Methods for treating source-separated urine and recovering nutrients in a suitable form had already been developed by various research labs. But experience with nutrient recovery from urine outside the laboratory – let alone in large-scale field trials – was extremely limited. The goal of existing laboratory methods had been to recover specific nutrients from urine. The Durban project prompted us to seek uses beyond laboratory experiments, also for the urine collected at Eawag headquarters. Rather than extracting individual nutrients, we developed a new approach – separating the water in urine from the nutrients. What then remains is a concentrated nutrient solution.
The production of fertilizer involves two steps. First, half of the nitrogen present in urine in the form of ammonia is converted to nitrate with the aid of bacteria. This process, known as nitrification, is also used at WWTPs. In the second step, the liquid is evaporated in a distiller to produce a concentrated nutrient solution. Partial nitrification is sufficient to stabilize the nitrogen. Without this process, nitrogen would be released in the form of gaseous ammonia during urine storage and distillation. Stabilization also removes the pungent odour .
In the basement of Forum Chriesbach, this method is used to treat the urine collected from staff toilets. The nitrifying bacteria are kept in two transparent plastic columns (Fig. 1). Here, suspended plastic particles provide a substrate for bacterial colonization. Compressed air is used to supply oxygen to the bacteria and, at the same time, to ensure that the urine in the container is well mixed. Stabilized urine then flows into a distiller, where 97 per cent of the volume is removed and returned as distilled water to the toilet flushing system. The remaining 3 per cent contain all the nutrients: 30 litres of highly concentrated liquid fertilizer is thus obtained from 1000 litres of urine .
Initial tests successful
The effectiveness of the urine-based fertilizer has already been demonstrated in initial tests: compared with synthetic fertilizers, its performance was excellent  (Fig. 5a). The product contains all the substances required for plant growth – not just the most important nutrients (nitrogen, phosphorus and potassium), but numerous trace elements, such as iron, zinc or boron, which also contribute to plant health (Fig. 5b). The fertilizer contains almost no heavy metals. Some pathogenic bacteria are inactivated during nitrification . In addition, the VUNA product is pasteurized during distillation, and any viruses surviving the nitrification process are inactivated as a result.
One challenge, however, remains to be overcome – pharmaceutical residues. Depending on the origin of the urine and the intended use of the fertilizer, these substances may need to be eliminated via an additional treatment step. In the laboratory, pharmaceutical residues have already been successfully removed with the aid of activated carbon. This method will now also be tested in our pilot plant.