*3.5. E*ff*ect of RSPRC Distribution in Reactor on Phosphorus Removal*

As mentioned in Table 6, phosphorus removal performance reached a low point under experimental condition a. The concentration of effluent P amounted to 0.34 mg/L and the phosphorus removal rate was merely 32%. Based on further analysis, the adsorbent was concentratedly distributed between adjacent diversion walls. Although the adsorbent can absorb phosphorus in the nearby wastewater with a higher efficiency, overall, P concentration of the wastewater is lower, and the solute concentration gradient affecting the diffusion rate is smaller. Therefore, the phosphorus in the wastewater is diffused with difficulty and it is unable to sufficiently contact the adsorbent. Ultimately, the adsorption capacity of RSPRC is not fully utilized [31,32]. Under the experimental condition d, the equal spacing of the adsorbent is evenly distributed between the adjacent diversion walls, which increases the contact area with the sewage. As a result, the phosphorus adsorbate in wastewater can be efficiently absorbed [33] and the role of the adsorbent can be completely utilized to increase the unit adsorption amount of the adsorbent. The effluent P concentration in experimental condition d was the lowest at only 0.08 mg/L and the phosphorus removal rate reached up to 84%. Ragheb observed that the removal of P increased with the increase in rpm to some extent and attributed it to the dispersal of the adsorbent particles in the aqueous solution, which leads to a reduced boundary mass transfer [34]. The dispersive distribution of the adsorbent increases the removal rate, whose mechanism is similar to that of the increase in rpm. In an attempt to achieve the best static adsorption phosphorus removal result in the concentric circular diversion wall adsorption reactor, the experimental condition d was selected as the adsorbent distribution in the subsequent experiments.


**Table 6.** Effect of experimental condition in reactor on phosphorus removal.

## *3.6. E*ff*ect of RSPRC Dosage in Reactor on Phosphorus Removal*

Figure 9 presents the experimental data on the effect of RSPRC dosage in the reactor on phosphorus removal. Interestingly, it was found that the P removal rate was almost the same as that of the former agitation experiment. However, Rastas and Hedstrom hold the view that the acquired absolute values for adsorption capacities from agitation experiments cannot be extrapolated to practical applications. They explained that the direct contact between the grains and solution differs and the agitation of the blast furnace slag may cause the destruction of the material, which may increase the sorption sites and thus the sorption capacity could be overestimated [35]. A possible explanation for the difference might be that the reactor realizes multi-stage series adsorption, which improves the removal rate and makes up for the effect of insufficient contact. With the increase in initial P concentration, the concentration of effluent phosphorus decreases when the dosage of adsorbent increases. If the initial P concentration is larger, the trend tends to be more obvious [30,36]. Further analysis indicates that when the P concentration in the wastewater is reduced to a certain extent, that the phosphorus removal performance of the adsorbent in the wastewater begins to decrease. On the condition that the P concentration drops to a very low value (0.36 mg/L), the effect of the adsorbent in this study is greatly inhibited, and it is impossible to effectively reduce the P concentration in the wastewater.

effluent is.

**Figure 9.** Effect of RSPRC dosage in the reactor on the effluent phosphorus concentration. **Figure 9.** Effect of RSPRC dosage in the reactor on the effluent phosphorus concentration.

#### *3.7. E*ff*ect of HRT in Reactor on Phosphorus Removal*

*3.7. Effect of HRT in Reactor on Phosphorus Removal*  The results of the effect of hydraulic retention time in reactor on phosphorus removal are presented in Figure 10. In cases where the HRT increases, the phosphorus removal rate increases significantly at a low adsorbent dosage, but with an increasing adsorbent dosage, the increase in the phosphorus removal rate of the adsorption reactor slows down [26,37]. The trend is similar to that of agitation experiments. Lu et al. achieved the same result. They explained that the rapid initial removal was mainly attributed to the precipitation of phosphate with exchangeable and dissolved Ca2+ rather than the adsorption. The liberated Ca2+ from the exchange site or from the dissolution of CaCO3, CaO and Ca(OH)2 were preferably precipitated by phosphate, which gave a high initial rate of adsorption. The exact contribution of the adsorption and precipitation phases to the removal of phosphate remains unclear [38]. Another possible explanation might be that when the HRT is short, the dosage of the adsorbent selected in the experiment cannot balance the adsorption reaction in the wastewater. In other words, the P concentration differs greatly from the equilibrium state. Although the reaction effect of the adsorbent has declined, the removal efficiency has not been excessively inhibited. An earlier study revealed that when the HRT is long, the instantaneous P concentration of the wastewater approaches the equilibrium state of the reaction, where the function of the adsorbent is obviously suppressed [39,40]. In cases where the dosage increases, the removal rate of phosphorus in the wastewater is still extremely slow. This is reflected in the fact that the longer the hydraulic The results of the effect of hydraulic retention time in reactor on phosphorus removal are presented in Figure 10. In cases where the HRT increases, the phosphorus removal rate increases significantly at a low adsorbent dosage, but with an increasing adsorbent dosage, the increase in the phosphorus removal rate of the adsorption reactor slows down [26,37]. The trend is similar to that of agitation experiments. Lu et al. achieved the same result. They explained that the rapid initial removal was mainly attributed to the precipitation of phosphate with exchangeable and dissolved Ca2<sup>+</sup> rather than the adsorption. The liberated Ca2<sup>+</sup> from the exchange site or from the dissolution of CaCO3, CaO and Ca(OH)<sup>2</sup> were preferably precipitated by phosphate, which gave a high initial rate of adsorption. The exact contribution of the adsorption and precipitation phases to the removal of phosphate remains unclear [38]. Another possible explanation might be that when the HRT is short, the dosage of the adsorbent selected in the experiment cannot balance the adsorption reaction in the wastewater. In other words, the P concentration differs greatly from the equilibrium state. Although the reaction effect of the adsorbent has declined, the removal efficiency has not been excessively inhibited. An earlier study revealed that when the HRT is long, the instantaneous P concentration of the wastewater approaches the equilibrium state of the reaction, where the function of the adsorbent is obviously suppressed [39,40]. In cases where the dosage increases, the removal rate of phosphorus in the wastewater is still extremely slow. This is reflected in the fact that the longer the hydraulic retention time is, the smaller the influence of the adsorbent dosage on the P concentration of the effluent is.

retention time is, the smaller the influence of the adsorbent dosage on the P concentration of the

**Figure 10.** Effect of HRT in the reactor on the effluent phosphorus concentration. **Figure 10.** Effect of HRT in the reactor on the effluent phosphorus concentration.

#### **4. Practical Applications and Future Research Perspective**

**4. Practical Applications and Future Research Perspective**  It is economically feasible to use RSPRC as the adsorbent for the deep dephosphorization of municipal wastewater. The self-developed concentric circular diversion wall adsorption reactor can effectively and stably control the effluent phosphorus concentration, within 0.10 mg/L, and create no secondary pollution. In view of the deficiencies found in the research, combined with the practical needs of sewage treatment, these findings provide the following insights for future research. Firstly, further studies need to be carried out in order to modify the materials of the adsorbent to enhance the phosphorus removal performance. Secondly, it is necessary to optimize the material formulation, improve the manufacturing process, and increase the strength of RSPRC. Thirdly, the study should be repeated using different adsorbents in the self-developed concentric circular diversion wall adsorption reactor, which would be a fruitful area for further work. Finally, further research could usefully explore how to optimize the structure of the reactor so that the reactor can be flexibly changed in practical applications as required. It is economically feasible to use RSPRC as the adsorbent for the deep dephosphorization of municipal wastewater. The self-developed concentric circular diversion wall adsorption reactor can effectively and stably control the effluent phosphorus concentration, within 0.10 mg/L, and create no secondary pollution. In view of the deficiencies found in the research, combined with the practical needs of sewage treatment, these findings provide the following insights for future research. Firstly, further studies need to be carried out in order to modify the materials of the adsorbent to enhance the phosphorus removal performance. Secondly, it is necessary to optimize the material formulation, improve the manufacturing process, and increase the strength of RSPRC. Thirdly, the study should be repeated using different adsorbents in the self-developed concentric circular diversion wall adsorption reactor, which would be a fruitful area for further work. Finally, further research could usefully explore how to optimize the structure of the reactor so that the reactor can be flexibly changed in practical applications as required.

#### **5. Conclusions 5. Conclusions**

This study set out to prepare and apply RSPRC to remove phosphorus in the self-developed concentric circular diversion wall adsorption reactor. The second aim was to investigate the relationship between phosphorus removal effect and several influencing factors. One of the more significant findings to emerge from this study is that the addition of activated clay can make the adsorbent possess a good decolorization performance and effectively improve the phosphorus removal performance of the adsorbent. The second major finding was that both the pseudo-first order and pseudo-second order kinetic models can better reflect the adsorption kinetics of adsorbents in different initial P concentrations, but the latter are more accurate. This study has also found that the phosphorus removal rate is the highest on the condition that the adsorbent is evenly distributed between the adjacent diversion walls. The research results provide a valuable reference for practical wastewater applications, and a feasible method for the application of the adsorbent in the advanced treatment of wastewater. This study set out to prepare and apply RSPRC to remove phosphorus in the self-developed concentric circular diversion wall adsorption reactor. The second aim was to investigate the relationship between phosphorus removal effect and several influencing factors. One of the more significant findings to emerge from this study is that the addition of activated clay can make the adsorbent possess a good decolorization performance and effectively improve the phosphorus removal performance of the adsorbent. The second major finding was that both the pseudo-first order and pseudo-second order kinetic models can better reflect the adsorption kinetics of adsorbents in different initial P concentrations, but the latter are more accurate. This study has also found that the phosphorus removal rate is the highest on the condition that the adsorbent is evenly distributed between the adjacent diversion walls. The research results provide a valuable reference for practical wastewater applications, and a feasible method for the application of the adsorbent in the advanced treatment of wastewater.

**Author Contributions:** Conceptualization, Y.L.; methodology, Y.L. and L.Z.; software, Y.L. and L.Z.; validation, Y.L. and L.Z.; formal analysis, L.Z.; investigation, L.Z.; resources, Y.L.; data curation, L.Z.; **Author Contributions:** Conceptualization, Y.L.; methodology, Y.L. and L.Z.; software, Y.L. and L.Z.; validation, Y.L. and L.Z.; formal analysis, L.Z.; investigation, L.Z.; resources, Y.L.; data curation, L.Z.; writing—original draft preparation, L.Z. and R.P.S.; writing—review and editing, R.P.S. and L.Z.; visualization, Y.L. and L.Z.; supervision, Y.L. and R.P.S.; project administration, Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to show their gratitude to all those who helped during the experimental period and writing of this manuscript. L.Z. would like to gratefully acknowledge the help of Rajendra Prasad Singh, School of Civil Engineering, Southeast University. I appreciate his patience, encouragement and professional instructions.

**Conflicts of Interest:** The authors declare no conflict of interest.
