Commercial swimming pools treatment

Ozone in pool water purification

Necessity of swimming pool water treatment

Various contaminants are introduced to the swimming pool from the swimmers and the surrounding environment. The contaminants range from waterborne infectious viruses and bacteria to dissolved and undissolved matter (mostly organic). This leads to a need for an effective water treatment to ensure a healthy swimming pool environment.

Table of contents

The aim of this page is to provide a wide overview of fundamentals in swimming pool water treament. As will be shown below, ozone opens up new possibilities significantly increasing water treatment performance and facilitating operation. 

  1. Chlorination problematics
  2. Existing treatment solutions
  3. Chlorination
  4. UV-treatment
  5. Ozonation
  6. The ozonation process
  7. PFD for swimming pool water ozonation
  8. Disinfectant overview and comparison
  9. Disinfectant efficiency comparison
  10. Disinfectant cost comparison
  11. Ozonetech advantages
  12. Process design

Chlorination problematics

Different methods have been used to reach a sufficiently high water quality. Among the most common methods are filter screens and sand filtration, chemical treatments (e.g. for flocculation and water softening), and disinfection with chlorine/ozone/UV. Different technologies are often combined depending on contaminant level and desired water quality. The by far most common disinfection method today is chlorination. However, chlorination is also associated with complications regarding health effects among swimmers. For example, in 2014 the Scientific Journal Environmental Science and Technology published an article which shows that uric acid, (and chemically similar compounds from body fluids) and chlorine form cyanogen chloride (CNCl) and trichloramine (NCl3). These two compounds are associated with mucous irritation (Lian et al. 2014) (Chen et al. 2007) and respiratory damage (NIOSH, September 2015) which may prevent swimmers from using the swimming pool. Other known by-products from chlorination are trihalomethanes (THMs) that cause health damage. By using other disinfectants than chlorination these effects can be mitigated. For example ozone treatment enables the use of very low amounts of chlorine, reducing water consumption, energy demand, and mitigating negative health effects caused by chlorine byproducts.

By-products from chlorination

By-products from chlorination: Uric acid, trichloramine & cyanogen chloride


Existing treatment solutions

A standard design for swimming pool water treatment is typically based on the following treatment steps; filtration, disinfection, chemical treatment, and water replacement.

Filtration is commonly performed in two stages where the first stage removes larger pollutants like hairs and the second stage removes small particles.

Disinfection leads to deactivation of microorganisms and chemical decomposition of organic material. This is commonly achieved by chlorination of the water treatment stream but can also be achieved by ozonation and UV-treatment. These different alternatives all have their advantages and drawbacks.

Additionally, chemical treatment is often used to soften the water (i.e. remove calcium and magnesium ions) or for flocculation (i.e. neutralization of negatively charged particles which causes them to form larger particles which can be removed by filtration.

Chlorination

So called “chlorine”, in colloquial language, actually covers an entire group of substances. Common chlorine compounds are e.g. chlorine gas, sodium hypochlorite (liquid), calcium hypochlorite (granular), in situ electrolysis of NaCl solution, chlorinated isocyanurates (stabilized chlorine), and chlorine dioxide. These chlorine substances all share the property of forming free chlorine in water solution which is readily available for water disinfection. Free chlorines are typically hypochlorite ions (OCl-) and hypochlorous acid (HOCl). 

In the chlorination process the level of chlorine has to be balanced to enable sufficient disinfection and at the same time minimize discomfort for swimming pool users. The main discomfort issues include mucous tissue irritation. According to the WHO the level of free chlorine should not exceed 3 mg/L in public and semi-public swimming pools. The maximum combined chlorine level for all temperatures at pH 7.2 – 7.6 should not exceed 0.4 mg/L. All chlorine based chemical treatments share the same problem; they all lead to buildup of combined chlorine which is the source for the above mentioned discomfort. Therefore, when using chlorination fresh makeup water has to be added regularly to the system to dilute and maintain an acceptable level of combined chlorine. When using chlorinated isocyanurates the maximum level of cyanuric acid proposed by the WHO is 100 mg/l. 

Chlorination oxidation agents

Chlorination oxidation agents: chlorine gas, hypochlorous acid & chlorinated isocyanurate

UV-treatment

Ultra-Violet radiation is another technology used for swimming pool water treatment. A UV-lamp is used to generate radiation in the UV-spectrum which efficiently deactivates microorganisms, virus, and algae by physically destroying the DNA. It is often used as a complement to chlorination which then enables a reduction of chlorine consumption. On the other hand, UV-radiation also breaks down some of the chlorine which therefore has an increasing effect on the demand for chlorine. 


Ozonation

Ozone is generated in-situ unlike traditional chemicals used for disinfection and cleaning. Ozonation utilizes naturally occurring oxygen which eliminates the need for chemical handling procedures. Ozone is produced by imposing a high voltage across a dielectric discharge gap (Corona Discharge) which ionizes oxygen atoms and forms ozone molecules. Ozone is a much stronger oxidant than chlorine and once applied ozone instantly reacts with contaminants, leaving no byproducts.
The main mechanisms for ozone disinfection include: destruction of microorganism cell walls, radical oxidation reactions, decomposition of nucleic acids (DNA and RNA), and breakage of carbon-nitrogen bonds (which are essential in most organic compounds like e.g. proteins). Two main factors influence the effectiveness of ozone disinfection, namely the contact time and ozone concentration. 

Ozone molecule

The ozone molecule


The ozonation process

The ozonation process itself involves five main steps, namely feed-gas preparation, ozone generation, contacting, ozone reaction, and ozone destruction:

PFD ozonation

Ozonation process overview

The feed-gas to the ozone generator is added either in the form of purified oxygen or atmospheric air. Purified oxygen can be delivered to the swimming pool site but is preferably also produced on-site from atmospheric air with an air separation unit generating a 93 % oxygen feed.

Using high purity oxygen feed in combination with an effective cooling system, Ozonetech offers a highly compact ozone generation design. Ozonetech supplies systems with production capacities ranging from 5-5000 grams O3/hour.

Ozone gas is effectively dissolved into the swimming pool water side stream using venturi injection. This way, as much ozone as possible is transferred into the water for subsequent disinfection in the reaction tank as depicted in the process flow diagram below.

The dissolved ozone is evenly distributed into the ozone reaction tank. This is where the disinfection process takes place. It is important to provide enough contact time to achieve efficient disinfection results. To optimize the disinfection process three key parameters have to be controlled, namely ozone concentration, contacting, and reaction time. The final step in the ozonation process involves ozone destruction of the off-gas to ensure a healthy working environment for operators.

PFD for swimming pool water ozonation

PFD for pool water ozonation

Process flow diagram for swimming pool water ozonation

Disinfectant overview and comparison

To give an overview of the most common treatment alternatives the benefits and challenges of ozone treatment, UV-treatment, and chlorination were compared in the table below:

Benefits and challenges of three typical disinfectants

BenefitsChallenges
Chlorination
  • Relatively low installation cost
  • Relatively effective disinfectant
  • Provides residual disinfection
  • Leads to formation of unhealthy compounds
  • High water and energy consumption
  • Emissions of chlorinated compounds
  • Requires continuous addition and handling of toxic chemicals
  • Corrosive effects
UV-treatment
  • Relatively low installation cost
  • No chemical use
  • No residual disinfection
  • No direct oxidation potential
  • Dosing complications
  • Fouling problems
  • Turbidity build-up
Ozone treatment
  • Very effective disinfectant
  • Low operation cost
  • Environmentally friendly
  • Greatly reduces the required chlorination amount
  • Biocidal efficiency not dependent on pH
  • No harmful residual byproducts
  • Increases sand filter effectiveness
  • Reduces water consumption
  • Higher installation costs
  • No residual disinfection


Disinfectant efficiency comparison

Based on CT-values for virus disinfection provided by EPA a graphical comparison is made between chlorine, chloramine, chlorine dioxide, and ozone. A low CT-value represents an efficient disinfectant.

CT-value comparison for main chemical disinfectants

CT-value comparison for main chemical disinfectants

Because of the high CT-value of chloramine an additional figure is presented below to show the relation between only ozone, chlorine dioxide, and chlorine.

CT-value comparison for main chemical disinfectants (except chloramine)

CT-value comparison for main chemical disinfectants (except chloramine)

The graphs clearly show the small amounts of ozone needed for disinfection, placing ozone on top as the most effective commercial disinfectant.

Disinfectant cost comparison

An estimated cost comparison between hypochlorite treatment and ozonation is shown below based on a 25 × 25 m commercial swimming pool which is operated 350 days per year. The comparison includes the hypochlorite chemical costs for chlorination and the power consumption costs for an ozone generator coupled with an oxygen concentrator.

Hypochlorite vs. ozone chemical cost

Cost comparison of chlorination vs. ozonation


Ozonetech advantages

Ozonetech offers premium state of the art technology with high reliability and efficiency, low energy consumption, and low maintenance cost. Other advantages with Ozonetech include:

  • Long lifetime of the ozone generator. This is partly due to the effective cooling system and the fact that concentrated, filtrated, and dried oxygen is fed to the generator.
  • Ozonetech offer turn-key facilities, ensuring reliable process operation.
  • Compact design and the possibility to increase disinfection capacity due to modular generator design.
  • High concentration of ozone, maximizing treatment effectiveness

Process design

The equations presented in the following paragraphs may be used to estimate the capacity requirements for a potential ozonation process for swimming pool water treatment.

Treatment capacity

To determine the capacity requirements of the water treatment system the most important factors include: swimming pool size, bathing load, pool type, and pool temperature. To calculate the flow rate/capacity of the treatment system the following equation may be used:

Water treatment capacity

Water treatment capacity

T is the time it takes for the equivalent of the entire swimming pool volume to be recirculated in the water treatment system. A value of d/T = 0.55 represents a “worst case” value and this value will be used for further reference. Also, an f-value of 0.8 is used as it is a common value for Olympic pools.
Ozonation is applied to a bypass stream which is usually in the range of 25 % of the main water treatment flow. This gives the following expression for the flowrate through the ozonation system, “Q(byflow)”, which requires ozonation:

Water by-flow for ozone treatment

Water by-flow for ozone treatment

For 28 ⁰C or 33-35 ⁰C the water requires an ozonation of 0.8 and 1.2 mg/l respectively. The contacting equipment allows for about 90 % dissolution efficiency of the generated ozone. However a dissolution efficiency of 0.8 may be used for extra margin. Furthermore the ozone generator capacity decreases over time. Hence, a decrease in capacity of 10 % can be used (to be conservative). To estimate the required ozone production, “Q(O3)”, of the generator the following formula could be used:

Required ozone demand

Required ozone demand

Reaction tank

To estimate the size of the reaction tank a minimum reaction time of 2.5 minutes should be used given an ozone concentration of 1 mg O3/L. This is equivalent to a CT-value of 2.5 (mg · min/L) which is definitely safe and sufficient for swimming pool water. As an example a by-flow (to the ozonation system) of 60 m3/h requires a reaction tank volume of 2.5 m3. 

Ozone tech