Treatment of Polluted Saline and Non-Saline Groundwater Through Hydrogenotrophic Denitrification

Hydrogenotrophic denitrification (or hydrogen-oxidising autotrophic denitrification) is an alternative method used to biologically remove nitrate and nitrite from polluted waters. Compared to conventional heterotrophic denitrification, which requires organic carbon as an electron donor, this process uses hydrogen gas as an electron donor to reduce nitrate/nitrite to nitrogen gas. In this regard, hydrogenotrophic denitrification bears a few engineering advantages for water treatment. For example, hydrogen gas is benign to humans, and hydrogen-utilising microbes have a low biomass yield in general.

The use of hydrogenotrophic denitrification may prove to be useful for New Zealand, which is experiencing increased groundwater pollution due to intensifying unsustainable agricultural activities and dairy farming. Furthermore, due to rising sea levels, seawater intrusion has become an added concern for coastal cities. Seawater intrusion increases the salinity of groundwater, which can hinder the activities of conventional biological groundwater treatment processes. It is evident that to use hydrogenotrophic denitrification to treat groundwater in New Zealand the process needs to be effective in both saline and non-saline conditions.

In this study indigenous hydrogenotrophic denitrifiers were enriched from different sources taken from saline and non-saline environments, respectively. The study proved to be successful, yielding a 97% nitrate removal in non-saline conditions with an average removal rate of 70±2 mg NO3--N L-1⋅d-1. In saline conditions (4% salinity) a 92% nitrate removal was achieved, with an average removal of 52±2 mg NO3--N L-1⋅d-1. Furthermore, 16S rRNA gene identification of the non-saline samples revealed that members of the genus Thauera were predominant in the non-saline enrichment culture with an abundance of 18% of the total microbial community. Many cultured species of the genus Thauera are known denitrifiers. In particular, some are capable of carrying out hydrogenotrophic denitrification as well as denitrification in aerobic environments.