As basis for a nutrient recovery in biological methanation, the rejection performance of a polyamide reverse osmosis membrane treating a multiple-salt solutions at 65°C is experimentally investigated. The samples are (a) composed of an abiotic salt solution and (b) of a biotic mixture of salt solution and archaea (Methanothermobacter thermautotrophicus). The rejection is calculated based on conductivity measurements. In diafiltration mode, the use of the membrane leads to 60-fold more time for the abiotic solution to reach 25% of the initial concentration, compared to a theoretical calculation without reverse osmosis. In normal operation mode, the membrane stabilizes at 90% rejection. The rejection performance of the membrane and the biotic solution is impeded by cell lysis and a resulting ion release. Quantitative analyses of the feed and permeate streams reveals an unequal retention of the different ion species. The group of better retained ions contains molybdenum, cobalt, magnesium, iron, nickel, and phosphorus; the group of lesser retained ions consists of sodium and potassium. An economic calculation shows that the membrane system can save 84% ± 12% of the cost of the ions used.