## 4 Conclusions  
The present study examines the removal efficiency of heavy metals (Cu, Ni, and Zn) in a mixed aqueous solution in a batch mode through a nanofiltration (NF) membrane and an electrocoagulation (EC) process and compares it with conventional chemical treatment processes. Solution pH is seen to significantly affect the removal efficiency in both the NF and EC processes. The highest permeate flux of 59.34 L/m\({}^{2}\)-h was observed at the experimental condition of pH 9.5, pressure 25 Bar, concentration 25 ppm and temperature 35 \({}^{\circ}\)C in NF process. The rejection rate of Zn, Ni and Cu was demonstrated as 95.32%, 94.98% and 96.93%, respectively. A marked synergistic effect of temperature and pressure has been observed, which increased the flux to a high value. The EC process has shown  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Factor** & **Name** & **Units** & **Low Actual** & **High Actual** & **Low Coded** & **High Coded** \\ \hline A & Conc. & ppm & 10.00 & 25.00 & \(-1.000\) & 1.000 \\ \hline B & Contact Time & Minute & 30.00 & 90.00 & \(-1.000\) & 1.000 \\ \hline C & Dosing Amount & g/L & 4.00 & 8.00 & \(-1.000\) & 1.000 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Factors and Range of Design for CaO and CaCO\({}_{3}\).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**S.N.**} & **Elements** & **Unit** & **CaO** & **CaCO\({}_{3}\)** & **EC** & **NF** \\  & **Name** & **Unit** & **(Maximum)** & **(Maximum)** & **(Maximum)** & **(Maximum)** \\ \hline
1 & Zn Removal & \% & 99.72 & 99.82 & 99.46 & 99.96 \\ \hline
2 & Ni Removal & \% & 99.96 & 43.46 & 98.14 & 99.63 \\ \hline
3 & Cu Removal & \% & 99.99 & 99.99 & 99.96 & 99.99 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Removal% Efficiency of Conventional (CaO and CaCO\({}_{3}\)) vs. EC and NF processes.  
a maximum removal of Zn (99.46%), Ni (98.14%), and Cu (99.87%) at the operational conditions viz., pH 6, time 60 (min), concentration 2.5 ppm, and current 1.5 A. The results for the EC process indicated that a lower concentration and approximately neutral pH helped the system to reach its full potential. Overall, both NF and EC processes have shown excellent removal for all the studied metal ions and the outcome of the experiments described above projects them as promising solutions in comparison to conventional chemical treatment approaches.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12010280/s1](https://www.mdpi.com/article/10.3390/app12010280/s1)]([https://www.mdpi.com/article/10.3390/app12010280/s1](https://www.mdpi.com/article/10.3390/app12010280/s1)), Table S1: Experimental design and responses of the NF process; Table S2: Experimental design and responses of the EC process.  
Conceptualization, J.K. and H.J.; methodology, J.K. and H.J.; software, J.K.; formal analysis, J.K. and S.K.M.; investigation, J.K.; resources, H.J. and S.K.M.; writing--original draft preparation, J.K.; writing, review and editing, H.J. and S.K.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
All relevant data are within the manuscript and available from the corresponding author on request.  
This work has been carried out with the Junior/Senior Research Fellowship of the University Grant Commission (UGC), New Delhi, India, under the Ph.D. programme. Authors are highly thankful to the Indian Institute of Technology Roorkee, Roorkee for providing the necessary support and encouragement for bringing out this publication.  
The authors declare no conflict of interest or state.