How Does a Septic System Work?

A septic system treats household wastewater in three stages: collection in a 1,000-1,500 gallon underground tank, biological breakdown by bacteria, and soil absorption through a drain field.

According to the EPA, more than one in five U.S. households depend on septic systems. In Texas alone, over 2.5 million homes use septic instead of municipal sewer. If your home isn't connected to a city sewer line, your septic system handles every drop of water that goes down a drain, gets flushed, or runs through the washing machine.

The system is simpler than most people expect. No moving parts in a conventional system. No electricity required. Just gravity, bacteria, and soil doing the work underground.

Stage	What Happens	Key Component	Timeframe
1. Collection	Wastewater separates into 3 layers	Septic tank (1,000-1,500 gal)	24-36 hours
2. Biological treatment	Anaerobic bacteria reduce solids 40-50%	Bacteria inside tank	Continuous
3. Drain field absorption	Soil filters and bacteria clean water	Perforated pipes in gravel	Days to weeks

What Happens When Wastewater Enters the Septic Tank?

All wastewater from your home flows into a buried concrete, fiberglass, or polyethylene tank that holds 1,000-1,500 gallons for most Texas homes.

Every drain in your house connects to a single main pipe that runs underground to the septic tank. The tank is usually buried 1-3 feet below the surface, anywhere from 10 to 50 feet from your house.

Inside the tank, wastewater separates into three layers:

• Scum layer (top): Fats, oils, grease, and anything lighter than water float to the surface
• Effluent layer (middle): Relatively clear liquid that will eventually flow to the drain field
• Sludge layer (bottom): Heavy solids settle to the bottom

The tank holds wastewater for 24-36 hours. During that time, anaerobic bacteria (bacteria that don't need oxygen) start breaking down organic matter in the sludge. This biological process reduces the volume of solids, but it doesn't eliminate them entirely. That's why pumping is necessary every 3-5 years to remove accumulated sludge.

An outlet baffle near the top of the tank lets the clear effluent flow out while keeping scum and sludge inside. Some tanks have an effluent filter at the outlet, which catches smaller particles before they reach the drain field.

How Do Bacteria Treat Waste Inside the Tank?

Bacteria inside the tank break down organic waste without oxygen (anaerobic digestion), reducing solids by about 40-50% before the liquid moves to the drain field.

The treatment happening inside your tank is biological, not chemical. Billions of naturally occurring bacteria consume organic waste. They don't need additives, supplements, or special products to do their job. In fact, pouring bleach, antibacterial soap, or chemical drain cleaners down the drain harms these bacteria and reduces the tank's treatment ability.

In a conventional system, the tank provides primary treatment only. The effluent leaving the tank still contains bacteria and nutrients that need further processing. That's where the drain field comes in.

In an aerobic system, a powered aerator pumps oxygen into a treatment chamber, supporting different bacteria that break down waste much faster and more thoroughly. Aerobic systems add a chlorination step to disinfect the effluent before it's pumped to spray heads in your yard. The result is cleaner effluent that's safe for surface dispersal.

How Does the Drain Field Treat and Absorb Wastewater?

Treated liquid from the tank flows through perforated pipes into gravel-filled trenches, where soil bacteria finish the treatment and cleaned water slowly percolates into the groundwater.

The drain field (also called a leach field) is a series of perforated pipes buried in gravel-filled trenches across your yard. For a 3-bedroom home in Texas, the drain field might cover 300-450 square feet.

As effluent trickles through the gravel and into the surrounding soil, two things happen:

Physical filtration. Soil particles trap bacteria, viruses, and remaining solid particles. Sand and loamy soils filter effectively. Clay soils filter well too, but absorb water much more slowly.

Biological treatment. Aerobic bacteria living in the top 2-4 feet of soil consume remaining organic matter and pathogens. By the time water reaches the groundwater table, it's been cleaned to near-drinking-water quality.

The drain field is the most expensive component to replace ($5,000-$20,000), and homeowner behavior directly impacts how long it lasts. A Texas Water Resources Institute study notes that conventional drain fields in Texas typically last 20-30 years with proper care. Overloading the system with too much water, driving vehicles over the field, or allowing solids to escape the tank will shorten its life significantly.

What Is the Difference Between Conventional and Aerobic Septic Systems?

Conventional systems use gravity and soil for treatment ($3,000-$8,000), while aerobic systems use powered aeration and chlorination ($10,000-$20,000) but work in poor soil conditions.

Texas has both types, but aerobic systems are more common here than in most states. The reason: clay soil. Many areas of Texas have soil that absorbs water too slowly for a conventional drain field to work. When a perc test (soil percolation test) shows slow drainage, TCEQ requires an aerobic system instead.

Feature	Conventional	Aerobic
How it treats waste	Anaerobic bacteria in tank, aerobic bacteria in soil	Powered aeration + chlorination in tank
Moving parts	None (gravity-fed)	Aerator pump, effluent pump, timer
Electricity needed	No	Yes
Maintenance contract	Recommended	Required by Texas law (quarterly)
Installation cost	$3,000-$8,000	$10,000-$20,000
Annual maintenance	$0-$400 (pumping every 3-5 years)	$275-$535 (maintenance contract)
Works in clay soil	Only if perc test passes	Yes, designed for poor soil
Drain field type	Buried gravel trenches	Spray heads on surface or drip lines

If your yard has spray heads that pop up periodically and spray water across the lawn, you have an aerobic system. If not, you likely have a conventional system.

How Does Texas Regulate Septic Systems?

TCEQ (Texas Commission on Environmental Quality) oversees all septic systems in Texas through the OSSF permit program, with county-level enforcement.

In Texas, septic systems are officially called "On-Site Sewage Facilities" or OSSFs. TCEQ sets the statewide rules under 30 TAC Chapter 285, but your county or local authority handles day-to-day enforcement.

Here's what the permit process looks like for a new system:

1. A licensed site evaluator performs a soil test (perc test) on your property
2. Based on the soil results and your home's bedroom count, a licensed installer designs the system
3. You or your installer applies for an OSSF permit from the county
4. After installation, the county inspects the system before it's covered with soil
5. For aerobic systems, you must sign a maintenance contract with a TCEQ-licensed provider

System sizing in Texas is based on bedroom count, not bathroom count or square footage. TCEQ uses 150 gallons per day per bedroom as the baseline:

Bedrooms	Daily Flow Estimate	Minimum Tank Size
1-2	300 gallons	1,000 gallons
3	450 gallons	1,000-1,250 gallons
4	600 gallons	1,250-1,500 gallons
5+	750+ gallons	1,500+ gallons

When Should You Call a Septic Professional?

Understanding how your system works helps you spot problems early. As Nathan Glavy, Extension Program Specialist at Texas A&M's Water Resources Institute, notes, "Inspecting and maintaining septic systems can help prevent negative impacts on local water quality and public health." All repairs beyond basic maintenance should be handled by a TCEQ-licensed professional.

Know your system basics:

• Where your tank is located
• When it was last pumped
• Whether you have conventional or aerobic
• Your county's permitting authority

Need help understanding your septic system in Texas? Find local professionals

Frequently Asked Questions

How long does a septic system last?

A well-maintained septic system lasts 25-30 years or longer. The concrete tank itself can last 40+ years. According to a Texas Water Resources Institute publication, conventional drain fields in Texas typically last 20-30 years depending on soil conditions and maintenance habits.

Does a septic system need electricity?

Conventional systems don't need electricity because they use gravity. Aerobic systems require electricity to run the aerator pump, effluent pump, and control panel. If you lose power, an aerobic system stops treating waste, and the alarm will eventually sound.

What should I not put in a septic system?

Avoid flushing anything besides human waste and toilet paper. No wipes (even "flushable" ones), feminine products, condoms, cat litter, or food scraps. In the kitchen, minimize grease down the drain, and avoid pouring household chemicals, paint, or medications into any drain.

Do I need a permit to install a septic system in Texas?

Yes. Every new septic installation in Texas requires an OSSF permit from your local permitting authority (usually the county). Repairs that change the system's capacity or footprint also need permits. Routine pumping does not require a permit.

How much does a septic system cost in Texas?

A conventional system costs $3,000-$8,000 installed, and an aerobic system costs $10,000-$20,000. Soil conditions determine which type you need, and that's the biggest cost driver. Site accessibility and local permit fees ($250-$500 in many counties) also affect the final price.

Can a septic system work without a drain field?

No. Every septic system needs a drain field or equivalent dispersal method to complete treatment. Conventional systems use buried gravel trenches, while aerobic systems may use spray heads or drip irrigation lines instead. Without a dispersal area, untreated wastewater has nowhere to go.

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Last updated: February 7, 2026 Reviewed by: Texas Septic Guide Editorial Team, TCEQ OSSF compliance specialists