Measuring turbidity in swimming pools

Swimmer safety can be affected by hazy water; lane markings or other features of the pool. The pool bottom should be clearly visible as water clarity helps to identify swimmers in danger and limit the risks of injury.

As soon as a swimming-pool shows some signs of cloudiness, the challenge for the pool operator is to identify the root cause and offer solutions to reduce the effect before it is too late. Ensuring disinfection and water balance are maintained in the pool water in addition to a clean and efficient filtration system will maintain turbidity levels within acceptable limits.

What is turbidity?

The turbidity of the water is an indication of the amount of particles accumulated and it is used to quantify the degree of cloudiness. These small suspended particles of soil, algae or other materials generally range in size from the microscopic level to about one millimetre. They are often invisible to the naked eye and can be introduced from a variety of sources including air pollution, dirt, pollen, bathers, source water, algae, or microbial organisms.

Why is my swimming pool cloudy?

1. When chlorine is used as a disinfectant, the formation of combined chlorine from the reaction of chlorine in the form of hypochlorous acid and organic wastes introduced from saliva, perspiration, urine in the form nitrogen or ammonia can be related to  a reduction in water clarity. Indeed, combined chlorine compounds have little disinfection potential and a lack of disinfection control can lead to particles and contaminants building up in the water creating algal bloom and cloudiness.

2. Water balance is vital for clean and clear pool water, and the parameters used to calculate the water balance Langelier Index need to be monitored:

  • pH above 7.8 can reduce the efficacy of the disinfectant at oxidizing bacteria, algae, and other organics leading to cloudiness. Adjusting the pH appropriately can remedy this situation.
  • Total Alkalinity is defined as the ability of the water to resist changes in pH. If Total Alkalinity is above 200mg/l, the oxidizing action may also decrease.
  • Total dissolved Solids (TDS) include calcium carbonates, dissolved organics, and inorganics materials. TDS levels of 1500ppm above water supply level can limit oxidation of organic material and cause cloudiness.
  • Algal growth linked to high levels of phosphates can significantly impact the cloudiness of the water and can be noticed as slimy walls. Maintaining phosphate levels below 0.3ppm is also best practice, as they are nutrients of algal.

3. Inadequate filtration is the most significant cause of cloudy water. Several variables can impact the effectiveness of the filtration system:

  • Inadequate filter sizing
  • Inadequate pump capacity and flow rate
  • Inadequate plumbing lines
  • Inadequate filter media
  • Deteriorated cartridges
  • Dirty filters
  • Low pool water levels
  • Poor suction
  • Dead spots
Issues related to water clarity Pool Water Treatment Advisory Group (PWTAG), 2014 American National Swimming Pool Foundation (NSPF), 2014
Disinfectant control For all pools using hypochlorite, and pH of 7.2. Maintain the level of free chlorine at 1mg/l or below, to an absolute minimum of 0.5mg/l. Combined chlorine levels should never be more than half the free chlorine, and never more than 1mg/l no matter what the level of free chlorine. These values can be achieved only where the pool is designed and engineered and operated well with effective pre-swim hygiene and not overloaded. For pool using chlorinated isocyanurates as disinfectant, free chlorine should be maintained at 2.5-5 mg/l and the cynuric acid at 50-100mg/l Maintain free chlorine ,levels below 5mg/l, with an ideal result between 2 and 4mg/l. Maintain combined chlorine levels below 0.2mg/l.
High pH Should be ideally 7.2 – 7.4 Should be ideally 7.4- 7.6, maximum 7.8
High Total Alkalinity Maintain levels below 150mg/l (measured as CaCO3) Maintain levels between 80 and 120 mg/l, maximum of 180mg/l
High Calcium Hardness Maintain levels below 150 mg/l (measured as CaCO3) Maintain levels between 200 and 400 mg/l with a maximum of 1000mg/l
High TDS Maintain levels below 1,000mg/l above the level in the source water. (PWTAG, 2014) Maximum 1500mg/l
High pH Should be ideally 7.2 – 7.4 Should be ideally 7.4 – 7.6, maximum 7.8


Applying enough chlorine to both overcome the chlorine demand and inactivate the microorganisms will help keeping turbidity at low levels, as chlorine is an effective oxidizer. If chlorine levels are not optimum, the use of strong oxidizers can break up organic contaminants and reduce combined chlorine. Flocculants help suspended particles to aggregate forming a floc within the flocculation process. The resulting floc can be then vacuumed or filtered.

What levels of turbidity are acceptable in a swimming-pool?

The Pool Water Treatment Advisory Group in the UK (PWTAG, 2014) states that the pool water treatment system should be capable of providing clarity of no more than 0.5 nephelometric turbidity units (NTU). This limit is followed internationally in particular by Germany, US, Canada, Spain and is mentioned in the Guidelines for Safe Recreational Water Environments Swimming Pools (2006) from the World Health organisation.

The National Sanitation Foundation standard for water clarity in pools is 0.5 nephelometric turbidity units (NTU). At times of peak bather loads, the turbidity is allowed to increase to 1.0NTU but must return to 0.5NTU within 6 hours (NSPF 2005).

Legislation in some countries now imposes an immediate pool closure when turbidity is too high. In Spain, a pool is closed if turbidity rises above 20 FNU, providing a pertinent example of the seriousness of turbidity in pool management.

How can I measure turbidity in a swimming pool?

In recent times, observations in the U.S utilise the Model Aquatic Health Code (August 2014). This recommends a four-inch square (10.2cm x 10.2cm) marker tile, in a contrasting colour to the pool floor or main suction outlet, to be located at the deepest part of the pool.

  • Inadequate pump capacity and flow rate
  • Portable hand-held nephelometers commonly referred to as turbidimeters are designed to be capable of stable, long-term calibrations with accuracy and can measure particulates considerably lower than with previous methods. The nephelometer uses the principle that light passing through a substance is reflected or scattered by particulate matter suspended in the substance. The light reflected at 90° to the light beam, which is specific to a nephelometric measurement, is received by photocells and measured based on calibration with a standard turbidity suspension. The results are expressed in terms of FNU (Formazin Nephelometric Units) – identical with NTU (Nephelometric Turbidity Units). The units of measurement for the EPA method are recorded as nephelometric turbidity units (NTU). The units of measurement for the standard ISO method are Formazin Nephelometric Units (FNU).


It is the responsibility of any aquatic facility operator to keep turbidity at the acceptable standards at all times. An inability to maintain and keep pool water clear and clean can result in unsafe conditions for swimmers and can lead to increased consumption of pool chemicals.


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