Influence of residual black liquor in pulp on wastewater pollution after bleaching process

Abstract

The influence of residual black liquor in pulp on wastewater pollution after the bleaching process was studied. The results show that the CODCr in bleaching effluent has a remarkable linearity with bleaching loss of pulps without residual black liquor. For pulps with some residual black liquor, more than 34% of the overall CODCr is produced by the residual black liquor. It follows that more effective washing to reduce the residual black liquor is an appropriate way to control the pollutant discharges from pulp and paper mill industry.

Full Article

Influence of Residual Black Liquor in Pulp on Wastewater Pollution after Bleaching Process

Tiantian Chen,^a Youming Li,^a,b Lirong Lei,^a,b Mingzhu Hong,^a Qihui Sun,^a and Yi Hou ^a,b,*

The influence of residual black liquor in pulp on wastewater pollution after the bleaching process was studied. The results show that the COD_Cr in bleaching effluent has a remarkable linearity with bleaching loss of pulps without residual black liquor. For pulps with some residual black liquor, more than 34% of the overall COD_Cr is produced by the residual black liquor. It follows that more effective washing to reduce the residual black liquor is an appropriate way to control the pollutant discharges from pulp and paper mill industry.

Keywords: Residual black liquor; Bleaching loss; COD_Cr; Wastewater pollution

Contact information: a: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China; b: National Engineering Research Center of Papermaking and Pollution Control, South China University of Technology, Guangzhou 510640, China;

* Corresponding author: ceyhou@scut.edu.cn

INTRODUCTION

In terms of fresh water usage, the pulp and paper mill industry is a water-intensive industry; it is ranked third in the world after metals and chemical industries (Merayo et al. 2013). Wood preparation, pulping, bleaching, and coating operations are the main sources of pollution (Mahesh et al. 2016). Effluents of the pulp and paper industry contain a number of toxic compounds. The rejection of this effluent in nature without any treatment is responsible of serious damage for the environment and constitutes a threat for human health (Lara et al. 2003).

Kraft (or sulfate) and soda are the two major alkaline processes to produce chemical pulps (Cardoso et al. 2009). Cellulose fibers are disassociated from lignin by chemical reactions, which occur in a pressurized digester where wood chips or fibers are heated and cooked with the cooking liquor, composed basically of NaOH (sodium hydroxide), and in the case of the kraft process, also sodium sulfide (Cao et al. 2011; Zheng et al. 2012). The products from the digester reactions are fibers and black liquor. Black liquor is one of the main by-products of pulp paper industry, which is considered a pollutant because it contains about 50% of lignin (Zaied and Bellakhal 2009).

Some new methods for the treatment of black liquor have been researched, such as the oxidizing action of the Fenton reagent in the presence of solar UV irradiation (Fe²⁺ /H₂O₂ /UV) (Torrades et al. 2003), ultra-filtration for the recovery of lignin starting from black liquor (Wallberg et al. 2005; Wallberg and Jönsson 2006; Wallberg et al. 2003), and the electrocoagulation method (El-Ashtoukhy et al. 2009; Khansorthong and Hunsom 2009; Zaied and Bellakhal 2009). But none of them is widely used industrially due to high cost. At present, soda recovery units (Licursi et al. 2015) have been mainly applied for the treatment of black liquor. However, limitations of equipment lead to only about 85% recovery ratio for black liquor. This means that more than 15% of the black liquor remains in the pulp, and unfortunately it will be passed forward into the bleaching process.

Bleaching of pulp using chlorine-based agents is still practiced in China and in other developing countries (Deshmukh et al. 2009). Due to the presence of chlorinated organic compounds, wastewater from bleaching units is toxic (Savant et al. 2006). These organochlorine compounds are collectively termed as adsorbable organic halides (AOX). AOX is a general parameter determining the total amount of organically bond halogens in wastewater. Many AOX compounds are persistent in the environment and show a significant toxic effect on human beings (Höfl et al. 1997; Barroca et al. 2001). About 500 different chlorinated organic compounds have been identified in paper mill effluent (Savant et al. 2006). Due to the universal harmful effect of organic halogens, many countries including China have a series of discharge standards for industrial wastewater (Xie et al. 2016). There are a number of approaches that have been explored to reduce the AOX level in paper mill effluent, which can be categorized as physic-chemical and biological (Kumar 2013). Physic-chemical approaches include adsorption, ultrafiltration, nanofiltration, and reverse osmosis (Patel and Suresh 2008). Biological techniques involve the use of diverse kinds of micro-organisms like bacteria, fungi, algae, and microbes of extreme habitats for reducing AOX and chromophores in pulp mill effluent (Belmonte et al. 2006; Ruggaber and Talley 2006; Morales et al. 2015). Although these methods of treatment seem effective for the wastewater of pulp industry, they present the disadvantage of being expensive because of their operating cost and the high cost for the chemical reagents used. How to reduce pollution emissions in the production process has become urgent for the pulp and paper industry.

There is a lack of literature about effects of remained black liquor entering the bleaching system on the COD_Cr of wastewater. This paper investigated the effects of residual black liquor in pulp on COD_Cr after the H-bleaching process, which is supposed to bring about fundamental changes in pollution control and treatments for pulping and paper-making industry in China.

EXPERIMENTAL

Raw Materials

Pinus koraiensis was used as the raw material, with alkaline cooking. The total solids content of the black liquor was 194.3 g/L. The three-stage adverse current washing method was adopted to wash the black liquor out of the pulp at 60 °C to simulate industrial washing. A stock suspension having 35% (or alternatively 30%) consistency has 0.72 (or alternatively 3.71) g of black liquor residual solids in the pulp suspension. As shown in Table 1, four samples were selected according to different degrees of cleanliness for further analysis. The No. 1 pulp was thoroughly washed without any residual black liquor (ideal condition) and has a Kappa index of 27.58, whiteness of 34.32 % ISO, and viscosity of 474 mL • g^-1. There are different amounts of black liquor in the No. 2, No. 3, and No. 4.

Table 1. Main Components and Cleanness in 20 g Slurry of Different Pulps

Bleaching Process of Pulp

In the pulp bleaching section, the amount of pulp was 20 g on a dry solids basis. Prior to experiments, these samples had been equilibrated to 10%. The runs were carried out with different doses of active chlorine (4%, 7%, 10%, 13%, 16%) and the other parameters were kept fixed. Single stage hypochlorite bleaching was adopted at 52 °C for 25 min in a water bath. Mixing was accomplished by means of stirring every 10 min in order to make the bleaching reaction better; 8% sodium hydroxide was applied to adjust the pH, and the pH values of the bleaching solution were measured before the reaction, during bleaching, and after the reaction (Diel et al. 2016).

Bleaching effluent was collected and the pulp was washed thoroughly with enough water to remove suspended matter and particles for the future study.

Bleaching Process of Black Liquor

To study the effects of residual black liquor on pollution loads in the bleaching process, the amount of black liquor of No. 1 (2, 3, 4) was 0 (2.4, 10.8, 23.5) g and the amount of solids of No. 1 (2, 3, 4) was 0 (0.4, 1.8, 3.9) g. The same bleaching method was applied to bleaching process of black liquor.

Other Analytical Method

Prior to experiments, all liquid effluent samples were filtered with a membrane filter (pore size 0.45 μm) to remove suspended matter and particles. The determination of the COD_Cr was carried out using a Hach spectrophotometer (DR2800, Hach, Loveland, CO, USA), according to standard methods (Karichappan et al. 2014). The pH was measured with a Sartorious PB-10 (Sartorius, Germany) pH meter.

RESULTS AND DISCUSSION

Kappa Index and Bleaching Loss of No. 1 and COD_cr of No. 1 after Bleaching

The No. 1 pulp sample without residual black liquor was H-bleached at different amounts of available chlorine of 4%, 7%，10%, 13%, and 16%, and Kappa index and bleaching loss of No. 1 and COD_cr of No. 1 after bleaching are shown in Table 2.

Table 2. Kappa Index and Bleaching Loss of No. 1 and COD_cr of No. 1 after Bleaching

As is shown in Table 2, with the increase of available chlorine, the kappa index of pulps decreased, indicating that lignin was disassociated from fibers. When the available chlorine was 16%, the removal rate of lignin was about 82.4%. The bleaching loss and COD_cr increased with the increase of the available chlorine. A linear relationship between bleaching loss and COD_cr was obtained, as is shown in Fig. 1.

Fig. 1. Relationship between bleaching loss and COD_cr in effluents for pulps without residual black liquor

For pulps without residual black liquor, the COD_cr in the wastewater are contributed to lignin and carbohydrates that are disassociated from the cellulose fibers (solid phase lignin). For the same fiber materials with the same bleaching conditions, the COD_cr contributed to solid phase lignin is calculated as COD₁ in the next experiments.

Kappa Index and Bleaching Loss of Pulp with Residual Black Liquor and COD_Cr of Pulp with Residual Black liquor after Bleaching

H-bleaching was applied to the No. 2, No. 3, and No. 4 pulp samples under different available chlorine dosages of 4%，7%，10%, 13%, and 16%. The kappa index and bleaching loss of No.2 (3, 4) and COD_Cr of No.2 (3, 4) are shown in Tables 3, 4 and 5, respectively. The COD₀ is the actual determination values in bleaching effluents, which is composed of COD₁ produced by solid phase lignin, and COD_L originated from the theoretical residual black liquor. According to Fig. 1, COD₁ can be calculated with bleaching loss.

To test the reliability of the results obtained by calculating the COD_L from COD₀ and COD₁, different amounts of black liquor were H-bleached in the same bleaching conditions, and the determination of COD_Cr in bleached residual black liquor was obtained as COD₂, as shown in Tables 3, 4, and 5.

The relative standard deviation (RSD) of the COD_L and COD₂ ranged from 0.10% to 3.15%. Thus, the calculated values ​​were in good agreement with the measured values, and it was feasible to calculate the COD₁ by Fig. 1 to determine the pollution load from solid phase lignin. For all pulp samples, with increasing available chlorine content, the kappa index decreased gradually.

For the same slurry, the COD_L from residue black liquor had a greater effect on the wastewater pollution load than COD₁ from solid phase lignin without No.2 pulp, but COD_L from residue black liquor in the wastewater pollution load of No.2 was still evident. This is because the residual black liquor mainly includes the small molecule lignin compounds, hemicellulose, and some organic degradation of sugars after cooking (Sun et al. 1999; Mussatto et al. 2007). However, with the increase of available chlorine, the increase of the COD_L is smaller than that of COD₁ in wastewater, because the amount of residue black liquor in the slurry is certain. Another possible reason is that aldehyde or alcohol organics in the black liquor are oxidized to the acid organics by adding one more oxygen, resulting in a smaller COD_L change.

Table 3. No. 2 Pulp Properties and Pollution Load after H-Bleaching

Table 4. No. 3 Pulp Properties and Pollution Load after H-Bleaching

Table 5. No. 4 Pulp Properties and Pollution Load after H-Bleaching

For different slurries at the same amount of available chlorine, with the increase of residual black liquor in the pulp, COD₀ and COD_L gradually increased and bleaching losses and COD₁ decreased. Thus, the liquid organics are more likely to react with bleaching agent than solid phase lignin; in a bleaching process for pulps with residual black liquor, most of bleaching agents react with the solubilized organics first. This reaction weakens the bleaching efficiency and wastes bleaching agents. A decrease in residual black liquor can reduce the dosage of bleaching agent, which may lessen the production of AOX and decrease the toxicity of wastewater (Rey et al. 2013; Kumar Chenna et al. 2016).

Contribution of Residual Black Liquor to COD_cr in Bleaching Effluents

The contribution of residual black liquor to COD_cr after H-bleaching is shown in Fig. 2. As shown in Fig. 2 and Tables 3, 4, and 5, for different slurries in the same amount of available chlorine, a greater amount of residual black liquor resulted in a greater COD_Cr. For the same slurry, with the increase of available chlorine content, the contribution of residual black liquor to COD_cr in the wastewater decreased gradually. But all COD_L/COD₀ ratio values were more than 34%. This result reveals that COD_cr from the residual black liquor was greater.

Fig. 2. Effect of black liquor on wastewater

CONCLUSIONS

1. In the H-bleaching process, the relationship of bleaching loss and COD_Cr in bleaching effluents is remarkably linear for pulps without residual black liquor.
2. For pulps with residual black liquor in H-bleaching process, most of bleaching agent reacts with residual black liquor in the pulp first. The results show that the COD_Cr produced by residual black liquor accounted for more than 34% of the overall COD_Cr.

ACKNOWLEDGMENTS

The authors are grateful for the support of the National Natural Science Foundation of China (21476091), State Key Laboratory of Pulp and Paper Engineering (Project Number 2015C02), the Science and Technology Planning Project of Guangdong Province (grant number 2015A020215009), Science and Technology Planning Project of FoShan in Guangdong Province (grant number 2015AG10011), and the Natural Science Foundation of Guangdong Province (2014A030310145).

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Article submitted: November 9, 2016; Peer review completed: January 7, 2017; Revised version received and accepted: January 25, 2017; Published: February 1, 2017.

DOI: 10.15376/biores.12.1.2031-2039