Reverse Osmosis System Basics

Reverse Osmosis Basics
Overview:
Osmosis
Reverse Osmosis Definition
RO Fundamentals
Reverse Osmosis Membrane
Parameters Affecting RO Systems
Air Gap Faucets
RO Maintenance
Quick ConnectFittings
Troubleshooting

Osmosis:
Osmosis is a natural phenomenon that provides water to all animal and vegetable cells to support life
Water moves from a high concentration of water (less sugar/salt dissolved in it) to a low concentration of water (more salt/sugar dissolved in it) across a semi-permeable membrane
This means that water can cross a selectively permeable membrane from a dilute solution (less dissolved in it) to a concentrated solution (more dissolved in it)

Osmosis: What is it?
Reference: //ehrig-privat.de
A semi-permeable membrane is a membrane which will allow only certain molecules or ions to pass through
Osmosis is very important in biology as it provides the primary means by which water is transported in and out of cells
Net Movement of Water Molecules

Reverse Osmosis: What is it?
RO is a form of filtration using osmosis in reverse
Water passes from a more concentrated solution to a less concentrated solution
To accomplish this a force or pressure MUST be applied
RO requires 1 psi of pressure for every 100 ppm of TDS
RO is used to remove dissolved solids from water but it can also improve taste, odor & color of water
RO membranes have the capability to remove particles as small as ions i.e. magnesium ions or sodium ions
RO membrane will reject most compounds based largely on size
Dissolved ions, such as salts, carry an electric charge and will also be rejected by membrane

Osmosis: How does it work?
Feed water flows into RO unit with the force of line pressure
Water is forced through membrane by cross flow filtration
Cross flow filtration is most commonly used in RO as it allows membrane to continually clean itself
Membrane then either rejects or repels contaminants
Two exit streams generated: waste & product stream
Waste stream consists of: A concentrate (reject) stream which carries contaminants (compounds too large to pass through membrane)
Permeate stream consists of: Product water which has been forced through the membrane & is virtually free of TDS

Reverse Osmosis Schematic

RO Membrane:

Reverse Osmosis Fundamentals:
FEED WATER: Main flow of impure water to be treated;
PRODUCT WATER: Portion of feed water which passes through membrane as permeate
REJECT WATER: Portion of feed water which did not pass through membrane; carries impurities to drain
MODULE: Combination of spiral round membrane element &
pressure vessel

Spiral Wound Membrane:
RO membrane is a method of fine filtration but much smaller
Membrane consists of tiny pores which are approximately 0.0005 microns in size
THEREFORE reverse osmosis membranes are capable of rejecting contaminants as small as 0.0001 to 0.00001 microns
Thin Film Composite Membrane (TFC)

RO Membrane:
Membrane is the work-horse of an reverse osmosis system
When choosing an RO system you must do so wisely – taking into account water quality is very important
Three main types of membranes:
CTA membrane: Cellulose acetate/triacetate blend
Provides lowest cost per gallon of water
Resists chlorine but has a lower flow rate limit in applications of high water consumption
TFC membrane: Thin film/thin layer composite
Can handle high flow, has a high rejection rate & can handle high water consumption
Cannot handle chlorine concentrations; water must be carbon pre-filtered

Membrane Comparison:

TFC Membrane:
TFC membranes are intolerant to chlorine – will cause deterioration of membrane
Must use a carbon pre-filter for TFC membranes
TFC membranes have a much higher rejection rate of many chemicals than CTA membranes
Carbon pre filters are vital in the life of this membrane & must be changed regularly

Parameters that affect RO:
Water Temperature: RO system ratings are based on 770F/ 250C water temperature
i.e. : RO system rated at 20 GPD (gallons per day) at 340F/10C will only produce 6 GPD
Feed Water Pressure: A lack in water pressure (less than 40 PSI) will reduce product water; this can be helped with the aid of a booster pump
Contaminants in Feed Water: Large particles, iron concentration or high levels of hardness in feed water can cause clogging of the membrane
Clogging of the membrane can decrease its life
Microorganisms: RO membranes can filter out bacteria, depending on the bacteria size, however this may cause fouling of the membrane as bacteria can begin to grow through pores of membrane causing BACTERIA CREEP – RO systems should not be installed on non-potable water

RO Production Rate vs. Feed Water Conditions:

RO Systems can remove the following contaminants:
Dissolved solids = TDS (salts & minerals) – UP TO 2000 ppm
Most Heavy metals (lead, mercury, silver)
Most radioactive elements
RO systems may not be capable of removing nitrates to recommended levels

Typical Rejection Ranges of Contaminants:

RO Treated Water:
RO systems will remove up to 97% (depending on parameters) of salts in water bringing it closer to a level of ‘pure water’
The purer the water, the more aggressive it will be with materials it comes in contact with
THEREFORE RO can be slightly corrosive
It’s recommended NEVER to use copper piping on RO water

Typical Consumer RO System:
POSTFILTER
RO membrane
AIR GAP FAUCET
PREFILTER
TANK
To
Drain

Under-the-counter RO System:

Air-Gap Faucets:
‘Air gap’ faucet is a plumbing device designed to provide a siphon break for an under-sink reverse osmosis unit
It is a valve used to isolate one part of a water system from the drain
Air gaps are designed to prevent backflow by incorporating a “backflow prevention” method called an ‘air gap’
Purpose of an air gap faucet is to assure that no water from a stopped-up sink can possibly be forced backward into your reverse osmosis system unit via its drain connection
Some plumbing codes do enforce the use of an Air Gap faucet

RO System Maintenance:
When to Replace Pre-filters:
When to Replace Post-filters:
Every 6 months for normal installations.
After a Boil Water Advisory (BWA)
Every 6 months, depending on water conditions.
Noticeable decrease in water production indicates that the prefilter should be changed.
After microbiological contamination, such as in a Boil Water Advisory (BWA)

RO System Maintenance:
When to Replace the Membrane
Typically, every 18 months to 3 years
Test using a TDS (total dissolved solids) meter. Replace a membrane below 80% TDS reduction.
This will depend on influent water quality, usage, pre & post filter maintenance
Maintain filters, lengthen life of membrane!!!

Troubleshooting:
Q: I have an RO system & there is water coming out the back of the faucet. What’s wrong?
A:You probably have an air gap style faucet. What is most likely happening is there may be a clog or kink in the RO drain line causing water to come through the hole at the back of the faucet
Q:What if my hardness level is more than 7 gpg?
A:The hardness level of your water will tend to foul the membrane of the system.
Q:Is it true that RO systems waste a lot of water?
A:With some RO systems it take as much as 10 gallons of water to produce one gallon. This is an extreme case but most ‘high quality’ systems will have a 3:1 ratio
Q:Should you use copper tubing with RO systems?
A:NO. RO systems reduce up to 97% of TDS in water. The aggressiveness of the purified water toward materials it comes in contact with is a function of its purity. The better the purity the higher its aggressiveness.

Troubleshooting:
Q: My RO continues to run. What is wrong?
A: Shut off valve could be defective OR
A: Check the storage tank; it might not have enough air in it. The tank should have 7 lbs of air in it when empty. Too much air will cause an RO to shut off permanently
Q: I have installed my RO but am not getting much water from faucet. I do hear water running. What is wrong?
A: For a residential system it takes a long time to product RO water. Faucet needs to be in ‘off’ position & then the tank will fill slowly. If a system is rated at 30 USGPM then it will take one full day to produce that 30 USGPM. The water you hear running is the reject stream going to drain.
Q: My RO system seems to work but when I want to get 3 gallons of water at once I can’t get anything. Is this normal?
A: Most RO systems come with a 3-4 gallon storage tank.

Troubleshooting:
No Permeate Water
Membrane Fouling
Prefilter Fouling
Inadequate Pressure
Slow Flow at the Faucet
Air Charge
Tank not filling
Installation of tank, tubing length
Inadequate Pressure

Quick Connect Fittings:
RO systems cannot be installed using copper piping as product water is slightly aggressive
Plastic tubing is most commonly used in RO installations
Pipe connections are accomplished using Quick Connect Fittings
Quick Connect fittingsdo NOT need any additional special tools
Simply cut tube square & insert into QCfitting to accomplish a secure connection

Quick Connect Fittings:
Prepare the connection: cut pipe square
ensuring that it is free from score marks
DO NOT USE A HACKSAW
To avoid damage to o-ring of QCfitting, make
sure burrs & sharp edges are removed

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Reverse Osmosis System Basics

by WaterFilters.NET on Jan 26, 2011

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