84715
Fosnrs 5: Quantifying Rates of Denitrification in the Biozone and Shallow Subsurface within Soil Treatment Units for Wastewater Reclamation.

See more from this Division: Oral sessions
See more from this Session: TRACK 1--Treatment and Fate of Contaminants: Nitrogen
Monday, April 7, 2014: 3:30 PM
Share |

Simon A Farrell1, Robert L Siegrist2 and Kathryn S Lowe2, (1)JVA, Inc., Boulder, CO
(2)Civil & Environmental Engineering, Colorado School of Mines, Golden, CO
A key constituent of concern affecting soil treatment unit (STU) design is nitrogen due to its potential transport to groundwater and adverse effects on human and ecosystem health.  The primary removal process for nitrogen entering the subsurface via a soil treatment unit is denitrification.  Complimentary to the FOSNRS project, research was completed at the Colorado School of Mines to evaluate denitrification in STUs and to what extent the N species in the effluent affect the potential and expressed rates.  Under laboratory conditions at the Mines Park Test Site in Golden, Colorado, four columns (15-cm diam. 60-cm long) were packed with soil classified as Seffner fine sand obtained from the Gulf Coast Research and Education Center, site of the FOSNRS test facility (s/si/cl fractions = 95/01/04 dry wt.%; pH = 5, TOC = 0 to 0.05 dry wt.%).  All four columns were dosed twice daily to yield a daily hydraulic loading rate of 2 cm/d.  Two columns were dosed with septic tank effluent (STE) and the other two were dosed with nitrified intermittent sand filter (ISF) effluent.  Effluent applied to the columns and percolate exiting the columns was characterized for water quality parameters including nitrogen species to facilitate a nitrogen mass balance.  When steady state was reached with respect to nitrification after 10 wks of operation, one STE and one ISF column were deconstructed and replicate samples were collected from depth intervals between 8 and 60 cm below the infiltrative surface (bis).  Samples were analyzed for water filled porosity (WFP) and denitrification rates (DNR).  DNR measurements were made to determine: 1) the representative DNR (DNRR) under the actual conditions in the column using static core acetylene inhibition and 2) the potential DNR (DNRP) under optimal conditions using denitrification enzyme activity.  At the time of sampling, the columns were achieving a very low total N removal (0-16%) based on influent and effluent sampling.  The low N removal efficiency was consistent with model predictions made using STUMOD-FL.  The average DOC levels in the percolates were similar (STE column = 6.4 mg-C/L vs. ISF column = 4.4 mg/L) but DOC removal in the STE column averaged 39% while in the ISF column it was only 6%.  Thus the STE provided a richer source of biodegradable organic matter.  The WFP within both columns varied from 37% (v/v) at 10-22 cm bis to 70% at 47-59 cm bis.  For the STE column, the average DNRR was 0.0020 mg-N/d per L of column pore volume (PV), which was only ~5% of the DNRP measured at 0.037 mg-N/d per L PV.  For the ISF column, the average DNRR was below reporting levels (RL) but the DNRP was 0.026 mg-N/d per L PV.  The DNRP measured within the native Seffner fine sand was zero; so effluent addition had increased the DNRP in both columns.

After 28 wks of continued operation, the remaining two soil columns were deconstructed and sampled at depth intervals between 0 and 60 cm bis.  DNR measured at >8 cm bis revealed rates similar to those observed after 10 wks of operation.  However, DNR measured at and close to the infiltrative surface were significantly higher than rates measured deeper in the columns (i.e., at >8 cm depth bis).  For the 0-1 cm depth in the STE column the average DNRR was 0.21 mg-N/d per L PV and the average DNRP was 1.35 mg-N/d per L PV.  For the 0-1 cm depth in the ISF column, the DNRR was <RL and the average DNRP was only 0.033 mg-N/d per L of PV, only ~2% of the DNRP in the 0-1 cm depth in the STE column and similar to rates measured at depths >8 cm bis.  

The results of this research revealed that rates of denitrification in native soil can be substantially increased by effluent application and can be elevated to higher levels in soil receiving STE compared to nitrified ISF.  Research is ongoing using molecular techniques to characterize the microbial communities within the samples collected and upon which DNR were measured.

See more from this Division: Oral sessions
See more from this Session: TRACK 1--Treatment and Fate of Contaminants: Nitrogen