The treatment of wastewater sludge with an alkaline material can be an effective stabilization process. Lime is the most widely used alkaline material in the wastewater industry. Lime has traditionally been used in wastewater

Wastewater Sludge Processing, By Izrail S. Turovskiy and P. K. Mathai Copyright © 2006 John Wiley & Sons, Inc.

treatment plants to raise the pH in stressed anaerobic digesters, to remove phosphorus in advanced wastewater treatment, and to condition sludge prior to vacuum filtration and pressure filter press dewatering. The original objective of lime conditioning of sludge was to improve its dewaterability, but in time it was observed that odors and pathogen levels were also reduced. Subsequently, lime addition became a major sludge stabilization alternative. Stabilization and disinfection of sludge with quicklime or hydrated lime are in wide use in Sweden, Finland, Germany, and Eastern European countries (Turovskiy, 1988). In the United States, lime stabilization or pasteurization is used to reduce odor, pathogens, and the putrecibility of sludge in PSRP (process to significantly reduce) and PFRP (process to further reduce pathogens). According to U.S. EPA's 1998 Need Survey (U.S. EPA, 1989), over 250 municipal wastewater treatment plants use lime to stabilize sludge. Larger treatment plants that use the process include those in Pittsburgh, Pennsylvania; Memphis, Tennessee (80 mgd); Toledo, Ohio (100 mgd); Washington, DC (300 mgd); and Las Vegas, Nevada (WEF, 1998).

The standard approach to lime stabilization is to add lime slurry to liquid sludge in sufficient quantity to achieve a pH of 12 or higher for two hours following addition. The high pH creates an environment that halts or substantially retards microbial reactions that could otherwise lead to odor production and vector attraction. This type of stabilization is classified by U.S. EPA as a PSRP that meets class B requirements as stated in the 40 CFR Part 503. Many of the advanced alkaline stabilization technologies in which dewatered sludge is treated with a dry alkaline material meet the class A requirements.

6.1.1 Advantages and Disadvantages

Both liquid lime and dry lime stabilization processes have several advantages over other sludge stabilization processes:

• The processes are reliable and easier to operate.

• They are capable of fast startup and shutdown.

• Pathogen reduction is reported to be as effective as or better than digestion processes.

• The processes greatly reduce odors if homogeneous lime feed and mixing occur.

• Stabilized sludge demonstrates improved dewaterability.

• Stabilized sludge may partially or fully replace liming agents used on acid soils for increasing alkalinity.

• The elevation of pH acts to fix or immobilize specific metal ions in sludge and soil and therefore restricts the possible uptake of metals by plants.

In addition to agricultural applications, beneficial end uses of lime-stabilized sludge include daily, intermediate, final, and vegetative cover in landfills; manufactured organic topsoil blends; bulk-fill applications such as slope stabilization and dike construction; and horticulture use in nurseries and sod farms. With respect to applying lime-stabilized biosolids to agricultural land, it may not be appropriate where soils are naturally alkaline, as in many parts of the western United States.

Lime stabilization has some disadvantages, including the following:

• Compared to digestion, there is no reduction in solid mass in lime stabilization.

• Increased mass from lime and chemical formation may result in higher transportation and ultimate disposal costs.

• Lime handling requires moderate operator attention and housekeeping because of the inherent dust in lime.

• Lime-stabilized biosolids have lower concentrations of nitrogen and phosphorus then do anaerobically digested biosolids.

• The lime stabilization process produces ammonia and possibly other odorous gases that may have to be treated before being exhausted.

• Dewatered sludge treated by lime can harden during storage.

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