The Importance of Water Balance In Pools – Part 3

A woman diving into a swimming pool, with the words, RWF Management, The Importance of Water Balance in Pools, Part 3

The previous installments of this article explored the collective impact of chlorine, pH, total alkalinity, and calcium hardness on maintaining safe and stable water conditions in aquatic facilities on board.

In this concluding article, we turn our attention to the remaining factors that influence the equilibrium of recreational water, Total Dissolved Solids and Temperature, to complete our comprehensive overview on the importance of water balance in pools.

Understanding Total Dissolved Solids

Keeping with the analogy used in the earlier parts of this article, if there were an awards ceremony similar to the Oscars for the field of maritime public health, it’s unlikely that total dissolved solids (TDS) would make any shortlist for recognition. While chlorine and pH are undoubtedly the stars of the water treatment show on board, the role of TDS is often overlooked.

Total dissolved solids refers to the overall concentration of dissolved substances within an aquatic venue’s water. Essentially, it’s an indicator of the total quantity of matter in an RWF that is not H20. Some of these substances will have been intentionally added to keep the water clean and safe, such as chlorine and pH increasers & reducers, while others may have been introduced into a facility through rain or organic waste from bathers. Consequently, TDS levels are in a constant state of flux, influenced by various environmental factors that alter the water’s chemical balance.

How to Measure TDS

To determine the amount of total dissolved solids in an RWF, one can utilize either a test strip or digital meter. It’s necessary to take two samples: the first from the source water in the pump room, and a second directly from the tub (i.e., the pool itself).

The true TDS value is found by calculating the difference in readings between these two samples. For example, if the reading from the pump room is 300 ppm, and the measurement from the tub is 1,200 ppm, the actual TDS level within the facility is 900 ppm.

The latest edition of the Model Aquatic Health Code (MAHC) recommends that the optimum concentration of total dissolved solids in recirculating freshwater RWFs should be approximately 1,500 ppm, however, there is no concrete scientific evidence to support any specific threshold as definitive. The MAHC’s recommended limit is based on the understanding that excessive levels of dissolved materials can cause problems in pools, including staining, cloudy water, a reduction in disinfectant efficiency, and algae growth.

How TDS Affects Water Chemistry

Detecting a rise in TDS values serves as an indicator of chemical overloading or a lack of dilution—meaning a sufficient volume of fresh water isn’t being added to the pool at regular intervals. Nonetheless, the total dissolved solids in an RWF will always be increasing due to the continuous influence of external elements on the water’s chemical composition.

In venues where the water in the entire system is routinely replaced, such as whirlpools spas on cruise ships which must be completely emptied and refilled at least every 72 hours, TDS-related issues are unlikely. Conversely, for swimming pools and spa pools that aren’t drained as often, monitoring TDS can be an effective way to identify the accumulation of contaminants.

High TDS levels can cause numerous issues, including:

  • Decreased chlorine efficiency: Excessive dissolved solids can hinder the ability of facility operators to maintain chlorine levels within their required range. The presence of these solids interfere with the efficacy of the free chlorine, compromising its disinfection powers. This occurs as the water reaches a high saturation point—a concept discussed later on in this article—which disrupts the chemical processes necessary for chlorine to act as an effective sanitizer.
  • Salty water: The water may acquire a noticeable salty taste when TDS thresholds are exceeded. This is often due to the presence of sodium (i.e., salt) in many of the chemicals utilized in recreational water facility management, such as sodium carbonate, sodium bicarbonate, sodium bisulfate, and sodium hypochlorite. Over time, the use of these chemicals results in a buildup of sodium levels in the pool, causing the water to taste salty.
  • Corrosiveness & scaling: Elevated levels of TDS may render the water aggressive, causing corrosion to metal fixtures, or the precipitation of mineral deposits on surfaces, piping, and equipment. Bathers are likely to report experiencing discomfort when using the facility, due to skin and eye irritations.
  • Cloudy water: Cloudiness in water isn’t directly caused by total dissolved solids, as they represent substances which have dissolved and are thus no longer visible. However, if the water becomes oversaturated, its clarity may be affected.

The Meaning of Saturation

Before discussing how the final component, temperature, affects water balance, it’s important to understand the concept of water saturation. In simple terms, it refers to the point at which a liquid cannot dissolve any more substances at a given temperature and pressure; it reaches a state where it’s in “equilibrium.” It’s akin to mixing sugar into coffee; initially, the sugar grains disappear, but add too much, and eventually the coffee reaches its saturation point and can’t dissolve any more.

When the water in an RWF is undersaturated or oversaturated, it will either eat away at the pool’s surfaces and equipment causing etching and corrosion, or leave limescale deposits—a chalky substance composed of calcium carbonate (CaCO3). The Langelier Saturation Index (LSI) is a useful tool that measures the saturation equilibrium of the calcium carbonate to predict water’s corrosive or scale-forming tendencies.

In a future article, we will explore the intricacies of using the Langelier Saturation Index to assess water balance in recreational water facilities. To summarize, an LSI value of zero signifies the water is in perfect balance, containing just the right amount of calcium carbonate. A negative LSI value indicates a deficiency, which results in corrosive water that seeks to extract calcium from the most readily available source: the pool’s infrastructure. On the other hand, a positive LSI value points to excess calcium carbonate, leading to potential scaling issues as the water precipitates the surplus.

Perhaps unsurprisingly, since they are the topic of discussion for this article, in order to calculate the LSI value, there are five things you need to know about the water in your pool: namely, pH, total alkalinity, calcium hardness, total dissolved solids, and temperature.

The Impact of Temperature on Water Balance

The temperature of water is an important factor in the Langelier Saturation Index. Water temperature affects the LSI value because the solubility of substances, like calcium carbonate, varies with temperature.

When the water is warmer, it dissolves more minerals, which consequently raises the LSI value, meaning that the water is less likely to corrode and more prone to forming scale. Conversely, cooler water holds fewer dissolved materials, resulting in a lower LSI value and more aggressive water; compared with warm water, cold water needs more calcium to maintain its balance.

For RWFs located on a vessel’s outer decks, water temperature can serve as a dynamic guide for maintaining proper chemical balance, both when sailing in warmer climates and colder conditions. Operators can utilize the temperature of a pool as a starting point for all chemical treatments. By doing so, they can predict how a facility’s calcium and alkalinity needs will fluctuate with the weather to prevent any adverse consequences such as corrosion or scaling.

Water naturally seeks a state of equilibrium: either you balance the water accordingly, or the water will balance itself. For example, if it lacks sufficient calcium and this is not supplemented through chemical treatment, the water will extract what it needs from its environment until it’s satisfied. Therefore, the goal of an RWF operator is to both establish, and consistently maintain, water balance in aquatic venues to prevent any damage from occurring.

Individuals responsible for the management of recreational water facilities on board are essentially the directors of their own water treatment show. Ensuring the chemical balance in aquatic venues is in perfect harmony, is crucial for both bather safety and the longevity of the facility’s infrastructure. Achieving this equilibrium will not only save time, money, and effort in the long run, but may also earn these “directors” accolades—a company’s nod to excellence, akin to receiving an Oscar for their stellar performance.