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What You Need to  Know

If your building plans here in Hawaii involve a new home or the addition of one or more new bedrooms to your existing home, chances are you’re going to need a new individual wastewater system (IWS) or an upgrade to your existing one.  There have been quite a few recent changes to the rules surrounding these systems and we here at Foresight would like to take the opportunity to provide some basic information about those rules, why they exist, and what they mean for your building project.

If your property has municipal or private sewer service, you need not read much further.  Individual wastewater systems are only allowed in areas where no sewer exists.  To learn about the planning requirements for connecting to a municipal sewer system, send us an email at


What is an Individual Wastewater System?

Essentially, an IWS is a self-contained means of disposal for all wastewater from a single-family home.  Until only a few years ago, this could be as simple as a cesspool, which is just a hole in the ground sealed with a concrete cap where all of a home’s wastewater was directed to.  Cesspools are quite fool-proof and require no maintenance if built correctly but they have become an environmental concern as our population has increased.  One can imagine the problem with discharging raw sewage -packed with bacteria and household chemicals- directly into the ground, especially in coastal areas where our highly porous soils do little to slow or break down our waste before it comes into contact with our near-shore waters.  It’s bad for our reefs, bad for our wildlife and bad for all of us too.

Recently the Hawaii Department of Health (DOH), which oversees the disposal of our wastewater, adopted new legislation banning the construction of new cesspools throughout the state.  Going forward, septic systems will take the place of cesspools in any building requiring a new IWS or an upgrade of an existing one.


A septic system is a more refined version of a cesspool.  Instead of being discharged directly into the ground, wastewater is first directed into an underground tank where bacteria break down and clean up the sewage through natural processes and then discharge this cleaner effluent to a series of perforated underground pipes placed in a gravel absorption bed (sometimes referred to as a leach field).  This bed of crushed stone evenly distributes what is left of the wastewater over a large area of shallow ground where it can be further broken down by soil bacteria before coming into contact with groundwater.

There are two types of septic systems allowed here in the islands- aerobic and anaerobic.  Both systems are comprised of a tank and absorption bed as described above but what sets them apart from one another is what is inside their respective tanks.


Anaerobic systems are much more common, simpler to install and maintain, and less expensive all around.  This is because a tank for an anaerobic system is simply that; a tank.  The bacteria which grow in this system do not require oxygen and break down wastewater without our help.  All that is needed is the occasional (every two to five years) pumping out of sludge which accumulates at the bottom of the tank over time and is a by-product of the bacterial processes which make the system work.


Though an anaerobic system is far better at reducing the environmental impact of our wastewater than a cesspool, it has its limits.  For properties at very low elevations, where groundwater levels are very close to the surface or properties within one thousand feet of a drinking water well, the discharge effluent from an anaerobic system is considered too dirty to safely mix with groundwater.  For situations such as these, aerobic systems are required.

An aerobic system can be thought of as a self-contained sewage treatment plant.  It is comprised of a tank and absorption bed, like an anaerobic system, but with a few more components within the tank itself.  These components allow for the growth of aerobic bacteria, which break down and disinfect wastewater to a much greater degree than anaerobic bacteria.  This means lower nutrient levels in discharge water, healthier reef ecosystems and less potential for drinking water contamination.  Aerobic systems are active however, and require electricity and monthly maintenance by a licensed wastewater contractor to ensure proper operation.  The tanks themselves are also more expensive to acquire and install because of their internal gadgets.  This increases upfront and ongoing costs of ownership and is the reason why these systems are normally specified only in areas where the DOH requires them.

How do the new rules affect my existing home?

If you’re not planning an addition of bedrooms to your existing home, you need not worry.  The DOH is allowing existing cesspools (installed and approved before the recent rule changes) to remain in service so long as no new bedrooms are added to the home.  Recently though, some lending agencies have denied mortgages to buyers looking to purchase a home in Hawaii with an existing cesspool for fear that the DOH might one day require that all existing cesspools be converted to septic systems.  While there is currently no word of such a requirement being implemented, it is making it more difficult for people to sell homes with cesspools and some have elected to convert to septic systems voluntarily in hopes of an easier home sale.


If you are planning to add one or more new bedrooms to your existing home, you will either need to abandon and fill your existing cesspool and install a new septic system or, if your home already has a septic system, you’ll need to increase its capacity.  This could mean installing a bigger tank (if the house will have a total of 5 bedrooms after the addition) and/or increasing the size of your existing absorption bed by digging it up and extending the underground perforated piping and gravel bed.  This kind of work is not inexpensive and may be impossible due to space limitations once a house is built.  Both the tank and absorption beds of any system must be a minimum of 5’ away from property boundaries and building footprints and 50’ away from any natural bodies of water.  Make sure you have the room to expand before getting too far along the design process.

How do the new rules affect my upcoming build?

Unless your property has municipal or private sewer service, you need a septic system.  No new cesspools are allowed anywhere in the State of Hawaii.

How big will my new septic system need to be?

Septic system sizing is based on occupancy of a building, not the number of bathrooms, as one might assume.  The DOH will allow up to 5 bedrooms and one kitchen within a residence to be served by a single septic system.  For one to four bedrooms, a 1000 gallon tank is required.  For 5 bedrooms, a 1500 gallon tank is required.  Absorption bed sizing is also based on the number of bedrooms.  On average, 75-100 square feet of absorption bed area is required per bedroom, depending on how porous or well-draining the soils on a given home site are.  For tight build sites, other methods exist for discharging effluent from septic tanks and are designed on a case-by-case basis.

How much is all of this going to cost me?

The million dollar question (we hope not).  The main cost involved in septic system installation is earthwork.  Though we always strive to design the most efficient and cost-effective system possible, Foresight’s fee to design a new system will likely be a whole lot less than a contractor’s price to install it.  For smaller systems in softer soils or imported fill, an anaerobic system can often be installed for less than $20,000.  For larger homes along the coast, where young pahoehoe lava flows must be broken up and excavated to install an aerobic system, costs can reach upwards of $50,000 for site work and equipment installation.  It’s the price for paradise.

Who can I call to install a new system?

Any contractor with the following license types can install or upgrade your system for you:

  • A – General Engineering

  • C-9 – Cesspool (Admittedly a misnomer)

  • C-37 – Plumber

  • C-37a – Sewer & Drain Line Contractor

  • C-43 – Sewer, Sewage Disposal, Drain & Pipe Laying Contractor

Often, plumbers will have a septic system installation company or equipment operator that they prefer to work with so it’s best to start by asking your plumber who they would like to work with.

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