Septic System vs. Cesspool: Which is Right for Your Hawaii Property?

If you own a home in Hawaii that is not connected to a municipal sewer line, your property relies on an on-site wastewater disposal method. Historically, many homes in Hawaii were built using cesspools. However, due to severe environmental and public health concerns, the state has banned the construction of new cesspools and mandated the conversion of all existing ones by 2050 under Act 125.

For homeowners facing this mandate, understanding the difference between outdated cesspools and modern, state-approved Individual Wastewater Systems (IWS) is crucial. At Advanced Compliance Solutions, Inc. (ACSI), we help property owners navigate these technical distinctions to select the right system for their lot. Here is a factual breakdown of how these systems differ.

The Outdated Technology: How a Cesspool Works

A cesspool is the most rudimentary form of wastewater disposal. It is essentially a pit dug into the ground, typically lined with perforated concrete rings, block, or stone.

The defining characteristic of a cesspool is its complete lack of wastewater treatment. When you flush a toilet or drain a sink, all the raw sewage - including solid sludge, floating scum, and liquid effluent - dumps directly into this single underground pit. Because the walls are perforated, the untreated liquid and scum drain directly into the surrounding soil. Over time, the solid sludge piles up at the bottom of the pit, requiring frequent pumping to prevent the system from backing up into the home.

This lack of filtration allows raw pathogens and excess nutrients to rapidly enter Hawaii's groundwater and nearshore ocean environments. This untreated discharge is a primary driver behind the state's 2050 conversion mandate.

The Standard Upgrade: How a Septic Tank Works

Unlike a cesspool, a modern septic system actively separates and treats wastewater before it reaches the environment. The core of this system is a watertight, underground septic tank.

When wastewater enters a septic tank, the system relies on gravity and time to separate the waste into three distinct layers. The heaviest solid waste sinks to the bottom to form a sludge layer. The lighter waste, such as oils and grease, floats to the top to form a scum layer. The middle layer consists of relatively clear liquid effluent.

Baffles installed inside the tank prevent the scum and sludge from escaping. Naturally occurring anaerobic bacteria inside the sealed tank slowly digest the organic matter in the scum and sludge. Only the middle layer of liquid effluent is allowed to flow out of the tank and into a designated drainfield (or leach field). As this effluent percolates through the soil in the drainfield, it undergoes a final, natural filtration process before reaching the groundwater. When properly maintained, the solid sludge never leaves the septic tank and is removed during routine professional pumping.

The Advanced Option: Aerobic Treatment Units (ATUs)

For properties with poor soil conditions, high water tables, or limited space for a traditional drainfield, the Hawaii Department of Health (DOH) approves the use of Aerobic Treatment Units (ATUs). You can review the full list of DOH-approved IWS treatment equipment at health.hawaii.gov/wastewater/home/iws/.

An ATU functions similarly to a small-scale municipal sewage plant. While a standard septic tank relies on anaerobic bacteria (which live without oxygen), an ATU uses a mechanical air pump to inject oxygen into the treatment chamber. This oxygen supports aerobic bacteria, which are significantly faster and more efficient at breaking down organic waste than their anaerobic counterparts.

Because the aerobic bacteria digest the waste so thoroughly, the effluent produced by an ATU is of a much higher quality than that of a standard septic tank. This allows the effluent to be safely discharged into smaller, more compact drainfields. However, because ATUs rely on mechanical components like air pumps and alarms, they require electricity to operate and generally mandate a routine service contract to ensure the mechanical parts remain functional.

Making the Right Choice for Your Property

If your home currently utilizes a cesspool, upgrading is not just an environmental choice - it is a legal requirement under Hawaii law. The Hawaii DOH approves several types of Individual Wastewater Systems, including standard septic tanks, ATUs, and passive aerobic systems.

Determining which system is right for your property requires a professional site evaluation. A Hawaii-licensed civil engineer must assess your lot size, soil composition, and proximity to sensitive water sources to design a compliant system.

As a full-service environmental engineering firm, ACSI handles every step of this process, from the initial engineering design and DOH permitting to the final excavation and installation. If you are ready to convert your cesspool and upgrade your property's infrastructure, contact ACSI today at 808-645-4672 or visit acsihawaii.com to schedule a consultation.