At higher cell concentrations, less volume of suspension would suffice if gas equilibration could be maintained at the higher consumption rates to avoid anaerobic conditions which could lead to a shift in metabolism. In the final analysis, the technical problem of gas transfer from the gas to the liquid phase determines the optimal cell concentration and, therefore, the required suspension volume. From data presently available, it can be concluded that, using the slow-growing _H. facilis_, the volume of suspension required to support one man is about 500 liters. Using _H. eutropha_, Schlegel ([ref.192]) calculated a suspension volume of 66 liters with 1 gram dry weight of bacteria per liter. In recent NASA-supported research, the amount of culture medium has been estimated using improved cultivation methods and conditions. For batch culture, the data show that from 10 to 66 liters would be required per man, with a best practical estimate of 20 liters at 9 to 10 grams dry weight of bacteria per liter ([ref.191]). For continuous culture using the turbidostat, the present data indicate a demand for some 30 liters of suspension, and a volume of 20 liters (at approximately 10 grams dry weight of bacteria per liter) as a realistic goal. In the foregoing section, the material balance for gases and water was discussed. It was shown that a close match could be obtained with these components of the closed environment. Less abundant, though no less important, are the nonwater components of urine and feces. The urine is important for the content of fixed nitrogen and other products of man’s metabolism and serves as a very effective substrate for cultivation of hydrogen bacteria. Maximum closure of the system necessitates utilization of the urea in urine as a nitrogen source. The average man produces 1.2 to 1.6 liters of urine per 24-hour period. This contains about 0.00005 gram per liter of iron, 0.113 gram per liter of magnesium, and 24.5 grams per liter of urea ([ref.193]). As shown in table X, each liter of bacterial medium requires 0.008 gram per liter of Fe(NH₄)₂(SO₄)₂, about 0.1 gram of MgSO₄·7H₂O, and 1.0 gram per liter of urea. In comparing the daily urine output with the estimated required ingredients of a bacterial medium, a relatively close balance is observed, with the exception of iron.