Here's a fun party question nobody asks: where does your toilet actually go? If you live in town on Oahu, the answer is usually "the city sewer." But for thousands of homes on the North Shore, the Windward side, the Waianae coast and plenty of other spots, the answer is "a hole in the yard." That hole has a name, and it's about to become a much bigger topic for Hawaii homeowners.
Let's break down the two systems you keep hearing about, without the engineering jargon.
The cesspool: a hole with ambition
A cesspool is an underground pit, usually lined with rock or precast concrete rings, with an open bottom. Everything that goes down your drains (toilets, showers, the kitchen sink, the washing machine) flows into it. Then the wastewater simply seeps out of the bottom and sides into the surrounding soil.
That's it. There's no tank to settle solids, no treatment step and no filter designed into the system. The Hawaii Department of Health describes cesspools as a way of disposing of untreated sewage, and that untreated wastewater can reach groundwater, streams and the ocean (Hawaii DOH).
Cesspools were common for decades because they were cheap and simple. Unfortunately, "cheap and simple" and "good for the reef" don't always go together.
The septic system: a tank and a field
A septic system has two main parts that do actual work.
1. The septic tank. This is a sealed, watertight tank buried in your yard. Wastewater flows in and sits there for a while. Heavy solids sink to the bottom and form a layer called sludge. Grease, oils and lighter material float to the top as scum. (Yes, the technical term really is "scum." Engineers are not poets.) The clearer liquid in the middle, called effluent, flows out the other end.
2. The leach field. Also called a drain field, this is a set of perforated pipes laid in gravel-filled trenches. The effluent trickles out through the holes, spreads through the gravel and soaks down into the soil. The soil does the final cleanup, filtering and breaking down much of what's left before the water reaches groundwater.
The tank keeps solids from clogging everything up, and the field spreads the liquid out so the soil can do its job. Teamwork.
Side by side
- Treatment: a cesspool provides essentially none; a septic system settles solids and uses the soil as a filter.
- Where the waste goes: a cesspool concentrates it in one spot; a septic system spreads treated liquid over a larger area.
- Maintenance: a septic tank needs periodic pumping to remove sludge. That is normal and expected.
- Legal status in Hawaii: new cesspools have been banned, and existing ones must be converted by 2050 under Act 125 of 2017 (DOH).
What about an ATU?
You may also hear about aerobic treatment units, or ATUs. Think of an ATU as a septic tank that went to the gym. It pumps air into the tank so oxygen-loving bacteria break down waste more thoroughly before the water heads to the disposal area. ATUs can be a good fit where a lot is small, the soil drains poorly or groundwater sits close to the surface. They do have moving parts, so they need power and regular servicing.
And if a public sewer line runs near your property, connecting to sewer may be an option too. Which choice fits your home depends on your lot, and a site evaluation is how you find out.
Why this matters in Hawaii specifically
Hawaii has roughly 83,000 cesspools, about 7,500 of them on Oahu, according to the state's Cesspool Conversion Working Group (CCWG final report, 2022). Many of us live close to the ocean, and many communities rely on groundwater for drinking water. That combination is why the state passed a law requiring every cesspool to be upgraded, converted or connected to sewer by 2050.
On Oahu, about 64% of cesspools are rated Priority 1, the highest-risk category, by the state's prioritization tool (CCWG briefing). You can see where your neighborhood falls on the Hawaii Cesspool Prioritization Tool.
How do I know which one I have?
If your home is older, isn't on the city sewer and nobody has ever mentioned a septic tank, there's a decent chance it's a cesspool. Your property records or a previous inspection report may say. Or you can skip the detective work: we'll come out, find it and tell you exactly what you've got.
New to all this? Our Cesspool 101 guide has a diagram that shows both systems side by side.