A typical scenario (an illustrative situation based on common post-flood problems, not a review from any specific customer)

Three days after the water left, the house in Bueng Kum still smelled like a street drain

Nid is an accountant in her early forties. She lives in a two-storey townhouse down a narrow soi in Bueng Kum with her husband, their eight-year-old daughter and her mother, who is in her seventies. In the last week of September 2026 the rain simply did not stop. Water crept in from the road, rose to ankle height across the ground-floor living room and stayed there for two or three days before it finally drained away. The family carried what they could upstairs, switched off the ground-floor breaker and slept squeezed together on the upper floor.

When the water receded, everyone pitched in. They shovelled out the mud, hosed the floor, scrubbed the walls with detergent and opened every door and window to let the air through. Nid thought the worst was over. But that night, once the house was closed up, a strange smell rose again: a stale, damp odour with a faint note of rotten eggs, exactly like the open drain at the side of the road. It was strongest in the downstairs bathroom and around the floor drain at the back of the house, but before long it had spread through the whole ground floor and drifted up the stairs into the bedrooms.

Her mother felt dizzy and her daughter refused to eat downstairs. Nid tried incense, air freshener, toilet cleaner down the drains and charcoal bags in every corner. The smell faded for an hour, then returned whenever the house was shut. Her question is the one asked in thousands of Bangkok homes: "We cleaned everything. Why does it still stink?"

This article explains where post-flood sewage and drain smells really come from, why the usual fixes do not finish the job, what the correct order of work looks like, and where the ozone odour and chemical-residue removal service from World Health Disinfection (WHD) fits into that process.

The context: Bangkok's floods of late September 2026

According to news reports, continuous heavy rain of around 300 mm fell on Bangkok in roughly 48 hours between 24 and 27 September 2026. On 26 September the Bangkok Metropolitan Administration declared all 50 districts a disaster zone. Eastern Bangkok was hit hardest, with Khlong Chan and Bueng Kum among the areas mentioned. Governor Chadchart said the canals were full and that draining the city would take two to three days after the rain stopped.

As of 30 September, reports said some roads, such as Si Burapha and Lat Phrao, still had 20 to 40 cm of standing water. Thai media compared the event with the great flood of 2011, and one Thai business report cited a Chamber of Commerce estimate of about 12,300 million baht in damage. Lives were also lost, and we extend our sincere condolences to the families affected.

For many households, what remains is a smell of sewage, drains and damp that disturbs sleep and sometimes signals a problem needing proper repair.

Where does the sewage and drain smell come from?

Post-flood odour is usually a blend of several problems. Knowing the causes helps you aim your effort at the right target.

1. Urban floodwater is mixed with wastewater

Bangkok floodwater flowed over roads and out of full canals and drains, mixing with sewage. It carried silt, grease, food and human waste into homes, and the residue settles in grout, wall bases, under cabinets and deep in plaster and timber.

2. Organic matter rotting without oxygen

Organic material in stagnant water or under mud is broken down by microbes that thrive without oxygen, releasing hydrogen sulphide (the rotten-egg smell), ammonia, other sulphur compounds and sour volatile fatty acids: together, "the sewage smell".

3. The water seal in your drains has gone

Properly installed household drains have a U-bend or trap that holds a small pool of water. That water is a seal that stops gases from the sewer below rising into the room. After a flood the seal can be sucked out when the outside water level drops quickly, pushed out by mud, or lost when a floor-drain cover is lifted during cleaning and never put back. The result is that gas from the main sewer and inspection chambers flows straight into bathrooms, kitchens and laundry areas.

4. Flooded septic tanks, cesspits and grease traps

When floodwater rises above a septic tank lid or vent, it can force waste back up; grease traps overflow with rancid fat. If the smell returns with every flush, you need a plumber or septic-pumping service. No spray or air treatment fixes a tank pushing waste back into the house.

5. Porous materials that soaked up dirty water

Render, gypsum, plywood, parquet, rugs, curtains, sofas and mattresses soak up dirty water like sponges. They look dry outside but stay damp inside, and the smell seeps out at night when the house is closed.

6. Mould and the musty smell that follows

After 24 to 48 hours in damp conditions, mould starts to grow on walls, under rugs and inside wooden cabinets. Public health agencies such as the CDC advise cleaning and drying as quickly as possible to limit mould. Mould gives off volatile compounds with an earthy, cellar-like odour, and when that mixes with sewage smell you get a complex stench that the nose struggles to pin down.

How these smells affect health, and who is most at risk

Smell is more than a nuisance. It tells you that the air contains substances your body can detect. At the levels usually found in homes after floodwater recedes, these compounds tend to cause irritation: headaches, nausea, stinging eyes and nose, a scratchy throat, coughing and poor sleep. People with allergies or asthma may find their symptoms flare more easily, particularly when mould spores are also in the air.

At high concentrations hydrogen sulphide is a dangerous gas, and at very high levels it can dull the sense of smell so you no longer notice it, which makes it more dangerous still. This tends to happen in confined spaces such as inspection chambers, septic pits or flooded basements. Never climb into a pit or enclosed space to clean it yourself. Call the authorities or a professional team with gas detectors and protective equipment.

People who need extra care

• Older adults, especially with heart or lung disease.
• Young children, who breathe faster and stay close to the floor.
• Pregnant women, often more sensitive to smells and nausea.
• People with allergies, asthma or chronic lung conditions.
• Anyone doing long hours of clean-up amid odour, mud and chemicals.

Most importantly: if anyone in the household develops a fever or muscle pain, especially in the calves, after wading through floodwater or mud (a warning sign for leptospirosis), or has diarrhoea, vomiting, difficulty breathing or unusual dizziness, leave the smelly area and see a doctor straight away. Removing odour from a home is not medical treatment and is never a substitute for medical care.

What happens when a house keeps smelling for weeks

Health: constant sewage and musty smells mean broken sleep, headaches and fatigue at the most physically demanding stage of recovery, when flood-related germs are still around.

Money: families spend heavily on air fresheners, charcoal and cleaning products, or throw away furniture that could have been saved, while the real cause may be a dry drain trap or a wall that is still wet.

Time and peace of mind: coming home to the same smell every evening makes the house feel like it is still not a home, and many people stay away longer or take extra leave.

Property and reputation: for rentals, ground-floor restaurants and shops, a lingering sewage smell can drive tenants and customers away for longer than the flood itself.

Family: in homes with older relatives and children, a smell that never leaves keeps everyone anxious about hygiene, food and sleep.

Why the usual fixes do not finish the job

1. Air fresheners, incense and scented candles mask, they do not remove

Fragrance only competes with the bad smell. The odour molecules remain, the stench returns as the perfume fades, and you add more volatile compounds to the air.

2. Charcoal and baking soda are too small for a whole floor

Absorbents work in a shoe cabinet or fridge, not across a whole floor of porous materials that keep releasing odour, and they stop working once saturated.

3. Pouring strong chemicals down the drain does not restore the seal

Many people pour toilet cleaner or caustic soda into drains to kill the smell. That can help briefly, but if the real problem is a dry trap, a missing cover or a mud blockage, the gas keeps coming. Worse, mixing products, for example chlorine-based bleach with acidic or ammonia-based cleaners, can release toxic gas. Never mix cleaning products.

4. Fans and open windows help, but are not enough

Ventilation is essential, but a fan only moves air. It does not break down odour trapped in materials, so the smell builds up again when windows close at night.

5. Painting over walls that are still damp and smelly

Painting over a damp wall seals moisture and organic matter inside. The paint blisters, the musty smell creeps back, and fresh paint fumes are added on top.

The right order: fix the cause first, then finish with ozone

Before we describe the WHD service, here is the key principle. Ozone treatment is a finishing step after mud removal, washing and drying. It does not replace cleaning and it is not a plumbing repair. The recommended order is:

1. Safety first. Have an electrician check the ground-floor wiring before switching power back on. Wear rubber gloves, boots and a mask while cleaning.
2. Remove all mud and wet debris, including rugs, mattresses and anything soaked in dirty water that cannot be saved.
3. Wash with clean water and detergent. Scrub grout lines, wall bases, under cabinets and anywhere water pooled.
4. Disinfect the cleaned surfaces with a disinfectant used as the label directs, such as the medical-grade Chemgene HLD4H disinfectant, at the dilution and contact time shown on the label.
5. Fix the drainage. Refill every trap with water, replace floor-drain covers, flush mud-blocked pipes, and have a plumber or pumping service check septic tanks, treatment units and grease traps.
6. Dry the house. Ventilate, run fans or dehumidifiers, and keep checking walls and floors until the dampness falls.
7. Ozone treatment for residual odour in the air and in materials, as the final step.

If you have completed steps one to six and the house still smells of sewage or damp, that is the moment when ozone delivers the most value.

How the WHD ozone odour and chemical-residue removal service works

According to World Health Disinfection's service information, ozone (O3) is a three-atom form of oxygen that reacts very readily. Released into a closed space, it oxidises odour molecules and breaks them apart at the source instead of covering them with fragrance. The service information also states that ozone tackles musty odours, volatile organic compounds (VOCs), and bacteria, viruses and mould, and that once the reaction is complete ozone reverts to oxygen, leaving no residue.

For a flooded home the key advantage is that ozone is a gas. It reaches places a cloth or brush cannot: behind built-in cabinets, under stairs, inside the void beneath a sink, the gap between floor and wall, and into the fibres of curtains or sofas that were worth keeping.

The WHD process step by step

1. Survey and questions. The team asks how high the water rose, how long it stayed, where the smell is strongest and what has already been done, to judge whether the home is ready or whether a root cause still needs fixing.
2. TVOC measurement before treatment. According to the service information, WHD measures total volatile organic compounds (TVOC) with calibrated instruments before treatment, so there is a reference figure rather than a judgement made by nose alone.
3. Preparing the space. People, pets and plants leave the area completely. Doors and windows are closed, the house is divided into zones, and cupboards or drawers that need gas access are opened.
4. Treatment with a Master Ozone Generator. Ozone is released into the empty, closed space at a concentration and duration set by the team for each zone's size and odour level.
5. Rest and ventilation. After the generator is switched off, time is allowed for ozone to revert to oxygen before the space is aired. According to the service information, the area is ready to use about two hours after treatment.
6. TVOC measurement after treatment. Before and after figures are compared so the owner can see a measured result.
7. Follow-up advice, such as keeping drain traps topped up, ventilation habits, and which spots to watch for mould over the next few weeks.

Safety rule: ozone treatment is only carried out in an empty, closed space. People, pets and plants must stay out for the whole treatment and may only return after the rest and ventilation period the team specifies. High concentrations of ozone irritate the airways, which is why the work must be done by trained staff.

See full details on the ozone odour and chemical-residue removal service page.

10 reasons to finish a sewage-smelling home with ozone

1. It breaks odour molecules at the source rather than spraying perfume on top, so the smell is not just hidden for an afternoon.
2. The gas reaches corners hands cannot, under stairs, behind cabinets, beneath sinks and between furniture, where sewage residue loves to hide.
3. It deals with musty mould odour at the same time. According to the service information, ozone works on musty smells, bacteria, viruses and mould.
4. No chemical residue is left behind. Ozone reverts to oxygen, which suits homes with young children and older relatives. Read more on the ozone chemical-residue removal page.
5. You get numbers, not just impressions. TVOC is measured with calibrated instruments before and after.
6. You are back home quickly. According to the service information, the space is ready about two hours after treatment, so families do not need to move out for days.
7. It also tackles leftover cleaning-chemical smells. After a flood many homes have used so many products that a chemical odour lingers, and ozone helps with that too.
8. It suits homes, condos, shops and offices. WHD serves residences, condominiums, offices, factories, hotels and storage spaces.
9. Professional equipment and control. The team uses a Master Ozone Generator and manages timing safely. See the WHD ozone service.
10. It gives back the feeling of home. Opening the front door without being hit by sewage smell lets a family use the ground floor normally again.

Example case: a noodle shop near Khlong Chan where customers smelled the drains before the owner did

This example case is a hypothetical scenario built from common problems to illustrate the process. It is not a real customer review.

Tom runs a noodle shop in a shophouse near Khlong Chan. After the late-September flood he closed for three days for a deep clean. On reopening day, regulars said the shop smelled like a drain; Tom had grown used to it.

Checking point by point revealed three layers to the problem. First, the grease trap under the dish sink had overflowed and was holding rancid fat. Second, the floor drain at the back had no water in its trap because mud had pushed it out. Third, the plywood panels under the counter had soaked up dirty water and were still damp.

The fix followed the order above: grease trap emptied, mouldy plywood replaced, trap refilled and cover replaced, floors washed and food-contact surfaces disinfected per the label, then several days of drying. Ozone was booked for a closed night, with all food and plants removed and nobody inside.

TVOC was measured before and after. In this example the reading fell and the drain smell did not reappear. The lesson: ozone worked because the causes were fixed first; skip the grease trap or drain seal and the smell returns however often you treat.

Checklist for clearing sewage and drain smells after the water goes down

Before ozone

• Have the ground-floor electrics checked by an electrician before use.
• Remove mud, wet waste, rugs, mattresses and anything beyond repair.
• Wash floors and walls with clean water and detergent, then disinfect surfaces according to the label.
• Pour clean water into every drain to restore the seal, including rarely used ones in storerooms or balconies.
• Make sure floor-drain covers are firmly in place and joints are not leaking.
• Have a plumber or pumping service inspect septic tanks, treatment units and grease traps.
• Never mix cleaning products, and never enter pits or confined spaces yourself.
• Dry the house with ventilation, fans or dehumidifiers.

On treatment day

• Make sure all people, pets and plants are out of the area.
• Seal or remove food, medicines and personal items you do not want exposed.
• Open cupboards, drawers and internal doors as the team advises.
• Close external doors and windows, and wait until the rest and ventilation period is over before going back in.

After ozone

• Keep ventilating for a while as advised.
• Pour water into little-used drains every week so the traps never dry out.
• Watch walls, corners and under cabinets. If mould returns, there is still moisture to deal with first.
• If the smell comes back in one spot only, recheck the drain or tank nearby.
• If anyone falls ill after wading through floodwater, see a doctor.

Frequently asked questions

1. If I use ozone, do I still need to fix the drains or empty the septic tank?

Yes. If gas is still coming from a dry trap, an overflowing pit or a malfunctioning tank, new odour will keep being produced. Ozone deals with odour already in the air and materials of the home, but it does not repair plumbing. Fix the cause first, always.

2. How dry does the house need to be?

Mud removed, surfaces washed and dried. If walls or floors are still soaking, let them dry first; the team will help judge readiness.

3. Can my family stay upstairs during treatment?

No. Treatment happens in an empty, closed space; people, pets and plants stay out until the rest and ventilation period ends.

4. How soon can we go back in?

According to the service information, the space is ready to use about two hours after treatment. Always follow what the team tells you on the day.

5. How do I know it worked?

Beyond what you can smell, WHD measures TVOC with calibrated instruments before and after treatment, according to the service information, giving you figures to compare.

6. Does ozone help with flood germs too?

According to the service information, ozone acts on bacteria, viruses and mould in the air and on surfaces the gas can reach. It is not medical treatment and does not replace washing and disinfecting surfaces with a labelled product. Anyone who is ill should see a doctor.

7. Will ozone protect my house from the next flood?

No. Ozone does not prevent flooding. It reduces odour and residual germs during recovery. To prepare for further heavy rain, follow announcements from the BMA and the relevant agencies.

Does your home still smell of sewage or drains after the flood?

Talk to the WHD team to identify the source of the smell, measure TVOC, and plan a safe ozone treatment in an empty, closed space.

See the Ozone Odour Removal Service

Call 065-556-6294  |  LINE @whd268  |  worldhealthdisinfection@gmail.com

Related pages

• Ozone odour and chemical-residue removal service
• Residential and building ozone disinfection
• Chemgene HLD4H medical-grade disinfectant

References

• Wikipedia: 2026 Bangkok floods
• Al Jazeera: Disaster zone declared across Bangkok as heavy rain triggers flooding
• CDC: Floods and clean-up guidance
• WHO fact sheets