Date of Award
12-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Environmental Engineering and Earth Science
Committee Chair/Advisor
Dr. Sudeep Popat
Committee Member
Dr. David Freedman
Committee Member
Dr. David Ladner
Committee Member
Dr. Mark Roberts
Abstract
Urine makes up approximately 1% of the total volume of municipal wastewater, but it contributes up to 75% of the nitrogen and 50% of the phosphorus received by publicly owned treatment works (POTWs). These elements are essential nutrients for plant growth and trigger algal blooms when released into natural bodies of water, resulting in rapid ecological degradation. Removing these nutrients from municipal wastewater is energy intensive and often operationally sensitive. The past few decades have seen an increase in focus on the source-separation and side-stream treatment of urine.
Urine characteristics differ greatly from those of municipal wastewater, presenting an opportunity to employ novel approaches for removal of nitrogen and phosphorus. The low volume per capita and high ionic conductivity of source-separated urine make it a viable candidate for electrochemical methods to be used for treatment. Electrochemistry presents an opportunity to tailor specific reactions and controlled rates while decreasing or eliminating the need for the addition of external chemicals. The potential to recover nitrogen and phosphorus as value-added fertilizers from urine presents an opportunity to reduce demand for traditional fertilizer production methods while simultaneously decreasing the costs required to remove these nutrients from municipal wastewater at POTWs. The strategies employed to treat source-separated urine largely depend on the age of the urine. Fresh urine is characterized by an aqueous solution and most of the nitrogen existing as urea. Urine aging is characterized by the hydrolysis of urea to ammonia by the urease enzyme, accompanied by an increase in pH and conductivity, as well as the precipitation of elements such as phosphorus, calcium, and magnesium.
One of the objectives of this dissertation was to evaluate nitrogen removal from aged urine using electrochemical ammonia stripping. A two-chamber electrochemical cell (EC) with the anode and cathode chambers separated by a cation exchange membrane (CEM) was used. The removal efficiency of ammonia was evaluated when feeding urine to either the anode or cathode chamber. Ammonia removal per power input was similar for cathode-fed and anode-fed systems at the highest current densities tested. Lower current densities favored the cathode-fed configuration for ammonia removal efficiency, reaching a maximum of 280 g NH3-N removed per kWh, albeit at reduced removal rates compared to higher currents. Despite the shortcomings of the ammonia stripping apparatus used in this study, NH3-N removal efficiency exceeded that of nitrification in conventional wastewater treatment (250 g NH3-N removed per kWh). The formation of chlorine (Cl2) and disinfection byproducts (DBPs) was also evaluated for each feeding method. Increasing applied current resulted in higher Cl2 concentrations and the formation of iodinated, brominated, and chlorinated DBPs. The highest current density tested, 8.6 A/L, resulted in 18.8 mg/L of total Cl2 and 2,455 μg/L of total DBPs. No Cl2 or DPBs were detected in cathode-fed samples.
The aging process of urine results in the uncontrolled precipitation of phosphorus and volatilization of nitrogen as free ammonia. While these phenomena represent a decrease in potentially recoverable nutrients, they are most readily observed as hard-to-remove scaling on plumbing fixtures and the generation of offensive odors. Stabilization of source-separated urine presents an opportunity for increased nutrient recovery while mitigating some of the problems commonly associated with urine diversion, collection, and storage.
The second objective of this dissertation was to demonstrate that hydrogen peroxide (H2O2) electrochemically synthesized in cathode-fed source-separated urine can successfully deactivate the urease enzyme and prevent the urine aging process. The pH increase associated with the reduction of oxygen (O2) in the urine resulted in the simultaneous precipitation of amorphous calcium-phosphate solids. The third objective of this research was to evaluate concomitant precipitation of phosphate and stabilization of urine with electrochemically synthesized H2O2 using a single-chamber EC equipped with a magnesium (Mg) anode and carbon-based gas diffusion electrode (GDE) cathode. The low reduction potential of the Mg anode generally mitigated the formation of Cl2 and did not oxidize the electrochemically synthesized H2O2. Over 98% of phosphate was precipitated out of solution at current densities over 0.71 A/L. The final pH of the urine, resultant of the amount of current applied, determined the speciation of precipitated solids. Phosphate was precipitated as primarily struvite at pH values less than 9. Above pH 10 the predominant solids precipitated were amorphous calcium and magnesium phosphate minerals. Treated urine exhibited stabilization if H2O2 residual persisted in solution, the duration of residual remaining depended on the initial concentration.
Mineral scaling was observed on GDE surfaces during phosphate precipitation experiments, increasing with successive experiments. The fourth objective of this dissertation was to evaluate the effect of mineral scaling on H2O2 electrochemical synthesis by GDE electrodes. No discernible decrease in H2O2 electrochemical synthesis efficiency was observed after sixteen batch cycles in any of the electrolytes tested. The greatest amount of scaling on GDEs was observed with synthetic urine samples at low current densities.
Recommended Citation
Arve, Philip, "Electrochemical Techniques for the Treatment of Source-Separated Urine" (2024). All Dissertations. 3822.
https://open.clemson.edu/all_dissertations/3822
