Large Community Filtration
Heavy uranium and arsenic contamination can be addressed through strongly basic anion exchange resins (AER). These resin layers are composed of positively charged polymers which attract negatively charged ions. At pH between 6-8, uranium exists in water primarily as a carbonate complex, which is negatively charged and has a tremendous affinity for strongly basic anion exchange resins (DeSilva, 2005). Anion exchange resins are only effective in filtering As(V), so if As(III) is present, it must be oxidized to As(V) prior to filtration. A manganese dioxide filter, which effectively oxidizes As(III) to As(V), could also be installed into the filtration system (Ohio Department of Health, 2012).
The anion exchange resin regeneration process is composed of four main steps: (1) backwash; (2) regeneration with brine; (3) slow water rinse; and, (4) fast water rinse (U.S. EPA, 2015). After cleaning the filter, the arsenic and uranium contaminants must be disposed of safely.
Anion exchange resin filters do not effectively eliminate microorganism contaminants, but UV light filters, which eliminate 99.9% of bacteria and viruses, can easily be attached to the filtration system (Clarke, 2006).
Figure 1: Anion Exchange Resin Filtration System (Original Diagram)
Region Specific Implementation Plan
One region in which this filtration system could be implemented is Bodaway, which is located in the Western Agency of the Navajo Nation. Bodaway has a population of approximately 1,704, according to the 2010 census (Navajo Nation Government, 2015). Although there is existing data on contaminant levels in wells throughout the Navajo Nation, all wells should be tested again for contaminants prior to filtration. Water from nearby contaminated sources, such as aquifers and wells, could be pumped into the filtration system, filtered of both arsenic and uranium, and stored in 3 15,000 gallon water tanks (see Figure 1 above). Although water resources are limited, some new wells can be drilled to obtain more water. The filtration system operates using a strongly basic anion exchange resin to filter both uranium and arsenic. (Clarke, 2006). Pumped water would first pass through UV light filters attached to the AER system to eliminate biological contaminants.
The pumps and filter could be powered using wind turbines, which are already being used on the Navajo Nation. Faucets could be attached to the tanks so that people could haul the clean water back to their homes. AER filtration systems can be implemented in any community across the Navajo Nation, as long as there are available water sources, such as wells, aquifers, rivers, and lakes. Water trucks could deliver water to homes which are far (outside of walking distance) from the central filtration system. 3 water trucks can supply water to Bodaway, each truck delivering water to houses within its respective region.
This implementation plan would effectively solve the contamination problems of people living relatively close to the AER filtration system, but would not be as effective for people living far from this system. Water trucks cannot operate 24 hours a day, and this solution is limited by the amount of available water in the region of Bodaway.
Cost of Implementation
This industrial anion exchange resin filtration system can filter water at 20 gallons/minute, which equates to 28,800 gallons per day (Samco, 2017). Assuming the average person uses about 50 gallons of water per day for all purposes, one system could supply water to approximately 576 people. For Bodaway, 3 separate filtration systems can supply water to the majority of people. One filtration system of this size and efficiency would cost around $200,000, so the total cost for the 3 systems is $600,000 (Samco, 2017). Each 15,000 gallon water storage tank would cost approximately $10,000 and a total of 9 storage tanks would be needed for the whole region of Bodaway. The UV light filters operate at approximately 5 gallons per minute and cost about $400 per system (Clarke, 2006). Therefore, 4 UV light filters must be installed in each AER system to maintain the 20 gallon per minute flow rate. Bodaway would need 12 UV filters, bringing the total to $4,800.
To power each filtration system and pump, a wind turbine could be used. The water pumps would take up most of the power, and a 3 HP pump can pump water at a rate well over the necessary 20 gallons per minute. For reference, 3 HP is approximately equivalent to 2.2 kW. A 10 kW wind turbine could power each system and would cost approximately $30,000, so the cost for 3 wind turbines is $90,000 (Windustry, 2012). Clean water can be delivered to homes using water trucks which are already being employed in areas of the Navajo Nation. A water truck with a 2,000 gallon storage capacity would cost approximately $40,000, so the total cost for 3 water trucks is $120,000 (Cosby Harrison Company, 2018). This places the total cost of implementing this solution plan at approximately $905,000 for the region of Bodaway. We estimate that annual maintenance costs for the AER filters would amount to about $3,000. Only about 2-3 workers, who could be Bodaway residents, would be needed to monitor each AER filtration system.
|Item||Unit Cost||Quantity||Total Cost|
|15,000 Gallon Storage Tank||$10,000||9||$90,000|
|UV Light Filter||$400||12||$4,800|