ARTICLE

Optimizing RO-Membrane
Performance In Produced-Water
Applications
The makeup of produced water generated
from oil, natural gas, or coal-seam-gas(CSG) production and other sources
creates many challenges for water reuseor discharge to the environment. Reverse
osmosis (RO) membranes are essential
to treatment, but not all industrial
applications warrant identical approaches.
Whether an RO membrane decisionmaker works for an organization
generating produced water or for an
RO service contractor treating it, thechallenges of maintaining manageablelevels in produced water retention ponds
are the same. Here are key factors thatshould be considered for their influenceson produced water chemistry, treatment
strategies, the ultimate cost of treatment,
and the ability to derive maximum value
from reused water at minimal total
lifecycle costs.
The Challenges Of Produced Water
Treating produced water generated
by oil drilling, gas drilling, or industrial
mining is more difficult than typicalseawater desalination in that it is typicallyladen with a host of components thatcannot simply be discharged back into
the environment. It must be treated toacceptable levels for discharge or for reusein the original production processes, inirrigation or aquifer recharge, or for otheruses. For example, produced water reusedin agricultural irrigation needs to maintainboron content below 1 mg/L to prevent itfrom killing irrigated plants. The delicate
balance of treatment hinges on a varietyof logistical and financial challenges,including:
Water Online • www.wateronline.com

Graphic courtesy of Toray Industries, Inc.

•••
•
•
•
•

the composition of the
produced water itself, in termsof contaminant type and loadconcentration,
regulatory requirements for theintended discharge or reuseapplication,
the types of RO membranes used
for treatment (Figure 1),energy costs associated with RO
performance,
membrane cleaning andmaintenance practices,
the cost of shipping/treating
concentrated brine remaining
after RO, and eventhe scarcity of water at remote
operating sites.

For well or mine operators and their water
treatment services, this article addresses

such issues from the perspectives oftreatment efficacy and cost efficiency aswell as other contractual factors relating
to treatment and discharge.
Membrane Performance
Considerations
One of produced water’s biggest
challenges is the fact that its composition
can easily change from application toapplication, from location to location,and even within a single location basedon variables of seasonality or production
volume.
Because of this variability, it is not practicalto compare membrane products by
manufacturer spec sheet “equivalence,”laboratory-condition QC ratings, orpurchase cost alone. The only quantitative
method to compare the relative cost
1

ARTICLE
effectiveness of membrane options is torun them side by side in the real-world
application to measure performance andlongevity under the produced water’sunique chemical composition and load
fluctuations.
With that in mind, here are somepreliminary issues to consider whenevaluating existing produced water
RO performance and optimizing cost
efficiency for future operations:•••

Pretreatment. The initial stepin optimizing produced water
treatment is not the RO systemitself. It is the pretreatment
system that prepares feedwater
so that RO elements can deliver
the most favorable and stableperformance. Ultrafiltration (UF)removes suspended materials
that act as foulants and ionexchange (IE) removes heavy
metals that can oxidize RO
membrane surfaces when they
react with monochloramines
used for biofouling control.
Permeate Quality. Whenevaluating produced water
treatment success, the essential
question is whether the permeate
quality meets expectations. Not
all membranes are designed
the same, nor do they performthe same in produced water
applications as they do inbrackish water or seawater
applications. Making appropriate
choices and balancing cleaning
maintenance and membrane
replacement according toperformance monitoring (Figure2) (instead of waiting until thepoint of failure) can extend
performance life and optimize
the balance between capital
expenses (CAPEX) and operating
expenses (OPEX).
Productivity Cost Implications.
Whether membrane acquisition
costs are the responsibility of
the well/mine operator or theRO treatment provider, both

Water Online • www.wateronline.com

Graphic courtesy of Toray Industries, Inc.
Figure 1. Reverse osmosis (RO) membranes with extremely fine pore sizes stop salts (NaCl) and other contaminantsfrom passing through the membrane with the clean permeate. Multiple filtration stages can concentrate theresidual brine to less than 10 percent of the original feedwater volume to minimize shipping and/or treatment
costs.

Graphic courtesy of Toray Industries, Inc.
Figure 2. Monitoring the dropoff in permeate flow rate (normalized to account for variable temperature and pHconditions) indicates when periodic cleanings are appropriate. The objective is to maintain maximum throughput
efficiency with minimal membrane cleaning or replacement disruptions. All-too-frequent cleanings can indicate
problems ranging from poor pretreatment design to membrane quality or aging issues.

the membrane chosen andthe way it is maintained canimpact all parties involved. The
true cost of handling produced
water depends not only on
membrane cost but also on thecumulative costs, inconveniences,and risks involved in failing toachieve regulatory complianceor generate water reuse
income or savings. That is why
it is important to evaluate
membrane performance on

the total associated costs that
get magnified as the size of an
application grows.
◦ Energy Costs. As RO
membranes foul or lose
efficiency, they can require
longer cycle times andmore energy to achieve
the desired throughput,elevating operating costs toa point where a maintenancecleaning or membrane
2

ARTICLE
replacement is warranted.
◦ Material And Maintenance
Costs. RO membrane costs
can vary by performance
grades required for theseverity of the applications.
Choosing an inappropriate
low-cost membrane tosave on CAPEX can impact
permeate quality or resultin much higher OPEX
maintenance costs. Premature
fouling can also run up
cleaning and replacement
cost and effort, even if it is
not a membrane issue alone.
It could indicate a needfor better pretreatment toremove as many foulants as
possible before they reach theRO membranes.
◦ Overlooked Costs. Asidefrom direct costs formembrane purchases andassociated labor/operating
costs, it is also important toaccount for hidden costs inthe process (e.g., excessive
maintenance downtime,lost productivity, shipping
costs for offsite brinetreatment, etc.). Poor upfront
contaminant removal andpoor membrane performancecan exacerbate these costs.

•Brine Handling Demands.
Better membrane performancecan reduce the volume of residual
brine that needs to be treatedafter RO treatment, saving bothtreatment and transportation
costs that can be magnified inremote operations.

Water Online • www.wateronline.com

by more than 10 percent, or
when differential pressure or
normalized salt passage increases
by more than 20 percent. For
optimum performance andlongevity, follow manufacturer
recommendations for operation
monitoring, flushing, andcleaning protocols.

Practical Guidelines For Optimum
Performance Efficiency
Although it is difficult to predict longterm membrane performance accurately,
it is reasonable to reference generalindustry experience to determine when
produced water RO treatment systems
are performing far below expectations.
Monitoring the makeup of both raw
produced water entering the RO process
and treated effluent leaving it provides
helpful data for identifying abnormalperformance. Daily readings should benormalized to account for variations
in salinity, in order to quantify the trueindication of declining performance.
•◦ Membrane Longevity.
Beyond lost productivity costs
associated with cleaning
frequency, the physicalimpacts of repetitive cleaning
can also shorten effectivemembrane life, resulting in
more frequent replacement
purchases.
•Seasonal Considerations.
Summertime fouling due toincreased biological growth canimpact performance, multiply
maintenance requirements, andraise the cost of maximizing ROperformance.

Rules Of Thumb For
Performance And Replacement.
In produced water applications,look for a minimum of six months
between service intervals, withan expected membrane life of
three to five years, depending on
the harshness of the feedwater.
Membranes should typically lastfrom six to 10 cleanings, basedon the aggressiveness of thefeedwater and the permeate
quality required.
Testing competitive membranes
side by side in the same process
and monitoring the permeate
quality and quantity for eachversion will also provide a true
picture of real-word performanceand value. This is more accurate
than making hypotheticalcomparisons of spec sheet
QC parameters based on lessaggressive water compositions.

•◦ Chemical Wash vs.
Aggressive Cleaning. At
early signs of reduced RO
permeate flow, perform agentle chemical wash with
an appropriate chemical forthe fouling problem — acidic
cleaning for metal hydroxides
and alkaline agents or
detergents for organic or
biological fouling. Repeat if
necessary. Doing so can beless disruptive to process
operations than aggressivefull-scale cleaning and cantypically restore normal
operating condition without
risk of shortening membranelife.

Cleaning/Maintenance
Schedules. Cleaning should beperformed whenever normalized
permeate flow rates decrease◦ Full Membrane
Replacement. RO membrane
performance naturally
decreases over time, basedon contaminant loads
in produced water andon cleaning frequency.
Monitoring performancechanges on a monthly basis(see below) makes it easier
to determine membrane
failure or when the cost ofperformance and frequency
of maintenance warrant
membrane replacement.
Optimal Strategies For Lowering Total
Lifecycle Cost
Establishing good protocols for measuring
permeate quality and quantity will
provide better guidelines for profitable
operating decisions and lowest total costof operation.
•Regular Monitoring. Performing
full water analyses on both3

ARTICLE
changes in feedwater makeup
entering the RO system and thequality of permeate coming outof each bank of an RO systemenables treatment system
operators to make timelydecisions about optimal system
operation and maintenance.
Tracking runtime and output
volume can highlight small
inefficiencies that drive up
energy costs and lead to more
frequent maintenance.
••An Ounce Of Prevention. Where
high concentrations of salts orcomponents foul membranes,
pretreatment steps such as UF
and IE can reduce the excess
load in the feedwater before it
reaches the RO membranes. This
contributes to longer membranelife and promotes cost-effective
overall performance in termsof membrane costs, minimized
downtime, and reduced laborcosts.
Balancing Operating Costs.
Tracking maintenance trends
makes it easier to strike an idealbalance between maintenance
downtime costs and membrane
replacement costs. One way
to spread the total costs of RO
operations more evenly fromyear to year is to replace a smaller
percentage of RO elements ateach cleaning instead of waiting
for a catastrophic failure or
replacing all elements at onetime. It usually makes sense to

Water Online • www.wateronline.com

Photo and graphic courtesy of Toray Industries, Inc.
Figure 3. The produced water at a Queensland CSG plant is treated by a series of cylinders fitted with RO
membrane cartridges that separate salts and other contaminants from the filtered permeate that exits througha central product tube. Three stages of RO filtration are outfitted with progressively finer membranes to produce
water clean enough to be used for agricultural irrigation in the otherwise arid location.

replace the highest-loading RO
elements in the treatment train
first and continue in order ofpriority until all elements arereplaced. Well-paced service
intervals on just one RO bank at
a time also enable each service
event to be of shorter duration(a day or two) and easier toaccommodate within normal
production schedules than one
major overhaul event lasting
weeks or months.
Proof In Performance
One example of successful RO treatment
for produced water is a CSG operation
in a water-challenged environmentof Queensland, Australia. There, highorganics, suspended solids, and TDS
ranging as high as 8,000 mg/L risk foulingRO membranes, while boron and fluoride
challenge regulatory-compliance andwater reuse concerns.
Two-stage pretreatment and a threestage 26-MGD treatment system using
low-fouling, high-tolerance, cross-linked
polyamide-composite RO membranes
protect precious groundwater resources
and prevent contamination of soil andshallow aquifers around the facility (Figure3). The system recovers more than 90
percent of produced water flow for reusein irrigation applications, with its highquality permeate exhibiting low levels ofboron (<1 mg/L), fluoride (<0.5 mg/L), andTDS (<175 mg/L).

4