Optimization of hybrid coagulation-ultrafiltration process for potable water treatment using response surface methodology

In order to optimize the operating conditions for a combined polyaluminum chloride (PACl) coagulation/flocculation and ultrafiltration process for treating potable water, the main, second order and interaction effects of PACl dose and flocculation retention time (FRT) on permeate turbidity, UV254 and membrane permeability were investigated using a 100 kDa hollow fiber membrane operated in the dead-end mode. A multilevel factorial design was used to determine the relevant ranges of the two factors for optimization. A 2 central composite design (CCD) was then used to develop mathematical correlation models for the optimum operating conditions. The main effect of PACl dose was the most significant factor on all the responses. For permeability, both the main effect of FRT and FRT–PACl dose interactions were found to be insignificant. The optimum PACl dose and FRT for the feed water were 20 mg/L and 14 min, respectively. Corresponding permeate turbidity, UV254 and permeability were 0.15± 0.01 NTU, 0.003± 0.001 cm 1 and 62.0± 9.52 Lm 2 h 1 bar , respectively. Experimental validation runs confirmed the reliability of the predicted optimal conditions thus implying that CCD models can be used to predict/optimize the quality and quantity of permeate from hybrid coagulation–ultrafiltration systems for potable water treatment. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/ws.2017.159 om http://iwaponline.com/ws/article-pdf/18/3/862/659684/ws018030862.pdf er 2020 S. G. Arhin (corresponding author) N. Banadda A. J. Komakech Department of Agricultural and Biosystems Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda E-mail: arhinsamuel32@gmail.com S. G. Arhin W. Pronk S. J. Marks Eawag: Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, Dübendorf 8600, Switzerland


INTRODUCTION
Ultrafiltration (UF) is widely applied as an effective technique for removing a diverse array of waterborne pathogens such as protozoa, bacteria and viruses from drinking water (Hill et al. ; Mull & Hill ). At the appropriate molecular weight cut-off (MWCO), UF membranes can produce bacteria-free drinking water, remove greater than 6 log of viruses, turbidity, a portion of natural organic matter (NOM) and disinfection byproduct precursors from the feed water (Liang et al. ; Mimoso et al. ). Compared with conventional sand filtration, UF has several advantages which include a higher treatment efficiency, smaller footprint, fewer chemicals demand, less sludge production and easy automation. Furthermore, recent advances in membrane technology have made UF a cost-effective alternative to other filtration techniques (Jeong et al. ; Mull & Hill ). It is therefore seen by communities as a safer treatment alternative (Guo et al.

).
Despite these advantages, membrane fouling still remains an important limitation for this technology (Arhin the membrane surface or within the membrane pores generating an increasing hydraulic resistance. Fouling reduces hydraulic permeability, increases transmembrane pressure and cleaning frequencies, which eventually lead to higher operating costs. Moreover, frequent membrane cleaning operations may lead to membrane deterioration causing reduction in the permeate quality and shorter membrane life span (Chon et al. ; Munla et al. ; Xu et al. ). In order to minimize fouling, pretreatment of feed water using techniques such as coagulation, adsorption, oxidation, bio-filtration, magnetic ion exchange resins and integrated pretreatment is practiced in full-scale and pilotscale UF plants (Gao et al. ). Among the various pretreatment techniques, coagulation is the most dominantly used due to its low cost, ease of operation and ability to significantly improve UF performance (Arhin et al. ). It is presumed that during coagulation, colloids are destabilized and clusters form larger flocs which are easily retained by UF membranes (Xiangli et al. ). Besides fouling minimization, the use of coagulation pretreatment has an additional beneficial effect since it can improve the removal efficiency of harmful contaminants such as viruses (Fiksdal & Leiknes ; Konieczny et al. ).
Although coagulation pretreatment is rapidly gaining attention, research evaluating its effect on membrane fouling has been inconsistent. Whereas several studies have reported improved membrane performance with coagulation pretreatment (Dong et  In this study, the optimum operating conditions for a hybrid PACl coagulation-UF system for potable water treatment were determined using RSM. The focus of this study was to establish the optimum PACl dose and FRT for enhancing both organic and inorganic foulants removal. To that end, the independent variables considered were PACl dose and FRT, and the process output was assessed in terms of

Raw water sampling and samples characterization
Surface water samples from Lake Victoria were collected at the raw water inlet to Ggaba II Water Treatment Works, Kampala, Uganda from November 2015 to April 2016.
Analytical tests were conducted to characterize the raw water based on temperature, pH, electrical conductivity, turbidity, color, and UV 254 . Temperature was determined with an Extech MO295 IR thermometer (Extech ® instruments, USA). pH and conductivity were determined using Hach No sample was kept for more than 3 days without performing analysis. Table 1 summarizes the characteristics of the feed water used for the study. Each parameter was determined at least in triplicate.

Preliminary batch experiments
Preliminary experiments were done to verify the statistical relevance of the two independent variable (PACl dose and FRT) and to determine their experimental ranges for optim-

Membrane filtration
Commercially available hollow tube UF membranes were used. The membrane specifications are provided in Table 2. The experimental setup is schematically illustrated in Figure 1. As shown in Figure (1): where Q is the permeate flow (L/h) and A is the membrane area (m 2 ). The membrane permeabilityafter 60 min of continuous filtrationwas subsequently determined by dividing the flux by the transmembrane pressure as shown in Equation (2): where TMP is the transmembrane pressure (Pa).

Experimental design for RSM
After the experimental ranges for the independent variables were obtained, RSM was used to determine the optimum factor settings for pre-coagulation. A 2 2 rotatable central composite design (CCD) consisting of factor combinations at two levels, four star points and five replicates at the center point was developed. The coagulant dose (coded as X 1 ) and FRT (coded as X 2 ) were set as the independent variables while the permeate turbidity (Coded as Y 1 ), UV 254 absorbance (coded as Y 2 ) and the membrane permeability (coded as Y 3 ) were set as the response variables. Randomization of the experimental runs was done to limit the effects of unexpected variability in the response. Table 3 shows the range and levels of the two independent variables for the response surface design.
The experimental data from RSM was fitted into the polynomial model indicated in Equation (3): where Y is the response variable; X 1 , X 2 ,…, X k are the independent variables affecting the response Y; β 0 , β i (i ¼ 1, 2,…, k), to Equation (4): where m is the number of responses.
The goal of the coagulation-UF process is to obtain a low turbidity and low UV 254 (organics) in the permeate while maintaining a relatively high permeability across the UF membrane. Therefore, the desirability function for turbidity and UV 254 were both the 'smaller-the-better' responses, which criterion is expressed in Equation (5) whereas the desirability function for permeability was the 'larger-thebetter' response, which transformation is described in Equation (6): where U is the upper acceptable value to the response, L is the lower acceptable value to the response, T is the target value, y is the response and t is the weight (Bezerra et al.

).
After the optimum conditions were obtained from the desirability function, four experimental replicates were per-      higher mixing volumes as well as a higher energy consumption and yet no improvement in water quality would be attained.
Therefore PACl doses ranging from 10 to 20 mg/L and FRT ranging from 5 to 15 min were used for RSM optimization.
Optimization of the hybrid coagulation-UF system Table 5 shows the layout and results of the 2 2 CCD. Irrespective of the factor settings used in this study, the color of permeate was zero (100% removal of color forming compounds). Hence, color was deemed an inappropriate response for assessing the effect of varying PACl dose and FRT on the hybrid coagulation-UF system. Three quadratic models representing the permeate turbidity, the presence of UV 254 absorbing organics and the permeability across the membrane were therefore obtained from the CCD and their statistical significance were examined by ANOVA. Fit summary output analysis revealed that the models were statistically significant (p-value <0.05) to describe the permeate turbidity, UV 254 and the membrane permeability.
As depicted in Table 6, the p-value for the mathematical model for turbidity implied that the model was highly significant. In addition to that, the multiple regression coefficient  As shown in Table 6, the main effect of PACl dose (X 1 ), the main effect of FRT (X 2 ), the second order effect of PACl dose (X 1 *X 1 ), the second order effect of FRT (X 2 *X 2 ) and the interaction of PACl dose and FRT (X 1 *X 2 ) were all found to be the significant factors affecting the permeate turbidity. The regression model for turbidity (Y 1 ) is given by Equation (7): Y 1 ¼ 0:6576 À 0:02435 X 1 À 0:03924 X 2 þ 0:000365 X 2 1 þ 0:001421 X 2 2 þ 0:000420 X 1 X 2 (7) Apart from turbidity, UV 254 is another important response variable for this study as it was used to connote the efficiency of pre-coagulation in removing organics from the feed water. As depicted in Table 6, the model for UV 254 was found to be very significant. Coupled with that, the R 2 showed that 97.06% of the variability in the response could be well explained by the model. Furthermore, the model was found The significant model terms for UV 254 were the main effect of PACl dose (X 1 ), the second order effect of PACl dose (X 1 *X 1 ), the main effect of FRT (X 2 ) and the second other effect of FRT (X 2 *X 2 ). The two level interactions of PACl dose and FRT was statistically insignificant (p-value ¼ 0.085) and was therefore eradicated using backward elimination procedures to improve the model (Table S1, Supplement, available online). Ultimately, the regression model for UV 254 absorbance (Y 2 ) was reduced to the form shown in Equation (8): Y 2 ¼ 0:01677 À 0:001364 X 1 À 0:000331 X 2 þ 0:000031 X 2 1 þ 0:000034 X 2 2 (8) Permeability as a third response variable was used to denote the fouling potentials of the pre-coagulated water.
Less fouling is accompanied by high hydraulic permeability while the converse is true for exacerbated fouling (Bae & Tak ). The empirical model for permeability was also found to be statistically significant (Table 6). An R 2 of 0.8100 was computed for the regression indicating that there is a strong correlation. To cap it all, the Lack-of-Fit test did not show inadequacy of the model implying that the model could adequately fit the experimental results.
The normal probability and surface plots for permeability are also shown in Figure 5. As depicted in response surface plot, permeability decreased around the center of the surface  It was also revealed that within the context of this study, the main effect of PACl dose (X 1 ), the second order effect of PACl dose (X 1 *X 1 ) and the second order effect of FRT (X 2 *X 2 ) were the significant factors affecting the permeability (Table 6). Therefore to improve the model, the backwards elimination procedure was once again employed to eradicate the insignificant factors (Table S1, Supplement).
The final empirical model for permeability is shown in Equation (9): Y 3 ¼ 181:4 À 13:19 X 1 À 6:50 X 2 þ 0:3797 X 2 1 þ 0:364 X 2 2 (9) The membrane permeability seemed to be dependent  (10)), which implies that the cake layer resistance is inversely proportional to the square of the particle diameter: where K c is the specific cake resistance, C c is solid concentration or solidosity of the cake in volume percentage and d p is the particle diameter.
This was also observed in previous studies demonstrating that a cake layer formed by smaller particles is much compact compared to that formed by large ones, which imply that smaller particles have a higher specific cake resistance (Lin et al. ; Chomiak et al. ). In addition to that, coagulant doses and FRTs that formed bigger flocs also contributed to higher removal of UV 254 organics.
Because humic NOM has more electron rich sites ( UV 254 rejections were sometimes accompanied by low permeability and vice versa. This phenomenon is very evident in standard orders 4, 5 and 12 (

CONCLUSIONS
The optimum operating conditions for a hybrid PACl coagulation-UF system for treating potable water from Lake Victoria were obtained using RSM. The focus of the study was to establish the optimum PACl dose and FRT for enhancing both organic and inorganic matter removal while simultaneously optimizing membrane permeability. Using RSM models, the optimum PACl dose and FRT for the feed water were 20.07 (≈20) mg/L and 13.64 (≈14) min, respectively. At this factor combination, turbidity, UV 254 and permeability of 0.15 ± 0.01 NTU, 0.0030 ± 0.0010 cm À1 and 62.0 ± 9.52 Lm À2 h À1 bar À1 were respectively predicted.
Experimental validations confirmed the reliability of these predicted optimum settings, indicating that RSM models can be used to predict optimum operating conditions for hybrid coagulation-UF process of the feed water at the bench scale. The effectiveness of the hybrid system appears to be influenced by a complex interaction between the coagulant dose and FRT. Coagulant doses and FRTs that formed smaller flocs offered high hydraulic resistance which led to low membrane permeability whereas agglomeration of particles into bigger flocs contributed to high membrane permeability.
Also, smaller flocs led to low turbidity and UV 254 rejection while the converse was true for bigger flocs. However, a tradeoff was found between the permeate turbidity, UV 254 and the permeability in that increase in one sometimes led to a decrease in the other. Consequently, a multi-criteria approach that finds the optimal compromises between turbidity, UV 254 and membrane permeability was used to simultaneously optimize the responses. This demonstrates that to maximize production, coagulation conditions should be set in order to achieve the optimum compromise between floc size, solute rejection and membrane permeability. The results presented here provide insights into simultaneous optimization of relevant operating factors in pilot or full scale coagulation-UF plants for drinking water treatment.