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Bioretention

Studies Results completed by the University of Maryland

Several research studies have now been completed. Some of the initial results are presented below.

Summary results from the two field experiments completed in 1997 & 1998 are given in the Table below. The copper and lead concentrations in the effluent at Greenbelt were less than or very near instrument detection limits (2 ľg/L), giving removals of 97ą2% (mean ą 1 standard deviation) and >95%, respectively. All of the zinc concentrations were below the detection limit (<25 ľg/L), for a removal of >95%.

At Landover, total lead was removed significantly, at about 70ą23% (16 ľg/L effluent); total copper was removed to a lesser extent (43ą11%). Zinc was also reduced with an average removal of 64ą42%. Effluent dissolved zinc averaged 390 ľg/L. Both copper and lead concentrations were significantly reduced by the treatment at Landover, but not to the extent found at Greenbelt

The total phosphorous removal at Greenbelt was 65ą8%. Effluent phosphorus levels were fairly constant over the sampling period. Removal of TKN was also constant at about 52ą7%. Ammonium removal was excellent, averaging 92ą7%; the mean effluent ammonium concentration was 0.22 mg/L as N. The removal for nitrate was poor, at only 16ą6%. This was not unexpected.

Effluent nutrient concentrations were observed to be below the input in all cases at Landover. Removal for phosphorus was 87ą2%, excellent removal, with effluent concentrations just above 0.1 mg/L P. This was better than the removal at Greenbelt. TKN removal was 67ą9% and nitrate removal averaged 15ą12%. Both showed some variation with time. These results are both comparable to those found at Greenbelt. Ammonium was not added to the runoff in the Landover study. In contrast to the metals, very similar nutrient removal efficiencies were found between the two facilities.

Pollutant Removal Summary Table for Field Studies

	Cu (ľg/L)	Pb (ľg/L)	Zn (ľg/L)	P (mg/L)	TKN (mg-N/L)	NH₄⁺(mg-N/L)	NO₃^-(mg-N/L)
_{^{Greenbelt Field Study}}
_^Input	_^66ą32	_^42ą35	_^530ą72	_^0.52	_^3.5	_^2.6	_^0.33
_{^{Average ą _{^{Std. Dev.}}}}	_^2ą1	_^<2	_^<25	_^0.18ą0.04	_^1.7ą0.23	_^0.22ą0.18	_^0.67ą0.49
_^Range	_^<2-4.2	_^<2-4.5	_^<25	_^0.15-0.24	_^1.4-1.9	_^0.08-0.53	_^0.25-0.30
_{^{Removal %}}	_^97ą2	_^>95	_^>95	_^65ą8	_^52ą7	_^92ą7	_^16ą6
*^_L^_andover Field Study*
_^Input	_^120ą27	_^54ą9.4	_^1100ą20	_^0.83	_^6.9ą0.81	_^-	_^1.3ą0.05
_{^{Average ą _{^{Std. Dev.}}}}	_^69ą9.4	_^16ą7	_^390ą440	_^0.11ą0.02	_^2.3ą0.64	_^-	_^1.1ą0.15
_^Range	_^55-85	_^6.7-26	_^120-1400	_^0.10-0.13	_^1.7-3.0	_^-	_^0.94-1.2
_{^{Removal %}}	_^43ą11	_^70ą23	_^64ą42	_^87ą2	_^67ą9	_^-	_^15ą12

Combining the effluent data from the two field, one large box, and two small box experiments yields pollutant removals at several different bioretention media depths. Input chemical concentrations, pH, and loadings were approximately equal in all cases. This allows direct comparison of the three different experimental scales and permits specific examination of the pollutant removal efficiencies as a function of bioretention depth.

Combined Studies-Lead

A plot showing average lead removals for each depth is shown to the left. Error bars represent ą the standard deviation for data collected over the application times. Results show excellent agreement among the various laboratory experiments. As well, excellent agreement was found between the laboratory box studies and the Greenbelt field data, even though the materials of design were not the same. Removals were all greater than 90% and variations were small. Nonetheless, results from the Landover study did not demonstrate the same degree of metals removal. There were several differences between the Landover and Greenbelt facilities, including differences in bioretention media, which could be responsible for the removal efficiency variations.

Companion studies have illustrated the importance of the mulch layer for metals uptake/sorption in bioretention. Accordingly, a shallow bioretention facility with several cm of mulch may be adequate for substantial removal of heavy metals from storm water runoff.

Combined Studies-Phosphorus

As expected, the overall scatter of the phosphorus data is greater than for lead. Nonetheless, very good agreement was noted among the laboratory data and the two field studies. In fact, the scatter among the various system scales was well within that exhibited by the three boxes that employed the same media. The phosphorus removals showed very good agreement among all types of experimentation. Better removal resulted from deeper bioretention through about 61 cm depth. At this point, the removal plateaued at about 70-85%. Most soils have a significant capacity to adsorb phosphorus at neutral pH and adsorption was likely the dominant phosphorus uptake mechanism.

2001-2002 Field Studies

Studies were completed on six different bioretention cells in 2001 & 2002. Six pollutants were monitored: oil/grease, suspended solids, total lead, total phosphorus, ammonia, and nitrogen. Results are shown in the Figure to the right. Excellent removal of oil/grease was found. Very good TSS and lead removal also was noted; these pollutants were correlated, as a large fraction of the lead was affiliated with TSS. Phosphorus removal was good, but variable among the six facilities. Ammonia removal was also somewhat variable, but poor. Nitrate removal was poor, as expected.

Right clicking on the image and viewing it directly will provide a clear figure.

Details of finalized research results are given in the publications listed below.

Bioretention and LID Publications

1. Davis, A.P., Shokouhian, M., Sharma, H. and Minami, C., "Laboratory Study of Biological Retention (Bioretention) for Urban Storm Water Management," Water Environ. Res., 73(1), 5-14 (2001).

2. 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.

3. Weinstein, N. Davis, A.P. and Veeramachaneni, R. "Low Impact Development (LID) Stormwater Management Approach for the Control of Diffuse Pollution from Urban Roadways," 5th International Conference Diffuse/Nonpoint Pollution and Watershed Management Proceedings, C.S. Melching and Emre Alp, Eds. 2001 International Water Association

4. Hsieh, C.-h. and Davis, A.P. " Engineering Bioretention for Treatment of Urban Storm Water Runoff," Watersheds 2002, Proceedings on CDROM Research Symposium,, Session 15, Ft. Lauderdale, FL, Feb. 2002.

5. Davis, A.P., Shokouhian, M., Sharma, H., Minami, C., and Winogradoff, D. "Water Quality Improvement through Bioretention: Lead, Copper, and Zinc," Water Environ. Res., 75(1), 73-82 (2003).

6. Kim, H., Seagren, E.A., and Davis, A.P. "Engineered Bioretention for Removal of Nitrate from Stormwater Runoff," Water Environ. Res., accepted for publication, December 2002.



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March 26, 2003