## 3 Results and Discussion
### Effect of Feed Concentration on Breakthrough Curve  
The performance of the adsorbent at different feed concentrations was tested, as shown in Figure 4 and Table 5, to evaluate the possibility of implementing the adsorption system at different locations in the WWTP. Experiments with a feed concentration of 10 mg P/L were conducted to evaluate the viability of implementing this process after chemical precipitation to form struvite [35], combining two processes (precipitation and adsorption) to recover P and reduce its concentration to the minimum possible.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Run**} & \multicolumn{2}{c}{\(FSB\)} & \multicolumn{2}{c}{\(FSB\)} & \multirow{2}{*}{\(\Delta t_{ads}\) (min)} & \multirow{2}{*}{\(\Delta t_{ads}\) (min)} & \multirow{2}{*}{\(RE_{Atds}\) (\%)} \\  & **Experimental** & **Predicted** & & & **Experimental** & **Predicted** & **(\%)** \\ \hline
1 & 0.309 & 0.309 & 0 & 122 & 121 & 0.82 \\
2 & 0.425 & 0.405 & 4.70 & 126 & 138 & 9.52 \\
3 & 0.436 & 0.455 & 4.36 & 196 & 185 & 5.62 \\
4 & 0.149 & 0.157 & 5.37 & 104 & 104 & 0.00 \\
5 & 0.239 & 0.253 & 5.88 & 131 & 121 & 7.63 \\
6 & 0.325 & 0.303 & 6.77 & 158 & 168 & 6.33 \\
7 & 0.067 & 0.058 & 13.4 & 84.9 & 85.9 & 1.18 \\
8 & 0.149 & 0.154 & 3.36 & 105 & 103 & 1.90 \\
9 & 0.201 & 0.204 & 1.49 & 148 & 150 & 1.35 \\ \hline
10 & 0.204 & 0.167 & 18.2 & 119 & 98.3 & 17.4 \\
11 & 0.352 & 0.334 & 5.11 & 155 & 144 & 7.09 \\ \hline Relative error: \(RE=\left|\frac{\sum_{\text{exp}}^{\text{exp}}-N_{pred}}{\sum_{exp}}\right|\).100. & & & & & \\ \end{tabular}
\end{table}
Table 4: Experimental and predicted results for \(FSB\) and \(\Delta t_{ads}\).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & \multicolumn{2}{c}{\(C_{0}\) (mg P/L)} & \\  & **1 (Run 12)** & **10 (Run 2)** & **40 (Run 13)** \\ \hline \(t_{bp}\) (min) & 139 & 40.4 & 3.34 \\ \(t_{ex}\) (min) & 329 & 166 & 52.9 \\ \(t_{st}\) (min) & 228 & 101 & 23.7 \\ \(\Delta t_{ads}\) (min) & 189 & 126 & 49.5 \\ \(FSB\) & 0.612 & 0.425 & 0.141 \\ \(q_{e}\) (mg P/kg) & 17.5 & 70.9 & 60.1 \\ \(MTZ\) (cm) & 17.3 & 22.7 & 28.1 \\ \(LUB\) (cm) & 11.6 & 17.3 & 25.6 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Parameters calculated to assess the process performance for different initial P concentrations [\(Q\) = 5 mL/min; \(h\) = 30 cm; \(20\pm\) 1 \({}^{\circ}\)C].  
As expected, the breakthrough time decreased sharply from around 140 to 3 min as the P concentration increased from 1 to 40 mg/L. The breakthrough curve becomes steeper, and the adsorbent reaches saturation faster, resulting in shorter exhaustion times. This behavior is likely due to the reduced mass transfer resistance at higher P concentrations [25; 34; 36]. In addition, the _FSB_ value is low when the column is fed at an initial concentration of 40 mg P/L. Around 85% of the bed is not used during the process. Thus, under these conditions, the adsorption process using CES700 is not advantageous. Its application is recommended for streams with concentrations between 1 and 10 mg P/L to maximize the efficiency of the adsorbent. The most effective way to apply this technique to WWTP is through its integration with chemical precipitation, implementing adsorption as a complementary step in a combined strategy.  
Regarding the influence of coexisting SO4\({}^{2-}\) ions, it was not possible to determine a breakthrough curve due to the difference in the magnitude of the ion concentrations. The inlet and outlet concentrations of SO4\({}^{2-}\) were the same for about 300 min of the experiment (\(Q\) = 5 mL/min, \(h\) = 30 cm), contrary to what was found previously in batch experiments [18]. The observed difference in SO4\({}^{2-}\) interference between the batch and fixed-bed experiments may be due to the longer contact time in batch systems, which allows greater competition between sulfate ions and phosphate ions. In contrast, continuous flow and shorter contact times in fixed-bed systems may limit such competition. However, further studies are required to understand this behavior better.