Bioretention (Speaker: Andy Erickson)

  • 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.
  • 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

Sweeping and leaf litter (Speaker: Sarah Hobbie)

  • Can the high-efficiency regenerative air machine be used solely for pick-up of high leaf litter or does it need to be combined with a mechanical brush sweeper?¬
    • In our Prior Lake study, we used a regenerative air sweeper on its own for leaf pickup.
  • To MPCA and presenters: What does the stormwater city of the future look like? How do we take these ideas to scale in a cost effective way?
    • We need to somehow close the P cycle. Currently trees are putting more P into the drainage system. We need to figure out how to harvest and use the P instead of washing it into our surface water systems. Upfront activities (P2 practices) are very effective ways to harvest this P.
  • Have there been studies done comparing the negative impacts of operating equipment for sweeping (carbon emissions) vs. the benefit of P removal?
    • We have not looked at this but it should be a relatively easy analysis to determine the significance of this trade-off
  • Have staff hours been considered in the cost of sweeping?
    • Yes
  • Are communities coordinating their water system and hydrant flushing process with their Fall tree litter removal? Each Fall we find lots of leaves are washed down drains due to the difficulty of coordinating these two activities.
    • We aren’t aware of specific coordination efforts but many cities seem to flush their hydrants in summer when leaf issues are less of a concern.
  • Have any communities that do dedicated leaf pick-up (versus regular sweeping) been considered? E.g., East Lansing does leaf pick-up like you just mentioned......with blowers, vactor trucks, front-end loaders, and dumptrucks...
  • Has your P loading model been tested for transferability? In other words, the 5-fold cross-validation wporked well in your study areas but have you tested it in other areas with different tree characteristics (species, density, etc)?
    • No but this would be a great next step
  • Interesting to note that your volunteers had shovels - not brooms. Shoveling the gutter looks very different than sweeping the street.
    • The appropriate tools will depend on how wet the leaf litter is. When things are really wet, and the leaf material is matted, shovels can work better than rakes or brooms. When things are dryer, rakes and brooms work better.
  • Any urban "forestry" comprehensive planning guidelines to minimize P loading?
    • We’ve done studies looking at different factors that might affect P loading, such as species and tree density. The most important factor seems to be size of the trees (i.e. tree canopy cover). Another issue is that different species drop leaves at different times of the year, so planning for this seems appropriate. For example, plant one species on a block and another on another block to create diversity but enhance sweeping efficiency.

Other questions

This page was last edited on 26 January 2018, at 20:16.