Energy-Efficient Nutrient Recovery from Real Urine for Hydroponics Based on Alkalinization, Full Nitrification in a Trickling Filter and Limited Supplementations

Human urine is considered a major stream of nitrogen mass flow in domestic wastewater, which is widely available and rich in valuable nutrient resources for hydroponic cultivation. In this study, a promising concept of nutrient recovery from real urine was proposed, including urine alkalinization by Ca(OH)₂, full nitrification in a trickling filter, and chemical supplementations. The steady performance of urine nitrification among different urine-collecting batches indicates the robustness of the trickling filter. An optimal hydraulic loading rate of 2.1 m³ m^–2 h^–1 sufficed the dissolved oxygen and urine circulation in the trickling filter, achieving a nitrate production rate of 223 ± 2 mg N L^–1 d^–1 with an efficiency of 90 ± 2% at pH 6 and 21 °C. The electrical energy consumption was only 1.15 kWh kg^–1 NO₃^–-N production at a short hydraulic retention time of 1 day. Among all of the three types of pH control reagents (i.e., Ca(OH)₂, CaCO₃, and K₂CO₃), K₂CO₃ could enhance the activity of ammonium-oxidizing bacteria by raising the inorganic carbon level in the trickling filter and subsequently result in the lowest supplementation of extra macronutrients (i.e., nitrogen, phosphorus, and magnesium) to the urine-sourced nutrient solution. Batch tests showed that the highest activity of ammonium-oxidizing and nitrite-oxidizing bacteria was in the bottom compartment of the trickling filter, consistent with the vertical stratification of their relative abundance. Overall, the proposed novel concept was demonstrated to be robust and energy-efficient in nutrient recovery from real urine for hydroponics.