Questions and answers January 9 2018 webinar
Bioretention (Speaker: Andy Erickson)
Bioretention
- If you use less compost aren't you dramatically effecting the infiltration rates and potentially increasing runoff or volume of water that can be stored?¬
- In most cases, no, because healthy vegetation will provide macropores and keep the top surface from becoming clogged up with accumulated sediment. Compost and organic matter actually hold water, not necessarily promote infiltration. As long as there is enough compost/organic matter to support the vegetation, the infiltration capacity should be maintained.
- How long is the iron amended sand layer effective for removing phosphorus?¬
- It will depend on the sizing of the practice, but in general the iron should have capacity to last 15 - 30 years or more for phosphorus capture. The oldest iron enhanced practice is less than 10 years old, so we don't know the longevity with much accuracy, but this is our best guestimate based on laboratory and field research. Some designs such as iron enhanced pond perimeter trenches might have a much shorter lifespan, perhaps only 5 - 10 years, because they receive a much larger amount of water per surface area of the filter.
- You suggested 3-5% compost , but earlier mentioned 3-5% organic matter. Typical composts have 50% organic content. Do you mean compost or OM? Are there studies looking at vegetation sustainability with the lower OM?
- Good point; my recommendation is 3 - 5% compost in the mix. I have seen few, if any, studies on the minimum amount of compost necessary to support the vegetation, though we may be studying this in the near future.
- Many watersheds allow biofiltration systems be upsized to meet their infiltration standard. Is this upsizing providing any benefits? What is the best approach when infiltration is not feasible? Pond upsizing vs. iron enhanced, etc. types of treatments.
- Upsizing often increases the volume that can be capture and infiltrated as well as disperses the sediment capture across a larger area. This often means more volume reduction due to the larger volume captured and less frequent maintenance due to larger area. The retention time within the media will likely increase because the depth on top of the biofiltration media will be less, resulting in better performance in terms of soluble pollutant removal, evapotranspiration, etc. If the system is properly designed and maintained, then it should perform as intended. If infiltration is not feasible, then biofiltration (with underdrains) is an option, even if it requires an impermeable liner to prevent any seepage into the underlying soils. Pond upsizing, iron enhanced systems, and other types of treatments could also be options, depending on the site and contributing watershed. Unfortunately, there is no “best” option for all scenarios.
- As the Iron Enriched sand takes on and binds to the nutrients, do they lose their effectiveness over time? If so, does it need to be removed and replaced at some point?
- There is a finite capacity to the iron amendment. The lifespan depends on how the system was designed and the characteristics of the water being treated. For example, there may be other constituents in the water that compete with phosphorus for binding sites and therefore decrease the lifespan and effectiveness of the practices for P treatment. If one collects samples of the outflow, one can determine if the performance is declining and identify when the system is nearing the end of its lifecycle. When it does, the media will likely need to be removed and replaced. Perhaps future research will discover methods to replenish the iron media while it is still within the filter.
- What is the additional cost for installation and maintenance for adding an iron filter?
- This depends on size and scale, but shipping costs can be large. The range for additional cost of including iron seems to be about 5 to 20% additional cost compared to a standard sand filter. More inspection will be needed, but this is not typically a significant additional cost.
- What about pretreatment for solids prior to bioretention treatment? Stormwater has very high TSS.
- Pretreatment is always recommended and often required because it will reduce loading to the bioretention practice and will reduce maintenance. Overall, this will increase performance of the bioretention practice as well as the life expectancy of the system.
- Is there a movement by watersheds and municipalities to promote or require use of Iron Enriched Sand?
- We aren’t aware of local government units requiring or promoting these practices, but may LGUs are now allowing these systems.
- How can you test the effectiveness of these soil amendments on a Bioinfiltration system (with no underdrain)?
- This can certainly be a challenge; however the practice is likely effective if it is infiltrating the design storm within a reasonable amount of time (12 - 48 hours). There are a few resources that show the performance of infiltration practices at removing pollutants, which can be used to support this assumption, including:
- Nieber, J.L., C.N. Arika, L. Lahti, J.S. Gulliver and P.T. Weiss. (2014). The Impact of Stormwater Infiltration Practices on Groundwater Quality. Report to the Metropolitan Council, St. Paul, MN, July 2014. http://hdl.handle.net/11299/169456.
- Weiss, P.T., G. LeFevre and J.S. Gulliver. (2008). Contamination of Soil and Groundwater Due to Stormwater Infiltration Practices. SAFL Project Report 515, June 2008. http://purl.umn.edu/115341.
- This can certainly be a challenge; however the practice is likely effective if it is infiltrating the design storm within a reasonable amount of time (12 - 48 hours). There are a few resources that show the performance of infiltration practices at removing pollutants, which can be used to support this assumption, including:
- Have you looked into bio-char at all? How does that work vs. Fe-enhanced sand?
- St. Anthony Falls Laboratory has not yet considered bio-char in depth, though we did evaluate bio-char and various other sorptive media on dissolved pollutant removal in the study below. In addition, we are aware of a field study on bio-char in combination with iron-enhanced sand by the Shingle Creek and West Mississippi Watershed Management Commission.
- Erickson, A.J., J.S. Gulliver, P.T. Weiss and W.A. Arnold. (2014). Enhanced Filter Media for Removal of Dissolved Contaminants from Stormwater. SAFL Project Report No. 572, University of Minnesota, Minneapolis, MN, September 2014. http://hdl.handle.net/11299/166940.
- http://www.shinglecreek.org/biochar-filters.html