Never thought about where your toilet goes? You're in good company. Here's the whole thing in plain language, no plumbing degree required.
The three words you need
Cesspool: an underground pit, often lined with rock or concrete rings and open at the bottom, that collects everything from your toilets, showers and sinks. It doesn't treat anything. The wastewater just soaks into the surrounding ground.
Septic tank: a sealed, watertight tank. Wastewater sits in it long enough for heavy solids to sink to the bottom (sludge) and grease and light stuff to float to the top (scum). The clearer liquid in the middle flows out.
Leach field (also called a drain field): a set of perforated pipes laid in trenches of gravel. The liquid from the tank trickles out of the pipes, and the soil underneath acts as a natural filter before the water reaches groundwater.
A cesspool lets raw wastewater soak into the ground. A septic system settles out solids in a tank, then spreads the liquid through a leach field so the soil can filter it.
Why Hawaii is converting
Hawaii has roughly 83,000 cesspools, including about 48,600 on Hawaii Island, 14,300 on Kauai, 11,000 on Maui, 7,500 on Oahu and 1,400 on Molokai (Cesspool Conversion Working Group final report, 2022). Because cesspools release untreated wastewater, they can affect drinking water, streams and coastal waters (Hawaii DOH).
The law: Act 125 (2017) requires every cesspool in the state to be upgraded, converted, or connected to sewer by 2050 (DOH).
The earlier dates: The Cesspool Conversion Working Group, created by Act 170 (2019), recommended Priority 1 cesspools be converted by 2030 and Priority 2 by 2035. That is a recommendation, not current law (CCWG report).
Priority levels
University of Hawaii researchers rank every cesspool as Priority 1, 2 or 3 based on risk to drinking water wells, streams, coastlines and groundwater. On Oahu about 64% are Priority 1 (CCWG briefing). You can look up your area on the Hawaii Cesspool Prioritization Tool.
Your options
Septic system: tank plus leach field, as above.
Aerobic treatment unit (ATU): a tank that adds air so bacteria break waste down further. Often used where soil, lot size or groundwater make a basic septic system a poor fit.
Sewer connection: if a public sewer line is available near your property.
Which one works depends on your lot. That's what the site evaluation figures out.
How a conversion works
Site evaluation: locate the cesspool and check soil, lot size and groundwater.
Design: a Hawaii-licensed professional engineer designs the system to Department of Health rules.
Permit: the design is submitted to the DOH Wastewater Branch for approval.
Install: a licensed contractor installs the new system and properly closes the old cesspool.
Inspection: final inspection and DOH approval to use.
Approved system options
The Hawaii Department of Health approves several kinds of replacement systems (DOH IWS). Here they are from most advanced (and generally most expensive) to most economical. Your lot decides which ones are possible.
Aerobic treatment unit (ATU) with nitrogen removal, plus disinfection. An air pump feeds bacteria that break waste down much more completely and remove nitrogen; UV or chlorine kills germs. Best for Priority 1 areas, shallow groundwater, and sites near the coast, wells or streams. Units discharging directly to groundwater must be certified to NSF/ANSI 245 for nitrogen reduction (DOH allowed technologies). Requires a service contract with inspections every 6 months (HAR 11-62-33.1) and uses electricity. Expect the high end of costs; operating costs run up to about $108/month for the most involved setups (CCWG briefing).
Bioreactor garden. Water leaving a septic tank flows through a lined garden where plants and microbes remove nitrogen. Approved in several sizes for nutrient-sensitive sites (DOH allowed technologies). Cost not published.
Standard ATU (NSF/ANSI 40 Class I). Same idea as above without the extra nitrogen removal. Used where poor soil, a high water table or a shoreline location rule out a basic leach field (Golden Penny guide). Same service-contract rules (HAR 11-62-33.1). Priced by custom quote.
Septic tank plus an advanced passive field (for example Eljen or Presby). Fabric and sand modules treat the water further so the field can be smaller, which helps on small lots and marginal soil (DOH allowed technologies). Maintenance is tank pumping. Cost not published.
Sewer connection. Only where a public sewer is available; the counties decide where sewer goes (CCWG briefing). Hookup cost not published.
Conventional septic tank with leach field, bed or seepage pit. The simplest option for lots with decent soil and enough depth to groundwater. Lowest operating cost, about $33/month at the low end (CCWG briefing).
Costs we can cite: DOH's historical septic and ATU installs ranged from about $10,000 (10th percentile) to $38,000 (90th percentile), averaging $23,000 (CCWG briefing); the full range runs from $9,000 to $60,000 or more (DOH finance report). A typical Oahu conversion runs $30,000 to $60,000 (Wastewater Conversations in Hawaii 2025). Your actual cost depends on your lot.
What about composting or incinerating toilets? They're DOH-approved, but they only handle toilet waste. Your shower, sink and laundry water still needs an approved treatment system, so they can't replace a cesspool on their own (DOH IWS).
Every system needs a Hawaii-licensed civil engineer, a percolation test and DOH plan approval (DOH allowed technologies).
Funding (as of October 2026)
Statewide loan program (Act 120, 2026): low-interest or forgivable loans for low- and moderate-income homeowners, run by the Hawaii Green Infrastructure Authority. It is still in development and not yet taking applications (DOH, HB1618).
Oahu: no Oahu-specific grant is currently available.
Kauai: County Grant 2 offers up to $20,000 reimbursement grants; deadline extended to Oct 23, 2026 (Kauai Now).
We give a property-specific estimate after a free site survey.
FAQ
Is a cesspool the same as a septic tank?No. A cesspool is a pit that lets untreated wastewater soak into the ground. A septic tank is a sealed tank that separates solids, and it sends the liquid to a leach field where the soil filters it.What is the deadline to convert my cesspool?Act 125 (2017) requires all cesspools in Hawaii to be upgraded, converted or connected to sewer by 2050. A state working group recommended earlier dates of 2030 for Priority 1 and 2035 for Priority 2, but those are recommendations, not law.How do I find out my cesspool's priority level?Use the University of Hawaii Sea Grant Hawaii Cesspool Prioritization Tool at seagrant.soest.hawaii.edu/cesspools-tool.Who designs and approves a new system?A Hawaii-licensed professional engineer designs it, and the Hawaii Department of Health Wastewater Branch reviews and approves it before and after installation.Is there funding?It depends on your island. As of October 2026, Kauai offers County Grant 2 (up to $20,000, deadline Oct 23, 2026), Hawaii County offers a pilot grant of up to $30,000 per project, and a USDA-backed 1% loan is listed for Maui. Oahu has no county grant. A statewide low-interest or forgivable loan program under Act 120 (2026) is being set up by the Hawaii Green Infrastructure Authority and is not yet accepting applications.How much does a conversion cost?It depends on your lot, soil and system type. We give a property-specific estimate after a free site survey.