Every floor drain in Croatia — from a hospital ICU to a hotel bathroom in Dubrovnik — relies on a few decilitres of water to block sewer gases. When that water evaporates the barrier is gone, and visitors walk into a space that "smells of the sewer". This is not a cleaning failure — it is physics.
How a trap works
The trap is the U-shaped section of pipe underneath every floor drain. Its job is simple — to hold a small column of water that physically separates the inside of the building from the sewer network. As long as that water is there, gases such as hydrogen sulphide (H₂S), methane and ammonia cannot get into the room.
Water reaches the trap naturally — every time someone washes their hands, takes a shower or mops the floor, part of that water ends up in the trap. In a building in normal use, the system keeps topping itself up.
The problem starts when the drain is not being used. Without water the barrier physically disappears — and sewer gases come straight into the room.
How much water a trap actually holds
It is a small quantity. A bathroom floor trap holds a few decilitres; the shallower traps under washbasins and shower trays hold only 50 to 100 ml. EN 1253 requires a minimum water seal depth of 50 mm — below that the barrier is not considered reliable.
That figure explains why the problem appears so quickly. A litre of water does not have to evaporate. Nor does the barrier fail all at once — as soon as the column drops below its design depth, some gas passes through, well before the trap is visibly empty.
Why it evaporates — and how fast
Water in a trap evaporates naturally. The rate depends on room temperature, relative humidity, air movement and the temperature of the sewer pipe below. In air-conditioned hotel rooms in Split or on Mljet, a trap can run dry after two to three weeks of complete inactivity. In dry, air-conditioned offices in winter, the rate doubles.
The most exposed are:
- Hotel rooms out of season — the Adriatic takes a significant share of its capacity offline from October to April
- School washrooms — a seven-week summer break is enough to dry out every single trap
- Plant and ancillary rooms — boiler rooms, corridors, garages
- Hospitality venues that close on Mondays
- Holiday homes and privately let apartments (Booking, Airbnb)
Where the problem shows up outside the hotel stock
Seasonal accommodation is the most visible case, but not the only one. The same pattern applies wherever occupancy varies:
- Conference and exhibition spaces between events
- Sports halls and swimming pools outside the competitive season
- Office floors on hybrid working patterns with fewer people on site
- Hospital wards and care homes during refurbishment or decanting
- Plant floors, server rooms and data centres
The last item is a special case. Where ventilation runs at strong negative pressure, air is drawn continuously through the trap and the evaporation rate roughly doubles. There, a trap runs dry in about ten days.
What the Croatian statistics say
According to the Croatian Bureau of Statistics (DZS), Croatia registered 1.2 million permanent beds in commercial accommodation in 2024. Most of that stock is seasonal. It means that from October to April, hundreds of thousands of rooms have inactive drains — and every one of them is a potential guest complaint about odour the moment the property reopens.
Traditional solutions — and why they fall short
Manual flushing. A caretaker or housekeeper walks the building and runs water into every drain. It works, but it depends on discipline, so something always gets missed. In a property with 200 rooms and 400 drains, that is a full-time job for one person.
Automatic trap primers top the trap up with water at intervals. They work until they fail — and failures are routine, because they depend on the plumbing and on electronics. A single trap primer can use more than 200,000 litres of water a year. Multiply that by the number of drains in a hotel.
Air fresheners. They mask the problem. Hydrogen sulphide still enters the room — the guest simply smells it through the lavender first.
Chemical drain treatments. Enzyme and chlorine products last 24 to 72 hours, after which evaporation carries on as before. Chlorine products have a further drawback — over the medium term they encourage biofilm growth inside the pipe, covered in more detail in biofilm in the sewer.
Glycerine or oil in the trap. A layer of low-volatility liquid on the water surface slows evaporation for a few weeks. It works for a long weekend; it does not work for a six-month closure, and in greasy kitchen drains it behaves unpredictably.
A passive mechanical barrier
Green Drain™ is a one-way silicone seal that fits directly into the body of the existing trap. Water still flows through the trap, but at rest the seal stays hermetically closed in the upward direction. Sewer gases physically cannot get past it. No water in the trap — no problem.
Installation takes 30 seconds, needs no tools and no changes to the plumbing. The seal lasts around five years (tested to ASSE 1072-2020 over more than 2,500 cycles).
- Resistant to temperatures from −40 °C to +100 °C
- Compatible with chlorine disinfectants up to 5%
- EU MDR 2017/745 Class I for hospitals
- EN 1253 specification for design documentation
When to consider the change
Three operational signals say a building is treating the symptom rather than the cause:
- At least one complaint about drain odour in the past 12 months.
- Trap flushing is already a line on the housekeeping or engineering rota.
- Occupancy varies — seasonally, by project, or because of refurbishment.
If any of the three applies, the trap will dry out regardless of how disciplined the staff are. A passive barrier changes the starting point, because it does not depend on whether anyone has walked through the room. Independent testing (SGS QDF25-0049810-01) also showed that the closed membrane blocks more than 99.9% of viral aerosols coming up the stack, which matters in healthcare and care settings.
Full overview: Green Drain™ — how a passive one-way drain seal works — sizes, materials, certification and how it is used in homes, hotels, schools and hospitals.