When a conventional septic system is not viable, an aerobic treatment unit (ATU) is often the alternative. Here is what you need to understand before your contractor proposes one.
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How a Conventional System Works
A conventional system relies on anaerobic (oxygen-free) bacterial decomposition. Wastewater sits in the tank while solids settle, oils float, and clarified liquid flows to the drain field. The soil provides the final treatment.
Conventional systems are passive â no moving parts, no electricity, no ongoing mechanical maintenance beyond pumping every 3 to 5 years. The lowest-cost option when soil supports it.
How an Aerobic Treatment Unit Works
An ATU injects air into the wastewater to accelerate bacterial decomposition, producing much cleaner effluent than a conventional tank. This cleaner effluent can be dispersed in locations where conventional systems would fail â smaller lots, poor soil, high water tables, and areas near water bodies.
Key Differences
- Effluent quality: ATU effluent is far cleaner â suitable for spray irrigation in some states
- Mechanical complexity: ATUs have pumps, blowers, and timers; conventional systems have no moving parts
- Electricity: ATUs require constant power; conventional systems do not
- Maintenance cost: ATUs require service contracts ($150 to $400 per year); conventional systems need only pumping
- Installation cost: ATUs run $10,000 to $20,000 or more versus $7,000 to $15,000 for conventional
Ongoing Maintenance
Most states require a maintenance contract with a licensed service provider for ATUs. Quarterly inspections, air pump filter cleaning, chlorine tablet replacement, and annual county reports are typical requirements. Neglecting ATU maintenance causes system failure and regulatory violations.