Swimming Pool UV-C Disinfection

Pool UV-C runs in a bypass reactor on the circulation loop — never as an in-basin installation. There are two distinct treatment goals, served by different lamp types:

1. Disinfection — 254 nm low-pressure UV as a complement to chlorine
2. Chloramine destruction — medium-pressure broadband UV (200–400 nm)

A single medium-pressure (or amalgam) installation can address both goals at once.

Why Pool UV-C — The Case for It

Lamp Strategy by Treatment Goal

Disinfection only (complementing chlorine)

• Low-pressure 254 nm emitters or amalgam lamps (for larger basins)
• Target dose in the reactor is commonly 40–60 mJ/cm² for disinfection duty
• High germicidal output at modest electrical input
• Typical for private pools and small hotel pools

Chloramine destruction (dechloramination)

• Medium-pressure broadband UV (200–400 nm) is required — low-pressure 254 nm cannot do this job effectively
• Low-pressure UV photolyses only monochloramine; medium-pressure polychromatic UV additionally breaks down di- and trichloramine, the species responsible for the irritant "pool smell"
• The current DIN 19643 mandates medium-pressure UV (UV-M) where UV is used to reduce combined chlorine in public pools
• A practical dechloramination design target is around 60 mJ/cm² averaged over the actual circulation flow (PWTAG TN31)
• Medium-pressure systems draw more power and use more expensive lamps than low-pressure systems

Trade-off to plan for: UV dechloramination is highly effective against irritant chloramines, but laboratory studies show UV treatment can also slightly increase the formation of volatile disinfection by-products (for example chloroform) during the subsequent post-chlorination step. Pool DBP and indoor-air management should be considered as a whole, not optimised for chloramine alone.

Combined installation

• A medium-pressure installation covers both disinfection and dechloramination
• Alternatively, a low-pressure unit plus a separate medium-pressure unit gives more flexibility at the cost of more plant-room space

System Sizing

UV reactors are sized to deliver the design dose at the actual circulation flow, not at a fraction of it. Key principles:

• The bypass flow must equal the circulation flow — a common error is sending only part of the circulation through the UV reactor, leaving the remainder untreated.
• Turnover time = basin volume ÷ circulation rate. Higher bather loads (many swimmers, higher temperature) justify shorter turnover.
• Larger public pools justify multi-lamp arrays or hydraulically validated bypass reactors. Lamp count and electrical rating are site-specific and should be derived from a hydraulic sizing calculation against the target dose — generic volume-to-wattage rules of thumb are not a substitute for design calculation.

Regulatory Aspects

• DIN 19643-1/2/3/4 — treatment of swimming and bathing pool water. Defines hygiene parameters (colony counts, Pseudomonas, Legionella, E. coli), combined-chlorine limits, and UV admissibility. The current edition mandates medium-pressure UV for dechloramination duty in public pools.
• Public-pool acceptance by the health authority — public pools are subject to periodic sampling and a functional verification of the UV installation.
• Private pools — generally not subject to a formal acceptance procedure, but liability considerations apply where a pool is let commercially (holiday rentals, etc.).

Pool Water Characteristics

Common Field Mistakes

1. Under-sizing for peak periods — a UV installation sized only for normal operation fails at the weekend visitor peak. Size with a comfortable buffer.
2. Expecting dechloramination from low-pressure UV — low-pressure 254 nm barely touches di- and trichloramine; effective dechloramination needs medium-pressure UV. Set customer expectations accordingly.
3. Bypass too small — if only part of the circulation passes through the UV reactor, the rest stays untreated. Bypass flow must equal the circulation rate, not a fraction of it.
4. Overlooking the quartz-sleeve cleaning interval — scale and biofilm on the quartz reduce UV throughput invisibly. Periodic visual inspection and cleaning are required.
5. Ignoring lamp end-of-life — UV output declines over the lamp's service life; track operating hours and replace lamps before output falls below the design margin.

Cross-References

• Process-Water Batch & Tank Types — multi-lamp arrangements in large tanks (analogous to pool multi-lamp layouts)
• Cooling Tower Legionella — analogous reasoning (recirculation hydraulics, Legionella)
• UV Lamp Technology — medium-pressure vs. low-pressure detail
• Wavelengths and Action Spectra — chloramine photolysis under medium-pressure broadband UV

Sources

• DIN 19643-1/2/3/4 — Treatment of swimming and bathing pool water (German standard)
• PWTAG Technical Note TN31 — Ultraviolet disinfection: specification, maintenance and validation
• Soltermann et al., Photolysis of inorganic chloramines and efficiency of trichloramine abatement by UV treatment of swimming pool water (Water Research)
• Cassan et al., Effects of medium-pressure UV lamps radiation on water quality in a chlorinated indoor swimming pool (Chemosphere)
• Spiliotopoulou et al., Secondary formation of disinfection by-products by UV treatment of swimming pool water (Science of the Total Environment)
• UV in Swimming Pools and Water Parks — Water Conditioning & Purification (industry overview, Cryptosporidium resistance and UV dose)