Water and Wastewater Engineering - Sciences Club

27-24 WATER AND WASTEWATER ENGINEERING
Lime Pretreatment
The addition of lime to liquid sludge is called pretreatment. This method is used for direct land
application of liquid sludge or as conditioning prior to dewatering. Lime pretreatment requires
more lime per unit weight of sludge processed than is necessary for dewatering. This is because
the chemical demand of the liquid requires a higher dose to achieve the required pH for disposition
on land.
Lime Posttreatment
The addition of lime with dewatered sludge in a pug mill, paddle mixer, or screw conveyor is
called posttreatment. E xcellent mixing is required to ensure contact between particles of sludge
and lime and to avoid pockets of putrescible material. When well mixed, the stabilized product is
a crumbly mixture that can be stored for long periods.
Alkaline Stabilization Design Practice
The design objective is to meet the regulatory requirements specified in the Code of Federal
Regulations (40 CFR Part 503). These are outlined in Chapter 18. To meet Class A criteria, sufficient
alkali must be added to maintain the pH above 12 for more than 72 hours to ensure pathogen
destruction. The temperature must be above 52 � C for at least 12 hours during the period the
pH is greater than 12. The solids must be air dried to over 50 percent after the 72-hour period at
elevated temperature.
To meet Class B requirements and to reduce vector attraction, sufficient alkali must be
added to raise the pH to at least 12 for two hours at 25 � C and maintain a pH of 11.5 for 22 more
hours (40 CFR 503.33(b)(6)). In theory, this should provide enough residual alkalinity so that
the pH does not drop below 11 for several days. The recommended design criteria are (U.S.
EPA, 1979):
• Treat the sludge in a liquid state.
• Bring the sludge to a pH of 12.5 by lime addition and maintain the pH above 12.5 for
30 minutes. This keeps the pH above 12 for two hours.
The amount of lime required is determined by the type of sludge, its chemical composition,
and the solids concentration. Example lime dosages are given in Table 27-9 . For specific applications
testing must be performed to determine the actual dosage requirements.
A conceptual layout for a lime stabilization facility is shown in Figure 27-10 . The mixing
tank is typically designed to operate in a batch process mode. The volume of the tank must be
large enough to hold the sludge for 30 minutes. The holding capacity may be considerably larger,
depending on the plant operating mode and shift responsibilities for treating the sludge. For a
Class A sludge, additional storage volume beyond the mixing tank must be provided to meet the
temperature and time requirements (52 � C for 12 hours).
Mixing may be either by air or by mechanical mixers. For air mixing, coarse bubble
diffusers are mounted along one wall of the tank to create a spiral roll. The air supply should be