## 2 Results and Discussion
### Residual Wastewater  
This study also aimed to investigate the phenolic compounds present in the residual wastewater after the isolation of EO, and the results are shown in Table 3 and in Figure 3.  
In their study, [PERSON] et al. (2015) pointed out the lack of information on the comprehensive profiling of the residual biomass left after the production of EOs and the importance of utilising their bioactive compounds and further converting them into value-added products that can, potentially, be used in various industries. Our previous study reported on the phenolic compounds detected in the residual water left after EO extraction by conventional hydro-distillation of various parts of the sea fennel plant ([PERSON] et al., 2015). Similar to this  
\begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Component** & **HD** & **MHD** \\ \hline
1. & gallic acid & 0.03 \(\pm\) 0.01 & 0.02 \(\pm\) 0.00 \\
2. & protocatechic acid & 0.05 \(\pm\) 0.02 & 0.04 \(\pm\) 0.00 \\
3 & neochlorogenic acid & 7.92 \(\pm\) 0.04 & 4.48 \(\pm\) 0.11 \\
4. & \(p\)-hydroxybenzoic acid & 0.78 \(\pm\) 0.06 & 0.53 \(\pm\) 0.02 \\
5. & chlorogenic acid & 13.67 \(\pm\) 0.03 & 22.18 \(\pm\) 0.03 \\
6. & cryptochlorogenic acid & 7.75 \(\pm\) 0.05 & 5.17 \(\pm\) 0.02 \\
7. & caffe acid & 0.04 \(\pm\) 0.01 & 0.03 \(\pm\) 0.02 \\
8. & ferulic acid & 0.11 \(\pm\) 0.00 & 0.22 \(\pm\) 0.00 \\
9. & sinapic acid & 0.16 \(\pm\) 0.01 & 0.11 \(\pm\) 0.00 \\
10. & rutin & 0.35 \(\pm\) 0.00 & 0.34 \(\pm\) 0.01 \\ \hline \end{tabular}
\end{table}
Table 3: Phenolic profile (mg/g of dry extract) of residual wastewater extracts after essential oil extraction by hydro-distillation (HD) and microwave-assisted hydro-distillation (MHD) detected by high performance liquid chromatography (HPLC).  
study, chlorogenic acid was the dominant compound with a concentration of 13.67 mg/g dry extract in the HD sample and 22.18 mg/g in the MHD sample. In Figure 4. the chemical structures of the dominant compounds in all investigated samples are presented.  
Figure 3: HPLC chromatogram of residual wastewater extracts after essential oil extraction by hydro-distillation (**a**) and microwave-assisted hydro-distillation (**b**). Peak assignment is given in Table 3.  
The presence of this bioactive phenolic compound in sea fennel samples has also been reported by other authors [13; 14; 26; 27; 28; 29]. As expected, the other two compounds present in the samples at high concentrations were cryptochlorogenic acid and neochlorogenic acid. However, these two compounds were found in lower concentrations in the MHD residual wastewater compared to the samples from HD, as were the levels of \(p\)-hydroxybenzoic acid and sinapic acid, while the content of ferulic acid was higher in the MHD wastewater extract. This is in agreement with the conclusions of [PERSON] et al. [30] and [PERSON] et al. [26], who reported that chlorogenic acid and its isomers are almost the only class of phenolic compounds in sea fennel. Of the flavonoids, only rutin was detected, and its concentration was not significantly different between the HD and MHD samples. In general, the sum of extracted identified phenolics were higher in the MHD sample than in HD, 33.11 and 30.87 mg/g, respectively, also indicating that this novel technique produces by-products with a higher amount of useful phenolic phytochemicals.