## 3 Results and Discussion
### Comparison with Chemical Precipitation and Adsorption Processes  
Chemical precipitation is a commonly used treatment process for the removal of heavy metals from industrial wastewater because it is relatively inexpensive and easy to operate. This process involves the precipitation of heavy metals in the form of hydroxide and sulfide. Hydroxide precipitation depends on pH adjustment (9-11) to basic conditions [58]. The metal ions dissolved in the solution are precipitated into the insoluble solid phase as metal hydroxide through the chemical reaction when quicklime (CaO) is employed as a precipitant. Yet another common treatment process viz., adsorption, on the other hand, is a mass transfer process involving the migration of the metal ions (adsorbate) from the wastewater to a solid surface (adsorbent, commonly CaCO3) and binding through physical (weak Van der Waals force) and chemical (strong covalent bond) adsorption mechanisms [59; 60]. With an idea to compare the performance of NF and EC processes with the above-stated routine ones, the present study employed concentration, contact time, and dosing amount as operational variables for the conventional chemical treatment process employing CaO and CaCO3. Twenty experiments were conducted for each process and Table 8 provides details of the experimental design. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Pollutants/Wastewater** & **E.C. Process** & **Metal Removal Efficiency** & **References** \\ \hline Synthetic wastewater & SS electrode & Zn (99.81\%), Ni (99.99\%), and Cu (99.98\%) & Current study \\ \hline Synthetic wastewater & Aluminum Electrode & Ni\({}^{2+}\) and Cu\({}^{2+}\) ions\% removal efficiency & [53] \\  & & ranged from 34.56–100\% & \\ \hline Electroplating Industry & Aluminium plates & Ni (99.75\%) & [54] \\   wastewater & Iron Electrodes & Co (99\%) Ni (97\%), Zn (99\%), Cu (97\%), & [55] \\  & & and Cd (99\%). & \\ \hline Smelting wastewater & Iron Plates & Zn\({}^{2+}\), Cd\({}^{2+}\), and Mn\({}^{2+}\) removal efficiency & [40] \\  & & 99.93\%, 97.15\%, and 85.46\% respectivilty & [40] \\ \hline Synthetic Smelting & Fe electrode & Zn\({}^{2+}\), Cd\({}^{2+}\), and Mn\({}^{2+}\) removal efficiency & [56] \\   Wastewater & & 99.5\%, 99.9\%, and 55\% respectively. & [51] \\ \hline Metal Plating Wastewater & Iron (carbon steel) plates & Cu\({}^{2+}\), Cr\({}^{3+}\), Ni\({}^{2+}\), and Zn\({}^{2+}\) over 97\% & [51] \\ \hline Waste Fountain Solution & Aluminium and Iron electrode & Ni (\(>\)95\%), Cu (\(>\)95\%), and Zn (\(>\)80\%) & [57] \\ \hline Metal Plating Effluent & SS Electrode & Ni (100\%) and Zn (100\%) & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Overview of metals ion removal efficiency by EC processes described in the literature. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Optimum Input Variables (Feed Solution)** & **Predicted Responses by RSM** & **Experimental Validation** & **Difference\%** \\ \hline pH & 7 & Zn Removal\% & 101.50 & 99.81 & 1.67 \\ \hline Time & 60 & Ni Removal\% & 94.45 & 99.99 & 5.86 \\ \hline Initial Conc. (ppm) & 24.923 & Cu Removal\% & 98.86 & 99.98 & 1.13 \\ \hline Current (Å) & 1.912 & – & – & – \\ \hline \hline \end{tabular}
\end{table} 
Table 7: EC Optimization of response through RSM. In CaO precipitation, the removal efficiency of the process was quite high for Zn, Ni, and Cu ions, as expected and as indicated by the results in Table 9 However, understandably, there are also many demerits in this process. It requires a large amount of chemical precipitant and produces a considerable amount of low-density sludge due to the poor settling properties, duly followed by further dewatering and disposal issues [60,61]. Aggregation of metal precipitates also has long term environmental impacts. Treated water also has a very high pH (10-12), so it cannot be further used in industrial process and requires treatment. Regarding the CaCO\({}_{3}\) adsorption process, the results presented in Table 9 indicate that the removal of Ni ions is not as efficient as for Cu and Zn ions.