## 3 Results and Discussion  
The SMAHS experimental results showed that the amount of nitrate removed and the volume of water treated per gram of adsorbent increased with flux (Table 2). This is mainly due to the increased amount of nitrate that entered the reactor per unit time, though at higher flux the contact time of solution with the adsorbent (retention time) was low and hence tended to produce reduced adsorption. Because of the decreased amount of nitrate that entered the vector per unit time at lower flux, the adsorption capacity decreased at lower flux. However, the reduction in adsorption capacity is less than 15% from 15 to 2.5 L/m\({}^{2}\) h. The performance of adsorbents was compared in terms of adsorption capacity and the volume of water treated per gram of adsorbent. Figure 2 shows the changes of effluent concentration with time. The Dowex-Fe performed the best out of the four adsorbents. The experiments were conducted till the effluent reached the WHO limit of 11.3 mg N/L and the duration was higher for Dowex-Fe for all flux. The Dowex-Fe reached the WHO limit in 30 h for low flux (2.5 LMH) whereas the concentration exceeded the limit in 10 h for AG corn cob. At high flux, Dowex-Fe reached the WHO limit in 7 h and AG corn cob reached it in 2.5 h. These results show that the operation time until the WHO limit is reached depends on the adsorption capacity of the adsorbents and the flux. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Flux (L/m\({}^{2}\) h)**} & \multirow{2}{*}{**adsorbent**} & **Volume of Water** & **Nitrate Adsorbed/Mass** \\  & & **Treated (L)** & **of Adsorbent (mg N/g)** \\ \hline \multirow{4}{*}{15} & Dowex-Fe & 14.5 & 19.2 \\  & Dowex & 10.8 & 12.8 \\  & AG coconut copra & 9.3 & 12.2 \\  & AG corn cob & 7.8 & 9.0 \\ \hline \multirow{4}{*}{10} & Dowex-Fe & 13.8 & 18.6 \\  & Dowex & 11.8 & 12.7 \\  & AG coconut copra & 9.9 & 11.8 \\  & AG corn cob & 7.9 & 8.8 \\ \hline \multirow{4}{*}{5} & Dowex-Fe & 13.2 & 18.1 \\  & Dowex & 8.9 & 11.5 \\  & AG coconut copra & 8.9 & 11.7 \\  & AG corn cob & 7.9 & 8.8 \\ \hline \multirow{4}{*}{2.5} & Dowex-Fe & 11.4 & 16.4 \\  & Dowex & 8.9 & 11.2 \\ \cline{1-1}  & AG coconut copra & 7.4 & 9.8 \\ \cline{1-1}  & AG corn cob & 6.5 & 7.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Nitrate removal performance and volume of water treated for four adsorbents at four different flux. Table 2 shows that the performance was in the order Dowex-Fe > Dowex \(\geq\) AG coconut copra > AG corn cob. This order agreed with the adsorption capacity order obtained from a column mode study [14; 15] but not with the Langmuir adsorption capacities obtained from a batch mode study (Table 1). The continuous column study used the same influent nitrate concentration of 20 mg N/L as in the SMAHS study whereas the Langmuir adsorption capacities in the static batch study were the maximum adsorption capacities calculated at very high equilibrium concentrations. These differences can explain the reason for the similarities/differences observed in the order of adsorbent performance observed in SMAHS continuous mode and batch mode studies. The SMAHS model fit to the experimental data is satisfactory (Figure 2) as evident from the high correlation (R\({}^{2}\)) values (Table 3). Deviation of some experimental data points from the model values were noticed at the lowest flux (2.5 L/m\({}^{2}\) h) for AG coconut copra and AG corn cob (Figure 3). The correlation coefficients (R\({}^{2}\)) were also low for AG corn cob (0.735) and AG coconut copra (0.744) at this flux. This may be due to the low mass transfer that occurred at low flux. Also, the corn cob and coconut copra are natural materials which were modified by amine grafting. This may not have uniform adsorption sites. On the other hand, Dowex is a commercial product which has beeb more uniformly studied and thus it gave rise to a higher correlation coefficient. The model parameters are given in Table 3. The Ds values were virtually the same for all four flux for each adsorbent.