## 5 Conclusions  
The project expanded on laboratory-scale research to design a full-scale passive treatment system for removing PO4\({}^{3-}\) from a tributary before it entered a drinking water reservoir. The system was comprised of eight treatment gates, with each gate containing four cartridges filled with approximately 11.35 kg of WTR. The cartridges were 3D printed from food-grade certified, biocompatible polycarbonate. Cartridges were installed in later April and early May and removed at the end of October. One season of service saw removal of 556.31 g PO4\({}^{3-}\) or approximately 3.68 g PO4\({}^{3-}\) removed/d. The gate system provided PO4\({}^{3-}\) sequestration in-stream and removed all sequestered PO4\({}^{3-}\) from the system when the media was refreshed. While there is room to improve gate, cartridge, and mesh bag design, the use of WTR to bind and remove PO4\({}^{3-}\) in a stream upstream of a drinking water reservoir shows incredible promise for controlling nutrient loading into water bodies by removing pollution upstream. This system has the potential for utilization in streams with known phosphorus issues, and provides measurable results showing the actual reduction in PO4\({}^{3-}\) from the water system. The novelty is a refillable, rechargeable system provides in-stream nutrient control that, with minimal upkeep, provides nutrient sequestration without permanently adding material to the waterways, reducing sedimentation and accelerated eutrophication of reservoirs and lakes.  
Though there are initial setup costs, once the system is in place the costs dramatically decrease. As the system relies on beneficially reused Al-WTR, which conventional WTPs pay to dispose, there is minimal cost associated with the procurement of the material, even when combined with the assumed costs associated with staff hours to collect, fill, and exchange cartridges. The system is affordable by even small utilities/communities, reduces accelerated sedimentation/ eutrophication associated with using alum or clay products for legacy phosphorus sequestration, provides measurable PO4\({}^{3-}\) removal, and permanently removes sequestered phosphorus from the watershed.  
The following are available on line at [[https://www.mdpi.com/article/10.3390/app11093743/s1](https://www.mdpi.com/article/10.3390/app11093743/s1)]([https://www.mdpi.com/article/10.3390/app11093743/s1](https://www.mdpi.com/article/10.3390/app11093743/s1)), Figure S1: Cartridge filled with Al-WTR placed in field 1 July-7 August 2019.  
Conceptualization: [PERSON], [PERSON], [PERSON]; methodology: [PERSON]; experiments: [PERSON]; validation: [PERSON], [PERSON], [PERSON]; writing--original draft preparation: [PERSON], [PERSON]; writing--review and editing: [PERSON], [PERSON]; funding acquisition: [PERSON], [PERSON]; project administration and supervision: [PERSON]. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by Ohio EPA 319 (Grant No. 17(H)EPA-17) and the Ohio Lake Erie Commission and Lake Erie Protection Fund (Grant No. 560-219).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** All relevant data are contained in the manuscript.  
**Acknowledgments:** The authors would like to thank [PERSON] for assistance with collecting and analyzing the field samples.  
**Conflicts of Interest:** The authors have no conflict of interest.