The Five Pillars of Pool Water Quality – What Large Bathing Facilities Must Control

Large bathing facilities and swimming pools are not ordinary water bodies. They host hundreds or thousands of bathers daily, each introducing sweat, skin cells, cosmetics, and occasional organic accidents. The water is recirculated, filtered, disinfected, and reused continuously. 

Under such relentless loading, water quality cannot be left to guesswork. Five core parameters—temperature, pH, turbidity, free residual chlorine, and urea—form the foundation of operational management, as stipulated by China's GB 37488-2019 standard for public places. Each tells a different story, and together they determine whether the water is safe, comfortable, and inviting.

Temperature is the most immediately felt parameter. For artificial pools, the standard range is 22–26°C. This is not merely about comfort; it directly influences disinfection chemistry. Higher temperatures accelerate chlorine decay, shortening its protective lifespan, while lower temperatures can cause bather discomfort and cold stress. Large facilities, with their enormous thermal mass, require active heat-exchange systems to maintain stability across changing weather and occupancy loads.

pH governs the entire chemical balance of the water. The mandated range is 7.0 to 7.8. This narrow window serves two critical purposes: it maximizes the germicidal efficiency of chlorine (which drops sharply above pH 7.8), and it protects bathers from skin and eye irritation—acidic water stings, while alkaline water leaves skin dry and hair sticky. In large pools, where bather load and makeup water fluctuate constantly, pH demands real-time adjustment via automated dosing systems.

Turbidity measures suspended particles—skin flakes, hair, dust, and microbial aggregates. The current standard is ≤1 NTU, significantly tighter than earlier limits. Low turbidity is not just about visual clarity; it is a safety prerequisite. Particles shield pathogens from disinfectants; the clearer the water, the more effectively chlorine reaches and destroys its targets. Large facilities depend on high-performance filtration systems—sand filters, membrane units—running continuously, often with coagulant aids, to keep turbidity in check under heavy bather loads.

Free residual chlorine is the primary disinfectant. The standard requires 0.3 to 1.0 mg/L. This concentration must be high enough to kill bacteria and viruses within 30 minutes of contact, yet low enough to avoid irritating chlorine odours, red eyes, and respiratory discomfort. In large pools, chlorine consumption is rapid and uneven; online monitors and automatic feeders are essential to maintain this narrow band across the entire operating day

Urea is the least discussed but most revealing parameter. It originates from sweat and urine, and the standard caps it at 3.5 mg/L. Urea itself is not acutely toxic, but it reacts with chlorine to form chloramines—compounds that produce the characteristic "chlorine smell," consume disinfectant, and irritate eyes and airways. Unlike other parameters, urea cannot be destroyed by filtration or extra chlorine; it must be diluted by continuous fresh-water make-up, typically at 5–10% of total pool volume per day.