A detailed study was conducted on nitrification using a bench top bioprocessor system proposed for water recycling of a urine-soap wastewater expected to be generated by crewmembers on International Space Station (ISS) or similar long-term space missions. The bioprocessor system consisted of two packed bed biofilm reactors; one anoxic reactor used for denitrification and one aerobic reactor used for nitrification. lnfluent wastewater was a mixture of dilute NASA whole body soap (2,300 mg/L) and urea (500 mg/L as organic nitrogen). During two months of steady-state operation, average chemical oxygen demand (COD) removal was greater than 95%, and average total nitrogen removal was 70%. We observed that high levels of nitrite consistently accumulated in the aerobic (nitrifying) reactor effluent, indicating incomplete nitrification as the typical end product of the reaction would be nitrate. Average levels of nitrite (NO2−) and nitrate (NO3−) observed in the aerobic reactor were 21 and 28 mg-N/L, respectively. Incomplete nitrification, or nitrite accumulation, may be advantageous when coupled with the anoxic denitrification process, during which nitrite are reduced to N2 gas. Aeration requirements for nitrification of the urea waste would be reduced, as are organic carbon requirements for denitrification. Furthermore, relying on a partial biological reaction would reduce the residence time and reactor volume required for wastewater processing.
Reactor conditions associated with incomplete nitrification were investigated, especially pH and oxidation-reduction potential (ORP), for purposes of process monitoring and control. Both pH and ORP were strongly correlated (R2 = 0.80) with the ratio of NO2− to total oxidized nitrogen species (NO3− + NO2−) in the aerobic reactor effluent. More than 50% of the oxidized nitrogen was NO2− when aerobic reactor pH ranged from 6.3 to 7.8 and ORP ranged from +0.01 to +0.17 volts. The ORP values observed were lower than those found in highly aerobic systems usually associated with biological nitrification, in spite of the fact that the observed dissolved oxygen concentration was greater than 6 mg/L. A further set of experiments, focusing on the nitrification step, were set up to improve nitrification efficiency and induce partial nitrification using pH control (above 7.7) and lowering oxygen supply to control ORP. The use of online measurements as indicators of process performance was further investigated. Nitrite accumulation was initially observed during reactor startup and steadily decreased as the reactors ran. Reducing the oxygen supply caused some nitrite accumulation but it was not sustained. Ammonia conversion was much higher in the reactor with pH control than the reactor without pH control. Both pH and ORP proved to be good indicators of process performance. ORP was correlated to the fraction of nitrogen converted to total oxidized nitrogen species (NO,), with an R value of 0.7 in both reactors.