Case Studies: Full-Scale Operations and Delivery Systems - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 338

may be operated at either remedial sites or at central waste treatment locations such
as TSDFs. The “ in situ ” methods are generally limited to remedial situations. “ In
situ ” methods introduce the binders and additives into the contaminated medium —
usually sludge or soil — using large-scale excavation, tilling, injection, or drilling
equipment modified for S/S chemical addition.
Each delivery system has advantages and disadvantages. Ex situ systems provide
proven control of reagent addition and mixing, enable easier quality control sam-
pling, and are more practical at shallow waste depths and where site access is limited.
Obviously, ex situ equipment and methods are used when waste is transported to a
TSDF for treatment and disposal. Large projects at great depth are more amenable
to in situ operation and are often less costly than ex situ operations.

13.3.1 M OBILE OR P ORTABLE S YSTEMS

13.3.1.1 Ex Situ Methods

Ex situ S/S systems are assemblies of mixers, chemical storage and feeding devices,
pumps, conveyors, and ancillary equipment. The actual treatment and disposal sce-
nario determines which combination to use. Mixer-based methods use the most
complex equipment, while pit-mixing and spray-on methods are the simplest. All
three techniques are used at fixed treatment sites such as TSDFs and waste generator
facilities. In the latter case, either mixer-based or spray-on techniques are used in
Totally Enclosed Treatment Facilities, which circumvent the need for extensive
permitting in certain instances. Economics usually govern the selection, subject to
a method's ability to achieve treatment objectives.
Mixer-Based Methods . A typical system is shown schematically in Figure 13.1.
The waste to be treated is conveyed by pump, mechanical conveyor, or other means
into a surge tank or feed hopper that feeds the waste into the mixer, where it is
mixed with the S/S reagents. Depending on the process used, one or more dry and/or
liquid components may be added to the waste in the mixer. The system may be set

REAGENT SILO

4

WASTE FEED HOPPER
1

BELT CONVEYOR

SURGE
HOPPER

9

10

5 ROTARY VANE FEEDER

6 SOLIDS FLOW METER

PUGMILL

7

WEIGH BELT
CONVEYOR

2

TWIN
SCREW FEEDER

3

8

LIQUID PUMP

FIGURE 13.1 Schematic of a mixer-based S/S system.