Aerobic Treatment Units (ATUs): The Solution for Challenging Lots in Hawaii

As Hawaii moves toward its 2050 mandate to convert all existing cesspools, many homeowners are discovering that their property cannot support a standard septic tank and drainfield. Whether due to a small yard, poor soil, or proximity to the ocean, these "challenging lots" require a more advanced engineering solution.

For these properties, the Hawaii Department of Health (DOH) frequently mandates the installation of an Aerobic Treatment Unit (ATU). At Advanced Compliance Solutions, Inc. (ACSI), we specialize in designing and installing ATUs for complex Hawaii properties. Here is a detailed look at why an ATU might be required for your lot, how it works, and what it means for your property.

When is an ATU Required in Hawaii?

A standard septic system relies on a large drainfield to filter wastewater through the soil. However, the DOH will not approve a standard system if the property cannot safely absorb the effluent. An ATU is typically required when a property has a small lot size without enough square footage for a traditional drainfield, when the home is located near the coastline or a drinking water well requiring a higher level of purification, when the natural groundwater is close to the surface preventing adequate vertical separation, or when the property sits on solid volcanic rock or dense clay that cannot filter or absorb effluent at the required rate.

How an Aerobic Treatment Unit Works

An ATU functions like a miniaturized municipal sewage treatment plant buried in your backyard. While a standard septic tank relies on anaerobic bacteria (which live without oxygen) to slowly digest waste, an ATU actively introduces oxygen into the system.

The system utilizes a mechanical air compressor or aerator to continuously pump oxygen into a central treatment chamber. This oxygen-rich environment allows aerobic bacteria to thrive. Aerobic bacteria are incredibly aggressive and efficient - they break down organic waste much faster and more thoroughly than anaerobic bacteria.

Because the waste is digested so completely, the liquid effluent that exits an ATU is highly purified. It is significantly cleaner and clearer than the effluent from a standard septic tank. Because the water is already treated to such a high standard, the DOH allows ATU effluent to be discharged into much smaller, compact drainfields, or even specialized shallow drip-irrigation systems.

Maintenance and Operating Requirements

While ATUs solve complex engineering problems, they are mechanical systems that require more oversight than a passive septic tank. The air compressor and internal alarms require a continuous electrical connection to operate. If the power goes out for an extended period, the aerobic bacteria can die off, temporarily reducing the system's effectiveness.

Because ATUs have moving parts, the Hawaii DOH generally requires homeowners to maintain an active, ongoing service contract with a certified maintenance provider. A technician must regularly inspect the air pump, check the sludge levels, and ensure the alarms are functioning correctly. Like any wastewater system, the solid sludge that cannot be digested will eventually build up and must be professionally pumped out, typically every 3 to 5 years depending on usage.

If you have been told your property is too small or too close to the water for a standard septic system, an ATU is likely the solution. ACSI's licensed civil engineers can evaluate your lot, design a compliant ATU system, and handle the complete installation. Contact ACSI today at 808-645-4672 or visit acsihawaii.com to explore your options.