Monthly Mechanics: Introduction to Septic Design

So you want to design septic systems for a living? Your answer is probably “omg no.” But consider: almost every household in rural America lacks a sewer connection and needs to provide its own wastewater treatment. In Vermont, that applies to over 50% of homes – maybe 150,000 units in all. And the number of people licensed to design septic systems in the state is relatively tiny – about one hundred professional engineers, and a few hundred other individuals who have passed a qualifying exam.

Moreover, wastewater systems – unlike the houses themselves – are strictly enforced in all communities. It’s an issue of public safety. If your house collapses, you’ll only hurt yourself. But if you dispose of your waste improperly, you’ll poison the water supply, potentially sickening lots of people and wildlife for years to come. So those several hundred designers have a captive market.

Usually the system takes the form of a septic tank, where solids (aka POOP) are allowed to settle out, followed by a field where effluent (the remaining wastewater without solids) safely drips into the soil. The first step in designing a wastewater system is to run a percolation test, or perc test, to find out if the existing soil on site is sufficient.

Here’s how a perc test works. First, dig a hole 10 inches deep and 6-8 inches wide. Place 1 inch of crushed stone on the bottom. Then start your timer and pour 6 inches of water into the hole as steadily as possible. (Experts recommend using a siphon.) You need to record how long it takes for the water level to drop a certain amount, with the distance depending on the type of soil you have. As soon as the water level drops the required amount, refill to 6 inches. Repeat a total of 7 times. The percolation rate is the average speed at which the water level drops.

If the percolation rate is faster than a prescribed rate (often 120 minutes per inch), then you can say that the soil percs, and it may be possible to construct a leach field on the existing ground. There are other factors to consider, including ground slope and isolation distance, to determine if a site is appropriate as is. If the soil doesn’t perc, then a mound system is required, which means trucking in appropriate soil to a depth determined by the needs of the system.

Septic1

Every person living in a house is assumed to generate wastewater in the amount of 70 gallons per day. Since the number of occupants can change over time, the design flow for a house depends on how many bedrooms it has. In Vermont, the first three bedrooms count for two persons each, and additional bedrooms count for one person each. Thus, a three-bedroom house has a design flow of 6*70=420 gallons per day, and a four-bedroom house has a design flow of 7*70=490 gallons per day. This is the volume a septic designer must prove the system can handle.

Interested yet? Read about septic designer licensing at the Vermont ANR website.

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