The Complete Guide to Biofilm in Hot Tubs


One of the most important parts of owning a hot tub is keeping every clean and sanitary. With each soak, thousands of bacteria are introduced, which can quickly develop into a problem.

In this article, we’re going to take a look at one of the most common problems spa owners face: biofilm. By the end, you’ll understand what biofilm is, why it forms, how to know if there’s a build-up, how to get rid of it, and the steps you can take to prevent it from forming.

What is biofilm?

Biofilms are living, reproducing microorganisms that exist as a colony or community. In other words, they are alive and have a complex social structure that both protects them and allows them to grow.

Biofilms consist of many species of bacteria, algae, protozoa, fungi, yeasts, and other microorganisms, along with non-living debris and corrosion products.

They form when certain microorganisms stick to the surface of an object in a moist environment and begin to reproduce. The microorganisms attach to the object’s surface by secreting a slimy, glue-like substance. 

Bacteria in biofilm secrete a plastic-like substance called extracellular polysaccharides (EPS). This substance acts as a protective layer around the cells in the biofilm and binds them together.

Biofilms can form on any surface, including metals, organic material, plastics, medical implants, and the walls of a hot tub. For biofilms to form, they require a combination of moisture, nutrients, and a surface on which to grow. 

Biofilms are very common in hot tubs. They feast on organic matter, such as carbon or dead skin cells. They are unhygienic and reduce the cleanliness of the water. Most bacterial infections that occur in hot tubs involve biofilms. Biofilms can harbor disease-causing bacteria that jeopardize the safety of bathers. 

Is biofilm harmful?

Biofilms are most certainly harmful, especially when the pathogens in biofilms become planktonic. Planktonic means they can move freely in the water and come into contact with bathers. However, the biofilm won’t be harmful if the pathogens are locked and can’t move freely.

Disease-causing germs such as Giardia and E. Coli can develop in biofilms. These microorganisms can make you sick, and in the worst cases, they can be fatal. For example, E. Coli can lead to painful stomach cramps, diarrhea, and vomiting. 

The most common symptoms of biofilm are rashes, itchiness, and tender skin. These symptoms typically disappear after a day or two, but if they persist or sores start forming on your body, this is a sign of something more serious. 

It could be folliculitis, a type of bacteria that thrives in a hot tub that isn’t properly sanitized. Folliculitis is often mild, but extreme cases will require antibiotics. So knowing what to look for and how to eliminate it is crucial.

The longer the biofilm in a spa goes untreated, the more dangerous it becomes. This is because the chlorine or bromine in the spa water rapidly breaks down as it fights biofilm. In turn, the water becomes more acidic and can irritate the skin.

Skin problems are not the only harmful effects of biofilm. Biofilm can also cause eye, ear, and respiratory infections. As the bacteria mix with the bubbles from the jets, they get aerated and rise as tiny droplets that can be inhaled or enter your eyes and ears.

Biofilm can also cause other health conditions like Legionnaires’ disease (a form of pneumonia infection) and urinary tract infection. 

Apart from health issues, biofilm affects hot tub performance by clinging to the inner surfaces of the plumbing, which reduces the water flow. Consequently, the pump might burn out from working excessively hard to circulate the water. 

In addition, the continuous build-up of slime on the surfaces of the hot tub can cause discolorations that are difficult to remove. It can also cause cloudy water, foamy water, and a visible oil ring at the hot tub’s waterline. 

Biofilm also greatly increases your chemical costs since you need to add more chemicals to eliminate the biofilm in the hot tub. Also, adding more chemicals can cause water discomfort, forcing you to drain and refill the hot tub more quickly.

What causes biofilm formation in a hot tub?

Environmental stresses

Bacteria form biofilms in response to environmental stresses like UV radiation, extreme pH, desiccation, high salt concentrations, extreme temperature, high pressure, limited nutrients, and antimicrobial agents. 

Level of use

Biofilm formation depends on how frequently you use your hot tub. If your hot tub has been sitting unused, biofilm will start to build up inside. Even if the hot tub is drained, biofilm will form in the water left inside the plumbing.

Not only does biofilm form when you don’t use your hot tub for a while, but it can also come from active, normal use of your spa. In addition, spas that have high usage are likely to experience biofilm build-up. 

Organic matter

Dead skin cells, cosmetics, body oils, body lotions, sweat, and other organic matter are building blocks of biofilm. They establish colonies in low turbulence areas of your circulation system and attach to surfaces when the pump shuts off.

Poor maintenance

If your hot tub is poorly maintained, biofilms will start to grow. Poorly maintained spas include those with old or dirty filters, inadequate sanitizer levels, and unbalanced water chemistry. It also includes hot tubs that are not shocked with a chlorine-based shock on a regular basis.

How do you know if your hot tub has biofilm?

Biofilm is rarely seen because it primarily builds up in the pipes and other internal parts of a hot tub. It’s almost impossible to identify it since it’s nearly microscopic, especially in its early stages. 

If you can disassemble your hot tub jets, you can inspect inside for any layers of oily or slimy substances, usually in a brownish shade. Also, hot tubs with a scum ring that develops around the edge of the waterline or behind the spa pillows may have a biofilm problem.

A biofilm build-up in a spa will show some or all of the following signs: cloudy water, bad smells, excess foam, or slime. Foam on the water’s surface can indicate that the water is getting full of total dissolved solids, which can eventually lead to biofilm if left unchecked.

Another indication of the presence of biofilm in a hot tub is a rotten egg odor. The smell of rotten eggs in your hot tub is most likely caused by hydrogen sulfide gas, which is caused by biofilm.

If you notice a plastic smell in your hot tub, biofilm is most likely present inside the plumbing. It usually forms when the manufacturer is testing the system to ensure that there are no leaks. This is very common in new hot tubs. 

During the test, water is passed through the plumbing system and then later, it is drained. Although the water has been drained, the plumbing is still damp and therefore is an ideal environment for contaminants such as biofilm to grow.

So when your new hot tub is filled and ready for use, biofilm can make its way from the plumbing system into your jets, filters, and other parts of your spa. Most reputable manufacturers sanitize and air-dry the piping after testing the hot tub to ensure that it is not a breeding ground for bacteria while sitting in storage.

How to get rid of biofilm

Remove filters 

The first thing to do before adding chemicals to the hot tub is to remove the filters and clean them thoroughly. Removing the filters also helps to prevent them from clogging as the biofilm remover gets to work.

Give the filters a quick rinse using a garden hose (never use a power washer). After rinsing, place them in a large bucket filled with diluted filter cleaner and leave them to soak for 24 hours.

After 24 hours, remove the filters from the bucket and rinse thoroughly with fresh, clean water before placing them back in your spa. If the filters remain dirty after soaking, you’ll need to replace them. Filters should be replaced every 12 months regardless of how much they’ve been used during that time.

Add plumbing cleaner

To remove the biofilm in the plumbing, you’ll need to add a plumbing cleaner, also known as a line flush. In my opinion, the best biofilm cleaner is Ahh-Some. I recommend it over other popular cleaners like Oh Yuk, Leisure Time, and BioGuard.

Ahh-Some Hot Tub Cleaner is a unique and environmentally-friendly gel formula that removes the build-up of bio-fouling inorganics, lotions, cosmetics, oils, exfoliated skin, and other contaminants from the plumbing, jets, and hot tub shell.

Add one teaspoon per 125 gallons (5g per 600 liters) of Ahh-Some to your hot tub, turn on all the jets, and allow it to circulate for at least an hour. 

If you’ve never used a line flush in your hot tub before, or the hot tub has been sitting unused for a long time, I recommend letting the flush circulate for several hours or, even better, overnight.

As the cleaner gets to work, it’ll cause scum to form on the surface of the hot tub water. This indicates that the cleaner is getting the job done and removing all the biofilm from the pipes. You should note that regular shocking can prevent biofilm formation, but it won’t kill a mature colony that has been allowed to grow.

Drain the water

After cleaning the hot tub, the next thing is to drain the water. To avoid the chance of electrocution, cut all power to the hot tub before draining the water.

Draining a hot tub can take a long time; however, you can speed up the process by using a submersible pump. If you don’t own a sump pump, you can use your garden hose by connecting it to the spa drain.

Ensure you drain the hot tub water into the sewer system. Don’t dispose of the water in a storm drain because they lead to natural bodies of water. Disposing spa water into storm drains harms fish and wildlife and carries hefty fines.

Clean the hot tub shell

Use a surface cleaner to clean the hot tub shell. Pay attention to any small places where bacteria could be hiding. Allow the cleaner to work for 15 minutes.

After using the surface cleaner, use a soft microfiber cloth to remove the chemical residue. Then rinse the hot tub shell thoroughly. If you don’t remove all the chemical residue, it might cause foaming after you refill the spa with fresh water.

Ensure that all the jets are open once you’ve finished cleaning. This helps to reduce the risk of water pressure problems caused by trapped air.

Remove all wastewater

After draining the hot tub, there’ll still be some water sitting in the footwells. Some hot tub owners ignore this and fill up their spas, but it’s advisable to get rid of every last drop of wastewater.

This prevents any biofilm or chemical cleaner from contaminating the fresh, new water. The most efficient way to remove the standing water is to use a wet/dry vac.

Refill with fresh water

Make sure you remember to close the spa drain before you start refilling the hot tub. Refill with fresh, clean water from your garden hose. Having an inline pre-filter attached to the hose can help filter out most of the impurities that affect your water chemistry.

Using an inline pre-filter will also prevent you from using so many chemicals in the hot tub. It even reduces the risk of mineral deposit build-up and staining.

How to prevent a build-up of biofilm

Balance the water

When your hot tub water is not balanced, the sanitizing chemicals won’t work properly, which then leads to biofilm formation. Therefore, you should test and adjust the pH and alkalinity levels in your spa water once a week. 

Balance the alkalinity first before moving on to the pH. The total alkalinity levels should be between 80-150 ppm. Meanwhile, the pH levels should be between 7.2 and 7.8, with the ideal range being between 7.4 and 7.6.

Shock regularly

Shocking your hot tub water every week prevents bacteria and microorganisms from building up in the plumbing. If you use a bromine sanitizer, adding shock once a week reactivates the bromides to keep your water clean and safe.

To prevent a build-up of biofilm, add non-chlorine shock after each use. This will help your sanitizer work more efficiently. Also, non-chlorine shock oxidizes any organic matter that has been introduced to the spa water. This helps to keep the water clearer for longer.

Clean the filters

To prevent biofilm from forming, rinse your filters with fresh water every week and rinse them with chemicals every month. Also, place them in a chemical soak every three months.

Keep the water circulating

Ensure the hot tub water is circulating in the spa when it’s not in use. This will help to prevent stagnation. In addition, leaving the jets open allows the spa water to flow continuously.

Keep your hot tub covered

Keep your hot tub covered when it’s not in use. An insulated cover prevents dirt and debris from entering the water, which helps to keep the water chemistry balanced. If the spa cover contains cracks or has become waterlogged, replace it to avoid contaminants from dripping into the water.

For extra protection, it’s advisable to combine the insulated cover with a floating thermal blanket. This prevents up to 95% evaporation, which keeps the water clean and fresh by not losing sanitizing chemicals to the air.

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