What is the ZVI Filter?
The Zero Valent Iron filter/sand filter (ZVI) can effectively filter contaminants such as uranium and arsenic from contaminated water on the Navajo Nation. The ZVI/sand filter works because the iron binds to the metal contaminants, thus adhering to the iron oxides. Two separate case studies in Saskatchewan, Canada proved that this type of filter can effectively remove both uranium and arsenic from well water (Mcmartin et al., 2013), (Goettinger et al., 2010). Figure 1: Zero Valent Iron Filter (Asong et al.)
How Does it Work?
In order to maximize the removal of arsenic from a water source, the ratio of ZVI fillings to sand should be between 40:60 and 50:50 (Goettinger et al., 2010). Furthermore, arsenic can only be filtered from water if the reaction occurs in an anoxic environment (one that lacks oxygen) (Sunbaek et al., 2005). However, the maximal environment for uranium filtration is a 30:70 ZVI fillings to sand ratio and the oxidation reaction that filters the uranium must occur in an oxic environment (Mcmartin et al., 2013), (Goettinger et al., 2010). A new technology, nanoscale zero-valent iron, has also been proven to effectively filter both uranium and arsenic (Crane et al., 2011). The nanoscale iron has a higher surface area to volume ratio, and thus is more effective in binding to contaminant metals (Crane et al., 2011). The ZVI filter requires periodic replacement of parts, but can last for at least a decade based according to one study, and does not have an established lifespan (Phillips et al., 2010).
However, even with this new nanoscale technology, one problem eliminates the ZVI/sand filter as an option for large scale implementation. Since the ZVI/sand filter must be in an oxic environment for uranium filtration and an anoxic environment for arsenic filtration, the implementation of this solution in an area that is contaminated with both metals would require complex engineering and may not be financially favorable, especially on the larger scale. This problem arises from the need to first filter the uranium in an oxic environment, remove oxygen, then remove arsenic. As a result, the final project would have to be made from the equivalent of two ZVI filters, along with a pump that removes oxygen from the water between the uranium and arsenic filtration sections, which results in increased costs. However, in a smaller region, the ZVI filter remains more cost-effective than the KAF filter despite these added technical costs.
The estimated cost of this filter for a 150 person community is about $110,000 (USD). This estimated cost was obtained by combining costs for a uranium filter and arsenic filter based on the two Canadian case studies, factoring this quantity down to a community of 150 people, and adding the cost of a 10000 gallon water storage tank to store the water that has been filtered (Mcmartin et al., 2013), (Goettinger et al., 2010). The estimated costs of the ZVI filter was specifically evaluated by doubling the cost of a ZVI filter based on the Canadian case study, and added to the costs of a vacuum pump to create an anoxic environment for arsenic removal and a water storage and for storing filtered water. In addition, the cost of a small region of biological activated carbon was added to filter out any remaining biological contaminants.
Where Can This Filter Be Used?
On the Navajo Nation, the ZVI/sand filter could be implemented on a small and decentralized scale so costs could be minimized. It could be a favorable solution to the KAF filter in regions of the Navajo Nation with a lower population (fewer than 300 people) near a source of water. Furthermore, the filtration process for a ZVI/sand filter is significantly slower than other similar filters, and requires a local source of water. Based on these restrictions, the ZVI filter could be a preferable option for very small towns in the Shiprock Agency and the northern region of the Western Agency. Because uranium removal requires an oxic environment and arsenic removal needs an anoxic environment, the filter would work best in an area that only needs filtering of one of the contaminants.