Abstract: The invention relates to a method for obtaining protein-containing animal feed from organic substances dissolved and/or suspended in water, where the substances are biologically processed in at least two series-connected stages (I, II) each having an activating tank (5, 7) and a settling tank (6, 8). In the first stage (I) a larger percentage of bacteria than protozoea prevails. The sludge is drained intermittently and alternatingly from the first and second stage (I, II), and conducted to a dehydrating device (9), and a part of the dehydrated sludge is returned to the input (5a, 7a) of that stage from which the sludge was taken. Into at least one of the activating tanks (5, 7), especially into the activating tank (5) of the first stage (I), additives promoting the growth of the micro organisms, especially trace elements and/or minerals, are placed.