This thesis concerns nitrification in different environments. Nitrification is a biochemical reaction of ammonium into nitrite and further nitrate. In drinking water distribution systems (DWDSs), nitrite formed in nitrification can be problematic because it is potentially harmful to humans. On the other hand, in wastewater treatment plants (WWTPs), nitrification is utilized in biological ammonium removal from wastewater. This thesis concerns the spatial and seasonal inspection of nitrite formation in full-scale DWDSs. In DWDSs, nitrite is formed by an added disinfection chemical, monochloramine, or ammonium in the raw water. Furthermore, this thesis inspects nitrification in relation to organic matter, in both WWTPs and in laboratory-scale simulated distribution systems (SDSs). The spatial analysis of water quality revealed that nitrite was forming rapidly after leaving the water treatment plant (WTP) in the DWDSs. The nitrite concentrations tended to be low in stagnating water. The normal low dose of monochloramine (0.35–0.4 mgCl2 L-1) was not high enough to prevent nitrite formation; however, it limited the maximum nitrite concentrations. Nitrite concentrations exhibited seasonal peaks either in the warm season or cold season, or there was no observable seasonal peak. The key drivers causing seasonality were water temperature and water age. The nitrite peaks in the cold season were caused by the decelerated ammonium oxidation. The dominant reaction at low water ages was ammonium oxidation into nitrite; and at high water ages, it was nitrite oxidation into nitrate. These results emphasize comprehensive and year-round nitrite monitoring in the DWDSs. In wastewater treatment, ammonium removal via nitrification was unexpectedly enhanced when the soluble organic matter concentrations were increased with pre-fermentation, retrofitted in a previously used pre-sedimentation basin. However, upon closer inspection, it was revealed that the organic load into the nitrification basin was reduced in the pre-fermentation line by the preceding denitrification and biological phosphorus removal, compared to the pre-sedimentation line. In non-disinfected conditions of tap water, decreasing the natural organic matter (NOM) in the water (TOC 1.0 mg L-1) prevented nitrite formation in SDSs, compared to unreduced NOM (TOC 1.6 mg L-1). When the results were interpreted with a pseudo first order reaction rate model, it was observed that the decreased nitrite concentrations were a result of enhanced nitrite oxidation. The maximum nitrite concentrations were strongly dependent on the ratio of the ammonium and nitrite oxidation activities. This study supports enhanced removal of NOM at the WTPs to also decrease the potentially harmful nitrite concentrations. The results the thesis provide more possibilities to control nitrification in DWDSs and at WWTPs.
|Translated title of the contribution||Nitrifikaatio talousveden jakelussa ja jäteveden puhdistuksessa – Syyt, seuraukset ja orgaanisen aineen vaikutus|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- drinking water distribution
- wastewater treatment