Overflow Turbidity

Automated controls for flocciJating reagents can use a feedforward mode based on feed turbidity and feed volumetric rate, or a feed-back mode incorporating a streaming current detector on the flocculated feed. Attempts to control coagulant addition on the basis of overflow turbidity generally have been less successful. Control for pH has been accomplished by feed-forward modes on the feed pH and by feed-back modes on the basis of clarifier feedwell or external reaction tank pH. Control loops based on measurement of feedwell pH are useful for control in apphcations in which flocculated sohds are internaUy recirculated within the clarifier feedwell. 

Automatic Control. In some industries, the waste streams can vary in composition over a relatively short lime period When the solids level of a slurry changes, the entire dosage response may change. Automatic systems are available for thickeners that adjust the dosage according to the incoming solids level, overflow turbidity, and streaming current potential. 

Overflow Turbidity Overflow turbidity can be used to control flocculant or coagulant. There may be some significant lag time between the actual flocculation process and when the clarified liquor reaches the overflow discharge point where the sensor is typically positioned. These sensors and meters are generally used as alarms or for trim only. 

Expected sedimentation objectives underflow slurry density or concentration overflow solids concentration (suspended sohds and/or turbidity) chemical treatment for soluble components (i.e., hardness, met s, anions, pH, etc.)... 

Equation (29) also implies that the fluid density and viscosity should be minimized. This suggests that clarification should improve at higher temperatures. One argument against this is the increasing solubility of some of the solid species (Fig. 7.42). More to the point in this development is that increased temperature, while increasing the difference in densities and reducing the frictional resistance to fall of the solids, also increases the rate of heat loss from the system. Nonuniform temperature in the clarifier results in thermal convection currents. These upset the streamline flow of solids toward the bottom of the clarifier and increase the turbidity of the overflow. 

Brine from the top of the clarifier overflows by gravity to the clarifier pump tank. A turbidity meter in this line can monitor the solids content and provide an alarm in case of an upset in the clarifier operation. Again, operation and control are similar to those discussed in connection with brine saturation. The top of the tank should be at least as high as the top of the clarifier overflow system. This provides more brine storage capacity as well as time to correct a pump failure or other simple operating problems. The normal level in the pump tank is about 50%. Level control options are as in Fig. 11.3 ... 

The conversions mentioned above are to be avoided whenever possible, because of inherent errors in such procedures, by using a method that would give the desired type of distribution directly. Different methods give different types of distributions and the selection of a method should be done on the basis of both the particle size and the type of distribution required. In common processing applications, for example, solid-fluid separations, it is the size distribution by mass that is usually of interest, because the separation efficiency is based on gravimetric means. There are, however, cases such as liquid clarification where turbidity of the overflow is of importance, and size distribution by surface or even by number would be more relevant. Figure 2.23 shows the four types of distribution in a diagram. 

Dredging operations cause the suspension of fines in the water column, called turbidity (see also section 3.4.3.3,11.3.4 and Appendix B.3.10). The cause of this turbidity is the stirring action by the dredge tool and the overflow of process water into the surrounding water by a Trailing Suction Hopper Dredger (TSHD) or the overflow of process water from a barge when hydrauhcally loaded. 

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