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Collecting sauna wastewater: how to choose a holding tank

You are building or renovating a sauna (saun) on a plot without mains sewerage, and the washing water has to go somewhere. In Estonia, sauna wastewater is wastewater in law: it may not simply run into a ditch or a gravel pit. The straightforward lawful route is a sealed sewage holding tank (reoveemahuti, in legal texts reovee kogumismahuti; UK readers may know it as a cesspool or cesspit) emptied by tanker (purgimine). It treats nothing and has no outlet. Because a sauna produces washing water only, a small tank lasts many sessions. Below: the infiltration rules, a per-session volume estimate, choosing and placing the tank, and what hair, soap and lint mean for it.

Sauna washing water is wastewater in law

The Estonian ministry guide (2015) calls washing, bathing and kitchen water greywater (hallvesi). The Water Act (Veeseadus) has no such category: under § 20, water from household use that exceeds the set emission limits and must be treated before discharge is wastewater (reovesi). By § 124 it must be treated on site, led to a treatment plant, or collected in a leak-tight holding tank and taken to a reception point.

Whether treatment with infiltration into the soil is open to you depends on the plot:

  • Sewage collection area (reoveekogumisala) of 2000 p.e. (population equivalents) or more: own treatment plants and discharging effluent into the soil are both prohibited (§ 124(4), § 128(8)). Without public sewerage, that leaves the holding tank.
  • Collection area below 2000 p.e. without public sewerage: a holding tank, or an own treatment plant with at least biological treatment (§ 124(3)).
  • Outside collection areas: either (§ 124(6)). Minister of the Environment Regulation No 61, § 8, allows infiltration only where groundwater is not put at risk, and at least 1.2 m above the highest groundwater level and bedrock. Where groundwater is protected, relatively protected or moderately protected, at least mechanical treatment is required (in the ministry guide, a septic tank); where it is unprotected or weakly protected, biological treatment. Washing water gets one concession: on weakly protected groundwater, mechanical treatment is enough for domestic wastewater that contains no flush-toilet water.

Why the municipality decides. It adopts the local rules for on-site wastewater treatment and removal (§ 104(7)), knows the collection-area boundaries, and receives the building notice (ehitusteatis), to which it may add conditions. Example only: Põlva municipality requires a qualified designer’s assessment of geology, groundwater and wells before an own treatment plant is built. Rules differ by municipality, so ask your own municipality first. A holding tank sidesteps the soil question; the ministry guide names small or periodic water use as a case where one fits.

How much water does a sauna session produce?

Best: read the water meter before and after a typical sauna evening, or count the buckets if water is carried in.

Fallback, with the assumption shown: the ministry guide puts sauna, bath and shower at 20–35 % of household water use and plans 100–150 L per person per day. That is 20 L (20 % of 100 L) to 52.5 L (35 % of 150 L) per person per day for all bathing. This article treats one sauna visit as one day’s bathing: about 20–50 L per person per session. This is a planning assumption, not an official figure, so your own meter reading always wins. A hot tub drained to the tank adds its whole volume at once.

sessions between emptyings = usable volume (L) ÷ (people per session × litres per person per session) emptyings per year = sessions per year ÷ sessions between emptyings

This is the standard holding-tank method with a day of use replaced by a session. For four bathers:

Tank Sessions at 80 L (4 × 20 L) Sessions at 200 L (4 × 50 L) Emptyings a year at 52 sessions (weekly)
2 m³ 25 10 about 2.1–5.2
3 m³ about 37 15 about 1.4–3.5

Usable volume is taken as nominal volume for simplicity. In practice it is somewhat below the nominal volume because the alarm level leaves a reserve, so real intervals are shorter; check the alarm height on the drawing for your model.

Choosing and placing the tank

The two smallest Vesmaco models are MK-2 D1100 (2 000 L, about 2.4 m long, 150 kg) and MK-3 D1100 (3 000 L, about 3.5 m, 200 kg), as shown in the model table. Both are horizontal reinforced-fibreglass (GRP) tanks 1100 mm in diameter, the smallest diameter in the range; their limits for other uses are in 2–3 m³ sewage holding tanks.

  • Cost = emptyings per year × your local price per visit. If the emptying company charges per visit rather than per cubic metre, fewer, fuller loads favour 3 m³; by the ministry guide even small tankers hold 4–5 m³.
  • Shared tanks. If a flush toilet or the house drains to the same tank, size it as household wastewater: see year-round homes or summer houses, which also covers leaving a tank over winter. A sauna rented to guests needs a peak-load calculation: see holiday rentals and campsites.
  • Alarm. The ministry guide calls a level sensor advisable; with irregular use, no routine reminds you. Vesmaco’s optional wireless alarm has an indoor receiver: put it in the house, not the sauna. The manufacturer’s sheet recommends a link test beyond 70 m.
  • Inlet. The inlet depth follows the sauna’s outlet pipe, and the buyer states it on the order drawing (on the MK-5 D1400 drawing, for example, up to 1500 mm); options can differ by model, so check the drawing for yours. The ministry guide advises insulation where cover is less than about 1.3 m.
  • Position. Saunas often stand at the far end of the plot or by the water. The ministry guide asks for tanker access in every season and says the truck must not drive over the tank. Any setback (kuja) comes from your municipality; there is no universal distance.
  • Paperwork. Under the Building Code, installing a holding tank requires a building notice filed through the Building Register (ehitisregister) at least 10 days before work starts, and a use notice (kasutusteatis) at least 10 days before the tank is taken into use. Your municipality may add requirements.

Hair, soap and lint: what they do in a holding tank

A holding tank has no filter, outlet or infiltration field, so hair, lint and soap scum cannot block anything inside it; they stay until the tanker pumps them out. Compare the ministry guide’s warning for septic systems: fine solids carried over from the septic tank gradually clog the infiltration field.

Hair and lint matter upstream, in the floor drain and the pipe to the tank: a strainer and an accessible trap are for your plumber to specify. In a sauna unused for weeks the trap can dry out and let tank air in; the order drawing offers a vent pipe with a carbon filter.

The order drawing gives the tank material’s tolerance as pH 3–10 and 60 °C. Check two sauna habits against that: emptying a wood-fired heater’s near-boiling water vessel straight into the drain, and concentrated caustic drain cleaner. If either could go beyond those limits, ask the supplier before it goes into the tank.

Frequently asked questions

Can sauna water just soak into the ground? It is only soap and water. Not untreated, and not everywhere: infiltration needs at least mechanical treatment and suitable ground, and is prohibited in collection areas of 2000 p.e. or more. Check your municipality’s rules.

Is 2 m³ enough for a sauna? For washing water only, by this article’s assumption it takes 10–25 sessions of four bathers. Measure your own use first.

We use a dry toilet at the sauna. Does that change anything? The ministry guide says a dry toilet alone is not enough: the remaining wastewater must be collected in a holding tank or treated in a suitable system. The tank then receives washing water only, so the arithmetic above applies.

Planning a tank for your sauna? Send us the number of bathers, sessions per year, anything else draining to the tank and the inlet depth, and request a quote. Use the enquiry form or call +372 641 8550; the site says replies usually come within 2–3 days. Vesmaco manufactures and delivers the tank; installation, which must follow Vesmaco’s installation guide, is for your installer, and emptying for a local emptying company.

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