Troubleshooting POU RO Systems: Problem -Cause-Solution

Reprinted with permission of the WQA.

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Problem Cause Solution
Little or no RO water production or quantity
  1. Feed valve is off or plugged.
  2. Plugged prefilter
  3. Scaled or fouled RO membrane (which can result from a plugged reject flow control).
  4. Storage tank air precharge pressure is too high.
  5. Undetected RO product water leak (e.g., from icemaker tubing line run under the house).
  6. The production capacity sizing exceeds capabilities at local pressure and temperature conditions are exceeded. Tech tip: To avoid a service call to determine this condition, ask the customer to measure the rate of RO production as follows: They should drain the tank or turn off the tank valve, then collect the steady drip from the open RO faucet in an 8 oz. measuring cup while timing it. Divide the number of minutes it takes to fill the 8 oz. cup into "90".  The result is the gallons per day production to an open container (the actual RO production available to the customer will be less due to the tank back pressure as it fills). Compare this amount with the customer's estimated usage.
  1. Open valve or remove debris from valve orifice.
  2. Replace prefilter cartridge.
  3. Replace membrane and perform required sanitizing procedures.
  4. Drain tank and adjust air precharge to supplier's specification (e.g., 5-7 psi).
  5. Find and repair leak.
  6. Install an RO booster pump with tank switch controls.


Little or no RO water flow from faucet.
  1. Storage tank air loss or ruptured diaphragm/bladder.
  2. Plugged carbon postfilter due to carbon fines collecting on the outlet media retainer. First verify acceptable flow before the postfilter or with the postfilter removed.
  3. Storage tank valve partially closed.
  4. Crimped tubing line to RO faucet.
  5. RO faucet valve is out of adjustment.
  1. Restore the correct empty tank air charge (e.g., 5-7 psi). If water remains in the tank after air is pumped into the air valve, the diaphragm or bladder is likely ruptured and the entire tank (or replaceable bladder, if applicable) needs to be replaced.
  2. Replace carbon postfilter.
  3. Open tank valve fully.
  4. Replace section of crimped tubing.
  5. Readjust per supplier's instructions.
Low RO water from refrigerator dispenser, but acceptable flow from RO faucet.
  1. Tubing from POU RO to refrigerator is too small (use 3/8" OD up to 25' and 1/2" over 25'), (see chapter 13).
  2. Refrigerator water inlet screen is plugged with scale and/or oxides from previous copper tubing connection.
  1. Replace tubing with larger size tubing using the guidelines in Chapter 13.
  2. Access the inlet screen (usually at back of refrigerator) and clean. Replace any copper tubing with plastic.
Bad tasting water from RO faucet
First, attempt to identify the taste -e.g., metallic, sour /astringent, plastic, rubber, rotten egg, musty, salty, same as tap water.
  1. Metallic - copper tubing used from product hookup.
  2. Bitter/astringent - excess CO2 in RO water lowers pH.
  3. Plastic/rubber - new plastic tubing after postfilter; diaphragm/bladder in tank; exhausted postfilter.
  4. Rotten egg - h2S in household water supply not removed by RO; system contaminated due to long period of nonuse or improper sanitizing.
  5. Musty - Water stale with high bacteria counts due to long periods of nonuse.
  6. Salty - A malfunctioning water softener, which is not properly rinsing after regeneration, allowing high TDS brine into the RO feedwater.
  7. Tap water taste - Th RO membrane is extremely scaled, deteriorated, or ruptured.
  1. Tap water taste - The RO membrane is extremely scaled, deteriorated, or ruptured.
  2. This may be a seasonal problem resulting from changes in local water chemistry. If the problem persists, try a calcite (or equivalent) "prolonged contact" media filter after the RO module to add alkalinity. Contact your supplier for his/her recommendation.
  3. First check the postfilter sealing gaskets to make sure bypass is not occurring. If okay, replace postfilter. If tubing is the cause, it may be wise to replace it with tubing from another supplier.
  4. H2S in the water supply must be removed prior to the RO system using conventional technologies such as aeration or oxidation / filtration. If nonpathogenic contamination of the RO system is suspected, replace all filters, "fast flush" the RO module to clean and sanitize purification assembly and tank (using procedures in Chapter 16) with double dose disinfectant. Verify acceptable RO performance when finished.
  5. Replace filters and sanitize purification assembly and tank (using procedures in Chapter 16) with double dose disinfectant. Verify acceptable RO performance when finished.
  6. Repair water softener.
  7. Determine the cause of membrane failure, correct the problem, and replace the membrane (or module).
Bad tasting water from refrigerator water dispenser.
  1. If the indicator only occasionally changes to yellow/red but returns to green most of the time, the cause may be TDS creep resulting from insufficient use of the system.
  2. Metallic - Copper tubing used for the connection from the RO system to the refrigerator.
  3. Plastic/rubber - Long runs of plastic tubing to the refrigerator impairs taste.
  1. Instruct the customer to frequently drain and discard one to two gallons from the dispenser until the tasted diminishes.
  2. Replace copper tubing with food grade plastic tubing.
  3. Install an in-line carbon filter at the refrigerator inlet.
RO water TDS too high
  1. Scaled, deteriorated, or ruptured RO membrane. A scaled membrane can result from plugged reject flow control. A deterioration can result from chlorine attack on a TFC membrane or high pH on a CTA membrane. A ruptured membrane can result from failed product check valve.
  2. Too low a feed pressure for the feed TDS. Recommend net pressure of 30 psi for TFC membranes and 40 psi for CTA membranes (see Chapter 4).
  3. The carbon postfilter is not sufficiently flushed on new installation or service.
  1. Replace the membrane (and associated cause of scaling, deterioration, or rupturing). Perform required sanitizing procedures.
  2. First check if the customer has a pressure regulator on their main house water line which can be reset to a higher pressure. If not, install an RO booster pump with  tank switch controls.
  3. Instruct the customer to drain about one quart of water from the RO faucet every several hours throughout a day.
Water quality indicator changes from green to yellow/red (also see RO water TDS too high).
  1. If the indicator only occasionally changes to yellow/red but returns to green most of the time, the cause may be TDS creep resulting from insufficient use of the system.
  2. If the indicator permanently changes to yellow/red, the RO membrane may be scaled, deteriorated, or ruptured.
  3. The water quality monitor set point is too low (or percent rejection monitor set point is too high) for the feedwater conditions.
  1. Have customer drain the RO tank until empty at least two times per week or find more uses for the water.
  2. Replace the membrane (and correct associated cause of scaling, deterioration, or rupturing). Perform required sanitizing procedures.
  3. Readjust the water quality monitor set point for approximately 75% rejection.
Gurgling or dripping sound coming from RO faucet air gap or from sink drain
First, inform customer that a slight, but unobjectionable, sound of RO reject water from air gap at base of faucet is normal.
  1. RO drain tubing from the air gap to the drainpipe drops lower than the connection point to the drainpipe, causing a siphoning effect.
  2. The vertical drainpipe is angled so the RO reject water drops   into the "pool" of standing water in the trap.
  3. The RO reject water flow control is defective, allowing excessive flow. Each type of flow control has its unique failure modes. Consult with your supplier.
  4. Very high feed pressure can result in excessive RO reject water.
  5. Oversized RO membrane for family drinking water needs High output RO membranes require correspondingly high reject flow (i.e. 3 to 5:1 ratio). Most faucet air gaps and drain arrangements are not capable of accommodating high flows without generating objectionable noise.
  1. Shorten the drain tubing so that it makes a gradual, continuously dropping run from the air gap to the drain connection.
  2. Reorient the drainpiping, if possible, so the RO reject water runs down the side of the drainpipe into the trap. Another solution is to change the position on the drainpipe so the RO reject water runs down the side of the drainpipe.
  3. Replace the RO flow control.
  4. Install a pressure regulator in the RO feed line.
  5. Reduce the membrane size to only produce sufficient water quantify for the customer's needs. The reject flow control should also be replaced to correspond to the new membrane product flow rate (i.e., 3 to 5:1 ratio).
Reject water comes out of air gap hole instead of into the drain.
  1. Dissolved air in the RO water from new filters.
  2. Dissolved air from main water supply produces minute bubbles in the RO water giving it a cloudy appearance.
  1. Instruct the customer that using the system for several days will eliminate air.
  2. Bubbles will dissipate as water sits in a container (may only be seasonal).
Cloudy ice

First, inform customer that ice clarity will generally greatly improve when using RO water, but will not be crystal clear.

  1. Dissolved air bubbles in RO water from new filters.
  2. Dissolved air bubbles from main water supply creates minute bubbles in the ice giving it a cloudy appearance.
  3. Automatic icemaker design limitations. The size and shape of the cube largely determines the degree to which air bubbles are retained in ice - rectangular and crescent shaped cubes are typically clearer than cylindrical ones.
  4. RO membrane not reducing TDS sufficiently.
  1. Use system for several days to eliminate air.
  2. The problem may be worsened by the type of icemaker cube (see #3 below). For the clearest cubes, instruct the customer to make them manually in plastic ice trays with the largest individual cubes.
  3. Short of replacing the refrigerator, the best solution may be to instruct the customer to make ice cubes manually in plastic ice trays with the largest individual cubes.
  4. Replace membrane and fix associated cause of failure. Perform required sanitizing procedures.
Automatic ice maker problems.
  1. Icemaker will not produce ice after new installation. RO water freezes up in the icemaker delivery tube due to a vary limited flow volume available after a new installation because the RO tank is typically drained. The icemaker requires the pressure and flow only available from a "substantially"filled RO storage tank.
  1. To prevent this from occurring after a new RO installation, major service, or emptying of the storage tank, the icemaker control arm or switch should remain in the "off" position until the RO tank is filled sufficiently.
  2. To correct an existing freeze-up, make sure icemaker is off (i.e., raise the ice level control arm) and the freezer temperature control is at warmest setting. Access the freezer compartment and use a hair dryer to melt ice in the delivery tube to the icemaker tray. When finished, make sure the RO has full tank of water. Turn the freezer temperature control back to normal, then turn on icemaker by lowering the control arm.

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