Septic Systems

By Caroline Gleason
Last Updated: 7/9/21

Introduction to Septic Systems

Septic systems are a kind of decentralized wastewater system commonly used in areas without a centralized sewer system. Conventional septic systems usually consist of a buried, water-tight container called a septic tank, and a shallow, covered soil absorption field called a drain field. These systems use a combination of technology and natural processes to partially treat household wastewater from bathrooms, kitchen drains, and laundry.

Septic systems are grouped under a number of terms. In addition to decentralized wastewater systems, you may see septic systems referred to as individual or private wastewater systems, onsite sewage treatment and disposal systems (OSTDS), or small community cluster systems. These umbrella terms often include other forms of shallow wastewater infrastructure, including cesspools.

There are a suite of pollution problems associated with conventional septic systems, including discharges of highly concentrated nutrients, including nitrogen, into groundwater and coastal waterways. Failure to conduct routine maintenance or provide adequate drain fields can also lead to pollution issues. Fortunately, new septic technologies are now available that are able to remove nearly 90% of the nitrogen in wastewater and address some issues in coastal areas with higher groundwater levels, which we will discuss later in this article.

How do conventional septic systems work?

Conventional septic systems usually consist of a septic tank, which is a buried, water-tight container, usually made from concrete, fiberglass, or polyethylene, and a drain field, which is a shallow, covered soil absorption field. Figure 1. Conventional Septic System. Image courtesy of EPA.

Wastewater from household use in bathrooms, kitchen sinks, and laundry machines flows into the septic tank through an inlet pipe (shown in Figure 2). The septic tank relies on naturally-occurring bacteria to digest organic matter in the wastewater, and gravity to separate the less dense, floatable matter (e.g. oils and grease) from the more dense solids (e.g. human waste) in the wastewater. Septic tanks are typically separated into two compartments. In the first compartment, the floatable matter, called scum, sits on top of the liquid wastewater while denser solids settle to the bottom of the tank to form sludge. Bacteria break down the wastewater solids into sludge through a process called ‘’anaerobic digestion’’, which happens in the absence of oxygen. Excess sludge may overflow into the second compartment, but it is prevented from escaping the tank by a T-shaped outlet (Figure 2). To keep the sludge from building up and causing the system to fail, homeowners are expected to get their septic tanks pumped every three to five years, depending on amount of use and on-site soil conditions.

The liquid wastewater, called effluent, is then discharged through the T-shaped outlet to the drain field. Effluent percolates through the drain field where the soil beneath serves as a natural filter; large particles within the effluent are trapped in voids within the soils, while positively-charged particles like chemicals and pathogens bind to negatively-charged soil particles. This filtration happens as the effluent flows downward toward the water table, which is the upper boundary of wet soil permanently saturated by groundwater. For this process to work, there needs to be at least two feet of dry, unsaturated soil under the drain field to filter the effluent before it reaches the water table and flows into the groundwater. When the depth of the drain field is too close to the water table, any nutrients or contaminants (including harmful disease-causing pathogens) present in the liquid effluent can directly pollute the groundwater, the surface waterways groundwater flows into and, eventually, the ocean. Figure 2. Septic Tank. Image courtesy of EPA.

Because successful septic treatment relies on this process of soil filtration, the design of the drain field depends on the depth of the seasonal water table (the uppermost depth of saturated soil during the wettest part of the year) and the characteristics of the soil on-site. In a standard septic system, effluent is discharged to a drain field through pipes nested in dug trenches that are filled with clean, porous gravel, then covered with landscaping fabric. Some drain fields forgo trenches in favor of a shallow drain field bed filled with the same clean, porous gravel. For areas with a shallow water table, chambers or even mound systems are preferred because they create more space for the effluent to percolate through soil before joining the groundwater. Read more about alternative drain fields on EPA’s Types of Septic Systems page.


Figure 3. Examples of adapted septic system designs. Top: Drip Distribution Septic System, with a drain field bed instead of dug trenches. Bottom: Mound Septic System, with piled soil, sand, and gravel to elevate the height of the drain field. Images courtesy of EPA.

It’s important to note that, while properly designed septic systems can adequately treat pathogens and bacteria in wastewater, none of these system designs thoroughly capture nutrients, which can be harmful to water systems nearby. The main nutrient of concern is nitrogen, which enters the system through human waste and can end up downstream, over-fertilize water bodies and lead to harmful algal blooms. Read more about nitrogen pollution and septic systems later in this article.

Current Use

Approximately 20% of households in the United States rely on a decentralized wastewater system, according to EPA. More than 60 million people are serviced by septic systems specifically. Septic use varies widely by region and state, with the highest concentration at 55% of households in Vermont and the lowest at 10% of households in California.

The region with the highest proportion of septic use is New England, where approximately 50% of households use septic systems. In addition to Vermont’s 55% of households, both New Hampshire and Maine report more than half of all households in each state use septic systems.

Reliance on septic is not confined to New England. For instance, when looking at absolute numbers, the state of Florida alone is responsible for over 12% of all septic systems in the US, according to the Florida Department of Health. About a third of households in the Southeastern US use septic; North Carolina reports around 48% of households are on septic, while Kentucky and South Carolina each report around 40%. Many communities in mid-Atlantic states like New York and New Jersey also rely on septic, particularly on Long Island. As of 2019, 75% of Suffolk County (the easternmost of Long Island’s four counties) is unsewered, with approximately 360,000 residential onsite sewage disposal systems (aka septic systems and cesspools). Long Island as a whole reports around 500,000 in-use decentralized wastewater systems total.

According to EPA, approximately one-third of all new development in the US is served by septic or other decentralized treatment systems. For many communities, especially in rural areas, decentralized wastewater systems alleviate the need for large, expensive infrastructure and energy costs associated with centralized sewer systems and wastewater treatment plants. Septic systems (if properly located, installed, and maintained) help reduce transmission of disease and exposure to harmful pathogens that can be found in wastewater. Septic systems also can help to reduce wastewater pollution entering and contaminating surface waterways and groundwater reserves.

That said, conventional septic systems are not perfect forms of wastewater treatment. They can fail, whether by improper installation, insufficient maintenance or rising groundwater levels. Even functioning at their best, conventional septic systems do not filter out the high concentrations of nitrogen in wastewater, which fuel harmful algal blooms and other environmental consequences when that wastewater enters surface waterways and the ocean. Since septic systems are so heavily used and relied upon, we must ask: how can we improve them?

Problems with Septic Systems

How do conventional septic systems fail?

Before we can figure out solutions to the problems conventional septic systems pose, we must understand how these systems fail.

For many homeowners, outside of sewage backing up into houses and onto yards, their septic systems are generally out of sight, out of mind. As Thomas Ruppert, a coastal planning specialist from Florida Sea Grant, summarized: “‘Working’ from the property owner’s perspective means the toilet flushes.” Sadly, this is often far from the truth.

Homeowners with septic systems are expected to pay a company to pump their system every three to five years to clean out the solid sludge. However, that doesn’t always happen. For some, it can be a (mistaken) point of pride to go ten, fifteen, even twenty years without having their systems pumped. This causes solids to build up in the tank, which leads them to overflow into the second compartment and possibly out into the drain field, ultimately causing “failure”. In other words, failure to routinely pump your septic system could lead to untreated human sewage leaking into the drain field and maybe even nearby properties.

Septic tanks can also crack. Cracked septic tanks or inlet pipes leak bacteria, nitrogen, and pathogens directly into the surrounding soils and groundwater, harming human health and the environment. This is a prevalent issue in wet, tropical climates as sandy soils become waterlogged. Tanks may also crack as a consequence of freeze and thaw cycles in more temperate climates.

Ultimately, even painstaking maintenance will not fix a septic tank that is too close to, or at, the water table. For septic treatment to work, there needs to be at least two feet of dry, unsaturated soil underneath the drain field to filter the wastewater before it reaches the water table and flows into the groundwater. Without this buffer of dry soil, the septic tank cannot effectively transport the effluent to the drain field, thus allowing nutrients and pathogens present in the wastewater to directly contaminate groundwater and the surface waterways it flows into. A submerged septic tank functions no more effectively than a cesspool, which is to say, not effectively at all.

Septic systems, once installed, also tend to be “out of sight, out of mind” for legislators and local regulating bodies. According to EPA, very few permitting agencies conduct regular inspections of septic systems after they are installed. This puts the onus onto homeowners to ensure their septic systems are functioning as best they can, and to watch out for signs that their system has failed.

Common signs of septic failure include:

• Water and sewage from toilets, drains and sinks backing up into the home
• Slow drainage of bathtubs, showers and sinks
• Gurgling sounds from the plumbing system
• Standing water or damp spots (ponding) near the septic tank or drain field
• Bad odors around the septic tank or drain field
• Bright green, spongy grass over the septic tank or drain field, even during periods of dry weather
• Algal blooms in nearby ponds or lakes
• High levels of nitrates or coliform bacteria in nearby water wells

If you observe any of these signs of failure in your own septic system, call your local septic pumpers and your local health department. For ways you can take care of your septic system, check out the “Personal Actions” section later in this article.

How do septic systems pollute coastal waters?

The #1 water quality problem in areas serviced by decentralized wastewater systems like septic systems and cesspools is nitrogen contamination of fresh and marine waters. When septic systems are situated properly with sufficient separation between the drain field and the groundwater, most of the pathogens (bacteria, viruses, etc) are filtered out or bind to dry soil particles as the effluent trickles down towards the groundwater. The effluent that eventually reaches the groundwater, however, remains heavily polluted with nitrogen (and some phosphorus). The now nitrogen-laden groundwater flows downstream towards surface waters like lakes, streams and, ultimately, the ocean.
Figure 4. Infographic: How do septic systems pollute in dry weather?

High concentrations of nitrogen in waterways have severe impacts on coastal ecosystems. Nitrogen acts as a fertilizer, causing algae populations to skyrocket in events called ‘’algal blooms’’. These increased populations of algae eventually die and sink to the bottom of the water body to decompose, which depletes the dissolved oxygen in the water and often causes mass die-offs of fish, turtles, manatees and other aquatic life from the lack of oxygen and food sources. When this happens on a large scale, affected areas are called ‘’dead zones’’.

Drops in dissolved oxygen are not the only consequence of ‘’eutrophication’’, or the over-enrichment of nitrogen and other nutrients in waterways. In tropical regions, the algae cover reefs, starving corals of sunlight and oxygen. The subsequent, sudden loss of reef habitat has repercussions for fish, with one study finding 83% of the most abundant species either severely reduced or completely eliminated following an algal bloom in the Gulf of Oman. In more temperate regions, high levels of nitrogen in the water lead to the decline of seagrass beds that provide nursery habitat for many important fisheries, as well as provide critical storage for atmospheric carbon as a ‘’blue carbon’’ ecosystem. Eutrophication can also lead to an increase in harmful algal blooms and red tides that produce toxins that contaminate shellfish, cause fish kills and other sea life to die, and even threaten human health with a whole suite of mild to severe symptoms.


Figure 5. Infographic: How do septic systems pollute in rainy weather?

Nitrogen pollution and its associated impacts are all happening during dry, “business as normal” conditions. The problems only get worse during wet weather, when rain and storm events cause local flooding. Saturated septic systems can discharge harmful, disease-causing pathogens into our recreational waters. Flood conditions saturate the soil with rainwater and runoff which raises the water table so that there is little or no dry soil between the drain field and the groundwater to filter out pathogens (less than the two foot minimum requirement). This allows these potentially harmful pathogens to directly contaminate groundwater. During flood conditions, septic drain fields can also get backed up when the surrounding soil becomes saturated, driving effluent backwards towards the septic tank and causing overflows that leach sewage into homes, onto yards and into stormwater runoff that ultimately ends up in the ocean.

Septic systems that are located close to the beach or surface waterways like rivers or creeks are more likely to put public health imminently at risk than systems located further inland. This is why it is very important not to swim for 72-hours after a heavy rain in communities that are not sewered. Many communities have developed management plans to prioritize waterfront zones to maximize the benefits of limited funding for wastewater upgrades, which we discuss further in the “Policy Actions” section of this article.

How do septic systems pollute drinking water?

Many areas of the country that rely on shallow wastewater infrastructure like septic systems are also more likely to rely on groundwater for their drinking water supply, according to EPA. The state of Florida, for example, is responsible for approximately 12% of all septic use in the United States and, according to the Florida Department of Health, 90% of the state’s drinking water comes from groundwater. This reliance on groundwater for both drinking water and wastewater discharge exacerbates the risk to human health were those septic systems to fail.

Threats to Septic Systems from Climate Change

Septic systems are already facing a number of climate-related threats, which promise to only get worse as our climate continues to warm. Flooding from increasingly severe coastal storms associated with climate change will cause septic systems and other types of sewage infrastructure to fail more frequently, releasing raw and under-treated sewage into coastal watersheds as surrounding soils become saturated during more frequent flood conditions. Flooded septic drain fields can also get backed up, driving effluent backwards towards the septic tank and causing overflows that leach sewage into homes, onto yards and into stormwater runoff that ultimately ends up in the ocean. Furthermore, many coastal regions in the US are already experiencing recurrent flooding at high tide during events aptly called high-tide flooding or sunny day flooding. Research shows that high-tide flooding events will become more common in areas already experiencing high-tide flooding, and will expand to currently unaffected areas as sea levels rise.

In addition to more frequent flooding events, sea level rise also brings concerns of more direct groundwater contamination from septic systems and other shallow wastewater infrastructure, even away from the coast. Projected levels of sea level rise are set to raise water tables which, in the case of septic systems, eliminates the pathogen filtration performed by dry soils. According to a study conducted across California, a coastal region with diverse topography and climate, 1 meter of sea-level rise is expected to expand the reach of areas flooded from below by approximately 50 to 130 meters inland, with low-lying coastal communities most at risk.

As mentioned above, many low-lying coastal communities in areas like New England, Long Island, and Florida rely on both decentralized wastewater systems like septic, and groundwater for their drinking water supplies. With conventional septic systems, elevation of the water table associated with sea level rise means more contaminants, nutrients, and disease-causing pathogens will enter water sources that are also used for drinking water, for recreation and by wildlife. This exposure will risk harm to both human health and the environment.

Current Regulations

Regulations for septic systems vary state by state. In many areas, local health departments are the agencies responsible for issuing permits to build and operate septic systems, provided that the system meets the standards set in each state's laws. Some states also include provisions to protect water resources in their septic regulations because of the potential impacts septic can have on waterways from nitrogen pollution. Before approving a new system, the local permitting agency typically conducts a site assessment to determine whether the soils can provide adequate treatment for the wastewater. However, once a system is installed, very few permitting agencies conduct regular inspections of active septic systems to ensure they are still functioning properly. This puts the onus on the homeowner to keep up with the required system maintenance and to look out for signs of failure, like those listed here and later in this article.

Solutions

Alternative Technologies

Where there is sewer infrastructure already in place or nearby, connecting homes and businesses that are on septic systems to sewers and sewage treatment plants is the preferred option, a process fittingly referred to as “Septics to Sewers.” A centralized wastewater system consists of a sewer system that transports wastewater to a central treatment plant. Sewer systems are either separated into stormwater sewers and sanitary sewers, or combined into a Combined Sewer Overflow that manages both stormwater and sanitary sewage or wastewater (although these are not recommended!). Wastewater that arrives at the central plant is treated before being discharged into the environment. Many counties and municipalities have successfully made the switch from septic to sewer over the years. Read more about one such success story from Monroe County in Florida, home of the Florida Keys, in the Case Studies section later in this article.

That said, centralized sewer systems are not always an easy, or even feasible, solution. Sewer systems can be costly to install in terms of money, time, and inconvenience - traffic disruptions not being an insignificant concern. Furthermore, for areas with unsuitable environmental conditions like shallow soils, impervious (unable for fluid to pass through) bedrock, or particularly vulnerable water tables, or even just where homes are too spread out, decentralized systems may make more sense. According to EPA, about one-third of all new development is served by septic or other decentralized treatment systems. We’re not going to completely phase out septic, so we should move towards advancements to these types of decentralized systems that make them more effective in reducing human health and environmental impacts.

Advanced treatment systems allow for additional treatment of wastewater beyond the limits of conventional septic, which helps reduce nitrogen pollution entering water sources. Individual systems are usually based around a septic tank, and therefore are often referred to as advanced septic technologies. There are two broad categories of advanced septic technologies: Aerobic or Advanced Treatment Units (ATUs) and Recirculating Media Filters (RMFs). Both categories use the addition of oxygen in the decomposition process to reduce nutrient loading and pathogens in the effluent before it is discharged to the drain field.

For areas better suited to a community-scale wastewater treatment system, package plants are modular systems that operate like miniature wastewater treatment plants. Package plants use a process called extended aeration to treat wastewater, which is a variation on the activated sludge process used by much larger plants. Standard package plants can produce high quality effluent that can be recycled as non-potable water for irrigation, aquaculture and industry applications.

Read more about these individual- and community-scale solutions in our article, Advanced Decentralized Wastewater Treatment Technologies.

Policy Actions

There are a number of local, state and federal policy initiatives to aid homeowners and communities in upgrading from conventional septic to more advanced systems. The goal of these policies is to increase and improve wastewater treatment capacity, ultimately protecting human health and the environment.

Some states, counties, and towns have issued mandates to upgrade or replace conventional septic systems to address imminent water quality concerns. In 1999, the state of Florida mandated Monroe County to transition from septic systems to a centralized sewer system to protect the fragile ecosystems of the Florida Keys. The $1 billion project received around $170 million in state funding to subsidize contributions from homeowners. These kinds of mandates have been issued for individual system upgrades as well: Suffolk County on Long Island, NY has called for use of advanced septic technologies in any new construction or renovations beginning July 1, 2019. Other approaches have focused on key problem areas, such as in Cape Cod, Massachusetts, where a 2023 rule requires landowners to upgrade or replace their septic systems in "Nitrogen Sensitive Areas".

To help defray the high cost of installing an advanced decentralized wastewater treatment system, many states, counties, and municipalities have implemented incentive programs to urge homeowners to upgrade their systems. Alongside their mandate for use of advanced septic in new construction and renovations, Suffolk County offers up to $30,000 to eligible homeowners who choose to upgrade their system through their Septic Improvement Program Grant and Loan Program. On a federal scale, the EPA Clean Water State Revolving Fund (SRF) also provides funding for upgrades to both centralized and decentralized wastewater systems. While this program is effective, it is severely underfunded and needs Congress to authorize and allocate significantly more funds. Learn more about Surfrider's campaign to increase federal funding to stop sewage spills at the beach, and ways you can help take action.

Personal Actions

If you have a conventional septic system, please practice proper maintenance and consider upgrading to a more advanced option. Is your community considering moving towards a centralized sewer system or packaged plant? Is a smaller-scale solution like advanced septic a better option for you? As mentioned above, there are many tax incentives and financial aid programs available to help alleviate the cost. Check with your state or local health department to see if there are any incentive programs in your area.

There are many actions we all can take to care for our wastewater systems, regardless of type. Good practices include:

• Only flush the three P’s: pee, poop, and (toilet) paper.
• Conserve water inside the house.
• Don’t pour cooking grease or oils down the drain; instead, collect it in a container, freeze it, and throw it in the trash.
• Try to avoid cleaners with petroleum additives or fillers, as they act like grease to cause blockages once they enter a wastewater system.
• Skip the powder detergents for the same reason.

To care for septic systems specifically, try to space out water use throughout the day to avoid flooding your septic tank and causing it to overflow. Use the “delayed start” feature on your washing machine or dishwasher to have it run during off-peak hours, like overnight, and try not to run multiple loads of laundry or dishes in a row. Check for leaks in your faucets and toilets (instructions on how to tell if your toilet is leaking can be found here). Use your garbage disposal sparingly; instead, try to catch and collect food scraps from the drain and compost them if possible - otherwise, throw them in the trash. If you or someone in your household is taking any major medications, like chemotherapy drugs, consider getting your septic tank pumped more often as those medications can upset the tank’s pH balance and kill off “good” bacteria that help break down solids. Don’t use any septic additives, especially ones that claim to dissolve solids in the tank to send out to the drain field: that’s the opposite of what we want from a functioning septic system!

It’s important to know where your septic system is located on your property and keep an eye out for any signs of failure, like those listed here. Take care to not drive over your septic system with heavy machinery (lawnmowers are generally okay, cars and trucks are not). Divert surface water (e.g. output from gutters, irrigation) from the septic field to keep the system as dry as possible. Be mindful of what you plant over the septic field; anything with large, deep root systems, like trees and shrubs, could damage your septic tank.

Florida Case Study: Where to go from conventional septic systems

With an estimated 2.6 million septic systems in use, the state of Florida is responsible for approximately 12% of all septic use in the United States, according to the Florida Department of Health. As reported in WUFT’s sewage series S-Storm, “the simple systems seemed harmless in the 1950s, before Florida’s population grew to today’s 21 million.” Now, out-of-date, leaky septic systems are a liability for the state, spreading pollution in a region already on the front lines of sea level rise, storm-related flooding, and other climate forces. Furthermore, Florida’s poor soils have less capacity for treating pathogens through soil filtration than soils in other parts of the country, according to University of Florida professor Mary Lusk. Conventional septic is simply not up to the task.

Monroe County, a county in southwest Florida that includes the islands of the Florida Keys, successfully made the switch from septic to a centralized sewer system. The fragile ecosystems of the Florida Keys were canaries in the coal mine of sewage pollution, so to speak; coral reefs declined, canals were polluted with wastewater, and blue-green algal blooms became commonplace. In 1999, the county was ordered by the state to upgrade their wastewater management to centralized sewer systems to address these water quality problems. Alongside about $170 million in state funding, homeowners were responsible for $4,500 of the cost for the approximately $1 billion project. Now, the Cudjoe Regional Wastewater System removes about 98% of nitrogen from sewage (a conventional septic system removes only around 30%) and the health of the county’s ecosystems has significantly improved. The treatment plant includes a deep injection well to dispose of tertiary-treated effluent 3,200 feet below the surface.

Often, the costs in money, time, and inconvenience of these large septic-to-sewer projects can be prohibitive. They can also be difficult to scale, particularly in very rural areas and in sprawling metropolitan areas already underlain with septic tanks, as described by Rachel Silverstein, executive director of the Miami Waterkeeper organization.

As reported by WUFT News, septic tank installations in Florida have been on the rise since 2018, following a decade-long decline. With the passage of the Florida Springs and Aquifer Protection Act of 2016, the state’s Department of Environmental Protection (DEP) implemented a grant program to pay licensed installers up to $10,000 to incentivize homeowners to upgrade their septic tanks to advanced systems designed to remove nitrogen from effluent. These grants funded over 1,000 upgrades since 2018 at a cost of $10 million, according to DEP. This program, however, was limited to just nine of Florida’s sixty-seven counties, and is no longer accepting applications. In order to make significant progress in reducing nitrogen pollution from septic, states like Florida must implement broad incentive programs to help homeowners with the cost of upgrading their conventional septic systems to more advanced technologies, including systems that add chlorine or UV radiation disinfection to new or existing tanks that currently carry a high price tag.

For more information on Florida’s transition away from conventional septic, check out these articles: Out of sight, still a blight and Flush Away! Florida Keys Celebrate Sewers.

Additional Resources

Nature Conservancy “Where Does ‘It’ Go When I Flush?”
WUFT’s S-Storm Project
EPA Types of Septic Systems
MSU Septics Webinar & Resources Page
TNC Reef Resilience Network’s Wastewater Pollution Toolkit