Methodology
Where the numbers come from
Water chemistry figures get copied between articles until they no longer match any authority. The defence against that is refusing to hold a number without recording where it came from, and being honest about the difference between a federal standard and an equipment specification.
7
Federal standard
A published EPA drinking water standard.
9
Government health agency
Stated by a government health agency, verified against its own page.
10
Engineering practice
An equipment operating limit or sizing convention in general use. Not a published health standard, and specifications differ between manufacturers.
The three tiers, and why the distinction matters
A federal standard and a manufacturer's operating limit are both numbers, and presenting them the same way is how a piece of engineering practice acquires the authority of regulation. Both are useful. They are not the same kind of claim.
The pH floor an oxidising filter needs is a good example. It is real, it decides purchases, and it varies between media because it is a property of equipment rather than of drinking water policy. Reporting it as though the EPA published it would be wrong, and reporting the EPA's iron standard as a vague rule of thumb would be equally wrong.
So every threshold on this site carries its tier, and the planner shows it alongside the figure.
The weakest numbers here, named
The softener compensation factors are the shakiest figures on this site, and they sit behind the claim it repeats most often. No extension service, EPA publication, NSF standard or public Water Quality Association document publishes one. The numbers in circulation come from softener manufacturers' own programming manuals, and those manuals disagree: iron between 4 and 5 grains per gallon per ppm, manganese between 2 and 4.
Both are still used, because ignoring iron entirely is a larger error than choosing imperfectly within a known range. Each is set where being wrong costs least: too high oversizes a softener and wastes some money, too low undersizes it, which is the failure the calculation exists to prevent.
What gets cited
A citation means a page that was read on the date recorded beside it. Sources that block automated access are not cited from memory, however well known the figure. Where authorities disagree, and on the iron level a softener can carry they do, the range is reported along with who says what rather than resolved into a single confident number.
Every threshold in the engine
| Threshold | Value | Basis | Source |
|---|---|---|---|
| Iron, staining thresholdEPA secondary standard, set for staining and taste rather than health. | 0.3 ppm | Federal standard | US EPAchecked 2026-09-14 |
| Manganese, staining thresholdEPA secondary standard. Black and grey staining begins around here. | 0.05 ppm | Federal standard | US EPAchecked 2026-09-14 |
| pH, lower boundLower bound of the EPA secondary range. Below it, water is corrosive to copper and galvanised plumbing. | 6.5 pH | Federal standard | US EPAchecked 2026-09-14 |
| pH, upper boundUpper bound of the EPA secondary range. | 8.5 pH | Federal standard | US EPAchecked 2026-09-14 |
| Total dissolved solids | 500 ppm | Federal standard | US EPAchecked 2026-09-14 |
| Nitrate, health standardEnforceable federal health standard. Infants under six months are the population at risk. | 10 ppm as N | Federal standard | US EPAchecked 2026-09-14 |
| Arsenic, health standard10 parts per billion. | 0.01 ppm | Federal standard | US EPAchecked 2026-09-14 |
| Manganese, health advisoryLifetime health advisory level rather than an enforceable standard. | 0.3 ppm | Government health agency | US EPAchecked 2026-09-14 |
| Hydrogen sulfide, ceiling for oxidising mediaAbove this, the answer moves to continuous chlorination or ozonation with filtration. | 6 ppm | Government health agency | Minnesota Department of Healthchecked 2026-09-14 |
| Hydrogen sulfide, ceiling for carbon | 1 ppm | Government health agency | Minnesota Department of Healthchecked 2026-09-14 |
| pH needed to oxidise ironRutgers Cooperative Extension states that where aeration is limited by a pressure system and pH is above 6.8, an oxidising filter can be an option. Oxidation speed collapses below it: 90% of iron oxidises in about 30 seconds at pH 8, an hour at pH 7 and roughly 100 hours at pH 6. | 6.8 pH | Government health agency | Rutgers Cooperative Extensionchecked 2026-09-14 |
| pH needed to oxidise manganeseHigher than the figure for iron, and the media minimum is a manufacturer specification rather than a published standard. The gap itself is well sourced: North Dakota State University Extension puts the optimum pH for chlorine oxidation at about 8.0 for iron and 8.5 for manganese, and the National Drinking Water Clearinghouse notes that oxidation of manganese by oxygen alone is very slow below pH 9.5. | 7.5 pH | Engineering practice | Equipment specification |
| Iron a softener can carry, cautious readingThe cautious reading. Above this, iron fouls standard resin and costs capacity permanently. | 1 ppm | Engineering practice | Equipment specification |
| Iron a softener can carry, vendor claimUpper figure claimed for iron-tolerant or fine-mesh resin. A vendor claim, not a design basis. | 3 ppm | Engineering practice | Equipment specification |
| Hardness worth softeningAbout 120 mg/L, where the USGS scale crosses from moderately hard into hard. | 7 gpg | Engineering practice | Equipment specification |
| UV, maximum turbidity | 1 NTU | Engineering practice | Equipment specification |
| UV, maximum iron | 0.3 ppm | Engineering practice | Equipment specification |
| UV, maximum manganese | 0.05 ppm | Engineering practice | Equipment specification |
| UV, maximum hardnessScale on the quartz sleeve blocks the light that does the work. | 7 gpg | Engineering practice | Equipment specification |
| Chlorine demand, iron | 0.64 mg Cl2 per mg Fe | Government health agency | University of Georgia Extensionchecked 2026-09-14 |
| Chlorine demand, manganese | 1.3 mg Cl2 per mg Mn | Government health agency | University of Georgia Extensionchecked 2026-09-14 |
| Chlorine demand, hydrogen sulfideClose to the ratio for oxidation to elemental sulfur. Complete oxidation to sulfate demands considerably more. | 2.2 mg Cl2 per mg H2S | Government health agency | University of Georgia Extensionchecked 2026-09-14 |
| Oxygen demand, iron | 0.14 mg O2 per mg Fe | Government health agency | National Drinking Water Clearinghouse, West Virginia Universitychecked 2026-09-14 |
| Oxygen demand, manganese | 0.27 mg O2 per mg Mn | Government health agency | National Drinking Water Clearinghouse, West Virginia Universitychecked 2026-09-14 |
| Softener capacity consumed by ironManufacturer programming manuals give 4 to 5 and no neutral authority publishes a figure. The lower end is used here. | 4 gpg per ppm | Engineering practice | Equipment specification |
| Softener capacity consumed by manganeseManufacturer programming manuals give 2 to 4 and no neutral authority publishes a figure. The upper end is used here, because undersizing is the error this calculation exists to prevent. | 4 gpg per ppm | Engineering practice | Equipment specification |
How the planner decides
- Health first. Bacteria, nitrate and arsenic produce a referral to a certified lab and a health department. They never produce a product recommendation, because that is not a decision a web form should make.
- Then what needs oxidising. Iron, manganese and hydrogen sulfide above their thresholds each call for oxidation, and manganese sets a higher pH target than iron does.
- Then whether pH allows it. If the water is below the target for what has to be oxidised, pH correction is inserted ahead of the filter, with a note that it raises hardness on the way through.
- Then which oxidiser. Air injection by default; chemical injection where sulfide, very high iron, organic iron or tannins put the water past what air handles.
- Then softening, sized on compensated hardness, counting only the iron and manganese that actually reach the resin.
- Then carbon, then ultraviolet, in that order, because UV needs water the stages above it have already cleared.
- Then flow. Backwash requirements are checked against what the well delivers, because a filter that cannot backwash will pack its bed and stop working.
What it refuses
Iron above 25 ppm, manganese above 5 ppm, pH below 5, hardness above 100 grains per gallon, a softener requirement past the largest standard cabinet, or a well that cannot backwash the smallest standard tank. Each returns an explanation and a recommendation to have a system designed.
Refusing is not a hedge. Packaged residential equipment has a range, and water outside it needs an engineer rather than a catalogue. A confident answer there would be worth less than no answer.
Corrections
Standards get revised and pages move. Any figure here that has gone out of date is worth reporting, and a citation that no longer resolves is a correction too.