Evaluation of a denitrifying bioreactor to improve water quality of tertiary treated water using DGT

Abstract

A proven nitrogen remediation strategy, denitrifying bioreactors are a low cost, edge-of-field approach for removing nitrate from subsurface drainage waters. The methodology to evaluate the performance of denitrifying bioreactors has remained practically unchanged for many years and is largely dependent upon grab sampling to evaluate their performance. In this study, a controlled flow denitrifying bioreactor treating wastewaters was used to evaluate a monitoring technique called Diffusive Gradients in Thin-Films (DGT) in place of grab sampling. Corbett et al., (in press) previously showed that DGT could replace high frequency grab sampling (e.g. hourly to daily) and significantly improve upon traditional low frequency sampling. Importantly, DGT provided NO3- concentrations comparable to high frequency grab sampling but require less time on site. In the current study, the bioreactor provided complete removal of nitrate within 4 meters with influent NO3- concentrations ranging from 9.7 to 15.9 mg NO3- L-1 over the four deployments. This study also demonstrated how grab samples can easily overestimate and underestimate NO3- removal with low frequency data providing a removal rate of 6.3 to 19.6 g N m-3 d-1 because of the infrequency of sampling. The time-weighted average concentrations calculated using Purolite AE520 DGT provided nitrate removal rates (NRR) that ranged from 5.7 to 9.4 g N m-3 d-1. This removal rate was higher than many other bioreactors most likely because influent wastewater contained dissolved organic carbon that could also support high denitrification. A Q10 of 3.9 was calculated using the NRR from this study and Corbett et al., (in press) illustrating that temperature was a determining factor in NO3- removal. A survey of redox reactions was also undertaken with oxygen, sulfate, methylmercury and total mercury measured. Sulfate reduction also measured for each deployment (average of 3.6 g S m-3 d-1; 1.7 to 5.10 g S m-3 d-1) via grab sampling. Methylmercury and total mercury concentrations were consistently below health guidelines. No phosphate reduction was observed. This study showed how DGT can provide more accurate NO3- removal data and were easier to use compared to traditional point-in-time grab sampling.