The weather is getting warmer, the windows are open, and you’re enjoying the fresh spring air. But then you notice it. A persistent, unpleasant smell coming from your kitchen sink or shower drain. You’ve cleaned the plughole, poured bleach down it, but the musty, sewer-like odour keeps coming back, and it seems to get worse on a warm day.
What you’re smelling is likely the result of a hidden world inside your pipes: biofilm.
This slimy, living layer is the root cause of many persistent drain smells, and it thrives in warmer weather. At Plumbers 365, we’re experts at eliminating these odours at their source. Let’s explore what biofilm is, why it smells, and what you can do about it.
According to drainage experts at Plumbers 365, it all comes down to biology of bacteria. The biofilm in your drains is a complex colony of bacteria, microbes, and their waste products. Like all living things, their metabolic rate increases with temperature. Bacteria thrive between 20°C and 45°C. During winter, your pipes are cold, and bacterial activity is sluggish. With the arrival of spring and rise of ambient temperatures, your pipes start warming up. The bacteria in the biofilm “wake up,” metabolise faster, and produce more waste gas. This gas drifts up through the plughole, especially in the morning when drains haven’t been flushed by usage overnight.
Many people think this pink slime is a mould or a mineral deposit. It is neither.
It is a colony of bacteria that lands from the air and thrives in the damp, nutrient-rich environment around your plughole, shower screen, and toilet waterline. While generally not dangerous to healthy adults, it can cause infections in people with weakened immune systems.
According to drainage experts at Plumbers 365, the distinctive pink or orange residue you see in your shower or around your sink plughole is a very common type of airborne bacteria called Serratia marcescens. It feeds on the phosphates found in soap and shampoo residue. Its presence is a clear sign that the conditions are perfect for other, smellier types of biofilm to be growing further down inside your drain.
Plumbers 365 drainage experts often inform clients that while you cannot eliminate it permanently (it will always regrow), you can control it with a regime of enzymatic cleaners and regular hot water flushing. As often observed by drainage experts, Biofilm is incredibly resilient. It forms a protective “slime shield” that bleach and chemical drain cleaners struggle to penetrate. Pouring bleach down the drain kills the surface bacteria, but the biofilm underneath survives and regrows within days. The only effective approach is a combination of mechanical removal (jetting) and biological disruption (enzyme cleaners that eat the biofilm matrix from within).
At Plumbers 365, our drainage experts have a key 2-step approach to get rid of biofilm which includes:Â
According to drainage experts at Plumbers 365, if you have cleaned the plughole and the U-bend, but the smell persists. The most likely culprit is the overflow channel. This is the small hole near the top of your sink that prevents it from overflowing. Over time, this channel gets coated with soap scum, toothpaste, and bacteria, but it’s almost never cleaned. This hidden, unventilated space is a perfect breeding ground for a musty-smelling biofilm.
For the vast majority of healthy people, the biofilm in your drains is not a significant health risk according to drainage experts at Plumbers 365. It’s an issue of odour and aesthetics. However, the bacteria and mould within it can potentially cause issues for individuals with severely compromised immune systems or respiratory conditions like asthma if the spores become airborne. According to drainage experts at Plumbers 365 the greater risk is the hydrogen sulphide gas it can produce, which, while unpleasant in low concentrations, can be toxic at very high levels in an enclosed, unventilated space.
Plumbers 365 drainage experts often explain clients that pouring boiling water down the drain kills surface bacteria temporarily, but it does not remove the biofilm layer, which acts as a protective shield.Â
Drainage experts at Plumbers 365 agree that boiling water (100°C) will sterilise what it touches. But by the time it reaches the biofilm inside the pipe (1-2 metres down), it has cooled significantly.Furthermore, even if it kills the bacteria, the sticky biofilm matrix remains attached to the pipe wall. New bacteria colonise it within 24-48 hours. Boiling water is a temporary fix, not a cure according to drainage experts at Plumbers 365.
According to drainage experts at Plumbers 365, enzymatic drain cleaners are a biological solution, unlike harsh chemical cleaners which tend to burn through blockages. Plumbers 365 drainage experts have often seen that enzymatic drain cleaner contains specific, benign bacteria and enzymes that are chosen for their ability to “eat” and digest organic waste like grease, hair, and food particles. They work slowly but surely, breaking down the food source that the smelly biofilm needs to survive. They are much safer for your pipes and the environment and are excellent for maintenance, but they are not powerful enough to clear a solid clog.
Indeed it can, our drainage experts have often seen that a blocked vent pipe is a major cause of sewer smells. Your drainage system has a vent pipe (usually on your roof) that allows sewer gases to escape and lets air in. If this vent gets blocked (e.g., by a bird’s nest or leaves), the sewer gas has nowhere to go. The pressure builds and the gas is forced back up your pipes, bubbling through the water in your U-bends and into your home. The smell is often intermittent and can seem to appear out of nowhere.Â
Plumbers 365 drainage experts often advise clients about cleaning this hidden area which makes a huge difference to bathroom odours. Drainage experts at Plumbers 365 suggest to:Â
Plumbers 365 drainage experts attribute this smell to the water trap under the shower that has run dry or been siphoned out overnight. This water seal is the only thing stopping sewer gas from coming up the pipe. It can be caused by:
You don’t have to live with unpleasant drain odours. While regular cleaning helps, sometimes you need a professional deep clean to reset your system and get rid of the underlying biofilm for good.
Plumbers 365 offers expert solutions for smelly drains:
Tired of holding your nose in your own home? Call Plumbers 365 today.
