Reverse osmosis remineralization: what the low TDS and pH readings mean, and whether a mineral cartridge is needed
RO strips 90 to 99% of dissolved solids per APEC. EPA's pH 6.5 to 8.5 and TDS 500 mg/L are ceilings with no mineral floor, so remineralize for taste.
- Verified
- Updated
- 9 min read
- 9 sources
Shop links on this page are affiliate links: a purchase may earn us a commission at no extra cost to you. Disclosure
Short answer
Reverse osmosis remineralization is optional, because no regulator sets a floor for minerals in drinking water. An RO membrane strips most dissolved solids: APEC’s FAQ puts its high-rejection membranes at 90 to 99 percent rejection, and iSpring rates the RCC7AK system at up to 97 percent TDS rejection under NSF/ANSI 58 test conditions. What comes out is low-TDS water that many owners describe as flat. EPA’s secondary drinking water standards give only ceilings, pH 6.5 to 8.5 and TDS 500 mg/L, and EPA does not enforce either one; there is no minimum for TDS, calcium or magnesium anywhere in that table. A remineralization cartridge such as the Waterdrop MNR35, or the calcite stage built into the APEC ROES-PH75, puts calcium back for taste and to make the water less aggressive to metal fittings, and no maker publishes how many mg/L its cartridge adds. Decide on a TDS meter reading and your own taste, not on a health claim.
Fact sheet
- Answer
- A reverse osmosis membrane rejects 90 to 99 percent of dissolved solids per APEC’s FAQ, and EPA’s secondary standards set only ceilings, pH 6.5 to 8.5 and TDS 500 mg/L, with no minimum for minerals. A remineralization cartridge is a taste and corrosion fix, not a requirement, and no maker publishes the mg/L it adds.
- Basis
- EPA National Secondary Drinking Water Standards: non-mandatory ceilings of pH 6.5 to 8.5 and TDS 500 mg/L, and no federal minimum for TDS or mineral content.
- Assumptions
- Feed water is municipal or softened well water inside the maker’s published feed limits
- Rejection figures are whole-system TDS figures under NSF/ANSI 58 test conditions, not per-ion figures
- Cartridge mineral additions are the makers’ qualitative lists; none of the three publishes an mg/L or pH figure
- Method
- Limits from EPA’s secondary standards page, rejection and cartridge figures from each maker’s product page or FAQ, the sodium figure from Penn State Extension, all read on 2026-09-12; nothing was tested.
- Primary source
- EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals (government or regulatory source)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Reverse osmosis remineralization: what the low TDS and pH readings mean, and whether a mineral cartridge is needed", https://ownerspec.com/water/guides/reverse-osmosis-remineralization-do-you-need-it/, facts verified 12 September 2026.
Every figure above also appears, with its source, in the page below. All sources ยท How OwnerSpec verifies numbers
The picks
Owns a Waterdrop G3P800, G3P600, D6, G2P600 or G2P400
Waterdrop MNR35
1,100 gallons or 12 months per Waterdrop; adds calcium, magnesium, sodium and potassium, no mg/L figure published; 1/4 in quick connect
Wants a tanked RO with the alkaline stage already in it
iSpring RCC7AK
75 GPD, 6 stages, up to 97 percent TDS rejection under NSF/ANSI 58 per iSpring; the FA15 stage is replaced with the 6-month F4AK set
Wants calcite pH correction built into the system
APEC ROES-PH75
6 stages, 75 GPD at 60 psi and 77 F; stage 6 calcite adds calcium carbonate and raises pH per APEC; WQA Gold Seal listed per APEC; feed max 2,000 ppm TDS
Measure tap and RO water before buying any cartridge
HM Digital TDS-3
0 to 9,990 ppm range, plus or minus 2 percent up to 5,000 ppm, factory calibrated with a 342 ppm NaCl solution per HM Digital
If the water tastes fine and nothing downstream of the RO faucet is staining, you can leave now: EPA sets no minimum mineral content, and nothing on this page is needed to meet a standard.
What does the reading tell you?
Total dissolved solids (TDS) is the sum of everything dissolved in the water, in mg/L, which a handheld meter reports as ppm.
| Reading or complaint | Likely cause | Confirm with |
|---|---|---|
| RO TDS reads a tenth of the tap reading or less | The membrane is working: APEC’s FAQ puts high-rejection membranes at 90 to 99 percent | Tap and RO readings on the same meter, same day |
| Water tastes flat | Low TDS: the calcium and magnesium tap water tastes of are what the membrane removed | A side-by-side glass; taste is the only test there is |
| pH strip reads low on RO water | Not a given: APEC’s own before-and-after reading was 8.12 at the tap and 8.06 after RO | A calibrated pH meter with the tap reading beside it, not a strip |
| Blue-green stain at the RO faucet or in the ice maker | Corrosion, the low-pH effect EPA lists in its secondary table | Inspect the copper or brass the RO line touches; if the whole house stains, read the blue-green stains guide |
| RO TDS climbing month after month | Membrane wearing out, which no mineral cartridge fixes | A monthly TDS log; see the RO sizing calculator for the tank and membrane figures |
What does the number mean?
EPA’s National Secondary Drinking Water Standards list pH at 6.5 to 8.5 and TDS at 500 mg/L, and EPA states that these secondary maximum contaminant levels are non-mandatory and not enforced. Both are ceilings. The table has 15 entries and none of them is a floor, so RO water at 10 ppm is inside every federal number.
The effects EPA records beside those entries are the only reasons to buy a cartridge: a bitter metallic taste and corrosion for low pH, salty taste, deposits and staining for high TDS. Neither line mentions water being too pure.
The claim that RO water always reads low pH has no primary source this page could find. APEC’s FAQ says RO may or may not reduce pH noticeably, and its own Southern California test read 8.12 before and 8.06 after. Measure yours.
On rejection, APEC’s FAQ puts its high-rejection membranes at 90 to 99 percent and most other membranes at 70 to 90 percent; that is a brand-wide FAQ claim, not a figure printed on the ROES-PH75 page.
iSpring prints up to 97 percent TDS rejection for the RCC7AK under NSF/ANSI 58 test conditions. At 97 percent, 300 ppm tap water leaves about 9 ppm.
Which treatment class fits?
| Reading or complaint | Treatment class | Why |
|---|---|---|
| Water tastes fine, fittings clean | None | EPA sets no minimum; a cartridge would add cost and a change interval for nothing |
| Flat taste, fittings clean | Inline mineral cartridge after the membrane (Waterdrop MNR35 type) | Waterdrop lists calcium, magnesium, sodium and potassium restored, no mg/L figure |
| Corrosion stain at fittings or a low pH meter reading | Calcite neutralizer stage (APEC ROES-PH75 stage 6 type) | APEC’s spec says the calcite stage adds calcium carbonate and increases pH |
| Wants a number to hold the cartridge to | A TDS meter before any cartridge | No maker publishes an mg/L or pH change, so your own before-and-after reading is the only spec |
Every figure on this page is the manufacturer’s published spec, read on 2026-09-12; nothing was bench-tested. None of the three makers prints how many mg/L the cartridge adds or how far it moves pH: Waterdrop and iSpring name the minerals, APEC names calcium carbonate and “increases pH”, and that is the whole spec.
Which cartridge fits which RO system?
| Option | Fits | Adds (maker’s list) | Capacity or interval | Certification, listed per the maker’s page |
|---|---|---|---|---|
| Waterdrop MNR35 | Waterdrop G3P800, G3P600, D6, G2P600, G2P400 | Calcium, magnesium, sodium, potassium | 1,100 gallons or 12 months | RoHS on the MNR35 page; the G3P800 system lists NSF/ANSI 42, 53, 58 and 372 |
| iSpring FA15, sold only in the F4AK 6-month set | iSpring RCC7AK and RCC7P-AK | Calcium, magnesium, potassium ions | 6 months as part of the F4AK set | NSF/ANSI 58 at system level for the RCC7AK |
| APEC ROES-PH75 stage 6 | Built into the ROES-PH75 only, no separate part number published | Calcium carbonate (calcite), raises pH | Not published on the product page | WQA Gold Seal at system level |
The Waterdrop MNR35 is the only true add-on here: Waterdrop rates it at 1,100 gallons or 12 months on a 1/4 in quick connect and lists only its own G3P800, G3P600, D6, G2P600 and G2P400 as compatible. Skip it on any other brand, and skip it if you want a guaranteed meter reading, because Waterdrop prints none.
The iSpring FA15 has no standalone listing: iSpring sells it only inside the F4AK 6-month set, so an owner of another RO cannot buy just this stage. The iSpring RCC7AK is the 75 GPD, 6-stage system that carries it; the RCC7 parts page covers its intervals.
The APEC ROES-PH75 builds the fix into stage 6 as a 10 in calcite neutralizer; APEC rates the 6-stage system at 75 GPD at 60 psi and 77 F, feed limit 2,000 ppm TDS, feed pH 2.0 to 11.0.
Calcite adds calcium carbonate only, so it raises pH without the magnesium or potassium the two cartridges above list. Skip it if the complaint is flat taste on a system you already own.
Does softened water into an RO leave sodium behind?
A softener does not remove solids; APEC’s FAQ describes the resin as swapping calcium and magnesium ions for sodium ions, and says its softeners are often paired with an RO drinking water filter downstream. Penn State Extension puts the sodium added at 7.5 milligrams per quart for each grain per gallon (gpg) of hardness removed.
Converted by this page, not by Penn State, 7.5 mg per 0.946 L quart is about 7.9 mg/L per gpg, so softening 15 gpg water puts roughly 119 mg/L of sodium at the RO inlet. The hard water guide explains the gpg figure; the salt usage calculator, what the softener consumes.
What the membrane leaves of that sodium is the number nobody prints: APEC’s 90 to 99 percent and iSpring’s up to 97 percent are whole-system TDS figures, and no product page here states a sodium-specific rejection rate, so this page will not invent one.
Before you buy anything
Do not order a mineral cartridge until you have two readings from one meter, the tap and the RO faucet, taken the same day. The HM Digital TDS-3 covers 0 to 9,990 ppm at plus or minus 2 percent up to 5,000 ppm, factory calibrated with a 342 ppm NaCl solution per HM Digital. If the RO reading is a tenth of the tap or less and the water tastes fine to you, there is nothing to buy. If taste is the complaint, the same meter is the only way to see what the Waterdrop MNR35 or the APEC calcite stage actually added, because neither maker prints the figure.
What should you do first?
- Read tap and RO TDS with the same meter on the same day and write both down; the ratio is your rejection rate.
- Inspect every copper or brass fitting the RO line touches for blue-green stain, the corrosion effect EPA lists for low pH.
- Match any cartridge to the system by the maker’s own compatibility list, not by the tubing size.
- If the house has a softener, multiply the hardness removed in gpg by 7.9 mg/L to know the sodium arriving at the RO inlet.
- Re-read the RO TDS 30 days after fitting a cartridge: the difference is the only mineral figure you will get.
Frequently asked questions
What is EPA's secondary standard for drinking water pH and TDS?
EPA's National Secondary Drinking Water Standards list pH at 6.5 to 8.5 and total dissolved solids at 500 mg/L. EPA calls these secondary maximum contaminant levels non-mandatory and does not enforce them; the table records the noticeable effects instead, a bitter metallic taste and corrosion at low pH, and a salty taste, deposits and staining at high TDS. Both figures are ceilings, not targets for RO water.
Does the EPA require a minimum amount of minerals in drinking water?
No. EPA's secondary standards table lists 15 nuisance parameters and every entry is a ceiling; there is no floor for TDS, calcium, magnesium or any other mineral, and EPA states it does not enforce the secondary levels at all. Low-TDS water from a reverse osmosis system therefore breaks no federal standard. Whether to add minerals back is a taste and plumbing question, not a compliance one.
How much sodium does a water softener add before the water reaches an RO system?
Penn State Extension puts the figure at 7.5 milligrams of sodium per quart for each grain per gallon of hardness removed, which works out to about 7.9 mg/L per gpg. Softening 15 gpg water therefore adds roughly 119 mg/L of sodium at the RO inlet. The RO membrane then rejects most of it, but no maker on this page publishes a sodium-specific rejection figure, only whole-system TDS rejection.
Can softened water be fed into a reverse osmosis system?
Yes. APEC's FAQ says its softeners are often paired with a reverse osmosis drinking water filter downstream, and the ROES-PH75 spec lists a feed limit of 2,000 ppm TDS and a feed pH range of 2.0 to 11.0. Softening changes neither number much: APEC describes the resin as swapping calcium and magnesium for sodium, not removing solids, so the TDS at the RO inlet stays close to the tap figure.
What percentage of dissolved solids does a residential RO membrane reject?
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. iSpring rates the RCC7AK system at up to 97 percent TDS rejection under NSF/ANSI 58 test conditions. Read those as whole-system TDS figures: neither maker prints a rejection rate for sodium or any single ion on the product page.
Sources
- EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals (government or regulatory source)
- Penn State Extension, Water Softening (university or extension service)
- APEC Water Systems, FAQ (membrane rejection, softener and RO pairing, pH before and after RO) (manufacturer or vendor document)
- APEC Water Systems, ROES-PH75 product page (stages, feed limits, certification) (manufacturer or vendor document)
- APEC Water Systems, MEM-ES-75 75 GPD Essence RO membrane (manufacturer or vendor document)
- Waterdrop, MNR35 remineralization filter product page (manufacturer or vendor document)
- Waterdrop, G3P800 reverse osmosis system product page (certifications) (manufacturer or vendor document)
- iSpring, replacement filters for RCC7AK and RCC7P-AK (F4AK set including the FA15 alkaline filter) (manufacturer or vendor document)
- HM Digital, TDS-3 handheld TDS meter product page (manufacturer or vendor document)