## 3 Nitrate and Bacterial Contamination in Dairy Cattle Drinking Water
### Water Treatment Options  
Many treatment methods are available for the removal of nitrate and bacterial contamination for drinking water applications, all with their benefits and drawbacks, making some methods better suited for different applications than others. Lactating dairy cattle have high water consumption rates, requiring an average of 45 to 128 L--even up to 189 L of water per day, depending on multiple factors such as ambient temperature, water content in feed, and milk production [32]. Troughs usually service approximately 20 cattle, so a trough should supply roughly 2649 L to 3785 L per day [33]. Point-of-use treatment methods for dairy cattle drinking water, therefore, would have to be adaptable for high flow rates in addition to being low-cost and low-maintenance to increase appeal for farmers. Some of the most common treatment methods for removing nitrate ions from water include distillation, ion exchange, RO, and electrodialysis. Distillation is the energy-intensive process of evaporating water and collecting the condensed water that is free of many contaminants including nitrate, bacteria, and some viruses as well [34]. While it produces high-quality water and is available for point-of-use systems, it is expensive and slow. Ion exchange is another treatment option for the removal of nitrate and other unwanted ions, and it can be scaled up or down for different capacity levels. Some of the drawbacks of this method, however, are that it requires the disposal of wasted brine solution, is subject to reduced removal efficiency with the presence of competing ions, requires chemical additions, and is not effective at removing bacteria [35]. The RO process uses pressure to force water through a semipermeable membrane and can remove both nitrate and bacteria (and many other contaminants) in both large- and small-scale applications, but it is usually energy intensive and expensive [36; 37]. In addition to the aforementioned physical removal methods, biological denitrification provides another treatment method. It is currently a less feasible option for treating water for several reasons including high maintenance requirements due to the addition of  
Figure 2: Flowchart of major inputs of nitrate and bacteria in cattle drinking water on dairy farms and their acute and long-term impacts. substrates and nutrients. Additionally, it is not a well-established treatment method, and it does not provide a method for bacterial removal, so it would need to be combined with another treatment method for animal drinking water applications. Similar to biological denitrification, chemical denitrification offers a treatment mechanism that does not physically remove nitrate into a separate waste stream and instead relies on reducing nitrate to less harmful forms, such as nitrogen gas. The chemical treatment method, however, has not been developed at full scale, depends on pH adjustments, and does not remove bacterial pollutants [2]. For dairy cattle drinking water sources with high nitrate concentrations, the most common recommendation is to change the water source or dilute the water with cleaner sources because options like distillation and RO may be too expensive [10]. While this may be feasible for cows drinking from surface waters, drinking water sourced from groundwater might not be easily changed or supplemented. Within large-scale water treatment plants, bacterial contamination is usually treated with the addition of chemical disinfectants such as chlorine but can also be treated using ozonation or ultraviolet (UV) disinfection [38]. Some of these same treatment options are available for treating individual drinking water troughs. It is recommended that farmers clean troughs regularly as infrequent cleaning allows bacteria to breed, but disinfect chemicals can also be added [9]. Some of the suggested disinfectants include bleach, sodium dichloroisocyanurate, calcium hypochlorite, and chlorine dioxide, and it is recommended to disinfect troughs at least twice a year.