What is Zero Liquid Discharge and How Does It
Work?
.
A ZLD treatment system
utilizes advanced technological water treatment processes to limit liquid waste
at the end of your industrial process to, as the name suggests, zero. Because
of the broad range of industries that use ZLD and the various waste streams
produced, ZLD is a highly custom process and these add ons will depend on your
facility’s individual needs.
SAMCO
It is usually
looked to as a last resort because it can be a complex process that requires a
high initial investment.
If a facility is
located on a site that has severe water scarcity issues and/or astronomical
discharge fees, it might be worthwhile to pursue, but in the instances it’s not
mandated (some local and/or federal regulations might require ZLD), careful
consideration must be made as to whether or not it will benefit your facility.
If
your facility does require or is considering it, you might be wondering, “what is zero liquid discharge and how does
it work?”
The complex answer
to this question is simplified and broken down for you below:
What is a zero
liquid discharge treatment system?
A ZLD treatment
system utilizes advanced technological water treatment processes to limit
liquid waste at the end of your industrial process to, as the name suggests,
zero.
An efficient and
well-designed ZLD treatment system should be able to:
o handle variations
in waste contamination and flow
o allow for required
chemical volumes adjustments
o recover around 95%
of your liquid waste for reuse
o treat and retrieve
valuable byproducts from your waste (i.e. salts and brines)
o produce a dry,
solid cake for disposal
A ZLD treatment
system will also help your facility meet stringent effluent requirements, such
as the U.S. Environmental Protection Agency’s Steam Electric Power Generating
Effluent Guidelines.
Just keep in mind
your facility’s requirements will vary based on whether you are discharging
into a publicly owned treatment works (POTW) or to the environment under a
National Pollutant Discharge Elimination System (NPDES permit).
What’s included in a basic ZLD treatment system?
The exact
components of a ZLD treatment system will largely depend on
(1.)
the volume of dissolved material present in the waste,
(2.)
the system’s required flow rate, and
(3.)
what specific contaminants are present. But in general, a basic ZLD treatment
system typically includes some type of:
o clarifier and/or
reactor to precipitate out metals, hardness, and silica
o chemical feed to
help facilitate the precipitation, flocculation, or coagulation of any metals
and suspended solids
o filter press to
concentrate secondary solid waste after pretreatment or alongside an evaporator
o ultrafiltration
(UF) to remove all the leftover trace amounts of suspended solids and prevent
fouling, scaling, and/or corrosion down the line of treatment
o reverse osmosis
(RO) to remove the bulk of dissolved solids from the water stream in the
primary phases of concentration
o brine concentrators
to further concentrate the reject RO stream or reject from electrodialysis to
further reduce waste volume
o evaporator for
vaporizing access water in the final phases of waste concentration before
crystallizer.
o crystallizer to
boil off any remaining liquid, leaving you with a dry, solid cake for disposal
Depending on the
needs of your plant and process, these standard components are usually
adequate, however, if your plant requires a system that provides a bit more
customization, there might be some features or technologies you will need to
add on.
Because of the
broad range of industries that use ZLD and the various waste streams produced,
ZLD is a highly custom process and these add ons will depend on your facility’s
individual needs.
How does a ZLD treatment system work?
Specific treatment
processes vary, but a typical ZLD treatment facility process will usually
include the following steps:
Pretreatment and conditioning
Pretreatment is
used to remove simple things from the wastewater stream that can be filtered or
precipitated out, conditioning the water and reducing the suspended solids and
materials that would otherwise scale and/or foul following treatment steps.
Typically this
treatment block consists of some type of clarifier and/or a reactor to
precipitate out metals, hardness, and silica.
This process starts
off with an assortment of mixing reactors, typically one or two reactors that
add specific chemicals to take out all the finer particles in the water by
combining them into heavier particles that settle out.
The most widely
used coagulates are aluminum-based such as alum and polyaluminum chloride.
Sometimes a slight
pH adjustment will help coagulate the particles, as well.
When coagulation is
complete, the water enters a flocculation chamber where the coagulated
particles are slowly stirred together with long-chain polymers (charged
molecules that grab all the colloidal and coagulated particles and pull them
together), creating visible, settleable particles that resemble snowflakes.
The gravity settler
(or sedimentation part of the ZLD treatment process) is typically a large
circular device where flocculated material and water flow into the chamber and
circulate from the center out.
In a very slow settling
process, the water rises to the top and overflows at the perimeter of the
clarifier, allowing the solids to settle down to the bottom of the clarifier
into a sludge blanket.
The solids are then
raked to the center of the clarifier into a cylindrical tube where a slow
mixing takes place and the sludge is pumped out of the bottom into a
sludge-handling or dewatering operation.
The settlers can
also be designed using a plate pack for smaller footprint.
Depending on the
material in the feed, additional reactors or chemistry may be required for the
reduction of metals or silica.
Careful
consideration must be given to the pretreatment step for a successful ZLD
system.
Ultrafiltration
(UF) can also be used after the clarifiers instead of the gravity sand filter,
or it can replace entire clarification process altogether.
Membranes have
become the newest technology for treatment, pumping water directly from the
wastewater source through the UF (post-chlorination) and eliminating the entire
clarifier/filtration train.
Out of this process
comes a liquid that is then filter-pressed into a solid, resulting in a
solution much lower in suspended solids and without the ability to scale up
concentration treatment.
Phase-one concentration
Concentrating in
the earlier stages of ZLD is usually done with membranes like reverse osmosis
(RO), brine concentrators, or electrodialysis.
The RO train will
capture the majority of dissolved solids that flow through the process, but as
mentioned in a prior article about common problems with ZLD, it’s important to
flow only pretreated water through the RO system, as allowing untreated water
to go through the semipermeable membranes will foul them quickly.
Brine
concentrators, on the other hand, are also used to remove dissolved solid waste
but they are usually able to handle brine with a much higher salt content than
RO.
They are pretty
efficient for turning out a reduced-volume waste.
Electrodialysis can
also be used at this part of the ZLD treatment system.
It’s a membrane
process that uses positively or negatively charged ions to allow charged
particles to flow through a semipermeable membrane and can be used in stages to
concentrate the brine. It is often used in conjunction with RO to yield
extremely high recovery rates.
Combined, these
technologies take this stream and concentrate it down to a high salinity while
pulling out up to 60–80% of the water.
Evaporation/crystallization
After the
concentration step is complete, the next step is generating a solid, which is
done through thermal processes or evaporation, where you evaporate all the
water off, collect it, and reuse it.
Adding acid at this
point will help to neutralize the solution so, when heating it, you can avoid
scaling and harming the heat exchangers.
Deaeration is often
used at this phase to release dissolved oxygen, carbon dioxide, and other
noncondensible gases.
The leftover waste
then goes from an evaporator to a crystallizer, which continues to boil off all
the water until all the impurities in the water crystallize and are filtered
out as a solid.
Recycled water distribution/solid waste treatment
If the treated
water is being reused in an industrial process, it’s typically pumped into a
holding tank where it can be used based on the demands of the facility.
The ZLD treatment
system should have purified the water enough to be reused safely in your
process.
The solid waste, at
this point, will enter a dewatering process that takes all the water out of the
sludge with filter or belt presses, yielding a solid cake.
The sludge is put
onto the press and runs between two belts that squeeze the water out, and the
sludge is then put into a big hopper that goes to either a landfill or a place
that reuses it.
The water from this
process is also typically reused.
In conclusion
SAMCO has over 40
years’ experience custom-designing and manufacturing industrial wastewater
treatment, so please feel free to reach out to us with your questions.
For more
information or to get in touch, contact us here. You can also visit our website
to set up a call with an engineer or request a quote.
We can walk you
through the steps for developing the proper solution and realistic cost for
your ZLD wastewater treatment system needs.
For more articles
on wastewater treatment, head on over to our blog. Some that might be of
interest to you include:
What Are the Best
Companies for Zero Liquid Discharge Equipment and Technology?
How to Choose the
Best ZLD System for Your Facility
How Much Will a
Zero Liquid Discharge System Cost Your Facility?
What Is a
Wastewater Treatability Study and How Does it Work?
How Much Does a
Wastewater Treatment System Cost? (Pricing, Factors, Etc.)
The Importance of
Wastewater Treatment for Your Facility: Is it Necessary?
How Do You Know If An Industrial Facility Needs a Wastewater Treatment System?
Whether
you’re looking to improve your water or wastewater, or have process separation
and purification needs to be met, here at SAMCO we’re ready to customize the
right solution for you, as we’ve been doing it for over 40 years.
Established
in Western New York in the 1950s as a soft water provider. Current owner
purchased the industrial portion of that company in 1987 to form CS Kimeric. In
1998, after identifying the need for an integrated solutions partner – a
company who could offer everything from concept to design fabrication – SAMCO
was born.
https://www.samcotech.com/what-is-zero-liquid-discharge-and-how-does-it-work/
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