Monopersulfate — the Good, the Bad, and the Ugly

Okay…I will finally admit it. I belong to the ILSWAC (I Love Spaghetti Westerns Addiction Club). I can’t get enough of badly written, badly acted, badly directed Westerns, usually made in Italy from the 1960s and 1970s. The “badder” the better.

Anyway, here’s the good, the bad, and the ugly about a treatment product being used in the pool/spa industry that has gained tremendous popularity in the past 10-ish years: potassium peroxy-monopersulfate (KHSO5), aka monopersulfate or non-chlorine shock. Now, I’m going to be very picky here, monopersulfate is NOT a non-chlorine “shock” since shock infers killing something, and monopersulfate doesn’t kill. It should correctly be referred to as “non-chlorine OXIDIZER” because the product keeps organics from our bodies (and other sources) from developing into combined chlorine (chloramines) — the odorous, itchy, rash-producing form of chlorine that is not a good thing to have. In fact, the industry usually recommends combined chlorine be removed once it reaches >0.2 ppm.

Here’s the good about monopersulfate…

• Contains no chlorine, so it doesn’t produce combined chlorine.
• Has no odor and doesn’t cause irritation.
• Doesn’t require premixing and dissolves fast in normal pool water temperatures (78°F–82°F).
• Simple to use — an oxidizing dose is 2 lbs./10,000 gallons.
• Can be used in chlorine and bromine environments.
• Can be used in hot water environments.
• No restrictions on what time of day to use.
• A short downtime after addition — usually one hour or less.
• By-products produced are harmless.

Here’s the bad about monopersulfate…

• Will not remove existing combined chlorine.
• Lowers total alkalinity.
• Adds to TDS significantly.

Here’s the ugly about monopersulfate…

• Does not sanitize or kill algae.
• Has a low pH of 2.3, so overdosing (more is NOT better) can cause overall pH to drop.
• Results in a false-high combined and total chlorine reading because monopersulfate reacts with the potassium in DPD #3. However, this false-high value can be avoided by using a reagent to mask the interference (contact your test kit supplier for more information).

So with nods to all you young, cigar-chewing Clint Eastwood wannabes out there, using monopersulfate can be a valuable tool in eliminating combined chlorine formation, thus reducing downtime for your aquatic facility.