Accelerating urea hydrolysis in fresh urine by modifying operating conditions of a sequencing batch reactor

Abstract

ABSTRACT A number of existing and emerging technologies can recover nitrogen from urine. A preliminary step in many nitrogen recovery processes is hydrolyzing urea to ammonium, a biologically-mediated process that can take days to weeks without intervention. The ability to achieve urea hydrolysis quickly and reliably would increase the feasibility of decentralized nitrogen recovery, especially where space and treatment time are constrained. The goal of this research was to determine whether urea hydrolysis could be accelerated by providing an inoculum containing microorganisms likely to have urease activity (feces or soil), providing a carrier to support attached growth (plastic carriers, granular activated carbon, or no carrier), and modifying the hydraulic retention time (HRT; 1.3, 2, and 4 days) and feeding frequency (Δt = 4, 24 h). Inoculated reactors achieved significantly more urea hydrolysis, and reactors inoculated with soil were able to sustain higher urea hydrolysis rates over time than those inoculated with feces. The mean zero-order rate constants (mM/hr) for reactors with a soil inoculum (15.1) were about three times higher than that of reactors with an inoculum of feces (4.9). A reactor with GAC and an inoculum of soil fed daily with fresh urine achieved greater than 90% hydrolysis with an HRT of 2 days; results suggest the HRT could be reduced to 16 h without reducing performance. No significant benefit was provided by increasing the frequency of feedings for the same HRT, likely because urease enzymes were saturated and operating at maximum hydrolysis rates during most of the reaction period.