanol for 1 h or washing with 0.3% NaOH and 100 mL of ultrapure water. The filtration device was manufactured by Millipore and operated in the batch mode.
The waste liquor was poured into the filtration equipment and magnetically stirred. After regulating the pressure of the pump, the water filtration equipment was stored at 25 °C, and the samples were collected for measuring the pH, turbidity, chromaticity, and conductivity to determine the amount of chemical oxygen demand (COD), soluble solids, ethanol, and soluble organic matter. Each experiment was repeated in triplicate.
Table 1. Specifications of the Separation Membranes
Fourier transform infrared (FTIR) spectrometry
Exactly 50 mL of the pulping waste liquor, previously evaporated, concentrated, and dried, was collected. Approximately 15 mg of the powder was used to make the sample according to the potassium bromide tableting technique. A Vector 22 FTIR spectrometer (Bruker, Karlsruhe, Germany) was used to measure the transmittance at wave numbers from 4000 cm-1 to 400 cm-1 and a resolution of 4 cm-1.
Gas chromatography analysis
First, the pulping waste liquor was extracted using methylene chloride and diethyl ether. Second, the organic phase was extracted using quantitative ethyl acetate after being dried in a vacuum dryer. Finally, the extracted samples were analyzed using an MS4000 GC-MS (Varian/Agilent, Santa Clara, CA, USA) (Lei et al. 2012a).
Determining the pulping waste liquor quality index
The dissolved oxygen concentration (DO), chemical oxygen demand (COD), biochemical oxygen demand (BOD), chromaticity, amount of total soluble solids, amount of soluble organic matter, turbidity, pH, conductivity, amount of ammonia nitrogen, total phosphorus, total phosphate, and reducing sugar content were measured using a published method (SEPA 2002).
The metal ion concentration was measured using an inductively coupled plasma mass spectrometer (ICP-MS, Stage vacuum interface technology, Houk and Fassel, Iowa, USA). Acid-soluble lignin content was measured at 205 nm (GB 10337 2008). The SiO2 content was investigated using the weight method (Zhou and Zhang 1992). The extracted hemicellulose was measured using ethanol precipitation (Bian et al. 2010).
RESULTS AND DISCUSSION
Characteristics of the Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping Waste Liquor
Table 2 shows the water quality indicators of the P-RC APMP waste liquor. The pH was 6.38, resulting from organic acid dissolved during the pre-impregnation process (Xiao and Feng 2012). The concentration of dissolved organic compounds was high, and the COD was 18108.1 mg·L-1. In addition, the value of BOD5/COD was 0.17, indicating poor biodegradability of the waste liquor. This may have been attributed to the following reasons. First, the soluble substances in the wood, such as alkali extract, chitosan, and furfural acids or other organic matter, were dissolved during the dipping process. Secondly, a small amount of debris fell into the waste liquor when the wood was crushed and torn during the extrusion process. Finally, the mechanical action may have caused lignin and drug residue to be drawn into the waste liquor, leading to a higher COD concentration and a higher total amount of solids (Liu and Hu 2007; Cai et al. 2010). The hemicellulose content of 11.6 g·L-1 was attributed to contamination after extraction and contained inorganic salts or organic matter with a low molecular weight (Liu and Hu 2007).
Table 2. Primary Wastewater Quality Indicators of Waste Liquor from Poplar Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping
COD, chemical oxygen demand; BOD, biochemical oxygen demand; TP, total phosphorus; SS, suspended solid
In addition, the waste liquor was dark brown, and its chromaticity was 679 CU, indicating a large quantity of chromophoric and auxochrome groups. These groups usually are dissolved during the prepregnation dissolution and alkali extraction processes of lignin (Garg et al. 2005; Leiviskä et al. 2008; Lei et al. 2010). Furthermore, inorganic elements, such as nitrogen, phosphorus, and metal ions (K+ and Mg2+) were found in the waste liquor.
Molecular Weight Distribution of Pollutants in the Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping Waste Liquor
In this experiment, ultra-filtration of pollutants in the waste of poplar P-RC APMP was carried out by ultrafiltration membrane with molecular weight of 5 kDa, 10 kDa, and 25 kDa, respectively. And the change of pollutants in waste liquid before and after ultrafiltration was measured. The results are shown in Table 3.
COD, chemical oxygen demand
Table 3 shows that the COD concentration, chromaticity, turbidity, total amount of soluble solids, and total amount of soluble organic matter decreased with decreasing membrane pore size (filter), relative to the molecular mass. However, the small components were the primary pollutants, and the 5 kDa fraction included a COD content of 1.35 × 104 mg·L-1. Compared to the original acidic waste liquor, the filtrate was alkaline after the high molecular weight (> 25 kDa) filter was applied, and the pH of other filtrates were neutral within the medium molecular weight range (< 10 kDa). There was no noticeable change in the conductivity, indicating that the salt content of the waste liquor was unaffected by ultra-filtration. The total dissolved solids exhibited no obvious variations, which was consistent with the change in the conductivity.
Table 4 shows the concentrations of the COD and total soluble solids and the color of different fractions of the waste liquor, measured using chemical methods.
Table 4. Contribution of Compositions with Different Molecular on Effluent Characteristics
(COD: Chemical Oxygen Demand)
The low-mass organic pollutants in the pulping waste liquor were the primary components of the COD; however, the pollutants with larger molecular weights were the main source of its chromaticity (Table 4). This result was attributed to the lignin molecules that were broken during the extraction process or the fiber fines and hemicelluloses that were degraded during the impregnation process. As a result, new chromophoric groups, auxochrome groups, and colored matter were produced (Xie and Zhan 2001). Therefore, the 25 kDa fraction of the waste liquor accounted for 42.6% of the waste liquor’s chromaticity, 9.6% of the total COD, and 34.1% of the total soluble solids.
The components of the 10 to 25 kDa fraction of the waste liquor accounted for 6.0% of the COD, 0.65% of the total soluble solids, and 12.1% of the chromaticity. The components of the 5 to 10 kDa fraction accounted for 13.2% of the COD, 1.3% of the total soluble solids, and 24.7% of the chromaticity of the waste liquid. The 5 kDa fraction of the waste liquid accounted for 70% of the COD and 20% of the residual chromaticity that had not been removed by ultrafiltration. The small compounds were a mixture of the degradation products of lignin, hemicellulose, wood extract, and fines (Liu et al. 2011).
Consequently, the COD, chromaticity, and total amount of soluble solids insufficiently described the pollution characteristics of the pulping waste liquid. Modern large-scale instruments can detect the compositions and concentrations of pollutants and perform quantitative and qualitative analyses, especially for toxic pollutants (Liu and Hu 2007).
Fourier transform infrared spectroscopy is a simple, non-destructive, fast, highly sensitive, and pollution-free process (Durig et al. 2008). Molecular groups and chemical groups in pulping waste liquor were detected using FTIR spectroscopy (Fig. 1 and Table 5).
Fig. 1. Fourier transform infrared spectrum of poplar wood after P-RC APMP
Figure 1 shows the spectroscopic proﬁles for wavenumbers in the region from 4000 to 400 cm-1. Hydroxyl groups were present between 3200 and 3645 cm-1, as shown in Table 5, because of the stretching vibrations of OH from hydrone, phenol, or alcohol (Lei et al. 2012a). There were two absorption peaks at 3074.79 and 3006.42 cm-1, which were attributed to the stretching of aromatic hydrocarbons (Andreas 2007). This result reflected the presence of numerous aromatic compounds in the pulping waste liquor. The bands at 2904.7 cm-1 and 2896.42 cm-1 were attributed to the stretching of olefin CH2 and alkane C-H, respectively. The bands at 1754.86, 1731.08, 1716.21, and 1692.43 cm-1 were attributed to the stretching vibrations of C=O from a carbonyl group, ester carbonyl group, or aldehyde group. The bands at 1668.65 cm-1 and 1644.86 cm-1 were attributed the stretching of olefin C=C. The absorption spectra of the C-C bonds in aromatic rings exhibited peaks at approximately 1575.79, 1505.14, 1457.58, and 1412.98 cm-1. In addition, peaks were centered at approximately 1258.4 to 1091.92 cm-1, 1068.14 cm-1, and 922.47 cm-1 for C-O-C, C-C, and O-H groups, respectively. Furthermore, the peaks at approximately 791.66 cm-1, 788.69 cm-1, and 696.53 cm-1 were assigned to the surface stretching of C-H bonds from the aromatic rings. The chromaticity of pulping waste liquid increased with carbonyl and hydroxyl groups, benzene rings, among others. In particular, carbonyl groups combined with benzene rings, forming conjugated double bonds, increased the chromaticity (Chen et al. 2007). In addition, hydroxyl groups were present as auxochrome groups in the waste liquor (Lei et al. 2012a).
In sum, the organic matter in the pulping waste liquor was resolved, generating hydroxyl groups, carbonyl groups, benzene rings, double bonds, carboxyl groups, esters, etc. Therefore, organic pollutants, such as aromatic compounds, alkanes, aldehydes, ketones, and esters (Lei et al. 2012a), were the primary components of the COD.
Characterization of the Organic Fraction of the Poplar Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping Waste Liquor
To confirm the primary pollutants and the distribution of organic pollutants in the waste liquor, experiments were performed using GC-MS. The ion chromatography of the organic matter, after liquid-liquid extraction using methylene chloride and ethyl ether, is shown in Fig. 2.
Fig. 2. Gas chromatography-mass spectrometry analysis of the poplar pre-conditioning refiner chemical alkaline peroxide mechanical pulping effluent
Target compounds were identified by matching their mass spectra with spectra obtained from the National Institute of Standards and Technology (NIST) library. Because of the lack of standard references for the different types of organic matter, the content of organic pollutants in water samples were expressed relative to the content determined by the GC-MS processing system. The results are shown in Table 6.
Table 6. Organic Pollutants in the Poplar Pre-Conditioning Refiner Chemical Alkaline Peroxide Mechanical Pulping Effluent According to Gas Chromatography-Mass Spectrometry
There were various organic pollutants in the P-RC APMP waste liquor (Fig. 2). The GC-MS data showed that the pulping waste liquor contained a large amount of organic compounds, primarily phenols, ketenes, alcohols, organic acids, among others (Table 6), which was consistent with the groups represented by the FTIR absorption peaks.
Lignin contains phenylpropane, which was connected by C-C and C-O bonds and exhibits numerous functional groups, including phenolic hydroxyl, methoxy, carboxyl, and carbonyl groups. Lignin was available for hydroxymethylation, carboxylation, and alkylation because the chemical bonds were broken during the chemical and extrusion processes (Lei et al. 2012b). Therefore, the compounds, guaiacol, lilac, alcohol, coniferyl alcohol and vanillin, were found in the waste liquor (Table 5).
The results of the analysis showed that there was a large amount of alkane (29.19%). Three major EPA pollutants (Fang and Hu 1993) were found in the waste liquor: octadecane, eicosane, and octacosanol. Because phenolic hydroxyl groups of lignin are hydrophilic, lignin decomposes to phenolic compounds easily. Therefore, six types of phenolic compounds were detected in the pulping waste liquor, totaling 7.7%. Because of the oxidation of methyl and hydroxylmethyl groups in benzene rings and side chains, aldehyde, carboxylic acid, and their derivatives were created. As a result, there were nine types of organic acids, including benzoic acid, benzene acetic acid, and cinnamic acid, totaling 43.17%. Moreover, 3-(2, 5-dimethyl phenyl) propionic acid, an unsaturated acid, was the most prevalent and reached 15.73% of the waste liquor. In addition, five types of aromatic aldehyde and nine types of ketone were detected, with relative contents of 7.55% and 9.76%, respectively. Finally, alcohols and ethers were found in the waste liquor; however, two types of priority control pollutants, diethyl phthalate and dibutyl phthalate, were not detected (Cox and Deshusses 1998). This was primarily related to the fact that lignin and its derivatives were broken down into low molecular weight organic compounds, while benzene rings and their side chains were fractured or combined to form phenols and organic acids after pharmaceutical impregnation with hydrogen peroxide. These results were consistent with the FTIR analysis.
According to the results of ultra-filtration, FTIR analysis, and GC-MS, low molecular weight organic compounds, especially aromatic compounds, such as carbonyl, carboxyl, and ether groups, were the primary pollutants in the pulping waste liquor. The higher content of aromatic compounds showed that these compounds were typical low-molecular-weight components of the pollution in pulping waste. These substances did not directly affect the chromaticity of the waste liquor; however, their biological (toxicity) effect on the biochemical treatment of waste liquor should be further studied.
- Poplar APMP P-RC pulping waste liquor was acidic as the pH was 6.38. The concentration of dissolved organic compounds was high, and the COD was 18108.1 mg·L-1. Besides, the value of BOD5/COD was 0.17, which indicates the poor biodegradability of the waste liquor. The waste liquor was dark brown and chromaticity was 679 CU. In addition, inorganic elements, such as nitrogen, phosphorus, and metal ions (K+and Mg2+) were found in the waste liquor.
- The 5 kDa fraction of the waste liquor components indicated that 70% of the COD and 20% of the residual chromaticity of the waste liquor was not removed. The physical and chemical properties of the pollutants with molecular weights below 5 kDa require further study.
- The organic pollutants in the pulping waste liquor were primarily benzene, hydroxyl, carboxyl, carbonyl, and conjugated double-bonding compounds, among others. The primary organic compounds were aromatic compounds, alkanes, ethers, esters, acids, etc. The primary chromophoric groups included carbonyl and double-bonded groups. The primary auxochrome groups included carboxyl, hydroxyl, and ether groups.
- The GC-MS data showed that the pulping waste liquor contained a large amount of organic compounds such as primarily phenols, ketenes, alcohols, organic acids and so on. The results of the analysis showed that there was a large amount of alkane (29.19%). Three major EPA pollutants were found in the waste liquor: octadecane, eicosane, and octacosanol.
The authors are grateful for financial support from the National Science and Technology Support Foundation of China (2011BAC11B04).
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Article submitted: June 7, 2016; Peer review completed: August 20, 2016; Revised version received: October 15, 2016; Accepted: November 2, 2016; Published: November 22, 2016.