## 3 Nitrate and Bacterial Contamination in Dairy Cattle Drinking Water
### Filter Media Options for Adsorption  
Filtration media selection is important to optimize the targeted removal of both nitrate and bacteria, affecting the pollutant removal efficiency as well as the overall cost of the treatment system. Several materials have been optimized for nitrate or bacteria filtration via chemical or physical treatment (i.e., activated carbon modified with aluminum hydroxchloride for improved bacteria removal). Further, chemically activated biochar has been found to be suitable for nitrate adsorption [12; 45]. While these materials are promising and may be more effective than natural materials, they are currently either not economically viable, commercially available, or both. High media costs--and thus overall treatment costs--may be a barrier for implementing water quality improvement measures. Choosing media for filtration that is commercially available, low cost, natural, and safe for cattle is important for improving the likelihood of farmers choosing filtration as a treatment option for improving the water quality for the health of their cattle. Activated charcoal (AC) is a commonly used filter medium for a wide number of applications because of its high surface area, porosity, and large number of active sites. The granular (>0.2 mm particles) and powdered forms (<0.2 mm particles) are commonly used to filter out organics, heavy metals, and factors affecting taste and odor in wastewater but are not often used for nitrate removal in conventional water treatment facilities [46; 47]. Raw activated carbon is initially negatively charged, which is not optimal for the adsorption of negatively charged nitrate ions or bacteria, but many different forms of physical or chemical activation and further modification methods have been explored at the lab scale for optimized nitrate and bacteria removal with AC-based adsorbents [36; 45; 48]. Commercially available forms are commonly activated with heat and steam or chemicals including zinc chloride, phosphoric acid, sodium hydroxide, and potassium hydroxide, which can still achieve acceptable levels of nitrate removal for animal drinking water applications [49]. For bacteria removal, activated charcoal has been studied and found to be effective, but long-term efficiency has been found to decrease, and in some instances, the AC can even become a source of bacterial contamination itself [45]. The removal efficiency for nitrate has also been shown to decrease over time, necessitating periodic replacement, as is true with most filter media [48]. Activated charcoal has been explored as a soil amendment, however, or even as an additive to bedding or liquid manure lagoons, which could provide a convenient means of disposal of the media after use [50]. Sand is another commonly used natural, cheap, and readily available filtration medium used for water treatment. In conventional water treatment plants, it is often used within a multi-media filtration system designed to filter out suspended solids and inorganics, and in lab-scale experiments, it has been used in conjunction with other media such as biochar to filter nitrate and reduce the need for backwashing [51; 52; 53]. Slow sand filters have been used to filter nitrates via biological denitrification. The low flow rate (0.18 L/min) allows for the growth of a biofilm capable of reducing nitrate as well as filtering out pathogens [54; 55]. Bentonite clay is another medium that is commonly used for adsorption within the context of water treatment. Raw forms have not been found to have high nitrate adsorption rates specifically, but bentonite is commonly used to prevent nitrogen leaching from lagoons on dairy farms, and modified forms have been able to achieve nitrate adsorption. Some studies have also evaluated it for its ability to filter pathogen indicators such as _E. coli_[12; 56; 57; 58; 59]. Additionally, bentonite has also been used as a soil amendment to enhance soil water and carbon retention, so it could be disposed of and recirculated easily onsite at a dairy farm [57; 58; 59].