If you’ve ever lived outside city limits in Texas, chances are you’ve relied on a septic system. But what exactly is a septic system—officially called an On-Site Sewage Facility (OSSF) by the Texas Commission on Environmental Quality (TCEQ)? Whether you’re building a new home, buying rural property, or developing land, understanding how these systems work and what’s required to install them can save you time, money, and stress.

This guide breaks down the OSSF design process, the soil evaluation, system components, and why different systems are used depending on your property and soil conditions.

OSSF Design Process

In most Texas counties, septic permitting follows TCEQ Chapter 285 regulations. Every OSSF design includes two major phases:

1. Field Observation (Site Evaluation)
2. System Design

Below is a breakdown of each step and what to expect.

1️⃣ Initial Planning & System Placement

Before digging a single hole, the designer must review state and county requirements and determine where each part of the system will go. Typical setbacks include:

• 100 feet from any water well

• 5 feet from property lines

• Clear of all buildings, driveways, pools, and future improvements

Placement must also consider:

• Topography

• Soil depth

• Drainage patterns

• Space for both the primary and reserve leach field

Tip: If you're developing rural land, check out our post on Texas Subdivision Groundwater Study Requirements, as water availability affects on-site wastewater planning.

2️⃣ Soil Evaluation (Required by TCEQ)

Once a preliminary layout is identified, the designer must dig at least two soil pits or drill two boreholes. This step is critical—the performance of a septic system depends entirely on the soil’s ability to absorb water.

Who Can Perform the Site Evaluation?

TCEQ allows the evaluation to be performed by any of the following licensed professionals:

• Licensed Professional Engineer (P.E.)

• Licensed Professional Geoscientist (P.G.)

• TCEQ-Licensed Site Evaluator

• Registered Sanitarian (R.S.)

These professionals identify soil type, texture, color, restrictive layers, and groundwater indicators. They also determine the soil loading rate, which directly determines the size of the leach field.

Main Components of an OSSF

Depending on the property and soil classification, you may have either a two-part or three-part system:

• Anaerobic Systems:
  Septic tank + leach field
  (Most rural homes fall into this category.)

• Aerobic Systems:
  Septic tank + aerobic treatment unit (ATU) + spray or drip dispersal system
  (Used when soils cannot support traditional leach fields.)

Septic Tanks

The septic tank receives all household wastewater and performs primary treatment:

• Solids settle to the bottom

• Fats and oils float to the top

• Bacteria begin breaking down waste

Partially clarified liquid exits to the drainfield

In Texas, septic tanks are usually precast concrete, though cast-in-place tanks are sometimes required—especially beneath pavement or in high-traffic areas.

Leach Field

This is where treated liquid from the septic tank enters the soil and disperses safely. The two most common systems in Texas are:

Perforated Pipe (Gravel Trench)

A traditional system that uses:

• Gravel-filled trenches

• A perforated distribution pipe

• Soil infiltration along the trench bottom and sides

Chamber Systems

In West Texas especially, chambers have become the preferred option because they:

• Install faster

• Require no gravel

• Qualify for a TCEQ-approved reduction in trench length

Perform well in clay soils and variable site conditions

Chambers are lighter, cleaner to install, and offer more storage volume—making them ideal for rural West Texas soils.

Aerobic Treatment Unit (ATU)

Some properties require an aerobic system, particularly where:

• Soils are shallow

• Clay or restrictive layers prevent adequate absorption

• Higher-strength wastewater (restaurants, commercial kitchens) is present

• Small lots limit available area for a traditional leach field

Aerobic systems inject oxygen into the wastewater to allow bacteria to break down solids more efficiently. They work well but:

• Cost more upfront

• Require routine maintenance

• Have more mechanical components

For these reasons, aerobic systems are only used when absolutely necessary.

Conclusion

If you’re planning to build a home or commercial project outside city limits, a properly designed septic system is essential to protecting your investment and complying with Texas law.

At Legacy Engineering, we handle the entire OSSF process—from soil evaluation and system design to county permitting and construction coordination. Whether you need a standard residential septic system or a complex commercial design, our team ensures your project meets TCEQ requirements and performs reliably for decades.

Ready to get started?

Contact Legacy Engineering today for professional septic system design, site evaluation, and rural development services.