Dewatering of source-separated human urine for nitrogen and water recovery using air gap membrane distillation (AGMD)

Abstract:

Human urine is the largest nutrient contributor to municipal wastewater’s nutrient load, which may cause sever eutrophication in public water bodies. Instead of direct releasing urine into the municipal wastewater, reclaiming nitrogen from urine is potentially capable of replacing 20-25% of commercial fertilizers and reducing the energy consumption for nutrient removal in wastewater by up to 30%. Membrane distillation (MD) has been considered as a promising dewatering process to concentrate the human urine due to its lower operating temperatures to distill water than conventional distillation, lower operation pressures than reverse osmosis, lower membrane fouling, and capability of harvesting or utilizing low-grade heat source. The present study systematically investigated the efficacy of nitrogen and water recovery of human urine in air gap membrane distillation (AGMD). The highest water vapor transfer and the least ammonia transfer to the permeate side in the AGMD was achieved with optimization of the feed solution pH, temperature, and total ammonia-nitrogen concentration. The results showed that with 20 ^oC on the coolant side, and a high temperature of 60 ^oC on the feed side, the process exhibited a high-water vapor flux (~24 L·m^-2·h^-1) and a low specific ammonia transfer (< 0.1 g/L) in AGMD system. A positive relationship between specific ammonia transfer and free ammonia concentration was identified under different total ammonia-nitrogen concentrations and pH. The acidified feed at pH 5 reduced the specific ammonia transfer. The experimental results were successfully analyzed by and consistent with the model prediction using Comsol Multiphysics 5.5.