Dry Lime Mixing

Mixing of dewatered sludge cake and lime (or other alkaline materials) is the most critical component of dry lime stabilization. The goal is to provide intimate contact between the sludge cake and lime and to produce a homogenized product. The mixing step is typically accomplished using a mechanical mixer such as a pug mill, plow blender, paddle mixer, or screw conveyor. Dewatered sludge and lime are introduced at the head end of the mixer. Sludge cake with a low moisture content and in large chunks, such as pressure filter press dewa-tered sludge, may require a "cake-breaker" conveyor to feed the sludge cake to the mixer. The proportioning of lime dosage to the rate at which cake is introduced is important. Dry lime can be supplied to the mixer with a screw conveyor if the distance from the bulk storage silo to the mixer is short, or with a pneumatic device. Large facilities with multiple mixing units convey lime pneumatically from the storage silo to a day tank (usually, a small storage silo) at each unit. Lime is then conveyed to the mixer using a volumetric or gravimetric feeder and a short screw conveyor.

A large number of variables affect the mixing process and, consequently, the characteristics of the product. The mixing characteristics of dewatered sludge cake vary with moisture content, conditioning chemicals used before dewatering, alkaline chemical type and dosage, temperature, mixing intensity, and mixing retention time. To adjust mixing intensity and retention time, mixers can be equipped with a variable-speed drive, adjustable mixer paddle configuration, weir plates, and other options. Many mixer manufacturers have mobile pilot testing units available. Whenever possible, this equipment should be used in the evaluation and selection of the proper and most efficient mixing equipment.

Design Example 6.1 Design a liquid lime stabilization system to treat a mixture of primary and thickened waste activated sludge for a wastewater treatment plant with the average flow of 3 mgd. The stabilized sludge is to be land applied. Stabilization and land application operate one shift a day (7 hours effective), five days a week. Assume the following design criteria:

Primary sludge:

average day sludge produced peak day sludge produced: dry solids concentration: specific gravity of sludge:

Thickened waste activated sludge:

average day sludge produced: 2000 lb/d peak day sludge produced: 3000 lb/d dry solids concentration: 3% specific gravity of sludge: 1.01

Note: Critical components should be sized to meet the critical condition of peak day sludge production. Two mixing tanks should be provided, each with the capacity to treat the peak day sludge production. While one tank is filling, sludge in the other is treated with lime, mixed for 30 minutes, held for about 2 hours, and then loaded in tank trucks for land application. Alternatively, stabilized sludge can be dewatered for disposal. Hydrated lime is used in this facility.

1. Mixing tank:

volume of primary sludge at peak day production = 4500 lb/d " (62.4 lb/ft3 )(0.04)(1.03) = 1750 ft3/d (50 m3/d)

: 3000 lb/d (1361 kg/d) 4500 lb/d (2041 kg/d) 4% 1.03

volume of WAS at peak day production _ 3000 lb/d

(62.4lb/ft3 )(0.03)(1.01) = 1587 ft3/d (45 m3/d) volume of concrete tank required

(62.4lb/ft3 )(0.03)(1.01) = 1587 ft3/d (45 m3/d) volume of concrete tank required

tank surface area with 8-ft liquid depth = 1363ft3 = 170ft2 (16m2)

With a 2-ft (0.6-m) freeboard, the tank size is 15 ft in diameter x 10 ft high (4.5 m x 3.0 m).

Note: Circular tanks are usually made of steel. Square concrete tanks may also be used, in which case each tank is 13 ft x 13 ft x 10 ft (4 m x 4 m x 3 m).

2. Mixing system: Air or mechanical mixing can be provided. Use an air-mixing criterion of 30 ft3 per 1000 ft3. Provide one blower per tank.

1000 ft3

If mechanical mixing is to be provided, mixer manufacturers should be consulted to design the proper mixer. A turbine mixer with a 5-ft-diam-eter impeller at 37 rpm and with a 7.5-hp motor may be provided.

3. Lime storage: Use a lime storage criterion of 30-day storage for average loading conditions.

Hydrated lime characteristics:

bulk density: 30 lb/ft3

Lime dosage (see Table 6.1):

primary sludge: 12% by dry weight WAS: 30% by dry weight

average daily lime required =

TABLE 6.1 Typical Lime Dosages for Liquid Lime Stabilization

Dry Solids Lime [Ca(OH)2]

Concentration, % % by Dry Weight Average pH

Type of Sludge Range Average Range Average Initial Final

TABLE 6.1 Typical Lime Dosages for Liquid Lime Stabilization

Dry Solids Lime [Ca(OH)2]

Concentration, % % by Dry Weight Average pH

Type of Sludge Range Average Range Average Initial Final










Waste activated









Anaerobically digested


















6 30lb/ft3

Note: A full truckload of hydrated lime is about 20 tons, which is about 1333 ft3. It is usually economical to deliver a full truckload of lime. Storage volume should be 150% of the lime delivery volume so that a delivery can be ordered when two-thirds of the silo volume is expended. Therefore, the storage silo should be sized for 2000 ft3 (57 m3), which is about 56 days of lime requirement.

4. Lime slurry: Assume that 20% hydrated lime slurry to be used in two batches at 10 minutes per batch.

lime feeder capacity = -

(2 batches/d)(5d/wk)(10min) = 75 lb/ min (34 kg/min)

Assume a 25% lime slurry with a specific gravity of 1.1.

Provide a slurry tank 4 ft in diameter x 5 ft high (1.5 ft of freeboard) (1.2 mx 1.5 m).

5. Long-term storage: Stabilized sludge generally needs to be stored during winter when sludge cannot be land-applied. Assume that 90 days of storage is required.

.4 , f , .4 3000 lb/d average-day volume of sludge=

(62.4 l^/ft3 )(0.04)(1.03) 2000 lb/ d + ( 62.4l^ft3 )( 0.03) (1.01) = 2225 ft3/d (63mVd)

Primary Sludge

Thickened WAS


Sludge Mixing System

Contact Tanks

Biosolids to Beneficial Use

Figure 6.5 Schematic of lime stabilization in design example.

volume of storage required = 2225ft3/d x 90d (36,421 m3)

Provide a storage tank 110 ft in diameter x 22 ft high (1 ft of freeboard) (33.5 mx 6.7 m).

Figure 6.5 is a schematic of the lime stabilization system in Design Example 6.1. Prior to stabilization, sludge feed should be passed through an in-line grinder. This improves sludge mixing and flow characteristics, protects downstream pumping, and eliminates unsightly conditions such as rags, sticks, and plastics at the disposal site. An odor control unit may be needed if air mixing is used to treat the ammonia stripped from the sludge.

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