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.

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