Flotation continuous settling

Liquid waste streams with a high-suspended solids content can be cleaned up by solids removal in clarifiers, thickeners, and liquid cyclones and by accelerated settling by inclined Chevron settlers or the like [73]. For waste streams with very finely divided solids in suspension (i.e., less than about 100 pm) dissolved air flotation techniques have been shown to be more efficient than methods employing sedimentation. Final dewatering of the sludges obtained may be carried out on a continuous filter or a centrifuge. The clarified water product can be accepted for more potential options of reuse or final disposal options than untreated water, and the separated solids may be burned or discarded to landfill, as appropriate [74]. 

The philosophy of pulp mill emission control is to recover as much fiber as possible before employing other treatment measures. Recovery is obtained by sedimentation and/or flotation methods. Sedimentation may be conducted in a pond, which is periodically drained and the settled material removed (as a semibatch process). Or a clarifier (which operates in a similar manner to a thickener) may be used for continuous operation. The sediment collected contains 90-95% water plus fiber and grit solids. For disposal the waste is dewatered to 50-60% solids in a filter or centrifuge and then burned, which accomplishes both disposal and can permit energy recovery. 

The total incoming flow of feed can be treated with air under pressure, but it is more usual to introduce the air into a recycle stream of cleaned liquid and to allow the bubbles to develop in the flotation chamber at the point where the recycle stream enters. The feed stream is pretreated with chemicals, if necessary, prior to its introduction to the flotation chamber where it meets the bubble blanket. The fine particles in the supension are captured by the bubbles and are floated up to the surface where they form a thick scum which is removed by a continuous mechanical scraper. Some solids will settle fi om most feed streams and it is usual to provide for the withdrawal of accumulated silt fiom the base of the chamber. Figure 7.13 shows a conventional diflused air flotation system housed in a rectangular tank and Figure 7.14 a DAF unit which incorporates an inclined plate settler to capture by sedimentation those particles that escape flotation. 

Real systems do not display the sharp boundaries of Fig. 7.48. One reason is that no real system is monodisperse and therefore individual particles do not all settle with the same velocity. Other reasons having to do with process control are the normal fluctuations in continuous operation and imperfect distribution of flow across the clarifier. There are also external factors that oppose the settling of particles. These include thermal convection currents that arise horn nonuniform temperatures and deaeration of the brine that leads to flotation of some of the suspended matter. To allow for these effects, it is customary to apply a safety factor to the calculated area. Seifert [93], for one, recommends using an area efficiency of 50%. 

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