Up to 80 percent of the nitrogen and 50 percent of the phosphorus in the wastewater streams that enter the wastewater treatment plant (WWTP) comes from urine but urine accounts for only one percent of the wastewater stream. If urine is collected and treated separately, water pollution can be controlled better, it might be possible to close nutrient cycles and WWTP’s can be dimensioned smaller and the risk of ammonium discharges from combined sewer overflows can be reduced.
In this work the possibilities are investigated to remove nitrogen from urine in a sequencing batch reactor (SBR) with the help of ammonium oxidizing bacteria, Anammox bacteria and heterotrophic bacteria.
The SBR was first operated with digester supernatant to replace the bulk volume. Hereafter it was operated with five time diluted urine. The urine was diluted five times to have about the same concentrations as in the digester supernatant. In the beginning of the operation of the reactor each batch started with an anoxic phase to facilitate the use of nitrate for the degradation of dissolved organic substrate. The goal was to reduce nitrite and COD concentrations concomitantly.
With experiments it could be shown that in increase of the air flow from 40 mL/min to 50 mL/min
did not increase the oxygen concentration in the reactor. It could also be shown that during the aerobic phases nitrite build up takes place; the Anammox bacteria are too slow to reduce all the nitrite that is produced by the ammonium oxidizing bacteria. In anoxic phases nitrite reduction takes place.
The efficiency of the reactor can be enhanced by operating the system with continuous aeration. In this way nitrifying bacteria and Anammox bacteria can work continuously what makes the batches shorter. However, when the system was changed to continuous aeration the nitrite concentration in the reactor increased a lot. This was caused by reduction of nitrate to nitrite by heterotrophic bacteria and nitrite production in the sedimentation phase. The initial anoxic phase was deleted and the batches ended with an anoxic phase for the Anammox to reduce the nitrite concentrations. It was not possible to operate the reactor with continuous aeration.
Higher pH values in the reactor have positive effects on the nitrogen removal rate but not at the reduction of the total nitrogen concentration. The pH in the reactor can be increased by exchanging more of the volume with urine, by adding basic chemicals or by stripping of CO₂. It could also be shown that long batches (about 30 hours) are not better for the nitrogen degradation compared to shorter batches (about 15 hours) because in the second half of the batch only seven percent more degradation is reached.