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RO Troubleshooting Guide

Reverse Osmosis Membranes:

Water flows through the RO membrane(s) under high pressure and travels from the feed end to the reject/ concentrate end of the membrane vessel. A portion of the feed water passes through the membrane and becomes purified RO permeate water. The contaminants that were previously in the feed water are concentrated and exit the membrane in the reject/ concentrate water flow. A portion of the concentrate flow may be used as recycle flow which is introduced back into the supply water at the pump suction.





The spiral wound membrane element is installed in a pressure vessel. A brine seal between the outside of the membrane and the inside of the pressure vessel prevents the feed supply water from flowing between the membrane and membrane pressure vessel (By-pass flow).  Membranes should always be installed with the brine seal on the feed end of the vessel, cupped towards the feed flow. Membranes should be installed and removed in the direction of flow on each of the vessel(s).



The reject/ concentrate water flow exits the vessel and feeds the next vessel(s) or is sent to the drain.  The permeate product water exits the vessel and is sent to a storage tank, which then is able to be transferred to point of use. An o-ring on each of the interconnectors and end cap adapters prevents the reject water from mixing with the permeate product water.

High pressure RO systems typically operate at pressures between 100 and 1000 psig. RO systems use a pump to provide the pressure to drive the reverse osmosis process.  In some cases a feed supply booster pump is also needed to increase incoming feed water pressures. These systems typically use multiple membrane vessels arranged in parallel and/or in several stages to provide the required quantity and quality of water.  The reject water from one vessel can be directed into series of membrane vessels for greater system efficiency.  An adequate reject/ concentrate flow must be maintained to prevent premature membrane fouling and scaling from occurring.

High pressure systems typically provide from 5 to 400 GPM of water with an efficiency of 0.3 - 6 gallons of reject water per gallon of permeate water produced. These systems typically remove more than 95% of the dissolved solids found in the feed supply water. Typically no treatment of the reject water is needed.


Factors That Effect the Operation of RO Systems:

  1. Total Dissolved Solids (TDS) of the feed water:
    The natural osmotic pressure of the feed water is approximately 1 psi for every 100 ppm.  This means that if the feed water TDS is 500 ppm it will take more than 5 psi of pressure just to begin the reverse osmosis process.
  2. Feed water temperature:
    Reverse osmosis is temperature dependent. The RO process occurs slower with cold water.  All RO membranes are rated with a feed water temperature of 77° F.  Higher operating pressures can be used to compensate for colder water temperatures.  For feed water colder than 77° F, it will take approximately 1.5% more pressure or the membrane will produce approximately 1.5% less water for each degree F. The system recovery must also be maintained to prevent fouling or scaling.
  3. Feed water pressure:
    Reverse osmosis is pressure dependent.  Raising the feed pressure produces more product water while lowering the pressure produces less product water. The system recovery must also be maintained to prevent fouling or scaling.
Pretreatment:

The RO feed supply water must be pretreated/ conditioned in order to prevent membrane damage and/or fouling. Proper pretreatment is crucial for any reliable operation of a RO system.

Pretreatment requirements vary depending on the source of the feed supply water. Pretreatment equipment is sold separately. A detailed water analysis is needed for proper pretreatment and RO design and sizing. The most common forms of pretreatment are described below:

Multi Media Filter - Used to remove large suspended solids (sediment) from the feed water. Backwashing the media removes the trapped particles. Backwash can be initiated by time, differential pressure or gallons used.

Water Softener - Used to remove calcium and magnesium from the feed water in order to prevent water hardness scaling. Regenerations can be initiated manually or automatic determined by the gallons used.

Activated Carbon Filter - Used to remove chlorine and organics from the feed water. Free chlorine will cause rapid irreversible damage to RO membranes.

Chemical Injection - Typically used to feed antiscalant, biocide or sodium bisulfate into the feed supply water

Prefilter Cartridge - Used to remove smaller suspended solids and trap any particles that may be generated by the other pretreatment. The cartridge(s) should be replaced when the pressure drop across the housing increases 5 - 7 psig over the clean cartridge pressure drop.

Iron & Manganese - These foulants should be completely removed. Special media filters and/or chemical treatment is commonly used. Regenerations can be initiated manually or automatic determined by the gallons used.

Silica: Silica forms a coating on membrane surfaces when the concentration exceeds its solubility. Additionally, the solubility is highly pH and temperature dependent. Silica fouling can be prevented with chemical injection and/or reducing the recovery. Standard recoveries are typically 65% - 75%.


Symptom Possible Cause Possible Solution
Low Operating Pressure
High Flow Rates Check the concentrate & recycle flows
Adjust if necessary
Clogged pre-filter Replace the cartridge filter (5 PSID)
Clogged pre-treatment Initiate a manual back wash
Increase backwash frequency
Replace control head and/or media
Electrical malfunction Check motor fuses & electrical circuit
Feed valve malfunction Clean and/or replace
Check air supply, if applicable
Check electrical controls, if applicable
Faulty pump operations Refer to pump operations/ maintenance manual
Low Permeate Flow Rate
Low operating pressure See above possible causes
Fouled or scaled membranes Clean or replace membranes
Damaged brine seals Check and replace
Flow meter accuracy Replace flow meter
Operating on cold water (55F) Install hot/cold feed water temperature valve
if more permeate is needed
Correct flows with pump discharge/ concentrate valves
Low Concentrate Flow
Concentrate valve setting Check and adjust concentrate valve setting
Flow meter accuracy Replace flow meter
Fouled or scaled membranes Clean or replace membranes
High Operating Pressure
Concentrate valve setting Check the concentrate & recycle flows
Adjust if necessary
Recycle valve setting Check and adjust recycle valve
Pump discharge valve setting Check and adjust valve setting
Pressure gauge accuracy Replace the pressure gauge
Fouled or scaled membranes Clean or replace membranes
Water Running When
Machine Is Off
Feed valve malfunction Clean and/or replace
Check air supply, if applicable
Check electrical controls, if applicable
Check solenoid valve’s diaphragm
RO Pump Not Functioning
No electrical supply Check electrical system
Pretreatment interlock malfunction
Check pretreatment interlock controls
Transfer tank interface malfunction
Check transfer tank interface controls
Pump/ motor failure Check and refer to pump operation manual
Replace defective unit
Impeller wear or damage Check and refer to pump operation manual
Replace defective part
Excessive Pressure Drop
Restricted flow in system Check for blockage in the concentrate flows
Severely fouled or scaled membranes
Clean or replace membranes
High Conductivity
Conductivity meter malfunction Calibrate or replace meter
Clean conductivity probe
Damaged O-Rings, Seals
Connectors
Inspect and replace damaged
o-rings, seals, connectors adapters
Fouled or scaled membranes Clean or replace membranes
Damaged membranes Replace membranes
Pretreatment malfunction Verify proper operations
Check chlorine levels
Check water hardness levels
Severely fouled prefilter
Replace cartridge filter
Feed water changes Check for incoming water changes
Bacteria/ Algae Growth
Inadequate pretreatment backwashes
Increase back wash frequency and/ or replace media
Add biocide to pretreatment chemical
dosing system, if applicable
Warm water temperatures Chemically clean the RO system and storage tank
Mechanically clean the flow meters
UV light malfunction Replace UV bulb annually
Inspect UV light system for proper operations
High Chlorine Levels
Insufficient back washes Increase back washes
Exhausted Carbon Re-bed carbon filter
Clogged pre-treatment Initiate a manual back wash
Increase backwash frequency
Replace control head and/or media
Control head malfunction Refer to manual, rebuild or replace
Feed water changes Check for incoming water changes
High Water Hardness
Insufficient regenerations Increase number of weekly regenerations
Exhausted resin
Re-bed water softeners
Control head malfunction Refer to manual, rebuild or replace
Feed water changes Check for incoming water changes
Insufficient brine draw or refill Refer to pretreatment operating manual
Chemical dosing malfunction Refer to dosing equipment operating manual


Routine Testing
Daily Adjust flow to maintain the proper flows, system recovery and Reject; refer to operating manual for system specifics
Daily Check and record all pressures, flows, conductivities, feed and permeate temperatures
Weekly Chlorine levels, pre and post carbon filter (Post filter > .05 mg/l)
Weekly Water hardness, pre and post softeners (Post filter > 1 – 2 GPG)


Testing Formulations
% Recovery = Permeate Flow
Feed Flow (Concentrate Flow + Permeate Flow)
% Rejection = (Average Conductivity – Permeate Conductivity)
Average Conductivity
Average Conductivity = (Feed Conductivity + Concentrate Conductivity)
2
Feed GPM = Permeate GPM + Concentrate GPM
Delta P = Feed PSI – Final PSI


Reverse Osmosis Required Data to Be Collected
Date
Name
Prefilter, PSI
Postfilter, PSI
Pump Suction, PSI
Pump Discharge, PSI
Feed Pressure, PSI
Intermed. Pressure, PSI
Final Pressure, PSI
Delta P, PSI
Permeate Pressure, PSI
Permeate, GPM
Concentrate, GPM
Recycle, GPM
Permeate Cond. (Panel)
Permeate Temp. (Sample)
Permeate Cond. (Sample)
Feed Cond. (Sample)
Concentrate Cond. (Sample)
Feed Temp. (F)
% Recovery
Average Conductivity
% Rejection
Chlorine (PPM)
Water Hardness (GPG)
Prefilter Change (Yes/ No)
System Clean/ Flush


Keeping Data Records:
A prerequisite for early detection of potential problems is proper record keeping and plant performance normalization. This includes proper calibration of all instruments. Without accurate readings it might be too late before a problem can be detected and corrected. Another key to successful troubleshooting is to understand the effects of changing system parameters on system performance. Some apparent plant problems are simply the normal reaction of the plant to changing feed or other operating parameters.

Identify the Cause:
Once a performance issue has been identified, the first step in solving the problem is to localize the problem if possible and try to identify the cause(s) of the problem by using the data collected on the records log sheet along with the use of some additional on-line testing. Consult your RO sales/ service manager for additional troubleshooting assistance.