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Active Wastewater Treatment in Decentralized Applications 

Dennis F. Hallahan, P.E., Technical Director, Infiltrator Water Technologies

Decentralized systems have come of age, and they offer many benefits:

  • Serve over 25 percent of the US population and 30 percent of new construction
  • A good solution for water scarcity: extract, treat and recharge water locally, completing the water cycle in a small footprint
  • Can be passive resulting in minimal energy consumption and carbon footprint
  • Can remain operational during and after natural disasters such as floods or hurricanes

The basic decentralized system consisting of a septic tank and a drainfield has performed extraordinarily well. However, in areas of environmental concern, such as estuaries, wetlands, proximity to lakes and other bodies of water, high water tables, or poor soils, decentralized systems historically struggled to provide a solution. Now, with advances in technology and design innovation, decentralized wastewater treatment systems can offer the same public health and environmental protection as centralized treatment systems. Treatment technologies that were once only available for large scale treatment systems are now available at the single-family size level. The benefits are now documented in US Environmental Protection Agency published fact sheets on decentralized systems. Further, decentralized systems, once known for only passive systems have expanded to include “Active” (Advanced) treatment options. 

The Decentralized Active Wastewater Treatment Model

Decentralized wastewater treatment involves the treatment of wastewater closer to the source, reducing the need for extensive and costly centralized sewer systems. This decentralized approach provides several advantages over traditional centralized systems. First, it offers greater flexibility and adaptability to meet the specific needs of different regions and communities. Small-scale treatment facilities can be designed and implemented based on local conditions, allowing for customized solutions. Additionally, decentralized systems minimize the risk of widespread contamination in case of failures or disasters, as the treatment is distributed across
multiple sites.

Active wastewater treatment in decentralized applications refers to the utilization of advanced treatment technologies to treat wastewater at smaller-scale, localized facilities. This approach is becoming increasingly popular due to its ability to provide efficient and sustainable wastewater management solutions for various settings, such as rural areas, small communities, industrial sites, and commercial developments.

One of the key components of Active wastewater treatment in decentralized applications is the utilization of innovative treatment technologies. These technologies are designed to achieve higher levels of treatment efficiency, remove contaminants of emerging concern, and minimize environmental impacts. Some of the Active treatment processes commonly employed include biological/organic reduction, nutrient removal, and resource
recovery techniques.

Reducing Organics with Extended Aeration

There are many treatment methods to reduce organics, one of the most common is extended aeration. The treatment processes take advantage of naturally occurring microbial communities to consume organics. Active treatment systems offer high-quality effluent and are effective in reducing BOD and TSS. The treated water can then be reused for non-potable purposes like irrigation or industrial processes.

Nutrient Removal

Nutrient removal is also crucial in environmentally sensitive areas. Excessive nutrient discharge, particularly of nitrogen and phosphorus, can lead to eutrophication in receiving water bodies, causing harmful algal blooms and oxygen depletion. Active  treatment technologies, such as biological nitrogen removal processes (e.g., nitrification and denitrification) and enhanced biological phosphorus removal, help achieve stringent nutrient removal targets. These processes use specialized bacteria to convert and remove nitrogen and phosphorus from the wastewater, mitigating environmental impacts.

Managed Aquifer Recharge

The ability to recharge groundwater and replenish aquifers is a noted environmental benefit of decentralized wastewater treatment systems according to the United States Environmental Protection Agency (USEPA). This growing trend is being recognized as Managed Aquifer Recharge (MAR).

With centralized treatment systems, water is extracted from a source, treated to potable standards, then pumped via a pipe distribution network. Once this water is consumed by a facility or homeowner, it is then classified as wastewater and ultimately discharged following centralized treatment to a river or ocean. Disposal of the treated wastewater commonly occurs in local waterways, where it mixes with the existing water flows and is not captured for reuse.

The extensive advancements in Active decentralized technology, has enhanced the capability of decentralized wastewater treatment systems to treat large volumes of water for commercial/cluster facilities to provide MAR. Today, there are decentralized systems with Active Treatment that have discharges more than 1 MGD (3,785 m3/d).

Active Decentralized System Operations and Maintenance

Furthermore, Active decentralized wastewater treatment facilities have embraced the concept of Operations and Maintenance (O&M) and the once Achilles-heel of the industry has been solved. Many codes require maintenance contracts and private industry is rushing to fill the void as it is seen as an opportunity to expand business. O&M can also be employed through public or semi-public entities such as new or existing utility districts. Other advancements to allow ease of O&M has been the integration of smart technologies and automation. Remote monitoring and control systems enable real-time data collection, analysis, and adjustment of treatment processes. This allows for efficient operation, predictive maintenance, and optimization of the treatment performance, enhancing overall system reliability and performance.

Case Study: Active Decentralized Treatment Provides Fast Response in Paradise, California

The 2018 “Camp Fire” devastated the community of Paradise, California, killing 85 people, destroying 11,000 homes, and displacing nearly 50,000 people. The Federal Emergency Management Agency (FEMA) needed to quickly stabilize the situation and support rehabilitation of the community. This required a 1500-person workforce housing camp, which included 400 temporary housing units, laundromat, and food preparation and dining facilities.

An emergency response was necessary with accelerated deadlines and extreme site limitations that included no sanitary sewer service and shallow bedrock. An Active wastewater treatment system was designed and installed to meet a design flow of 100,000 gallons-per-day. The sand filter specifying Advanced Enviro-Septic (AES) receives gravity-flow influent to four, 40,000-gallon septic tanks configured in series. The effluent is then split into four treatment trains to facilitate isolation during maintenance. Four lined AES beds provide passive, secondary treatment. Treated effluent is collected and gravity-distributed to the UV disinfection units, each followed by a pump tank. These pumps distribute purified effluent to two evapotranspiration ponds, which allow for possible reuse.

The low maintenance, high flow, decentralized wastewater treatment system allowed for full occupancy of the FEMA work housing camp quickly. This provided needed resources close to the devastated community.


Active wastewater treatment in decentralized applications offers a sustainable and efficient solution for wastewater management. By utilizing Active treatment technologies, such as advanced treatment, nutrient removal, and resource recovery techniques, decentralized facilities can achieve higher treatment efficiency, remove emerging contaminants, provide managed aquifer recharge, and minimize environmental impacts. The integration of smart technologies and decentralized energy generation further enhances the performance and sustainability of these systems. As communities and industries continue to seek cost-effective and environmentally friendly wastewater management solutions, advanced, active decentralized treatment approaches will play a crucial role in meeting these evolving needs.

Dennis F. Hallahan, P.E.,  is the Technical Director of Infiltrator Water Technologies. Dennis has over 30 years of experience with the design, construction of decentralized wastewater treatment systems. He has authored numerous articles for on-site industry magazines and regularly gives presentations nationally on the science and fundamentals of on-site wastewater treatment systems. Dennis also serves on various national industry association wastewater committees.