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Mercury
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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.
8.0 TYPES OF
PRETREATMENT SYSTEMS
8.1 General
Considerations
There are many
different types of wastewater pretreatment systems that can
treat specific and multiple contaminant problems. The complexity
and installed cost of a complete pretreatment system will depend
on many factors, including:
- the volume and
nature of the wastewater
- the allowable
contaminant levels in the final discharge
- the desired degree
of pretreatment system automation
- the difficulty of
system installation within an existing facility.
For example, one might
imagine that a laundry facility would require a relatively
simple pretreatment system for its wastewater. A local
industrial laundry, however, recently installed a wastewater
pretreatment system that consisted of seven different unit
operations to remove regulated contaminants from its wastewater.
The unit operations were equalization, solids removal
(screening), pH adjustment, oil skimming, dissolved air
flotation with chemical treatment, sand filtration, and
activated carbon filtration.
A facility that must
remove heavy metals from a wastewater stream might use pH
adjustment to precipitate the metals from solution. Most heavy
metals will precipitate out of solution as hydroxide salts at
higher (alkaline) pH levels when sodium hydroxide is used as a
pH adjustment reagent. Hexavalent chromium, on the other hand,
is very soluble in wastewater and must be chemically reduced at
lower (acidic) pH levels to the trivalent form before
precipitation is attempted. Thus, the need for two different pH
conditions for the chromium reduction and precipitation steps
means that a pretreatment system for hexavalent chromium must
have two separate reaction tanks. Often, a third tank is needed
for final neutralization before discharge.
In addition, heavy
metal removal to low levels (in the µg/L or ppb range) can be
achieved by reverse osmosis (RO) or ion exchange pretreatment
processes. If there are organic compounds or bacterial activity
in the wastestream, however, the RO membranes or ion exchange
resins may become fouled, thus reducing flow through the system
or allowing the metals to pass through the system, respectively.
Therefore, the offending organic compounds or bacterial activity
should be controlled prior to these pretreatment processes.
The levels of organic
compounds can be reduced or eliminated by implementing a source
reduction program or by an initial pretreatment step such as
carbon adsorption. A source reduction approach is generally
preferred since it can lower the capital and operating costs of
the pretreatment system. Bacterial activity can be controlled by
chemical additions (i.e., oxidizers such as bleach or
peroxides), high intensity ultraviolet light, high temperature
exposure, or even filtration.
At times, a wastewater
stream may contain chemical agents that can interfere with the
heavy metal removal process. These chemical agents are called
chelators or complexing agents. Source reduction steps may be
needed to reduce or eliminate these agents. Alternately, the
specific wastewater streams that contain these agents may have
to be segregated from the main wastewater stream. The segregated
streams could then either be piped into a separate specialized
treatment unit within the pretreatment system or be collected
and shipped to a licensed disposal facility.
Some form of
pretreatment system for mercury removal may be needed if source
reduction efforts alone fail to solve the problem adequately.
Since mercury is a complex wastewater contaminant, multiple
pretreatment unit processes may be necessary to reduce mercury
levels sufficiently to reach compliance with discharge limits.
For example, filtration would be an excellent candidate for an
initial pretreatment unit process because mercury readily binds
with particulate matter in wastewater. Depending upon the
performance of the filter system in reducing mercury
concentrations, subsequent pretreatment system unit operations
may be smaller, have lower operating costs, or may be eliminated
altogether.
For more detailed
information on the characteristics of mercury in wastewater and
on the performance of several pretreatment technologies in
bench-scale feasibility tests on a clinical laboratory
wastewater, please see the MWRA/MASCO Mercury Work Group,
Technology Identification Subgroup Report, which is a companion
to this Manual.
8.2
Types of Pretreatment Unit Operations
Usually, processes used
in a wastewater pretreatment system can be placed into four
categories as follows:
Biological Processes
- Processes where living microbial organisms are used to
metabolize organic wastes into carbon dioxide, water, methane
gas, simple organic acids, and microbial matter. Aerobic
microbial organisms require oxygen for their metabolisms.
Anaerobic microbial organisms live in oxygen-limited
environments. For municipal wastewater treatment, POTW’s use
both aerobic and anaerobic biological processes.
Chemical Processes -
Processes that alter the chemical structure of the
constituents of the wastewater so they can be removed from the
wastewater stream before discharge. An example is heavy metal
precipitation by pH adjustment.
Physical Processes -
Processes that separate components of wastewater without
altering the chemical structure of the constituent materials.
Examples are dissolved air flotation (DAF), reverse osmosis, and
filtration.
Thermal Processes - Processes
that operate at high temperature to reduce the volume of wastes
and breakdown the toxic components into simpler less toxic
forms. These processes are typically expensive to operate
because of high energy costs, but they can be very efficient for
certain types of pollutants.
The following are
specific types of unit operations used in wastewater
pretreatment systems:
1. Aerobic or
Anaerobic Pretreatment - Operations that use aerobic
bacteria or anaerobic bacteria to reduce organic wastes in
wastewater. Levels of the organic wastes are usually measured in
terms of wastewater biochemical oxygen demand (BOD) and chemical
oxygen demand (COD). Aerobic pretreatment requires a source of
oxygen and can produce significant quantities of biomass
(sludge). At times, high levels of organic wastes can more
economically be treated by an anaerobic process. Many inorganic
contaminants, such as heavy metals, can be adsorbed onto the
biosolids produced during the treatment process. Because of
sensitivity of the bacteria to sudden changes in conditions,
protection of the bacteria by various initial physical or
chemical process operations may be needed.
2. Disinfection
(chemical, thermal, or UV sterilization) - Used to reduce or
eliminate bacterial or viral activity in a waste stream.
Chemical methods usually involve the use of oxidizers such as
hypochlorite (bleach), permanganate, and peroxides. Some unit
operations - such as ion exchange and membrane filtration (see
below for descriptions) - can be adversely affected by
oxidizers. Thermal disinfection is highly effective, but is
usually impractical for large streams because of cost
considerations. Ultraviolet light (UV) sterilization is
especially useful and economical for smaller flows, but may be
ineffective on cysts and spores.
Depending upon the
specific pretreatment technology, disinfection may be used
before a mercury removal step. In some systems, disinfection by
oxidizers can serve a dual purpose: prevention of biological
growth in the adsorbent media and oxidation of complexed mercury
species to more readily removed ionic forms.
3. Clarification -
Used to remove settleable solids from a wastewater stream. At
times, this gravity separation process is chemically enhanced by
adding polymers under controlled conditions to cause
agglomeration of the solids into larger particles for faster and
more efficient settling.
4. Simple Filtration
- Used to remove particulate matter (usually greater than 5
microns in size) from a wastewater stream. Filtration systems in
this category would include bag type, depth or fiber wound
cartridges, and graded sand and diatomaceous earth filter media.
Filtration is often used for wastestreams high in particulate
matter that could disturb subsequent unit operations. Since
mercury has a high tendency to bind to particulate matter,
coarse and fine filtration may be routine as initial unit
operations in a mercury pretreatment system.
5. Membrane
Filtration (micro or nano) - Used to remove smaller
particulate matter, down to the 0.1 micron range. These systems
can employ organic membranes (cellulose-based) or synthetic
membranes. The organic membranes can be adversely affected by
organic solvents, chlorine, and other oxidants. The membrane
pore size can be compared with 0.45 microns that is typically
defined in laboratory analyses of wastewater samples as the
differential point between dissolved matter and suspended
solids. Membrane filtration has been successful in applications
of precipitated metals and free oil removal from wastewater.
6. Reverse Osmosis
(RO) - Used to remove sub-micron particulate and high
molecular weight ions. Hospitals often use RO units for
desalinization of incoming city water (often used with ion
exchange processes). In wastewater pretreatment applications, RO
membranes can readily be fouled by oil and grease and suspended
solids. The membranes can also be adversely affected by organic
solvents and chlorine or other oxidants.
7. Ion Exchange - Used
to remove dissolved ionic compounds. Ion exchange resins usually
require specific pH ranges for good operation and tend to be
expensive, require regeneration, and are susceptible to
degradation by oxidizers and to fouling by suspended solids, oil
and grease, and organic compounds.
8. Dissolved Air
Flotation (DAF) - Used to remove light particulate from a
waste stream by infusing fine air bubbles into a holding tank.
The air bubbles attach to the particulate and lift them to the
wastewater surface where they can be skimmed off. DAF treatment
is often used for fats, oil, and grease (FOG) removal after
de-emulsification.
9. Adsorption - Used
to remove high molecular weight compounds from air and
wastewater streams. The process uses a surface-active medium,
the most common of which is activated carbon. Often used for
removing low concentrations of volatile and non-volatile organic
contaminants (solvents, pesticides, PCB, and phenols ) and
inorganic contaminants (such as mercury and other heavy metals)
from wastewater.
10. Chemical
Precipitation/Redox Reactions - Conversion of a dissolved
pollutant to an insoluble form. Typical of these reactions is
hydroxide precipitation of heavy metals from metal finishing
wastewater. The reactions are usually followed by a particulate
separation process, such as gravity clarification or filtration,
to remove the formed solid particulate from the waste stream.
11. Neutralization -
A unit operation that adjusts the pH of a wastewater stream
by adding acids or alkalies to produce a solution that is near
neutral (pH = 7 standard units (su)) or within an acceptable
range for discharge. The MWRA has an allowable discharge pH
range of 5.5 to 10.5 su.
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