Module 10 Deep Dive: Pools, Spas & Recreational Health

The chlorine math, turnover times, and fecal-response protocols that make Module 10 one of the most number-heavy sections on the REHS exam.

Module 10 rewards candidates who memorize numbers cold and punishes candidates who half-remember them. Unlike food safety, where you can often reason your way to the right answer, pool and spa questions usually have one exact correct figure and three distractors that are close enough to trip you up. There's no partial credit for "somewhere around 2 ppm."

This module also leans harder on 22 CCR and the California Building Code than most other sections, so the citations matter as much as the numbers themselves.

The numbers you have to know cold

These are the values most likely to show up as a direct recall question, not a scenario:

Breakpoint chlorination

Combined chlorine, better known as chloramines, is the weak-sanitizing byproduct that causes the "chlorine smell" and eye irritation people blame on too much chlorine, when it's usually too little. The fix is breakpoint chlorination: raising free chlorine to roughly 10 times the combined-chlorine reading to oxidize the chloramines and restore a clean free-chlorine residual.

Expect a calculation question here. If combined chlorine reads 0.6 ppm, the exam wants you to know you're raising free chlorine by roughly 6 ppm, not adding a fixed dose regardless of the reading.

Fecal accident response

This is the section most candidates under-prepare for because it has three different protocols depending on stool type and whether CYA is present. Per CDC guidance, which California pool operators follow:

• Formed stool, no Cryptosporidium suspected, no CYA in use: raise free chlorine to ≥2 ppm, hold at pH 7.5 for ≈25 minutes.
• Diarrheal stool, Crypto suspected, no CYA in use: 20 ppm free chlorine for 12.75 hours.
• Diarrheal stool, Crypto suspected, CYA in use: 40 ppm free chlorine for 30 hours, since CYA suppresses chlorine's effective killing power and requires a longer contact time to reach equivalent disinfection.

The exam's favorite trap here is giving you a diarrheal-stool scenario with CYA in use and offering the no-CYA answer (20 ppm × 12.75 hr) as a distractor. Read for CYA status before you pick an answer.

Cryptosporidium: why it dominates this module

Crypto oocysts are highly chlorine-resistant, which is exactly why the fecal-response protocols above exist and why they scale so aggressively when Crypto is suspected. Standard chlorination that kills bacteria in seconds can take over a day to neutralize Crypto at typical operating concentrations. Control relies on filtration, supplemental disinfection like UV or ozone, and the hyperchlorination protocols above, not routine chlorine levels alone.

Entrapment prevention: the VGB Act

The Virginia Graeme Baker Pool & Spa Safety Act (2008) mandates anti-entrapment drain covers and secondary suction-entrapment protection, codified in California at H&SC §116064.2. The current consensus standard for compliant drain covers is ANSI/APSP-16. If a question describes a pool with a single, non-compliant drain cover and no secondary protection, that's a VGB violation regardless of how well the water chemistry checks out elsewhere.

A related but separate requirement: certain public pools must have an AED on site under H&SC §116045(b). Don't confuse the drain-cover requirement with the AED requirement; they come from different code sections and test as separate facts.

Filtration and testing basics

Typical maximum filtration rates vary by media type, and the exam expects you to know the rough order even if not the exact figure: sand filters run around 5 gpm/ft² (high-rate sand systems up to 15), DE filters around 2 gpm/ft², and cartridge filters around 0.375 gpm/ft². Cartridge is the slowest and DE sits in the middle.

For testing, the DPD method is the standard field test for both free and combined chlorine residual. And don't overlook a low-frequency but real exam topic: depth markings are required both on the deck and at or above the waterline, so bathers in the water can see them too, not just people standing on deck.

How to study this module efficiently

Build a single reference table with every number in this article, plus the citation next to each one. Module 10 questions are frequently pure recall, so flashcard-style repetition works better here than it does for scenario-heavy modules like Food Protection. Once you have the numbers cold, layer in the fecal-response decision tree specifically, since that's the part most candidates get right on individual facts but wrong when they have to pick the correct protocol under time pressure.

The bottom line

Module 10 is unusually learnable because nearly everything in it reduces to a specific number tied to a specific condition. Get the CYA-dependent chlorine minimums, the turnover times, and the three-tier fecal response memorized, and you've covered the large majority of what this module actually tests.