Stormwater runoff is a primary
non-point source of pollution that contains many pollutants that have
deleterious effects on the environment. To manage stormwater, different
techniques that target specific stormwater pollutants may be implemented in
watersheds. One such treatment practice is called bioretention, or rain
gardens. This thesis discusses the use of bioretention media as a treatment
technique for the removal of dissolved toxic metals and investigates the
release of phosphorus. A review of previous research and the laboratory
experiments will be discussed in two chapters **.
Chapter 1 reviews the literature on the
concentrations, sources, and effects of dissolved toxic metals found in
stormwater runoff. The review also
discusses relevant management practices and parameters related to the removal
of toxic metals using materials commonly found in bioretention practices, such
as MNDOT Grade 2 compost and C-33 sand. Third, Chapter 1 reviews the sorption
mechanisms and important variables that aid or hinder sorption of toxic metals
to organic materials. Lastly, previous
research on sorption of cadmium, copper, lead, and zinc to organic and
inorganic sorbent materials is discussed.
Batch and column experiment were
performed to investigate the removal of cadmium, copper, and zinc form
synthetic stormwater by compost-amended sand. The results of these experiments
are discussed in Chapter 2. The batch sorption capacities for Cd and Zn are
2.13 mg/g and 3.82 mg/g, respectively, for Minnesota Compost 1 and 0.02 and
0.07 mg/g, respectively for sand. Copper precipitates as tenorite (CuO) at the pH
of the stormwater (7.2), so a sorption capacity was not computed. Column
studies using four different ratios of compost (0, 10, 30, and 50%, by volume)
in sand were conducted to develop metal breakthrough curves. The breakthrough
curves for Cd and Zn were fit to the Thomas Model. The resulting sorption
capacities are 0.07, 0.23, 0.37, 0.78 mg Cd/g and 0.10, 0.23, 0.33, 0.61 mg
Zn/g for 0, 10, 30, and 50% compost fractions, respectively. These sorption
capacities, when adjusted for mass of sand and compost, are consistent with the
sorption capacities determined from the batch experiments. Assuming
representative values for precipitation and dissolved metal concentrations, the
estimated lifespan of bioretention cells for removal of Cd and Zn ranged from 24
to greater than 95 years for bed depths of 5 to 15 cm for a bioretention
practice constructed with 30% compost.
Copper was removed in the columns due to filtration and no breakthrough
occurred in the duration of the study.
In the batch and column studies, concentrations
of phosphorus exceeded the initial concentrations indicating that phosphorus is
exported from the bioretention media to the infiltrating stormwater. The
phosphorus concentrations exiting the columns were initially high (0.5 mg P/L),
but then decreased to a steady state value of 0.20 - 29 mg P/L (that exceeded
the influent value of 0.13 ± 0.03mg P/L) for the remainder of the
experiment. The total yearly load
exported from a bioretention practice containing 30% compost is 1.44 g
dissolved phosphorus per year for every square meter of bioretention area.
Overall, the results suggest that
bioretention cells are not likely to fail because of loss of dissolved toxic
metal removal capacity as the breakthrough times on the order of hundreds of
years far exceed the typical design life of engineering systems of 30 years.
Although only one compost was tested in the column experiments, the similarity
in batch sorption capacities for several compost samples obtained from
Minnesota and around the country suggest that the source of compost is not a
strong factor in determining dissolved metal removal performance. For metals
that are in particulate form or particle-associated, removals will be dictated
by the filtration performance of the bioretention cell which is a function of
particle size, bioretention media grain size and porosity, and other factors.
Copper, 72% of which was in the particulate form in the column influent, was
effectively removed by the bioretention columns and removal improved with increasing
fraction of compost. Nevertheless, it is
difficult to extrapolate these results to other particle-associated metals and
other bioretention practices. Finally, one significant concern regarding
bioretention media is that not only are nutrient not removed effectively, but
the compost that is key to metals removal may actually release nutrients (i.e.,
phosphorus). Thus, it is important to consider the installation of alternative
media beneath the compost-amended sand to remove phosphorus, such as iron-amended
sand. Such hybrid approaches require more investigation ***.
* The Executive Summary copied from my thesis.
** I'm smirking as I copy and paste this.
*** Let me know if you want some more....
