----------------------------------------------------------------- O₂ input, kg 0.862 ----------------------------------------------------------------- CO₂ output, kg 1.056 ----------------------------------------------------------------- Drinking water, liters 2.5 ----------------------------------------------------------------- Food rehydrating water, liters 1 ----------------------------------------------------------------- Caloric value of food, kcal 3000 ----------------------------------------------------------------- Water output: ----------------------------------------------------------------- Urine, liters 1.6 ----------------------------------------------------------------- Respiration and perspiration, liters 2.13 ----------------------------------------------------------------- Feces, kg 0.09 ----------------------------------------------------------------- Total heat output, Btu 11 100 ----------------------------------------------------------------- Two types of biological regenerative systems have been proposed. The photosynthetic closed ecological system was proposed as early as 1951. This involves the use of single-celled algae or higher plants, including floating aquatic and terrestrial plants, and requires the interaction of light energy with CO₂ and H₂O to produce O₂ and plant cells. Another system, proposed in 1961, involves electrolysis of water into oxygen and hydrogen, and the concurrent use of _Hydrogenomonas_ bacteria which take up hydrogen, some oxygen, carbon dioxide, and urine yielding water and bacterial cells.