Abstract: Two samples of water are taken at two predetermined levels from a variable turbidity region disposed just below the sludge bed of a sewage treatment apparatus. The samples are alternately fed to a constant-level vessel whereat the turbidity variation between the samples is measured. The rate and duration of the sludge extraction operation are controlled as a function of such turbidity variation.

Abstract: A settling apparatus includes a decanter having a sludge bed therein. A raw water inlet leads raw water to be treated into the decanter, such that the raw water passes upwardly through the sludge bed, whereby impurities in the water are removed in the form of settling sludge, a portion of which collects in a concentrator. A vacuum source is connected to the decanter to allow for pulsating introduction of the raw water into the sludge bed. At least one siphon extends into the concentrator and is selectively connectable to the vacuum source to withdraw collected sludge from the concentrator and pass such sludge to a sludge removal station. The siphon is also selectively connectible to the vacuum source and the raw water inlet to cause raw water to be passed through the siphon and wash the concentrator and the interior of the decanter.