Nitrogen stabilization in urine

During the storage of source-separated urine, urea − the main dissolved compound of urine − is degraded by bacteria to ammonia and bicarbonate. As a result, 90% of the nitrogen in stored urine is ammonium (NH₄⁺/NH₃) and the pH reaches values around 9. These conditions facilitate ammonia loss by volatilization. A biological process − nitrification − can stabilize stored urine by oxidizing ammonium to nitrate. The amount of ammonium to be oxidized depends on the alkalinity. In stored urine, the available alkalinity is sufficient to nitrify about 50% of ammonium to nitrate.

The two bacterial groups involved – ammonium oxidizing bacteria and nitrite oxidizing bacteria – are very sensitive towards changes in pH and nitrogen concentrations. Biofilm reactors tend to be more suitable for stable nitrate production due to their higher resilience to influent changes. In a moving bed biofilm reactor (MBBR) nitrate production rates of 380 gN/m³/d were achieved without pH control. Only 1% of the nitrogen in the effluent was nitrite.

Membrane aerated biofilm reactor (MABR). Silicone tubing is used as biofilm support and for diffusive oxygen supply (photo: Kai Udert)

We observed that biofilm reactors with counter diffusion of oxygen and ammonium facilitate the growth of acid-tolerant ammonium oxidizing bacteria. In a membrane-aerated biofilm reactor (MABR) fed with ammonium nitrite, biological ammonium oxidation in combination with chemical nitrite oxidation resulted in pH values as low as 2.5. All of the nitrite was oxidized according to the following mechanism:

2 HNO₂ ↔ NO + NO₂ + H₂O

NO + 0.5 O₂ → NO₂

2 NO₂ + H₂O → HNO₂ + NO₃^- + H⁺

An alternative process for urine nitrification could consist of an upstream biological reactor for ammonium oxidation to nitrite combined with a downstream chemical reactor for nitrite oxidation by acid dosage. This process promises to be highly resilient because biological ammonium oxidation in a sequencing batch reactor (SBR) is very stable. However, the chemical reactor has to be aerated in a way that minimizes the emission of environmentally harmful gases such as nitric oxide (NO) and nitrous oxide (N₂O).

Nitrification is an ideal pretreatment for the production of a highly concentrated urine fertilizer via evaporation. After urine has been stabilized during nitrification, the dissolved compounds can be recovered in a powder by evaporating the water without significant ammonia loss.

Literature:

Udert K.M., Larsen T.A., and Gujer W. (2005) Chemical nitrite oxidation in acid solution as a consequence of microbial ammonium oxidation. Environmental Science and Technology 39(11), 4066-4075.

Udert K.M., Fux C., Münster M., Larsen T.A., Siegrist H., and Gujer W. (2003) Nitrification and autotrophic denitrification of source-separated urine. Water Science and Technology 48(1), 119-130.