Decentralized wastewater systems, also called decentralized wastewater treatment systems, collect, treat, and reuse or dispose of wastewater from small or spread-out communities, buildings, and homes in remote areas or on individual public or private properties. Wastewater is created when buildings have access to a water supply or when water is available nearby.

These systems treat, reuse, or dispose of wastewater close to where it was created. Their purpose is to protect public health and the environment by greatly reducing health and environmental risks.

They are called "decentralized wastewater treatment systems" because the main challenge is selecting the right treatment or disposal method. A common name for these systems is DEWATS.

Background

Centralized wastewater systems are the most common type used in large, developed cities and have been used for a long time to manage wastewater. These systems use large pipe networks, called sewerage, to collect wastewater and move it over long distances to one or more treatment plants. Stormwater can be handled in two ways: through combined sewers, which mix stormwater with wastewater, or through separate stormwater drains, which use two different sets of pipes—one for wastewater and one for stormwater. After treatment, the cleaned water is usually released into natural water sources like rivers or lakes. In some cases, the treated water is reused for other purposes, such as irrigation, and is called reclaimed water.

The main difference between centralized and decentralized systems is how wastewater is moved. In decentralized systems, wastewater is treated and reused or disposed of near where it is created. This results in a smaller network of pipes, sometimes just one pipe. Because the network is small, different methods can be used to move the wastewater, such as gravity sewers, pressurized sewers, or vacuum sewers. The amount of wastewater in these systems is usually small and can vary a lot over time.

In areas with existing infrastructure, decentralized systems can be a good option when upgrading or replacing old systems. This is because centralized systems are often large, long-lasting, and difficult to change once built. Many different types of hybrid systems, which combine elements of both centralized and decentralized approaches, are possible.

Decentralized systems are especially useful in new urban areas where infrastructure, like roads or water systems, is not yet built or will be completed later. In some places, wastewater systems are built years after homes are built, so temporary decentralized systems are needed to avoid health and environmental problems. These systems are helpful because they can be used locally as needed and can cover large areas at lower costs.

Decentralized systems allow for separating different types of wastewater based on their source, such as black water (toilet waste), greywater (from sinks and showers), and urine. This requires separate pipes to move each type of wastewater, so each can be treated differently and used safely for reuse or disposal.

In developing countries, where many areas lack proper infrastructure, decentralized wastewater systems are often promoted because they use technologies that are easier to operate and maintain. These systems also require less money to build compared to centralized systems.

Types

Different levels of decentralization exist based on the size of the area being served:

• Decentralization at the level of a suburb or satellite township in an urban area – these systems are called small centralized systems when used in small towns or rural areas. However, when used in parts of medium or large cities that already have centralized systems, the overall system is called a hybrid system. In this case, decentralization is used only for some areas within the larger system.
• Decentralization at the neighborhood level – this includes groups of homes, gated communities, small districts, and similar areas that use vacuum sewers for waste removal.
• Decentralization at the "on-site" level (on-site sanitation) – these systems are fully contained within one property and serve one or more buildings located there.

Wastewater treatment options

These systems are usually used at the location where wastewater is created because very little wastewater is produced. However, they need suitable soil that allows water to soak into the ground and a low water level underground. If not used correctly, they can cause serious pollution in groundwater.

• Pit latrines are used when there is very little water and wastewater is hard to produce. They are the most common way to handle waste in areas with limited resources.
• Septic tanks are the most common system for treating wastewater on-site. They work well when there is enough water and good soil and groundwater conditions.

These systems need a lot of space because the natural processes that clean the water happen slowly. For this reason, they are better suited for warmer areas, as the cleaning process depends on temperature. These methods are more reliable when the amount of wastewater changes and do not need complicated care or operation. Constructed wetlands are best used at the location of the waste or in neighborhoods, while stabilization ponds can be a good option for small towns or rural areas.

There are many types of wastewater treatment plants that use different methods. Small treatment plants in decentralized systems use similar technology to medium or large plants. For on-site use, compact systems are made with separate sections for each process. However, designing and managing small treatment plants, especially in neighborhoods or at individual sites, is challenging for engineers. These challenges include dealing with changes in water flow, needing skilled workers to operate and maintain many small plants, and higher costs per person.

Regulations and management

Water pollution rules, such as laws, guidelines, or official orders, set the required level of treatment for wastewater so that the treated water meets standards for safe disposal or reuse. Treated wastewater can be released into natural water sources or absorbed into the ground. These rules also describe how wastewater systems should be designed and operated, as well as the consequences for not following them. Large, centralized systems are built and managed by local government officials to meet these rules. In smaller systems or hybrid systems in towns or rural areas, management is often handled similarly.

For systems that handle wastewater on-site or in small groups of buildings, the entire wastewater system is located on private property. The owner is responsible for designing, building, operating, and maintaining the system. In many cases, specialized companies may handle these tasks. Local officials issue permits and may help by collecting waste, providing certificates or licenses for approved treatment equipment, or working with qualified private companies. From a rule-making perspective, the quality of treated wastewater for reuse, discharge, or disposal is the responsibility of local or national government officials. This can be difficult if many systems need to be monitored. It is important for the owner to maintain the system properly, especially if the treated water is reused. Common problems often occur when sludge is not removed regularly or when too much water flows through the system due to an increase in the number of people using it or higher water usage.

Urban planning and infrastructure issues

Wastewater systems are part of the basic systems in cities and towns. They are important during the planning process for these areas. Information about homes, roads, stormwater drainage, water supply, and electricity systems is needed to create wastewater systems that work well over time. In some systems that handle wastewater in smaller areas, stormwater may not be properly managed, leading to flooding. When these smaller systems are planned, stormwater drainage should be connected to road systems.

In areas without existing infrastructure, it is hard to create long-lasting sanitation solutions. For example, pit latrines and septic tanks need regular cleaning, which usually requires special trucks that need roads to access them. Managing waste from these systems includes collecting, moving, treating, and reusing the waste in a way that is safe and sustainable.

When planning new or updated cities and towns, it is best to consider different types of wastewater systems, including smaller ones. A good plan should use reliable and cost-effective methods. In many cases, larger systems that handle wastewater from many areas may be better, depending on factors like land size, land shape, underground water levels, and weather. However, smaller systems can also be useful if used properly. They can help protect the environment and reuse treated water for other purposes, such as watering plants or cleaning. This helps provide new water sources and reduce harm to nature. People's understanding and support are important when choosing the right wastewater system for a specific area.

Examples

One example of local wastewater treatment is the "DEWATS technology," promoted by the German NGO BORDA. This system has been used in many countries in Southeast Asia and South Africa. It uses processes that break down waste without oxygen, such as anaerobic baffled reactors and anaerobic filters, followed by treatment in ponds or wetlands with oxygen. Studies in South Africa showed that the system's ability to clean water was less effective than expected.

A study of a local wastewater system that reuses treated water was done at the Botswana Technology Centre in Gaborone, Botswana. This system serves one building in an area with a city sewer system. Wastewater from the building is treated in a system that includes a septic tank, a planted rock filter, a bio-filter, and a surface flow wetland. The cleaned water is used to irrigate nearby green spaces, but the study found that water flowed out of the wetland only during heavy rains. This shows the importance of carefully planning how much and what type of wastewater will be produced when designing local systems.

Founded in 2013, the Peru-based NGO EcoSwell works on rural development projects, including water and sanitation in Peru. They operate in the Lobitos district of the Talara region, an area with little water and high drought risk. EcoSwell helps build local wastewater systems with community members and interns, using technologies like communal biodigesters, dry toilets, and projects that reuse greywater (wastewater from sinks and showers). They also work on reforestation and constructed wetlands to naturally clean wastewater and reduce harmful germs.