Stabilization of urine by nitrification in a Moving Bed Biofilm Reactor

Abstract: Municipal wastewater contains a lot of compounds that could be seen as resources. Nitrogen, phosphorus and potassium are common ingredients in fertilizers and can all be found in wastewater. Nowadays only a small part of the nutrients found in wastewater are used on arable land. Urine contains the most of the nitrogen, phosphorus and potassium secreted by humans. Source separation of urine is a way to facilitate recovery of nutrients in wastewater and to lower the load on existing wastewater treatment plants. Fresh urine contains urea which may undergo ureolysis and form ammonia during storage. In order to prevent ammonia volatilization during further treatment such as volume reduction, urine needs to be stabilized. One method for stabilization of urine is nitrification. Nitrification is a biological process performed in two steps. In the first step, ammonia is oxidized to nitrous acid by ammonia oxidizing bacteria. This oxidation lowers the pH, which prevents ammonia from vaporizing. The second step is performed by nitrite oxidizing bacteria which oxidize nitrite to nitrate. Since ammonia and not ammonium is the substrate for the ammonia oxidizing bacteria only a part of the ammonia will be oxidized as pH drops. Ammonia and nitrate are common compounds in fertilizers. In 2025, the housing area Sege Park in the city of Malmö should function as an example for sustainable city development. One goal is that the area should accommodate at least one test facility for source separation of urine. Before the construction of the facility can begin VA SYD, the regional water and sewer organisation, wants to investigate different volume reduction methods. Because ammonia may vaporize during volume reduction, they also want to investigate different stabilization methods. The purpose of this work was to study the start-up process of a nitrification reactor for stabilization of urine. A bench-scale moving bed biofilm reactor was run for 57 days. The highest nitrification rate measured was 0.97 g N m-2 d-1. Nitrite accumulation, which is very problematic for a nitrification reactor, occurred two times. The first nitrite accumulation was handled by increased air flow and decreased load. To get rid of the second nitrite accumulation the solution in the reactor was diluted until the nitrite concentration was not measurable (< 0.6 mg L-1). The dilution resulted in a second start-up of the reactor. The two nitrite accumulations occurred at inlet nitrogen concentrations of 1,220 mg N L-1 and 1,320 mg N L-1 respectively. The load was 2.10 g N m-2 d-1 before the first nitrite accumulation and 1.54 g N m-2 d-1 before the second nitrite accumulation. After the second start-up the reactor ran stably and the inlet nitrogen concentration was increased to a maximum of 1,450 mg N L-1. In order to find signs of process instabilities early and to prevent nitrite accumulations it is important to measure pH and dissolved oxygen in the reactor. Both parameters may indicate if something happens with the process. If any or both of the parameters are regulated, flow changes can probably be studied instead. It may also help to regularly measure the nitrite concentration in the reactor in order to be able to prevent nitrite accumulation from inhibiting the nitrification process.