The Role Of Mixing

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.


Sludge conditioning is a process whereby sludge solids are treated with chemicals or various other means to prepare the sludge for dewatering processes. Chemical conditioning (sludge conditioning) prepares the sludge for better and more economical treatment with vacuum filters or centrifuges. Many chemicals have been used such as sulfuric acid, alum, chlorinated copperas, ferrous sulfate, and ferric chloride with or without lime, and others (refer back to discussions in Chapter 8). The local cost of the various chemicals is usually the determining factor. In recent years the price of ferric chloride has been reduced to a point where it is the one most commonly used. The addition of the chemical to the sludge lowers or raises its pH value to a point where small particles coagulate into larger ones and the water in the sludge solids is given up most readily. There is no one pH value best for all sludge. Different sludge such as primary, various secondary and digested sludge and different sludge of the same type have different optimum pH values which must be determined for each sludge by trial and error. Tanks for dissolving acid salts, such as ferric chloride, are lined with rubber or other acid-proof material. Intimate mixing of sludge and coagulant is essential for proper conditioning.

Feeders are also necessary for applying the chemicals needed for proper chemical conditioning. The most frequently encountered conditioning practice is the use of ferric chloride either alone or in combination with lime. The use of polymers is rapidly gaining widespread acceptance. Although ferric chloride and lime are normally used in combination, it is not unusual for them to be applied individually. Lime alone is a fairly popular conditioner for raw primary sludge and ferric chloride alone has been used for conditioning activated sludges. Lime treatment to a pH of 10.4 or above has the added advantage of providing a significant degree (over 99 percent) of disinfection of the sludge. Organic polymer coagulants, and coagulant aids have been developed in the past 20 years and are rapidly gaining acceptance for sludge conditioning. These polymers are of three basic types:

• Anionic (negative charge) - serve as coagulants aids to inorganic Aluminum and Iron coagulants by increasing the rate of flocculation, size, and toughness of particles.

• Cationic (positive charge) — serve as primary coagulants alone or in combination with inorganic coagulants such as aluminum sulfate.

• Nonionic (equal amounts of positively and negatively charged groups in monomers) — serve as coagulant aids in a manner similar to that of both anionic and cationic polymers. The popularity of polymers is primarily due to their ease in handling, small storage space requirements, and their effectiveness. All of the inorganic coagulants are difficult to handle and their corrosive nature can cause maintenance problems in the storing, handling, and feeding systems in addition to the safety hazards inherent in their handling.

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