Shocking a pool usually requires chlorine. Lots of chlorine.
But there is a chlorine-free alternative shock treatment that’s both uniquely useful and widely misunderstood.
In this article, we’ll cover non-chlorine shock, the benefits of using it, and when you should consider it over chlorine-based shock.
Quick answer: Non-chlorine shock, or MPS, is a shock treatment that breaks down non-living organics in the water. It’s useful under certain conditions but it’s not a sanitizer and can’t replace traditional chlorine shock.
What is Non-Chlorine Shock?
Non-chlorine shock is also often called MPS, short for monopersulfate.
As its name suggests, it’s a chlorine-free shock treatment — although it comes in 1 lb bags of white, granules that look very much like chlorine shock.
That’s where the similarity ends, however. Traditional shock is a sanitizer and an oxidizer, meaning it tackles non-living and living contaminants in the water, including bacteria, viruses, and algae.
Non-chlorine shock, or MPS, is only an oxidizer, meaning it can only break down non-living contaminants such as sweat, saliva, lotions, and sunscreen.
Chlorine shock is the germ killer, and non-chlorine shock is the waste handler.
What Are the Benefits of Chlorine Shock?
If you’re thinking non-chlorine shock is just a less powerful version of regular shock, you’re not entirely wrong, but there are some benefits.
Let’s break it down:
It Makes Your Chlorine Last Longer
Chlorine gets used up as it sanitizes and oxidizes all contaminants in the water.
While non-chlorine shock only oxidizes contaminants (and only certain types at that), it still takes some of the load off your chlorine. After all, the less “stuff” that needs to be oxidized, the less chlorine is consumed.
Non-chlorine shock is particularly useful here because it works much faster than chlorine-based shock, meaning it’s able to eliminate specific contaminants in the water before chlorine even knows about it.
Note: MPS shock also doesn’t affect your free chlorine, so it will remain constant. The same is true if you use bromine instead of chlorine.
It Lets You Swim Much Sooner
Chlorinated shock generally elevates your free chlorine to unsafe levels, at least for a few hours after application.
Non-chlorine shock reacts much faster than chlorine, so it can “finish” sooner than a traditional shock treatment — often in as little as 15 minutes from when it’s added to the water.
It also specifically targets non-living matter, and while high concentrations can still be considered unsafe for swimming, it’s still far less irritating to humans compared to chlorine.
As an added benefit, MPS shock can be added to your water any time of day because it won’t be burned up by the sun.
It Prevents a Build Up of Chloramine
Chloramine is what you end up with when your chlorine gets used up (it’s also commonly referred to as combined chlorine).
Too much chloramine will cause your water to become cloudy and irritate your eyes, as well as lead to an unpleasant “chlorine” smell, so it’s best kept in check.
Since MPS isn’t chlorine, it doesn’t form chloramine when it reacts in the water — but it goes even further than that.
Non-chlorine shock targets and oxidizes contaminants that chlorine would have otherwise reacted with to form chloramine, thereby preventing at least some chloramine formation caused by your residual sanitizer.
It Doesn’t Over-Stabilize Your Water
Some chlorine shock products (namely dichlor and trichlor) are stabilized, meaning they contain high levels of cyanuric acid to protect chlorine from breaking down under the sun’s UV light.
Not only is this counter-intuitive for shocking since you want the chlorine to burn off quickly, it can also lead to too much stabilizer in the water — especially given how much chlorine is used during a shock treatment.
This matters because too much cyanuric acid in the pool can “lock up” the free chlorine, causing it to become significantly weakened and unable to effectively sanitize your water.
Non-chlorine shock doesn’t contain any stabilizer, so you don’t have to worry about it locking up your chlorine.
It Doesn’t Over-Saturate Your Water
Another common type of chlorine-based shock is calcium hypochlorite. While it doesn’t contain cyanuric acid, it does contain lots of calcium that will persist in the water long after the shock treatment has dissipated.
Calcium isn’t necessarily a problem for pools. In fact, some level of calcium is often recommended to avoid leaching and etching caused by undersaturated (or calcium-starved) water.
The problem is, too much calcium will oversaturate your water, leading to high calcium hardness, cloudy water, and eventually scaling that can damage your pool surfaces and equipment.
You guessed it, non-chlorine shock doesn’t contain any calcium either.
What Are the Drawbacks of Chlorine Shock?
Aside from MPS not being a sanitizer, there are a few downsides to using non-chlorine shock that aren’t the case when using traditional shock.
Let’s break it down:
It Doesn’t Sanitize Your Water
We touched on this already but it certainly bears repeating.
Unlike chlorinated shock, non-chlorine shock is NOT a sanitizer, so it can only tackle certain types of non-living contaminants. Nothing more.
In practice, that means it can’t be used to deal with serious contamination. This is by far the single most important difference to be aware of when considering chlorine-free shock.
It Can’t Break Down Chloramine
Yes, MPS shock can prevent chloramine formation by oxidizing certain contaminants before chlorine has a chance to combine with it.
However, it can’t break down chloramines that have already formed; it’s just not strong enough.
While you can ignore a build-up of chloramine for a while, at a certain point the clarity and harshness of your water will become problematic. Chlorine is always needed eventually.
It Adds Sulfates to Your Water
Unsurprisingly, MPS shock (monopersulfate) contains sulfate. Sulfate doesn’t pose a problem at lower concentrations, but overusing non-chlorine shock will eventually lead to a high concentration.
While it’s hard to pinpoint the exact threshold, at some point, sulfate can damage concrete or plaster surfaces and even corrode metal.
A high sulfate level is particularly concerning for saltwater pools as it will degrade the plate coatings on the saltwater generator.
It May Show Up as Chlorine
Strangely, even though chlorine-free shock isn’t chlorine, it can still identify as chlorine in your pool test results.
Depending on the type of testing kit you’re using, it will either show up as combined chlorine or free chlorine, which makes balancing your residual chlorine level a challenge.
Waiting for the MPS shock to dissipate is an easy solution to this problem, but it can certainly catch you off-guard if you’re not prepared for it.
So When Should You Use Non-Chlorine Shock?
When you weigh up the pros and cons, it only makes sense to use MPS shock in a few scenarios, and even they assume certain conditions in your water.
1. When You’re in a Hurry
Shocking the pool can put it out of action for the rest of the day, but that’s not the case with non-chlorine shock.
If you can hold off for 15 minutes or so, a quick dose of non-chlorine shock can often be enough to tide you over, at least for a short while.
Of course, this assumes you aren’t dealing with serious contamination. Things like animals in the pool (dead or alive) and early-stage algae can’t be treated with MPS shock; they require a strong sanitizer.
2. When the Pool Gets a Lot of Use
High pool usage adds plenty of contaminants to the water.
This includes non-living organics such as sweat, saliva, urine, and even potentially urine, as well as inorganics such as sunscreens, lotions, and dirt.
Fortunately, non-chlorine shock specializes in dealing with these types of contaminants. Assuming you add it to your water quickly enough, it will do a pretty good cleanup job before chlorine arrives at the scene.
This will help to maintain your water on two fronts; preserving chlorine and preventing chloramine formation.
3. When You Have High Stabilizer or Calcium Levels
Despite adding sulfate to your water, non-chlorine shock doesn’t contribute cyanuric acid or calcium like other shock treatments.
In other words, it may work as a temporary substitute when dealing with high stabilizer or calcium levels, as it can help prevent exacerbating these conditions while still offering some oxidation of non-living contaminants.
Again, this is heavily situational and it won’t cover serious contamination, but it might buy you a little time.
The Bottom Line
Typically, when you shock a pool, you’re trying to “reset” the water.
That means sanitizing all the living organics—like bacteria, viruses, and algae— and oxidizing all the non-living organics — like sweat, urine, and lotions.
But since MPS shock is only an oxidizer, it can only do half the job. It’s also a weaker oxidizer compared to chlorine, and it’s not quite strong enough to deal with a build-up of chloramine (used-up chlorine) in the water.
Overall, despite the many benefits of non-chlorine shock, it can’t fully replace chlorine shock and should only be used alongside it.
