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To limit eutrophication, wastewater treatment plants use biological methods to convert degraded nitrogen to nitrogen gas. Deammonification, or partial nitritation in combination with anammox, has been shown to be an energy efficient process. This process is currently implemented in approximately 150 full-scale plants, and mainly on reject waters, the liquid fraction after dewatering of anaerobic digestion at municipal wastewater treatment plants. Implementation has been impeded by the slow growth of anammox bacteria, and 99% of the full-scale plants using the process have been using different methods to inoculate the process with anammox bacteria from elsewhere. Separate reject water installations, however, have shown high nitrous oxide emissions, which could increase the total carbon footprint.

The objective of this thesis was to develop and validate a start-up concept using the moving bed biofilm reactor (MBBR) technique applied to reject water, and to investigate how the operational strategies could be optimized to limit potential nitrous oxide emissions. The results show that a one-stage deammonification process based on the MBBR technology with indigenous anammox bacteria originating from the reject water can be set up within a applicable time frame (<100 days). This was validated in two laboratory reactors and in two full-scale studies. Reject water originating from both mesophilic and thermophilic digested sludge was used. Anammox growth and nitrogen reduction were detected with fluorescence in situ hybridization (FISH) and chemical analysis, respectively. The start-up time was 72 days in the laboratory and 120 days in full-scale. In laboratory scale, there was no improvement in start-up time when adding external anammox inoculum. Results from a screening study of seven reject waters and their content of anammox bacteria using qPCR indicated the presence of 10⁴–10⁵ genome units anammox per mL in reject water, which could be sufficient for starting up deammonification plants within an applicable time frame.

A final case study shows the potential of decreasing nitrous oxide emissions when a full-scale plant treating reject water was modified from nitrification/denitrification using a Sequencing Batch Reactor (SBR) to a deammonification process using the MBBR technique. The nitrous oxide emissions decreased from 10% to 0.1–0.7% of total nitrogen load with the change of operation mode. Further optimization by pH set point led to lower emission values. This effect is thought to be linked to the lower aeration ratio and increase in complete denitrification of dissolved nitrous oxide at higher pH.