Aerobic Septic System: How It Works, What It Costs, and How to Maintain It

If you've heard the term 'aerobic septic system' and aren't sure how it differs from a regular septic tank — you're not alone. Aerobic systems look similar from the outside, cost significantly more to install, and require quarterly service contracts. But in the right situation, they're not optional: many California counties now require them on parcels where soil conditions, lot size, or proximity to waterways make a conventional system inadequate.

This guide breaks down exactly how aerobic systems work, what separates them from conventional septic tanks, what they cost to install and maintain, and what problems to watch for. If you already have one — or are considering a property that has one — this is the guide to start with.

What Is an Aerobic Septic System?

An aerobic septic system (also called an aerobic treatment unit, or ATU) is an advanced onsite wastewater treatment system that uses oxygen injection to accelerate and enhance the breakdown of sewage. The key difference from a conventional septic tank is that aerobic systems introduce air into the treatment process, which allows aerobic (oxygen-loving) bacteria to do the work — rather than the slower anaerobic (oxygen-free) bacteria in a conventional tank.

The result is treated effluent that is dramatically cleaner. A well-functioning aerobic system can produce effluent that is 85–95% treated before it ever reaches the soil — compared to roughly 30–40% treatment from a conventional septic tank. That cleaner output is why aerobic systems are permitted in situations where a conventional system would pose a health or environmental risk.

How an Aerobic Septic System Works: The 3-Stage Process

Most aerobic treatment units operate in three stages, each housed in a separate chamber within the same tank unit or in connected tanks.

Stage 1: Pre-Treatment (Trash Tank)

Raw sewage enters the first chamber, which functions similarly to a conventional septic tank. Heavy solids settle to the bottom and form sludge. Lighter materials like grease float to the top. The pre-settled liquid effluent flows into the second chamber. This stage reduces the solids load before biological treatment begins.

Stage 2: Aerobic Treatment (Aeration Chamber)

This is where aerobic systems differ entirely from conventional tanks. An electric air pump — either a surface-mounted compressor or a submersible aerator — constantly injects air into the treatment chamber. Aerobic bacteria thrive in this oxygen-rich environment and consume organic matter far more aggressively than anaerobic bacteria. The result is highly treated liquid effluent and a much smaller volume of sludge.

This stage is what drives the need for electricity and maintenance. The air pump runs continuously, 24 hours a day, 365 days a year. If the pump fails, the aerobic bacteria die off rapidly and the system reverts to anaerobic conditions within days — producing foul odors and significantly reduced treatment quality.

Stage 3: Clarification and Disinfection

Treated effluent flows into a clarification chamber where any remaining suspended solids settle out. From there, most systems pass the effluent through a disinfection component — typically a chlorination tablet dispenser or UV light — before releasing it to the dispersal field. This disinfection step is what allows aerobic systems to be used with surface spray dispersal, a method that's not permitted with conventional septic effluent.

Aerobic Septic System vs. Conventional Septic Tank: Key Differences

Understanding the practical differences helps you know what to expect if you're buying, selling, or maintaining a property with an aerobic system.

• Treatment quality: Aerobic systems treat effluent to 85–95% completion vs. 30–40% for conventional tanks
• Electricity required: Aerobic systems run an air pump 24/7 — adding $20–$50/month to utility bills
• Maintenance: Aerobic systems require quarterly service contracts (mandatory in most CA counties); conventional systems need pump-outs every 3–5 years
• Cost: Aerobic systems cost $10,000–$20,000+ to install vs. $5,000–$12,000 for conventional
• Moving parts: Aerobic systems have air pumps, motors, chlorinators, and alarms that can fail; conventional tanks have none
• Dispersal flexibility: Aerobic systems can use surface spray heads; conventional systems require subsurface drain fields
• Permitted locations: Aerobic systems are approved for sites where conventional systems cannot legally be installed

When an Aerobic System Is Required (or the Better Choice)

Aerobic systems aren't chosen for performance alone — they're often the only legally permitted option. California counties, including Stanislaus, San Joaquin, Merced, and surrounding Central Valley counties, require aerobic treatment in specific situations:

• Small lot size: When a parcel doesn't have enough room for a conventional drain field setback from property lines, wells, or structures
• Poor soil conditions: Clay-heavy soil or high water tables that don't allow adequate percolation for a conventional leach field
• Proximity to water: Properties near rivers, lakes, or waterways where stricter effluent quality is required to protect water quality
• Repair situations: When an existing drain field fails on a lot too small for conventional replacement
• High water table: Areas where the seasonal high water table is too close to the surface for a conventional system
• Steeper slopes: Terrain that makes conventional installation impractical

In Central California, aerobic systems are increasingly common as rural parcels get subdivided and lots shrink. If you're buying rural property and the listing mentions an 'ATU,' 'aerobic treatment unit,' or 'advanced treatment system,' you're looking at an aerobic system — and you should budget accordingly for its ongoing maintenance costs.

Aerobic Septic System Cost: Installation and Annual Maintenance

Installation Cost

Aerobic septic system installation costs significantly more than conventional systems. In California's Central Valley, expect to pay:

• Basic aerobic system (ATU only, existing drain field): $10,000–$14,000
• Full aerobic system with new drain field: $14,000–$20,000
• Aerobic system with spray dispersal field: $16,000–$25,000+
• Aerobic system on challenging terrain or with permits: $20,000–$35,000+

The higher cost reflects the more complex equipment (aeration unit, disinfection system, alarms, control panel), the county permitting process, and the engineered design that most California counties require for aerobic installations. Costs vary significantly based on lot conditions, local permit fees, and whether existing infrastructure can be reused.

Ongoing Maintenance Cost

This is where aerobic systems diverge most sharply from conventional tanks. Most California counties legally require an annual service contract for aerobic systems, and quarterly inspections are the minimum standard. Budget for:

• Quarterly service contract: $400–$700/year (covers inspections, adjustments, chlorine tablets)
• Air pump replacement: $300–$600 every 5–10 years (or sooner if not maintained)
• Chlorinator/UV lamp replacement: $100–$300 every 1–3 years
• Control panel repairs: $200–$800 as needed
• Tank pump-out (sludge removal): $300–$600 every 2–4 years
• Electricity: $20–$50/month added to utility bill

Total annual cost for a properly maintained aerobic system typically runs $600–$1,200/year, not counting electricity — compared to $150–$300/year averaged out for a conventional system (factoring in pump-outs every 3–5 years). Over a 20-year span, the maintenance cost difference can exceed $15,000.

Aerobic Septic System Maintenance: What's Required

Aerobic system maintenance has two layers: what the service contractor does during quarterly visits, and what the homeowner needs to do between visits.

What Quarterly Service Visits Include

• Inspect all mechanical components: air pump, control panel, floats, and alarms
• Check aeration function — confirm air is actually reaching the treatment chamber
• Measure sludge and scum levels in each chamber
• Replenish chlorination tablets or check UV lamp operation
• Test effluent quality visually and sometimes with meters
• Document findings and submit reports to the county (required in most jurisdictions)
• Flag any needed repairs before they become failures

What Homeowners Should Do Between Visits

• Check the alarm panel monthly — a flashing or audible alarm means something needs attention
• Avoid flushing anything other than toilet paper and human waste — the treatment bacteria are sensitive to harsh chemicals
• Use septic-safe cleaning products — bleach, antibacterial soaps, and strong disinfectants can disrupt the aerobic bacteria colony
• Don't plant trees or shrubs near the system or spray heads
• Keep the area around spray heads clear of tall grass or debris
• Know where your access lids are — contractors need quick access during service visits
• Keep the service contractor's number posted — air pump failure is time-sensitive

Common Aerobic Septic System Problems

Because aerobic systems have mechanical components, they fail differently than conventional tanks. Here are the most common problems and what they mean:

Alarm Going Off

Aerobic systems have built-in alarms that trigger when something is wrong — high water levels, power loss to the pump, or a component failure. Don't ignore an alarm. Some indicate minor issues (like a chlorine tablet that ran out); others signal imminent failure. Call your service contractor the same day.

Foul Odors

Odors from an aerobic system almost always signal that the aeration process has been disrupted. The most common causes are air pump failure, the introduction of high concentrations of disinfecting chemicals (like bleach) that kill the aerobic bacteria, or improper loading. If you smell sewage odors near your system, contact your service contractor promptly — this is not a wait-and-see issue.

Spray Heads Not Working (or Spraying at Wrong Times)

Aerobic systems with surface spray dispersal use a timer-controlled pump to distribute effluent to spray heads. If heads are clogged, off-schedule, or pointing in the wrong direction, effluent can pool in the yard, spray toward structures, or create wet spots that pose a health risk. This requires a service visit to inspect the dispersal field and timer controls.

High Water Level Alarm

A high water level typically means either the system is receiving more water than it can process (check for leaking toilets, running faucets, or water softener discharge into the system) or the dispersal field is failing. Both require prompt investigation.

Air Pump Failure

Air pumps are the workhorse of aerobic systems and typically last 5–10 years with regular maintenance. When a pump fails, the aerobic bacteria die off within a few days, and the system essentially becomes a conventional (untreated) septic tank until the pump is replaced. Replacement is urgent — typically $300–$600 for the pump plus labor.

Buying or Selling a Property With an Aerobic System

Aerobic systems require extra scrutiny in real estate transactions. As a buyer, you should:

• Request the full service history and maintenance records — gaps indicate neglect and potential hidden damage
• Verify the county permit and maintenance contract are current — operating without a current contract can mean violations
• Have the system inspected by a licensed septic professional before closing, not just a general home inspector
• Budget for the ongoing maintenance costs when calculating affordability — this is $600–$1,200/year that a sewer-connected home doesn't pay
• Confirm the dispersal field is functional — replacing a failed spray field or drain field on an aerobic system can cost $8,000–$15,000+

As a seller, up-to-date maintenance records are one of the most valuable documents you can provide. A well-documented, recently serviced aerobic system removes a major buyer objection. A system with gaps in service records — or worse, a lapsed maintenance contract — will often require seller concessions or a price reduction.

Frequently Asked Questions About Aerobic Septic Systems

How long does an aerobic septic system last?

The treatment tank itself can last 20–30 years. Mechanical components have shorter lifespans: air pumps typically last 5–10 years, control panels 10–15 years, and chlorinators vary. With regular maintenance, a well-installed aerobic system can function reliably for 20+ years. Neglected systems often require major repairs or replacement within 10–15 years.

Can I convert a conventional septic system to aerobic?

Yes, in many cases. If your conventional drain field is failing and a full replacement isn't feasible, converting to an aerobic system is often the permitted solution. The existing tank may be repurposed as the pre-treatment stage, and a new aerobic treatment unit is added. This is common in Central California when conventional systems fail on smaller or more constrained parcels.

Is aerobic septic system effluent safe for vegetable gardens?

Even with disinfection, aerobic effluent is not considered safe for direct contact with edible crops. California regulations prohibit using aerobic spray dispersal in areas where effluent can contact vegetables or fruit. Maintain the same separation rules as conventional systems: keep gardens away from spray heads and never use spray effluent for irrigation near food crops.

What happens if I stop the maintenance contract?

In most California counties, aerobic systems are required by permit to maintain an active service contract. Letting a contract lapse can result in notices of violation from the county environmental health department, fines, and required system repairs or upgrades at your expense. Beyond legal consequences, unmaintained aerobic systems deteriorate rapidly — often requiring thousands of dollars in repairs that regular maintenance would have prevented.

Do aerobic septic systems smell?

A properly functioning aerobic system should have minimal odor. The aerobic treatment process produces far less hydrogen sulfide (the 'rotten egg' gas) than anaerobic systems. If you're noticing significant odors, it's almost always a sign that the aeration is disrupted or the system needs service. Surface spray systems can occasionally produce a slight earthy smell after dispersal cycles, which is normal.