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SAMCO
Well-designed RO/NF
units are compact and demand relatively little maintenance, making them an
attractive alternative to conventional treatment trains.
Even despite the
merits of today’s efficient RO/NF technologies, it pays to understand common
pitfalls associated with them.
If your facility
currently uses RO/NF or is considering purchasing an RO or NF system, the
following article will help you to understand the most common problems
impacting reverse osmosis and nanofiltration, and some possible solutions.
RO/NF membrane fouling
With the smallest
pore sizes of any membrane filtration technology, RO/NF are particularly prone
to premature membrane fouling.
Without adequate
pretreatment and process monitoring steps in place, RO/NF membrane
fouling can reduce unit service life, compromise permeate quality, and increase
operational costs.
Preventative steps
and/or remediation strategies for membrane fouling depend on the types of
contaminants present in the process or waste stream.
Common types of
fouling include:
Particulate and colloidal fouling
In many cases
increased pressure differential measurements provide early indication of
particulate fouling in RO/NF membranes.
Common particulate
contaminants include bacteria, viruses, sediment, macromolecules, iron oxides,
salts, and colloidal silica.
In many cases,
particulate fouling of RO/NF units can be prevented by applying appropriate
upstream filtration, which can include media filtration, microfiltration
(MF), and/or ultrafiltration (UF), depending on the sizes and
geometric shapes of particles present.
For streams with
colloidal particles, it is sometimes necessary to apply an inorganic coagulant
to separate out suspended solids.
Commonly-used
coagulants include aluminum sulfate, aluminum chloride, sodium aluminate, and
ferric chloride, and the use of each will vary depending upon the contaminants
present in the feed stream.
The use of
coagulant chemicals must be monitored closely, however, as coagulants can
also lead to RO/NF membrane fouling if allowed to proceed downstream,
where they can react with antiscalants or other substances and collect on the
membrane.
Biofouling
As biofilm
accumulates on the membrane surface, greater pressure is needed to force water
through, resulting in higher energy costs, and eventual damage to the RO/NF
membrane element.
Key symptoms of
biofouling include increased differential pressure from feed to concentrate,
and decreased membrane flux.
RO/NF membrane
elements tend to provide the warm, low-flow environments suitable to biological
growth, making them particularly susceptible to biofouling. Some
common solutions for control and prevention of biofouling include:
· Biofiltration to remove nutrients from the feed stream;
· Chlorination to chemically destroy biological contaminants; and
· Fouling-resistant membranes that prevent microbials from
clinging to the RO/NF element.
Scaling or precipitation fouling
Scaling or
precipitation fouling occurs when membrane pores are blocked by crystallized
salts, oxides, or hydroxides that have precipitated from solution.
Scaling is among
the most common forms of fouling in RO/NF elements, especially by divalent
calcium (Ca2+) and Magnesium (Mg2+) ions.
Like other forms of
fouling, scaling and precipitation fouling can compromise the efficiency of an
RO/NF unit, and, over time, can irreversibly damage membrane elements.
Control and
prevention strategies for scale and precipitation fouling focus on inhibiting
crystal growth, resulting in particles that are small enough to be
carried away in the reject stream.
Control methods
include:
· Acid injection to control calcium carbonate scale;
· Water softening, or the addition of lime to feed water as a
means of reducing hardness, alkalinity, and silica to prevent scale crystal
formation; and
· Scale inhibition, or the injection of a specialized chemical
substance into the feed stream to inhibit the growth of salt crystals.
[Download our
free RO/NF membrane system e-book.]
Membrane material compatibility
RO/NF membranes are
fabricated from a variety of materials, including cellulose acetate, polyamide,
and polysulfone, among others.
In order to prevent
premature failure, a number of important process factors — including industrial
application, pH, substances present, temperature, feed pressure, and biological
load — should be considered in the selection of a membrane material.
An example of this
is the use of polyamide membranes.
While widely used
in RO/NF units, polyamide
membranes are easily damaged by chlorine, making them a poor option
in applications where chlorine is needed for disinfection purposes.
In order to prevent
chemical attack and oxidation, all process characteristics must be carefully
considered when selecting a membrane material.
Reject water discharge
Disposal of the
concentrated waste streams produced by RO can be challenging, especially if your
facility is subject to zero liquid
discharge (ZLD) regulations, or if you face high costs for sewer or
surface water discharge.
A popular solution
to mitigate disposal costs is the reduction of RO waste stream volume through
evaporation.
Overall impact of proper design on preventing common problems
Many of the common problems
impacting RO/NF can be avoided through careful design that takes process
conditions into account.
While we’ve already
discussed how pretreatment is essential for minimizing operational and
maintenance issues, care must also be given to other system design elements,
including:
· Flux: Flux is the volume of water to pass through a
membrane in a given amount of time, often expressed as the number of gallons of
water per square foot of membrane per day (GSFD). Flux is used to determine the
number of membrane elements needed for an application, and is affected by feed
water quality, temperature, and salt concentration.
· Flow rate: Generally measured in gallons
per minute (GPM), feed and permeate flow rates are critical measures for
efficient RO/NF operation. Good RO/NF system design takes water source into
account, for example, when processing surface waters with high colloids, an
optimal flow rate may be 10 – 14 GPM/ft2 of membrane.
· Array: An array is the physical arrangement of pressure
vessels in an RO/NF system. An array can entail multiple stages, with multiple
pressure vessels in each stage. Generally speaking, the higher a recovery rate
demanded by a facility, the greater the number of stages in its array.
A trusted engineer
can help you to weigh these and other factors to achieve optimal RO/NF
performance, maintenance, and energy costs, both immediately and in the long
term.
How can SAMCO help?
SAMCO has over 40
years’ experience custom-designing and manufacturing RO/NF systems for a range
of industries and solutions, so please feel free to reach out to us with your
questions.
For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your RO/NF treatment system needs
SAMCO promises complete
partner and customer satisfaction through our comprehensive, project-based
approach that delivers customized industrial solutions.
At
SAMCO, we anticipate the needs of industry, and respond with forward-thinking
solutions. Our focus on industrial applications began in 1987 with the founding
of Northeast equipment supplier and systems servicer CS Kimeric. Acquired from
a Western New York soft water provider with over 30 years in the business, CS
Kimeric was established to provide specialized service for industrial
applications. Over the course of the next decade, it became clear that
industrial clients would benefit from working with a partner capable of
delivering comprehensive, concept-to-completion solutions. In 1998, founder and
CEO Richard Posa established SAMCO as an integrated provider of design,
fabrication, startup, and maintenance services.
Today,
SAMCO serves the process water needs of clients across the nation and globe
from its headquarters in Buffalo, New York. Leveraging the collective skills of
experienced chemical, civil, environmental, electrical, mechanical and process
engineers, chemists, and skilled tradesmen, SAMCO blends a culture of teamwork,
commitment and passion to help solve your unique industrial water treatment
needs.
