Septic System: The Design

A conventional septic system consists of a septic tank and an absorptive drain field. I’ll cover sizing and locating both in this articles. I’ll also refer and link to Texas Administrative Code Chapter 285 fairly often. It’s the statewide code for on-site sewage facilities otherwise referred to here as septic systems.

Sizing the Tank

A septic tank must be able hold a minimum of three days worth of wastewater. The length time wastewater is held before being released into the drain field is referred to as Hydraulic Retention time or HRT.

Table III in the appendices of Chapter 285 lists waste water usage rates based on the number of bedrooms in a house. Note, the code does allow some leeway for usage rates. Even though our cabin with one bedroom falls under the first category with a usage rate of 180 gallons per a day, our county has their own informal “tiny house” usage rate of 100 gallons per a day. I wrote in the previous variance articles that we had negotiated down to 80 gallons per a day but that ultimately didn’t work out. See my story of rejection below.

100 gallons per a day X 3 days = 300 gallons.

However, per Table II of the appendices of Chapter 285, 750 gallons is the minimum septic tank size.

Sizing the drain field

A standard drain field is a trench, 2 – 3 feet wide and a foot and a half to three and a half feet deep. The bottom of trench must be level to within one inch over every 25 feet. There are several options for what goes in the trench but I opted for leaching chambers. Leaching chambers are bottomless chambers installed with the open bottom in direct contact with the bottom of the trench.

Leaching Chamber

The required length of a drain field trench is largely determined by the soil class in the drain field area.

Chapter 285 requires that a site evaluator or professional engineer dig two holes two feet deeper than the proposed trench depth to observe the characteristics of the soil. My engineer advised me that it would significantly less expensive if I were to dig the holes. So, I dug two four feet deep holes with my post hole digger and called the engineer back.

Soil Class Table

Chapter 285 identifies 4 soil classes with class I being divided into two sub types. It has been determined how many gallons of water a square foot of each soil class can absorb in one day.

The engineer came out, confirmed the depth of the holes and jotted down some notes. He said the soil looked like Class III but he would take a baggie of the spoils and let me know. He also confirmed that there was no sign of ground water seepage or a layer of bedrock or other restrictive horizon. Either of these two cases would have ruled out using a standard absorptive drain field.

Later that day he confirmed that it was class III. We’ll take a look back at that soil class table focusing on that last column.

The Drain field calculation

To calculate the how long our trench needs to be we first need to know how many square feet of of absorption area (A) we need. We’ll divide our waste water usage rate of 100 gallons per a day by the absorption rate of square foot of Class III soil of 0.20 gallons per a day to get 500 square feet.

A = 100 gallons per a day / 0.20 gallons per day per square foot = 500 square feet

Now, looking at this profile of a trench. Water is absorbed through the bottom of the trench and up to 1 feet on either side. So a 3 wide trench has 5 feet of sides absorbs waste water. So, 500 square feet divided by 5 feet equals 100 feet of trench.

500 square feet / 5 feet = 100 feet

However, Chapter 285 allows a 25% reduction in trench length if leaching chambers are used bringing my trench length down to 75 feet. I found a leaching chamber that came in 5 foot lengths so that worked our perfectly.

0.75 X (500 square feet / 5 feet) = 75 feet of 3 foot wide trench

Out of curiosity, here are some calculations for different soil types:

Class Ib

A = 100 gpd / 0.38 gpd sqft = ~264 sqft

0.75 X (264 sqft / 5 ft) = ~40 feet of 3 foot wide trench

Class IV

A = 100 gpd / 0.10 gpd sqft = 1000 sqft

0.75 X (1000 sqft / 5 ft) = 150 feet of 3 foot wide trench (Yikes!)

As much as I would love to have class Ib soil I’m happy to have class III.

Site Plan included with design submittal

Drain field details

Due to space limitations I decided to use three 25 foot long trenches. There must be at least 3 feet of undisturbed soil between each trenches so I went with 4 feet. My total drain field area was 25 feet by 17 feet

Each trench must be level within 1” for every 25’ per section 285. For this reason trenches should parallel the contour of the land. Our drain field is located on a slight slope so the bottom of the first trench is about a foot higher than the bottom of the last trench. The solution was to use two PVC elbows at then end up the upper trenches to create an overflow threshold so that when an upper trench fills up with water it overflows into the one below.

Other design considerations include:

  • You need to run 3” schedule 40 PVC pipe from a two way sewer clean out at the house to the septic tank with a minimum of 1/8” per 1 foot of run. There is no maximum slope.
  • There must be a clean out within 5’ of any 90° bend in the pipe running to the septic. It preferable not to have a 90° bend at all and use two 45° bends instead.
  • There must be 5’ of pipe between the septic tank outlet and the drain field trench.
  • There must be at least 1 foot of drop from the septic tank outlet to bottom of the drain field trench.
Elevation Profile included with design submittal

Submitting and resubmitting my design

My application was rejected several times. The biggest kicker was that I ultimately had to design the drain field to accommodate 100 gallons of water per day instead of 80 because the development services had an informal “tiny house” rule. I also had mistakenly listed the engineer I consulted with for the variance and had advised me on the final design as the designer so the county wanted his seal of approval on the design documents. So, I had to hire him again(for an hour of billable time) to send sternly worded letter to the county stating that he was merely an advisor and not the designer and that a professional engineer is not required to design a conventional septic system in Texas.

Other points I had to modify:

  • First revision: In addition to the 100 gallon rule and engineer’s seal debacle, the county also requires an effluent filter be used in the septic tank outlet tee and noted on the design document.
  • Second revision: I had originally planned to use a distribution box to evenly disperse water to all the trenches but the county said it wasn’t suitable for trenches at different depths. This was mildly irritating as it had to been on my initial design but didn’t get rejected until my second revision. There was also a dearth of information on using distribution boxes with sloped drainfields
  • Third revision: I had the trenches at the same elevation but that was rejected because the first trench was too deep. E-mailing back and forth the county official suggested the overflow pipe linking the trenches.

With my fourth revision I received my authorization to construct valid for one year. The whole process from submitting my first design took about six weeks. Here’s the final design submittal.