Aerobic vs Conventional Septic Systems: Which Do You Have? (And Why It Matters in Texas)

A lot of Williamson County homeowners aren't entirely sure what kind of septic system they have. They know there's a tank buried somewhere in the yard, and they know it needs pumping every few years. Beyond that, the details are fuzzy.

That's fine — until the system needs service, until you're buying or selling a home, or until the county sends you a notice about a required maintenance contract you didn't know you needed.

Conventional and aerobic systems are fundamentally different in how they treat wastewater, what they require from you as an owner, and what happens when they're not maintained properly. Here's how to tell them apart — and why it matters.

The Short Version

Conventional systems use natural bacterial activity in the soil to treat wastewater passively. They have no moving parts, no electrical components, and relatively minimal ongoing maintenance beyond periodic pumping.

Aerobic treatment systems (ATS) use an electric air pump to inject oxygen into the wastewater, accelerating bacterial breakdown. They treat water to a higher standard than conventional systems — clean enough, in Texas, to be dispersed via surface spray irrigation. But they have more components, require electricity to operate, and in Williamson County (and under Texas state law), require an ongoing maintenance contract with a licensed provider.

How to Tell Which One You Have

The fastest way: step outside and look for a control panel or alarm box near the tank or mounted on the side of your house. It will have indicator lights — typically labeled "Power," "Air," and "Alarm" — and sometimes an audible alarm buzzer.

If you see that panel, you have an aerobic system.

No panel? Look for spray heads scattered around your yard, usually on short risers that pop up above the grass. These are the irrigation heads that disperse treated effluent from an aerobic system. If you find them, you have an aerobic system.

If you have neither — no panel, no spray heads — you almost certainly have a conventional system with a subsurface drain field.

When in doubt, you can pull the original OSSF permit from the Williamson County Environmental Health Department. The permit will identify the system type, tank size, and drain field layout.

Conventional Septic Systems: How They Work

A conventional system has two main components: the septic tank and the drain field (sometimes called a leach field or absorption field).

Wastewater from the house flows into the tank. Heavy solids settle to the bottom and form a sludge layer. Lighter material floats to the top as a scum layer. The liquid in the middle — called effluent — flows out through the outlet baffle into the drain field.

The drain field is a network of perforated pipes buried in gravel-filled trenches. Effluent seeps out of the pipes, filters through the gravel, and then percolates into the soil below, where naturally occurring bacteria in the soil complete the treatment process.

When it works well, it requires little attention. The main maintenance task is pumping the tank every 3–5 years to remove the accumulated sludge and scum before they build up enough to overflow into the drain field pipes.

Conventional Systems in Williamson County

Soil type matters enormously for drain field performance. Williamson County has significant variability: the blackland prairie clay soils common in eastern areas (Taylor, Hutto) absorb slowly and saturate during wet periods; caliche-heavy soils in western areas (Georgetown, Liberty Hill, Leander) have a calcium carbonate layer that can sit close to the surface and restrict drainage. A site evaluation is required before installing any new conventional system because soil conditions can differ dramatically between neighboring lots.

Aerobic Treatment Systems: How They Work

An aerobic treatment system adds a critical step conventional systems don't have: active aeration.

Wastewater enters a pre-treatment chamber, then moves into an aeration chamber where an electric air pump continuously injects oxygen. The oxygen accelerates aerobic bacterial activity, breaking down organic matter more completely than a conventional tank's anaerobic process. Treated water then passes through a clarifier chamber, is disinfected (most Texas systems use chlorine tablets), and is pumped to spray heads that distribute it over a designated area of the yard.

The treated effluent from a properly functioning aerobic system is significantly cleaner than conventional tank output — typically meeting secondary treatment standards. That's why it can be surface-sprayed rather than requiring deep soil absorption.

Why Aerobic Systems Are Common in Williamson County

The county's rapid growth over the past two decades, combined with those problematic soil types, created situations where conventional drain fields simply weren't viable on certain lots. Aerobic systems solve that problem because the treated water quality is high enough for surface application — no deep soil absorption required.

You'll see aerobic systems most commonly on:

Lots with shallow caliche or restrictive soil layers
Smaller lots where there isn't enough square footage for a conventional drain field
Newer construction in Liberty Hill, Georgetown, and Leander's growth corridors where difficult soils are prevalent

The Key Difference for Homeowners: Maintenance Requirements

This is where the two system types diverge significantly.

Conventional System Maintenance

Pump every 3–5 years (depending on household size and usage)
Don't flush items that kill beneficial bacteria (bleach in large quantities, chemical drain cleaners, medications)
Protect the drain field from compaction — no parking or building over it
No ongoing maintenance contract required

That's essentially it. A well-designed conventional system on suitable soil, maintained with regular pumping, can operate for 20–30 years with minimal intervention.

Aerobic System Maintenance: What Texas Requires

This is the part that surprises many homeowners — especially those who bought a home with an aerobic system without realizing what they were taking on.

Under TCEQ rules (and enforced by Williamson County's OSSF program), aerobic system owners must:

Maintain an active service contract with a TCEQ-licensed maintenance provider. This is not optional. It's a state and county legal requirement. The contractor must inspect your system at least once every four months.
Keep chlorine tablets stocked in the chlorinator tube. Running out allows untreated effluent to spray onto your yard — a health hazard and a code violation.
Respond to alarm conditions. When the alarm panel on your system activates, it needs to be diagnosed and addressed, not silenced and ignored.
Allow the maintenance provider to submit inspection reports to the county. Your provider files these on your behalf, but you're responsible for having the contract in place.

If your maintenance contract lapses — even if the system is functioning fine — you're out of compliance and subject to a notice of violation from the county. For more detail on how this works, see our post on Williamson County septic regulations.

The ongoing maintenance contract typically runs $150–$300 per year depending on the provider and contract terms, in addition to the cost of chlorine tablets and any repair calls.

Cost Comparison

	Conventional	Aerobic
Installation	Lower (if soil allows)	Higher ($10,000–$20,000+)
Pumping frequency	Every 3–5 years	Every 3–5 years (pump tank)
Annual maintenance cost	Low (pumping only)	Moderate ($150–$300/yr contract + chlorine)
Required maintenance contract	No	Yes — legally required
Electricity required	No	Yes (air pump runs continuously)
Repair complexity	Simpler	More components, higher repair cost
Effective lifespan	20–30 years (drain field)	Mechanical components 10–20 years

Which System Is "Better"?

Neither is categorically better — they solve different problems. Conventional systems are simpler, cheaper to maintain, and have fewer failure points. Aerobic systems make on-site treatment possible where conventional drain fields won't work. The common mistake is treating an aerobic system like a conventional one — skipping the maintenance contract, letting chlorine run out, ignoring alarm signals. That's how small, fixable issues turn into expensive repairs.

If you're not sure what system you have, or if your aerobic system is due for a maintenance visit, contact us to reach a licensed contractor who can assess it and confirm you're in compliance with Williamson County requirements.