Fixture Units or Bedrooms for large single family residences 2

[gsisoils]

I have been providing onsite waste waters system design for large commercial, residential, industrial projects. However, I can never find information on septic tank sizing recommendations for large single family residences (approximatley six to seven thousand square feet). I know that everone ask how many bedrooms, some are two others are up to seven. However, each of residence has a similar seventy to ninty fixture unit count. I have always used a minimum three to five day hold time due to flow variation. Is there any resources that address a specific issue such as this?

 

[RWF7437]

If you're in the US your County Sanitarian is probably the best source for specific requirements for septic systems in your area.

 

[Spartan5]

Bedrooms and fixtures don't make wastewater, people do. While you can sometimes get a feel for what the immediate occupancy of a building is going to be, you don't know what the future holds. To that extent it is good to be conservative in your design.

I think a good rule of thumb is to assume 2 people per proposed bedroom and a 50 gallon per day per capita wastewater flow. You can then use your retention time to dial in how conservative you want your design to be. If it is typical for buildings in those areas to be underutilized, use 3 days. If you want to be more conservative, use 5 days. In a 6 bedroom house of 6,000 square feet we are talking the difference between 1,800 gallons and 3,000 gallons of tankage. I would err to the side of caution myself and go with 3,000 gallons.

3,000 gallons of tankage might be conservative for the typical daily use of the system. However, large houses like this can be used to entertain a great number of people. This will cause a surge of wastewater through the system which has the potential to "flush" the septic tank.

Effluent filters are always a good idea for any system. I would definitely use one in this application to protect the absorption system from solids carryover in the event the system gets hit with a large surge. I'd couple it with a simple highwater alarm also to make the homeowner aware that the filter needs maintenance. This will protect them from backups in the event the filter clogs.

 

[PEDARRIN2]

The International Code Council has a Private Sewage Disposal Code that should address a lot of your questions. In the county in which I live, the Health Department (which typically has jurisdiction over septic systems) uses this code.

Take a look at it.

 

[kchamilton]

You may want to take a look at the Pitkin County (colorado)health department ISDS regs. They govern the Aspen area where there are many large homes. They up the flows per bedroom based on square footage of the house and have higher peaking factors as well.