The design process
The design of passive treatment systems is a somewhat inexact science
due to the variety of water chemistries requiring treatment and the
variety of materials that can be used in construction. For chemically
simple coal drainage (relatively mild pH water containing iron and manganese
and little or no aluminum), engineers and scientists at the former US
Bureau of Mines developed "cookbook" design criteria (Hedin,
et al., 1994) for aerobic systems that are still being followed (sometimes
inappropriately) today. Wildeman, et al., (1993) developed a phased
design protocol that is appropriate for more complex acidic as well
as neutral to net alkaline drainage chemistries.
These two approaches represent end points in a design philosophy continuum.
The inherent danger in any "cookbook" design approach is a
typical inability to properly address situations lying outside the range
of conditions that were originally used to develop the standardized
design criteria. The treatment of low pH water containing dissolved
aluminum is especially problematic and outside the original US Bureau
of Mines design criteria which addressed the issue by suggesting restrictions
in the application of anoxic limestone drains (ALDs). A precise and
reliable aluminum design guideline has yet to be developed for ALDs
and probably should not be even considered. That is because of the complexity
of aluminum chemistry. While iron can be more or less be precipitated
aerobically as ferric hydroxide or anaerobically as a sulfide or carbonate,
the list of aluminum mineral species found in nature (and thereby possible
in a passive treatment system) is extensive.
The "cookbook" design challenge represented by the individual
case of aluminum is multiplied many fold when additional heavy metal
contributions are considered, as may be the case for some MIW sources
at metal mines. Adding the effects of varying anionic concentrations
and water temperature further reinforces the futility of considering
cookbook approaches to passive treatment design. Still, the design engineer
must start somewhere.
The situation is not as bleak as it may sound. The mining, chemistry
and other industries have used a phased design process, probably since
the dawn of engineering. The concept is simple: start small, learn from
failures, and build on successes until the data required to properly
design a full-scale treatment system is obtained. With that data, the
risks of the full-scale system failure or less than optimum performance
are significantly reduced. Wildeman, et al. (1993) proposed a design
protocol that included laboratory-, bench- and pilot-scale phases. The
approach has been used at over three dozen mine drainage sites.
A phased-approach design project typically begins in the laboratory
with static tests, graduating to final testing phases (bench and pilot)
performed at the site on the actual MIW. Bench scale testing will determine
if the treatment technology is a viable solution for the MIW and will
narrow initial design variables for the field pilot. A proper bench
scale test will certainly reduce the duration of the more costly field
pilot test. Field pilot test duration can range from days, to months,
to years, depending on the nature of the technology. Depending on the
nature of the equipment and personnel needed, significant costs may
be incurred during the field pilot tests: about $500 to $1,000 per week,
mostly for sampling and analysis. Compare this to $5,000-$10,000 per
week for active treatment pilot tests. More detailed descriptions of
testing phase activities follow.
Phases of testing
- Lab Scale Testing - This phase of testing is usually conducted in
the laboratory. It might include:
- Paste pH and redox testing of passive treatment material substrates,
- Static bottle tests to isolate and identify beneficial bacteria
for a given cell type (aerobic or anaerobic), and
- Static limestone "cubitainer" tests for limestone consumption/alkalinity
determination.
- Bench Scale Testing - This phase of testing is typically performed
in the controlled environment of a laboratory but can be conducted
in the field. It is most appropriate for evaluating the dynamic response
of different mixtures of organic substrates, system configurations
or metal loading rates. This level of testing should be relatively
inexpensive to set up; most of the cost should be allocated to sampling
and analysis. To keep costs down, bench-scale test units can be constructed
with off-the-shelf items like trash cans and kiddie wading pools,
items typically found at do-it-yourself/home improvement stores and
gardening centers. Once the range of dynamic variables has been narrowed,
one should proceed to onsite pilot testing.* Field Pilot Scale Testing
- This phase of testing is performed at the site, on the actual MIW.
Information gathered during these tests should provide an accurate
operating cost estimate as well as final capital cost data. If the
field pilot study does not meet the necessary discharge standards,
another treatment technology should be considered or added on. It
is also important to determine the sludge characteristics during this
phase, will the sludge be a hazardous or non-hazardous? Can the treatment
sludge be disposed of on the mine site? Sludge management and organic
substrate replacement may comprise the principle "operating"
costs of a passive treatment system. Upon completion of the field
pilot test, full-scale design should take into consideration seasonal
fluctuations in flow rate and seasonal fluctuations in chemical composition
that may not have occurred during a shorter pilot test. Equalization
ponds or tanks should be included in the design to handle these fluctuations.
It is important to note that there are two equally important aspects of
full-scale passive treatment system design: bio-geochemistry and filtration.
The bench and pilot test results should have yielded the conditions necessary
to establish the proper bio-geochemistry or dominant geo-ecosystem in
a given treatment cell to develop stable chemical precipitates. However,
constructing an ideal bio-geochemical environment is a wasted effort if
the metal precipitates formed are flushed out of the system because of
inefficient filtration. Among other factors, this aspect of a proper system
design is influenced by the grain-size distribution and compacted density
of organic substrates, the settling and flocculating characteristics of
the precipitates, and the retention times of the settling cells.
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