Mechanisms of iron sulfide-based autotrophic denitrification for removal of nutrients and emerging contaminants from carbon-deficient wastewater
Bai, Yang
Bai, Yang
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Publication Date
2023-09-19
Type
Thesis
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Abstract
Autotrophic denitrification is a biological nitrogen removal process in which nitrate (NO3 - ) is reduced to nitrogen gas (N2) by chemoautotrophic denitrifying bacteria in the presence of inorganic electron donors (i.e. reduced sulfur compounds, hydrogen gas, and iron). Recently, autotrophic denitrification with iron sulfides as the electron donors has gained increasing attention. Iron sulfides are a variety of natural minerals and synthetic compounds consisting of sulfur (S) and iron (Fe) elements. Iron sulfides-based autotrophic denitrification (IAD) has a unique advantage in terms of simultaneous NO3 - and phosphorus (PO4 3- ) removal with low operation cost and sludge production. However, the unclear mechanism and slow kinetics of IAD limit its engineering application in wastewater treatment plants. The objectives of this PhD research included: (1) study of the NO3 - transformation pathways coupled with sulfur and iron cycles in the FeS-based autotrophic denitrification process; (2) investigation of enhanced nutrients removal in an innovative constructed FeS-S0 coupled autotrophic denitrification (ISAD) system; and (3) exploration of the potential of FeS-based autotrophic denitrification for emerging contaminants removal. The results showed that NO3 - can be chemically reduced to ammonium (NH4 +) by FeS under room temperature and neutral pH conditions. With the inoculation of autotrophic microorganisms in the IAD biofilter, the nitrogen-transformation network was complex and consisted of chemical reduction, autotrophic denitrification, dissimilatory NO3 - reduction to NH4 + (DNRA) and sulfate reducing NH4 + oxidation (Sulfammox). Different groups of functional microorganisms were involved in these biochemical processes and engaged in the cycles of N, S, and Fe in the IAD biofilter. The coupled FeS-S0 autotrophic denitrification biofilter showed superior N and P removal performance; the highest NO3 - removal rate was up to 960 mg/L/d and the PO4 3- removal efficiency was 100% at 1 hr hydraulic retention time (HRT). Metagenomic sequencing results revealed that the high-rate NO3 - and PO4 3- removal performance was due to the collaborative denitrification community formed in the coupled system, which consisted of nitrate-dependent iron-oxidizing (NDFO) bacteria, sulfur-oxidizing bacteria (SOB), and DNRA bacteria. When the initial tetracycline concentration was 50 mg/L, higher NO3 - removal efficiency (>98%) and higher NO3 - removal rate constant (0.54 d-1 ) were shown in the IAD system, compared with Na2S2O3-based system. The ameliorative autotrophic denitrification performance in the IAD system under high tetracycline stress was due to the effective removal of TC by FeS through rapid adsorption and reduction. As a result, simultaneous tetracycline and nutrients (NO3 - and PO4 3- ) removal was achieved in the IAD system. This PhD research provides fundamental insights into iron sulfides-based autotrophic denitrification and would make significant contributions to wastewater treatment to meet more stringent nutrient permits and greatly lower carbon footprints. It will also open new horizons for removing emerging contaminants from wastewaters.
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NUI Galway