Suffolk County Government Crest

Suffolk County Stormwater Management Program

Edward P. Romaine, County Executive

Department of Public Works

Department of Economic Development and Planning

All the water in your house that gets flushed or drained enters your septic system, where it undergoes 4 steps before being released back into the ground.

Step One: Through the House Sewer Line

There are two types of water in every household that end up flowing through the septic system. The water that is used for example by your bathroom and kitchen sinks, washing machines, and dishwashers, is known as Graywater. The water that is flushed down the drain from your bathroom toilets, showers or bathtubs is called Wastewater.  All water however travels through plumbing lines that run throughout the house. All of these discharge-based plumbing lines flow downward, perhaps aided by electric pumps, but typically aided by just gravity, and they all end up combining into a single master drainage line, called the house sewer line. This house sewer line is your main plumbing connection that directs the flow of both graywater and wastewater directly to your septic tank. Keeping the amount of water you and your family use on a daily basis in mind is important if you’re into being green and not wasting water, or even if you’re just into saving money by not wasting water. It’s important to remember that for every gallon of water that gets flushed down the toilet, every gallon that drains down of the sink, or every gallon that gets used in the washing machine, the same total volume of water is discharged into and out of your septic tank. On a daily basis, that can quickly lead to a whole lot of water flowing through those pipes and through your system. One very simple equation that we can all remember pretty easily is: “One Gallon Flushed In = One Gallon Flushed Out!”

Step Two: Into the Septic Tank

Your septic tank is a device of vital importance. Commercially, it is known as an “Individual Residential Wastewater Treatment System.” Now don’t worry, it’s possible, very likely even, that many of you may have never actually seen your septic tank, only knowing that you are pretty sure that you might have one buried somewhere on your property.  Well, it’s actually this ‘out of sight - out of mind’ device that treats nearly all of our household’s raw sewage and wastewater before it is discharged back into the ground, where over time it will slowly disperse and flow downward, until once again reaching the groundwater and recharging our drinking water supply. Your septic tank is the first line of defense against the contamination of our drinking water supply by harmful substances and pathogenic microorganisms. So, now I bet you can see why your septic tank is so important. Your septic tank is responsible for the first stage, and most important stage, of the sewage treatment and contaminant removal processes and therefore is an essential part of keeping Long Island’s groundwater clean and safe for human consumption. One of the most basic functions of the septic tank is to perform a process known as density sorting, where the raw sewage that enters the tank from your house sewer line is sorted into three separate layers, which as the name of the process implies, are entirely based on the density of its constituents. The three layers are called the Scum layer, the Wastewater layer, and the Sludge layer.

  • The Scum Layer – is the thin, and almost foamy top layer, which contains organic and inorganic floatables, toxic gaseous elements from the bio-degradation processes and other low density waste that enters the tank.
  • The Wastewater Layer – is the wide, liquid middle layer, which contains a large portion of dissolved solids, and is composed of mainly the wastewater and tiny particulate matter that has undergone or is still undergoing some degree of bio-degradation.
  • The Sludge Layer – is the very bottom layer of the tank, which contains the densest waste that consists of mostly non-degradable inorganic materials and the newest organic solids that are just beginning to undergo bio-degradation.

Once the flow of wastewater from the house sewer line passes through the inlet baffle and reaches the inside the septic tank, the raw sewage begins to bio-degrade through multiple different decomposition processes that are performed by a variety of different forms of bacteria. These different forms of bacteria can thrive in the specific conditions created within each of the separate layers of the tank. For example, anaerobic bacteria, which thrive in the anoxic conditions found in the scum and sludge layers, will reproduce solely in these layers and consume the organic matter in them by utilizing a process called anaerobic digestion. Most of the solid organic matter will be converted into treated liquid effluent, and as a by-product the process will also produce nitrates, phosphates and a small amount of other decomposition-based gases. A portion of indigestible scum and sludge that cannot be decomposed will remain in the tank, which must eventually be pumped out by a licensed septic professional. The treated wastewater, now called effluent, is able to travel out of the septic tank through the outlet baffle and to the next part of the system.

Step Three: Through the Distribution System

The effluent flows out of the septic tank and into a distribution device. This piece of equipment evenly distributes the effluent outflow from the septic tank through a soil absorption system. Distribution devices are typically used in larger systems, where there is a requirement for more than one seepage pit or multiple absorption beds.  The simplest form of distribution system incorporates a very slight downhill gradient in order to create a gravity fed even flow of effluent. More complex distribution devices use pressurized systems to allow for short periods of rapid distribution with a period of downtime, or a dosing distribution type of system that stores the effluent from the septic tank and periodically discharges into the soil absorption system through the use of siphons or pumps. Once in the pipes, discharge is performed by gravity instead of internal pressurization, and the amount of flow per discharge is reduced to nearly75% of the designed daily flow rate.

Step Four: The Soil Absorption System

This is where the final phase of wastewater treatment occurs through both physical and biological means once again, but this time it occurs in the soil. Bacterial biodegradation processes by both aerobic and anaerobic microorganisms in the surrounding soils continue to break down any organics that made it past the outlet baffle in the septic tank while also continuing the de-nitrification process on the effluent. Hydrodynamic dispersion and diffusion processes lessen the concentrations of harmful inorganic and chemical pollutants, such as nitrates and phosphates, as the effluent travels further from the source. Any possible harmful pathogenic microorganisms are typically stopped during soil treatment, due to the actual size of the organisms. The average soil type on Long Island is known as Sandy Loam, which is strong mix of mostly sand and silt sized particles with clays mixed in. Larger grain sizes such as gravel are uncommon in a typical coring of the stratum. The prevailing small grain sizes are able to physically prevent the movement of many pathogenic microorganisms from reaching the groundwater table and contaminating our drinking water supply. Although viruses are much smaller, it is very rare for them to travel far enough to contaminate the groundwater near a public supply well.

So now that we know the basics of how a septic system works, lets take a look at each component in a bit more detail, starting with What is a Septic System?