Existing Treatment Systems

Sewage pipe. Photo © Joe Miller
Managing sanitary waste has been a concern since the earliest settled civilizations. Historically, sewage and household wastewaters were discharged into the nearest waterways, taking advantage of dilution and oxidation as treatment. This idea of “self-purification” was not incorrect; many contaminants can be removed by natural processes with sufficient exposure, time, and dilution. However, population growth, and the increasing contaminants in sewage, have rendered this approach inadequate. The discovery of sewage-borne illnesses resulted in sanitation development with the goal to separate sewage from drinking water to protect people’s health. ref Many treatment systems have since been developed to help stop raw sewage from entering oceans. Below is an introduction to common sewage treatment systems used today.

wastewater treatment plant

Wastewater treatment plant in California, USA. Photo © Michael Layefsky, Flickr

Centralized Wastewater Treatment Plants (WWTPs) and Sewers


Densely populated areas and industrialized cities primarily rely on centralized wastewater treatment plants (WWTPs) to receive and treat sewage. Intricate networks of underground sewage pipes bring sewage from homes and buildings to the WWTP using gravity and pumps. These large facilities are expensive to build, run, and maintain. Technologies and treatment capacities of these systems are rarely upgraded after initial investments. This is also true for sewer pipes, which are subject to frequent leaks and overflows. Establishing sewer pipe infrastructure is expensive, especially outside of densely populated urban areas. There is also an increasing risk of malfunction induced by increased rainfall, rising water tables, and sea levels. In addition to initial construction costs, upgrades to this extensive infrastructure are costly, and typically the responsibility of a municipality or local government.

Wastewater treatment plant aerial

Wastewater treatment plant from above. Photo © Alex de Haas, Flickr

Large cities generate not only large volumes of sewage, but stormwater as well. In areas lacking the absorption and retention that soils, grasslands, forests, and other natural features offer, precipitation has nowhere to go, so it flows over impervious surfaces, accumulating debris and contaminants, resulting in polluted urban runoff. In response, many cities built combined sewers to collect and transport stormwater to the same centralized wastewater treatment plants as sewage. While this seems efficient, it increases the vulnerability of all components of the system. Storms and heavy rain often exceed the capacity of pipes, holding tanks, and treatment systems, leading to large discharges of untreated wastewater, including raw sewage, into waterways. In the United States, 40 million people are served by combined sewers, which discharge over 3 trillion liters of untreated sewage and stormwater runoff annually in combined sewer overflow (CSO) events. ref

Watch the Wastewater 101 Webinar for more information on sewage management:


Once sewage arrives at a WWTP, it undergoes several stages of treatment before discharge.

  • Primary, or physical, treatment begins with screening: sewage is passed through screens to remove large solids. Effluent is then brought to settling tanks where gravity helps to settle out additional suspended solids.
  • Secondary, or biological, treatment aims to remove organic matter from sewage before disinfection. Oxygen and microorganisms are used to catalyze and promote biochemical reactions that break down contaminants. This process models natural systems and is made more efficient by aeration or exposure to additional oxygen. Oxygen is necessary for decomposition, and aeration helps eliminate dissolved gases. These reactions eventually encourage remaining particles to settle out. Common techniques for biological treatment include trickling filters and activated sludge, which increase the surface area available to microorganisms, as well as their density.
  • Tertiary, or chemical, treatment is used to promote further settling and nutrient removal. Added polymers attract pollutants to create clumps while carbon or charcoal filters catalyze physical adsorption to reduce nutrients.
  • Finally, effluent is disinfected to neutralize any remaining pathogens. While chlorine is one of the most common disinfectants, UV or ozone may be preferred to minimize residual chemical concentrations. ref

The flow of effluent through a WWTP from intake pipes to discharge. Source: opens in a new windowMallik and Arefin 2018

Primary and secondary treatment are required in the US by the Clean Water Act (CWA) and the number of facilities incorporating tertiary treatment is increasing. ref In addition, limitations on nutrient concentrations in effluent are being implemented at state and facility levels to address nutrient loading and the resulting eutrophication. Treatment criteria are helpful, but not enough to protect marine ecosystems from pollution.

Centralized System Overview

  • Centralized systems increase the efficiency of treatment, consolidate the costs of maintenance, and minimize nonpoint source pollution
  • Shortcomings of centralized systems include significant initial investment, costly and technical upkeep, capacity limitations, susceptibility to leaks, vulnerability to weather, and inadequate removal of nutrients

Decentralized Treatment Systems


Onsite wastewater treatment systems (OWTS) are localized, small-scale systems for managing sewage where centralized systems are either inappropriate or have not been constructed. Hydrology, geology, and geography (as well as finances, politics, and regulations) can dictate whether a sewer and centralized system are possible or if onsite wastewater treatment systems (OWTS) are more suitable. Areas with dispersed residences, shallow soils, impervious bedrock, or vulnerable water tables are often served by OWTS. These systems can be costly for individual homeowners, however in some places installation and maintenance costs can be reduced by subsidies or local incentives.


The flow of sewage through a conventional onsite septic system. Source: EPA Office of Water 2002

Onsite wastewater treatment systems (OWTS) collect, treat, and discharge wastewater effluent at the site where it is generated. Many types of onsite treatment systems exist, but the following three types are the widely used globally:

  • Cesspools, which have just one containment and treatment step. Here, dug or built pits collect effluent for natural settling and treatment. Cesspools are shown to be ineffective, provide inadequate treatment, and are being phased out, replaced, and even disallowed in many places.
  • Septic systems typically contain a holding tank for raw effluent and a dispersal method to provide additional treatment for effluent as it is discharged. As well as capturing sewage, tanks promote settling and anaerobic treatment. An additional aerobic tank chamber is becoming more common to enhance biological treatment and nutrient removal, and some septic systems even have recirculation pumps to move effluent between the aerobic and anaerobic environments. Dispersal is critical for slowing the flow of effluent into the environment.
  • Drain fields are a dispersal technique that promote opportunities for additional treatment of effluent by microorganisms in soil, gravel or other materials before discharge into the ground or surface waters. The video below from The Nature Conservancy Long Island provides a more detailed explanation of septic and cesspool systems.

Another OWTS option are container-based systems, which similarly collect and store sewage on site, then require waste to be transported elsewhere for treatment. These systems are predominantly found in areas with limited infrastructure and include pit latrines, which need to be emptied once they are full, and bucket toilets, which are emptied daily. Treatment of waste collected from container-based options ranges from the conventional treatment processes outlined above, to new resource recovery practices, to no treatment at all. Successful container-based options are described in more detail in Emerging Solutions.

Septic systems and cesspools are not designed to address nutrients and are inadequate in removing them from effluent and may be hazardous for the marine environments in coastal areas. Although there have been many technology advances to address nutrient removal in OWTS, there is a lack of regulation on nutrients in sewage effluent globally. Overlooked leaks and malfunctions cause nonpoint source pollution which is difficult to detect and lacks consequences for noncompliance, leaving little opportunity for enforcement. Upgrades to OWTS systems to include enhanced nutrient reduction are demonstrating more cost efficiency than building new, large-scale sewage treatment facilities.

Decentralized System Overview

  • Decentralized treatments are individualized, operating at smaller scales and serving sparser populations
  • Shortcomings of decentralized systems include frequent mismanagement or oversight leading to noncompliance, inadequate removal of nutrients and emerging contaminants, and no treatment of stormwater


Discharge from an outfall pipe. Photo © pixabay

After treatment from either centralized or decentralized systems, treated effluent is discharged directly to nearby water bodies or into the ground. Outfall pipes are used to discharge effluent directly into rivers and oceans, while drain fields, soils, wetlands, and vegetation slow percolation of effluent into groundwaters. Contamination to oceans caused by effluent is dependent on both the level of treatment it receives prior to discharge and the discharge strategy used. Advanced nutrient reduction techniques and nature-based solutions can achieve additional treatment and slow the flow of effluent. On the other hand, discharge of inadequately treated sewage presents increased hazards for human, organism, and ecosystem health. While large-scale treatment plants in coastal areas often discharge treated or raw effluent directly into the ocean, sewage pollution from smaller containment systems also occurs, through groundwater discharge and leaching, often going unnoticed.

Considerations for System Selection

Infrastructure, resources, geology, population size, cultural norms, and politics all influence the selection of sewage treatment systems. For example, a WWTP is suitable in areas with high structural and population density, existing pipe networks (or the resources and suitable geology to install them successfully), and capacity for highly technical maintenance. Alternatively, OWTS are more appropriate where piped sewers do not exist and there is more distance between sources of sewage (homes, businesses, etc.). Existing sewage infrastructure influences system suitability, as upgrades can often be easier and more cost effective to implement than development of new infrastructure. See this case study of work to centralize sewage treatment on the island of Rotan, Honduras.

Decision support tools for selecting the best system based on local contexts are lacking, making it difficult to adequately manage sewage pollution and sanitation needs. Considerations for system selection should include existing infrastructure, community resources, social and cultural expectations, political support or constraints, the local geology and hydrology, and many other factors. Visit the opens in a new windowSustainable Sanitation and Water Management Toolbox to learn more about sanitation systems and technologies.

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