Enhanced nitrate removal and side effects of methanol dosing in denitrifying bioreactors

Abstract

The nitrate removal efficiency of denitrifying bioreactors decreases when the carbon supply from woodchips is insufficient, particularly during large nitrate pulses. This study aimed to assess the effects of methanol dosing, as an external carbon source, on nitrate removal rates in mesocosm-scale bioreactors while monitoring the secondary effects of dosing that may occur. A secondary goal was to quantify sulfate reduction rates and methanol consumption in the presence and absence of nitrate to aid in determining the dosing load to minimize undesirable effects such as methanol entering receiving waters and minimizing sulfate reduction. We continuously dosed the bioreactors with nitrate (∼20 mg NO3−-N L−1), methanol (∼35 mg CH3OH-C L−1), and sulfate (∼9 mg SO42−-S L−1), which was already present in the tap water. In a long-term controlled mesocosm experiment, we established three bioreactor treatments to investigate nitrate, methanol, and sulfate removal rates with and without the presence of nitrate and methanol. Compared to the woodchip control treatment, methanol dosing resulted in an approximately fourfold increase in nitrate removal rates from 7 to 27 g N m−3 day−1. Methanol dosing increased sulfate removal rates, from average sulfate removal rates of 1.5 g SO42−-S m−3 day−1 under nitrate-prevailing conditions to 5.5 g SO42−-S m−3 day−1 removal rates under nitrate-limiting conditions compared to woodchip control removal of 0.3 g SO42−-S m−3 day−1. Mean methanol removal rates were 23 g CH3OH-C m−3 day−1 under nitrate-prevailing conditions compared to 18 g CH3OH-C m−3 day−1 in the woodchip control experiment. Improved nitrate removal rates, as well as methanol consumption and sulfate removal rates, might be leveraged to develop innovative low-footprint bioreactors.