Flocculation velocity

In the experimental phase, 900 ML of stationary-phase E. coli cells were removed from the chemostat and placed in a stirrer-reactor assembly, which provided both rapid-mix and flocculation velocity gradients. A 100-mL aliquot of a specified PEI concentration was then added to the E. coli suspension during a 2-min rapid mix (G = 190 sec ). The suspension was flocculated for 60 min at G = 20 sec with turbidities and particle size distributions recorded before flocculant addition and after flocculation. The initial suspension had a pH = 7.0 ( 0.1), ionic strength I = 0.06M NaCl, T = 25°C, and a cell concentration of 2.5 ( 0.5) X 10" cells/mL before flocculation. The flocculation velocity gradient (G = 20 sec ) was set by regulating the angular velocity of a two-pronged stirrer to yield the desired torque. 

This G, representing the number of collisions against one particle during one second, is a direct measure of the flocculation velocity. The value of G when V — 0, i.e., when there is no interaction between the particles, except a very steep attraction when the particles touch each other, is equal to... 

For dujdz = 1 sec and a particle diameter of 10 cm, the ratio Jjlis of the order of and completely negligible but when the diameter of at least one of the particles is larger than 1 (x the ratio Jbccomes larger than unity and for particles of 10 jx the orthokinetic flocculation is far more important than the normal perikinetic flocculation The enhanced flocculation velocity under the influence of agitation is thus seen to be present irrespective of the fact whether the system is monodispersed or poly-dispersed. 

The influence of particle radius and flocculation velocity on the Schul2 e-Hardy rule is not very pronounced (c/. Figs, 3 and 4),... 

The larger particles of suspensions sediment rather rapidly. The flocculation velocity of a suspension is slow, owing to the relatively smalinumber of particles. Therefore the sedimentation velocities of a flocculated and a stable suspension do not differ pronouncedly, although in special cases this difference is applied (blood sedimentation). 

The natural process of bringing particles and polyelectrolytes together by Brownian motion, ie, perikinetic flocculation, often is assisted by orthokinetic flocculation which increases particle coUisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment like gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling the floes to increase the rate of particle—particle coUisions through the increase in soUds concentration. 

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

Subsequent to stock preparation and proper dilution, the paper furnish usually is fed to the paper machine through one or more screens or other devices to remove dirt and fiber bundles. It then enters a flow spreader which provides a uniform flowing stream and which is the width of the paper machine. The flow spreader, or manifold, discharges the slurry into a headbox, where fiber flocculation is minimised by microturbulence and where the proper pressure head is provided to cause the slurry to flow at the proper velocity through the slice and onto the moving Fourdrinier wire. 

Figure 1 shows the particulate loading of a pipe containing gas and particulates where the nonuniformity induced by a disturbance, ie, a 90° bend, is obvious (2). A profile of concentration gradients in a long, straight, horizontal pipe containing suspended soHds is shown in Figure 2. Segregation occurs as a result of particle mass. Certain impurities, eg, metal-rich particulates, however, occur near the bottom of the pipe others, eg, oily flocculates, occur near the top (3). Moreover, the distribution may be affected by Hquid-velocity disturbances and pipe roughness.

MUNICIPAL WATER treatment). Scale-up of orthokinetic flocculators, generally in the form of paddle devices, is based on the product of mean velocity gradient and time, for a constant volume concentration of the flocculating particles. 

In other words, the lower the mass of the particle, the higher its velocity, because the average energy of any particle at a given temperature is constant, kT. A dispersed particle is always in random thermal motion (Brownian motion) due to coUisions with other particles and with the walls of the container (4). If the particles coUide with enough energy and are not well dispersed, they will coagulate or flocculate. 

The sequence, flocculation — coalescence — separation, is compHcated by the fact that creaming or sedimentation occurs and that this process is determined by the droplet size. The sedimentation velocity is monitored by the oppositely directed forces which form the buoyancy and the viscous drag of the continuous phase on the droplet ... 

Collection efficiency is normally increased by increasing the gas throughput (Drijver, op. cit.). However, if the entering dust is flocculated, increased gas velocities may cause deflocculation in the cyclone, so that efficiency remains the same or actually decreases. Also, variations in design proportions that result in increased collection effi-... 

The parameter used to design rapid mix and flocculation systems is the root mean square velocity gradient G, which is defined by equation... 

Equipment typically consists of concentric circular compartments for mixing, flocculation and settling. Velocity gradients in the mixing and flocculation compartments are developed by turbine pumping within the unit and by velocity dissipation at baffles. For ideal flexibihty it is desirable to independently control the intensity of mixing and sludge scraper drive speed in the different compartments. 

This angle plate gravity separator removes suspensions of solids from a dilute liquid. The unit is more compact than a box-type settler due to the increased capacity achiev ed by the multiple parallel plates. The concept is fairly standard (U.S. Patent 1,458.805—year 1923) but there are variations in some details. For effective operation, the unit must receive the mixture with definite particles having a settling velocity. The units are not totally effective for flocculants or coagulated masses that may have a tendency to be buoyant. 

In brewing beer and SCP production, several highly flocculent yeast strains are used. The special yeast strains are easily separated without the use of any expensive separation process.6 The settling velocity is defined by ... 

Its unique compact and efficient design is made possible by the use of the principle of zero velocity eliminating internal water turbulence (see below). The flocculated water thus stands still in the flotation tank for optimum clarification. The unit is complete with automatic backwash filter in which dirty backwash water is recycled back to the unit inlet for reprocessing. The average waste flow from the process is less than 1.0% of the incoming raw water. 

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