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Advanced Treatment Units (ATUs)

Wastewater Treatment: The Need for Change

Wastewater (sewage) is generated from many daily activities such as washing clothes and dishes, preparing food, taking a bath or shower, washing our hands, and using the bathroom. But where does it go? Rarely do we stop to think about where our wastewater goes. In Hawai'i, there are two main "destinations" (more or less), Cesspools and Septic Tanks.

Cesspools are underground holes of raw human waste. There are over 90,000 cesspools being used in Hawai'i today. This untreated sewage is discharged directly into the ground, contaminating our ocean and streams, killing our coral reefs, and exposing our community to disease-causing pathogens and nitrates. Further, Large-capacity cesspools have been banned by the EPA since 2005, but Hawaii only began enforcing the ban in 2017, with the passage of Act 125, requiring the replacement of all cesspools in the Islands by 2050.


Septic tank-soil absorption systems are the most widely-used method of on-site domestic waste disposal. If properly designed, constructed, and operated septic tank systems have demonstrated an efficient and economical alternative of waste water treatment in Hawaii. However, because of their widespread use in unsuitable situations (areas prone to flooding, and/or sea level rise) they have also demonstrated high risk of contamination to ground and surface waters.

Q. Is there a Viable solution for Hawai'i?

A. Yes, We think so... Meet the Aerobic Treatment Unit.

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What is an Aerobic Treament Unit?

Aerobic Treatment Units (ATUs) are similar to standard septic systems in that they use natural processes to treat wastewater. But unlike conventional systems, ATUs also use oxygen to break down organic matter, much the same as municipal wastewater treatment systems, but in a scaled-down version.

Because ATUs decompose organic solids quickly, the wastewater leaving the system is cleaner. ATUs are useful in environmentally sensitive areas or locations that are less suitable for conventional or gravity flow septic systems, such as inappropriate soil conditions where the water table is too high to allow the drain field to operate effectively.

How does it work?

The ATU uses a process called suspended growth, which takes place in two compartments:

The Aeration Chamber in which air is forced and mixed with the wastewater - creating an environment where bacteria are free-floating in liquid and grows as they digest the solids (suspended growth).

The Second Chamber where solids, that the bacteria are unable to digest, settle. The two chambers are connected, so these undigested solids can be returned to the aeration chamber, either by gravity or a pump. It is this process of return and mixing that is important for effective operation.

Benefits from using ATU's

A higher level of treatment than septic tank

Helps protect water resources in areas of failing septic systems or cesspools

The best alternative for sites unsuitable for septic systems (flood-prone areas, areas close to the water table, small lot size)

May extend the life of drain field

Reduces the ammonia discharged to receiving water

Uses fewer chemicals

Smaller leach fields

Cesspool Upgrade Income Tax Credit

In June of 2015, Hawai'i state Legislature approved Act 120, cordially called the Cesspool Tax Credit. Act 120 provides a temporary income tax credit for the cost of upgrading or converting a qualified cesspool to a septic system or an aerobic treatment unit system, or for the connection to a sewer system. According to the State's Wastewater Branch, a taxpayer may apply for a tax credit of up to $10,000 for each qualified cesspool. The tax credits are available for five years, beginning in the tax year 2016 and ending December 31, 2020. 

Ocean Water


If you are ready for a more effective, eco-friendly alternative to a conventional septic system , PAL is here to help you get started. 

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