Department Process Engineering

In contrast to phosphorus, nitrogen is an unlimited resource. 78% of the atmosphere consists of dinitrogen, which can be transformed into ammonium with the help of the Haber-Bosch process. In rural areas and in regions with a strong need for nitrogen fertilizer, direct recycling of urine to agriculture can be economical. In many urban areas, however, direct application of urine for nitrogen recovery is not economically interesting because of the lack of agricultural land and the need for costly transport out of urban areas.

For these reasons, we not only focus on nitrogen recovery but also on nitrogen removal from urine. In this project we investigate the one-stage nitritation/anammox process for decentralized nitrogen removal from urine. The process is resource-efficient and has been used in many wastewater treatment plants to remove nitrogen from digester supernatant. However, small on-site reactors pose new challenges. The process stability and resilience tends to be small because of low diversity within the different bacterial groups. An added challenge associated with urine is the high content of biodegradable organic substances, which fosters the growth of heterotrophic bacteria.

The key process bacteria are aerobic ammonium oxidizing bacteria, anaerobic ammonium oxidizers (anammox bacteria), heterotrophic bacteria and – under non-ideal process conditions – nitrite oxidizing bacteria. Microbiological methods (e.g. PCR and DGGE) are used to determine the diversity of the microbial community, to identify the bacteria and to quantify the main organisms and their enzymatic activity. Besides low performance, process instability can produce harmful intermediates such as the microbial toxicant nitric oxide (NO) or the atmospheric pollutant nitrous oxide (N2O).

So far, nitrogen removal rates of 430 mgN/L/d at removal efficiencies of 92% have been achieved in a sequencing batch reactor (SBR). In the next project phase, a rotating biological contactor (RBC) will be operated to compare the process stability and performance of the two reactors.  können.

Literature

Udert K.M., Kind E., Teunissen M., Jenni S., Larsen T.A. (2008) Effect of heterotrophic growth on nitritation/anammox in a single sequencing batch reactor. Water Science and Technology 58(2), 277-284.