Case Studies: Full-Scale Operations and Delivery Systems - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 340

is transported from the ash collection system on a conveyor belt, and little additional
mechanical equipment is required. This results in a simple, inexpensive stabilization
of soluble metals (e.g., lead) in the ash. Spraying with a cement slurry is also used,
with or without additional reagents.

13.3.1.2 In Situ Methods

In situ methods were probably the first to be used for non-nuclear remedial projects,
long before RCRA, the Land Disposal Restrictions Program, Superfund, and the
other legislative and regulatory tools that created a hazardous waste industry. In
those days, S/S was used primarily for solidification of liquid and semi-liquid wastes
in ponds or “lagoons” so that those collection systems could be reused, or the land
area used for another purpose such as construction. Backhoes were always available,
and solidification with cement, lime, flyash, or other inexpensive binders was easy
and inexpensive. Such simple systems are still used for that purpose. However, much
remediation work now requires a more sophisticated approach, and in situ technology
has had to keep pace with the changes.
In addition to just S/S processing, both ex situ and in situ methods are capable
of conducting sequential treatment operations; for example, pre-treatment to oxidize
or reduce a constituent followed by solidification. However, containment between
the operations is easier with in situ processing, and waste handling is essentially
eliminated. Stripping volatile organics from contaminated media prior to metal
stabilization is one multi-method operation especially amenable to in situ treatment.
With ex situ treatment, two different treatment systems must be mobilized to the
site and operated; with in situ treatment, the same basic equipment often does both
more cost-effectively.
Backhoe-Based Methods. The first such implement used for in situ treatment
was the familiar backhoe. Figure 13.2 shows this operation at a large remedial
operation, where several backhoes are in use at one time. Reagents were poured on
the surface of the area to be treated and mixed to the required depth with the backhoe,
or tilled in with a harrow for treatment of shallow layers. This, obviously, was not

FIGURE 13.2 In situ mixing with backhoes.