Soil amendments to enhance phosphorus sorption
Principal mechanisms for phosphorus (P) removal in bioretention are the filtration of particulate-bound P and chemical sorption of dissolved P (e.g. summarized in Hunt et al 2012). Most stormwater control measures (SCMs) capture particulate P by settling or filtration, but leave dissolved P (typically phosphates), which is most bio-available, untreated. This untreated P accounts on average for 45 percent of total phosphorus in stormwater runoff and can be up to 95% of the total phosphorus, depending, for example, on the storm event (Erickson et al 2012). Phosphorus sorbing materials contain a metal cation (typically di or trivalent) that reacts with dissolved phosphorus to create an insoluble compound by adsorption or precipitation or both (Buda et al 2012). Soil components and amendments that have been shown to be effective to increase chemical sorption of dissolved P include:
- Iron filings (Erickson et al 2012)
- Steel wool (Erickson et al 2007)
- Native iron rich soils such as those in the Piedmont of the Mid and Southern Atlantic USA (Hunt et al 2012), or Krasnozem soil in Australia (Lucas and Greenway 2011)
- Water treatment residuals (WTRs), which are a by-product of drinking water treatment and a source of aluminum and iron hydroxides (O’Neill and Davis 2012 a and b, Hinman and Wulkan 2012, Lucas and Greenway 2011, Lucas and Greenway 2010)
- Sorptive media (Imbrium) (Balch et al 2013)
Buda et al (2012) provide a literature review of P-sorption amendments. Characteristics of ideal P-sorption amendments include low cost, high availability, low toxicity to soil and water resources, potential for reuse as a soil amendment once fully saturated (Buda et al 2012), and no toxicity to plants, wildlife, or children. It is also crucial that soil amendments do not negatively impact soil infiltration rate and ability to grow vigorous plants. Some P sorptive amendments, such as water treatment residuals (WTRs), are waste products turned into a resource to reduce P in bioretention (or agricultural) soils.Results from much of the research to date on use of P-sorbing materials to reduce nutrients in stormwater effluent are promising so far, but much remains to be learned about lifespan and long term effects of P sorbing materials on soils and plants.
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Benefits
P sorptive amendments have been shown to provide effective P retention for the expected lifetime of bioretention facilities (e.g. Lucas and Greenway 2011, O’Neill and Davis, 2012 a and b). Even with sorptive amendments, the presence of healthy vegetation plays a crucial role in extending P reduction lifespan (Lucas and Greenway 2011).”Vegetation is essential not only for uptake, but even more for its effects on extending sorption capacity” (Lucas and Greenway 2011).
Types of P-sorbing materials
The primary P-sorbing chemicals are calcium (Ca), aluminum (Al) and iron (Fe). These are found in a variety of materials.
Limestone or calcareous sand
Combinations of C 33 sand with limestone or calcareous sand were tested in laboratory columns by Erickson et al (2007). Limestone or calcareous sand is not recommended as a P sorptive amendment in bioretention facilities because it clogged the columns, resulting in hydraulic failure.
Drinking Water Treatment Residuals (WTS)
Drinking-water treatment residuals are primarily sediment, metal (aluminum, iron or calcium) oxide/hydroxides, activated carbon, and lime removed from raw water during the water purification process (Birikorang et al., 2009). WTRs are increasingly being used to control phosphorus in soils where phosphorus leaching may be problematic for water quality. Kawczyinski and Achtermann (1991) reported that landfilling is the predominant disposal method, followed by land application, sanitary sewer disposal, direct stream discharge, and lagooning. WTRs contain high concentrations of amorphous aluminum (Al) or iron (Fe), making them potential amendments for sorbing soil phosphorus.
Aluminum-based Water Treatment Residuals (WTRs)
O’Neill and Davis (2012a and 2012b) recommend a bioretention soil media of 5 percent WTR, 3 percent triple-shredded hardwood bark mulch, and 92 percent loamy sand for P reduction on the basis of batch, minicolumn, and large column studies. The life expectancy for this media was 20 years. In a comparison of bioretention soil medias (BSM’s) with varying fines concentrations, they found that increasing the concentration of sand (i.e. decreasing fines) improved P reduction. They also found that hardwood bark mulch, a source of organic matter typically low in P, further improved P reduction (O’Neill and Davis 2012a). The authors contend that an oxalate-extractable aluminum-, iron-, and phosphorus based metric, the oxalate ratio, can be used to predict P sorption capacity, and suggest that a media oxalate ratio of 20-40 is expected to meet P adsorption requirements for nutrient sensitive watersheds.
This media adsorbed 88.5 percent of the applied P mass, compared to a non-WTR amended control media for which effluent P mass increased 71.2 increased. “This media consistently produced total phosphorus effluent mean event concentrations < 25 micrograms per liter and exhibited a maximum effluent concentration of only 70 micrograms per liter” (O’Neill and Davis 2012 b). “Concentrations of P as low as 25 micrograms P per liter may be necessary to reduce eutrophication risk depending on receiving water conditions (U.S. Environmental Protection Agency (US EPA 1986) in O’Neill and Davis 2012a). References to additional studies are found in O’Neill and Davis (2012a and 2012b).
Iron-based Water Treatment Residuals (WTRs)
As reviewed in O’Neill and Davis (2012 a), one study of iron based WTRs found iron based WTRs to be ineffective to P reduction because they solubilized and released all adsorbed P in reducing conditions (Stumm and Morgan 1996 in O’Neill and Davis 2012 a), but another more recent study found this may not be the case (Shober and Sims 2009 in O’Neill and Davis 2012 a). According to Dr. Allen Davis, iron based water treatment residuals “should work just as well, maybe better than Al. The concern with Fe is that if the media becomes anaerobic due to flooding or any other reason, the Fe can be reduced and will dissolve. It adds another layer of complexity to the system.” This concern can be addressed by designing the bioretention practice to ensure the layer where P sorbtion will occur stays aerobic.