Every rainy season, or before and after a typhoon, many Hong Kong households and shops run into a nightmare: even though no water is being used, sewage surges back up out of the toilet or floor drains instead. This is "sewage backflow". At best it fills the home with foul odours and floods the floor; at worst, foul water backs up and overruns an entire shop, causing heavy losses. Sewage backflow usually comes on fast and without warning, so knowing how to respond immediately and judge the cause is what lets you reduce the damage.
Where does sewage backflow come from?
Put simply, sewage is meant to flow downward and away, but when the downstream drain is blocked or full, the sewage has nowhere to go and instead surges back up from the lowest floor drain (usually a floor drain or the toilet of a ground-floor unit). Common causes include:
- The public sewer or septic tank silts up or overflows, jamming the whole branch drain.
- Heavy rain overloads the stormwater drains and sewers at the same time, causing the water level to rise sharply.
- Grease and debris build up in the pipes over time; they just about hold up normally, but blow out as soon as heavy rain falls.
- Lower-lying units or village houses naturally become the "outlet" for the sewage.
5 emergency steps for sewage backflow
- Stop using water immediately — don't flush the toilet or turn on taps, to reduce the volume of sewage.
- Move valuables and electrical appliances off the floor, and cut the power in the affected area to avoid electric shock.
- Use sandbags, towels or a waterproof plug to temporarily seal off the floor drain and slow down the surge of sewage.
- Open windows to ventilate, and wear a mask and gloves. Sewage contains large amounts of bacteria — never clean it up with bare hands.
- Take photos to record the scene (for insurance and liability purposes), then contact a professional drainage company to come and inspect.
Why is the rainy season especially prone to backflow?
Hong Kong's summer rainstorms are concentrated, dropping a large amount of rain in a short time, and the drainage systems of many older buildings and New Territories village houses far exceed the load they were designed for back then. Pipes that are already half silted up can just about cope on a clear day, but once heavy rain falls, the upstream water volume surges while the downstream can't drain fast enough, so the sewage backs up from the lowest point. That's why doing a drain inspection and cleaning before the rainy season is the most practical prevention.
When must you call a professional?
- The backflow keeps happening, more than once.
- Multiple floor drains (kitchen, toilet, floor) back up at the same time, meaning the blockage is in the main drain rather than a single spot.
- It involves the septic tank, public drains, or multiple units, so the scope is large.
- What backs up is foul/faecal water rather than just wastewater, posing a high hygiene risk.
In cases like these, using drain cleaner yourself is almost useless. You need professional high-pressure water jetting to clear it, and if necessary a CCTV pipe survey to pinpoint the blockage, so that it can be resolved thoroughly and avoid backflow again after a few more downpours.
Don't wait until you're flooded to call someone
Sewage backflow is often the "breaking point" of long-term drain silting — it doesn't happen overnight. Rather than waiting until you're flooded and out of pocket before scrambling to find someone, it's better to arrange a drain inspection and high-pressure cleaning before the rainy season, as a precaution.
Motion Waste Management Limited (Drainage Services Department (DSD) registration number P0038) has served all of Hong Kong since 1992, providing emergency drain clearing, sewage backflow handling, and septic tank and public drain unblocking services. Don't panic when backflow strikes — you can first describe the situation via WhatsApp or the quote form, and we will assess first and then quote, with transparent charges, to help you get back to normal as soon as possible.