Sludge thickening

In a centrifuge technique, various centrifuge types are used for sludge thickening. 

Table 16. Performance of Sludge Thickening and Dewatering Equipment...

O. E. Albertson, S/udge Thic/zening, Manua/ of Practice ED1, Task Eorce on Sludge Thickening, Water Pollution Control Eederation, Washington,... 

The bubble size in these cells tends to be the smallest (10 to 50 Im) as compared to the dissolved-air and dispersed-air flotation systems. Also, very httle turbulence is created by the bubble formation. Accordingly, this method is attractive for the separation of small particles and fragile floes. To date, electroflotation has been applied to effluent treatment and sludge thickening. However, because of their bubble generation capacity, these units are found to be economically attractive for small installations in the flow-rate range of 10 to 20 mVh. Electroflotation is not expected to be suitable for potable water treatment because of the possible heavy metal contamination that can arise due to the dissolution of the electrodes. 

Detention Time Waste activated sludge only, after sludge thickening. 10 -15 days volumetric displacement time. If sludge temperatures are much less than 60°F, more capacity should be provided. Primary sludge mixed with waste activated or trickling filter humus. 20 days displacement time in moderate climates. 

FIGURE 27.8 The DAF full-flow pressurization (total pressurization) system. [From WEF, Sludge Thickening, Manual of Practice No. FD-1, Water Environment Federation (formerly Water Pollution Control Federation), Washington, DC, 1980, pp. 33-66. With permission.]... 

The above-cited studies demonstrate the performance of a particular unit system for the treatment of specific type of waste stream. A particular unit system alone may not be able to treat the wastewater to a level of effluent standard prescribed for its safe disposal. Hence a number of pretreatments, such as screening, sedimentation, equalization, and neutralization, and post-treatment units such as secondary sedimentation, sludge thickening, digestion and disposal, disinfection, and so on, are extremely important for complete treatment. The effluent treatment and disposal facilities adopted by various types of pharmaceutical industries are described in the following sections. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

Plant 000012 produces 3.9 x 10" kkg/year (8.7 x lO lb/year) of emulsion crumb rubber, primarily neoprene. The contact wastewater flow rate is approximately 8.45 m /day (2.25 X 10 gpd) and includes all air pollution control equipment, sanitary waste, maintenance and equipment cleanup, and direct contact wastewater. The treatment process consists of activated sludge, secondary clarification, sludge thickening, and aerobic sludge digestion. Noncontact wastewater, with a flow rate of approximately 1.31 x 10 m /day (3.46 x 10 gpd), is used on a once-through basis and is remrned directly to the river source. Contact wastewater is also returned to the surface stream after treatment. 

In the sludge-thickening and dewatering section, polymer is added to thicken the sludge from 1 percent total suspended solids content to about 3 percent. Ferric chloride and diato-maceous earth are then added, prior to dewatering in conventional filter presses. The filter cake will have a total solids content of about 20 percent, which is high enough that no free liquids will be present. 

As evidenced from this batch analysis, varions degrees of sludge thickening can occur. Table 5.3 shows some data that indicate the varions thickening, expressed as percent solids, that can result in varions nnits. 

Example Activated sludge thickening under denitrification condition... 

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