Settlers sedimentation

The settler. In this unit, gravitational settling frequently occurs and, in addition, coalescence of droplets must take place. Baffles are fitted at the inlet in order to aid distribution. The rates of sedimentation and coalescence increase with drop size, and therefore excessive agitation resulting in the formation of very small drops should be avoided. The height of the dispersion band ZB is influenced by the throughput since a minimum residence time is required for coalescence to occur. This height Zb is related to the dispersed and continuous phase superficial velocities, //,/ and uc by ... 

The degree of cell separation is an important parameter to be evaluated in perfusion systems. This can be done through the use of some concepts as cell separation efficiency, grade efficiency, and cut size. These concepts are applicable to any equipment whose performance remains constant if the operational conditions do not change. They are valid, therefore, for equipment such as sedimenting centrifuges, hydrocyclones, gravitational settlers, etc. 

Lamella settlers are gravity settlers that use a large number of inclined flat plates, closely packed, so that the distance between the plates is small. The gravitational action makes the cells move in the direction of the lower surface of the individual separating space between two plates. Once settled, the particles slide down in a layer towards the plate periphery and then into the sludge hopper. For the same required sedimentation area, lamella settlers are much more compact than vertical ones. Theoretically, their total sedimentation area is the sum of the horizontally projected areas of all plates. In practice, however, only 50% of this total area is effective [137]. The main problem with inclined settlers is that the cells tend to adhere to the plates. Special coating of the plate surface and vibration of the whole lamella pack may alleviate this problem. 

Thompson and Wilson [78] operated a 21-L air-lift perfusion reactor coupled to an external lamella settler. The sedimentation device bearing an angle of inclination of 30° to the vertical was maintained at 37°C. They found a reasonable agreement between the theoretical and experimental values of breakthrough for viable and nonviable cells in the harvest stream. As expected (see Eq. 8), the maximum perfusion rates increased with an increasing settling area. 

The system used by Stevens et al. [79] consisted of an air-lift reactor with an external lamella settler. They did not need to pump the cell suspension through the settler, since free flow convection was achieved by cooling the cell suspension (20 °C) before entering the sedimentation device. As also found by other authors, they could achieve selective retention of viable cells by varying the perfusion rate. 

In the sedimenter or gravity settler, the particles in the feed suspension settle due to difference in densities between the particles and the fluid. The settling particle velocity reaches a constant value - the terminal velocity - shortly after the start of sedimentation. The terminal velocity is defined by the following balance of forces acting on the particle ... 

To provide this finished water quality, a system incorporating flocculation, settling, and mixed-media filtration is used. Fig. 6 schematically illustrates the flow of the system used. After flocculation, the coagulated water is processed through a shallow-depth sedimentation basin using tube settlers. Tube settlers provide efficient sedimentation in greatly reduced detention times. The tube settlers are essentially a honeycomb of 1-in. trays operating in parallel. The... 

There are three basic designs for clarifiers (also known as "gravity clarifiers") solids-contact, inclined-plate settlers, and sedimentation. The advantages of each type of unit are ... 

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]. 

Lamella Settler Up to 3.0 Up to 7.5 Inclined tubes or parallel plates to increase sedimentation Various... 

The formed elemental sulfur is then separated from the liquid by sedimentation in a settler (S). Here, aggregates of sulfur are formed, which can grow to particles with a diameter of up to 3 mm but which can also be easily ground down [40]. 

Industrial separators nearly all provide for the continuous removal of settled solids. The separation may be partial or very nearly complete. A settler that removes virtually all the particles from a liquid is known as a clarifier, whereas a device that separates the solids into two fractions is called a classifier. The same principles of sedimentation apply to both kinds of equipment. 

The most frequently used settlers are classified according to the flow direction into horizontal, radial and vertical types. In the treatment and purification wastewater plants with a higher capacity, only continuous operation settlers are used. Figure 3.50 illustrates a vertical sedimentation tank with a sludge scraper. 

The secondary sludge is usually pumped from the after-sedimentation into the primary settlers, and processed together with the primary sludge as a mixed sludge. Here, the primary sludge is the prevalent material. 

An interesting situation arises if it is desired to avoid the upward propagation of kinematic waves by moving the sediment downward at a rate such that the upward-propagating concentration differences are stationary relative to the container walls. The downward motion of the sediment is obtained by its continuous withdrawal uniformly over the settler cross section. The process is termed continuous thickening. The continuous sedimentation process is thus composed of the batch gravitational flux and solid convective flux pu. This is illustrated in Fig. 5.4.7, where the total solids flux curve is the sum of the batch flux and the convective flux the shape of the curve of Fig. 5.4.5 illustrates the batch flux (Petty 1975). 

Most early observations of sedimentation in inclined channels indicated that a quasi-steady interface shape between the clarified layer and suspension was formed rapidly in times short compared with the characteristic suspension settling time. Moreover, the clarified liquid layer thickness below the upper channel wall was observed to be much thinner than the channel width b. Most of the clarified fluid accumulated above the horizontal interface at the top of the suspension. It was this kinematic shock interface that was observed to fall with a vertical velocity larger than the hindered settling velocity U measured in vertical settlers. 

It is of interest that in the continuous operation of inclined settlers, termed lamella settlers, where the sediment is withdrawn continuously from the bottom of the channel and clarified product continuously from the top of the channel, that two modes of operation have been shown to exist for the same rate of clarified flow (Probstein et al. 1981, Leung Probstein 1983). In one mode the feed suspension layer expands down the channel subcritical mode), as in Fig. 5.4.8, whereas in the other mode supercritical mode) the layer contracts. The appearance of one mode or the other depends on the geometry and the manner in which the suspension is fed into the settler and the clarified liquid withdrawn. We would suggest that under appropriate conditions there exist two types of steady solutions for all continuous sedimentation processes. 

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