The most common vacuum filter consists of a large, horizontally mounted rotating drum, which is covered by a porous cloth or metal coils (see Figure 3-7). The bottom portion of the drum is submerged in a vat of biosolids. As the drum rotates, biosolids are picked up on the porous medium under a vacuum. The drum is divided into sections, and vacuum is applied by a rotary valve. The filter operates in three zones:

• Cake formation • Cake dewatering • Cake discharge

Vacuum filters can be successfully used with organic (polyelectrolyte) conditioning, inorganic (lime and ferric chloride) conditioning, and thermal conditioning.