# Evaluation of Nitrogen and Phosphorus Removal
## 5 Conclusions  
The design of this treatment system is novel, as it is the first reported combined MBBR system and woodchip bioreactor study treating silage bunker runoff. As a whole, the treatment system successfully reduced TN mass load by 76% throughout the study duration. Most reduction of TN took place in the bioreactors, compared to the tanks, despite low nitrate concentrations entering the bioreactors. DRW may have been the primary driver for N transformations and removal. While the system's design specifically targeted treatment of N species, moderate P removal was also observed; 26% of the TP mass load and 19% of SRP mass load over the sampling season. Overall, the system was an effective treatment option for high concentrations of N and P in the influent silage bunker runoff evaluated during the third year of operation. Other treatment options or treatment tank designs should be considered prior to runoff entering the bioreactor to maximize the transformation of N into \(\text{NO}_{3}{}^{-}\). To increase hydraulic retention time and nitrification performance of the treatment tanks, future designs could consider a decreased flow rate through the system that incorporates substantial storage up-stream of the treatment tanks. Simple hydraulic control (e.g., an orifice) could then be used to slowly release this stored runoff through the treatment system in between storm events at a rate that would allow for full nitrification to occur. Such modifications would likely improve performance but could also increase installation costs. Longevity of the woodchips should be monitored on an annual basis to ensure a readily available source of C for denitrification. Issues of unequal flow splitting and extra unintended bypass to the high flow path around treatment tanks highlight the importance of proper operation and maintenance of silage bunker runoff treatment systems. Areas for solids settling that can easily and frequently be cleaned out are critical to effective performance [6]. In other water-quality contexts, where N is already in the form of NO\({}_{3}\)\({}^{-}\), this combined MBBR and woodchip bioreactor system may also offer a promising solution. The following are available online at [[http://www.mdpi.com/2076-3417/10/14/4789/s1](http://www.mdpi.com/2076-3417/10/14/4789/s1)]([http://www.mdpi.com/2076-3417/10/14/4789/s1](http://www.mdpi.com/2076-3417/10/14/4789/s1)). Flow injection analysis methods used for water quality analysis. Table S2: Average of all event mean concentrations (EMC) in 2019 sampled storm events (corresponding standard deviation and sample size in parenthesis), and percent concentration reductions for each part of the system (i.e., pre-treatment tanks with MBBR, WB & EB) compared to System Inflow. Conceptualization, J.W.F. and S.E.H. ; Data curation, J.C.S. ; Formal analysis, J.C.S. ; Funding acquisition, J.W.F. and S.E.H. ; Investigation, J.C.S. ; Methodology, J.C.S., J.W.F. and S.E.H. ; Project administration, J.W.F. and S.E.H. ; Supervision, J.W.F. and S.E.H. ; Validation, J.W.F. and S.E.H. ; Visualization, J.C.S. ; Writing--original draft, J.C.S. ; Writing--review and editing, J.W.F. and S.E.H. All authors have read and agreed to the published version of the manuscript. This research was funded by the University of Vermont College of Agriculture and Life Sciences, and USDA NIFA Hatch Grant VT-H02504. The authors would like to acknowledge [PERSON], [PERSON], and [PERSON] for laboratory assistance. [PERSON] was the graduate research assistant for this project in 2017 and provided recommendations for troubleshooting and improvement of the system. Authors are grateful for assistance with field and lab work from [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON]. Thanks to [PERSON] and staff at UVM Paul R. Miller Research Complex for maintenance of the system. AnoxKaldnessTM-Veolia Water Technologies AB donated media used in the MBBR tank. The authors declare no conflict of interest.