Well owner guides

Chlorine Dioxide vs Bleach for Well Shocking

There are two chemistries for shocking a private well: the chlorine everyone has used for decades, and the chlorine dioxide that municipal plants moved to. They are different molecules that fail and succeed in different places, and if your well keeps re-fouling, the difference is probably why.

What the incumbents actually ship

Nearly every well sanitizer kit on the market is chlorine: either jugs of sodium hypochlorite, which is laundry bleach, or bags of calcium hypochlorite pellets, which is pool shock at 65 to 75% available chlorine. The extension-service method doses that chlorine to 100 to 200 ppm in the well and holds it overnight. It is cheap, it is familiar, and when the water is clean and slightly acidic it works.

Four places the chemistry splits

Killing power per ppm. A chlorine dioxide molecule accepts 5 electrons in oxidation; hypochlorite accepts 2. The maker of Selectrocide, Selective Micro Technologies, publishes lab data showing equivalent disinfection at 5 ppm of chlorine dioxide versus 1,000 ppm of bleach, a 200-fold difference. That is why our shock dose is 20 ppm while the bleach method needs 100 to 200 ppm, and why the bleach method leaves you flushing chlorine for days.

pH. Above a pH of about 7.5, chlorine converts to the hypochlorite ion and gives up most of its killing power. Most groundwater sits between 7 and 8.5. Chlorine dioxide's strength does not depend on pH, so it delivers the same kill in the water your well has, not the water the bleach label imagines.

Biofilm. Iron bacteria and sulfur bacteria live inside a slime matrix that chlorine burns on the surface and leaves alive underneath, which is the engine of the shock-and-relapse cycle. The manufacturer's published data shows chlorine dioxide penetrating and stripping biofilm. If your symptom is orange slime or a rotten egg smell, this is the difference that decides the outcome.

What is left behind. Chlorine reacts with organic material to form chlorinated byproducts like trihalomethanes, and at shock strength it corrodes pump wiring, seals, and fixtures. Chlorine dioxide breaks down to simple salts and oxygen and does not form those byproducts, so the water clears to normal taste as soon as the residual flushes out.

The chemistry city water plants use, sized to your well.

Answer a few questions and we pull your well's dimensions from your state's official well record, then pack a chlorine dioxide kit to those exact numbers from $399. A contractor charges $1,500 to $3,000 for the same cleaning.

Find my well See what's in the kit

How it ships is part of the chemistry

Chlorine strong enough to matter is a hazardous liquid or a reactive dry oxidizer, which is why the store jug is 5% and mostly water. Selectrocide ships as a sealed dry sachet that generates 99.9% pure chlorine dioxide right in the mixing water through a patented micro-reactor (US Patent 12,036,525), with the reaction byproducts locked inside the spent pouch. Full-strength disinfectant arrives legally at your door, and the dose is measured by counting sachets instead of guessing at jug math. Our kit counts them for you, from your well's official record, at the manufacturer's published 20 ppm shock dose, and every number is on the sources page.

Straight answers

Is chlorine dioxide safe to use in a drinking water well?

The chlorine dioxide our kit ships, Selectrocide, is EPA registered and NSF/ANSI 60 certified for drinking water treatment, and municipal water plants dose the same chemistry into city water every day. Like any disinfectant it demands respect during the treatment itself: gloves, eye protection, and flushing to a verified zero residual before you drink the water again.

Is chlorine dioxide the same thing as bleach?

No. Bleach is sodium hypochlorite, and chlorine pellets are calcium hypochlorite. Chlorine dioxide is a different molecule with a different kill mechanism: it accepts 5 electrons in oxidation against hypochlorite's 2, stays selective instead of burning up on the first iron atom it meets, and does not form chlorinated byproducts like trihalomethanes.

Why do contractors still use bleach to shock wells?

Because it is cheap, it is on every store shelf, and shock chlorination is the method the extension guides have published for decades. It does kill bacteria when it is dosed and held correctly. Its weaknesses show up in the wells that keep re-fouling: alkaline pH, high iron, and established biofilm are exactly the conditions where hypochlorite underperforms.

Can I just buy chlorine dioxide sachets and skip the kit?

You can buy Selectrocide sachets on their own. What you would still need is the part people get wrong: your well's real volume from its state record, the sachet count sized to that volume at the manufacturer's shock dose, the residual test that proves the treatment held, PPE, and instructions with your numbers written in. That sizing is the kit.

Keep reading

Why we do not just tell you to pour bleach

Ask around and you will hear the same advice: dump a jug of 5% laundry bleach down the well. Contractors say it because bleach is cheap and everywhere, and every other well kit on the market is bleach or chlorine pellets in a box. Here is the honest comparison.

Bleach and chlorine pellets, what everyone else ships

  • Bleach needs brute force. Extension guides call for 100 to 200 ppm of chlorine to shock a well, and the maker of Selectrocide reports bleach needs 1,000 ppm to match what chlorine dioxide does at 5.
  • It gives up in real well water. Above a pH of about 7.5, chlorine converts to the hypochlorite ion and loses most of its killing power, and most well water sits on that alkaline side.
  • It burns itself up on iron and organics before it ever reaches the bacteria, which is why the smell hangs around while the problem stays.
  • It scorches the surface of the slime layer that iron bacteria build and leaves the colony underneath alive to grow back.
  • At shock strength it is corrosive to the pump wiring, the casing, the seals, and every fixture the flush runs through, and it leaves chlorinated byproducts like trihalomethanes behind.
  • Bleach strong enough to matter is a hazardous liquid that nobody will ship to your door. The 5% jug at the store is mostly water, and the pellet kits are pool chemicals in a smaller bag.

Selectrocide chlorine dioxide, what this kit ships

  • A chlorine dioxide molecule accepts 5 electrons in oxidation. Bleach accepts 2. The maker's lab data shows equivalent disinfection at 5 ppm versus 1,000 ppm of bleach, a 200-fold difference.
  • It keeps its full sanitizing strength across the pH range real wells actually have. There is no pH cliff at 7.5.
  • It penetrates and strips biofilm instead of glancing off it. That slime is exactly what iron bacteria build, so the treatment reaches the colony, not just its skin.
  • It arrives as a sealed dry sachet and generates 99.9% pure chlorine dioxide in water at your wellhead (US Patent 12,036,525), so full-strength product ships legally to your door.
  • It is EPA registered and NSF/ANSI 60 certified for drinking water treatment, the same chemistry many city water plants dose every day.
  • When the work is done it breaks down into simple salts and oxygen. No trihalomethanes, no chloramines, and no bleach taste hanging around for days.

The performance numbers above come from Selective Micro Technologies, the maker of Selectrocide, and their published material at selectivemicro.com. Chlorine dioxide still deserves respect: the kit includes gloves and eye protection, and the instructions keep you upwind of the wellhead while the treatment works. Every dosing number is on the sources page.