Stabilization of Urea in Urine through the Electrochemical Generation of Hydrogen Peroxide

Abstract

Urine represents a small fraction, usually 1-2%, of the total volume of a typical wastewater stream, but contributes over half of the nitrogen. This nitrogen, which is predominantly ammonium, must be removed prior to discharging the treated wastewater into the environment. Traditional nitrogen removal processes are often energy intensive and operationally sensitive. Source separation of urine presents an opportunity to divert most of the nitrogen received by a traditional wastewater treatment plant and allow direct treatment of this highly concentrated waste stream. Source separation of urine is not without its own operational difficulties though; flushless urinals often succumb to clogging and foul odors. This is in part due to the hydrolysis of urea by the urease enzyme which converts urea, the principal nitrogen compound in urine, to ammonia. This in turn produces odors and precipitates minerals which lead to mineral scaling. Source separated urine is also susceptible to biological growth, which along with scaling, are the primary causes of plumbing fixture failures in source separation installations. Source separated urine has been identified as a potential candidate for resource recovery, but the instability of fresh urine presents operational hurdles that must be overcome. The hydrolysis of urea by urease may be inhibited by dosing fresh urine with a sufficient volume of strong acid or base, or by strong oxidants such as hydrogen peroxide. These stabilizers are hazardous materials, and bulk storage of these chemicals presents inherent danger. We have investigated the use of electrochemical cells capable of producing hydrogen peroxide in-situ in fresh urine to determine the feasibility of this technology for urine stabilization. Urine is fed to the cathode chamber where a gas diffusion electrode reduces atmospheric oxygen to hydrogen peroxide, which deactivates any urease present in the urine. An added benefit to using hydrogen peroxide is its biocidal properties; urine samples treated with this approach displayed no biological growth after extended storage. Previous experiments have utilized exogenously added jack bean urease to simulate conditions present in source separated urine collection systems; control samples with no hydrogen peroxide present exhibited complete disappearance of urea, while samples with electrochemically synthesized hydrogen peroxide remained stable. Current experiments are exploring the long-term stability of treated urine after exposure to ureolytic bacteria to better simulate real world applications of this treatment approach.