Patent EP-0968965-A1 (EP)
IPC: C02F3/12
Filed: 1998-06-29
Abstract: For the biological processing and cleaning of sewage, active sludge sediment is returned to the aeration basin after the initial settling phase and before the dwell phase. The return of the active sludge sediment is during or pref. after the clear water has been drawn off. During the dwell phase, a gas with an oxygen content is fed into the settling basin. A feed of a gas with an oxygen content into the aeration basin is interrupted by throttling at the start of the return feed of the active sludge from the settling basin, and the gas feed remains blocked during the duration of the sludge return action or at intervals while the active sludge flows back. The time intervals are ≤ 10% of the cycle time of the operation in the settling basin, and pref. ≤ 5%, and the return flow duration for the active sludge is ≤ 10 % of the settling time and pref. ≤ 5%. The water forced out of the aeration basin is fed into the settling basin, below the sludge level. A gas with an oxygen content is fed into the settling basin and aeration basin simultaneously. The assembly has at least two pairs of aeration and settling basins in parallel, which are in use in offset time periods. The clear water take-off time duration is measured, to set the feed of gas with an oxygen content during the next cycle using an algorithm. During times where there is a low feed, sludge is fed into the aeration basin from a preceding anaerobic sewage processing stage. An Independent claim is included for a sewage processing assembly with a return flow system (42) at the settling basin (26) to pass back active settled sludge. The return flow system (42) has at least one suction opening (48) at the lower section of the settling basin (26). Preferred Features: At least one suction opening (48) is at the base zone (44) of the settling basin (26), or there are a number of suction openings for the return flow system (42) at horizontal intervals. The outflow (58) of the return flow system (42) at the aeration basin (10) is at a horizontal gap from the overflow, at the opposite basin wall to the overflow, and is below the min. water level (38) by a vertical gap of 10-90% of the min. water level (38) height and pref. 20-40%. The whole length of the pipe (50) in the return flow system (42) is below the lowest water level (38), and at least part of the return flow pipe (50) length is within the overflow channel (24) between the aeration basin (10) and the settling basin (26). The entry opening (40) of the overflow (24) is at the min. water level (38). The overflow has a horizontal section (28) between the aeration (10) and settling (26) basins and, in the settling basin (26), has a kink (30) and a vertical section (32). The overflow (24) opening (34) at the settling basin (26) is below the sludge level (36) of the settled activated sludge. The feed opening (20) of the feed channel at the aeration basin (10) is at its base (22). A compressed air lifter is at the return flow system (42) and also at the take-off (66) for the clear water and surplus sludge (82). The take-off (66) for the clear water has an upper (68) and lower (70) pipe curve, with an air feed at the upper pipe curve (68), linked to the feed of a gas with an oxygen content into the settling basin (26) through a throttle. The take-offs (66,82) are fed with a transport gas through a common control valve. The throttle is located after the control valve so that the vol. for each take-off can be set independently. A float (80) at the settling basin (26) controls the min. water level (38), to close the control valve when the water is at the lowest level.