Supervisor: keemia instituudi teadur Taavo Tenno, PhD
Opponent: prof William Hogland, PhD (Linnaeus Ülikool, Rootsi)
Autotrophic nitrogen removal technologies have been developed to save energy on aeration and on addition of organic carbon sources. In this study the technology of anaerobic ammonium oxidizing (anammox) bacteria enrichment and the performance of a moving bed biofilm reactor for the treatment of nitrogen-rich wastewater was studied in detail. Reject water the anaerobic digester effluent of the Tallinn Wastewater Treatment Plant was used as a substrate and also as a source of anammox bacteria. The anammox organisms were enriched onto the biofilm carriers' by selective environmental conditions and substrate loading. Satisfactory autotrophic total nitrogen (TN) removal rates (1 kg N m-3 d-1) were achieved for the treatment of digester effluent showing a potential for a full-scale application of the developed technology. The anammox process started up with carriers having nitrifying biomass on it showed a somewhat better stability against increased nitrite concentrations. Rapid overcoming from nitrite inhibition or tolerance to higher nitrite concentrations is important for sustaining a stable TN removal rate in anammox process operated at high substrate loading. We have demonstrated the acceleration of post-inhibition recovery of the anammox process by optimum (small) amounts of intermediate metabolites (hydrazine and hydroxylamine) on the anammox process, which is a matter of great interest as it can be used in case of inhibition in water treatment systems. After the start-up of two-stage deammonification systems, the nitritation process was developed from the nitrifying process by limiting the activity of nitrite oxidizing bacteria to minimum. A suitable technique for that was application of a combination of low HRT, increased free ammonia and inter¬mittent aeration. The deammonification process in a single reactor encountered problems when nitrite was further oxidized into nitrate in a form mainly unavailable to anammox organisms. Nitrite oxidation into nitrate was avoided by means of applying increased free ammonia (FA) concentration and also by increased HCO3- (CO2) concentrations. The methods developed for the start-up of one and two-step deammonification processes and for anammox bacteria enrichment and inhibition recovery can be applied for a full-scale cost-saving treatment.