Roles of dissimilatory nitrate reduction to ammonium in biological nitrogen removal system

Abstract

Biological nitrogen removal system is a wastewater treatment process that normally utilizes nitrification-denitrification to convert nitrogen wastes to unharmful gaseous products. However, apart from these two pathways, dissimilatory nitrate reduction to ammonium (DNRA) can also compete with denitrification for nitrate and yield ammonium waste as the end product. The aim of this research is to extend knowledge on the DNRA pathway, including the study on its occurrence and microorganisms responsible for the process. To observe the presence of DNRA, microbial sludge from aquacultures which utilized biological nitrogen removal processes was applied to examine the pathway of DNRA in these systems. The results revealed that the two microbial sludge from low and high C/NO3- aquaculture systems performed DNRA with different inducers, i.e. nitrate and nitrite. The addition of sulfide was found to enhance ammonium production in the low C/NO3- sludge, whereas sulfide did not have positive effect on DNRA in the high C/NO3- sludge incubations. This suggests that different DNRA microorganisms were responsible for the process in the low and high C/NO3- microbial sludge, since the DNRA microorganisms can be further classified into respiratory- and fermentative-types DNRA. As the high C/NO3- ecosystem has been observed to sustain DNRA over denitrification, enrichment cultures maintaining under low and high C/NO3- ratios were operated to monitor microbial community selected for each environment. The results from Illumina MiSeq 16S rRNA gene sequencing revealed that microbial populations closely related to Sulfurospirillum and the family Lachnospiraceae were the dominant microorganisms in the high C/NO3- culture. Additionally, primers targeting the fermentative-type DNRA (F-DNRA) microorganisms were developed in this study by using NADH-dependent nitrite reductase large subunit, or nirB gene, as a marker for primer detection. By applying the designed primers, target DNA and cDNA sequences were detected in the high C/NO3- culture samples, indicating the presence and activity of potential F-DNRA microorganisms under the high C/NO3- ecosystem.