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CLOUDY POOL WATER 101: The causes and solutions for uninvited water

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Since 1990, HydroTher Hot Tubs have been the #1 choice of architects, consultants, designers and facility operators for commercial aquatic applications.

Cloudy water plagues both pool owners and veteran pool operators. It can dampen the excitement felt at spring openings or disappoint a pool owner who has just returned home from an extended period away. It can appear just before algae blooms or appear a few days after a pool party or major storm. Not only is cloudy water uninviting, but it may also be a symptom of important issues that warrant investigation. Cloudy water can interfere with visibility. Swimmers using a pool in which key features, lane dividers, or drains are obscured by clouding can also be physically harmful.

For the healthiest swimming conditions, being on top of the water balance, sanitizer, routine cleaning, and oxidizing schedule will help pool owners prevent costly and frustrating remediations. With the 2023 season drawing near, the following are a few strategies to consider for preventing and treating cloudy water.

Defining cloudy water

The pool industry uses the term turbidity to quantify cloudiness by analyzing the presence of suspended particles in water, in nephelometric turbidity units (NTU). The Pool & Hot Tub Alliance’s (PHTA’s) Pool & Spa Operator Handbook advises pool water turbidity to be kept below one NTU during peak pool season, and
for interventions to be able to reduce it below 0.5 NTU in eight hours. For pool operators or owners without instruments to test turbidity, simply inspect the pool and its surfaces while standing on the pool deck. The pool’s steps, deep end, and main drain should be clearly visible. Being unable to see these structures can be hazardous to the safety of users.

Understanding the cause

Diagnosing the cause will solve the problem easier and normal operations can return. Any visible debris should be removed from the water and skimmers to allow for better circulation patterns. This will also prevent organic stains from appearing and remove unnecessary distractions from the sanitizer. Testing the water can point toward any problems with chemical imbalance, low sanitizer residuals, or oxidizer demands. Even if the issue is related to one of these problems, understanding the health and functionality of the circulation and filtration system is crucial.

Adequate water circulation and filtration

In a pool, keeping the water moving is important for the distribution of chemicals and removal of contaminants. The bulk of windborne debris, algae spores, and swimmer waste accumulate along the surface where the disinfectant residual is often at its lowest. This water passes through the skimmer to the filter where undissolved particles are removed. The water may then pass through any chemical or sanitizer feeders where the sanitizer is added before being reintroduced to the pool.

The filtration system should allow for multiple complete turnovers of the water. The flow rate should be adjusted to allow for one turnover every six to eight hours. This allows up to three to four turnovers over a 24-hour period. For pool owners concerned with energy efficiency, a variable-speed pump (VSP) adjusted to a low speed during the night can keep the water moving and filtering.

Filter maintenance

An important, yet, neglected routine is filter maintenance. Each pool season should begin with a clean filter and periodic assessments of its performance throughout the season. Too often, filters will only be cleaned upon encountering cloudy water. Cloudy water, not caused by inadequate sanitization or improper balance, might be due to an underperforming or overly soiled filter. A quick examination of the filter pressure gauge reading above 137 kPa (20 psi) or low flow coming from the returns can be indicators of a filter needing service.

Simply rinsing a cartridge filter or backwashing a sand filter may leave behind more complex organic or mineral waste products. When to clean them can often depend on the type of filter or certain treatments that have been performed recently. Each filter type comes with unique challenges. Sand filters struggle more with tinier particles and might need additional help with filter aids, such as charged cellulose or diatomaceous earth (DE), which can sit atop the sand bed to better capture debris. This filter media may also develop channels or cracks running through the sand bed, allowing unfiltered water to pass through.

Cartridge filters might be able to capture smaller debris compared to sand, but it might need more cleaning or replacement after significant events. Dead algae and clarifier treatments often clog up these filters quickly. It is a good practice to have a second set of fresh cartridges to replace soiled ones while they are being cleaned.

Choosing the right type of cleaner will help when rinsing is not enough. If the buildup is mostly comprised of waste from heavy use, such as organic material or body oils, an alkaline formula with detergents and surfactants is preferred. Accumulated contaminants can also be region-specific. Pools in a calcium-rich water region or with a higher concentration of metals will need an acidic cleaner to neutralize these minerals. Keeping the filter healthy will go a long way in maintaining clean and clear water. If the problem is not alleviated by a clean filter, the issue might be mechanical due to cracked laterals or manifolds.

Coagulants and flocculants

Cloudy water in pools forms due to the accumulation of microscopic contaminants which are too small for the filter to collect. In recreational water, these contaminants usually have negative charges. Debris remains in colloidal suspension, where the similar charges combined repel each other so the particulates cannot settle.

Particulates will either need to be broken down by oxidizers, bridged with cationic coagulants for the filter to remove, or heavy enough to drop to the pool floor with flocculants. Coagulants, since they are slow acting, should be used as soon as water begins to haze, or even in regular maintenance to stop problems from forming. A flocculant, which can only be used in pools that allow vacuuming to waste, is meant to treat extreme cloudiness. They are effective in getting rid of dead algae following algicidal treatments. Water restoration is quicker, but vacuuming is necessary to remove material from the bottom. This keeps flocculant usage limited to sand or some DE filters if the manufacturer recommends.

Coagulants can work wonders for both preventive and curative treatments, but often take time to improve. In many instances, the mechanisms of coagulants and flocculants occur in succession. Coagulation neutralizes the negative charge on contaminants to bind them together into small clumps that are more manageable for the filter, whereas flocculation binds them into heavier and larger clumps that will eventually separate from the water.

Although these particles are larger, it is still unlikely to see them with the naked eye. What will be evident is clearer water surrounding the weighted particles. This reveals the contaminant charge has been neutralized for the flocculant mechanism to take place and drop out the particulates. Aluminum-based salts are high performing flocculants and clarifiers that function under most chemical balances. Some commercial flocculant materials may require more dosages and pH alterations.

Coagulants are commercialized in two forms: natural and synthetic. Both have similar and efficient mechanisms, but many pool operators might prefer one over the other. For a more natural route, chitosan is an option. In addition to the aquatics industry, it is used by many others including pharmaceutical, biochemical, cosmetic, and water treatment.

Chitosan is produced by extracting chitin, the supportive material of crustaceans recycled from the food preparation industry. Through some chemical processing, this biodegradable polysaccharide chain of glucosamine and a positively charged amino group becomes chitosan. It has a carbohydrate structure not unlike cellulose. It floats along the top few layers of the water where it seeks attractive contaminants such as oils, metals, or dirt to neutralize their charges and coagulate them into larger clumps for better filtration. Unlike some remedial products, chitosan will function regardless of the water chemistry and will not be impacted by sanitizer residual.

Synthetic alternatives to chitosan are often comprised of ammonium co-polymers. Their long chains stick to and bridge together entrapped particles, enlarging them for the filter to hold onto easier. Like chitosan, they also function under a wide range of pH environments. Maintenance technicians should exercise caution when dosing with these products as an overcorrection can cause the negatively charged material to be coated in a shell of positively charged material, resulting in a situation resembling the original problem.

Similarly, charged particles will not settle out or reach the filter. The now positively charged coating makes it harder to remove. For some high-powered clarifiers, synthetic polymers can be paired with additional clarifiers. These materials work together in a more concentrated dosage, resulting in a faster response. They are sometimes available in dosages better suited for weekly maintenance. Some products are even designed for use when a stronger trouble-shooter may be needed, such as spring opening.


Deciding how to treat cloudy water can be frustrating, but timely intervention can help avoid costly treatment plans. Prevention will always be key. Weekly activities such as brushing, remaining on top of sanitization, and adequate circulation may seem inconvenient, but it will work wonders. It is much like housekeeping. A little tidying each day is certainly preferable over wasting away multiple days with deep cleans.

By Emily Johnson

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