On-site urine treatment combining Ca(OH)₂ dissolution and dehydration with ambient air

We present the results of three field tests and three laboratory tests of a new physical-chemical urine treatment system, which can recover all nutrients, while pathogens are inactivated. The system consists of two steps. In the first reactor, biological processes including urea hydrolysis are prevented by mixing fresh urine with calcium hydroxide (Ca(OH)₂). Due to the high pH value and the high availability of calcium, phosphate can be recovered by precipitation. The high pH value also fosters the inactivation of microorganisms, including pathogens. In the second reactor, water is evaporated at low energy consumption by blowing unheated ambient air over the stabilized urine. Stabilization in the first reactor was successful in all field and laboratory tests. The pH value remained between 12 and 13, except for short dips due to shortages of Ca(OH)₂. Nearly all phosphorus (92-96%) precipitated and could be recovered as calcium phosphate in the first reactor, while nitrogen and potassium overflowed with the urine into the evaporation reactor. The efficiency of the second treatment step was very different for field and laboratory experiments and depended on the duration of the experiment. During a four-day laboratory test, nitrogen recovery was 98%. In contrast, nitrogen recovery was only around 20% in the long-term field experiments. The high nitrogen losses occurred, because biological urea hydrolysis was not inhibited anymore, when the pH value in the second reactor decreased due to the dissolution of high amounts of carbon dioxide from the ambient air. Potassium was not subject to any significant loss, and the measured recovery in the solid evaporation product was 98%. Evaporation rates ranged between 50 g m^-2 h^-1 (RH = 82±13%, T = 12±6°C) and 130 g m^-2 h^-1 (RH = 60±19%, T = 24±5°C) in the three field tests. Apart from some disturbances due to low supply of Ca(OH)₂, the urine module functioned without any substantial failures and was simple to maintain. The minimum consumption of Ca(OH)₂ at full capacity was 6 g·L^-1 urine and the electricity demand was 150 Wh kg^-1 water evaporated from urine, resulting in operational costs of 0.05 EUR pers^-1 d^-1.