The froth from flotation is rich in particles but high in moisture. The froth is also rich in surface active materials. Foam fractionation has often been proposed as a purification step for molecules that seek the gas-water interface, but the yields of product are not particularly high and protein products tend to be unstable when subjected to interfacial forces.

Biochemical engineers tend to overlook sedimentation because its performance is so poor with microbial cells. As a unit operation, sedimentation offers low capital and operating costs, but it is inefficient for capturing biological materials that have very slow settling rates. Despite its shortcomings, sedimentation is widely used in water and waste treatment to collect inorganic precipitates and organic flocs, and in the chemical process industries to concentrate suspensions of dense particles.

Conventional sedimentation has no value for the biochemical engineer for particles with low settling velocities if they must fall more than a meter or so. For fine, light particles, the alternatives to sedimentation are centrifugation, filtration, and ultrafiltration. Environmental engineers avoid these operations because handling large volumes to collect a small amount of solids from very dilute environmental process streams is costly.

Lamellar Settlers

Lamellar sedimentation refers to gravity collection with a short sedimentation path. The first lamellar settlers were bundles of tubes, but the side walls did not aid settling, and modern designs use stacks of plates. Inclining the plates causes the particles to slide to a hopper for collection. As the angle increases, more time is required for a settling particle to reach the lower plate, so steep inclines should be avoided. Sewage sludges require about 60 ° inclination, but microbial cells such as yeast may slide at 30 ° . Lamellar settlers will eventually foul with cells that slide poorly, but cleaning by bubbling air through the plates is quite effective.

This sketch of a lamellar settler with 60 ° inclination shows some problems. Particles will slide down quite well, but note the fine green construction lines. There is a long distance for a particle to fall before it reaches a surface. If the angle were 90 °, it would have to fall all the way to the hopper.

Note that with 45 °, the distance to fall is less. Also you have to decide how much area is needed. The 60 ° unit needs to get taller to increase the collection area; the 45 ° unit gets wider AND taller to increase area.

With 30 ° inclination, the area requires mostly getting wider as it increases. Note that the distance for a particle to fall is considerably shorter. This would be the way to go except that most types of particles will not slide off well, and cleaning by blasting air through would need to be frequent.

You can appreciate designing a lamellar setter by experimenting with the following Applet:

Large, heavy-duty lamellar settlers are available commercially, but they are not promoted for collecting light biological particles. Popular designs have a serious problem with interference between the feed stream and collected cells. As the cells fall from one plate and encounter moving liquid, they can be resuspended. In other words, the collected cells have to fall through the inlet flow to adjacent plates. Some designs use downflow; this has the potentially more serious problem of resuspension of cells in the exit stream. The feed can be distributed to the sides of the plates for less interference with the sliding cells. Other means for reducing entry effects are baffles or distributors to direct the feed stream to each plate without interference with collected cells. These measures are only partially effective and add to the complexity and cost of the units.

The cross-flow design with inclined plates practically eliminates plate-to-plate interferences. The feed is distributed to the entire cross section of one end, and the suspension flows independently through each element. Baffles in the collection hopper prevent flow parallel to the plates. Concentrated cells do not fall directly through an inlet stream.

Because shallow depth sedimentation achieves high performance at low cost, it should become more widely used with biological systems. Fabrication should be very easy, but assembly on-site is recommended. To survive shipping, a unit would have to be fairly rugged, but flimsy construction would be very satisfactory in service. Buoyancy of the plates reduces strain on their fastenings, and simple spacers or clamps would hold the plates in position. There are no serious corrosion problems in most biological systems, thus there is a wide choice of construction materials.

Think about the hopper that collects the solids. If there were no impediment to flow, the path of least resistance would be through the hopper, and little fluid would pass through the parallel plates. You should be able to suggest ways of mounting baffles to overcome this problem. Another point to consider is distribution of feed and collection of effluent. If you allow the feed to jet into the plates, sedimentation efficiency will suffer. The solution to this problem is not easy, but your engineering approach should be to have a distributor with many openings. The openings should be small to have a large head loss at each hole so that other head losses will have minor effects on the flow rates and each hole will have about the same discharge rate. The holes should point away from the plates to minimize disturbances in the settling region.

For additional reading on this topic see H. R. Bungay and M. P. Millspaugh, (1984) "Cross Flow Lamellar Settlers for Microbial Cells", Biotechnol. Bioengr. 26 640-641