Inclined settler

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. 

Fig. 4 Bioreactor with inclined settler to recycle concentrated biomass to the reactor.

It is of interest that in the continuous operation of inclined settlers, termed lamella settlers, where the sediment is withdrawn continuously from the bottom of the channel and clarified product continuously from the top of the channel, that two modes of operation have been shown to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other depends on the geometry and the manner in which the suspension is fed into the settler and the clarified liquid withdrawn.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. 

Agarwala JP (1994) Inclined settler classification of particles and transport of ions through liquid membranes with an electric field. Ph.D. Thesis, University of Colorado... 

Note We have generally employed particle size density functions ff rp), fi rp) and fzirp), where the probability density function depends on the random variable tp, the particle radius. In the analysis considered here for inclined settlers, we are dealing with density functions ffJJpa), ( Upzt) and f-JJJp. Since, by relation (6.3.1), the relation between Up and tp is (if Stokes law is valid)... 

The factor C and the exponent y must be determined by experiments with pilot settlers. These terms combine all the unknown parameters. The cross-sectional area, S, of the settler should be chosen in such a way that the superficial velocity, of the dispersed phase does not exceed 5-9mms . Often aids for coalescing (e.g., packages of inclined plates or filter cartridges made of metal or plastic) (Fig. 9.23) are used. There are several ways of predetermining coalescence [31-33]. Furthermore, in special cases electrical fields and centrifuges are used to support coalescence [34]. 

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. 

A process for phase separation based on density differences. A commercial lamella settler for suspensions or emulsions comprises a stack of parallel plates spaced apart from each other and inclined from the horizontal. The space between each set of plates forms a separate settling zone. The feed is pumped into these spaces, at a point near the longitudinal middle of the plates. The less dense phases rise to the underside of the upper plates... 

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... 

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. 

The froth contains typically about 60% bitumen, 30% water, and 10% solids. To remove solids and water, the froth is diluted with a diluent, in both existing operations this is locally produced naphtha. Bitumen density is about the same as that of water. Addition of a light solvent lowers the density of the oil phase and, at die same time, lowers its viscosity, making water and solids separation possible. In the froth-treatment operation, scroll and disk centrifuges as well as incline plate settlers (IPSs) are used for cleaning the froth diluted with naphtha. The product, which usually contains less than 2% water and less flian 0.4% fine solids, is then fed to upgrading. Here, in die first operation, the diluent is recovered and recycled to froth treatment. 

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. 

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