Mixing plays an important role in digester operation Without well-mixed systems, the processes cannot acceptable levels of efficiency. There are a number of methods or combination of methods whereby proper mixing is attained. These include:
• Stirring by rotating paddles and scum breaker arms.
• Forced circulation of sludge and/or supernatant by pumps or by draft tubes with impeller.
• Discharge of compressed sludge gas from diffusers at the bottom of the digestion tank..
Mixing may be either intermittent or continuous, but however effected it provides all working organisms their proper food requirements and helps maintain uniform temperature. Intermittent mixing allows separation and removal of supernatant from a single stage digester. With continuous mixing the digestion proceeds at a higher rate
Mixing serves several purposes:
The incoming raw solids are intimately mixed with the actively digesting sludge -
• Promotes contact between food and organisms.
• Scum formation is reduced.
• The transmission of heat from internal coils or other heating devices to the sludge is improved.
• Distributes alkalinity throughout tank, aiding in pH control.
• Continuously disperses and dilutes inhibitory materials in feed sludge.
throughout the entire tank, thus reducing the tank capacity needed. Such continuous mixing requires a second digester or storage tank into which digesting sludge may be moved to make room for fresh sludge in the first digester and to make possible separation and removal of supernatant in the secondary digester.
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