5 Criteria for Sizing a Septic System

A prerequisite to getting a building permit is a septic systems study and report undertaken by an engineer (see "The Septic System Report" - April 30).

Well Peter MacIntosh, the engineer that conducted the study provided a report about two weeks ago. Turns out that I have the perfect location for a septic system - enough area and the right soil.

The report covers the following considerations:

1. Size of the Cottage
The size of the required septic system is directly related to the size of the Cottage. More precisely is the number of people regularly using the cottage, hence, the volume of waste generated. The engineer uses the number of bedrooms as a proxy for the number of people. In our case we will have three bedrooms which is a proxy for four to six people on a regular basis.

This is a save number given the cottage will mostly be used on weekends and primarily three months of the year. Thus the weekends that boost the number beyond six - those when the whole (extended) family is there for a get together, sons, grandchildren, cousins and their friends - averages out with the weeks there is no one at the Cottage.

2. Topography of the site
It is important that there be a reasonably level area the right size for the septic tank and the seepage bed. While there are system designs for a slope, they start to be a bit more pricey. It is also better that the location of the septic system be a bit lower than the cottage.

3. Soil & Percolation
The type of soil that the seepage bed will be placed in is very important. The fluids coming out of the of the septic tank need a place to go and disperse. So it is important the that soil have a high percolation rate - that is the rate at which water flows through the soil. Certain kinds of soil are great and others are verybad for septic systems. Other than solid rock, clay soils are the worst - water just pools and doesn't drain away very well. Sand is the holy grail. We are fortunate as the soil up at the lot is a gravely sand. The engineer indicates that the percolation rate for this type of soil is less that 4 min/cm and considered to be very permeable.

4. Water supply
The septic system must be positioned as to not foul your source of drinking water. I intend to draw my water from the lake which is more than 75 feet from the septic system. In the future I may consider a well. The engineer indicated the best place for the well. Any source of water must be at least 50 feet from the seepage bed.

5. Choice of System
The choice of system is determined by the engineer from the above requirements. Our system is a 850G NQ approved prefabricated concrete septic tank and a modified seepage bed utilizing a new higher efficiency infiltration chamber system. This system allows for a seepage bed that is up to 30% smaller that the classic tile and stone system. I will describe this new system in a future post.

Bottomline is that with the septic engineering report I was able to secure the building permit. In fact I had the permit the next day.

Coming up in the next few posts:\
  • Getting the building permit
  • Investigating the infiltration chamber system