Possible risks for the functioning of cyclic processes in the experimental model of a closed ecosystem.

The purpose of the present study is to consider a number of possible risks that may emerge when processed human wastes are involved into mass exchange processes as fertilizers for plants cultivated in the experimental model of the closed ecosystem (CEEM). The problems relating to the disruption of cycling processes in closed ecosystems can be tentatively divided into two groups: the problems that can be rather easily overcome and the chronic problems. Addition of plant inedible biomass to the soil-like substrate (SLS) can result in a decrease in plant productivity because of allelopathic interactions and enhanced growth of microorganisms. The 30% decrease in wheat productivity by the end of long-duration experiments in the CEEM, with plants grown on quasi-non-renewed solutions based on liquid products prepared by physicochemical mineralization of human wastes, was caused by lower resistance of the plants affected by toxicants accumulated in the solution because of incomplete mineralization of the wastes. The reason for the differences between the macronutrient inflows and outflows was that the donor of human wastes followed a European-type diet while the system produced only part of the plant-based diet. Moreover, macronutrients were partly sorbed in rooting substrates and became unavailable to plants: the substrates in the system retained about 50% of the Ca and 20% ÷ 25% of the Mg, Na, and P inflows over one cycle. These problems are temporary and can be minimized in the foreseeable future.

A small closed ecosystem with an estimated portion of human metabolism.

The study describes a small closed ecosystem used to test technologies to be further employed in full-scale manned closed ecosystems. The experimental ecosystem is designed to use a certain portion of human metabolism, which is included in the gas, water, and organic waste loops of the system. In this experimental ecosystem, gas and water loops are fully closed, and the model enables processing of human waste and plant inedible biomass. A physicochemical method is used to remove pollutants from the air in the system. A human takes part in the gas exchange of the system through its respiration loop. This experimental ecosystem can be used for testing and improving new technologies to be further used in the future space stations.