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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dagnew, Martha

Abstract

The main objectives of this study were: (i) to investigate dynamic specific denitrification rates (SDNR) from nitrite at various chemical oxygen demand (COD) / nitrogen (N) ratios using municipal wastewater (MWW) and acetate, and to study the kinetic parameters, (ii) to investigate the dynamic specific denitrification rates (SDNRs) from nitrite at various COD/N ratios in a system that uses methanol as a complementary carbon source, and studying the effect in case if nitrate replaced nitrite, and (iii) to investigate and compare between the nitrite shunt and N-PDN-anammox technologies through studying the dynamic specific denitrification rates (SDNRs) from nitrate and nitrite at various COD/N ratios in two systems that use acetate as carbon source, to investigate the difference in denitrifying microbial diversity and structure grown in nitrite denitrifying and partial denitrification environment, respectively.

Summary for Lay Audience

First, A sequencing batch reactor (SBR) continuously fed with primary effluent and nitrite solution were operated at a hydraulic retention time of 8.4 hrs and solids retention time of 26-30 days for 3 months. Influent MWW characteristics varied significantly during the study, i.e. 200-810 mgCOD/L and 6-80 mgN/L. The SDNR from the SBR (MWW) were compared with those determined in four batch reactors using acetate. The SDNR was directly related to COD/N until a maximum SDNR (mgNO2-N/mgVSS/d) of 0.07 for MWW and 0.4 for acetate occurred at COD/N ratios of 6 and 13, respectively; beyond this COD/N ratio, SDNR decreased. The biomass yield coefficients (mgVSS/mgCOD) were 0.33 for MWW and 0.51 for acetate. The relationships of SDNR with COD/N and F/M ratios were developed. This study considered to be the first study, in denitrification from nitrite with MWW, to determine the kinetic parameters for denitrification from nitrate and to model a direct relation between SDNR and F/M ratio.

Second, an SBR fed with primary effluent, and nitrite solution was operated at an HRT of 8.4 hrs and SRT of 16-17 days for 110 days. The SDNR from the SBR was compared with those determined in 23 batch reactors employing MWW, methanol, nitrite, and nitrate. The maximum SDNR was 0.52 mgN/mgVSS/d corresponding to COD/N ratios of 5.9 in the SBR, which was much higher than the literature reported value of 0.07 mgN/mgVSS-d for systems fed with just municipal wastewater (MWW). The nitrite SDNR using nitrite acclimatized biomass in the batch-test with MWW showed comparable SDNR. However, the maximum nitrite SDNR was lower than the observed batch nitrate 1.06 mgNO3-N/mgVSS-d SDNR using the nitrite acclimatized biomass from the SBR. The nitrite denitrifier yields were 0.39 and 0.34 mgVSS/mgCOD for the SBR and offline batch tests with complementary methanol and pure methanol, respectively. The half-saturation coefficient of nitrite (KNO2) during post-denitrification with methanol was found to be 9.2 mg/L, which confirmed the lower SDNR values at lower nitrite concentrations below the KNO2. The higher SDNR rates could be translated into 28% and 30% capital and operating cost savings, respectively; hence the addition of methanol as a complementary carbon source for nitrite denitrification can be considered as a mainstream process even for conditions carbon is not limiting in the MWW.

Last, a study is conducted to investigate and compare the dynamic specific denitrification rates (SDNRs) from nitrate and nitrite at various COD/N ratios in two systems that use acetate as carbon source. SBR1 (47 d-SRT) and SBR2(31 d-SRT) were operated for 126 days, with HRT of 16 hrs, and fed with nitrite and nitrate, respectively. The maximum SDNRNO2and SDNRNO3 during the cyclic tests were 0.69 and 2.67 mgN/mgVSS-d in SBR1 and SBR2, respectively. However, the maximum SDNRNO21occurred during nitrate denitrification was 1.27 mgN/mgVSS-d in SBR2 which is double the rate in SBR1 due to nitrite accumulation stress and the difference in microbial structure and diversity in SBR2. The optimum operation condition for PDN was found to be at minimum COD/N of 2.7-3 with 79% nitrite accumulation. Higher COD/N of 6.7 and 9.8 also accumulate nitrite, but nitrite accumulated was denitrified with a maximum SDNRNO22 of 1.04 mg N/mgVSS-d at CODN of 9.8. Kinetic parameters were also determined for nitrite in offline batch experiments, and the half-saturation coefficient KNO2 was found to be 3.17 mgN/L and the maximum specific growth rate of 0.77 d-1. The taxonomic analysis showed zoogloea genus (42% RA) in SBR1 was more responsible for nitrite denitrification, while thauera genus in SBR2 with RA of 10% was more responsible for PDN. We relate the difference in microbial structure to the NiR reductase and NaR reductase distribution among microbial structure indicating NiR reductase enzymes is higher in zoogolea genera than NaR reductase , whereas NaR is higher than NiR in thauera.

Available for download on Tuesday, January 31, 2023

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