the quantity of the sludge flowing through the under or overflow can be regulated the sludge drained off through the overflow 10 is removed as overflow sludge the sludge flowing through the underflow 11 is conveyed to an aerator 13 where it picks up atmospheric oxygen by flowing past aeration openings and becoming saturated or supersaturated with oxygen this saturated sludge is introduced into a mixing vessel 19 where together with the liquid from the circular activation basin 3 which has been aerated in a similar installation it is intermixed with the latter in the case of certain sewage particularly chemical sewage the more vigorous bacteria colonies are to be found in the overflow 10 so that in these cases the overflow is aerated and the underflow 11 is drained off as overflow sludge since the liquid and the sludge manifest different oxygen requirements as well as different reaction times this installation allows an optimal gasification the intermingled liquid is now channeled to a circular area 16 in which a partial degasification is set by enlargement of the free surface or a reaction time is set by the existing statistical settling time before the liquid is once again aerated by a siphon 20 and conveyed to the actual activation basin 3 the lower opening of the siphon 20 manifests a lateral arm 21 fig2 so that.