How do you know whether the septic tank outside your home is big enough for your household? Bedroom counts give us a starting point, but we also need to understand your water use and the tank’s actual treatment capacity. Here, we walk through a four-bedroom example and explain what those numbers mean when you own or are buying an acreage.
Key takeaways
- Bedrooms help us estimate future demand. Your actual water use is what matters. e.g., we have seen a couple that uses more water than a family of four.
- Working capacity is the number that matters. The 48-hour rule for proper septic tank settling and separation is simply 2x your household’s peak water use. Something that can be calculated and confirmed.
- The four-bedroom table specifies 940 Imperial gallons. The video’s 900-gallon example explains retention time simply; there is a 40 gallon calculation for septic tank sludge accumulation over 3 years of use.
- Your water-use habits belong in your household’s water profile. Include all water use: guests, laundry and any filtration equipment sending reject or backwash water into the tank.
- The whole system has to fit. Tank capacity, delivery, distribution and the receiving soil all need to support the household water use that dictates the demand on your system’s wastewater processing.
Why bedrooms are the starting point
When a new home is being designed, we may not know who will eventually live there. Bedroom counts give the designer an occupancy allowance to work from. Alberta’s 2021 Standard of Practice uses the following starting point for a typical single-family home.
| Bedrooms | Design occupancy allowance |
|---|---|
| 1 | 2 people |
| 2 | 4 people |
| 3 | 4.5 people |
| 4 | 6 people |
For three or more bedrooms, the allowance is 1.5 people per bedroom. The fraction is a design allowance; it doesn’t mean someone expects half a person to move in. It gives us a consistent way to estimate use before the household is known.
Once you know who is living in the home, you can also look at their actual water use. Both pieces of information are useful. Today’s occupants help us understand operation, while the home’s design allowance helps us plan for the use the property is intended to support.
The four-bedroom example
Our four-bedroom example allows for six people. Using the standard’s 50 Imperial gallons per person for mean daily use and 75 for peak daily use, the arithmetic looks like this.
| Daily flow | Calculation | Imperial gallons |
|---|---|---|
| Mean or average | 6 people × 50 | 300 per day |
| Peak design allowance | 6 people × 75 | 450 per day |
We use Imperial gallons here to match the Alberta reference. One Imperial gallon is approximately 4.55 litres, so be careful when comparing a Canadian design with equipment literature that uses US gallons.
The peak allowance gives the design room for busier days. It is also a starting point: extra fixtures, unusually high water use and equipment discharges need to be included when they add to the expected workload.
Why the tank needs working room
In a conventional septic tank, gravity helps heavier solids settle and lighter fats, oils and grease float. The liquid between those layers continues toward the field for further treatment. Giving the tank adequate time and space for separation helps keep material in the tank that should not be carried into the distribution system.
For a given liquid volume, increasing the amount of wastewater passing through shortens the average time it remains in the tank. That is the relationship I’m illustrating with the 48-hour example in the video.
At 450 Imperial gallons per day, two days of liquid volume would be 900 Imperial gallons. It is a straightforward way to show why tank volume and water use belong in the same conversation. Actual settling performance also depends on the tank’s arrangement, flow patterns and the space occupied by accumulated solids.
Where the 940 gallon figure fits
Alberta’s four-bedroom table specifies 940 Imperial gallons of septic tank working capacity for the standard residential case. The standard accounts for sludge and scum storage as well as wastewater flow. The video’s 900-gallon calculation illustrates retention time; it is not the complete regulatory sizing calculation.
That working capacity is the treatment portion of the tank. Air space and a separate pump, siphon or effluent chamber do not count toward it. So a tank described as having a large total volume has a smaller settling capacity.
This is also why pumping and maintenance matter and where the extra 40 gallon calculation comes from. As sludge and scum build up, they take up space that was available for liquid and separation. A tank’s dimensions stay the same, but its operating volume capacity changes between cleanouts.
Your household may use water differently
The bedroom calculation gives you a reference. Your own household gives us the real picture. How many people live there? How often do you host overnight guests? Are long showers and large baths part of normal use? Does most of the laundry happen on one day? Do you have a one-to-one water-to-reject ratio reverse osmosis water filtration system that rejects 1 litre of water, for every 1 litre of usable home water, which effectively doubles your household water use. If that reject water goes into your septic tank, it obviously will have an impact, as it doubles processing volume.
All water-treatment equipment belongs in the conversation and final calculation. Iron filters, water softeners and other equipment may send backwash or regeneration water. We need the equipment’s actual discharge volumes, schedule and composition.
A useful starting record includes normal days, laundry days and a period with guests. If you have a water meter, tell us where it sits in relation to the treatment equipment and outdoor irrigation. That helps us work out how much of the measured water actually becomes septic wastewater.
What this means for an older acreage
A system installed ten, twenty or thirty years ago needs to be understood on its own terms. We look for the original design, tank model and working capacity, then compare those with the home and household today. Has the home gained bedrooms, bathrooms or occupants? Has a water-treatment system been added? Have the tank’s internal components stayed intact?
An older tank may have plenty of capacity. Another may be operating close to its limits. Age alone doesn’t answer the question, and today’s bedroom count doesn’t tell us what was installed years ago.
The tank is one part of the assessment. The delivery system must move the effluent properly, and the field and receiving soil must be able to accept and treat it. A larger tank won’t, by itself, increase the amount of water the soil can handle.
You don’t need to arrive with all the answers. Bring whatever records you have, tell us how your household uses water and let us establish what is there. That gives you a much better basis for deciding whether maintenance, restoration or a change to the system makes sense.
Buying an acreage or wondering whether your current system fits your household? Contact us to discuss your tank capacity, water use and field performance.