Every water treatment company says its system works. Few show the numbers from local homes.
We have them. At Culligan of Red Wing, our technicians measured water before and after treatment, at homes from Red Wing to Ellsworth, and wrote the results down.
Here’s what 846 paired checks and 143 iron tests say about treating water in southeastern Minnesota and western Wisconsin, including where the numbers stop.
Why we measure twice
A water test tells you what’s in your water. It doesn’t tell you whether your equipment is fixing anything.
So on service visits, our technicians take two total dissolved solids (TDS) readings: one from the tap, one from the reverse osmosis (RO) water. The gap between them shows how hard the membrane is working. If the gap shrinks over time, something needs service.
What our local checks found
| What we checked | Records | Result |
| Paired tap and RO-water TDS checks | 846 | 93% median TDS reduction |
| Checks that met our 79% reduction target | 846 | 96% met it |
| Treated-water iron tests | 143 | 80% below 0.3 ppm |
Source: Culligan of Red Wing service records, May 2025 to August 2026.
The 79% target is our own service benchmark. A check that clears it means the system is doing its job. The 4% that missed it are why we service equipment instead of just installing it.
What 93% looks like in your town
TDS varies a lot across our service area. The typical Red Wing tap reading was 370 ppm, and 29% of tested Red Wing homes ran above 500 ppm. That’s EPA’s secondary guideline, which covers taste, deposits, and staining. Ellsworth’s typical reading was 396 ppm. Zumbrota’s was 292.
Apply a 93% reduction to Red Wing’s 370 ppm, and you land near 26 ppm. That’s arithmetic, not a measured result, but it shows the scale. The water you drink and cook with ends up far lower in dissolved solids than the water coming into your home.
RO treats drinking water at one tap, usually the kitchen. Hardness is a different job. With a median of 19 GPG across the region, whole-home scale protection usually means a softener. Many homes in Red Wing, Cannon Falls, and Lake City end up with both.
Where the numbers stop
A TDS drop is a good sign. It isn’t a guarantee about every contaminant.
TDS measures dissolved minerals and salts as a group. It can’t tell nitrate from calcium. In our data, 19% of nitrate field readings across the region came in at 10 ppm or higher. Those are screening results, and our form doesn’t say whether they’re nitrate-nitrogen or nitrate ion.
EPA’s limit is 10 mg/L as nitrate-nitrogen. If you’re on a private well and your reading is high, confirm it with a certified lab before you decide anything.
That’s why we test first and match treatment to the problem, instead of selling one system to everyone.
Iron: most results are clean; some aren’t
Treated water came in under EPA’s 0.3 ppm iron guideline in 80% of 143 tests. That leaves one in five readings at or above it. Iron filters need regular service, and a skipped visit shows up in the results.
These tests come from homes already using iron treatment, so they say nothing about how common iron is in your town. They do show that treatment works when it’s sized and serviced correctly.
“Culligan of Red Wing did a great job changing out our RO filters after Lake City’s boil water order was lifted. Both technicians were polite, knowledgeable, and they quickly completed the filter change-out. It’s so nice to receive great service like this!” – Mike Stori.
Start with a Free Consultation
Before you buy a system, find out what it needs to fix. Get one of our Culligan specialists to test your water at your home, explain the results in plain language, and recommend treatment based on what’s there. We install it and keep checking it.
Reverse Osmosis FAQs
Does reverse osmosis work on well water?
It can reduce dissolved solids well. The right setup depends on what’s in your well, so test first. Iron and hardness often need treatment ahead of an RO system.
What does a 93% TDS reduction mean?
It’s the median drop between tap and RO-water readings across 846 service checks. A 370 ppm tap would read about 26 ppm after RO.
Does lower TDS mean the water is safe?
Not by itself. TDS doesn’t identify specific contaminants. Use a certified lab for health questions such as nitrate.
How often should RO equipment be serviced?
We check performance during service visits. A shrinking gap between tap and RO readings is the signal to act.
Do I need an RO system if I have a softener?
They do different jobs. A softener handles hardness for the whole house. RO improves drinking and cooking water at one tap.
Does Culligan serve my town?
We serve southeastern Minnesota and western Wisconsin, including Red Wing, Lake City, Cannon Falls, Zumbrota, Wabasha, Goodhue, and Ellsworth.
