What 'IWS' actually means
Individual Wastewater System (IWS) is the Hawaiʻi DOH umbrella term for any on-site wastewater treatment system that is not a cesspool and not connected to municipal sewer. Septic is one type of IWS. ATU is another. There are four IWS configurations permitted under Hawaiʻi DOH on Kauaʻi, and the percolation result on your specific lot determines which one you can install.
Option 1 — Conventional gravity septic
How it works: a septic tank separates solids and floats grease, then effluent flows by gravity to a buried leach field where soil bacteria finish treatment.
Best for: parcels with good percolation (15–60 minutes/inch), enough slope for gravity flow, and adequate setbacks from water sources.
Cost on Kauaʻi: project-specific — we quote after a site walk.
Maintenance: pump every 3–5 years. No mechanical parts in the field.
Option 2 — Aerobic treatment unit (ATU)
How it works: an ATU is a septic tank with active aeration that grows bacteria fast and discharges much cleaner effluent. The leach field is smaller because the water leaving the tank is already mostly treated.
Best for: parcels with poor percolation, high water table, small lots, or properties inside the Priority 1 / shoreline buffer where DOH wants extra treatment.
Cost: project-specific — quoted after a site walk.
Maintenance: annual service contract required by DOH; blower replacement every 5–10 years.
Option 3 — Drip dispersal
How it works: treated effluent is pressure-distributed through small-diameter drip tubing across a shallow dispersal field, like a slow drip-irrigation network.
Best for: sites with shallow soil over rock (common on South Shore Kauaʻi), steep slopes where a conventional leach field won't fit, or properties where preserving landscape is a priority.
Cost: project-specific — quoted after a site walk.
Maintenance: filter cleaning 1–2x/year; pump and controls service annually.
Option 4 — Elevated (mound) system
How it works: when native soil can't accept effluent (high groundwater, hardpan, or shallow rock), an engineered sand-and-gravel mound is built above grade to serve as the treatment medium.
Best for: low-elevation parcels near streams or wetlands, sites with seasonally high water tables, and Hanalei/Wailua floodplain properties.
Cost: project-specific — mound systems plus landscape integration are quoted after a site walk.
Maintenance: pump chamber service annually; visual inspection of the mound seasonally.
How DOH and your engineer decide
Percolation test results, soil profile from test pits, depth to groundwater, distance to surface water, lot size, and slope are all entered into the DOH IWS sizing worksheet. The output points to one (sometimes two) of the four system types as approvable for your site. You don't get to pick — your soil and your lot do.
That's why a written estimate before percolation testing is always a range. Once we have your perc results, the system type is decided and the cost narrows quickly.
Talk to Kauaʻi Excavation about your specific lot
We've installed every one of these four system types across Kauaʻi. Call (808) 855-3478 for a free site assessment — we'll tell you which configurations DOH is likely to approve for your address before you spend a dollar on percolation testing.
Got a Kauaʻi project to dig?
Call for a written, project-specific estimate.