Designing Bioretention for Water Quality Treatment

Designing Treatment Media for Emerging Stormwater Contaminants Based on Water and Soil ChemistryShirley E. Clark, Ph.D., P.E., D. WRERobert E. Pitt, Ph.D., P.E., BCEE, D. WRECurrent PA GuidanceMany guidance documents apply expected pollutant removals based on literature. However, typically presented as efficiencies of removal and have been misinterpreted and misapplied. Also difficult to remove 85% of pollutants in clean water.Do not address metals or problematic organics.

2Total Nitrate-Nitrogen (TN)

International BMP DatabaseTMDL needs concentrationsWhy Does the Literature Report Widely Different Efficiencies for Bioretention?FACTORSInfluent Water Chemistry GeoChemical Reaction EquilibriumGeoChemical Reaction KineticsMicrobiogeochemical InteractionsVegetationInfluent Water ChemistryTypes of Pollutants in Stormwater RunoffSolidsWide range of sizes from colloidal to sands/gravelsPollutants (fraction of total load) that are associated with/attached to solidsMetals, Phosphorus, Organics, BacteriaPollutants that are dissolved or unbound to solidsRemainder of total load of metals, phosphorus, organics, bacteriaNitrates, Nitrites, Ammonia, Chloride, etc.

Suspended SolidsToxicity

Total PhosphorusAssociations of Pollutants with Specific Particle Sizes

From: Morquecho 20056Influent Water ChemistryMetals Partitioning Why Different for Representative Metals?

% Ionic% BoundZinc1585Copper7030Cadmium1090Lead1288p = 0.004Zinc

Inorganic distribution No DOM assumed (0rganic complexation greatly changes distribution)

What form are those compounds that pass through a 0.45-m filter?INFLUENT WATER QUALITY AND TREATABILITYHowever, there is a caveat to that simple statement. The dissolved fraction is not as simple as a group of ions floating in water. In fact, many of the dissolved fraction pollutants are association with colloidal material, potentially organic in nature. As noted by these results, for zinc, cadmium and lead, these are associated with organic colloids and that information is important in determining the effectiveness of removal. That may require more contact time or the use of a different media to ensure removal. Well get to the importance of other characteristics in a minute.9Soil Chemistry

www.seafriends.org.nz/enviro/ soil/geosoil.htm Soil Chemistry EffectsGOAL: Remove pollutants in the upper layers of the media (deeper penetration into the soil profile, greater likelihood of groundwater contamination or transport out of the device through an underdrain). Potentialproperties of interest in predicting removal:Soil and water pHPollutant forms (relationship to solids loading and PSD)Cation Exchange Capacity (CEC) [and Anion Exchange Capacity (AEC)]Mineral matterOrganic contentPhosphorus contentOxidizing or reducing environmentSalinity and Sodium Adsorption Ratio (SAR)

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General Rule of Thumb:For nutrients, if soil is Low, it is more likely to remove nutrients from runoff.

If soil is High, leaching more likely.Example Media TypesSand relatively inert (without modification)Common modifications are iron oxide and manganese oxide coatings to improve pollutant retention.

Ion-exchangers/Zeolites lattice structureInterested in exchanging out ions with stronger attractive forces (particularly ones that have a higher valence/charge state)

Zeogarden.comExample Media TypesActivated carbon made from a variety of carbon sourcesReacts with chemicals through hydrogenbonding and van der Waals forcesTypically attraction through dipole interactionsPeakpureair.com

http://www.chemistry.wustl.edu/~courses/genchem/Tutorials/Water/Adsorption.htmExample Media TypesOrganic Non-Activated Media (soil, peat, compost, biosolids)Function of base materialSoils mixture of organic matter from organic debris and weathering of parent material (rock)Less weathering products (Ca, Mg, Na, K) & more relatively insoluble elements such as Fe & Al than original rock. Most chemically active: colloidal clays & organic matter. Organic fraction < 10% of soil mass by weight.Reservoir for plant nutrients, nitrogen, phosphorus, and sulfurIncreases soil water holding and cation exchange capacitiesEnhances soil aggregation and structure. Summarizing Types of MediaOrganic Non-Activated Media (soil, peat, compost, biosolids)Soils mixture of organic matter from organic debris and weathering of parent material (rock)Soil pH affects nutrient transformations and the solubility of nutrients. Phosphorus most available in slightly acid to slightly alkaline soils, while all essential micronutrients, except molybdenum, become more available with decreasing pH. Aluminum, manganese, and even iron can become sufficiently soluble at pH < 5.5 to become toxic to plants. Bacteria generally tend to be most active in slightly acid to alkaline conditions.

Design ConsiderationsContact Time Reaction Kinetics

19Minimal filtered metal removal when contact time