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November 2010 (vol. 5, issue 6)
Recommendation for Bioretention Redesign
Funded by the Minnesota Pollutation Control Agency
Bioretention facilities, or rain gardens, are a useful and aesthetically appealing option for managing storm water runoff. Rain gardens have been placed alongside roadways or next to buildings to allow rainwater to infiltrate and decrease peak runoff discharges. Infiltration of the storm water runoff through the rain garden media may also remove oils and greases, suspended solids, and heavy metals. Typically, these facilities have been constructed using engineered soils or a combination of sand and compost. The ease of acquiring rain garden media and the relative small size of a rain garden make implementing this management practice attractive. Many municipalities and other groups have encouraged both local homeowners and other site developers to build rain gardens. For example, Kansas City, Kansas and Melbourne, Australia have started initiatives to build 10,000 rain gardens, respectively. Thus, the number of rain gardens in the U.S. is expected to grow rapidly in the coming years.
The effectiveness of organic, leaf-based compost at simultaneously removing heavy metals, such as cadmium, copper, and zinc, is currently being studied at the St Anthony Falls Laboratory (SAFL). A small amount of compost was mixed in a bottle with a synthetic storm water solution whose chemistry (e.g., pH) matches what one would find in storm water runoff. After initial kinetic studies (e.g., Figure 1), the dosage of compost was varied to obtain sorption isotherms. The sorption results were then used to determine an estimated life span of a typical rain garden.
Figure 1: Cadmium removed from 250 mL of synthetic stormwater by 1 gram of compost (Ct=0 = 500 μg/L)
The top 6 inches of a rain garden, assuming a ratio of compost to sand of 30/70% by volume, can effectively remove cadmium for 80 years assuming typical precipitation levels for Minneapolis, MN and storm water cadmium concentrations reported in the literature. Likewise, the top 6 inches of a rain garden will last 145 years removing zinc from storm water runoff. Thus, the compost in rain gardens should be very effective at removing dissolved toxic metals. Other studies, including ongoing work in our laboratory, found that rain gardens can also remove petroleum hydrocarbons.
Unfortunately, the material used to construct rain gardens may not be suitable for removing all necessary storm water pollutants. For example, we also observed that compost may be a source of phosphorus, which is the target of many storm water management practices. A batch experiment was performed in which a small amount of organic, leaf-based compost was added to a synthetic stormwater and sampled over time. Figure 2 shows that phosphorus leaches off of compost and that the longer the compost is in contact with water, more phosphorus is released from the compost.
Figure 2: Phosphorus release from 1.0 g of compost added to 250 mL of synthetic stormwater.
So if compost leaches phosphorus, then there is a concern that the released phosphorous may eventually make its way into surface waters. Fortunately, there is a solution to the leaching of phosphorus. Work done at the St. Anthony Falls Laboratory by Andrew Erickson demonstrated that filtration with sand amended with iron filings is effective at removing phosphorus from stormwater. A sand filtration basin amended with 5% iron filings, by weight, may remove at least 80% of total phosphorus for up to 30 years of service under a hydraulic loading rate of 5.6 m/yr (Figure 3). There are other enhancements currently being tested, but so far iron has proved to be the most cost-effective solution.
Figure 3: Phosphorus Removal efficiency of mixtures of iron and sand.
It is thus possible to combine the results from both of these investigations into a hybrid bioretention system that removes both dissolved metals and phosphorus. Bioretention facilities can be designed as a two-stage system where the top layer is constructed of compost amended sand and the bottom layer is comprised of iron-enhanced sand (Figure 4). An appropriately sized layer of filter fabric may need to be placed between the two layers to minimize mixing of the two materials. Storm water runoff will infiltrate through the compost-amended sand, where the suspended solids and dissolved metals will be removed. The water will then infiltrate through the iron-enhanced sand where any phosphorus in the storm water runoff and any phosphorus that has leached off of the compost will be removed by the iron filings. Lastly, adequate oxygen is needed to continuously oxidize the iron filings to produce the Fe(III) required for phosphorus removal. If the dissolved oxygen in the stormwater reaching the iron-enhanced sand layer is inadequate, then an underdrain that is open to the air is needed.
Figure 4: Proposed design of a bioretention facility.
Rain gardens can be an effective best management practice for treating storm water runoff. When designed and built properly, they can be aesthetically appealing and long lasting. The proposed two-stage rain garden design can filter suspended solids, remove dissolved metals and hydrocarbons in the upper layer, and remove dissolved phosphorus in the lower layer. The key is proper media selection, installation, and maintenance so that rain garden will continue to infiltrate water quickly throughout the desired lifespan of the installation. Overall, rain gardens appear to be effective pollution prevention devices.
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- How do you envision performance changing from a simple batch test to a more realistic experiment to real-life application?