Water and nutrients recovery from synthetic source-separated human urine using AGMD

Human urine is one of the largest nutrient contributors to the municipal wastewater's nutrient load, which, if not properly managed, may cause severe eutrophication in natural waters. Reclaiming the nutrients from source-separated urine could potentially substitute 20–25% of commercial fertilizers and save up to 30% of the energy consumption for biological nutrient removal. The present study investigated the performances of urine separation and water recovery using air gap membrane distillation (AGMD), which recovers water via vapor conversion and transfer across the membrane and concentrates urine salts. The effects of operation conditions (e.g., the temperatures and flow rates of feed and coolant) on water permeate flux were examined. The results show that a high permeate flux (~14 L·m⁻²·h⁻¹) and a low specific ammonia transfer (< 0.1 g·L⁻¹) were achieved under optimal operations of the feed flow rate and the temperatures. Increasing feed flow or cold coolant flow or the thermal gradients can substantially increase permeate flux. Moreover, the Stefan diffusion model was used to compare and validate the experimental observations. The actual operational parameters and membrane properties (e.g., feed/coolant temperatures, air gap thickness, and membrane thickness) were employed in the model calculation without any fitting or unknown parameters. The modeling and experimental results matched well and highlights the promising potential of this model for guiding the design and operation of this AGDM process for urine separation and water recovery.