Aerobic Treatment Unit Deep Service

Why this matters

An aerobic treatment unit (ATU) is the EPA-recognized advanced-treatment option when a conventional septic system cannot meet site conditions: high water table, shallow soils, lakefront setbacks, or stringent state nitrogen-discharge limits. ATUs deliver near-tertiary effluent quality (BOD under 30 mg/L, TSS under 30 mg/L, often nitrogen reduction) by oxygenating wastewater with aerator pumps that allow aerobic bacteria to digest waste much faster than the anaerobic process in a conventional tank. The trade-off: ATUs require service contracts (most states mandate semi-annual or quarterly inspections), the aerator is mechanical and fails, and improper service produces effluent quality that violates the customer's discharge permit. A septic contractor who can service ATUs deeply (not just inspect them) commands premium service rates because the trade is specialized.

When to service

Service an ATU on the schedule mandated by:

• State / county discharge permit (often quarterly or semi-annual)
• Manufacturer's service contract
• ATU alarm condition (immediate service)
• Annual contract bundle

NSF/ANSI 40 certification requires ATU systems to be serviced by a "trained service provider" - typically the original installer or a manufacturer-trained technician.

Common ATU technologies

Each technology has different service procedures. Manufacturer-specific training is typical for the more complex units (MBR especially).

Service components

A typical ATU has:

• Treatment tank (the aerobic chamber)
• Aerator (compressor / air pump pushing air through diffusers)
• Effluent filter
• Sludge return pump (some models)
• Disinfection system (chlorinator or UV)
• Control panel with alarm
• Effluent pump (delivers treated effluent to drainfield or surface discharge)
• Sampling port

Tools and materials

• Disposable nitrile gloves and chemical-resistant outer gloves
• Eye protection
• Long-handled tank inspection tools
• Sludge judge (depth measurement tool)
• pH and DO (dissolved oxygen) meter or test kit
• Effluent sample bottle
• Replacement diffuser membranes
• Replacement filter media (per system)
• Air pump or aerator parts
• Chlorination tablets (if equipped)
• UV bulb (if equipped)
• Hose for tank flushing
• Multimeter (electrical diagnostics)
• Phone with manufacturer service manual

Safety

ATU work involves untreated to partially-treated wastewater with bacterial, viral, and parasitic load. Splash, aerosol inhalation, and contamination of cuts are real risks.

Some ATUs use methane gas; ventilate the working area. Confined-space entry into a tank requires OSHA confined-space protocols (atmospheric testing, attendant, retrieval system).

Procedure

Step 1: Check the control panel

Before opening any tank:

• Note the alarm state (active or clear)
• Note any historical faults (most panels log recent events)
• Verify the aerator is operating
• Check the run-hour counter for the aerator
• Verify the effluent pump (if present) is cycling

A panel with an active alarm has specific guidance per the panel; address the specific fault.

Step 2: Inspect the aerator

The aerator is the heart of the system. Failures here kill the entire treatment process.

For air-pump aerators (compressor + diffuser):

• Verify the air pump is running (visible motion or audible humming)
• Check filter on air intake; clean or replace if clogged
• Inspect diffuser membranes (typically in the tank; may require viewing through inspection ports)
• Check air flow rate (some pumps have flow indicators; otherwise measure with a flow meter)

Diffusers degrade over 3 to 7 years; clogged or torn diffusers reduce aeration efficiency. Replace per manufacturer schedule.

For mechanical surface aerators (rare in modern residential ATU):

• Inspect for proper rotation
• Check shaft bearings
• Verify impeller condition

Step 3: Test dissolved oxygen (DO)

DO in the aerobic chamber should be 2 to 4 mg/L for proper bacterial activity.

• Insert DO probe into the chamber (some chambers have access ports; others require lifting the cover)
• Read DO and note temperature
• Compare to manufacturer spec

Low DO indicates:

• Aerator not working at full capacity
• Air flow restricted (filter, hose)
• Tank organic load exceeding aeration capacity (overloading)
• Diffuser failure

Adjust aerator or replace components as needed.

Step 4: Inspect effluent filter

The effluent filter prevents sludge from reaching the drainfield or downstream components.

• Locate the filter (typically inline between treatment tank and effluent pump or drainfield)
• Remove and inspect
• Rinse with hose (do not soak in disinfectant; bacterial population is intentional)
• Reinstall

Replace the filter media if it is broken or beyond cleaning.

Step 5: Inspect sludge and scum levels

In the treatment tank:

• Measure sludge depth with sludge judge
• Measure scum depth
• Compare to manufacturer spec

For most ATUs, sludge over 12 to 18 inches indicates pumping is needed. Scum over 4 to 6 inches similar.

Pump-out is a separate service call (typically by the same contractor with a vacuum truck).

Step 6: Test effluent quality

Some service contracts include effluent sampling for laboratory analysis:

• Collect a sample at the effluent discharge point (after disinfection if equipped)
• Sample size: typically 250 to 500 mL in a clean, sterile bottle
• Refrigerate during transport
• Lab tests: BOD, TSS, fecal coliform, sometimes nitrogen and phosphorus

A failed effluent test (over limits) triggers system corrective action. The customer's discharge permit may require reporting.

For basic service, a field test:

References

• 40 CFR Part 503 (Standards for the Use or Disposal of Sewage Sludge).
• EPA Onsite Wastewater Treatment Systems Manual.
• NSF/ANSI 40 (Residential Wastewater Treatment Systems).
• NSF/ANSI 245 (Wastewater Treatment Systems - Nitrogen Reduction).
• OSHA 29 CFR 1910.146 (Permit-Required Confined Spaces).
• State / county septic regulations.
• Manufacturer service literature: Bio-Microbics MicroFAST, Norweco Singulair, AdvanTex AX, Aquaklear, MicroSepTec, Hoot, Jet.
• Manuall internal: Septic Aerobic System Service, Septic Alarm Troubleshooting, Septic System Troubleshooting.