Risk of nitrous oxide emissions and potential of bioaugmentation when treating digester supernatant via nitrification-denitrification

Abstract

This thesis examines two different impacts of sidestream treatment of digester supernatant via nitrification-denitrification in a sequenced batch reactor (SBR). One of the impacts is the detrimental formation of nitrous oxide, and the other is the positive boosting of nitrifiers to the mainstream process through bioaugmentation. The studies have been carried out in a full-scale wastewater treatment plant.

Different operating conditions were investigated in order to find thresholds for where the formation of nitrous oxide is obviously increased: low oxygen concentration during nitrification and low dosage of external carbon during denitrification. It was found that the nitrous oxide formation during nitrification was sharply increased when the oxygen concentration was lower than 1.0–1.5 mg O2/L. It was also found that it is important to maintain a sufficient dosage of external carbon during denitrification to avoid formation of nitrous oxide during anoxic conditions. The emissions of nitrous oxide were considerably lowered with a carbon dosage corresponding to more than 4 kg COD/kg TN in the influent than with a carbon dosage of lower than 2.5 kg COD/kg TN. Nitrifier denitrification and incomplete denitrification are believed to be the main pathways under oxic and anoxic conditions, respectively. The nitrous oxide emissions were also modeled. It was shown that the model was capable of partly reproducing the emissions. However, additional work is required to predict the emissions with high certainty by simulation.

The boosting of nitrifiers from a sidestream reactor to the mainstream process has been studied: bioaugmentation. The effect of bioaugmentation was evaluated through nitrification rate measurements and analyses of nitrifiers by molecular methods. The measurements demonstrated that the nitrification rate increased by more than 40% during the coldest weeks and 25% during the whole studied period. The molecular methods showed an increased abundance of nitrifiers of 25% during the whole studied period, and thus consistent with the results from the nitrification rate measurements. Furthermore, the total number of nitrifying species increased during the bioaugmentation.

Abstract (Swedish)

This thesis examines two different impacts of sidestream treatment of digester supernatant via nitrification-denitrification in a sequenced batch reactor (SBR). One of the impacts is the detrimental formation of nitrous oxide, and the other is the positive boosting of nitrifiers to the mainstream process through bioaugmentation. The studies have been carried out in a full-scale wastewater treatment plant.

Different operating conditions were investigated in order to find thresholds for where the formation of nitrous oxide is obviously increased: low oxygen concentration during nitrification and low dosage of external carbon during denitrification. It was found that the nitrous oxide formation during nitrification was sharply increased when the oxygen concentration was lower than 1.0–1.5 mg O2/L. It was also found that it is important to maintain a sufficient dosage of external carbon during denitrification to avoid formation of nitrous oxide during anoxic conditions. The emissions of nitrous oxide were considerably lowered with a carbon dosage corresponding to more than 4 kg COD/kg TN in the influent than with a carbon dosage of lower than 2.5 kg COD/kg TN. Nitrifier denitrification and incomplete denitrification are believed to be the main pathways under oxic and anoxic conditions, respectively. The nitrous oxide emissions were also modeled. It was shown that the model was capable of partly reproducing the emissions. However, additional work is required to predict the emissions with high certainty by simulation.

The boosting of nitrifiers from a sidestream reactor to the mainstream process has been studied: bioaugmentation. The effect of bioaugmentation was evaluated through nitrification rate measurements and analyses of nitrifiers by molecular methods. The measurements demonstrated that the nitrification rate increased by more than 40% during the coldest weeks and 25% during the whole studied period. The molecular methods showed an increased abundance of nitrifiers of 25% during the whole studied period, and thus consistent with the results from the nitrification rate measurements. Furthermore, the total number of nitrifying species increased during the bioaugmentation.

• author: Stenström, Fredrik
• supervisor: Jes la Cour Jansen
• opponent: Professor Jardin, Norbert, Ruhrverband, Essen, Germany
• organization: Division of Chemical Engineering
• publishing date: 2017-10-24
• type: Thesis
• publication status: published
• subject: Engineering and Technology
• keywords: Bioaugmentation, Digester supernatant, Inoculation, Nitrification-denitrification, Nitrifiers, Nitrous oxide emissions, Reject water, Bioaugmentation, Digester supernatant, Inoculation, Nitrification-denitrification, Nitrifiers, Nitrous oxide emissions, Reject water
• pages: 140 pages
• defense location: lecture hall KC:B, Kemicentrum, Naturvetarvägen 14, Lund University, Faculty of Engineering LTH, Lund
• defense date: 2017-11-17 13:00:00
• ISBN: 978-91-7422-545-7
• ISBN: 978-91-7422-546-4
• language: English
• LU publication?: yes
• id: b60a0edf-814d-4758-84be-c4af6d1ed53d
• date added to LUP: 2017-10-23 17:55:08
• date last changed: 2018-11-21 21:35:32