St. Louis lime-softening leaves a magnesium-dominant scale, hardness runs 80 to 206 mg/L, and winter road salt sprays the lower elevations.
St. Louis softens its water with lime, which strips calcium but leaves magnesium, so the water, hard on average, dries into a magnesium-dominant scale. Winter road salt sprays the lower walls, and July to October is the best stretch to wash. South City's repeating housing keeps routes efficient, while dense historic streets price on pane count and access; a single-story exterior runs $300 to $550.
The utility's 2025 report gives total hardness as 129 mg/L on average, across 80 to 206 — hard — but a bare figure would mislead in this city, and the reason is the treatment rather than the source. Lime softening strips calcium hardness far more effectively than magnesium, so the water arriving at a tap has a composition inverted from the usual — proportionally more magnesium than calcium. Magnesium deposits are the harder of the two to remove, which means this water can behave worse on glass than its total suggests. Assume a tenacious deposit, keep panes wet, and do not expect a mild acid to do the whole job.
| Hardness | 80–206 mg/L as CaCO₃ Averaging 129 mg/L; hard, as published by City of St. Louis Water Division. |
|---|---|
| pH | 9.52 (range 8.52–10.05) |
| Alkalinity | 55 mg/L as CaCO₃ (range 3–173) |
| Iron | not detected |
| Manganese | not detected |
| Sodium | 45.4 mg/L (range 26.8–62.5) |
| Chloride | 26.3 mg/L (range 5.5–37.6) |
| Total dissolved solids | 298 mg/L (range 216–388) |
| Disinfectant | chloramine |
| Source and treatment | Surface water, per the EPA SDWIS federal record · Lime and soda ash softening with sedimentation, rapid sand filtration and sludge treatment, pre and post chlorination with chloramines as the residual, and fluoridation · The plant softens. |
Not in the report: corrosion index.
SOURCE · City of St. Louis Water Division 2025 Consumer Confidence Report, optional monitoring (secondary contaminants) table · How we source these numbers
Winter is the structural fact: reliable work runs roughly April to November, and everything accumulated in the closed months arrives at once on the first spring visit, salt included. Inside the season, mineral deposition is a continuous baseline rather than an event, so recurring schedules make sense here in a way they do not in soft-water cities. Shaded elevations add organic film through humid summers, and the older brick neighbourhoods hold it longer because the streets are narrow and the light is poor.
MISSOURI SEASON BY SEASON →St. Louis treats two rivers at two plants and softens with lime at the head of the process — and because the intakes sit close to the confluence, the Mississippi plant is largely handling Missouri River water anyway. Lime softening is the fact that matters: it removes calcium preferentially and leaves magnesium behind, so the remaining hardness is unusually magnesium-dominant, which resists a mild acid. Above ground it is brick density, no side access, and a South City stock consistent enough to make route density real.
St. Louis treats water from two different rivers at two plants — one on the Mississippi north of the city and one on the Missouri well west of it — and the utility softens at the head of the treatment process rather than simply filtering.
There is a wrinkle that makes both plants more similar than the map suggests: the water reaching the Mississippi intake is primarily Missouri River water, because the two rivers have not fully mixed that close to their confluence.
Lime softening is the fact that changes everything about the residue here, and it does so in a way a bare hardness figure hides completely.
Softening with lime removes calcium hardness preferentially. It is far less effective against magnesium, so what comes out of the process is water whose remaining hardness is disproportionately magnesium rather than calcium.
That is the opposite of the usual composition, and it matters because magnesium deposits are more tenacious than calcium ones. A mild acid that lifts ordinary calcium scale will underperform on this water.
The utility's own 2025 report puts total hardness at 129 mg/L on average, ranging from 80 to 206, with 74 of it non-carbonate — exactly the residue lime softening leaves, weighted toward magnesium rather than calcium.
A range that wide across one system is itself informative: two plants on two rivers, softening at the head, and river conditions that swing with flow mean the delivered figure is not a constant.
Above ground the city is defined by brick and by density. Whole neighbourhoods of pre-1900 rowhousing share party walls and have no side access, so upper panes come off the street or from inside.
The consistency of the South City stock is the commercial opportunity — mile after mile of near-identical two-flats and bungalows makes route density achievable in a way that a varied housing stock never allows.
Continental climate closes reliable exterior work through the middle of winter and adds road salt to lower elevations for months, so the first spring service is reliably the heaviest of the year.
Summer humidity is high enough that shaded north elevations hold organic film through the growing season, on top of whatever the water leaves.
Route density is the whole economics of this market and South City is where it is achievable, because the stock repeats. Against that, the dense historic neighbourhoods price on pane count and access rather than area, and Lafayette Square and the Central West End add heritage pacing on ornate original sash. Downtown high-rise prices on rigging and out-of-hours access. The short season concentrates the year's revenue into eight months and capacity inside them is what is really being sold.
Ground-floor work in the newer parts of the metro is reasonable homeowner territory. The two local obstacles are real. The water leaves a magnesium-heavy deposit that punishes letting a pane dry mid-job and does not fully respond to an ordinary vinegar remedy. And the rowhouse neighbourhoods have no side access at all, which is a structural problem rather than a difficulty of nerve — upper sashes come off the street or from inside, and many are painted shut.
OPEN COST ESTIMATOR →2025 annual mean at the Blair Street monitor, Blair STREET: 3247 Blair Street, St. Louis, MO 63107, 4.6 km north-east of the city (EPA site 29-510-0085).
2025 annual mean at the Forest Park monitor, Forest PARK: McKinely Dr., St. Louis, MO 63110, 3.2 km west of the city (EPA site 29-510-0094).
Largest air release: Afton Chemical Corp, 501 Monsanto AVE (chemicals), 1.8 km outside the city limits, 53,468 lb in 2024.
St. Louis can put a meter on any flat-rate customer who allows water to be wasted or negligently lets it run, and the Water Commissioner can shut off the supply until satisfied the waste will stop.
St. Louis Revised Code §23.10.010: abuse requires meter · St. Louis Revised Code §23.10.030: water shut off · checked September 2026
The Metropolitan Sewer District's Ordinance 15048 governs St. Louis storm sewers: nobody may put any water that isn't entirely storm water into a separate storm sewer or watercourse unless an NPDES permit covers it or it is exempt, and even then it must not breach water-quality standards or become a nuisance.
MSD (St. Louis) Ordinance No. 15048, Article IV Section One: prohibited discharges to separate storm sewers and watercourses · checked September 2026
In a St. Louis city historic district or on a landmark site, the Building Commissioner may not issue a permit for construction, alteration or demolition until the Cultural Resources Director finds the work meets the district or landmark standards, or the Preservation Board or the director recommends approval.
St. Louis Revised Code §24.20.030: determination of compliance required before permit · checked September 2026
Wet-only handling. Peak late April. Heavier in southern Missouri pine areas than northern Missouri agricultural belt.
The utility's 2025 report gives total hardness as 129 mg/L on average, across 80 to 206 — hard — but a bare figure would mislead in this city, and the reason is the treatment rather than the source.
Residential window cleaning in St. Louis typically runs $10–16 per pane or $300–550 for a standard single-story exterior. Route density is the whole economics of this market and South City is where it is achievable, because the stock repeats.
St. Louis's drier, frost-free months are July to October, by NOAA's 1991–2020 normals for the Cahokia St. Louis Airport station, where May brings the most rain. The calendar on this page marks July (after the April 12 bloom) and October (after smoke season and before frost). Frost nights usually run from October 26 to April 8.
Yes, if the rinse is squeegeed off before it dries: City of St. Louis Water Division publishes dissolved solids of 298 mg/L, between this site's two dissolved-solids flags. Water left to dry leaves mineral scale; fresh spots lift with a mild acid. Treatment starts with lime, soda ash softening with sedimentation, rapid sand filtration and sludge treatment.
In St. Louis the dominant residue patterns include magnesium-dominant scale left by lime softening and winter road salt and street spray on lower elevations. Winter is the structural fact: reliable work runs roughly April to November, and everything accumulated in the closed months arrives at once on the first spring visit, salt included.
Ground-floor work in the newer parts of the metro is reasonable homeowner territory. The two local obstacles are real. The water leaves a magnesium-heavy deposit that punishes letting a pane dry mid-job and does not fully respond to an ordinary vinegar remedy.
Early 20th-century office towers, warehouse conversions and civic stock beside the arch grounds. Large continuous elevations, building management scheduling and genuine high-rise work. Central West End and Lafayette Square have their own housing stock and glazing as well.
Just over the Missouri line