Why Does Tap Water Smell Different During the Rainy Season?

Geosmin – The Invisible Compound Behind the Musty Smell in Rainy-Season Water

As the first rains of the season arrive, many households notice an unusual odor in their tap water described as earthy, musty, or damp. This phenomenon is especially common in areas near rivers, lakes, or where water treatment systems are impacted by heavy rainfall.

The primary cause? A naturally occurring compound called geosmin.

🌱 What is Geosmin?

Geosmin is a naturally occurring organic compound produced by certain microorganisms, particularly Streptomyces (soil bacteria in the Actinobacteria group) and cyanobacteria (also known as blue-green algae) found in water. It is released during microbial metabolism or when these microorganisms die and rupture - a process often triggered by sudden environmental changes like heavy rain or fluctuating water levels.

Once released, geosmin dissolves into the water supply and leads to that unmistakable “damp earth” or “musty” smell, even when the water still meets national safety standards for household use.

Interestingly, geosmin poses no health risks, but its odor is incredibly noticeable. Most people can detect it at concentrations as low as 15 nanograms per liter (15 ng/L), and individuals with a sensitive palate can detect it at just 5 ng/L - equivalent to a teaspoon diluted in over 200 Olympic-sized swimming pools. Even trace amounts at the nanogram level are enough to drastically affect the sensory experience of drinking water.

💡 To put it into perspective: Just one teaspoon of geosmin can produce an earthy smell across hundreds of swimming pools. This makes the compound extremely difficult to eliminate using conventional filtration.

🧪 Why Is Geosmin So Challenging to Remove?

Because even minuscule concentrations can significantly alter the taste and smell of water, removing geosmin is highly challenging. Not all filtration technologies are equipped to handle it, and many traditional methods are practically ineffective.

🔹 Conventional Filtration Methods Fall Short

Basic filtration methods such as sediment filters (PP filters), sand filters, or mechanical filtration—are designed to remove large particles or suspended solids, not dissolved molecules like geosmin. These conventional filters are not designed to capture dissolved compounds at the molecular level.

Even standard activated carbon filters often struggle to maintain performance in the face of fluctuating geosmin levels. Without specialized design, these materials can quickly become saturated and ineffective.

That’s why the removal of geosmin requires advanced technologies - designed specifically to target and neutralize trace-level odor compounds. The most effective solutions include Reverse Osmosis (RO), ozone oxidation, and next-generation adsorption media like BWT Bewasorb.

Only these molecular-level technologies can deliver consistent, measurable results for odor control, even at extremely low geosmin concentrations detectable by the human nose.

💧 BWT’s Advanced Solutions for Rainy Season Water Quality

Every filtration technology developed by BWT is the result of extensive research and rigorous real-world testing, designed to achieve optimal performance — even against challenging compounds like geosmin. Below are the solutions BWT currently implements to effectively address geosmin-related odor issues during the rainy season:

🔹 1. Reverse Osmosis (RO) Technology

Reverse Osmosis uses a semi-permeable membrane with pore sizes as small as 0.0001 microns to filter out bacteria, heavy metals, and dissolved impurities. Under certain conditions, RO systems can also effectively remove geosmin—especially when combined with activated carbon or ozone pre-treatment.

However, since geosmin is low in molecular weight and partially volatile, RO efficiency can vary depending on membrane structure, operating pressure, and system configuration. For best results, BWT systems combine RO with specialized multi-stage filtration, tailored to geosmin removal.

RO-purified water is often extremely pure - colorless, odorless, and tasteless. But it can also lack essential minerals beneficial to the human body. That’s why BWT recommends combining RO with mineral-enhancing cartridges like:

• BWT Woda-Pure Mineralizer XL Mg + CUF Protect
• BWT AQA Drink MPC500

These cartridges:

• Reintroduce natural magnesium, which supports metabolism and energy production.
• Improve the taste of water, making it fresher and more enjoyable, especially for children, seniors, and those who drink large amounts daily.

With the combination of deep filtration and mineral restoration, BWT provides water that is not just clean, but health-enhancing and palatable. It’s a comprehensive solution, especially valuable during the rainy season, when tap water often carries unpleasant odors and impurities.

🔹 2. Ozone Treatment

Ozone is an extremely powerful oxidizing agent, capable of breaking down odor-causing compounds like geosmin, while also killing bacteria, viruses, and hard-to-remove contaminants. In the case of geosmin, ozone directly attacks its chemical bonds, neutralizing its smell-producing properties.

That said, ozone is most effective in centralized water treatment facilities. For home use, improper ozone levels can result in residual ozone in drinking water, which could pose health risks.

Therefore, home systems using ozone must include a residual ozone neutralizer to ensure complete safety for household use.

🔹 3. BWT Bewasorb – Advanced Adsorption Technology

BWT Bewasorb is a proprietary filtration material developed exclusively by BWT. It’s engineered to adsorb odor-causing molecules like geosmin, making it highly effective as a supplemental stage in multi-layer filtration systems.

While it may not eliminate geosmin as completely as RO or ozone, Bewasorb significantly reduces musty smells, especially during long rainy seasons when geosmin levels in the water source spike.

Advantages of BWT Bewasorb:

• Easily integrated into residential and commercial filtration systems
• Requires no electricity, emits no byproducts, and is safe for everyday use
• Ideal for regions prone to biological water fluctuations or microbial activity

☔ Embrace the Rainy Season with Confidence

Clean, safe, and great-tasting water is the foundation of a healthy life. At BWT, we continually innovate to meet the demands of nature and protect your family’s well-being—rain or shine.

From advanced RO systems to ozone and specialized adsorption media, BWT delivers sustainable, reliable solutions to ensure your tap water is fresh, odor-free, and perfectly suited for everyday living.