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How often should a sewage holding tank be emptied? Worked examples

A sewage holding tank (Estonian reoveemahuti; in legal texts reovee kogumismahuti; UK readers may know it as a cesspool or cesspit) treats nothing and has no outlet: every litre stays in it until a tanker pumps it out (purgimine). So there is no calendar answer to how often to empty a holding tank. The interval is the tank’s usable volume divided by your household’s daily wastewater. Below: the formula, worked examples with Vesmaco’s 3, 5 and 10 m³ tanks, why septic-tank intervals do not apply, when to order the tanker, seasonal use, and how to need fewer visits.

The emptying-interval formula

days between emptyings = usable volume (L) ÷ (persons × litres per person per day)
emptyings per year     = 365 × occupancy share ÷ days between emptyings

Each input is an assumption you can check:

  • Litres per day. Almost everything that passes your water meter ends up in the tank (garden watering from the same meter excepted), so use it: a month’s m³ × 1,000 ÷ number of days. Without a meter, take the planning range in the Ministry of the Environment’s 2015 guide to rural wastewater systems (official, though dated): 100–150 L of wastewater per resident per day, and calculate with both ends.
  • Usable volume. Taken here as the nominal volume. In practice you order before the tank is full, and an alarm sensor, if fitted, sits below the top, so the usable volume is somewhat below the nominal volume and real intervals are somewhat shorter. Check the drawing and the alarm height for your model.
  • Occupancy share. 1 for a permanent home; otherwise the share of the year the property is in use.
  • Household wastewater only. If rainwater, drainage water or groundwater gets in, the tank fills faster than the formula says.

Your receipts are the real-life check. Each municipality adopts rules for on-site wastewater treatment and removal (reovee kohtkäitluse ja äraveo eeskiri, Water Act § 104(7)). Saaremaa vald’s rules, for example, require an invoice at every emptying showing address, volume removed and date, kept by the owner for at least two years; Keila linn sets at least three years. These are examples; rules differ by municipality, so check your own. The dates between receipts give your actual interval.

Worked examples for 3, 5 and 10 m³

Assumptions: permanent occupancy, 100–150 L per person per day, usable volume = nominal volume. Diameter does not change the result: MK-5 D1100 and MK-5 D1400 hold the same 5 m³, MK-10 D1400 and MK-10 D1600 the same 10 m³ (see the model table).

Tank (Vesmaco model) 2 people (200–300 L/day) 4 people (400–600 L/day) 6 people (600–900 L/day)
3 m³ (MK-3 D1100) 10–15 days · 24–37 a year 5–7.5 days · 49–73 a year 3.3–5 days · 73–110 a year
5 m³ (MK-5 D1100 / D1400) 17–25 days · 15–22 a year 8–12.5 days · 29–44 a year 5.5–8 days · 44–66 a year
10 m³ (MK-10 D1400 / D1600) 33–50 days · 7–11 a year 17–25 days · 15–22 a year 11–17 days · 22–33 a year

A couple with 3 m³. 3,000 ÷ 300 = 10 days; 3,000 ÷ 200 = 15 days: a tanker every one and a half to two weeks, 24–37 times a year.

The official benchmark: four people with 5 m³. The ministry guide names at least 5 m³ as the optimal size for a four-person family living permanently, and says the tank is then emptied once a week. The formula agrees: 5,000 ÷ 600 = 8.3 days, 5,000 ÷ 400 = 12.5 days. Allowing for the tanker’s notice time, that is roughly weekly: 29–44 visits and 146–219 m³ removed a year.

One special case: in a sewage collection area (reoveekogumisala) of 2,000 population equivalents or more that has no public sewerage yet, the Public Water Supply and Sewerage Act (ÜVVKS) requires the tank to hold at least two weeks’ wastewater (§ 47(7)); the water company must organise the emptying service (purgimisteenus) on request, and you are entitled to it twice per calendar month (§ 47(10)). On planning figures, 5 m³ does not cover two weeks for four people.

Four people with meter data and 10 m³. Illustration: the meter shows 13.5 m³ in 30 days, or 450 L a day. 10,000 ÷ 450 = 22 days, about 16 visits a year; with 5 m³ it would be every 11 days, about 33 visits. The catch is the truck: the 2015 guide says small tankers carry 4–5 m³ and the largest about 10 m³, so a full 10 m³ tank needs a large tanker or two loads; ask local emptying companies what they run today. See tanker access for holding tank emptying; to work back from the interval you want to a volume, see what size sewage holding tank you need.

Why septic-tank intervals do not apply

Advice such as “empty it twice a year” comes from septic tanks (septik). A septic tank settles solids and passes the liquid on to an infiltration or filter system, so the truck removes sludge. The ministry guide accordingly says a septic tank is desludged as needed, but at least twice a year.

A holding tank has no outlet and collects the whole daily flow. For it, the same guide says: empty as needed, check the level with a dipstick, order the tanker when the wastewater passes about half the tank’s diameter, and never let it overflow. On a septic-tank schedule, four people’s 5 m³ holding tank would overflow within two weeks.

The reverse is a warning sign. If a “holding tank” in a lived-in house needs the tanker far less often than the formula predicts for its volume and household, the wastewater may be going somewhere. The Water Act (§ 124(2)) requires a leak-tight (lekkekindel) tank, so have it checked.

Level alarm: order the tanker at the right time

The emptying company needs notice, so you order before the tank is full:

order-by day = days until full − notice days

Four people (400–600 L a day) and three days’ notice (an assumption; ask your emptying company):

Tank Full after Order by day (after emptying) Reserve for the notice period
3 m³ 5–7.5 days 2–4.5 1.2–1.8 m³, 40–60% of the tank
5 m³ 8.3–12.5 days 5.3–9.5 1.2–1.8 m³, 24–36%
10 m³ 16.7–25 days 13.7–22 1.2–1.8 m³, 12–18%

The reserve in litres is the same whatever the tank, so a small tank gives up a far larger share of its volume. Without an alarm, the guide’s half-diameter dipstick rule keeps about half the tank in reserve: close to the need for 3 m³, generous for 10 m³. The guide calls a level sensor advisable, and Vesmaco’s order drawing lists an overfill alarm as an option. On the Smartel Tank-Check RF-431 wireless alarm, the sensor cable length sets the alarm level (manufacturer’s sheet): set it so that the volume above the sensor covers notice days × daily wastewater. Choosing an alarm is covered in holding tank level alarms.

Seasonal use and winter

Seasonal use changes the number of visits a year, not how fast the tank fills while you are there. Two people at a summer house produce 400–600 L per weekend (2 people × 2 days × 100–150 L), so 5 m³ lasts about 8–12.5 weekends, or 17–25 days of a holiday. For the year, 60 days of use (occupancy share 60/365) gives 365 × 60/365 ÷ 17–25 days = 2.4–3.6 tankfuls: plan for three or four visits. Holiday water use can differ from the planning range, so your first season’s receipts will give a better figure.

The ministry guide says the tanker must be able to reach the tank in every season, and Saaremaa’s rules, for example, require tanks that can be emptied year-round with access for the truck. Ask your emptying company about winter notice times and order with that margin. Closing a holiday home for the winter where groundwater can be high? Do not have the tank pumped empty out of habit; see the second question below.

How to reduce the number of emptyings

The cubic metres removed each year equal the wastewater you produce; visits fall only if less water goes in or more is removed per visit.

  1. Repair leaks at once. A running cistern or dripping tap fills the tank day and night.
  2. Keep other water out. Rainwater, drainage water and snowmelt do not belong in the tank. If receipts show clearly more cubic metres than your meter recorded, water is getting in from elsewhere.
  3. Order by level, not by date. Fixed dates pay for visits to half-full tanks.
  4. Use less water. Four people at 100 rather than 150 L per person per day need about 29 instead of 44 visits a year with 5 m³.
  5. Choose a larger tank at purchase or replacement. It splits the same cubic metres into fewer, larger visits. That helps where the charge is per visit and a big enough tanker can reach you; where it is per m³, as under ÜVVKS § 48(2), size does not change the bill. A buried tank cannot be enlarged.

For the budget, annual cost = emptyings per year × your local price per visit (or m³ per year × price per m³); see holding tank running costs.

Frequently asked questions

How often is a 5 m³ tank emptied with a family of four? On the ministry guide’s planning range, 5 m³ is full in 8–12.5 days, so in a permanent home about once a week (the guide’s own benchmark), or 29–44 times a year. With meter data: 5,000 L ÷ your litres per day.

Should it be emptied completely? A holding tank has no treatment process that needs liquid left in, and the emptier the tank, the longer the next interval. The exception is high groundwater: the ministry guide calls for a concrete slab and anchoring so that an empty or half-empty tank is not lifted. If you do not know how a tank in wet ground was anchored, ask the manufacturer or your installer before it is pumped out whether it may be emptied completely and what minimum level to leave; water left in as ballast shortens the interval.

Who may transport the wastewater? An emptying company that meets your municipality’s rules. You must arrange transport to the reception point (purgimissõlm) named in the municipality’s water and sewerage development plan (Water Act § 124(2)), and discharging wastewater into the public sewer without the water company’s permission is unlawful use of the service (ÜVVKS § 43(1) p 4). Municipal rules add detail: Saaremaa vald and Keila linn require the emptying company to have a contract with the water company; Kohila vald allows only carriers holding a municipal carrier card (vedaja kaart).

Emptying too often? Ask about a larger model. Send us the number of people, a water-meter reading or your receipt dates, and the largest tanker load available locally, and we will suggest Vesmaco volumes from 2 to 15 m³ with their drawings. Use the quote form or call +372 641 8550. Vesmaco manufactures and delivers the tanks; installation is done by your installer and emptying by your local emptying company.

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