The order of operations

Most treatment mistakes are sequencing mistakes, not product mistakes. Water passes through each stage in physical order, and each stage assumes the one before it did its job.

Order Stage Trigger Why it sits here
1 Neutraliser pH below 6.5 Acidic water corrodes plumbing and degrades everything downstream
2 Oxidation + iron filter Oxidised iron, or dissolved iron above 5 mg/L Oxidised iron fouls softener resin. Dissolved iron below 5 mg/L can go through a softener; above it, remove upstream
3 Sulphur treatment Rotten-egg smell Air injection or catalytic carbon; usually shares the stage-2 vessel
4 Softener Hardness in the hard band or above Removes calcium and magnesium by cation exchange
5 UV disinfection Bacteria present Requires clear water — must follow all particulate removal
6 Point-of-use RO Nitrate, arsenic, or high TDS Treats drinking water only; not economic whole-house

The single most common error is putting a softener first because it is the product people have heard of. Penn State Extension puts the limit plainly: “Oxidized forms of iron and manganese will foul the softener resin.” On well water with oxidised iron, a softener-first install is a resin bed being fouled from day one. See iron in well water for the glass test that tells you which form you have.

Where the trigger numbers come from

Every threshold above is a published federal figure, not a rule of thumb:

  • Iron at 0.3 mg/L and manganese at 0.05 mg/L are EPA Secondary Maximum Contaminant Levels — the point where EPA lists staining and metallic taste as noticeable effects.
  • pH 6.5 to 8.5 is EPA’s secondary range; below it, EPA lists corrosion.
  • Nitrate at 10 mg/L and arsenic at 0.010 mg/L are enforceable primary standards set on health grounds.
  • The hardness bands are USGS: soft to 60 mg/L, moderately hard to 120, hard to 180, very hard above that.

Note the difference in status. Secondary standards are aesthetic guidelines that EPA does not enforce — exceeding one means staining and taste, not a health risk. Primary standards are enforceable health limits. We keep those apart because conflating them is how filtration gets sold on fear.

Softener sizing, worked

Softener capacity is measured in grains of hardness removed between regenerations.

capacity needed = hardness (gpg) x gallons per day x days per cycle

Household water use runs about 75 gallons per person per day. So:

Household Hardness Daily grains 7-day cycle Buy
2 people 10 gpg 1,500 10,500 16,000 grain
4 people 15 gpg 4,500 31,500 32,000 grain
4 people 25 gpg 7,500 52,500 64,000 grain
6 people 20 gpg 9,000 63,000 64,000 grain

If iron is going through the softener, add roughly 5 grains per gallon of rated capacity for each 1 ppm of dissolved iron. At 2 ppm iron that is the equivalent of another 10 gpg of hardness — which is usually the argument for removing the iron upstream instead.

The grains-per-gallon conversion, derived

Most sites assert “17.1” without saying where it comes from. It falls out of two unit definitions:

1 grain      = 1/7000 pound = 64.79891 mg
1 US gallon  = 3.785411784 L

1 grain per gallon = 64.79891 mg / 3.785411784 L = 17.118 mg/L

So USGS’s 120 mg/L boundary between moderately hard and hard is 7.0 grains per gallon — which is where softener sizing normally starts being worth the running cost.

What this method cannot do

It sizes equipment from numbers you already have. It cannot tell you what is in your water.

If you are on a private well, nobody is testing it for you. EPA’s primary standards are legally enforceable for public water systems and do not apply to private wells at all. The numbers above are still the right reference points — there is simply no one obliged to measure against them on your behalf.

And a caution on regional hardness maps: the widely-reproduced USGS national hardness map illustrates data from 1975, and USGS states plainly that those data “are not designed to be used to make local decisions or decisions on the scale of individual homeowner property.” Use it to know whether your region trends hard. Do not use it to size a softener.

On certification claims

When you get to choosing equipment, one fact does most of the work: under NSF/ANSI 42 and 53, certification claims vary by product. A system “certified to NSF/ANSI 53” is not certified against all fifty-plus contaminants in that standard — only against the specific claims listed for that model. Check the individual claim, not the standard number on the box.