Cesspool vs. Septic System vs. ATU
Direct answer: What is the practical difference between a cesspool, a conventional septic system, and an aerobic treatment unit, and who decides which replacement system fits a property?
The short comparison
| System | What happens to wastewater | Mechanical equipment | Homeowner implication |
|---|---|---|---|
| Cesspool | Raw wastewater is discharged into a pit or chamber without treatment. | Usually none. | Substandard system that must ultimately be replaced or connected to sewer under Hawaiʻi law. |
| Conventional septic | A watertight tank separates solids; effluent moves to a soil absorption or disposal area for further treatment and dispersal. | Often gravity-based, though some designs use pumps. | Needs proper siting, pumping, records, and protection of the disposal area. |
| ATU | A treatment unit introduces oxygen and biological treatment before effluent reaches the disposal component. | Typically includes a blower, pump, controls, alarm, or other components. | Requires electricity, more frequent attention, and manufacturer or service requirements. |
Why a cesspool is different
Hawaiʻi DOH 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. A cesspool is not simply an older version of a septic tank.
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 ATU differs
An aerobic treatment unit adds oxygen and active biological treatment. Hawaiʻi DOH 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.
An ATU is not automatically required on every conversion, and it is not automatically better for every property. It can be appropriate when the approved design needs a higher treatment level or when site constraints make a conventional arrangement unsuitable.
Who selects the replacement system
The property owner should not choose solely from an online comparison or contractor preference. A Hawaiʻi-licensed civil engineer evaluates the site and wastewater requirements, prepares the design, and submits it to DOH. The accepted system must fit the property and comply with the current rules.
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?
Official sources
- Hawaiʻi Department of Health Wastewater Branch — Cesspools in Hawaiʻi
- Hawaiʻi Department of Health Wastewater Branch — Individual Wastewater Systems
- U.S. EPA — How Septic Systems Work
- Hawaiʻi Administrative Rules Chapter 11-62, unofficial searchable version
This article provides general homeowner information, not legal, engineering, real-estate, lending, or health advice. Property conditions and current agency requirements control. Hawaiʻi Cesspool Help coordinates between homeowners and licensed professionals and does not provide engineering services directly.