Available at: https://digitalcommons.calpoly.edu/theses/1908
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Development of urban areas continues to increase, disrupting the natural ecosystem and the pathways for water to drain into downstream water bodies. As the amount of impervious area increases, pollutants can accumulate on the surface and enter the water cycle by stormwater. In most cities, the stormwater flows into a storm drain that is discharged into a water body. Low Impact Development technology has been developed to treat stormwater prior to discharge downstream. A bioretention cell is used to treat stormwater pollutants such as nitrate, phosphorus, total suspended solids, and metals. Past research has indicated that the removal efficiency of nitrate by bioretention cells greatly varies from a 75% reduction in nitrate concentration to the addition of nitrate in the effluent from leaching of the bioretention cell. It is important to remove nitrate from water because it can cause negative environmental and human effects. Excess nitrate in the environment can lead to eutrophication, resulting in the death of fish. If ingested by infants, nitrate can cause “blue baby syndrome” leading to death. One area of study that focuses on the removal of nitrate from stormwater is maximizing the efficiency of the designed soil media. The addition of a clay amendment could lead to higher removal efficiencies. The use of clay nanoparticles, or nanoclays, can maximize the amount of surface area available for adsorbance potentially increasing the amount of nitrate removed from water. The goal of this study was to identify a nanoclay with high adsorbance by testing its nitrate removal efficiency and then determining if it would be feasible to add to a bioretention cell by calculating the hydraulic conductivity to compare to industry design values. This study analyzed a montmorillonite clay, a bentonite nanoclay, and a pre-modified trimethyl stearyl ammonium nanoclay as a 1% w/w added amendment to a Nevada sand to determine the number of pore volumes required until the system reached breakthrough. The pre-modified nanoclay required the highest amount of water, 19 pore volumes, until breakthrough was reached. The fraction of nanoclay was increased to 2% w/w and breakthrough was not seen in the volume of water that was filtered through the system. The hydraulic conductivity for this nanoclay/sand mixture was 14 in/hr, greater than most minimum design values. Since the results so far indicated that this mixture would efficiently remove nitrate and still meet the minimum hydraulic conductivity, a synthetic stormwater solution was filtered through the column to test the nitrate removal when it is competing with other pollutant ions. This resulted in a projected breakthrough of 27 pore volumes and a 9 in/hr associated hydraulic conductivity. Future research can be completed to assess the best methodology to homogeneously mix the nanoclay particles with the sand to prevent leaching of the nanoclays. The next step in optimizing a bioretention cell for water quality treatment would be to focus research on how plants affect the system. If plants are able to remove nitrate from the system entirely, the lifetime of the bioretention cell could increase.