Urine nitrification: Start-up with high strength urine

Sammanfattning: The importance of recycling nutrients becomes more important as the finite resources starts running out, and recycling of urine nutrients is an important factor in developing a sustainable society that takes advantage of nutrients already existing in the system. Urine only represents one percent of the domestic wastewater volumetric flow, but contains 80 % of the nitrogen, 50 % of the phosphorus and 60 % of the potassium of the total wastewater. Urine nitrification with following distillation is one of the promising methods of retrieving the nutrients from urine on a large scale. The end product is a very concentrated liquid or dry fertiliser that can be used on agricultural lands and substitute the commercial fertilisers springing from finite sources and industrial nitrogen fixation. In this study, the aim was to develop the urine nitrification process by studying the dynamics during start-up with high strength urine. Two reactors were operated simultaneously with the only difference in influent concentrations of urine; one with 25 % urine to water and the second one with 50 % urine. The inflow was controlled with pH regulated pumps. The reactors showed similar patterns with an initial rapid increase in nitrification rate which decreased significantly after a few days of operation, probably due to the high concentrations of nitrate, ammonium and salts in both reactors. The nitrification rates increased when the pH set-point was adjusted from 6.2 to 6.3, which influenced the availability of the substrate ammonia. During changes in pH the nitrite concentrations increased with a subsequent increase of nitrous oxide concentration. The maximum nitrification rate was 0.63 g NO3--N m-2 d-1 with a total nitrogen concentration of approximately 4,400 mgN L-1 in the reactor. In order to incorporate a fast start-up with high strength urine it is recommended to regulate the inflow pump with a pH set-point of 6.3, possibly higher, and after a sufficient nitrification rate is reached the pH set-point should be set to lower values to keep a stable balance between ammonia and nitrite oxidising bacteria. By having low nitrite accumulation nitrogen loss in form of nitrous oxide emissions can be avoided.