Surfaces, inclined, settling

Tube and plate settlers have been used for many years as sedimentation units. There are a variety of designs, but basically all of them use submerged inclined surfaces with relatively close spacing that increases the separation surface area in a vessel that is smaller than conventional sedimentation units (Fig. 3.6). The water to be treated is passed between the surfaces at velocities which permit suspended sohds to settle and to coalesce on the lower tube or plate surface. The angle of inclination (45° or more) enables the settled sohds to slide downward into a sludge-concentration compartment located at the bottom of the treatment unit. 

Pearce(11) studied the effect of a downward-facing surface by considering an inclined tube as shown in Figure 5.5. Starting with a suspension reaching a level AA, if the sludge line falls to a new level BB, then material will tend to settle out from the whole of the... 

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. 

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. 

The elastic properties of the surface affect the rebound of particles contacting the surface under free-settling conditions [11]. Experiments have been performed on the adhesion of freely falling spherical glass particles, in relation to the angle of inclination of a surface painted with a chlorinated PVC enamel. 

The results here presented are valid for particles falling freely in air. The settling rate of particles smaller than 10 m in diameter is insignificant (order of a few cm/ sec). Such particles cannot fall vertically they are only able to come into contact with the surface as a result of inertial, diffusion, and Brownian motion. In this case, not only vertical surfaces, but also plates inclined at ce > 90° may become dusty. 

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