Circular bioprocess for phosphorus nutrient recovery to grow lettuce in lunar space

Abstract

A looming global scarcity of the nutrient phosphorus (P) requires solutions to prevent losses via recycling and recovery. One approach is to address this practically in lunar space. Here we propose for the first time a ‘circular’ solution that uses P-fertiliser fed to lettuce-hydroponics to grow at scale to feed six (6) astronauts. The P-feed is created via minerals leaching from P-containing moon crust. Phosphorus in the non-edible part of the lettuce is recycled via mineralisation of the roots to ash. The ash permits the recovery also of potassium (K). Judicious experimentation to determine mineralisation at various temperatures (250 to 750 °C) confirmed that a ‘low’ treatment at 250 °C was optimal for P recovery supported via leaching with an organic acid. The leached solution contained, respectively, 117 mg P L⁻¹ and 1083 mg K L⁻¹. Significantly, for the P-bioprocess, a Material Circular Indicator (MCI) score was computed to be 0.59, a value similar to processes in ‘Circular Mars Biofactories’, and a factor of 2.7 with respect to conventional terrestrial extraction. A holistic sustainability assessment via Life Cycle Assessment (LCA) demonstrated a global warming potential (GWP) of 0.3 kg CO₂-eq. g⁻¹ and favourable E-factor (mass ratio of waste to product) 0.02. We reliably estimate N nutrient productivity via non-thermal plasma N₂-fixation in a N–P–K fertiliser and production in a moon colony, and conclude the hydroponic proposal will provide an N–P–K fertiliser of, respectively, 248, 47, and 350 mg L⁻¹, in lunar space to grow lettuce for six astronauts. Findings will be of benefit and interest to a wide range of engineers and researchers for growing food in circular fashion and under extreme conditions.