The water cycle is an important part of the controlled ecological life support system (CELSS), which involves the supply of drinking water and plant irrigation water and the recycling of urine, sanitary wastewater, and air condensate water. Using urine and sanitary wastewater for plant culture is an effective way of wastewater reuse, but there are three main challenges. Firstly, some salt content in wastewater, especially sodium chloride, is high, but it is not necessary for plant growth and may inhibit plant growth. Secondly, the wastewater contains organic components such as surfactants, which also has a negative impact on plant growth. Thirdly, if wastewater is used as the main nutrient source, the proportion of mineral elements in wastewater might be inconsistent with the proportion of plant growth needs, which will also affect the normal growth of plants.

In a research paper recently published in Space: Science & Technology, Liangchang Zhang verified the feasibility of reusing treated wastewater (including sanitary wastewater and urine wastewater) as a plant hydroponic medium.

Two hydroponic media were used for hydroponic cultivation. One was the reused medium prepared by recovering domestic wastewater from CELSS, and the other was the standard plant nutrient solution Hoagland nutrient solution. The composition of the two nutrient solutions was shown in Table 2. The reused medium had a higher concentration of nitrogen (N) and potassium (K) and roughly equal magnesium (Mg) and sulfur (S) concentrations.

Two kinds of crops, apium graveolens Linn (celery and mesembryanthemum cordifolium L.F. (cordifolium) were selected in this study, which showed good salt tolerance in previous research. Celery was cultivated in box A and C, while cordifolium was planted in box B and D. For box A and B, the plants were cultivated with reused medium, and for box C and D, Hoagland nutrient solution was used.

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"Is it feasible to resume high salinity wastewater as a plant nutrient medium for plant hydroponics in CELSS?" in Space Science & Technology, DOI 10.34133/2022/9853421.