Sizing: Sewage Flow, Lot Size and Field Length
Systems are sized on estimated daily sewage flow combined with soil permeability. Bedroom count, not the number of people living there, drives residential sizing.
19 questions in this section
- How is my system size determined?
- How much flow is assumed for a house?
- Why is it based on bedrooms and not on how many people live here?
- What is the minimum lot size for a septic system?
- How is lot width measured?
- How long does my disposal field need to be?
- What is the minimum total field length?
- How many concrete leaching units would I need instead?
- What size septic tank goes with each flow?
- What is the maximum flow a conventional system can handle?
- Does a cottage get sized differently from a house?
- What if I add a jacuzzi or soaker tub?
- What if I install a garbage grinder or in-sink food disposal?
- What about high-flow showers?
- Will low-flow fixtures let me install a smaller system?
- What is the minimum design flow for a commercial or institutional building?
- Can real metered water data be used instead of the table?
- What is the expected strength of household wastewater?
- How many toilet flushes are assumed per person?
How is my system size determined?
By the estimated daily sewage flow the system will receive, combined with the soil conditions and permeability found in the test pit. The field is sized on the soil with the slowest permeability within 1.2 metres (4 feet) below the distribution pipe.
How much flow is assumed for a house?
One bedroom, 750 L/day. Two bedrooms, 1,022 L/day. Three bedrooms, 1,365 L/day. Four bedrooms, 1,705 L/day. Each additional bedroom adds 350 L/day.
Why is it based on bedrooms and not on how many people live here?
Because the system has to serve the house over its whole life, not just your household. A future family may be larger. Sizing on bedrooms builds in that capacity.
What is the minimum lot size for a septic system?
For flows up to 1,365 L/day (a three-bedroom house), the minimum lot is 4,000 square metres, with a minimum width of 54 metres and a minimum depth of 38 metres. Larger flows require larger lots - up to 8,050 square metres for flows of 4,091 to 5,460 L/day.
How is lot width measured?
Width is measured parallel to the road and is not necessarily measured at the road frontage. Depth is measured perpendicular to the road.
How long does my disposal field need to be?
For a three-bedroom house (up to 1,365 L/day) using pipe and stone: 60 metres of distribution pipe in Category A soil, 82 metres in Category B, and 100 metres in Category C. Poorer soil means a longer field.
What is the minimum total field length?
60 metres (200 feet) for a pipe and stone system. If a minimum design flow of 500 L/day is used for a non-residential building, the minimum system length is still at least 60 metres.
How many concrete leaching units would I need instead?
For up to 1,365 L/day: 6 units in Category A soil, 8 in Category B, 10 in Category C. The units are 4 feet by 8 feet. Larger flows scale up to 24, 32 or 40 units.
What size septic tank goes with each flow?
Up to 1,365 L/day requires a 3,410 L working capacity tank. From 1,366 to 2,055 L/day, 4,090 L. From 2,056 to 4,090 L/day, 6,136 L. From 4,091 to 5,460 L/day, 8,180 L.
What is the maximum flow a conventional system can handle?
5,460 litres per day (1,200 imperial gallons per day). Anything above that requires a non-conventional, engineered system.
Does a cottage get sized differently from a house?
No. Individual cottages are sized as per residential flows in Appendix D. Seasonal use is not treated as temporary use.
What if I add a jacuzzi or soaker tub?
Fill-and-drain style hydro-massage and soaker tubs add flow calculated as the volume of the tub in litres minus 340 L, then multiplied by two. That extra volume must be included in the design.
What if I install a garbage grinder or in-sink food disposal?
For a residence, expect a 5 percent increase in peak daily wastewater volume, a 30 percent increase in wastewater strength, and a 50 percent increase in the sludge storage volume required in the septic tank. It is one of the most consequential fixtures you can add.
What about high-flow showers?
For showers discharging more than 13 litres (3 imperial gallons) per minute, add 50 litres for every 6 litres per minute, or portion thereof, above that threshold.
Will low-flow fixtures let me install a smaller system?
No. Peak daily flow estimates cannot be reduced because water-saving fixtures are used, unless the increased wastewater strength is properly accounted for. Using less water concentrates the waste, so no reduction in treatment area is applied.
What is the minimum design flow for a commercial or institutional building?
500 litres per day, with a minimum system length of at least 60 metres.
Can real metered water data be used instead of the table?
Yes, under conditions. The data should cover the most recent 6 to 12 months of continuous operation or three full seasons, come from a comparable facility, and include a 20 to 50 percent increase factor for future growth and peaks. With less than 6 to 12 months of data, or only period averages, the figures should be increased by up to 100 percent.
What is the expected strength of household wastewater?
As 30-day maximum averages: BOD5 of 300 mg/L, total suspended solids of 350 mg/L, and fats, oils and greases of 35 mg/L. These are based on mass loadings of 50 to 60 grams of BOD5 or TSS per person per day.
How many toilet flushes are assumed per person?
Five flushes per resident per day for residential design.
Have a question about your own property? General answers only go so far — your soil, slope, water table and setbacks are specific to your lot. Call (506) 233-6272 or send us the details.
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