Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 117

effective approach is to avoid producing the contamination in the first place.
A factor as specific and seemingly mundane as particle size may be the most
important limiting characteristic for application of treatment technologies to
certain wastes (e.g., contaminated sediments). It reminds us that green designs
are only as good as their attention to minute details. Looking at the tables,
we see the peril of “one size fits all” thinking. Most treatment technologies
work well on sandy soils and sediments. The presence of fine-grained material
adversely affects treatment system emission controls because it increases particulate
generation during thermal drying, it is more difficult to dewater, and it has greater
attraction to the contaminants (particularly, clays). Clayey sediments that are
cohesive also present material-handling problems in most processing systems. The
solids content generally ranges from high [i.e., usually the in situ solids content
(30 to 60% solids by weight)] to low [e.g., hydraulically dredged sediments (10 to
30% solids by weight)]. Treatment of slurries is better for lower solids contents,
but this can be achieved even for high solids contents by water addition at the time
of processing. It is more difficult to change a lower to a higher solids content, but
evaporative and dewatering approaches, such as those used for municipal sludges,
may be employed. Also, thermal and dehalogenation processes are decreasingly
efficient as solids content is reduced. More water means increased chemical costs
and increased need for wastewater treatment.
We must be familiar with every potential contaminant in the life cycle. We
must understand how it is generated and how it changes in space and time. Again,
a quick review of the tables shows that elevated levels of organic compounds or
heavy metals in high concentrations can be drivers in deciding on the appro-
priate technological and operational solution but also as indicators or possible
ways to prevent pollution. Higher total organic carbon (TOC) contents favor
incineration and oxidation processes. The TOC can be the contaminant of con-
cern or any organic, since they are combustibles with caloric value. Conversely,
higher metal concentrations may make a technology less favorable by increasing
the contaminant mobility of certain metal species following application of the
technology.
A number of other factors may affect the selection of a treatment technology
in ways other than its effectiveness for treatment (some are listed in Table 3.4). For
example, vitrification and supercritical water oxidation have been used only for
relatively small projects and would require more of a proven track record before
being implemented for full-scale sediment projects. Regulatory compliance and
community perception are always a part of decisions regarding an incineration
system. Land-use considerations, including the amount of acreage needed, are
commonly confronted in solidification and solid-phase bioremediation projects
(as they are in sludge farming and land application). Disposing of ash and other
residues following treatment must be part of any process. Treating water effluent
and air emissions must be part of the decontamination decision-making process.