Submerged-Membrane Bioreactor for the Treatment of NASA Greywater

Potable water represents a key component of life support for extended-duration space missions and the continued operation of the International Space Station. Treatment systems for the recycling of wastewater collected from routine hygiene, urine and humidity condensate must be efficient, low maintenance and have reasonable power requirements. Current physiochemical systems are energy inefficient and require consumables that add to stowage space and mass. Biological pretreatment systems offer a potential means of improving wastewater treatment by using microorganisms to utilize contaminants as either as source or sink for electrons; processes well understood in terrestrial wastewater treatment processes. Membrane bioreactors present a possible means of implementing biological wastewater processing in a microgravity compatible manner.;A novel submerged-membrane bioreactor was constructed to examine their treatment efficiency of a NASA wastewater ersatz. The bioreactor utilizes both anoxic and aerobic processes for carbonaceous and nitrogenous species removal. While complete biological nitrification proved difficult due to free ammonia inhibition, by acidifying reactor feed consistent nitrification was observed. An unexpected observation was nitrite accumulation, typically a transient species in wastewater treatment. Kinetic modeling suggests that nitrite accumulation results from inhibition of nitrite oxidizing bacteria due to a combination of free ammonia and nitrous acid.;Organic carbon removal averaged 92.0 +/- 5.2% and 91.0 +/- 8.2% for the bioreactors, less than the treatment criteria of 95% but similar to other biological treatment systems. Denitrification averaged 46.1 +/- 7.8% and 39.7 +/- 8.8% indicating that the feed is electron donor limited. Addition of exogenous electron donating substrates, namely hydrogen gas and methanol, showed insignificant improvement in denitrification for hydrogen gas but significant improvement with methanol, denitrification efficiency increasing to 61.8 +/- 10.3%. Analysis of effluent organic nitrogen indicates that the bioreactors are effective at removing organic nitrogen species, a key challenged encountered with current physiochemical systems.;A direct comparison with another biological treatment system demonstrated similar organic carbon removal efficiencies of 91.8 +/- 2.8% and 93.1 +/- 3.5%, but significantly enhanced denitrification efficiencies of 76.0 +/- 7.7% and 68.0 +/- 6.9%. The enhanced denitrification is consistent with improved denitrification due to shortcut biological nitrogen removal via nitrite and the observed nitrite accumulation. The comparison indicates that the submerged-membrane efficiently removes carbonaceous and nitrogenous wastewater constituents and also that nitrite accumulation is a potentially valuable area of research for improving nitrogen removal in biological pretreatment systems for NASA wastewater.