# Evaluation of Nitrogen and Phosphorus Removal
## Materials and Methods  
### Study Site Description  
A treatment system for silage bunker runoff (Figure 1) was installed in 2017 at the University of Vermont Paul R. Miller Research Complex in South Burlington, VT, a small dairy farm with 45 milking cows and an equine center used for research and teaching. The site is located within the Potash Brook watershed, which drains to Lake Champlain. The drainage area of the system is 2767 m\({}^{2}\) consisting of silage storage bunkers, where varying amounts of corn and hay is ensiled and stored under plastic tarps throughout the year, as well as a paved asphalt entry area for heavy equipment. Silage leachate and silage bunker runoff flow downslope away from the stored silage and are directed to the treatment system. A diversion sperm in the asphalt prevents other production area runoff from entering the treatment system. The system consists of a particle separating screen assembly, a sequence of three treatment tanks for containment of low flows, settling and aeration, woodchip bioreactors, and an earthen infiltration basin (Figure 2). Figure 1: Plan view of the silage bunker runoff treatment system located east of the silage storage bunkers. Treatment tanks are enlarged to show underground location. Storm-driven runoff from the bunker area enters the treatment system (Figure 2) through a stainless-steel screen assembly that has a series of three screens with mesh holes of decreasing size (7.2 cm\({}^{2}\), 3 cm\({}^{2}\), 0.79 cm\({}^{2}\)) to block silage particles and other large solids transported in the runoff. After the screen assembly, runoff and leachate flow through a series of three treatment tanks that overflow into each subsequent tank during storms. Tank 1 has a storage capacity of 7.57 m\({}^{3}\) and is intended for BOD removal, settling of organic solids, and to prevent clogging of the downstream system components. Tank 2 is a 3.76-m\({}^{3}\) MBBR tank equipped with a regenerative blower, which operates every other hour. This tank is oxygenated to allow for reduction of BOD and N transformations; specifically, mineralization of organic N to NH\({}_{4}\)+ and nitrification of NH\({}_{4}\)+ to NO\({}_{3}\)-. In May 2019, cylindrical media carriers were added to fill 40% of the Tank 2 size by volume (1514.2 L) which each have a diameter of 2.5 cm, height of 0.4 cm, and a projected surface area of 800 m\({}^{2}\)/m\({}^{3}\) (AnoxKaldnessTM-Veolia Water Technologies AB). Tank 2 was inoculated with 18.9 L of a liquid suspension containing ammonia oxidizing microbial strains (MICROCAT-XNL, Bioscience, Inc., Allentown, PA, USA) for establishment of nitrifying bacteria populations. A third 3.76-m\({}^{3}\) tank, Tank 3, is for settling of any remaining solids. Paired side-by-side denitrifying woodchip bioreactors referred to as East bioreactor (EB) and West bioreactor (WB) are each 9.1 m by 12.2 m with a depth of 1.4 m. The woodchip bioreactors are continuously saturated, the bottom, side walls, and center barrier splitting the two bioreactors are lined with 1.1-mm ethylene propylene diene monomer (EPDM) and contain mixed hardwoododchips (approximate size of individual chips: 5 cm \(\times\) 5 cm \(\times\) 0.6 cm). The species composition of the woodchips was 60% Ash (_Fraximus_ sp. ), 20% Yellow Birch (_Betula alleghaniensis_), and 20% Silver Maple (_Acer saccharium_), determined by a local woodchip supplier, and was based on available feedstock species at the time of construction. The bioreactors receive effluent that is split after leaving Tank 3 through two sets of pipes distributed across the top perimeter of each bioreactor. The effluent percolates vertically into the woodchips in each bioreactor. The bioreactors remain saturated and internal water depth is governed by an adjustable water level control structure. When the woodchip bioreactors discharge, their outlets flow through parallel 100-mm diameter underdrain pipes into an earthen infiltration basin.