Caretakers in Croatian schools know it better than anyone — on the first day of September, the school smells. Parents notice it the moment they walk in, and the pupils get used to it. The explanation lies neither in the cleaning nor in a fault in the drainage, but in seven weeks of summer holiday and dry traps across the 891 primary schools and 447 secondary schools in Croatia (Croatian Bureau of Statistics — DZS, 2024).
The arithmetic of the summer break
The average Croatian school closes in mid-July and reopens in the first week of September. That is 7–8 weeks with the washrooms completely out of use. Buildings with technical maintenance work or sports activities are the exception, but most rural and suburban schools simply lock the doors.
Summer temperatures over that period accelerate evaporation. In Zagreb conditions in July (an average of 22–25 °C and low humidity), 100–150 ml of water evaporates from a trap in five to seven days, and a standard trap holds only 200–300 ml. After two weeks there is no water left.
And then another five weeks of an empty building. Sewer gases enter the space day after day. Carpets, sports equipment in the halls, books in the library — everything absorbs the odour.
Summer is not the only break
The school calendar is published each year by the Ministry of Science and Education, and alongside the summer holiday it contains several more periods in which not a single litre of water passes through the school's drains. Added together, the washrooms stand unused for close to a third of the year.
| Period | Duration | State of the trap on return |
|---|---|---|
| Summer break | 7–8 weeks | Completely dry |
| Autumn break | a few days | Partial drop in level |
| Winter break, first part | around 2 weeks | Dry or borderline |
| Winter break, second part | around a week | Partial drop in level |
| Spring break | around a week | Partial drop in level |
| Weekends and public holidays | 2–3 days, all year round | Measurable evaporation |
So this is not only a September problem. After every winter break the drains in rarely used rooms end up in the same state, simply on a smaller scale.
Where the smell comes from
The sewer network below the school produces gas continuously — mainly hydrogen sulphide (H₂S), methane and ammonia. While the water seal in the trap is intact, the gas stays in the pipe. Once the seal evaporates, that same gas enters the classroom through the empty trap.
The odour threshold for hydrogen sulphide is extremely low, around 0.5 ppb (parts per billion). Concentrations far below any exposure limit are still clearly noticeable to pupils and staff, which is why complaints arrive even when measurement shows nothing of concern.
The pattern repeats year after year:
- A strong sewer odour in ground-floor classrooms and the washrooms next to them
- A persistent odour in the kitchen and canteen for the first day or two of term
- Drain flies around traps that have stood dry all summer
- The first complaints from parents and staff within hours of the building reopening
The smell intensifies when the heating or the ventilation is started for the first time in September. Warmer air and negative pressure in the building draw the gas up the stack towards the empty traps.
The specific places in a school
The most exposed points, in order of priority:
- Pupils' washrooms — the largest number of drains and the lowest flow outside term time
- Sports halls and changing rooms — the same problem as the washrooms, with the added odour load from the equipment
- The kitchen and canteen — drains in the production area and in the staff washrooms
- The library and classrooms with sinks — small drains that are easily forgotten
- Ancillary rooms — boiler room, basements, cloakrooms
What caretakers do at the moment
The traditional protocol: a week before term starts, the caretaker walks through every washroom and runs water into each drain for around thirty seconds, long enough to refill the traps.
The problems:
- The sheer number of drains — a smaller school has 30–50 points, a large city grammar school more than 100
- Forgotten zones — wood stores, basements and inspection chambers, the easiest to skip and usually the worst offenders
- It does nothing for the winter and spring holidays — another four to six weeks a year
- The water in the traps starts evaporating again immediately — during September itself some of the traps are dry once more
Why chemicals and ventilation do not help
Alongside flushing, schools usually reach for three more measures. None of them touches the cause.
- Chemical drain products. Chlorine and enzyme preparations applied during the big summer clean last 24 to 72 hours. Chlorine also encourages biofilm growth inside the pipe over the medium term.
- Ventilation. Open windows and extract fans dilute the air in the room, but the source stays open.
- Reacting to complaints. A complaint from staff triggers another drain treatment. The cycle repeats at the next holiday.
They share the same weakness: anything that depends on water sitting in the trap fails as soon as the building stands empty for long enough. The summer deep clean is not organised badly — it is simply done before the traps have finished drying out.
A passive seal means a one-off protocol
The Green Drain™ silicone seal stays closed through the whole summer break, the winter holidays and any other period of inactivity. It needs no contact with water — it works mechanically.
Practical implementation for a school:
- Drain inventory (1–2 hours)
- Ordering by size (8 standard sizes plus 3 dual-fit combinations cover 100% of cases)
- Installation (around 30 seconds per drain — the whole building in a single working day)
- Annual visual inspection (5–10 minutes)
Cost for an average school (50 drains): a one-off €1,000–1,500 at exclusive Croatian distributor pricing. Spread over the five-year service life of the seal, that works out at €200–300 a year for the entire school, or around €5 per drain.
The ASSE 1072 specification for public tenders
The Croatian public procurement system (EOJN, the Electronic Public Procurement Classifieds) accepts the ASSE 1072-2020 specification as the reference standard for waterless trap seals. The specification prescribes flow testing, service life testing and evaporation testing. Green Drain™ holds a valid IAPMO 9301 listing and every certificate needed for inclusion in tender documentation.
Who pays for it, and from which budget
Running costs and building maintenance for schools are funded by the founding authority — the city, municipality or county — through decentralised funding for primary and secondary education. In that structure passive seals count as routine maintenance rather than capital investment: there is no work on the pipework, no design documentation and no notification of works.
Set against what is already being spent every year:
- Chemical drain products and air fresheners — a recurring line in every annual procurement plan
- Additional cleaning ordered after complaints — an unplanned cost that nevertheless turns up regularly
- Caretaker hours — flushing every drain before reopening takes one to two hours per closure, or eight to fifteen hours a year
A one-off purchase that removes those lines for five years pays for itself within two years on direct cost alone, before any allowance for pupil and staff comfort. If the sanitary inspectorate or the school medical officer notes drain odour in a report, a documented installation with test reports is the answer that closes the subject. A detailed calculation for buildings with trap primers is set out in the ROI of replacing trap primers, and the overview for education is on the schools and nurseries page.
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.