Effects of influent composition and substrate type on nitrate and Escherichia coli removal in laboratory-scale denitrifying bioreactors

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Shannon E Brink (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: Escherichia coli (E. coli) and nitrate (NO3-) are common contaminants found in waters down gradient of wastewater sources such as septic tanks and agricultural operations. Carbon-rich subsurface denitrifying bioreactors are water treatment technologies that have the potential to decrease concentrations of NO3- and pathogenic pollutants released to water resources. Recent reports have indicated the potential for laboratory scale denitrifying woodchip bioreactors to significantly decrease E. coli concentrations at hydraulic retention times (HRTs) >10 hours. The goals of the current column study are to compare E. coli and NO3-+nitrite (NOx-N) removal efficiency of various substrates (gravel, peanut shells, or woodchips) receiving influent water containing either NO3-+E. coli, E. coli only, or NO3- only, at a target HRT of 6 hours.\nNOx-N removal efficiency was significantly higher in woodchips (25%-30%) than both peanut shells (8%-18%) and gravel (-2%-2%), regardless of the presence of E. coli. Total nitrogen (TN) removal efficiency trends support that NOx- removal efficiency represent N removed from the system and not simply transformed to another species. E. coli removal efficiency was significantly higher (p<0.05) in gravel (medians of 88%-96%) than peanut shells (70%-86%) and woodchips (30%-67%), regardless of whether NO3- was added. When NO3-+E. \ncoli was treated, both woody substrates performed similarly. When influent water had E. coli only, peanut shells significantly outperformed woodchips. E. coli removal efficiency of woodchips and gravel significantly decreased when NO3- was not added. \nTypically, effluent dissolved organic carbon (DOC) concentrations were significantly higher than influent concentrations\; however, when influent water contained E. coli only, only woodchips significantly increased DOC concentrations. Effluent DOC concentrations never varied significantly by substrate type when receiving the same influent water. \nIn both scenarios where only one pollutant was added to influent water, peanut shells resulted in significantly higher ammonium (NH4+) concentrations than that of the influent. Peanut shells also released significantly more NH4+ when E. coli only was treated compared to when NO3-+E. coli was treated, whereas woodchips released significantly less when E. coli only was treated, and gravel remained unaffected by changes in influent composition. \nIn most scenarios, all substrates resulted in effective removal of NOx and/or E. coli. Woodchips always removed the most NOx and gravel always removed the most E. coli followed closely by peanut shells. Woodchips and gravel both achieved significantly better E. coli removal when NO3- was added to influent water, but peanut shells were not impacted by the addition. The findings reported here indicate that a paired-media approach may be best to treat for both contaminants and supports the need for further research into E. coli removal mechanisms and NOx- removal using novel substrates at varying HRTs and at the pilot- and field-scales.

Additional Information

Publication

Thesis

Language: English

Date: 2023

Subjects

Nitrate, E. coli, remediation, denitrifying bioreactor, wood chips, peanut shells, gravel

Email this document to