Advanced Decentralized Wastewater Systems

From Beachapedia

By Caroline Gleason
Last Updated: 7/27/21

Introduction to Decentralized Wastewater Systems

Decentralized wastewater systems are individual or community-scale types of wastewater infrastructure, commonly used in areas without a centralized sewer system. Included under the umbrella of conventional decentralized wastewater systems are septic systems, which use a combination of technology and natural processes to treat household wastewater, and cesspools, which do not treat wastewater and instead only collect and dispose of it.

Conventional septic systems (and cesspools, to a smaller, less successful extent) use naturally-occurring bacteria within the system to break down wastewater solids through a process called anaerobic decomposition. Wastewater discharge, called effluent, then percolates through dry soil which filters out harmful, disease-causing pathogens from the wastewater before it flows into the groundwater at the water table (the boundary of soil saturated with groundwater).

Current Use

Approximately 20% of households in the United States rely on a decentralized wastewater system, according to EPA. [1] As mentioned above, decentralized wastewater systems are commonly used in areas lacking a centralized sewer system, which transports wastewater from homes and businesses to a central wastewater treatment plant. Decentralized systems are spread out across the United States, with highly concentrated use in areas like New England, where around 50% of all households rely on a decentralized wastewater system, and Florida, which is responsible for approximately 12% of all septic systems in the country. [2] [3] As of 2019, 75% of Suffolk County, NY (the easternmost of Long Island’s four counties) is unsewered, with approximately 360,000 residential onsite sewage disposal systems in use. [4] Around 250,000 of those systems are cesspools. [5]

Problems with Conventional Decentralized Wastewater Systems

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 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. As cesspools are basically lined pits in the ground that receive household wastewater, when cesspools are situated properly and functioning as best they can, the liquid effluent leaches out of the pit without any pretreatment and into the surrounding soil. In both cases, the effluent that eventually reaches the groundwater 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 1. Infographic: How do septic systems pollute in dry weather?

Figure 2. Infographic: How do cesspools 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. [6] In more temperate regions, high levels of nitrogen in the water lead to the decline of seagrass beds which 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.

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. Cesspools and saturated septic systems can discharge harmful, disease-causing pathogens into our recreational waters and potential drinking water sources. 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 or cesspool 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.

Figure 3. Infographic: How do septic systems pollute in rainy weather?

Figure 4. Infographic: How do cesspools pollute in rainy weather?

Because of these problems, many local, state and federal policy initiatives have been implemented to encourage and aid homeowners and communities in upgrading from cesspools or conventional septic to more advanced systems. The goal of these policies and the technologies described below is to increase and improve wastewater treatment capacity, ultimately protecting human health and the environment.

Types of Advanced Decentralized Treatment Systems

Where there is sewer infrastructure already in place or nearby, connecting homes and businesses that are on decentralized systems to sewers and sewage treatment plants is the preferred option. This is especially the case in areas that have advanced centralized wastewater treatment facilities that produce recycled water instead of just discharging treated effluent. 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, particularly vulnerable water tables, or even just where homes are too spread out, decentralized systems may just make more sense. According to EPA, approximately one-third of all new development in the US is served by septic or other decentralized treatment systems. [7] Since septic systems are able to provide some level of treatment and are so heavily used and relied upon, we should move towards advancements to these types of decentralized systems that make them more effective in reducing human health and environmental impacts.

Advanced Septic

Advanced treatment systems allow for additional treatment of wastewater beyond the limits of conventional septic (and certainly beyond the non-treatment of cesspools), which helps reduce the release of pathogens and nitrogen pollution entering water sources. Individual systems are usually based around a septic tank, and are often referred to as advanced septic systems. Because they discharge a higher quality effluent, advanced septic can be particularly useful in coastal areas with shallow water tables, as they rely less on dry soils for sufficient removal of pathogens.

There are two broad categories of advanced septic technologies: Aerobic or Advanced Treatment Units (also referred to as ATUs), and Recirculating Media Filters (RMFs).

Aerobic Treatment Units (ATUs)

ATUs are the most commonly implemented type of advanced septic. These systems use the addition of oxygen to more efficiently break down organic matter in an aerobic (oxygen-rich) environment. Because aerobic decomposition can be quicker and more effective than anaerobic conventional septic systems, ATUs reduce nitrogen pollution and pathogens in effluent before discharging it to the drain field (more on this process later). [8] [9]
Aerobic treatment unit.jpg
Figure 5. Aerobic Treatment Unit (ATU). Image courtesy of EPA.

There are three different methods ATUs use to break down organic matter, depending on the type of system:

Suspended Growth Systems have two compartments: a main compartment, called the aeration chamber, and a secondary overflow compartment, called the settling chamber. Air is pumped into the aeration chamber, where it mixes with the wastewater to create an aerobic environment for free-floating bacteria to grow as they digest the organic solids (thus, suspended growth). The settling chamber is where the solids that were not digested by the bacteria settle based on density. The two compartments are connected to allow for the bacteria and undigested solids in the second chamber to return after settling, by gravity or pump, to the aeration chamber where aerobic treatment can reoccur. [10] [11]

Fixed Film Systems use a porous surface like fabric, styrofoam, or gravel as a medium to support the biomass film that digests organic matter in the wastewater. These systems fall into two broad categories: flexible media or stationary media. Flexible media systems move the media relative to the wastewater, allowing the biomass to alternate between immersion in the wastewater and exposure to the air. Stationary media systems instead fix the media in place and vary the flow of wastewater to allow the biomass to alternate between air exposure and immersion in the wastewater. In both systems, the biomass needs to be exposed to both air and wastewater for aerobic digestion to occur. [12]

Sequencing Batch Reactor Systems, the least common of the three types of ATUs, use a variation of the activated sludge treatment process to cycle between phases of aerobic and anaerobic (without oxygen) decomposition, but within the same chamber. Wastewater enters the tank through an inlet valve. During the anaerobic cycle, solids settle in a low-oxygen environment within the chamber. Then, air is bubbled through the chamber to start a cycle of aerobic decomposition. The length of the cycle varies depending on the size of the system and the quality of the wastewater input. The air is then shut off to allow for another anaerobic settling cycle, and the process repeats. Sequencing Batch Reactor Systems, while not widely used, can be a great solution when space is constrained as they only require one chamber. [13] [14]

Overall, ATUs provide a higher level of treatment than a conventional septic system, which helps to protect local water resources from nitrogen pollution and contamination by harmful pathogens. They can be a great solution for sites that are not suited to a conventional septic system, like in areas with high water tables or poor soils for filtration, and can extend the life of a drain field by discharging higher quality effluent that requires less filtration.

That said, ATUs are more expensive to install and operate than conventional septic systems. They require electricity and use mechanical parts that can break, which means that these systems can require more maintenance than a conventional system, as well. Many states, counties and municipalities have implemented tax credits or financial assistance programs to help homeowners cover the costs associated with these more advanced systems; read more about these programs in the Incentives section later in this article.

Recirculating Media Filters (RMFs)

Sand filter septic system.jpg
Figure 6. Sand Filter Septic System, a type of Recirculating Media Filter (RMF). Image courtesy of EPA.

As the name suggests, recirculating media filter systems use a "medium" like sand, peat, or gravel to filter effluent leaving a septic tank before it is released into the soil. A wide variety of materials can be used, though sand is the most reliable and widely used (shown in Figure 6 above). [15] RMFs treat effluent by pumping it into a water tight liner or container, containing a coarse sand, gravel, textile, or peat filter. The effluent is oxygenated (exposed to air) as it moves through the media filter, which breaks down ammonia from wastewater into nitrates, a process called nitrification. Effluent then flows through a recirculation tank (depicted as a pump tank in Figure 6 above), where low-oxygen conditions let anaerobic bacteria break down those nitrates into nitrogen gas, which is released back to the atmosphere. The effluent moves (by gravity or pump) between the media filter and the recirculating tank multiple times before it is sent to the drain field for further soil treatment. [16]

RMFs have been used since the 1970s as community-scale wastewater systems for flows of over 5,000 gallons per day.[17] Large systems can support wastewater flows of up to 36,000 gallons per day. [18] Use of individual-scale RMFs (flows of less than 1,200 gallons per day) has been on the rise as well. [19] Similar to ATUs, media filters are more effective than conventional septic systems, but are also more expensive than conventional septic. Because they perform a high level of nutrient treatment, RMFs are particularly well-suited to areas with high water tables, shallow bedrock, or close to the coasts, though are not as effective at removing fecal coliform (an indicator of sewage pollution) as other single-pass (not recirculated) sand and peat systems like these. [20] [21] They can also be used to recover drain fields that have failed due to excessive nutrient pollution, which could help salvage a conventional septic system that is no longer functioning. [22]

Other Advanced Treatment Systems

Nitrogen Removal Systems

Nitrogen removal systems employ engineering to mimic the natural nitrogen cycle, using both aerobic and anaerobic decomposition, similar to some centralized wastewater systems. First, aerobic bacteria transform the organic ammonia (NH4+) in the wastewater into nitrites and nitrates (NO2- and NO3-, respectively). This step is called the nitrification stage.[23] These nitrites and nitrates are then put into an anoxic (without oxygen) environment along with a source of carbon (most commonly methanol) to start the denitrification stage. The oxygen molecules from the nitrites and nitrates bond to the carbon molecules from the methanol, leaving behind nitrogen gas that can return to the atmosphere. [24]

In increasing efficiency of nitrogen removal, these systems become increasingly complex, which means they require more oversight and management than more simple wastewater systems. [25] Because of this additional attention required, many manufacturers of nitrogen-removal systems encourage designing systems to serve multiple homes to provide an economy of scale. [26] As with most advanced septic systems, nitrogen-removal systems are also more expensive to add onto a standard septic system. For cost and upkeep, these systems may be best suited for small community-scale wastewater systems.

Ultraviolet (UV) Disinfection Systems

Some systems use ultraviolet (UV) radiation to disinfect and remove harmful pathogens from the septic tank. This process mimics the treatments used in much larger wastewater treatment plants. Radiation from UV rays penetrates the cell walls of pathogenic organisms, preventing the ability of those cells to reproduce. [27] [28] This inactivates many pathogens, keeping them from causing diseases like gastroenteritis, leptospirosis, and cholera, to name a few. [29]

For UV disinfection to work, wastewater must be sufficiently treated before UV radiation is applied; if the wastewater is too cloudy, the UV rays won’t effectively disinfect the pathogens. The UV process itself has no lingering effects that could be harmful to humans or aquatic life, making it a safer alternative to chemical treatments like chlorination. [30] [31] Like many other advanced wastewater treatment methods, adding UV disinfection either to an existing septic system or a new advanced septic system will be more expensive than a conventional septic system. Read more about UV disinfection here.

Package Plants

Package plants are a community-scale wastewater treatment system that operate like miniature wastewater treatment plants, treating up to 100,000 gallons of wastewater per day. They use a treatment process called extended aeration, which is a variation on the activated sludge process used in much larger treatment plants. Effluent discharged from the plant after treatment can be reused as non-potable water for irrigation, aquaculture, and industry applications, or can undergo additional treatment in areas that require a higher quality effluent.[32] [33]

Package plants are modular systems, meaning they can be easily adapted to suit the needs of each community. These systems are well-suited to housing subdivisions or trailer parks, recreational parks, remote construction sites, and even small- and medium-sized cities. A typical mid-sized package plant is divided into four sections: an equalizer tank, a sludge holding tank, an aeration tank, and a clarifier. Plants may also have a chlorination or disinfection tank, a nitrogen removal tank, or some other kind of tertiary wastewater treatment added on.[34] [35]

TreatmentProcessFlowChart 6-29-16(002).png
Figure 8. Package Plant Treatment Process. Image courtesy of Pollution Control Systems, Inc..

How do package plants work?

Wastewater flows into the plant’s equalization tank, where it passes through a screen that filters out many of the solids coming into the system. That filtered wastewater then enters the aeration chamber, the largest of the plant’s standard compartments. This is where the untreated wastewater is mixed with an active biomass that digests organic matter. This mixing occurs in a slow rolling action driven by diffusers along the bottom of the tank. The chambers are designed to help drive the rolling motion of the water and eliminate any ‘dead zones’ in the tank where anaerobic decomposition could occur, therefore ensuring the entire volume of wastewater is aerated. An added benefit of this design is that it limits the amount of scum and froth that can accumulate within the tank, which also maximizes efficiency by reducing the amount of solids for bacteria to digest. The oxygen pumped into the tank through the diffuser, along with the rolling action, ensures there is enough oxygen in the system for bacteria to oxidize (break down) the wastewater into carbon dioxide, water and a stable sludge byproduct.

Once the wastewater has been aerated, it flows into the clarifier, where it is held for a period called the retention time. During this treatment phase, any solids still present in the wastewater settle to the bottom of the clarifier. Pumps inside the tank return the solids (and accompanying bacteria) to the aeration chamber as activated sludge. Returning activated solids to the aerobic environment of the aeration chamber maintains the maximum efficiency of the biological process, and mimics the procedures of much larger wastewater treatment plants.

Treated effluent flows from the clarifier to a disinfection chamber for final treatment via chlorination or ultraviolet (UV) disinfection before being discharged. Some plants use an additional tertiary filtration method to produce a higher quality effluent in areas where required.

You can watch a video tour of a standard package plant here.

Recycling Potential

Because package plants can produce a high quality effluent as well as a stable solid byproduct of the treatment process, these systems have great potential for wastewater recycling. Even without a tertiary treatment step, a standard plant’s extended aeration and disinfection processes produce an effluent that can be reused as non-potable water for use in irrigation, aquaculture, or other industry. The solid byproduct that accumulates in the sludge holding tank can also be recycled and added to soils as a fertilizer.

Local Mandates and Incentive Programs


There are a number of local, state and federal policy initiatives to aid homeowners and communities in upgrading from cesspools or conventional septic to more advanced systems like those described here. 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 some cases, these mandates have required a transition to centralized sewer systems (read more about Monroe County’s Septic to Sewer project here), but mandates have been issued for individual system upgrades as well. Suffolk County on Long Island, NY, for example, has called for use of advanced septic technologies in any new construction or renovations beginning July 1, 2019. [36]

To help defray the high cost of installing an advanced decentralized wastewater treatment system, many states, counties, and municipalities have implemented incentive programs to encourage 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. After Florida passed the 2016 Springs and Aquifer Protection Act, the state’s Department of Environmental Protection (DEP) implemented a grant program to pay licensed installers up to $10,000 to incentivize homeowners in nine springs-centered counties to upgrade to advanced systems designed to remove nitrogen from effluent. These grants have funded over 1,000 upgrades since 2018 at a cost of $10 million, according to DEP. [37]

Another way for governments to provide incentives and financial aid to homeowners is through tax credits. Following legislation phasing out the use of cesspool in Hawai’i, the Hawai’i Department of Health was instructed to work with the state’s Department of Taxation to provide tax incentives and financial assistance for homeowners to upgrade their wastewater treatment from cesspools. This program included a temporary tax credit of up to $10,000 for each qualified cesspool to help cover the cost of converting to a septic or aerobic treatment unit (ATU) system, or connecting to a central sewer system. The credit could be claimed from tax years 2016 to 2020. 21 [38]

Check with your state or local health department’s wastewater branch to see if there are any incentive programs in your area.


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