Surfactants effect on aeroponics and important mass balances of regenerative life support system - Lettuce case study

In order to be able to permanently settle other celestial bodies, it is necessary to create an efficient and closed life support system. Such a system will allow high autonomy and significantly reduce operating costs in a future colony. Grey water is a major waste stream in terms of the water volume generated by colonists, and its reuse is necessary. One possibility is to reuse it in aeroponic cultivation without treatment, which will in turn reduce its transported mass. The article focuses on water, carbon and other elements (N, P, K, Ca, Mg, Na) that are included in the mass balance of part of a life support system containing the aeroponic cultivation of lettuce that is supplied with clean water and water contaminated with one surfactant, crewmembers and a urine nitrification reactor. Three surfactants (Sodium Laureth Sulfate (SLES), Sodium Dodecylbenzene Sulfonate (SDBS), Sodium Methyl Cocoyl Taurate (SMCT)) in two concentrations, reflecting concentrated and diluted grey water, were tested. The growth of lettuce exposed to low concentrations (ca. 0.07 g·L^-1) of surfactants has a moderate (SLES, SMCT) or no (SDBS) impact on the mass of grown plant. Exposition to high concentrations (>1.0 g·L^-1) led to the complete failure of cultivation (SDBS, SMCT) or to very limited growth (SLES). In all cases, exposition to surfactants in low concentrations causes differences in the mineral composition of lettuce. In most cases, crops cultivated on water containing surfactants were characterized by a decreased harvest index (the ratio of edible biomass to total biomass). For the most cultivations the relative mass balances of water, C, N, P and K were generally unaffected by surfactants, while for Ca, Mg and Na, noticeable differences occurred. The data provided in the paper gives clues about which surfactant could be used in a future extraterrestrial colony.