Reverse osmosis water tastes bad: 5 causes by taste

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Short answer

Reverse osmosis water that tastes bad is diagnosed by the taste itself. Flat or “distilled” is normal: EPA’s WaterSense performance overview states that NSF/ANSI 58 requires an RO system to remove at least 75 percent of total dissolved solids, so the calcium and magnesium that give tap water its taste are gone. Plastic or rubber means a new tank, tubing or filters that have not been flushed; run and discard the number of tank-fulls the manual states. Salty or bitter means the membrane’s rejection has dropped: read feed and product TDS with one meter, and if rejection sits below the 90 to 99 percent APEC publishes for its high-rejection membranes, the membrane is the suspect. Rotten egg points at biofilm in the storage tank, or at hydrogen sulfide in the feed water, which Nebraska Extension G1490 says passes the membrane untouched. Chlorine means the carbon post-filter is spent. Buy a TDS meter before any part.

If the water only tastes flat and nothing else is wrong, you can leave now: that is the membrane doing its job, and the only thing to buy is nothing, or a mineral cartridge if you prefer the old taste.

What does the taste tell you?

Total dissolved solids (TDS) is everything dissolved in the water, in mg/L, which a handheld meter reports as ppm. Rejection is the share of feed TDS the membrane keeps out, and Nebraska Extension G1490 defines it as the percentage of a contaminant not allowed through the membrane.

Taste or smell	Likely cause	Confirm with
Flat, “distilled”	Normal. NSF/ANSI 58 requires at least 75 percent TDS removal per EPA’s WaterSense overview	RO TDS well below tap TDS, no other symptom
Plastic or rubber	New tank, tubing or filters off-gassing, not yet flushed	Worst right after install; fades after the tank-fulls the manual says to discard
Salty or bitter	Rejection failure: membrane fouled, scaled or past its life	Feed and product TDS on one meter; rejection below the maker’s published figure (APEC: 90 to 99 percent)
Rotten egg	Biofilm in the storage tank, or hydrogen sulfide in the feed water	If the kitchen tap smells too, the feed water carries it and the membrane never removes it per Nebraska Extension G1490; if only the RO faucet smells, the tank or tubing has grown biofilm
Chlorine or chemical	Spent carbon post-filter no longer polishing residual chlorine before the faucet	Taste returns near the post-filter’s due date (APEC’s schedule: stage 4 carbon post-filter every 2 to 3 years, stage 5 carbon post-filter every 3 to 5 years, stage numbering by model)

Two of these rows share a symptom with a different cause, so the confirm column is the whole diagnosis. Rotten egg at every tap is a well-water problem, covered in the rotten egg smell guide, not an RO problem.

What does the number mean?

EPA’s WaterSense performance overview states that the WaterSense specification requires a minimum TDS reduction of 75 percent, the same baseline NSF/ANSI 58 sets. So a certified system leaves at most 25 percent of the feed TDS in the product water, and most leave far less.

APEC’s FAQ states that its high-rejection membranes deliver 90 to 99 percent rejection and that most other membranes deliver 70 to 90 percent. Those are APEC’s figures for APEC membranes; your maker publishes its own, and iSpring’s FAQ could not be read for this page, so no iSpring figure appears here.

The arithmetic is the one the TDS meter guide already walks through: feed TDS minus product TDS, divided by feed TDS, times 100. Tap water at 300 ppm and RO water at 15 ppm is 95 percent rejection; the same tap at 90 ppm out is 70 percent, and the membrane is done.

EPA’s secondary drinking water standards list TDS at 500 mg/L, and EPA states that it does not enforce these secondary maximum contaminant levels. That is an aesthetic ceiling for taste and deposits; RO water sits far below it, and the flat taste is the absence of minerals, not a fault. The remineralization guide covers whether to add any back.

A meter reads bulk conductivity, not any single substance. A salty taste with a clean TDS reading, or a rotten-egg smell with any reading, is not something the meter can name; the what RO removes table lists which contaminants need a lab test instead.

Which treatment class fits?

Symptom	Treatment class	Deciding number
Flat taste, nothing else wrong	None, or an optional remineralization cartridge	EPA’s 500 mg/L TDS line is an unenforced ceiling; low TDS breaks no standard
Plastic or rubber taste	Flush per the installation manual	The manual’s tank-full count; no maker page read for this page states one, so this page will not invent it
Salty or bitter taste	Test rejection, replace the membrane if below the maker’s figure	Below 90 percent on an APEC high-rejection membrane, or below whatever your maker prints
Rotten egg at the RO faucet only	Sanitize the tank and system per the manual	APEC RO-90: 2 to 4 oz of hydrogen peroxide or household bleach in the first-stage housing, two tank-fulls to drain
Rotten egg at every tap	Treat sulfur upstream; the RO is not the fix	Hydrogen sulfide passes the membrane per Nebraska Extension G1490
Chlorine or chemical taste	Replace the carbon post-filter	APEC’s maintenance schedule: stage 4 carbon post-filter every 2 to 3 years, stage 5 carbon post-filter every 3 to 5 years, stage 6 pH+ post-filter every 6 to 12 months; earlier if the taste returns

Sanitizing is unscented household bleach or hydrogen peroxide from the shelf; no dedicated household RO sanitizing kit could be confirmed on Amazon for this page, so none is linked. The post-filter is a system-specific part: the APEC ROES-50 parts page and the iSpring RCC7 parts page carry the numbers for those two systems.

Nebraska Extension G1490 adds a cause of biofilm nobody expects: on unchlorinated feed water, a carbon pre-filter can encourage microbial growth on the membrane surface. On a well with no chlorine, a sulfur smell that returns after every sanitizing cycle is that, and the fix is upstream disinfection, not another bottle of bleach.

Before you buy anything

Do not order a membrane, a post-filter or a mineral cartridge until you have two TDS readings from one meter, the kitchen tap and the RO faucet, on the same day. The HM Digital TDS-3 reads 0 to 9,990 ppm at plus or minus 2 percent in the 0 to 5,000 ppm band per HM Digital, which covers any tap. Its limit is the one to know: it reads conductivity, so it will confirm or clear the membrane, and it will tell you nothing about a chlorine or rotten-egg taste. Those two are diagnosed by the post-filter’s age and by which taps smell, and both fixes cost less than a membrane you did not need.

What should you do first?

Name the taste from the first table and run the confirm test for that row before buying anything.
Read tap and RO TDS on one meter the same day; divide the difference by the tap reading for the rejection rate, and compare it with your maker’s published figure.
On a new install, run and discard the number of tank-fulls the manual states before judging the taste at all.
Smell the kitchen tap as well as the RO faucet: sulfur at both is a feed-water problem; sulfur at the RO faucet alone is a sanitizing job per the manual.
Check the post-filter’s install date against the maker’s interval, and replace it before replacing anything else if a chlorine taste has come back.

Frequently asked questions

Why does reverse osmosis water taste flat?

Because the membrane removed the minerals tap water tastes of. EPA's WaterSense performance overview states that NSF/ANSI 58 requires an RO system to remove at least 75 percent of total dissolved solids, and APEC publishes 90 to 99 percent for its high-rejection membranes. Flat water with a low TDS reading and no other symptom is the system working; a remineralization cartridge is a taste choice, not a repair.

What rejection rate should a reverse osmosis membrane hit?

APEC's FAQ states its high-rejection membranes deliver 90 to 99 percent rejection and that most other membranes deliver 70 to 90 percent. Rejection is the feed TDS minus the product TDS, divided by the feed TDS. Read both with one meter on the same day; a result below the figure your maker publishes for your membrane means the membrane is fouled, scaled or past its life.

Does reverse osmosis remove the rotten-egg smell of hydrogen sulfide?

No. Nebraska Extension G1490 lists hydrogen sulfide among the dissolved gases an RO membrane does not remove; it passes through untouched. If the water at every tap in the house smells of sulfur, the fix is upstream of the RO. If only the RO faucet smells and the kitchen tap is fine, the storage tank or tubing has grown biofilm and needs sanitizing per the manual.

How do I sanitize a reverse osmosis storage tank that tastes or smells off?

Follow your own manual; the dose and sequence differ by maker. APEC's RO-90 instructions call for 2 to 4 oz of hydrogen peroxide or common household bleach in the first-stage housing with the pre-filters and membrane removed, then two full tanks run to drain before the filters go back. APEC times this with a filter change, and no maker read for this page publishes a yearly interval.

Is a high TDS reading in tap water a health problem?

EPA lists total dissolved solids at 500 mg/L in its secondary drinking water standards, and states that it does not enforce these secondary maximum contaminant levels; they are aesthetic guidelines for taste, deposits and staining, not health limits. A TDS meter reads bulk conductivity only, so it cannot identify any single contaminant; a lab panel does that.

Fact sheet and how to cite this page

Answer: The taste names the cause. Flat is normal, because NSF/ANSI 58 requires an RO system to remove at least 75 percent of TDS per EPA’s WaterSense summary. Salty or bitter means rejection has dropped below the 90 to 99 percent APEC publishes for its membranes, rotten egg means tank biofilm or feed hydrogen sulfide that passes the membrane, and chlorine means a spent carbon post-filter.
Basis: NSF/ANSI 58 minimum TDS reduction of 75 percent, as summarized in EPA’s WaterSense performance overview for point-of-use RO systems; EPA’s secondary standard for TDS of 500 mg/L, which EPA does not enforce.
Assumptions: Rejection is calculated as feed TDS minus product TDS, divided by feed TDS, from two readings on one meter the same day
Membrane rejection and post-filter intervals are APEC’s published figures for APEC systems; other makers publish their own
Sanitizing dose and flush counts come from the owner’s manual for the specific system
Method: Standards from EPA’s WaterSense RO summary and secondary standards page, hydrogen sulfide behaviour from Nebraska Extension G1490, rejection and maintenance figures from APEC’s FAQ and sanitizing instructions, meter spec from HM Digital, all read on 2026-09-15; nothing was tested.
Primary source: EPA WaterSense, Performance Overview: Point-of-Use Reverse Osmosis Systems (government or regulatory source)
Facts verified: 15 September 2026
Last updated: 15 September 2026
Canonical URL: https://ownerspec.com/water/guides/reverse-osmosis-water-tastes-bad/
Cite as: OwnerSpec, "Reverse osmosis water tastes bad: what flat, plastic, salty, rotten-egg and chlorine tastes each mean and what fixes them", https://ownerspec.com/water/guides/reverse-osmosis-water-tastes-bad/, facts verified 15 September 2026.

Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers

Sources

EPA WaterSense, Performance Overview: Point-of-Use Reverse Osmosis Systems (government or regulatory source)
EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals (government or regulatory source)
University of Nebraska-Lincoln Extension, G1490, Drinking Water Treatment: Reverse Osmosis (university or extension service)
APEC Water Systems, FAQ (membrane rejection rates, post-filter maintenance schedule) (manufacturer or vendor document)
APEC Water Systems, RO-90 sanitizing instructions (manufacturer or vendor document)
HM Digital, TDS-3 product and spec page (manufacturer or vendor document)