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Why should rainwater be kept out of a sewage holding tank?

A sewage holding tank (Estonian reoveemahuti, in legal texts reovee kogumismahuti; UK readers may know it as a cesspool or cesspit) has no outlet. Everything that flows in stays until a tanker pumps it out (purgimine), and you pay for every load. Rainwater in a holding tank is therefore clean water you pay to have hauled away as sewage. A connected roof can add cubic metres after a single wet day, and some municipalities’ rules explicitly require storm water to be kept out. This article shows how to work out the extra volume, what it does to emptying, what the rules say and how to keep rain out.

Rainwater takes up space meant for sewage

The Ministry of the Environment’s 2015 guide to rural wastewater systems (official, though dated) is clear that a holding tank treats nothing and must have no outflow. Anything that reaches it takes up space meant for household wastewater:

  • a roof downpipe connected to the sewer pipe;
  • a yard, garage or driveway drain, or a foundation drain;
  • surface water or snowmelt running in through a cover or riser top that sits too low.

Once rainwater mixes with sewage, it is sewage. The Water Act (Veeseadus) takes a similar view of sewer networks: storm water discharged into a combined sewerage system is also deemed wastewater (§ 20).

How much water does a roof add?

The arithmetic is simple, because 1 mm of rain on 1 m² is 1 litre:

extra litres = catchment area (m²) × rainfall (mm)

Assumptions: the area is the plan area (the ground the roof covers) and all the water reaching the drain ends up in the tank, so treat the result as an upper estimate.

Connected area 10 mm of rain 20 mm of rain 50 mm of rain
50 m² (shed, porch or paved yard) 500 L 1 000 L 2 500 L
100 m² (small house roof) 1 000 L 2 000 L 5 000 L
150 m² (larger house roof) 1 500 L 3 000 L 7 500 L

For your own plot, use your roof’s plan area and rainfall figures for your area from the Estonian Weather Service (Riigi Ilmateenistus).

What it does to emptying and cost

Take four people living permanently with a 5 m³ tank. At the ministry guide’s planning range of 100–150 litres per person per day, they produce 400–600 litres a day. Treating nominal volume as usable for simplicity (in practice usable volume is somewhat below nominal, because the alarm is set below the top — check the alarm height on your model’s drawing), the tank lasts:

5 000 L ÷ 600 L/day ≈ 8 days   ·   5 000 L ÷ 400 L/day = 12.5 days

Now connect a 100 m² roof. One 20 mm rainy day sends 2 000 L into the tank: 40% of its volume, or three to five days of household use gone in a few hours. A 50 mm downpour on that roof is 5 000 L, enough to fill the whole tank on its own.

This has three consequences:

  1. More loads. Every 5 m³ of rainwater means one extra emptying of a 5 m³ tank. Annual cost = emptyings per year × your local price per visit, so each extra load adds to the bill (budget method: holding tank running costs).
  2. Overflow risk. Heavy rain can fill the tank faster than a tanker can be booked, and the ministry guide warns never to let a holding tank overflow.
  3. Unreliable intervals. Intervals calculated from water use, as in how often a holding tank should be emptied, stop working. A bigger tank only stores more rainwater you still pay to remove.

What Estonian rules say about storm water

National law treats storm water as a separate stream:

  • Water Act. § 129(2) defines storm water as water that has fallen as precipitation and is collected and discharged through construction works, including ditches. § 129(1) says that solutions releasing storm water at source should be preferred. § 124(2) requires a holding tank to be leak-tight (lekkekindel).
  • Public Water Supply and Sewerage Act (ÜVVKS). Where public networks exist, consumers have the right to lead storm water into a separate storm-water sewer (§ 33(1)); leading it into the public sewer without the water company’s permission is unlawful use (§ 43(1) p 6). Worth knowing if mains sewerage later reaches your area.

For a holding tank, the detail is in your municipality’s rules for on-site wastewater treatment and removal (reovee kohtkäitluse ja äraveo eeskiri, adopted under Water Act § 104(7)). Two examples:

  • Saaremaa vald: rain and snowmelt water must be kept from getting into on-site facilities, which include holding tanks (§ 4(3)).
  • Kohila vald: storm water must be kept from getting into on-site facilities (§ 4(4)), and tank hatches must be intact and close tightly (§ 4(5)).

These are examples only. Check your own municipality’s rules, and ask the municipality where rainwater from your plot may go.

How to keep rainwater out

  1. Separate the pipes. Downpipes, yard drains and foundation drains should not join the sewer pipe that runs to the tank. Lead roof water to a rain barrel, a soakaway or surface drainage away from the tank and the house, as your municipality allows and without sending it onto a neighbour’s plot.
  2. Keep the riser top above the ground. Finish the ground so it slopes away from the riser and puddles cannot form over the cover. Vesmaco’s base kit includes a plastic cover, and the order drawings list EN 124 covers as an option for areas with loads. For how high the riser top should sit above finished ground, check the drawing for your model or ask the supplier.
  3. Keep the cover seated and undamaged, especially before autumn rain and spring snowmelt.
  4. Check after heavy rain. If the level jumps, run a garden hose into a suspect drain while someone watches the inlet pipe from above the riser with a torch.

If the tank also rises in dry weather, the cause may be groundwater or a leak rather than rain. The step-by-step checks are in holding tank filling too quickly?, and warning signs of a tank that is no longer tight are in is your holding tank leaking?

Frequently asked questions

Can I let just a little rainwater in, such as from a porch roof? Even a 20 m² porch adds 200 litres for every 10 mm of rain, and municipal rules such as Saaremaa’s and Kohila’s say storm water must be kept out of on-site facilities. Check your municipality’s rules and lead it elsewhere.

Will I notice if a downpipe is connected to the tank? Usually: the level jumps after rain and intervals shrink in wet months. If receipts show clearly more cubic metres removed than your water meter recorded, outside water is getting in.

Can rainwater be collected in a tank instead? Only in a separate rainwater tank with no connection to the sewage system or the holding tank. Ask your municipality what is allowed on your plot.

Planning a new holding tank or replacing an old one? See Vesmaco’s reinforced fibreglass (GRP) holding tanks from 2 to 15 m³ on reoveemahutid.ee. For questions about a specific model, use the quote request form or call +372 641 8550.

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