Biofiltration Applications

Biofiltration is a method capable of addressing both biological contaminants and arsenic contamination. Abandoned uranium mines have caused much of the water to exceed standard limits of both uranium and arsenic levels (Gies, 2016). These heavy metals are dangerous in consumption over Major Contamination Levels (MCL) of 10 µg/L for arsenic and 30 µg/L for uranium, as determined by the EPA in 2006 (Gies, 2016) .

Because current research has not found a significant link between this method of filtration and remediating uranium, we propose this as a method of filtration available on a smaller scale for use by communities that do not wish to utilize water from mechanical filtration, such as the Kanchan Arsenic Filter (KAF), anion exchange resin (AER) filter, or Zero Valent Iron (ZVI) filter, or whose water does not exceed uranium MCL levels.

Biofiltration uses hyperaccumulators, or plants that are capable of growing in soils with heavy metals and absorbing these metals through their roots to remove contaminants (Peer, 2005). We have determined that Typha Latifolia, or cattails, are the best plant option for the Navajo Nation because they are native to North America, including the Navajo Nation, and because they are effective in removing arsenic from water. Although they often live in wet climates, they have also been used in the Navajo Nation (B. Francis, personal communication, Sept. 14 2018). They can reduce the concentrations of arsenic by 89% (Markham, 2009) and also aid in the desalination of water (Janen, 2010).

How Bioswales Work

The diagram below outlines the procedure by which water is purified within a bioswale. Water from the pipe travels through rocks and is poured onto plants, which are usually specialized species of hyperaccumulator plants. These plants soak up certain contaminants in the water, leaving the water that leaches down into the sandy loam layer cleaner. The geotextile fabric filters out mulch and loam particles from the water, and the water exits the drain pipe.

(Taylor and Stamets, 2013)

Overview of Biofiltration Plan

  1. Pipe transports water from a river or lake to the bioswale.
  2. The contaminated water then runs through cattails to remove arsenic and biological contaminants. This occurs through intentional irrigation such as pumping onto bioswales. Alternatively, water hauled, collected from groundwater, or collected from rainwater can be poured onto bioswales.
  3. After soil is filtered out by the geotextile fabric (see figure above), water exits the bioswale through the outflow pipe and enters a well.
  4. People living nearby come and collect water for their individual households, much like how it is currently done in many areas.

Financial Considerations

Compared to other filtration systems such as reverse osmosis, biofiltration is relatively cheap. The cost of each bioswale is the sum of the price of the plants used in the bioswale, installation cost, and maintenance fee. The installation cost includes pipe material, construction, and required land.

Cattails are very cheap, costing less than $6 per 100 plants on Pond Megastore and less than $2 per 50 seeds from Seedville on Amazon, so the cost of plants used in the bioswale will be very small.

  • PVC pipes cost about $3-$4 per foot, but PVC pipes have lower quality than copper pipes. Copper pipes, which will last longer due to their strength, cost $10-$20 per foot on average according to FIXR. The implementation of either type of pipe requires a one-time installation fee.
  • Because the bioswales would sit on top of pre-existing wells or water collection sources in the Navajo Nation, the cost of additional land needed for the system is negligible.
  • After the first implementation, maintenance charges would not be much of a concern either. Cattails are predicted to be able to absorb arsenic for up to 50 years, so it’s recommended new cattails are planted just every ten years (Markham, 2017). 
  • One maintenance cost is the cost of disposing of cattails properly. The most feasible, environmentally sound, and economic method of disposal is incineration which destroys organic matter and releases metals. These metals are often available for recovery after incineration. The cost of incineration is between 200 and 250 $/1000 kg of biomass (Sas-Nowosielska et. al).
  • Bioremediation farms currently being used on a small scale in the Navajo Nation cost approximately $300 per farm (B. Francis, personal communication, Sept. 14, 2018).

Implementation Areas

We determined the following criteria in predicting places in the Navajo Nation where biofiltration would work best and be the most helpful for citizens.
  • If pre-existing wells are used, in order to benefit the largest number of people possible, bioswales should be implemented at wells that are used by the most people. In some cases, this means more densely populated areas.
  • If water from a river or lake is used, in order to minimize the cost, the bioswale must be built close to the body of water, as shorter distance means less pipe material used.
  • Cattails can’t filter out uranium, so it is safer to have different filtration systems that remove both uranium and arsenic near abandoned uranium mines. Therefore, we are not prioritizing the construction of bioswales near water with higher concentrations of uranium.

Based on the above criteria, using GIS mapping of abandoned uranium wells, density of wells, and population density, as well as population data (U.S. Census Bureau, 2010) and a map of rivers and lakes in the Navajo Nation (Gulch, 2016), we identified a few areas that we believe biofiltration would be most readily-implemented and impactful in. Within these listed areas, bioswales would be most feasible closest to the body of water or connected to a pre-existing well and most impactful for more densely populated areas along the water or the most-used wells.

Navajo Nation Chapter Agency Distance to Nearest Abandoned Uranium Mine Body of Water in Chapter Population Well Density
Leupp Western ~10 miles Little Colorado River 1,584 Relatively High
Upper Fruitland Shiprock ~30 miles San Juan River 2,934 High
Navajo Mountain Western ~50 miles Lake Powell 636 Very low

Diné Feedback

Because biofiltration uses natural resources, we believe that many Diné communities are more likely to embrace biofiltration than other industrial types of filtration such as Kanchan Arsenic Filter (KAF), Anion Exchange Resin Filter, or Zero Valent Iron (ZVI) Filter. Also, Diné have practiced traditional farming since the 1600s, (The Post-Pueblo Period: A.D. 1300 to Late 1700s) and created their own bioswales in the past (B. Francis, personal communication, Sept. 14, 2018) suggesting their willingness to implement this solution.