Advanced Treatment Units Explained: When Contractors Need an Aerobic Septic System
Some jobs will not pass with a conventional septic system. The lot is too small for a full drain field, the soil will not perc, the water table sits too high, or the property is close enough to a lake, river, or aquifer that the county demands cleaner effluent.
Whenever that happens, the answer is usually an advanced treatment unit, or ATU.
This guide is written for the crews who have to spec, permit, and stand behind these systems. It covers what an ATU is, how it treats wastewater, when a county will require one, and what a complete, permit-ready installation actually includes. Tank Depot is the authorized distributor for the Norwesco and BioMicrobics MicroFAST Flex ATU across Texas and Florida, and this is the plain-language version of what our team walks contractors through every week. The goal is simple: help you recognize an ATU job, quote it completely, and install it so it passes the first inspection.
What an Advanced Treatment Unit Is
An advanced treatment unit, also called an aerobic treatment unit, is an onsite system that cleans household wastewater to a much higher standard than a conventional septic tank. The difference is oxygen. A conventional tank relies on anaerobic bacteria, which work slowly in a low-oxygen environment, then leans on the soil in the drain field to finish the job.
An ATU actively pumps air into the wastewater, which feeds aerobic bacteria, the same kind used in municipal treatment plants. Those bacteria digest waste faster and more completely, so the water leaving the tank is far cleaner before it ever reaches the ground.
Think of it as a small treatment plant buried in the yard. Instead of trusting the dirt to do the cleaning, the ATU does most of the work first. That is why counties allow advanced treatment on sites where a conventional system would never be permitted. To compare live products, spec sheets, and state approvals, see the Tank Depot Advanced Treatment Unit (ATU).
How an ATU Treats Wastewater
Every ATU follows the same basic sequence. Knowing it helps you explain the system to a homeowner or an inspector with confidence.
- Settling: Household wastewater enters the tank first. Heavy solids sink to form sludge and lighter scum floats to the top, the same as the first stage of any septic tank. An optional trash screen keeps solids out of the treatment zone.
- Aeration: An above-ground blower pushes oxygen into the treatment chamber. This is the step that sets an ATU apart. The oxygen feeds aerobic bacteria so they break down organic matter, nutrients, and pathogens quickly.
- Submerged media: Inside the chamber, a fixed media insert gives bacteria a large surface to grow on. Attached growth on the media and free-floating growth in the water happen at once, which keeps treatment efficient.
- Discharge: Treated effluent passes a built-in baffle and leaves the tank. On most permitted jobs it then passes through disinfection, a tablet chlorinator or a UV unit, before it reaches the dispersal field.
The only moving part in the whole process is the blower, which is a big reason these systems are reliable and easy to service.
When an ATU Is Required
This is the heart of the decision. A county or state pushes a project from conventional to advanced treatment when the site cannot safely rely on soil alone. The common triggers are similar across most markets:
- Restricted lot size, where there is not enough room for a conventional drain field based on the soil and percolation rate.
- Poor soil percolation, such as the heavy clay common across North Texas and the Gulf Coast, or the shallow rocky ground of the Hill Country, both of which can fail a perc test.
- High water table, a defining challenge in low-lying Houston-area sites and across coastal South Florida.
- Proximity to water, meaning lakes, rivers, creeks, wetlands, or canals where stricter effluent quality protects the water.
- Aquifer recharge zones, such as Edwards Aquifer counties in Central Texas, where nitrogen-reducing systems are frequently required.
- Local ordinance or replacement rules, where some counties require every new system, or every replacement of a failed system, to be an ATU regardless of site conditions.
In Texas, these rules sit under the TCEQ program in 30 TAC Chapter 285, and many counties act as authorized agents with their own added requirements.
In Florida, onsite systems fall under Department of Health Rule 64E-6 and DEP Rule 62-6, and the state enhanced nutrient reduction rules under HB 1379 require NSF 245 nitrogen-reducing units in certain areas.
One honest caveat worth passing to every customer: product approval is not the same as permit approval. The unit is approved at the state level, but each installation still needs a site-specific permit from the local authorized agent.
For a contractor, the practical move is to read the site early. Ask for the site evaluation, the soil or perc report, and the county before you price anything. If the lot is tight, the ground is clay or rock, the water table is shallow, or the property sits near water or in a recharge zone, plan for advanced treatment and a full system from the start. Catching that at the bid stage, rather than after a failed perc test, saves a redesign and a hard conversation with the owner.
NSF 40 vs NSF 245 and Why Nitrogen Matters
Two certifications decide whether a unit can be permitted. NSF/ANSI 40 covers general treatment quality and Class I performance, and it is the baseline for TCEQ and FDEP approval. NSF/ANSI 245 is the nitrogen-reduction standard, certifying that a system cuts total nitrogen by at least half.
Nitrogen is the reason so many waterfront and recharge-zone jobs need an ATU. Too much of it reaching a lake, river, or aquifer drives algae growth and degrades water quality, so sensitive areas set a nitrogen limit that a conventional tank cannot meet. The Tank Depot Norwesco and BioMicrobics MicroFAST Flex units are certified to both NSF 40 and NSF 245 and reduce total nitrogen by about 55 percent in testing, which clears the common 50 percent threshold.
For contractors, the takeaway is simple: if the site is in a nitrogen-sensitive zone, you need a 245-certified unit, and this one qualifies.
What a Complete, Permit-Ready ATU Job Includes
The ATU tank is the heart of the system, but it never goes in the ground alone. Quoting only the tank is the fastest way to lose money and time on a job. A permit-ready installation almost always includes:
• The ATU tank with the FAST media insert, pre-assembled at Tank Depot so it arrives ready to set.
• The approved aerator (the blower) that supplies the oxygen. For the MicroFAST Flex, the only approved aerators are the Fuji Electric VFC200 and the Mapro CL20HS. A Hiblow or other substitute can put the unit approval at risk, so this is not a place to swap parts. • A separate pump chamber. The ATU tank does not include one. A separate tank collects treated effluent and doses it to the dispersal field on a timer. • An effluent pump and a timed dosing control panel sized to the design flow and head.
• Disinfection, a tablet chlorinator or UV unit, required on most spray and drip jobs before effluent leaves the system.
• Dispersal, either low-pressure dosing pipe or subsurface drip, chosen for the soil, lot, and county. Drip is common on the rocky ground of the Hill Country and across many Florida sites.
• Risers, lids, and inspection ports. These bring access to grade and support the required maintenance visits. Many Texas counties require green lids so inspectors can identify an ATU on sight.
Tank Depot stocks the tank, the approved aerators, pump tanks, pumps, risers, dispersal parts, and accessories, so a contractor can source the whole system in one place. Explore the full ATU lineup and the drain field and dispersal parts to build a complete quote.
Quoting the complete system also protects you. When the advanced treatment tank, the approved aerator, the pump chamber, the control panel, disinfection, and the dispersal parts are all on one order, there are fewer surprises at inspection and fewer trips back to the supplier. It is cleaner for the customer and cleaner for your schedule.
HDPE vs Concrete for the Tank Housing
Most contractors grew up on concrete tanks, so the switch to high-density polyethylene deserves a plain explanation. HDPE is far lighter, which means no crane on most jobs, faster installation, and lower excavation cost. It will not crack, it does not corrode from the hydrogen sulfide gas that eats concrete over time, and roots cannot penetrate it. The seamless molded shell also avoids the joints where concrete tanks can leak.
For the full material breakdown, see our guide on plastic vs concrete septic tanks. For ATU work, the low-profile Norwesco housing matters most: its shallow shape is built for the high water tables of coastal Florida and low-lying Texas, where a taller tank simply will not fit.
Sizing and Where the Advanced Treatment Units Are Approved
Residential ATUs are sized by daily flow, which tracks with bedrooms and square footage. The MicroFAST Flex comes in a 400 gallons per day [1,514 L] unit and a 500 gallons per day [1,893 L] unit, housed in Norwesco HDPE tanks. As a rule of thumb, the 400 GPD unit fits smaller homes and the 500 GPD unit covers larger ones, but the exact bedroom and square-foot limits differ slightly between Texas and Florida, so confirm the count against the current approval before you quote.
Approval also varies by state. The MicroFAST Flex is currently approved in ten states, including Texas and Florida where Tank Depot stocks it, along with Georgia, Kentucky, Virginia, Alabama, Arkansas, Idaho, Louisiana, and Washington. For live sizing and the exact tank that matches each approval, see the ATU product pages.
Certification, Maintenance, and What Tank Depot Handles
Advanced treatment comes with responsibilities that a conventional tank does not. In Texas, installing an ATU requires an Installer II license, and every ATU owner must keep a maintenance contract with a licensed provider who inspects the system on a set schedule, checks the aerator, and logs chlorine readings with the county.
Florida has its own maintenance-contract rules. The single moving part, the blower, typically runs for years before it needs a rebuild or replacement, and the fixed media is self-regulating, so day-to-day upkeep is light.
Tank Depot supports contractors on both ends of the job.
We pre-assemble the BioMicrobics media insert into the Norwesco tank before pickup, so the unit is quality-checked and ready to drop in with no field assembly of the treatment module. We also run installer certification training for the MicroFAST Flex, so crews that are not yet certified can get qualified and add ATU work to what they offer.
If you are weighing whether to add advanced treatment to your services, that training is the place to start. Keeping the approved aerator on the job and the maintenance records current is not just paperwork; it is what keeps the system approval intact if an inspector or the county ever asks.
Talk to the Tank Depot ATU Team
Have a job that needs advanced treatment? Our ATU specialists can match the right unit to the site, build a complete system quote, and get your crew certified.
Call an ATU expert at (954) 526-6990, or request a free Advanced Treatment Unit (ATU) quote.
A note for homeowners: Tank Depot supplies these systems to licensed contractors rather than selling them direct. Share your name, phone number, location, and email, and we are glad to pass your project to a reliable local contractor who can take it from there.
