Cesspool vs. Septic System vs. Aerobic Treatment Unit
A cesspool receives wastewater without the treatment provided by an approved individual wastewater system (IWS). A conventional septic system uses a tank and disposal component, while an aerobic treatment unit (ATU) adds mechanical treatment. A Hawaiʻi licensed civil engineer must evaluate the property and design the system submitted to the Hawaiʻi Department of Health (DOH).
The short comparison
| System | What happens to wastewater | Mechanical equipment | What the owner should know |
|---|---|---|---|
| Cesspool | Untreated wastewater enters a pit or chamber and seeps into the surrounding ground. | Usually no treatment equipment. | Cesspools are subject to Hawaiʻi's statewide conversion, upgrade, or sewer connection requirement before January 1, 2050 unless an exemption applies. |
| Conventional septic | A watertight tank separates solids. Liquid then moves to a disposal area where soil provides further treatment and dispersal. | Many systems use gravity. Some designs include a pump. | The tank needs pumping and the disposal area must remain accessible and protected from damage. |
| ATU | An aerobic treatment unit adds oxygen and biological treatment before the liquid reaches the disposal component. | Common parts include a blower, pump, controls, and alarm. | The system needs electricity, service, and maintenance based on the approved equipment and operating requirements. |
Why a cesspool is different
Hawaiʻi Department of Health describes cesspools as substandard systems that do not treat wastewater. They concentrate raw sewage in one place and can affect groundwater, drinking water sources, streams, and coastal waters. Their design differs from a septic tank and disposal system.
Because there is no treatment tank and engineered disposal field comparable to an approved IWS, pumping a cesspool does not convert it into a compliant septic system.
How a conventional septic system works
A typical septic system begins with a watertight tank. Solids settle, oils and grease float, and the liquid effluent exits to a disposal area where the soil provides additional treatment and dispersal. The tank and disposal component must be sized and located for the property’s wastewater load and site conditions.
Septic systems still require maintenance. Solids accumulate in the tank, records should be retained, and the disposal area should be protected from vehicles, structures, roots, and excess water.
How an aerobic treatment unit differs
An aerobic treatment unit adds oxygen and active biological treatment. Hawaiʻi Department of Health lists ATUs among approved IWS treatment technologies. Because the unit relies on mechanical or electrical equipment, the owner should expect controls, alarms, service needs, and ongoing operation and maintenance responsibilities.
DOH approval of an ATU technology does not establish that it suits every parcel. The engineer selects treatment and disposal components based on the property, wastewater flow, site conditions, and current requirements.
Who selects the replacement system
A Hawaiʻi licensed civil engineer evaluates the site and wastewater requirements, prepares the design, and submits it to DOH. An online comparison or contractor preference cannot determine the system for a particular parcel.
Important factors can include lot size, slope, soil and percolation conditions, setbacks, wells and water features, available disposal area, dwelling count, wastewater flow, access, power, and maintenance capability.
Questions to compare real proposals
- What treatment and disposal components are included?
- Why does the engineer recommend this system for this property?
- Does the design use gravity, pumps, blowers, controls, or alarms?
- What inspections, pumping, service, testing, or maintenance are expected?
- What happens during a power outage or equipment failure?
- Where will future maintenance access be located?
- What records and final approvals will the owner receive?
Need help applying this comparison to a property?
Send the address or tax map key, the number of buildings, and what you know about the existing system. A professional can then evaluate the site rather than relying on a general chart.
Official sources
- Hawaiʻi Department of Health Wastewater Branch: Cesspools in Hawaiʻi
- Hawaiʻi Department of Health Wastewater Branch: Individual Wastewater Systems
- Hawaiʻi Department of Health Wastewater Branch: Allowed individual wastewater systems
- U.S. Environmental Protection Agency: How Septic Systems Work
- Hawaiʻi Administrative Rules Chapter 11-62, unofficial searchable version