Sludge blanket clarifiers

The design of the sludge-blanket clarifiers used primarily in the water industry is based on the jar test and a simple measurement of the blanket expansion and settling rate (12). Different versions of the jar test exist, but essentially it consists of a bank of stirred beakers used as a series flocculator to optimize the flocculant addition that produces the maximum floc-setfling rate. Visual floc-size evaluation is usually included. 

Design and operation of recirculation systems can be compHcated. Problems are avoided by using a sludge-blanket clarifier, in which feed enters below a blanket of accumulated and flocculated soflds which become fluidized in the zone-settling regime by the upflowing feed. Feed soflds are trapped in the blanket. The soflds content of the blanket continuously increases and part must be bled off in order to maintain the mass balance. 

Sludge-blanket clarifiers are difficult to start up because the first blanket must be estabUshed, and large-scale units require extensive excavation. Sizes range from 600 x 600 mm to 50 x 50 m. Precipitation and crystallization can be carried out in similar hopper-designed units, having overflow rates of 80 m/h or higher. 

Clarifiers typically are used in chemical precipitation and biological treatment processes to remove precipitated metal soHds and suspended biological soHds. To prevent the sludge blanket from becoming too thick or heavy, part of the sludge blanket is removed continuously or intermittently from the system and thickened prior to disposal. 

Figure 11. Solids contact clarifier without sludge blanket filtration. ...

Hopper-tank sludge-blanket clarifier, 22 61-62 Hops, 3 569-572... 

Sludge-blanket clarifiers, 22 56, 61-62 Sludge control, crude tank, 26 708-711 Sludge dewatering... 

The potential exists for denitrification to occur in the secondary clarifier sludge bed in the refinery wastewater treatment plant under anoxic conditions. The biological reduction rate of nitrate to nitrogen in the sludge blanket is typically slow due to limited soluble Chemical Oxygen Demand (COD), the food source for denitrifying... 

Figure 8.4 Spiracone sludge-blanket clarifier. Courtesy and registered trademark of Siemens Water Technologies Corp., Warrendale, PA.

Steam is used to heat the process. Figure 8.19 shows the cross section of a sludge-blanket (upflow) hot process softener. The operations of the sludge blanket hot process unit is similar to that for sludge-blanket clarifiers (see Chapter 8.1.1.1). Figure 8.20 shows the cross section of a downflow hot process softener. The downflow units rely on recirulation pumps to provide sludge contact. 

In practice, two types of plants are generally used for chemical precipitation hardness removal One type uses a sludge blanket contact mechanism to facilitate the precipitation reaction. The second type consists of a flash mix, a flocculation basin, and a sedimentation basin. The former is called a solids-contact clarifier. The latter arrangement of flash mix, flocculation, and sedimentation were discussed in previous chapters on unit operations. 

Solids-contact clarifier—A clarifier that uses a sludge blanket contact mechanism to facilitate the precipitation reaction. 

Ives, K. J., Theory of operation of sludge blanket clarifiers . Proc. Inst. Civ. Engrs, 39, 243-260 (1968)... 

Ide, T. and Kataoka, K., A technical innovation in sludge blanket clarifiers . 

---