Abstract: Underdrain units in a sand medium or dual medium filtration system include efficient air, water and combined air/water cleaning. Air, water or both are delivered in backflow direction from a very low position on the underdrain unit, as well as water from a higher position, so as to effectively remove blocking contaminants on the outside the underdrain unit and in the surrounding filter media.

Abstract: Rapid removal of settled solids in secondary clarifiers is critical in sewage treatment plants having nutrient removal permits, especially phosphorus, to prevent solids from surfacing after going aerobic and releasing phosphorus. Release of phosphorus could impact the plant's permit limit. This is addressed by the described design of rake blade flights and squeegees that quickly move solids on the clarifier floor to the sludge pickup tubes, usually in one revolution. In a preferred form the blade flights are curved and may be spiral in shape, causing gathered sludge to advance outwardly toward the pickup tube at each flight, usually in one revolution. Previous blade flights included angled linear sections that progressively move the sludge in incremental movements with each revolution of the clarifier mechanism.

Abstract: Rapid removal of settled solids in secondary clarifiers is critical in sewage treatment plants having nutrient removal permits, especially phosphorus, to prevent solids from surfacing after going aerobic and releasing phosphorus. Release of phosphorus could impact the plant's permit limit. This is addressed by the described design of rake blade flights and squeegees that quickly move solids on the clarifier floor to the sludge pickup tubes, usually in one revolution. In a preferred form the blade flights are curved and may be spiral in shape, causing gathered sludge to advance outwardly toward the pickup tube at each flight, usually in one revolution. Previous blade flights included angled linear sections that progressively move the sludge in incremental movements with each revolution of the clarifier mechanism.

Abstract: Underdrain units in a sand medium or dual medium filtration system include efficient air, water and combined air/water cleaning. Air, water or both are delivered in backflow direction from a very low position on the underdrain unit, as well as water from a higher position, so as to effectively remove blocking contaminants on the outside the underdrain unit and in the surrounding filter media.

Abstract: An underdrain filtration system has improved perforation structure, the perforations being positioned to admit the filtrate into a plenum defined by elongated underdrain rib structures of the system. Instead of drilled or laser-cut holes or slits, the slits are formed by stamping discs from the stainless steel plate, such that the discs extend parallel to the surrounding flat area but are raised just sufficiently to define a plurality of slits under each disc, preferably two to four slits, each being an arc of slightly less than 180°, 120° or 90°, respectively. A pair of metal bridges remain to integrally connect the raised disc and the surrounding flat field. Consistent perforation width is achieved, at, for example, 0.010″ width varying by less than 0.001″ in either direction, improving consistency of pressure drop.

Abstract: An underdrain filtration system has improved perforation structure, the perforations being positioned to admit the filtrate into a plenum defined by elongated underdrain rib structures of the system. Instead of drilled or laser-cut holes or slits, the slits are formed by stamping discs from the stainless steel plate, such that the discs extend parallel to the surrounding flat area but are raised just sufficiently to define a plurality of slits under each disc, preferably two to four slits, each being an arc of slightly less than 180°, 120° or 90°, respectively. A pair of metal bridges remain to integrally connect the raised disc and the surrounding flat field. Consistent perforation width is achieved, at, for example, 0.010″ width varying by less than 0.001″ in either direction, improving consistency of pressure drop.

Abstract: An underdrain filtration system has improved perforation structure, the perforations being positioned to admit the filtrate into a plenum defined by elongated underdrain rib structures of the system. Instead of drilled or laser-cut holes or slits, the slits are formed by stamping discs from the stainless steel plate, such that the discs extend parallel to the surrounding flat area but are raised just sufficiently to define a plurality of slits under each disc, preferably two to four slits, each being an arc of slightly less than 180°, 120° or 90°, respectively. A pair of metal bridges remain to integrally connect the raised disc and the surrounding flat field. Consistent perforation width is achieved, at, for example, 0.010″ width varying by less than 0.001″ in either direction, improving consistency of pressure drop.

Abstract: An underdrain unit of the type having air and water backwash for backflushing a granular filtration system has improved retention structure for retaining folded, perforated stainless steel underdrain plates against the floor of the tank drain. The retention structure is in the form of hollow horizontal tubes which not only hold the folded plates in place but also serve to deliver backwashing air. Thus, instead of air delivery tubes being mounted above a series of brackets that hold the folded plates in place against the floor, the air delivery in the system of the invention is via the same structural element which retains the folded plates. Embodiments are described for gravel bed systems and for sand filter systems.