Hydrogen-driven Autotrophic Denitrification of Wastewater Using a Membrance Biofilm Process

Marc Russenberger1, Kaleigh Biss2, Hukerenui Bonnet1, Jiabao Wendy Qi1, Rob Fullerton1, 3, Shan Yi4, Wei-Qin Zhuang1, 

1 Department of Civil and Environmental Engineering, University of Auckland, Auckland 1142, New Zealand

2 Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL33620, USA

3 Beca, Auckland 1010, New Zealand

4 Department of Chemical and Materials Engineering, University of Auckland, Auckland 1142, New Zealand

Nitrate contamination in water resources, stemming from agricultural intensification, excessive use of inorganic nitrogenous fertilizer, and inadequate municipal wastewater nitrate removal, poses serious environmental and health risks globally. Conventional heterotrophic denitrification has been used to remove nitrate from wastewater, but these processes often result in significant nitrous oxide emissions-a potent greenhouse gas (GHG) 298 times more potent than carbon dioxide. Moreover, some processes consume considerable energy by recycling nitrate-laden wastewater to mix with the primary effluent in pre-anoxic tanks to use its biochemical oxygen demand (BOD) for heterotrophic denitrification. Given the global urgency surrounding greenhouse gas emissions and New Zealand’s commitment to the Paris 2050 emissions goal, a net-zero-emission nitrate removal process is essential.

Hydrogenotrophic denitrification is a promising bioprocess that uses hydrogen-oxidizingautotrophic bacteria to reduce nitrate to dinitrogen gas. Previous studies have shown that hydrogenotrophic denitrification can significantly reduce the production of direct and indirect greenhouse gas emissions. Due to no organic carbon being required by autotrophic microbes, there is no risk of organic carbon carryover or additional carbon dioxide production as in heterotrophic denitrification processes. Therefore, hydrogen can be provided in excess relative to the nitrate concentrations and stoichiometric needs of autotrophic denitrifiers, driving complete denitrification. However, technical challenges such as low water solubility and the explosive nature of hydrogen hinder the application of this novel wastewater treatment technology. 

This research aims to develop a robust and efficient hydrogenotrophic denitrification process to minimize nitrous oxide emissions and energy consumption while maintaining wastewater discharge standards. Results from bench-scale testing using an innovative 20- liter membrane biofilm reactor (MBfR) highlighted that it could effectively and safely provide hydrogen through direct diffusion to hydrogen-oxidizing denitrifying bacteria (HODB) biofilm. Additionally, our results demonstrated that return-activated sludge from Rosedale Wastewater Treatment Plant (WWTP) could seed an MBfR effectively. The process achieved a peak denitrification rate of 41 mg NO3-N/L·d when operated in a batch mode with an applied hydrogen pressure of 5 psi. The denitrification rate could significantly increase when the MBfR was operated in continuous-flow mode. We noticed that pH control played an essential role in stabilizing performance and increasing denitrification rates. Overall, our MBfR-based hydrogenotrophic denitrification process can be a sustainable process to drop into existing wastewater treatment trains as a post-anoxic step.