## 4 Discussion  
### Effect of KOH Activation of Hydrochar on E. coli Removal  
The performance of an adsorbent mainly depends on its surface, which provides internal pore structure and adsorptive sites [30]. Several attractive or repulsive forces between _E. coli_ cells and the adsorbent surface regulate their mutual interaction [31]. Considering the characteristics of _E. coli_, the adsorptive removal will be facilitated when the adsorbent possesses a highly porous surface with a positive (less negative) surface charge and hydrophobic properties. It has been reported that the KOH activation of hydrochars derived from plant materials increased the surface roughness, resulting in enhanced heavy metal or _E. coli_ removal from artificially contaminated influents [17; 19]. However, the BET and SEM analyses carried out in this research could not explain the improvement in _E. coli_ removal efficiency induced by the KOH activation. The modification in the surface morphology of the hydrochar by the KOH treatment was insignificant (Figure 4). Moreover, the specific surface area of the activated hydrochar was 27% less than that of the raw hydrochar (See Section 3.1.4). A possible explanation for the decrease in the specific surface area derived from the KOH activation and subsequent washing processes can be the collapse of micro and mesopores, resulting in \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Pore Volume**} & \\ \cline{2-3}
**Content** & \multicolumn{2}{c}{**First**} & \multicolumn{2}{c}{**Second**} \\ \cline{2-3}  & **24 h idle** & **72 h idle** & \\ \hline Sand & 52.1 \(\pm\) 13.3 (\(n\) = 46) & 66.9 \(\pm\) 14.7 (\(n\) = 12) & 17.2 \(\pm\) 8.64 \(\lx@math@degree\) (\(n\) = 60) & 36.5 \(\pm\) 10.1 (\(n\) = 60) \\ Raw & 23.0 \(\pm\) 17.3 \(\dagger\) (\(n\) = 44) & 35.1 \(\pm\) 12.9 (\(n\) = 10) & 20.0 \(\pm\) 12.5 \(\lx@math@degree\) (\(n\) = 56) & 24.4 \(\pm\) 10.5 \(\dagger\) (\(n\) = 56) \\ Activated & 92.8 \(\pm\) 5.0 \(\ddagger\) (\(n\) = 46) & 92.6 \(\pm\) 8.0 \(\ddagger\) (\(n\) = 12) & 90.0 \(\pm\) 9.1 \(\ddagger\) (\(n\) = 60) & \(\pm\) 7.5 \(\ddagger\) (\(n\) = 60) \\ \hline \hline \end{tabular}  
Within each column, values followed by the symbol \(\lx@math@degree\) are not significantly different using [PERSON]’s test at \(p\) \(<\) 0.05. Within each row, values followed by the same symbol \(\dagger\) or \(\ddagger\) are not significantly different using [PERSON]’s test at \(p\) \(<\) 0.05.  
\end{table} 
Table 2: Removal efficiency (average% \(\pm\) standard deviation) of _E. coli_ in large column experiments during 30 days of intermittent flushing in duplicate columns of each treatment. Figure 6: _E. coli_ removal efficiencies from large columns during 30 days of flushing with a flow rate of 33.3 mL (1.35 cm)/min. the development of a macroporous structure on the hydrochar surface. However, a large surface area of an adsorbent, mainly consisting of micro (<2 nm) and mesopores (2-50 nm), may not be a direct indicator for efficient _E. coli_ removal, considering the size _E. coli_ cells (\(\sim\)2 \(\upmu\)m). Further investigations on the surface area and surface properties of the macroopres are thus recommended in order to obtain a better understanding of the _E. coli_ removal mechanisms by (KOH treated) biochar. Because the investigations carried out to compare the characteristics of raw and activated hydrochar showed only minor differences, the improved _E. coli_ removal performance obtained from the KOH activation could result from the increase in hydrophobicity of the hydrochar surface, which enforces the hydrophobic attraction. Hydrochar consists of a hydrophobic core and a hydrophilic outer surface [26; 32]. The KOH activation carried out in this research would have brought more hydrophobic surfaces into contact with _E. coli_ cells by removing the hydrophilic surface coatings formed by recondensation and repolymerization of water-soluble substances during the HTC process [27].