Graywater processing in recirculating hydroponic systems: Phytotoxicity, surfactant degradation, and bacterial dynamics

Incorporation of human hygiene water (graywater) into hydroponic plant production systems, and subsequent recovery of the water transpired by the plants, is one potential means for water purification and recycling in bioregenerative life support systems under development for long duration space missions. Surfactant phytotoxicity and the potential for growth of human-associated microorganisms were assessed in studies of wheat and lettuce in controlled environmental chambers to provide baseline information for future studies with actual graywater streams. Igepon TC-42 (sodium N-coconut acid-N-methyl taurate), a surfactant designated for use on the International Space Station and a common ingredient of soaps and detergents, was added to plant systems in three different modes: (1) pulse addition of 875 mg m^-2 growing area once a day, (2) continuous addition of 875 mg m^-2 over the course of a day, and (3) variable addition of 0-3000 mg m^-2 d^-1 based on plant water demand. The survival of three human-associated bacteria (Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa) in the plant nutrient delivery systems were monitored following introduction 6 (wheat) or 3 (lettuce) days after planting (DAP). Igepon rapidly disappeared (i.e., a half-life of less than 1 h) following an initial adaptation period lasting less than 2 days. Microbial degradation of Igepon was supported by appearance of the degradation intermediate methyl taurine and an increase in the numbers of bacteria able to grow on media containing Igepon as the sole carbon source in the Igepon treated systems relative to the control. Wheat growth was not significantly affected by any of the Igepon treatments, but lettuce yield was significantly reduced in the pulse and continuous treatments. E. coli and S. aureus decreased below detection limits within 3-5 days within the systems, but P. aeruginosa persisted in the rhizosphere, nutrient solution, and nutrient delivery system biofilm for the duration of the wheat (70-day) and lettuce (28-day) experiments.