Achieving Excellence in High Purity Water Since 1985
TechNotes
PUREFLOW
The Official Journal of the PFI High Purity Water Conference & Seminar Series
Spring 2014
Basics of RO Troubleshooting by Adam Holferty
Process Engineer - Water Treatment
Reverse osmosis (RO) technology has been in widespread to pass water from one side of the membrane to the other.
application for 30 to 40 years, and the industry’s understanding A semipermeable mem-
and acceptance of this technology has grown significantly over brane, true to its name, is a barrier
this period. Process industry professionals now recognize the value through which certain compo-
that a reverse osmosis unit adds to a water system, in terms of both nents may pass while others are
production quality and reduction in the recurring costs associated rejected. In the case of a reverse
with ion exchange systems. It is surprising then, 40 years after its osmosis membrane, water is
inception, how poorly many reverse osmosis systems are designed freely allowed to pass, while
and operated. Any potential operational cost savings are thrown dissolved & suspended contami- Semipermeable Membrane
away by excessive cleaning and membrane replacement caused nants such as dirt, minerals or
by unsound operation or design. This article is meant to provide a bacteria are mostly rejected.
reverse osmosis operator or maintenance person with an overview Reverse Osmosis then, is the reversal of the osmosis process:
of basic RO monitoring and troubleshooting techniques that can water passes from a solution of high TDS to low TDS. This means we
improve quality production, reduce downtime, and prevent costly RO can use membranes to remove dissolved solids from water by applied
membrane element destruction. pressure. A high pressure pump
is the driving force behind
Reverse Osmosis Basics water production in every RO
While this article is intended for someone who is already familiar unit. There must be enough
with reverse osmosis units, a brief introduction may prove beneficial pressure applied to the feed
to the uninitiated. Osmosis is a naturally occurring phenomenon water in order to overcome the
in which water has a tendency to pass through a semi-permeable osmotic pressure developed by
membrane from a solution of low total dissolved solids (TDS) into the dissolved salts present. The
a solution of high total dissolved solids. Flow of water through the more salts present in the water, the Reverse Osmosis
membrane is dependent on the osmotic pressure of the solution, higher the osmotic pressure and
essentially a term describing the magnitude of a solution’s tendency the higher the required feed pressure for the RO unit.
This explains the significant difference between RO units de- membrane it is referred to as scaling. The two most common scaling
signed for use on city water or surface water, and RO units designed compounds are calcium carbonate and calcium sulfate.
to desalinate seawater. The dissolved solids in seawater could be 100 Fouling is another common issue that can affect membrane
times the dissolved solids present in a lake or a river. performance. Fouling is a general term used to describe any
The reverse osmosis suspended solids that accumulate on the membrane surface or in
process is an example of a the membrane feed channel and reduce water production. Fouling
cross flow filtration system. can be caused by dirt, sand, silt, clay, bacteria or any other small
Typical media or cartridge suspended solids. Fouling caused by bacteria or living organisms
filtration uses a full flow is often called “biofouling” to distinguish it from fouling caused
design, where all of the by inorganic compounds. Biofouling is often one of the trickiest
feed water passes through problems to diagnose and treat in an RO unit. The main difference
the filtration media and is between scaling and fouling is in the type of constituents in the
collected as product. In a Full Flow vs. Cross Flow water that are creating the problem. Fouling is caused by suspended
cross flow system, only a solids (typically greater than 0.01 microns), while scaling is caused by
portion of the feed water is collected as product, and the rest is used dissolved solids (salts) exceeding their solubility.
to sweep away the dissolved solids that don’t pass through the mem- Scaling and fouling are both fundamentally the same type
brane. This cross flow prevents premature blockage of the membrane of issue, particulate plugging the membrane surface resulting in
surface and failure of the water system. reduced flow. The third category, chemical attack, refers to chemical
RO units consist of a number of membrane elements, housed destruction of the membrane surface. The membrane is a composite
in pressure vessels, a high of several layers of different types of material designed to give it the
pressure feed pump, valves, unique salt rejecting properties. The top layer is extremely thin and
piping and instrumentation fragile, and certain chemicals can destroy it. As damage accumulates
to control the various flows, the quality of the product water declines. Oxidizing agents such
monitor critical parameters as free chlorine or high concentrations of disinfectants or cleaning
and perform basic mainte- chemicals can produce this effect. While scaling and fouling can
nance functions. Although both be remediated with proper chemical cleaning, chemical attack
units vary in size, materials is irreversible, and replacing membrane elements is the only way to
of construction and controls Green = Permeate Blue = Concentrate return the system to start-up quality.
capabilities, these are the Pink = Feed/Recycle
common components found in different designs. Monitoring
Monitoring, recording system data and trending selected param-
Scaling, Fouling & Chemical Attack eters on an RO unit is one of the most effective tools for identifying
The issues that adversely affect RO system operation can almost problems. The following items, at a minimum, should be measured
always be grouped into three main categories: and logged on a daily basis.
• Scaling • Fouling • Chemical Attack • Conductivity (Quality measurement)
- Feed water
In order to produce water, an RO unit takes feed water with some
- Permeate (product water)
amount of dissolved solids (salts) and splits it into two streams: a
• Pressures
purified stream (product/permeate) and a dirty stream (concentrate/
- Membrane Feed (housing inlet piping)
reject). Since the RO unit is rejecting the dissolved solids in the feed
- Interstage (between housings)
water, the concentrate stream has a significantly higher concentra-
- Concentrate (housing outlet piping)
tion than it did to start with. It is common to see about four times the
• Flows
amount of dissolved minerals in the concentrate stream as in the feed
- Feed flow
stream. How does this become an issue? Imagine pouring a table-
- Permeate flow
spoon of salt into a glass of water. After some brief stirring action the
- Concentrate flow
glass of water would appear clear again, because the salt has been
• Temperature
completely dissolved in the water. Now imagine what would happen
- Feed water temperature
if you poured five pounds of salt in the same glass. Some of the salt
would dissolve as before, but after a certain point, it would stop, and The above parameters provide enough data to compute the
you would see a glass full of solid salt crystals. three critical operational indicators for an RO unit. These indicators
Every compound are:
has a limit to the amount • Normalized permeate flow
that can be dissolved • Pressure drop per stage
in water before crystal- • Normalized salt rejection
line solids start to form.
Normalized permeate flow is a calculated value. This parameter
Some compounds have a
must be normalized because there are several factors that influence
very high limit and some
permeate flow: feed pressure, feed conductivity and feed tempera-
compounds have a very Feed Water Concentration
ture. A higher feed pressure increases the amount of water produced.
low limit. The com-
This applied pressure is the force that is overcoming the osmotic
pounds that have a low limit are the ones that are most troublesome.
pressure and pushing water through the membrane. It makes sense
These solid crystalline salts can cause issues with RO operation. The
that a stronger push causes more water to flow through. Higher
membranes only operate effectively when the surface is kept clean.
feed water conductivity reduces the amount of water produced.
If there are solids blocking the surface, there is less area to make
The osmotic pressure counteracts the applied pressure so a greater
water. When dissolved solids are precipitating on the surface of the
feed conductivity = higher osmotic pressure = less water produc- independently. A decline in normalized permeate flow is enough to
tion. Temperature also affects permeate production. A higher water indicate a problem but it doesn’t necessarily point to whether the
temperature lowers the viscosity of the water, which reduces the problem is scaling or fouling. Fouling almost always occurs first at the
required feed pressure to produce a given amount of permeate. All lead end of an RO unit. The housings in the first stage are more likely
of these variables must be taken into account when normalizing the to foul before the last stage. Scaling however is more likely to occur
permeate flow rate. Otherwise it would be difficult to tell whether at the back end of an RO unit. This is where the highest concentration
the production of an RO unit is declining due to a problem, or to a of dissolved solids occurs, so exceeding the saturation limit in the last
decrease in the feed water temperature. Normalized permeate flow stage is much more probable. If a decline in normalized permeate
then can be determined as follows: flow is observed and a pressure drop increase occurs across the first
stage, but not the second, the problem is likely some sort of fouling.
Normalized Net driving pressure original Temperature On the other side, if the NPF is going down and an increase in pres-
Permeate = X Correction X Permeate
Flow Net driving pressure current Factor Flow sure drop across the second stage occurs but not the first stage, then
scaling is most likely the issue.
When the membrane is brand new and completely free of any The third critical operation parameter is percent salt rejection.
dissolved or suspended contaminants the normalized permeate This value indicates how well membranes are rejecting the dissolved
flow is at its maximum. Over time, as contaminants begin to plug the solids in the feed stream. It can be approximated as follows:
membrane surface, more pressure is required to produce the same
% Salt Feed Conductivity – Permeate Conductivity
amount of product water. The normalized permeate flow declines. Rejection = X 100
Normalized permeate flow (NPF) is the single most effective Feed Conductity
measure for scaling or fouling. NPF can also be used to identify chem- This parameter is especially useful for identifying scaling or
chemical attack problems. If the membrane is being oxidized by free
chlorine for instance, the salt rejection decreases. As the rejecting lay-
er is damaged, more salt passes through. Similarly, if the membranes
are heavily scaled, a decline in salt rejection is observed. This decline
can be explained by two factors. First, an accumulation of crystalline
salt material on the membrane surface creates localized high concen-
tration spots that increase salt diffusion through the membrane. This
can be attributed to a phenomenon called concentration polariza-
tion. There is a very thin layer (only a few microns) at the surface of
the membrane where the crossflow is much lower than in the rest of
ical attack. If the surface of the membrane is deteriorating, the water the feed channel. This layer is referred to as a boundary layer. The rate
can pass through easier. In this situation it is possible to see normal- of dissolved solids diffusion into the boundary layer is higher than
ized permeate flow increase, which should not occur under normal the rate of diffusion. This results in a localized concentration increase
conditions. If a significant increase in normalized permeate flow is typically between 10 and 20 percent. This in turn increases the salt
observed, then tests for potentially oxidizing substances should be diffusion through the membrane and compromises the salt rejection.
performed on the concentrate water. If left un-checked, the product Secondly, it is possible that sharp crystalline scale could scratch the
quality will eventually deteriorate to a point where membranes must extremely thin membrane surface, opening up holes for more dis-
be replaced. solved solids to pass through.
Pressure drop can also be used for monitoring & troubleshoot- The following table summarizes the expected indicators to help
ing an RO system. Pressure drop is determined by taking the inlet diagnose a problem.
pressure & subtracting the reject outlet pressure:
PROBLEM SYMPTOM
Pressure drop = Inlet pressure - Reject outlet pressure Scaling Decrease in normalized permeate flow
Increase in pressure drop across last stage
In a unit with new elements, the pressure drop is due to the Decline in % salt rejection
friction and energy loss of the water as it flows through the feed
Fouling Decrease in normalized permeate flow
water spacer, vessel, pipe and valves. Every unit has a normal amount
Increase in pressure drop across last stage
of pressure drop which should be recorded at start-up. As the feed
Little to no effect on % salt rejection
water channel becomes plugged with scaling or fouling material,
the pressure drop rises. As the spaces through which the water must Chemical Attack Increase in normalized permeate flow
pass become more constrained, more friction and energy loss results, Little to no effect in pressure drop
which presents itself as a higher pressure drop in the unit. Decline in % salt rejection
It is especially useful to measure pressure drop across each stage
This article is meant to provide basic troubleshooting techniques
needed to properly operate and maintain a reverse osmosis system.
These techniques do not require costly instrumentation or software
packages, and they can be applied to RO systems of any size or type.
There are a variety of advanced techniques not discussed in this
article, including membrane autopsies, element probing and housing
profiling, but the information presented covers the majority of prob-
lems encountered when operating an RO unit. The most crucial part
of maintaining a system in proper operation is consistent monitoring
and collection of data. Analyzing the data collected is the easy part.
1241 Jay Lane
The Official Journal of the
Graham, NC 27253
PFI High Purity Water
Conference & Seminar Series
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lo
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