Septic tank for 3, 4 or 6 people: sizing by occupants and peak flow
"Which septic tank do I need for a 180-square-metre house?" sounds like a reasonable question, but it has no answer: a treatment plant takes litres, not square metres. Two numbers do the sizing — the daily wastewater volume and the maximum peak discharge — and it is the second one that more often sets the limit. Here is how to get from the number of occupants to a specific model.
Daily volume: people, not floor area
The volume of wastewater equals the volume of water used: almost everything that enters the house through the pipe leaves through the drain. The daily load is therefore calculated from the number of permanent residents multiplied by the specific water use per person.
That per-person figure depends on how the house is equipped. A house with one bathroom and no domestic appliances stays near the lower end of the usual 150–250 litres per person per day. Two bathrooms, a bathtub instead of a shower, a dishwasher and a washing machine push the figure towards the upper end. A whirlpool bath or a swimming pool is counted separately, outside this calculation: that discharge is not domestic wastewater and is never routed into a biological plant.
Count permanent residents, not beds. A guest room occupied two weeks a year does not set the daily load. Rounding down is unwise too: a house is built for decades, and a household changes more often than the plant does.
Peak discharge: the second number that decides
The daily volume is an average, and wastewater does not arrive as an even trickle. The morning rush in a house with two bathrooms, or a bathtub emptied in the evening, produces a peak several times the average hourly flow.
Peak discharge is the volume that arrives within a few minutes:
- ✓a bathtub — usually 150–200 litres per emptying;
- ✓a washing machine — 40–60 litres per cycle;
- ✓a dishwasher — 10–20 litres;
- ✓a shower — 40–80 litres per use.
Do not add all of it up; add only what realistically coincides. A bathtub and a washing machine often run together in the evening, whereas two bathtubs emptied one after the other are rare. Compare the resulting figure with what the plant can take in one go.
Why this matters more than the average flow: if the peak volume exceeds what the plant can accept and hold, the water passes through the aeration zone faster than the sludge can treat it. On paper the daily capacity has not been exceeded, yet turbid water and sludge carry-over appear at the outlet. That is what a typical sizing error looks like — the plant was chosen by the number of occupants, and in the first week after the family moves in it starts carrying sludge out into the drainage ditch.
The range by number of users
SOLVIA Bio models differ in the number of users and in daily capacity. The range steps up evenly — 200 litres per day per user, the upper end of ordinary domestic water use.
| Model | Users | Capacity, l/day |
|---|---|---|
| SOLVIA Bio 3 Aero | up to 3 | 600 |
| SOLVIA Bio 3 Long | up to 3 | 600 |
| SOLVIA Bio 4 Long | up to 4 | 800 |
| SOLVIA Bio 5 Long | up to 5 | 1,000 |
| SOLVIA Bio 6 Long | up to 6 | 1,200 |
| SOLVIA Bio 8 Long | up to 8 | 1,600 |
Read the table with one caveat. "Up to 6 people" and "1,200 l/day" are the same limit stated twice, and they line up only at 200 litres per person. Take a house with four residents and two bathrooms, where real use is closer to 250 litres per person: on litres it lands in the five-user model, not the four-user one. Go by the litres — the number of users is a convenient label for them, not a criterion of its own.
Two models cover "up to 3 people": Bio 3 Aero and Bio 3 Long. They match on both users and capacity, and the specifications do not say how the two versions differ. For the load calculation that makes no difference — both cover the same figure — but if the distinction matters on a particular site, settle it before ordering rather than guessing from the description.
When to take one step up the range
Spare capacity costs money, so you buy it for specific reasons, not just in case. There are four:
- ✓The calculation landed on the boundary. The daily volume came out equal to the model capacity, or within a few per cent of it. A boundary is not an operating point: the plant needs room for uneven loading.
- ✓The house is equipped for the upper end of water use. Two bathrooms, a bathtub, a dishwasher and a washing machine, regular guests.
- ✓The family is expected to grow, or part of the house will be let. Digging up the plot and replacing the tank costs more than buying one size larger once.
- ✓Peak discharge is at the limit. It is worked out separately from the daily volume, and it often rules out a model that fitted on daily figures.
The opposite mistake also happens. A plant chosen with a large margin runs permanently underloaded, and the activated sludge goes short of food: the biomass never builds up to its design level and treatment quality wanders. Twice the capacity "just to be sure" is not caution, it is the other sizing error. A sensible margin is one step up the range, not two.
What besides occupant numbers drives the choice
The number of occupants settles the model but not the whole scheme. Three more things are worked out at the same time.
Occupancy pattern. Biological treatment lives on a steady inflow of wastewater and on electricity for the aeration. Year-round occupancy suits it; a weekend house that stands idle five days out of seven is hard on it.
How the treated water leaves. Across the range it is either gravity or pumped. Gravity needs the receiving point to sit lower than the outlet; otherwise the scheme gains a pump, and with it a dependence on electricity and one more item to service.
Room for the tank. These are vertical units in a single shell: the footprint is small, but you need depth. Check the groundwater level at its seasonal peak, and check that a vacuum truck can pull up to the access neck — the hose has to reach.
What next
To pick a model, gather five figures:
- ✓the number of permanent residents;
- ✓how the house is equipped: bathrooms, bath or shower, washing machine and dishwasher;
- ✓the calculated daily wastewater volume in litres;
- ✓the maximum peak discharge — the sum of the discharges that realistically coincide;
- ✓the occupancy pattern: year-round, seasonal or weekends only.
With those numbers in hand you can look through septic tanks for the home and match your calculated daily volume against the model capacities. If the plot itself has not been surveyed yet — the soil, the groundwater level and where the treated water will go are all still unknown — start there instead: sizing a model by occupant numbers takes five minutes, while the disposal scheme is dictated by the site, and the site rules out more options than capacity ever will.
