## 3 Results and Discussion
### Study of the Removal Efficiency  
The validated HS-GC-MS method in SIM mode was used for the analyses of the treated industrial waters for the inspection of treatment efficiency. The peak areas of the identified compounds in treated water samples were compared with the peak areas obtained for the raw industrial effluent. The evaluations of the data showed significant elimination of the VOCs in all studied treatment systems.  
The removal efficiency factors (REFi) [19] were calculated for the characterization of different treatment procedures, for the studied individual compounds:  
REFi= (1 - A (Mi, TW)/ A (Mi, IW))-100%  
where A (Mi, TW) is the peak area of individual VOCs determined in the headspace gas taken above the treated water sample, and A (Mi, IW) is the peak area of individual VOCs from the headspace gas taken from the industrial water sample (raw sample).  
Significant removal efficiency was obtained when the REF exceeded a percentage of 70%. Removal efficiency was characterized as moderate when the obtained REF was in the range of 30-70%. Below 30%, the removal of selected VOCs was evaluated as weak. Table 3 contains the removal efficiency factors for all studied VOCs for different treatment procedures.  
The most abundant compound in industrial water effluent was toluene, and it was shown that the amount was significantly eliminated during the treatment process. In the case of using sample dilution 1:3 before the treatment process (Treatment 3 and 4), significant  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Treatment Method** & **COD Removal Efficiency [\%]** & **Specific Compounds’ Removal** & **References** \\ \hline Coagulation-flocculation & 59 & X & [35] \\ Coagulation-flocculation & 86–88 & X & [36] \\ Coagulation-flocculation & 94.1 & X & [37] \\ Batch reactor (carbon electrodes) & 66 & X & [38] \\ Electrooxidation (Ti/Pt anode) & 81 & X & [39] \\ Electrooxidation (Ti/IrO\({}_{2}\) anode) & 75 & X & [40] \\ Photo-Fenton process (solar radiation) & 99.5 & X & [41] \\ Electro-thermochemical process & 95–99 & VOCs & This study \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of different treatment processes used for paint-industrial effluents.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Identifed Compounds** & **REFi (TW1) [\%]** & **REFi (TW2) [\%]** & **REFi (TW3) [\%]** & **REFi (TW4) [\%]** \\ \hline Toluene & 55 & 80 & 78 & 90 \\ Ethylbenzene & 84 & 91 & 86 & 94 \\ Xylenes & 43 & 47 & 89 & 90 \\ Chlorobenzene & 90 & 96 & 81 & 100 \\ \hline \hline \end{tabular} Note: TW1—dilution 1:1 (sample: deionized water _V:V_), working electric current: 20 A; TW2—dilution 1:1 (sample: deionized water _V:V_), working electric current: 30 A; TW3—dilution 1:3 (sample: deionized water _V:V_), working electric current: 20 A; TW4—dilution 1:3 (sample: deionized water, _V:V_), working electric current: 30 A.  
\end{table}
Table 3: Removal efficiency factors (REFi) for the identified VOCs for studied water treatment technology procedures (TW).  
removal was obtained for all of the studied compounds. On the other hand, when the sample dilution ratio was 1:1 (Treatment 1 and 2), moderate removal was acquired for Toluene and the sum of the xylenes. In all cases, the REFs were significantly higher when the working electric current of 30 A was applied during the treatment procedure for both dilution ratios. For all the VOCs, significant removal was shown in the case of procedure 4. It can be observed that the worst results were obtained when the sample dilution ratio was 1:1 and 20 A of the working electric current was applied. Compared with untreated industrial water, the peak areas of the studied compounds decreased in all cases (Figure 4).