Composting inedible crop residue for advanced life support systems: Nutrient extraction and recycling for hydroponic plant growth

In-vessel composting is being testing as a solid waste processing approach as part of the development of Advanced Life Support (ALS) systems to maintain humans during long-term space missions such as a lunar or Mars base. Preliminary studies conducted at Kennedy Space Center have focused on composting of inedible residue from hydroponically grown plants, a large and unique waste stream generated as part of biomass production within ALS systems. This report focuses on the 1) nutrient mass balance during the composting process, 2) the efficiency of nutrient leaching from the compost, and 3) the effects of recycling leachate to hydroponic systems. Over 90% of most elements (K, Ca, Mg, and P), but only 64% of N, present in the feed material was present in the compost after 21 days. Nitrogen losses were most likely due to denitrification, although untrapped volatilized ammonia was another potential sink. Recovery of nutrients from the compost by leaching ranged from 13% for Ca to 100% for K, and values were generally higher than previously reported data for other composts, particularly for N and P. Use of compost leachate in hydroponic systems supporting wheat plants resulted in increased microbial cell density in the rhizosphere, one potential mechanism for observed reductions in plant growth.