Title: Advancing Membrane Technologies for Recovery of Phosphorus and Nitrogen from Human Urine
Description: The existing linear economy approach to nutrient management has clear shortcomings including high expenditures for nutrient extraction and production of fertilizer as well as additional costs for nutrient removal at downstream waste water treatment plants (WWTPs) to prevent the pollution of aquatic environments. In a circular nutrient economy, phosphorus (P) and nitrogen (N) are removed from waste streams and captured as valuable fertilizer products in order to more sustainably reuse the resources in closed-loops and simultaneously protect receiving aquatic environments from harmful P and N emissions. The overarching aim of this thesis is to understand strategic approaches for nutrient recovery from wastewater and advance membrane technologies for P and N reclamation. The studies i.) approach nutrient recovery on a system-level to recognize optimal waste streams to target for P and N separation, ii.) advance membrane-based processes for nutrient recovery, and iii.) examine the economic viability of the nutrient recovery techniques.The thesis presents a thermodynamic and energy analysis of nutrient recovery from various waste streams of fresh and hydrolyzed urine, greywater, domestic wastewater, and secondary treated wastewater effluent.

The analysis revealed comparative advantages in theoretical energy intensities for P and N recovery from nutrient-dense waste streams, such as fresh and hydrolyzed urine, compared to other more dilute sources. The thesis quantifies efficiencies required by separation techniques for nutrient reclamation to be competitive with the energy requirements of the prevailing industrial fertilizer production methods, i.e., phosphate mining and nitrogen fixation by the Haber-Bosch process. The dissertation examines and advances the performance of membrane-based processes for separation and recovery of P and N from diverted human urine. Donnan dialysis (DD), an ion-exchange membrane-based process, can capture and enrich orthophosphate, HxPO4(3−x)−, from source-separated urine. This work demonstrates the transport of Cl− driver ions down a concentration gradient, across an ion-exchange membrane to set up an electrochemical potential gradient that drives the transport of target HxPO4(3−x)− in the opposite direction, enabling P capture.

Importantly, H2PO4− is transported against an orthophosphate concentration gradient, which achieves uphill transport of P. The thesis also provides a framework to better understand the impact of different ions in the water matrix on P recovery potential and kinetics. The thesis presents a novel operation of membrane distillation (MD) — isothermal membrane distillation with acidic collector (IMD-AC) — to selectively recover volatile ammonia, NH3, from hydrolyzed urine. The innovative isothermal and acidic collector features, respectively, suppressed undesired water permeation and enhanced ammonia vapor flux relative to conventional membrane distillation (CMD). The elimination of water flux in IMD-AC resulted in ≈95% savings in vaporization energy consumption relative to CMD. Critically, IMD-AC achieved uphill transport of ammoniacal nitrogen, i.e., transport against a concentration gradient, demonstrating the promising potential of the technique for N recovery. The dissertation proposes an integrated bipolar membrane electrodialysis (BPM-ED), DD, and IMD-AC system to drive the separation and recovery of orthophosphate and ammoniacal nitrogen from human urine.

This work elucidates the role of pH and nutrient speciation (i.e., H2PO4− versus HPO42− and NH4+ versus NH3) on the performance of DD and IMD-AC. In the proposed configuration, BPM-ED generates acids and bases in situ to strategically control the pH of urine streams to benefit DD and IMD-AC performances. Strategic pH modification can enhance orthophosphate transport and selectivity in DD as well as ammonia transport and recovery potential in IMD-AC. Importantly, the analysis quantifies comparable specific energy consumptions of the proposed integrated membrane-based process to the existing approaches to P and N management. This thesis presents a preliminary economic assessment of onsite nutrient recovery employing DD and IMD-AC for respective P and N recovery from diverted urine. The analysis reveals opportunities to utilize widely-available waste chemical streams and recovered thermal energy to improve the economic viability of nutrient recovery. The largest capital expenditures are urine diversion toilets and additional piping for source-separation.

Preliminary analysis demonstrates that employing urine diversion in public sanitation rooms, as opposed to private bathrooms, can reduce these capital expenditures. Furthermore, realizing savings from avoided costs for downstream nutrient removal at centralized wastewater treatment plants in addition to fertilizer revenue can enhance the economic viability of the approach. Overall, this dissertation critically informs nutrient recovery approaches and advances membrane-based processes for P and N reclamation to facilitate a paradigm shift from an inefficient linear nutrient economy to a sustainable circular nutrient economy. The work reveals opportunities to minimize energy intensity for nutrient separation, advance the performance of membrane-based techniques for selective and energy-efficient nutrient recovery from urine, and enhance the cost-competitiveness of nutrient reclamation. The findings of this work support nutrient recovery efforts and provide important insights that can be applied to other separation and resource recovery endeavors.