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Bioretention

Engineering Bioretention for Nitrate Removal

 

This research study began March 1999 and is now completed.  It was sponsored by the Maryland Water Resources Research Center.  Co-Principle Investigator on this project was Dr. Eric Seagren

 

Maryland Water Resources Research Center

 

 

This research examines the fate of nitrate in model bioretention systems, with a focus on the biological transformation and removal of nitrate.  Specifically, the modification of bioretention system was evaluated.  The overall goal of this study was to systematically examine the removal of nitrate from urban runoff by re-engineering the concept of bioretention.  In this evaluation, conditions to optimize the denitrification reaction were determined so that design parameters could be established for use in bioretention systems. Thus, the specific objectives of this research were to:

  1. Determine an electron donor and carbon source that is stable for a long period of time in the subsurface, but does not limit the denitrification process. 

  2. Optimize the system with the electron donor that gave the best nitrate removal efficiency and effluent quality by varying nitrate loading and hydraulic retention time.

  3. Evaluate the performance of the optimized system under conditions of intermittent loadings, such as are expected in the field.

  4. Scale up the optimized condition to a pilot scale bioretention facility.

 

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Providing an appropriate electron donor is a key environmental factor affecting denitrification.  The electron donor should be stable for a long period of time in the subsurface, but still should not limit the denitrification process, which means that it should be a readily metabolizable solid. Furthermore, low cost and ready availability are required from the economic perspective.  Column studies using various electron donors for denitrification were performed in order to select promising electron donor candidates for bioretention.  The results of the first phase of experiments indicate that on the basis of nitrate removal efficiency, as well as the effluent water quality (TKN and turbidity), newspaper, woodchips and “small sulfur”/limestone were the best electron donor candidates for supporting denitrification.  
 

 

The second task was to optimize the system by varying nitrate loading and hydraulic retention time.  Newspaper, woodchips and “small sulfur” were selected from the first phase experimental sets.  Throughout the second phase of experiments, the newspaper demonstrated better N removal efficiency than the other two materials at all three nitrate concentrations and at all five flow rates.  Newspaper was selected as overall the best electron donor substrate out of the materials studied.    Based on nitrate loading and flow rate studies, the design of the anoxic zone in bioretention can be determined by selecting an optimum volumetric nitrogen loading.   The optimum volumetric nitrate nitrogen loading for newspaper based on the mass of electron donor added in these studies was approximately 17 mg/L-day.  

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The third task of this work was to evaluate the performance of the optimized system under conditions of intermittent loadings, which are expected in the field.  This is a unique challenge of bioretention that distinguishes it from many other engineered systems for biological denitrification.  The initial recoveries of columns after two dormant periods, 30 days and 84 days, were studied by measuring initial effluent nitrate concentrationsStudies of viability after these dormant periods demonstrated that a bioretention system engineered using newspaper as an electron donor for biological denitrification should be effective under conditions of intermittent loadings.  Specifically, rapid initial recoveries were observed after extreme dormant periods, with a return to >90% nitrate removal efficiency within 14.5 hours after a 30-day dormant period and within 30 hours after an 84-day dormant period.    
 

 

A pilot scale bioretention study was completed in the final task.  The reactor consisted of a 76-cm long by 40-cm wide plastic box with sufficient depth for up to 36 cm of material and a 10 cm freeboard.  Pilot-scale bioretention studies confirmed the effectiveness of the proposed design to reengineer bioretention, demonstrating nitrate and nitrite removals of 70% to 80%.

 

 

This work was completed by MS student Hunho Kim.

 

Publications from this research project.

Kim, H., Seagren, E.A., and Davis, A.P., "Engineered Bioretention for Removal of Nitrate from Stormwater Runoff," WEFTEC 2000 Conference Proceedings on CDROM Research Symposium, Nitrogen Removal, Session 19, Anaheim CA, October 2000.

Kim, H., Seagren, E.A., and Davis, A.P. "Engineered Bioretention for Removal of Nitrate from Stormwater Runoff," Water Environ. Res., 75(4), 355-367 (2003).

All LID Publications

 
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August 29, 2003