Water Reduction and Nutrient Reconcentration of Hydrolyzed Urine via Direct-Contact Membrane Distillation: Ammonia Loss and Its Control

Abstract

Water flushing increases the urine transportation cost in a source-separation system, a new concept in sustainable management of municipal wastewater. This study investigates the reduction of water and the reconcentrations of nutrients from hydrolyzed urine via direct-contact membrane distillation (DCMD), especially addressing the loss of ammonia. High rejections of phosphate and K were achieved (>97%) when hydrolyzed urine was concentrated by 17.8 times. However, total ammonia in the permeate increased to 11.0 g N L−1, resulting in a low rejection (31%) of total ammonia and reducing the quality of water generated. Many factors, including temperature difference, urine dilution ratio, urine pH, urea hydrolysis, and urine stabilization via nitritation, were investigated, aiming to minimize the ammonia loss. Full nitritation of urine induced low pH and reduced the concentrations of total ammonia and chemical oxygen demand. Accordingly, the rejection of total ammonia reached 94% in the concentrated urine, and solutes in the permeate water were significantly reduced. Therefore, biological stabilization of urine prior to the DCMD process is recommended for minimizing the ammonia loss and improving the quality of permeate water generated.