Designing and Evaluating Novel Approaches to Nitrogen Recovery from Source-Separated Urine
Urine is a “waste” stream that contains the majority of nitrogen, phosphorus, and potassium present in domestic wastewater. Separate collection and treatment of urine can potentially reduce treatment costs, environmental pollution due to nutrient discharges, and dependence on energy-intensive synthetic fertilizers. Closing the nitrogen loop between conventional fertilizer production and wastewater treatment can improve system efficiencies in industrialized settings and generate revenue that can be used to enhance sanitation access in developing communities. Harnessing the benefits of urine separation requires selective separation of nitrogen from other urine constituents. In this dissertation, two unit processes were characterized to recover nitrogen as ammonium from urine: ion exchange and electrochemical stripping. Observations were made at several scales, from molecular-scale mechanisms to process performance metrics and systems-level evaluation. In sum, this dissertation furthers understanding of two processes for recovering nitrogen as ammonium from urine as well as the overall source separation and resource recovery paradigm.
Ion exchange is an established technique for separating charged species from solutions and has recently been applied to urine. Household cartridges could be employed to concentrate nitrogen at the source of urine production. Four adsorbents were compared using equilibrium isotherms and fixed-bed columns; Dowex Mac 3, a synthetic ammonium-specific resin, exhibited the highest adsorption density and regeneration efficiency. Based on comparing synthetic urine solutions of increasing complexity and similarity to real urine, competition from sodium and potassium did not significantly decrease ammonium adsorption. Because urine composition varies, several models were compared to experimental data to predict adsorption isotherms. Flow rate did not significantly affect ammonium adsorption nor regeneration; higher ammonium concentration in urine and proton activity in regenerants increased adsorption and regeneration efficiencies. Only trace organics that were positively charged at urine pH values (~9) adsorbed, and only two of ten compounds studied were detected in the ammonium sulfate product. Potassium recovery via cation exchange and phosphate recovery via struvite precipitation and anion exchange did not significantly affect ammonium adsorption, indicating potential implementation of complete nutrient recovery treatment trains.
Electrochemical stripping was shown to recover nitrogen as ammonium sulfate from urine based on the charge and volatility of ammonia. This novel approach combines electrodialysis with membrane stripping and selectively recovered 93% of nitrogen in batch studies at lower energy consumption than conventional ammonia stripping. Based on continuous-flow nitrogen fluxes, transport from the cathode to the trap chamber was identified as the rate-limiting step of electrochemical stripping. Synthetic urine solutions were used to determine the effects of urine composition and operating parameters on nitrogen flux, recovery efficiencies, and competing reactions. Electrochemically produced chlorine was found to be a loss mechanism for nitrogen but was mitigated by organic compounds present in urine. Nitrogen was selectively separated from other urine constituents, including sodium, potassium, inorganic anions, trace organic compounds, and metals.
Building on the molecular and process-scale insights in the laboratory, ion exchange for nitrogen recovery was evaluated in a container-based sanitation system in Nairobi, Kenya. No losses in performance metrics were observed over ten adsorption-regeneration cycles or with locally-produced columns ten times larger than lab scale. Parameters that can be economically and regularly measured to indicate process performance, such as absorbance and electrical conductivity, were identified as useful proxies for urine and ammonium breakthrough. While ammonium sulfate produced from urine was slightly more expensive than trucking urine to a local waste treatment facility, urine-derived fertilizers could be produced for less than conventional fertilizers in Kenya. Producing fertilizer from waste could reduce costs of sanitation provision and increase access to fertilizer.
Insights from this dissertation are useful to researchers and practitioners interested in closing loops to generate revenue from “waste” streams and reduce environmental impacts of excreta treatment. Adding fundamental understanding of two nitrogen recovery processes informs the design of pilot and demonstration scale installations to separately collect and treat urine at household and building scales. Beyond urine treatment, ion exchange and electrochemical stripping can be applied to other nitrogen-rich waste streams such as anaerobic digester effluent, landfill leachate, and food processing wastes.