ULTRA-PURE, READY-MADE CHLORINE DIOXIDE SOLUTION
REMOVES BIOFOULING AND PREVENTS REGROWTH
WITHOUT DAMAGE TO MEMBRANES
Thomas E. McWhorter, CDG Environmental LLC., 301 Broadway Suite 420
Bethlehem, PA 18015 215-327-4748
[email protected]
Introduction
Published test results demonstrate that pure chlorine dioxide supplied in ready-to-use
aqueous solution can remove bio-fouling and prevent its regrowth on TFC membranes and
filters in reverse osmosis or ultrafiltration system without damage to membranes. Previous
results showing membrane damage from chlorine dioxide are believed to be caused by
impurities such as chlorine or chlorous acid in chlorine dioxide made at the point of use.
Biofouling in Membrane Systems
One of the biggest problems plaguing the fast-growing field of membrane-based water
purification is bio-fouling. Bacteria and other microorganisms form a slimy layer that
adheres to the surfaces of membranes, filters, pipes and other components of a reverse
osmosis or ultra-filtration system. Biofilm consists of bacteria and other microorganisms
bound together and to a surface by polysaccharide “glue”. Biofilm formation is especially
problematic in water containing high levels of nutrients such as tertiary municipal
wastewater and other wastewater streams. Biofilm blocks the flow of water through
membranes and filters and necessitates frequent shut-down for off-line cleaning or
replacement of elements. Off-line cleaning often results in damage to expensive
membranes.
Biofilm usually forms most quickly on the feed side of the membrane, but it can also
form in the permeate stream or even within the membrane.1 Biofilm in the permeate
stream can reach a thickness where it sloughs off, resulting in bacteria-laden particles in
the product stream.
Chlorine, hypochlorite, and related chemicals are not effective at controlling biofilm and
can quickly damage membranes.1 Some disinfectants are only partly effective against
biofilm in part because the disinfectants cannot penetrate the polysaccharide matrix.
Therefore, when membrane performance is compromised by biofilm, the membrane must
be taken off-line for mechanical cleaning and disinfection. During the shutdown for offline cleaning the total system must be shut down, causing loss of production, or a
secondary set of membranes must be provided, often at substantial cost. Off-line
disinfectants never remove 100% of the biofilm so biofilm re-seeds itself and grows back
faster after every cleaning.1
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In many membrane systems the water is pre-filtered using cartridge filters to remove
particles as small as a fraction of a micron before the water passes through the
membranes. These filters are often subject to rapid bio-fouling by microorganisms that
grow in the nutrient-rich media. This fouling usually necessitates shutdown and
replacement of filter cartridges.
Chlorine Dioxide
Chlorine Dioxide (ClO2) has been used for more than 70 years in disinfection of drinking
water and oxidation of certain drinking water contaminants. It is a broad range biocide
and selective oxidant. In aqueous solution it reacts with other compounds by taking an
electron to convert ClO2 (dissolved gas) to ClO2- (dissolved chlorite ion).
Environmental Fate
When ClO2 reacts in solution, it produces chlorite (ClO2-) and chloride (Cl-) ions. The
USEPA has established standards for acceptable levels of ClO2 and ClO2- in drinking
water and water discharged to the environment. Simple, off-the-shelf equipment is
available to monitor the required parameters. Over time, in the presence of reducing
chemicals, ClO2- reacts further to become Cl-. Thus the ultimate environmental fate of
ClO2 is the generally benign chloride ion.
Selective Reaction
An example of chlorine dioxide’s selective oxidation capability is its ability to rapidly
oxidize manganese from the water-soluble Mn-2 to the insoluble Mn-4 state without fully
oxidizing the metal to produce soluble Mn-8 (permanganate). Many drinking water
companies use chlorine dioxide in this reaction to convert dissolved manganese to
particles that can be removed by filtration.
ClO2 is also frequently used to replace chlorination in drinking water where chlorine
reacts with organics in the feed water to produce trihalomethanes and haloacetic acids
(THMs and HAAs) which are increasingly regulated as carcinogens. Chlorine dioxide
does not chlorinate the hydrocarbons. Unlike chlorine, chlorine dioxide does not react
with ammonia, which is often present in large concentrations in processed wastewater.
Traditional ClO2 Manufacturing Options
Historically, ClO2 was considered unstable. Popular wisdom was that it could not be
shipped and must be produced at the point of use. When stored as a dissolved gas in
water, the product lifetime usually ranged from hours to a few days. In such
circumstances, chlorine dioxide in less than ton quantities was usually produced near the
point of use in one of a variety of processes2.
One of the most common processes is the “acid/chlorite” or two-component process in
which an aqueous solution of sodium chlorite is mixed with one of a variety of acids
(typically hydrochloric acid) in Reaction 1. Solutions of sodium chlorite are typically
marketed as “stabilized sodium chlorite” and acid is marketed as “activator” for use in
this reaction.
5NaClO2 + 4HCl => 4ClO2 + 5NaCl + 2H2O
Reaction 1
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This reaction proceeds very slowly and typically produces substantial quantities of byproduct impurities such as chlorous acid (HClO2), as well as unreacted HCl.
A second common manufacturing technique is the three component system that mixes
sodium chlorite with hydrochloric acid and sodium hypochlorite (bleach) in Reaction 2.
2NaClO2 + NaOCl + HCl =>2ClO2 + 2NaCl + NaOH
Reaction 2
This reaction overcomes some of the problems of Reaction 1. However, bleach (NaOCl)
is notoriously unstable and the reactor must be frequently readjusted to accommodate
variations in bleach concentration. As a result, the product usually contains impurities
such as chlorine or NaOCl.
The “Rio Linda” generator mixes aqueous sodium chlorite with gaseous chlorine in an
eductor to produce chlorine dioxide in a water stream in the reaction
2NaClO2 + Cl2 => 2ClO2 + 2NaCl
Reaction 3
Even slight mis-adjustments to this system can result in elemental chlorine in the
resulting product and damage to membranes.
The “Gas:Solid” system passes a stream of dilute chlorine gas over a matrix containing
solid sodium chlorite. The resulting dilute chlorine dioxide stream produced by Reaction
3 is essentially 100% pure as long as the solid sodium chlorite is replenished frequently
enough. If the solid sodium chlorite is not replenished as needed, chlorine gas will break
through the bed and damage membranes.
The processes described above represent the most commonly-used approaches to on-site
production of chlorine dioxide at a scale up to one ton/day or so. It is clear that if any of
these processes are used to produce chlorine dioxide for disinfecting membrane systems,
the membrane can be exposed to damaging chemicals on an on-going basis and/or during
upset conditions.
Chlorine Dioxide for Disinfecting Membranes and Filters and Preventing Bio-fouling
Chlorine dioxide has long been known as an effective disinfectant for removing and
preventing biofilm and for cleaning membranes and filters.3 Low doses of chlorine
dioxide (< 1ppm in most cases and typically 0.25 ppm) applied continuously, prevent the
growth of biofilm and can penetrate polysaccharide slime to remove existing biofilm.
Chlorine dioxide can also be useful in off-line cleaning of filters or membranes.
Chlorine dioxide exists in water as a dissolved gas. Therefore chlorine dioxide introduced
into a feed-water stream of a membrane will disinfect the feed stream and the feed side of
the membrane. It will also permeate the membrane and kill any microorganisms within
the structure of the membrane. Then it will be carried along with the permeate stream and
disinfect the permeate side of the membrane and the product water.
Chlorine dioxide can be introduced to an RO system upstream of cartridge filters and can
greatly increase the useful life of cartridge filters by controlling or eliminating biofouling. In this application, chlorine dioxide passes through the cartridge filters and into
the membrane modules where it disinfects and prevents biofilm on the membranes.
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Because chlorine dioxide is normally unstable, in the past it was always made near the
point of use. The traditional processes for making chlorine dioxide always produced
some level of chlorine contamination and/or contamination by other chemicals that can
quickly damage membranes. Even chlorine dioxide generators that normally produced
chlorine-free chlorine dioxide sometimes produced chlorine contaminants during process
upsets. Though it was well-known3 that pure chlorine dioxide does not damage TFC
membranes, the use of chlorine dioxide for disinfection of membrane systems was very
limited.
Ultra-pure Ready-to-use Chlorine Dioxide Solution
Until recently, membrane manufacturers warned against use of chlorine dioxide for
disinfecting membranes because chlorine-free chlorine dioxide was not available. A few
years ago, CDG Environmental, LLC, of Bethlehem, PA developed a storage-stable,
shippable chlorine-free chlorine dioxide solution containing 3000 ppm of ultra-pure
chlorine dioxide in water. This patented disinfectant is marketed as CDG Solution 3000
by CDG. It is also marketed by distributors under different brand names.
Solution 3000 is stable in storage and shipment with a shelf-life of 9 months or longer at
room temperature. It is made in a patented process that includes dissolving ultra-pure
chlorine dioxide gas in ultra-pure water under strict quality control monitoring.
Concentrations of elemental chlorine in Solution 3000 are less than 10 ppm in 3000 ppm
of dissolved chlorine dioxide. Thus, if Solution 3000 is used at a typical 0.25 ppm dose in
a membrane system, elemental chlorine concentration in contact with membranes will be
less than 0. 9 ppb (less than 1 part per billion).
Examples of Membrane Test Results
Many membrane users are adapting their systems to Solution 3000. A few have published
the results of their testing.
Published results of tests on full-scale commercial RO plants with TFC membranes have
shown that the use of chlorine-free chlorine dioxide for disinfecting membrane systems
often greatly reduces the operating cost of the membrane system. In one plant4,
continuous application of chlorine dioxide at a dose of 0.20 – 0.25 ppm resulted in an
increase in filter cartridge life from two weeks to eight weeks while simultaneously
reducing pressure drop across the cartridges and allowing replacement of 5 micron
cartridges with 1 micron cartridges. In the same plant, the chlorine dioxide passed
through the filter cartridges and into the RO modules. Before chlorine dioxide use, the
membranes were sent off-site for cleaning every 2-3 weeks. After the use of chlorine
dioxide, the membranes did not have to be sent for off-site cleaning during the 12-month
duration of the trial. Over the duration of the trial, there was no deterioration in permeate
quality and no apparent oxidative damage to the membranes upon post-trial examination.
As a dissolved gas ultra-pure chlorine dioxide passed through micro-filters and the
membranes with essentially no change in concentration so the same chlorine dioxide
dose controlled biological contamination and biofilm in the feed stream, the permeate
stream and the reject stream.
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In another plant trial1, the use of chlorine dioxide resulted in increased flux through the
membrane with negligible change in permeate conductivity over the 19-month trial.
Of course, the success of chlorine dioxide in improving membrane performance is
dependent on the extent to which bio-fouling is the limiting factor in membrane
performance. The manufacturer or its distributors can be helpful for full-scale or pilotscale application testing as well as design guidelines on applications equipment, piping
and other installation considerations.
Shipping and Application
Solution 3000 is shipped in ready-to-use polymeric containers ranging in size from 5 –
330 gallons. Containers can be supplied with specially designed Colder Fittings that
permit connection and disconnection without exposing workers to volatile gas from the
solution. The concentrated product 3000 can be metered into the process stream using
standard metering pumps and off-the-shelf controls. Solution 3000 is storage-stable for at
least nine months at room temperature and can be stored at temperatures ranging from
32oF to 140oF.
References
(1)
(2)
(3)
(4)
Dimotsis, George, Field trial experience using chlorine dioxide as cleaner for
biofilm control in an RO application, Dripping Wet Water San Antonio, Texas
Simpson, Gregory D. PhD, (2005), “Practical Chlorine Dioxide” pp 67-75
Dow Chemical Company Product Guideline (1998), Form No. 609-01010-498QRP
CH 172-041-E-498
Zupanovich, John, Bishop David and Doak, Andrew, Chemtreat, Inc., Duff,
Andrew, Covanta Energy (2012) “Microbiological Control in Thin Film
Composite Membrane Systems Using a High Purity Chlorine Dioxide System”
American Water Works Membrane Technology Conference Proceedings
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Chlorine Dioxide Solution
Eliminates Biofouling without
Damage to Membranes
Tom McWhorter
CDG Environmental LLC.
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“Biofilm is the last great challenge in
reverse osmosis system design and
operation.”
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Biofilm
• Bacteria, algae, other microorganisms
• Bound together and to surfaces with a
polysaccharide “glue”
• Lives and grows on nutrients in water
• Most problematic in high nutrient streams
like tertiary wastewater
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Biofilm Effects
• Clogs membranes and filters
• Reduces flux through membrane or filter
• Requires frequent shutdown for offline
cleaning or replacement
• Can penetrate membranes and grow on
permeate side and inside membrane
• Most biocides will not penetrate the film
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Consequences of Biofilm
• Clogs membranes and filters
• Reduces flux and/or increases pressure
requirements
• Can grow inside membrane structure
• Can penetrate membrane and enter
permeate.
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Shortcomings of Remedies
• Most disinfectants cannot penetrate biofilm
• Offline cleaning is expensive and may
damage membranes.
– Backup membranes and replacement of filters
– Downtime
– Not effective against embedded organisms
– Does not remove 100% - grows back faster
after every cleaning
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Chlorine Dioxide
•
•
•
•
Broad range biocide
Effective at very low concentration
Environmental fate is chloride ion
Selective oxidant
– Does not form THM or HAA
– Does not react with ammonia
– Does not oxidize bromide to bromate
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Chlorine Dioxide
• Exists as a non-ionic dissolved gas
• Penetrates and destroys biofilm
• Permeates membranes
– Disinfects feed, permeate and waste stream
– Disinfects inside membrane structure
• Prevents biofouling of membranes, filters,
piping and other wetted surfaces.
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Chlorine Dioxide
• Does not damage TFC (polyamide)
membranes
• Previously reported damage caused by
chlorine and other contaminants
• In the past pure chlorine dioxide was
unavailable
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Chlorine Dioxide
• Traditionally unstable – made at point of
use
– Acid/chlorite reactors
– Acid/chlorite/bleach reactors
– Chlorine gas/chlorite reactors
• All produce chlorine and/or other
contaminants if not maintained and
adjusted properly
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New Chlorine Dioxide Supply
• Ready-made solution
• 3000 ppm chlorine dioxide + ultra-pure
water
• Storage stable for > 9 months at room
temperature
• Application requires only chemical feed
pump
• Essentially zero chlorine or other
contaminants
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Application
• Delivered in plastic containers 5 – 330 gal.
• Recommended 0.1 – 1.0 ppm chlorine
dioxide continuously
• Apply continuously upstream of filters
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Case 1 Application
• Tertiary treated waste water processed for
cooling tower make-up
• Chlorine dioxide applied upstream of filters
• Applied at 0.1 – 0.25 ppm depending on
seasonal factors
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Case 1 Filter Results
• Micro-filter changed 5 micron to 1 micron
• Lower pressure drop across filters
• Filter life improved from 12-14 days to 8
weeks
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Case 1 Membrane Results
• Lower membrane feed pressure with
slower rate of increase
• Membrane life approximately doubled
• 15 months(25,000 ppm-hours) with no
membrane damage
– Based on rejection rate
– Based on microscopic analysis
– Accelerated lab testing shows similar result
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Case 2- Application
• Clarified river water
• TFC Membranes
• Continuous application to already fouled
membranes
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Case 2 - Results
• Former DNBPA disinfectant ineffective
• Chlorine-free chlorine dioxide does not
damage membrane after 19 months
• Flux increases, pressure declines
approaching original performance
• Chlorine dioxide concentration unchanged
by passage through membrane
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Conclusions
• Pure chlorine dioxide does not damage
TFC membranes at recommended doses.
• Restores membrane performance
• Effective for biofilm elimination
• Chlorine dioxide permeates membrane
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Plans
• Accelerated test of long term effects on
membranes
• Other membrane materials and processes
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