|
Mercury
Work Group
Phase II Reports >> Pretreatment Manual
Facilities Loadings
| Pretreatment Manual |
Tech. Identification
Hg Management Guidebook | Mercury Products
Database
For more
information, contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
6.0 WASTEWATER CHARACTERIZATION
6.1
General Considerations
An
important step in a source reduction program and in the
selection of a pretreatment system is to learn the physical and
chemical characteristics of the process wastewater stream in
question. The complexity of the characterization effort may vary
depending upon the nature and size of the facility and upon the
type and extent of the discharge problem. The study of the
wastewater stream’s characteristics may help identify
contaminants that are present in the various contributing
industrial processes. The measured levels of these contaminants
can be compared with the limits of applicable sewer discharge
regulations.
Beyond
the contaminants subject to regulation, some contaminants can
interfere with the proper operations of certain wastewater
pretreatment systems. If individual process waste streams
contain interfering contaminants, the waste streams could be
either reduced, segregated from the other streams, or
eliminated. On the other hand, the pretreatment system may be
designed to work effectively with the identified interfering
contaminants.
A
wastewater characterization study that examines these issues can
help to set an overall approach to achieving compliance with
regulations. Such an overall approach may involve a combination
of source reduction, source segregation, and pretreatment. An
experienced consulting engineering firm may be employed to
perform the wastewater characterization study and to help in the
development and execution of the overall approach.
Because
of the potential for cost savings, source reduction
possibilities would usually be examined first for reducing or
removing the regulated contaminants. Technically and
economically feasible source reduction options would be
determined and implemented.
Therefore,
one might consider conducting a source reduction and water
conservation audit in conjunction with a wastewater
characterization study conducted in some form both before and
after the audit. The source reduction and water conservation
audit may yield the following results:
A
better understanding of the materials used and discharged from
the facility.
-
Opportunities
for product substitution may be found.
-
Options
to reduce pollutant loads may be found making possible a
simpler, less expensive pretreatment system than originally
planned.
-
Possibilities
for achieving compliance without pretreatment may be
identified.
-
Processes
where water is being wasted or could be reused may be
revealed.
The
wastewater characterization studies should be done by a
qualified professional using a certified analytical laboratory.
Certified analytical laboratories must meet minimum performance
standards and must pass periodic proficiency tests. Certified
laboratories can be identified by referring to the Massachusetts
DEP. Analytical laboratories can also be found in the local
phone directory or over the Internet. The laboratories can be
asked to verify that they are certified in the desired
wastewater analyses. The analytical methods used by the
laboratories should conform to EPA approved methods. Ideally,
several laboratories should be investigated and their costs
compared. If desired, a contract can be prepared, with review by
an attorney.
To
conduct the wastewater characterization study, identify a
representative site to sample the wastewater stream that may be
connected to a pretreatment system. The wastewater sampling site
should be selected that is specific to the process and is not
mixed with sanitary wastes or any other non-process wastes. This
site should be upstream of any existing pretreatment operations
and should be easily accessible by sampling personnel. Ideally,
the site should have electric power nearby for lighting and
sampling equipment operation.
At the
selected site, a spigot fitted with a 3/8 inch barb (maximum O.D.
size) should be installed. As shown in Figure 2, Recommended
Sampling Port for Special Wastes, the spigot may be placed at
the bottom of the pipe to allow sampling of low wastewater
flows. If the spigot is installed in this manner, a volume of
liquid (that may contain settled particles) should first be
purged to obtain representative and uncontaminated samples of
the waste stream.
To
collect the samples, Silastic or Teflon tubing is often
connected to the barb. Sampling should be done during high and
low flow periods of the process day. It is best to sample each
site on several different days to help identify variations in
waste stream characteristics.
Recommended
Sampling Port for Special Wastes
Figure
2
Note: This
figure is derived from the 1995 Infrastructure Subcommittee
Maintenance Guidebook of the MWRA/MASCO Mercury Work Group. A
sampling port for other types of piping systems may be similarly
installed.
Composite
samples are taken as time or flow proportional samples. Such
sampling is a collection or "composite" of individual
samples taken at regular intervals of time or flow during a
process day (up to 24 hours). Composite samples are often
collected by an automatic sampling unit programmed to collect
individual samples of wastewater at selected intervals.
Generally, the sampling unit automatically purges the sample
connection and tubing before collecting a sample.
Flow
proportional sampling is the preferred method of composite
sample collection, but this is not always possible since the
required flow meters may not be available. Properly taken
composite samples are usually considered to represent the
wastewater over the course of a process day.
In the
automatic sampler, the individually collected wastewater samples
are often deposited and held in a single clean glass jar packed
in ice (a temporary preservation medium). When the sampling
event is complete, the composite wastewater sample is measured
for pH and temperature and is poured into appropriate sample
bottles. These bottles can be made of plastic, clear glass, or
amber glass depending on the analysis to be done on that
particular sample. The samples are then chemically preserved, if
necessary, and put on ice for transfer to the analytical
laboratory. Some parameters that are preferably measured using
composite samples are total suspended solids (TSS), biochemical
oxygen demand (BOD), sulfates, semi-volatile organics, and heavy
metals (total and dissolved) including mercury.
Grab
samples are single, instantaneous collections of wastewater that
represent the composition of the wastewater being analyzed at a
particular sampling location and time. Certain parameters
including pH, volatile organics (VOA), petroleum hydrocarbons (PHC),
and fats, oil, and grease (FOG) must be taken as grab samples to
avoid losses or other changes in sample characteristics. If a
wastewater stream is highly variable or intermittent, grab
samples may be selectively taken and analyzed during a specific
operating period to obtain an accurate characterization of the
changing wastewater composition including its extremes. Before
collection of a grab sample, it is important that sample
connection and any connected tubing be thoroughly purged so that
the sample represents the waste stream.
Batch
discharges may require special sampling techniques to obtain
representative samples. Typically, a batch discharger collects
process wastewater over a portion or an entire day in a holding
tank. The collected wastewater is discharged to the sewer after
being neutralized or treated for compliance with permit
requirements. In such cases, after the batch has been thoroughly
agitated or mixed, grab samples can be taken at the beginning,
middle and end of the discharge and can be used to prepare a
manual composite (average) sample of the collected and treated
wastewater.
6.2
Mercury Species in Wastewater and Mercury Speciation Testing
For
wastewater containing mercury, a wastewater characterization
study should include determination of the chemical species and
physical forms of mercury that may be present. Mercury in
wastewater may exist in three chemical species: metallic, ionic,
and organic. These mercury species should be understood because
some pretreatment technologies can effectively remove only
certain species. In addition, the various species of mercury may
bind to particulate matter in the wastewater to form physical
agglomerates containing mercury.
Metallic
mercury is typically found in thermometers, manometers,
sphygmometers, fluorescent lamps and switching devices. This
form of mercury is a silver-colored liquid at room temperature
with a specific gravity of 13 (i.e., it is 13 times
heavier than water), and it is only slightly soluble in water.
Metallic mercury slowly vaporizes at room temperature and can
cause dangerous vapor concentrations in enclosed rooms. The
vapor form of metallic mercury is readily absorbed through the
lungs and is very toxic. Metallic mercury may be combined with
other metals to form amalgams (alloys).
Ionic
mercury exists when mercury atoms form covalent bonds with
halogens and other inorganic ligands (complex ions). Ionic
mercury can exist in two forms. With a single atom and an
overall +2 charge (Hg ++ ), the ionic mercury is in
the mercuric form. The mercurous form is diatomic with an
overall +2 charge (Hg 2 ++ ). The mercuric
form readily forms salts (e.g., mercuric chloride - HgCl 2 )
that are soluble in water. Mercuric chloride and Calomel (mercurous
chloride - Hg 2 Cl 2 ) are often used in
medical applications.
Organic
mercury (typified by methyl mercury) consists of mercury atoms
covalently bonded to organic groups. Often called organomercuric
compounds, these forms of mercury are quite soluble in water and
wastewater and are extremely toxic to aquatic life. These
compounds are readily absorbed by fish from their aqueous
environment and tend to become highly concentrated (bioaccumulated)
in the fish tissues. If fish having bioaccumulated organic
mercury are consumed, there can be major human health concerns.
In addition, inorganic mercury in the environment can be
converted by microbiological activity into methyl mercury
compounds that can be absorbed by fish.
The
various species of mercury can bind to the particulate matter
that may exist in ambient water or wastewater. Particulate-bound
mercury can move through the food chain through ingestion
(filter feeding organisms) or through re-conversion to dissolved
forms. Mercury-laden particulate matter can range in size from
tens of microns to sub-micron (colloidal). Typical EPA
methodology (Methods 200.7, 200.9, and 245.1) separate dissolved
from particulate mercury by filtration through a 0.45 micron (µm)
membrane filter.
As a
physical species of mercury (instead of the previous chemical
species), particulate mercury can often be a significant
fraction of total mercury in a wastewater stream. Moreover,
accumulations of metallic mercury or mercury-laden solids in
plumbing systems (at elbows, traps, and other points) can cause
chronic mercury contamination of the wastewater stream.
In
analytical testing of wastewater samples, total mercury
concentrations are usually determined by analytical laboratories
using EPA Method 245.1. Analytical laboratories typically
achieve a detection limit of 0.2 µg/L (ppb). This EPA method is
the analytical method of choice because most applicable federal,
state, and local regulations address total mercury
concentrations.
For the
various mercury species that may be present in a wastewater
stream, concentrations of particulate mercury are the easiest to
quantify. Particulate mercury concentrations in wastewater
samples are not directly measured, however, but are determined
as mathematical differences in analytical test results of total
mercury and dissolved mercury. Dissolved mercury concentrations
are determined using EPA Method 245.1 on wastewater samples that
have been initially filtered through a 0.45 micron (µm) filter5.
Standard EPA methods dictate that a 0.45 micron (mm) filter be
used for this filtration step, although some laboratories
recommend an additional test with a smaller filter such as 0.2
mm because particulate mercury is such an important species of
mercury in wastewater.6
Analytical
tests that separate the chemical species of mercury (i.e.,
metallic mercury, methyl mercury, free ionic mercury, and
loosely complexed mercury) are not routine or standard
laboratory procedures as compared with the above applications of
EPA Method 245.1. The mercury speciation techniques combine
inorganic (inductively coupled plasma (ICP) and cold vapor
atomic absorption (CVAA)) and organic (high pressure liquid
chromatography (HPLC)) techniques to separate and quantify the
various species of mercury. In some instances, the techniques
include quantitative / qualitative tests and intuitive
interpretation of the results. An alternate technique to EPA
Method 245.1 has been proposed in EPA Method 1631 that uses
bromide reduction of the various mercury compounds and
pre-concentration with a gold amalgam before cold vapor
analysis. With the gold amalgam concentration step, laboratories
such as Frontier Geosciences, Inc. of Seattle, WA, can measure
mercury concentrations in ambient water samples to detection
limits of 0.00005 to 0.0002 µg/L (ppb).
The
following is a brief description of the advanced speciation
techniques. Elemental mercury can be determined by direct
amalgamation onto gold and atomic fluorescence analysis.
"Free" mercury (free ionic or loosely bound inorganic
mercury) in a sample can be determined by using a mild reduction
of mercuric ions to elemental mercury before the direct
amalgamation step. In addition, but of lesser importance, methyl
mercury can be determined through distillation (most common) or
solvent extraction. These methods of mercury speciation will
tentatively identify the important species of mercury in a
wastewater stream. The methods require proficient and careful
laboratory techniques that will be more time consuming than EPA
Method 245.1, resulting in higher analytical costs.
As an
alternative to using these rigorous analytical methods to detect
mercury speciation, the following qualitative procedure may be
used as part of a source reduction audit of a facility. (Please
note that this approach will not yield the same high quality
results as the above methods.) EPA Method 245.1 should first be
used on properly-collected samples of the wastestream to measure
the typical total mercury concentrations. Dissolved mercury
levels could also be determined as described above. Then,
identify all mercury-containing compounds discharged into the
wastewater stream from each process operation. For each of these
mercury compounds, assign a mercury speciation.
For
example, if the only known mercury-containing chemical in the
wastewater stream is thimerosal, assume that most of the mercury
will be present as an organomercuric compound. Similarly, if
reagents are known to contain mercuric salts, then mercuric ions
will be present. Usually, most waste streams will contain
several forms of mercury that may change over time. Even so, for
purposes of selection of candidate wastewater treatment
processes, it is helpful to estimate the percentage of each form
of mercury that is likely to be present in the wastestream.
5Typically,
dissolved mercury would be comprised of ionic mercury and any
mercury compounds that can pass through the filter.
6Refer to the
MWRA/MASCO Mercury Work Group, Technology identifiction Subgroup
Report, for further information on mercury speciation testing
relative to a bench-scale mercury removal feasibility test
project.
RETURN
TO PRETREATMENT MANUAL
TABLE OF CONTENTS
|
