Primary coagulants need good mixing Carefully read over all equipment manuals and know the operating ranges and limitations Experiment with different feed points Don't feed polymers too close to chlorine or other oxidants Use dilution water with polymers Split up feeding of polymers Watch blowdown carefully Watch the centerwell for troubleshooting and to observe the need for any chemical dosage changes Optimize the operating variables (e.g., sludge bed depth, turbine speed, chemical feed rate, etc.)
The sludge-blanket clarifier (Figure 24) goes one step further, by passing the water up from the bottom of the clarifier through a blanket of suspended solids that acts as a filter. The inverted cone within the clarifier produces an increasing cross-sectional area from the bottom of the clarifier to the top. Thus, the upward velocity of the water decreases as it approaches the top. At some point, the upward velocity of the water exactly balances the downward velocity of a solid particle and the particle is suspended, with heavier particles suspended closer to the bottom. As the water containing flocculated solids passes up through this blanket, the particles are absorbed onto the larger floe, which increases the floe size and drops it down to a lower level. It eventually falls to the bottom of the clarifier to be recirculated or drawn off.
Although these processes seem relatively simple, especially in relation to many chemical manufacturing operations or unit processes, there are a number of operational problems that can make the life of an operator miserable. Excessive floe carryover is a very common problem. This is most often associated with hydraulic overload or unexpected flow surge conditions. You can tackle this problem by relying on equalize flow (metering the flow of the clarifier), which will help to dampen out surges. Unfortunately, hydraulic overload conditions are not the only causes of excessive floe carryover. Other reasons many be thermal currents, short-circuiting effects, low density floe, chemical feed problems. Another common operator problem is simply no floe in the centerwell. This can result from underfeeding of chemicals or a loss of the sludge bed recirculation. Refer to the sidebar discussion on this page for some general corrective actions you can try.
You will have to investigate and apply trial and error field tests to resolve some of these problems. When new equipment are installed, it is wise to spend time during a shake-down and start-up period to explore the operational limitations of the process and train operators on how to handle these types of problems.
Corrective Actions for Floe Problems
Excessive Floe Carryover
Cause: Hydraulic overloading; flow surges.
Corrective Action: Equalize flow to eliminate surges.
Cause-. Thermal currents.
Corrective Action: Equalize flow.
Corrective Action: Use tracer dye to confirm and identify exact nature of problem. Check clarifier internals such as mixing baffles and modify as needed. Cause: Low density floe.
Corrective Action: Increase coagulant/flocculant doses. Cause: Chemcial feed problems.
Corrective Action: Check pump and meter settings. Confirm thast setting correspond with anticipated chemical feed rates. No Floe in Centerwell Cause: Chemical underfed.
Corrective Action: Check operation of feed pump. Check pump settings. Cause: Sludge bed recirculation lost. Corrective Action: Increase recirculation slowly.
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