Such installations are used in the purification of sewage in sewage treatment plants. So-called suspended matter, primarily sands and other mineral substances, are also contained in sewage in addition to organic contents. These inorganic substances can not be converted in the biological sewage treatment and can have a disadvantageous effect on the operational safety of sewage treatment plants and sludge treatment apparatuses. A separation of the sands and other mineral substances also carried along in the sewage from organic substances is therefore necessary. In sewage treatment plants the sewage is first conducted through a rake or a sieve that retains coarse contaminating substances. The sand entrained in the sewage is subsequently separated in sand trap installations by sedimentation. To this end non-aerated sand trap installations and aerated sand traps are known in addition to circular sand trap installations.
The non-aerated long sand trap consisting substantially of an elongated trough through which the flow passes horizontally is primarily used in fairly large sewage treatment plants. Its cross section can be rectangular or trapezoidal. The setting of an appropriate flow speed causes the entrained sand to settle on the bottom part while the specifically lighter organic substances are held suspended by the flow and washed away. Consequently, the sand trap must have an appropriate length. This design of a sand trap installation has the disadvantage that organic components also settle in addition to the mineral components. For example, a separation of fat is not possible with these installations. In particular, it is, however, difficult in non-aerated long sand trap installations to maintain a flow speed that is independent from the inflowing amount of water. The separation performance of a non-aerated longitudinal sand trap is therefore heavily dependent on the amount of inflow.
An improved separation between organic and inorganic substances independent of the inflow could be achieved by using aerated sand traps. In this case the settling of organic substances is limited by a uniform circulating flow. The blowing of air in the vicinity of the bottom part generates a rolling flow in the sewage so that a circumferential speed of the sewage that is advantageous for the settling of suspended matter is achieved independently of the inflow on the bottom part of the basin. In addition, a fat chamber can be arranged in aerated sand trap installations that is separated from the sand trap by a slotted separating wall. Also, the greatest part of the fatty components can be separated by the floating action of the aeration. The settling of organic substances can be largely avoided and a constant separating performance independent of the inflow could be achieved by aerated sand traps. However, the turbulences produced by the aeration also hold sand in addition to the organic substances in a suspended state so that the retention in particular of fine sands becomes worse with an increasing charging of air.
The present invention therefore addresses the problem of achieving a good separation of mineral particles of all grain sizes independently of the inflow.