Biofiltration Media Optimization: Phase I

Project overview

Phosphorus is a nutrient that plants need in order to grow. However, when phosphorus concentrations are too high in lakes, ponds, or other waterways, it fuels excessive plant and algal growth, which leads to the collapse of aquatic ecosystems. In Minnesota, stormwater managers are therefore required to reduce the amount of phosphorus entering natural waterbodies. Biofiltration basins are often employed for this purpose.

Biofiltration basins are excavated depressions that are lined with an engineered media mix; the media mix is designed to filter stormwater while also supporting plant growth. The plants and the media are both intended to chemically and biologically capture the phosphorus carried into the basin by the stormwater. However, the most commonly used biofiltration media mixes have been shown to export more phosphate than they retain, potentially contributing to water quality impairments.

This study aimed to determine which locally sourced biofiltration media components are capable of removing phosphorus from stormwater runoff while also supporting plant growth. Mesocosm studies of various biofiltration media mixes were completed in the outdoor spillway adjacent
to St. Anthony Falls Laboratory. Clean, washed concrete sand was used as an experimental control, and seven different biofiltration media amendments were added to clean washed concrete sand in various ratios (by volume) as follows:

  • 10% food residue compost + 90% clean washed sand,
  • 20% food residue compost + 80% clean washed sand,
  • 10% leaf compost + 90% clean washed sand,
  • 20% leaf compost + 80% clean washed sand,
  • 15% biochar + 20% leaf compost + 65% clean washed sand,
  • 5% spent lime + 20% leaf compost + 75% clean washed sand,
  • 5% iron + 20% leaf compost + 75% clean washed sand,
  • 20% sphagnum peat + 80% clean washed sand, and
  • 20% reed sedge peat + 80% clean washed sand.


These media mixes were evaluated for filtration rate, nutrient output (phosphate and nitrate), and vegetation growth during one rainy season. Fourteen simulated events each released approximately five gallons of phosphorus-enhanced water during the summer and autumn of 2019. Flow rate through the mesocosms was measured and samples of the influent and effluent water were tested for nitrate and phosphate. In between events, settling, vegetation growth, and rainfall movement through each mesocosm were also monitored.

Research questions

  • Can we identify any locally available and sustainable biofiltration media components that effectively filter stormwater?
  • Can we identify any locally available and sustainable biofiltration media components that  support plant growth and microbial function?
  • Can we identify any locally available and sustainable biofiltration media components that do not release phosphate?
  • Are there simple tests or metrics that practitioners can use to reliably predict phosphorus export from a media mix sample?

Research findings

  1. Filtration Rate: Compared to flow through 100% sand (baseline control), flow through all media mixes was similar. There is no clear evidence that any of these mixtures have a major advantage or disadvantage for filtration rate within the first year of use. Further testing in subsequent years will reveal whether these patterns change as the mixtures age.
  2. Supporting Vegetation Growth: Spent lime had the tallest vegetation and the most biomass, which were statistically greater than 100% sand. All of the iron-free, compost-amended mixes had more vegetation biomass than the iron, peat, and sand media mixes. The peat and, notably, the iron media mixes, produced statistically similar amounts of biomass as the 100% sand. This suggests that adding iron limits the vegetation support provided by compost when iron and compost are mixed together.
  3. Limiting Phosphate Release: The addition of 10% or more compost (leaf or food) to the media mixes significantly increased the effluent’s phosphate concentration. Spent lime and biochar additions reduced this release slightly, but only the addition of iron fully mitigated phosphate release from leaf-based compost. When leaf or food compost is replaced with peat, the effluent’s phosphate concentrations were reduced compared to the influent.
  4. Simple Tests or Metrics: The research team did not identify any tests or metrics that can be used to predict phosphate release from compost. Two phosphorus test kits were investigated for use in field batch experiments, but both failed to provide reliable measurements of the phosphate concentration of compost mixed with clean water. Compost samples were also submitted to a soil analytical lab to measure Olsen Phosphorus, Bray Phosphorus, phosphorus by ICP-OES, and Mehlich III Phosphorus. Finally, the compost samples were also analyzed according to the Solvita manufacturer instructions to measure CO2 respiration, NH3 respiration, and Solvita Maturity Index. None of these methods appeared to be strong predictors of phosphate release from compost.
     

Key innovations/contributions

This research quantified phosphate export from bioretention media mixes and demonstrated that mixes that were able to support healthy plant growth exported more phosphate than they captured.

What does this mean for Minnesota?

Understanding the limitations of conventional biofiltration media mixes provides a foundation upon which to develop additional research projects that will lead to better bioretention basin designs. A second phase of this project will investigate whether the patterns documented in this project are consistent across multiple years, and how media mixes respond to road salt. To read about phase II of this work, visit Biofiltration media optimization – Phase II: Multi-year performance, impacts of road salt, and optimized organic ratio. These efforts will prevent excess phosphorus from polluting Minnesota’s surface waters.  

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