Optimization of the Coagulation-Flocculation Process For

water research 45 (2011) 5633e5640 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Optimization of the coagulation-flocculation process for pulp mill wastewater treatment using a combination of uniform design and response surface methodology Jian-Ping Wang a,b, Yong-Zhen Chen a, Yi Wang b, Shi-Jie Yuan a, Han-Qing Yu a,* a Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China b Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA article info abstract Article history: Pulp mill wastewater was treated using the coagulation-flocculation process with Received 7 June 2011 aluminum chloride as the coagulant and a modified natural polymer, starch-g-PAM-g- Received in revised form PDMC [polyacrylamide and poly (2-methacryloyloxyethyl) trimethyl ammonium chloride], 6 August 2011 as the flocculant. A novel approach with a combination of response surface methodology Accepted 14 August 2011 (RSM) and uniform design (UD) was employed to evaluate the effects and interactions of Available online 1 September 2011 three main influential factors, coagulant dosage, flocculant dosage and pH, on the treatment efficiency in terms of the supernatant turbidity and lignin removals as well as the Keywords: water recovery. The optimal conditions obtained from the compromise of the three Coagulation-flocculation desirable responses, supernatant turbidity removal, lignin removal and water recovery Optimization efficiency, were as follows: coagulant dosage of 871 mg/L, flocculant dosage of 22.3 mg/L Pulp mill wastewater and pH 8.35. Confirmation experiments demonstrated that such a combination of the UD Response surface methodology and RSM is a powerful and useful approach for optimizing the coagulation-flocculation Uniform design process for the pulp mill wastewater treatment. ª 2011 Elsevier Ltd. All rights reserved. 1. Introduction treatment of palm oil mill effluent (Ahmad et al., 2005), textile wastewater (Meric et al., 2005) and abattoir wastewater The gross output of paper and paperboard is 79.8 million tons (Amuda and Alade, 2006), etc. Recently, coagulation- in 2008 in China, and about 22% of them are using straw pulp flocculation or flocculation processes have also been exten- as staffs. The wastewater generated by the pulp mills is not sively used for the treatment of pulp mill wastewater. In such easy to treat due to the presence of a large amount of chem- studies, polyaluminium chloride (PAC), chitosan, polymeric icals, such as many sodium salts of organic acids. In addition, phosphate-aluminum chloride, cationic and anionic poly- a large amount of lignin present in the wastewater, which acrylamides (PAMs) and polydiallyldimethylammonium causes colority, turbidity and high COD (chemical oxygen chloride (polyDADMAC) have all been tested as a flocculant in demand), is usually wasted and overburdens the treatment the flocculation process, and various levels of removal effi- process. Thus, both the lignin reclamation and wastewater ciency for turbidity and lignin have been achieved (Razali treatment are crucial. An efficient and cost-effective process et al., 2011; Renault et al., 2009; Wong et al., 2006; Zheng for the treatment of pulp mill wastewater should be pursued. et al., 2011). In the coagulation-flocculation process, the effi- Coagulation-flocculation is a simple and efficient method for ciency is governed by various factors, such as the type and wastewater treatment, and has been widely used for the dosage of coagulant/flocculant (Desjardins et al., 2002; Hu * Corresponding author. Fax: þ86 551 3601592. E-mail address: [email protected] (H.-Q. Yu). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.08.023 5634 water research 45 (2011) 5633e5640 et al., 2005; Nandy et al., 2003; Spicer and Pratsinis, 1996; Wang et al., 2002), pH (Elmaleh et al., 1996; Miller et al., 2008; 2. Materials and methods Rohrsetzer et al., 1998; Syu et al., 2003), mixing speed and time (Gurses et al., 2003; Rossini et al., 1999), temperature and 2.1. Chemicals and operation retention time (Howe et al., 2006; Zhu et al., 2004). A proper optimization of these factors could significantly increase its Aluminum chloride, hydrochloric acid and sodium hydroxide, treatment efficiency. purchased from Shanghai Chemical Reagent Co., China, were Response surface methodology (RSM) is an efficient way to of analytical reagent grade and used without further purifi- achieve such an optimization by analyzing and modeling the cation. The flocculant, starch-g-PAM-g-PDMC, was prepared effects of multiple variables and their responses and finally as follows: starch, (2-methacryloyloxyethyl) trimethyl optimizing the process. This method has been widely used for ammonium chloride and acrylamide were dissolved in water the optimization of various processes in food chemistry, in Pyrex glass vessels, and were heated in a preset water bath material science, chemical engineering and biotechnology using potassium persulphate as the initiator after deoxyge- (Granato et al., 2010; Hong et al., 2011; Liu et al., 2010; Singh nating. Thereafter, the sample solutions were precipitated in et al., 2010). In the traditional experimental design acetone and separated by filtration. Homopolymers formed in approaches used in RSM, such as central composite design, the reactions were removed using the soxhlet extraction with an increase in experimental factors, the number of coef- method in ethanol. All the grafted samples were dried in ficients of the quadratic model equation increases exponen- a vacuum oven at 50 C until a constant weight. The grafting tially and so does the number of experimental trials (Cheng percentage and the cationic degree of the graft copolymer Â À3 et al., 2002). To overcome this shortcoming of RSM, uniform used in this experiment was 269% and 1.96 10 mol/g, design (UD) can be used to investigate more factors with respectively. The point of zero charge of the graft copolymer substantially fewer experimental trials, since it determines the was measured as 7.80. Its molecular structure is as shown in number of the experimental trials only by the level of factors, Fig. 1 and its image is illustrated in Fig. S1 (Appendix). rather than by the number of factors. The UD method was first Pulp mill wastewater was blending black liquor from the proposed by Fang (1978). Compared to the traditional experi- primary sedimentation tank of Guoyang Paper and Pulp Mill mental design methods, UD is capable of selecting experi- Co., China. The stuffs of the paper were wheat straw. The mental points uniformly in the experimental region and highly initial pH, chemical oxygen demand (COD) and turbidity were representative in the experimental domain; it imposes no 6.99, 1358 mg/L and 1209 NTU, respectively. The average sizes strong assumption on the model and may be used when the of the colloid particles in the wastewater were 544 nm. underlying model between the responses and factors is The coagulation-flocculation experiments were carried out unknown or partially unknown; it can accommodates the using the jar test method in 1-L beakers. After the coagulant largest possible number of levels for each factor among all (stock solution of 50.0 g/L) was added with a dosage varying e experimental designs (Leung et al., 2000; Li et al., 2003; Wen from nil to 2100 mg/L, the solution pH was adjusted to 2.5 11.5 et al., 2005; Zhang et al., 1998). A combination of UD and RSM by adding 0.1 mol/L HCl or NaOH solutions. Then, the floccu- would be able to achieve the optimization of a complex lant at a concentration of 1.0 g/L was added with a dosage multivariate process with the fewest multilevel experiments. varying from nil to 48 mg/L. The sample was immediately Therefore, in this study, UD and RSM were integrated to opti- stirred at a constant speed of 200 rpm for 2 min, followed by mize the coagulation-flocculation process for pulp mill a slow stirring at 40 rpm for 10 min; thereafter, a settlement for wastewater treatment. 5 min was performed. After that, samples were taken from the The selection of high efficient coagulants and flocculants is water level around 2 cm underneath the surface for measuring essential for a successful coagulation-flocculation process. In the turbidity and lignin concentrations of the supernatant. this work, on the basis of our previous studies (Wang et al., Meanwhile, the volume of produced sludge was calculated directly from the reading on the beakers, and then the volume 2007, 2009), the conventional coagulant Al(OH)3 was chosen as the coagulant, while a novel modified natural polymer, of the recovered clean water was calculated accordingly. starch-g-PAM-g-PDMC [polyacrylamide and poly (2-methacryloyloxyethyl) trimethyl ammonium chloride] with both 2.2. Experimental design and data analysis strong charge neutralization and bridging abilities, was used m as the flocculant. UD tables can be described as Un(q ), where U, n, q and m stand The main objective of this work was to treat the pulp mill for the UD, the number of experimental trials, the number of wastewater using the coagulation-flocculation process, which levels and the maximum number of factors, respectively. For was optimized using an integrative UD-RSM approach. a given measure of uniformity M, a uniform design has the Removal efficiencies of both supernatant turbidity and lignin, smallest M-value overall fractional factorial design with n and recovery efficiency of clean water were chosen as the runs and mq-level factors. dependent output variables. The compromise optimal condi- CH3 CONH2 tions for these three responses were obtained using the SO CH2 C CH2 C desirability function approach. The novel optimization m H n strategy used for the pulp mill wastewater treatment process COOC2H4N(CH3)3Cl in this study is expected to provide valuable information for other complicated systems in environmental engineering and Fig.

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