A group of technologies that are described here as "high-rate solids contact clarifiers" have had limited reported use for SFBW treatment in the US. Four different solids contact clarifiers are discussed in Foundation Report # 3114 calling attention to their potential for SFBW treatment.
1. Two stage processes with internal and external solids recycle and contact prior to tubes or plates:
Untreated water and treatment chemicals first enter a mixed solids contact module followed by a gravity clarification module with tubes. Clarified water passes through the tubes and a portion of the solids collected in the clarification module are recirculated to the solids contact module. Solids concentration typically produced from these processes are on the order of 3 to 5 percent. As with DAF, these solids may not need thickening prior to dewatering. This desirable result will reduce the net impact on plant footprint when adding one of these processes for SFBW treatment.
2. Two stage process with tube settlers prior to an upflow (buoyant media) clarifier:
This process includes chemical addition followed by removal of particulate material in two clarification stages. The first stage includes a tube settler section followed by passage of the partially clarified water upward through a bed of buoyant plastic media that the manufacturer describes as an "adsorption clarifier" (AC). Five to ten percent of solids collected from tube settler section are recirculated to the front of the process, thereby promoting additional solids contact prior to clarification in the tube settler portion of the process. The subsequent AC portion of the process provides additional mixing, contact flocculation, and solids removal.
3. Two stage processes with microsand ballasted solids recycle and contact prior to tubes or plates:
Sand ballasted flocculation/clarification systems include a flocculation stage, analogous to the solids contact stage of other solids contact processes (e.g., CONTRAFAST and DensaDeg), followed by a clarification step. The microsand added prior to the flocculation stage provides additional surfaces for collisions with particulate material in untreated water and the resulting particulate and microsand floc create large, heavy particles that settle rapidly, allowing the clarification step to be operated at a high surface overflow rate. A cyclone separates the higher density microsand particles from other solids so the sand can be recycled.