much higher than for the three processes in
combination.
It follows, then, that it would not be prudent
to spend effort, time, and money to conduct a
large-scale investigation into technologies for
treating wastewaters from metal plating,
beyond the line of thinking outlined previously.
The pollutants in these wastewaters are not
organic, are, therefore, not biodegradable, and
extensive experience has shown that:
1. Chemical precipitation is the most costeffective
method for removing the bulk of
the dissolved metals.
2. Sand, diatomaceous earth, or other media
filtration is the most cost-effective “next
step” to follow the chemical precipitation
process.
3. If still further reduction in metals concentration
is required, ion exchange is the best
candidate.
Having said that, it must now be said that, in
many cases, substances in certain metal-plating
wastewaters require more than straightforward
alkaline precipitation, filtration, and ion
exchange. For instance, if chelating agents are
present, it may be necessary to destroy or
otherwise inactivate them, in order to expose
the metal ions to the full effect of the precipitating
anions. In other cases, if the concentration
of organic matter is high, it may interfere
with the precipitation process and have to be
removed by biological or other treatment
before the metals removal steps.
Figure 8-1 presents a categorization of the
components of industrial wastewater and preliminary
selections of treatment technologies, based
on the appropriateness of the mechanism of each
technology compared to the fundamental properties
of the pollutants. Different versions of
Figure 8-1 could be generated by beginning with
a characterization other than dissolved or undissolved;
for instance, organic or inorganic, but all
versions would ultimately result in the same list
of appropriate treatment technologies.
In Figure 8-1, the first level of categorization
of pollutant characteristics is that of dissolved
or undissolved state. For instance,
trichloroethylene as a pollutant in wastewater
would be dissolved (albeit to only very low
concentrations), organic, nonbiodegradable,
and volatile. Candidate technologies, then,
would be stripping, activated carbon adsorption,
and chemical oxidation.
Figure 8-2 presents a schematic of “the industrial
waste system,” showing that raw materials,
water, and air enter the system, and as a result of
the industrial process(es), products and byproducts
exit the system, along with airborne
wastes, waterborne wastes, and solid wastes.
Because discharge permits are required for each
of the waste-bearing discharges, treatment systems
are required. Each of the treatment systems
has an input, the waste stream, and one or
more outputs. The output from any of the treatment
systems could be an air discharge, a waterborne
discharge, and/or a solid waste stream.
Principle and Nonprinciple Treatment
Mechanisms
Most treatment technologies remove substances
other than the target substances. For
instance, “biological treatment” can effectively
remove a certain amount of metal ions from
wastewater. Because metal ions do not particularly
like to be dissolved in water (they are
hydrophobic), they are driven by the second
law of thermodynamics to be adsorbed on the
surface of solids—just about any solid—
including activated sludge solids.
This mechanism for removing metals from
wastewater is often undesirable because the
presence of the metal ions in the waste sludge
may render the sludge unsuitable for a desired
disposal method. Composting with wood chips
to produce a horticultural soil conditioner is an
example. Another example is simple disposal
in an ordinary municipal landfill. In other
instances, the removal of metal ions by absorption
on biological solids from a wastewater that
has very low concentrations of them may serve
as a fortuitous polishing step. In any case, biological
treatment is a “principle” technology
for removing organics from wastewaters, and
its unintended removal of metal ions is a “nonprinciple”
mechanism.