23 JULY 2003
2-4.4.1 Degritting and Grinding. Degritting and grinding of sludge are pretreatment
methods, typically applicable to municipal wastewater, that remove materials such as
trash ahead of the sludge dewater system and associated ancillary equipment to reduce
wear and prevent clogging of hydraulic equipment (i.e., piping and pumps). These
pretreatment methods may have only limited use at HTRW sites. Additional information
on specific requirements for degritting and grinding equipment is presented in the
following publications Sludge Dewatering (WPCF 1983) and Design of Municipal
Wastewater Treatment Plants (WEF 1992).
2-4.4.2 Sludge Thickening. Although not typically required for filter presses, sludge
thickening processes are often used to concentrate combined or separate sludge
streams. The primary goal of thickening is to reduce the volumetric loading to, and
increase the efficiency of, subsequent sludge processes. For primary and chemical
sludge applications, the sludge is usually thickened at the source, such as within a
clarifier or by gravity thickening. Biological sludge may be thickened by the use of
gravity, dissolved air flotation (DAF), centrifuge, gravity belt, or rotary drum.
Typically, sludge thickening is not required for filter presses because filter
press dewatering devices can handle sludge streams with low solids content (i.e., 2%
solids). However, if thickening is required, the following references provide detailed
descriptions of design considerations and guidelines: EPA (1982a), EPA (1987), WEF
(1992), WPCF (1983), and GLUMRB (1990).
2-4.4.3 Sludge Stabilization. Sludge stabilization is primarily used to make biological
sludge less odorous and putrescible and to reduce the pathogenic organism content
prior to final disposal. Stabilization treatment processes include anaerobic digestion,
aerobic digestion, wet air oxidation, and lime stabilization. If sludge stabilization is
required, the following references provide detailed descriptions of design considerations
and guidelines: EPA (1982a), EPA (1987), WEF (1992), WPCF (1983), and GLUMRB
2-4.4.4 Thermal Conditioning. Thermal conditioning can be used to enhance the
dewatering characteristics of the sludge by applying heat or pressure, or both, in a
confined vessel. This process results in the coagulation of the sludge by breaking down
its gel-like structure and allowing the bound water to separate from the solid particles.
Thermal conditioning is primarily suitable for biological sludge that cannot be stabilized
biologically because of toxic materials.
The typical thermal conditioning process is a continuous flow that heats the
sludge to 180 to 210 degrees C (350 to 400 degrees F) in a reactor under pressures of
1720 to 2760 kPa (250 to 400 psig) for 15 to 40 minutes (EPA 1987). Typically the heat
treatment (HT) process and the low-pressure oxidation (LPO) process are used. In the
LPO process air is added, while no air is added in the HT process. Further details of
these processes are provided in EPA (1979, 1982a, 1987), WEF (1992), and WPCF