Impact of new technologies on the sustainable reuse of treated wastewater within metropolitan and urban areas

Introduction

Conventional centralized wastewater collection systems are generally arranged to route wastewater to remote locations, often near water bodies for treatment and dispersal to the environment or reuse including nonpotable and potable reuse. However, because treatment plants have historically been located in relatively remote locations, the opportunities for the sustainable beneficial reuse of treated wastewater are often inhibited by infrastructure costs for storing, construction costs for pipelines, and pumping cost for transporting treated wastewater (reclaimed water) to the points of use (i.e., typically away from the treatment plant location). If treated wastewater is to be used sustainably in the future, conventional wastewater management must be rethought to incorporate upstream satellite water reuse systems. Historically, satellite treatment has often not been considered feasible because of the land requirements for conventional primary and secondary treatment within metropolitan and urban areas. However, with new treatment technologies with low footprint requirements the implementation of satellite treatment is now feasible in most metropolitan and urban areas. The purpose of this paper is to consider reuse opportunities within metropolitan and urban areas, the types of satellite treatment systems that can be used, the new advanced primary and secondary treatment technologies, and the impact of new technologies on the implementation of satellite facilities in metropolitan and urban areas. Challenges for implementation of satellite treatment are also discussed.

Reuse opportunities within metropolitan and urban areas

Within metropolitan and urban areas, reclaimed water can be used in several different applications as identified in Table 1. In reviewing the reuse applications in Table 1, it is evident that many of the uses are seasonal and/or dependent on site specific reuse opportunities. Some reuse applications depend on the implementation of separate piping systems for the distribution of the treated effluent. Although indirect potable reuse, through groundwater recharge and surface water augmentation, and direct potable reuse account for only a small percentage of the reclaimed water that is now used, it is inevitable, as will be discussed subsequently, that these uses will become dominant in the future (Tchobanoglous, et al., 2011; 2015).

TABLE 1. Water reuse in metropolitan and urban areas: typical reuse opportunities, constraints, concerns, and potential. ^a

Source: Adapted from Tchobanoglous, et al., 2003.

Distributed wastewater infrastructure

In the literature, the terms distributed, satellite, and decentralized wastewater management systems are often used interchangeably which leads to confusion. For purposes of this discussion, distributed wastewater systems include both decentralized and satellite wastewater management approaches. Distributed systems are often characterized by a centralized management approach to ensure that a responsible management entity (RME) is involved in monitoring wastewater treatment operations (WEF, 2024). Decentralized and satellite wastewater systems are defined below.

Decentralized wastewater management facilities

Decentralized wastewater management facilities are independent and are not connected directly to a larger central wastewater treatment system. Individual onsite wastewater management systems, small cluster systems, and small communities without long pipelines are all characterized as decentralized systems. Currently approximately 25 percent of the US population reside in homes served by individual onsite systems, typically comprised of a septic tank and soil absorption system (e.g., leachfield). In some cases, the term decentralized has also been used to describe the use of multiple stand-alone treatment plants within a metropolitan area or governmental jurisdiction. The stand-alone wastewater treatment plants used in New York City are illustrated on Figure 1. Because the focus of this discussion is on satellite treatment systems, decentralized small and stand-alone treatment systems are not considered further in this paper. However, decentralized technology has an important role in sustainable water management in isolated areas (Tchobanoglous and Leverenz, 2012).

Satellite treatment facilities

Wastewater treatment facilities located upstream within a centralized collection system are known as satellite treatment facilities. There are two basic types of satellite systems as Illustrated on Figure 2: the extraction type and the interception type. In the extraction type varying amounts of wastewater are extracted from a wastewater collection system, treated, and used for specific local applications. In the interception type the wastewater is intercepted for treatment and reuse before it reaches the centralized collection system. In both systems, screenings, suspended and waste biological solids are discharged back to the collection system for treatment at a downstream centralized facility. In the interception type excess wastewater is bypassed to the collection system.

Examples of satellite systems

Four examples satellite systems used in conjunction with centralized facilities are: (1) the City of Los Angeles, (2) the County of Los Angeles, (3) the City of San Diego, and (4) in Europe, the Ellinikon Urban Regeneration Project (EURP) in Greece. One of the largest satellite treatment facilities in the United States is the Donald C. Tillman Water Reclamation Plant located in Los Angeles, CA with a capacity of 80 Mgal/d (see Figure 3). Phases I and II were completed in 1984 and 1991, respectively. The product water is used in different applications including in-plant uses; water for the Japanese Garden (Coordinates 34.184°N, 118.480°W), located adjacent to the plant; water for Lake Balboa; water to maintain flow in the Los Angeles River; and water for groundwater recharge. The Tillman plant is currently being expanded to include an advanced wastewater treatment (AWT) facility to produce purified water to replenish groundwater supplies by means of surface spreading at the Hansen Spreading Grounds (James, 2024; City of Los Angeles, 2026). In the case of the Los Angeles County Sanitation Districts (LACSD), six satellite, four stand-alone, and one centralized wastewater treatment facility are employed. Groundwater replenishment with imported river water, local stormwater runoff, and reclaimed water from the satellite plants has been used since 1962 (CSDLAC, 2026). In San Diego, The North City Reclamation Plant (NCRP), a satellite plant, was built in the mid 1990’s. Initially the plant was built to enhance local reuse of the treated effluent, primarily for golf course irrigation. Currently, an AWT facility is being built adjacent to the NCRP to produce advance treated water for discharge to San Vincenti Reservoir for indirect potable reuse as part of the Cities Pure Water Program (Coordinates 32.878°N, 117.197°W) (City of San Diego, 2026).

In Europe, one of the largest satellite projects is located at the former Athens airport in Greece, which closed in 1996 (Coordinates 37.8899°N, 23.7398°E). The project comprises an area of approximately 1,500 acres (ac) [6.2 million square meters (Mm²)] and includes a developed area of 670 ac (2.7 Mm²), 500 ac (2 Mm²) of metropolitan parks, 150 ac (0.6 Mm²) of green open spaces, 2 mi (3.2 km) of coastline and a marina (Giotis, 2016). Sustainable water management includes runoff collection from natural streams and the reclamation and reuse of water. Wastewater from the area is currently treated with the wastewater from the greater Athens basin on the Island of Psyttalia located in the Saronikos Gulf about 9.9 mi (16 km) west of the port of Piraeus. A new satellite wastewater treatment plant is under construction which will intercept all wastewater produced from the developed area within the EURP and additional wastewater which will be extracted from the coastal wastewater collection system to meet the irrigation demand. Thus, the satellite WWTP will function as both an interception and extraction type facility. Biosolids from the new plant will be discharged downstream to the coastal collection system for treatment at the Psyttalia WWTP (LAMDA Development, 2026).

New technologies for satellite treatment systems

Most of the satellite treatment systems shown on Figure 3 were built when sufficient land was available and affordable for the use of conventional wastewater treatment systems and where suitable reuse and or dispersal options were available for the treated effluent. However, with urban sprawl over the last 35 years, few large open areas are available for the use of conventional technologies. Fortunately, several new innovative wastewater treatment technologies have been developed that can be implemented with significantly smaller footprints, improved treatment performance, and reduced energy consumption. Most notable are the new technologies for advanced primary treatment (APT) and advanced secondary treatment (AST). The new APT and AST are technologies considered in the following two sections will have a major impact in bringing about the sustainable management of treated wastewater through beneficial reuse in metropolitan and urban areas. Further, smaller footprint of APT and AST technologies helps to mitigate the impact of increased cost of land.

Innovations in primary treatment

Representative APT technologies that have been developed and tested to replace conventional primary clarifiers include:

• Cloth disc filter (CDF) (as supplied by Aqua Aerobics)
• Proteus primary filter (PPF) (as supplied by Tomorrow Water)
• Compressible medium (CMBF) (as supplied by WesTech)
• Saturated air flotation (SAF) (as suppled Komline-Sanderson Corporation)
• Micro screen (MSc) (as supplied by Huber)
• Microsieve (MSi) (as developed by the Technical University of Crete)

These APT technologies were selected for illustrative purposes because their implementation and performance have been studied and documented extensively in the literature (Avants, 2012; Mansell, et al., 2018; Caliskaner et al., 2022; Caliskaner et al., 2024; Tchobanoglous, et al., 2022; Tsamoutsoglou, et al., 2024; Dursun, et al., 2025; Leverenz, et al., 2025).

Description of APT technologies

Schematic diagrams of the APT technologies are presented on Figure 4. A brief functional description of each technology is provided in Table 2. Additional operational details may be found in references cited above.

TABLE 2. Abbreviated descriptions of the operational details of APT technologies.

In the APT technologies depicted in Figure 4, except for the SAF process, all the other technologies involve some form of filtration. While the performance of the SAF process has been studied and verified, additional consideration of thickened sludge management at satellite facilities is required.

Performance of APT technologies

Typical performance data for the APT technologies for the removal of total suspended solids (TSS) and total chemical oxygen demand (tCOD) are presented in Table 3 and on Figure 5. While the performance of the SAF process has been studied, additional performance verification testing is ongoing (Caliskaner, 2026). In reviewing the data presented in Table 3, the performance of the micro screen (MSc) and microsieve (MSi) is equivalent to performance achieved with primary sedimentation but is not comparable to the other APT technologies. Nevertheless, the MSc and MSi technologies are included, because of the significant footprint reduction that can be achieved (90%) as compared to primary sedimentation. The performance of the APT technologies with respect to the removal of TSS and tCOD will depend on the degree of solubilization that occurs within the collection system during transport.

TABLE 3. Typical performance data and aerial footprint of APT technologies compared to conventional primary sedimentation facilities. ^a

Sources: Avants, 2012; Caliskaner et al., 2022; Caliskaner et al., 2024; Tsamoutsoglou et al., 2024; Dursun, et al., 2025.

Typical long-term performance for the removal of TSS and tCOD for the CDF is shown on Figure 5. Similar constituent removal performance was observed for the Proteus primary filter and the compressible medium filter (Caliskaner et al., 2024; Dursun, et al., 2025). Perhaps the most striking thing about the removal data plotted on Figure 5 is the stability of the TSS concentration in the filter effluent, regardless of the influent variability. Also, what is not reflected in Figure 5 is the difference between the particle size distribution in the filtered effluent as compared to the influent particle size distribution which has a significant effect on the biological treatment kinetics. The particle size distribution is similar to that obtained with primary effluent filtration (Matsumoto et al., 1982; Caliskaner et al., 2021).

With constant influent flow, reduced variability in the TSS and tCOD loadings, and an altered particle size distribution, the treatment performance of the AST technologies can be optimized as compared to treating primary sedimentation tank effluent (Tchobanoglous and Leverenz, 2024).

Advantages of APT technologies in satellite treatment systems

In addition to outstanding treatment performance, another major advantage of the APT technologies is that there is no need for ancillary processes such as thickening that would be required in a conventional wastewater treatment application. In addition, the TSS removed by the APT technology is returned to the collection system, which improves the downstream production of digester gas. Successful implementation of APT technologies can provide substantial benefits over conventional primary wastewater treatment including, footprint reduction, energy-savings, capital and operational cost savings, and capacity increase. It is interesting to note that of the many hundreds of wastewater treatment flow diagrams found in the engineering literature, little or no mention is made of the use of APT technologies, an example of how slow change occurs in the wastewater treatment field.

Innovations in biological treatment

Over the past 25 years, the number of new biological treatment process that have been developed or are currently under development is staggering. Representative advanced secondary treatment (AST) technologies include:

• Membrane bioreactor (MBR)
• Membrane aerated biofilm reactor (MABR)
• Aerobic granular sludge (AGS)
• Moving bed biofilm reactor (MBBR)
• Fixed film biofilm reactor (FFBR)

These AST technologies listed above were selected for illustrative purposes; their implementation and performance have been documented extensively in the literature (de Kreuk et al., 2007; Judd, 2011; van Loosdrecht and Brdjanovic, 2014; Li et al., 2023; Sravan et al., 2024; Tari et al., 2025; Dursun, et al., 2025). Further, the intent here is not to discuss the operational characteristics and performance of the AST technologies, which are well known, but rather to consider their aerial footprint relative to convention activated sludge technologies for satellite treatment applications. Typical aerial footprints for AST technologies are reported in Table 4. A range of percentages values is given, as the values reported in the literature often include primary sedimentation and other ancillary facilities.

TABLE 4. Typical aerial footprint of AST technologies compared to conventional secondary treatment with activated sludge.

In general, of the technologies listed in Table 4, the MBR, MABR, and AGS processes will have the smallest footprint relative to conventional activated sludge.

Impact of new technologies

The impact of the implementation of the new APT and AST technologies described above, in both satellite and centralized applications, will be in reduced space requirements, improved wastewater treatment performance, and reduced energy consumption for biological treatment. In addition, when these technologies are used in centralized systems, enhanced energy production results from the additional TSS removed by the APT technologies.

Reduced space requirements

Using the information from Tables 3 and 4, the approximate reduction in aerial footprint for satellite treatment as compared to CST is illustrated schematically on Figure 6. The specifics of process selection will depend on local conditions. Although the APT unit is shown adjacent to the AST it can, because of its small size be located near the point of wastewater extraction or interception.

Of the various possible combinations of APT and AST technologies discussed above, the coupling of the CDF with an MBR will result in the smallest footprint (see Figure 6). Depending on the type of treatment required, the combined APT and AST technologies, the required footprint would be about 40% of a conventional CST (Dursun, et al., 2025). Further, given that the footprint of a conventional activated sludge process is about 50% for the total footprint occupied by a conventional WWTP [see Figure 6A], the footprint required for a combined APT and ASP satellite treatment process including controls will be about 75%–80% less for the same hydraulic capacity. With reduced footprint combinations, APT and AST technologies can now be used in locations previously thought unsuitable because of site space limitations.

Improved treatment performance

The operation of satellite reclamation plants can be optimized because they are designed to be operated at a constant flowrate without sludge processing facilities. With reduced variability in solids loading because of improved APT performance, enhanced AST treatment performance has been observed (Caliskaner et al., 2022; Dursun, et al., 2025). Without return flows it will be possible to produce high quality treated wastewater effluent for further treatment in an advanced water treatment facility (AWTF) for indirect and direct potable reuse applications. The choice of AST technology will depend on the effluent quality required for the intended beneficial uses. For example, by co-locating an AWTF next to a satellite wastewater treatment plant highly treated water can be produced and used for indirect potable reuse through groundwater and/or surface water augmentation.

Reduced energy consumption

When operated under constant flow, APT can reduce both the magnitude and variability of TSS and tCOD loadings to secondary treatment; as a result, secondary-treatment energy consumption is estimated to decrease by approximately 20%–40% (Caliskaner et al., 2024). Beyond reduced aeration demands, APT can also improve the energy balance of centralized WWTFs by capturing a larger fraction of neutrally buoyant primary suspended solids—materials that typically have high calorific value and can be redirected to anaerobic digestion. As a result, energy self-sustainable wastewater treatment can become more attainable, as illustrated below.

The chemical energy in municipal wastewater, based on the organic fraction of COD, is approximately 15 MJ/kg COD (Tchobanoglous et al., 2013). For a representative organic COD concentration of 500 kg/1,000 m³, the corresponding energy wastewater content is 7,500 MJ/1,000 m³ of wastewater treated (15 MJ/kg × 500 kg COD/1,000 m³). By comparison, the energy demand for large WWTFs (without and with nutrient removal) is typically in the range of 1,000–2,000 MJ/1,000 m³, with higher values for smaller facilities (Tchobanoglous et al., 2013; Raucher and Tchobanoglous, 2014). Thus, the energy embedded in wastewater is on the order of 3.75–7.5 times the energy required for treatment; the practical challenge is how to recover that energy.

In conventional wastewater treatment, approximately 25% of the original energy is recovered as digester gas and 25% remains in the residual sludge. Under APT conditions, the corresponding fractions, based on the added COD, are 41.6% (digester gas) and 21% (residual sludge). Assuming 40% conversion efficiency from biogas to electricity, the recoverable electrical energy from digester gas is approximately 1,248 MJ/1,000 m³ [7,500 MJ/1,000 m³ x (0.416 × 0.4)]. In addition, because 55%–65% of total plant energy demand is typically associated with biological treatment, a 35% reduction in biological-treatment energy demand (with APT) translates to an overall plant energy reduction of roughly 20%. Under this assumption, total energy requirements for wastewater treatment decrease to 800–1,600 MJ/1,000 m³. Further gains are possible if biogas conversion efficiency is increased using fuel cells (potentially up to 60%), or with a combined heat and power unit (potentially up to 80%), Additional energy can also be recovered from biosolids (Gikas, 2017; Biosolids2energy, 2026).

Remote control of satellite treatment systems

With new control systems employing real-time data visualization and AI operating models, it will be possible to remotely control the operation of new satellite wastewater treatment systems, minimizing the number of on-site full-time staff needed for plant operation, as compared to CST.

Reduced pumping costs for recycled water

With the use of satellite facilities, pumping costs to potential recycling opportunities will generally be reduced as compared to pumping from a centralized WWTP, and especially in those situations where the centralized facility is located remotely.

Challenges for distributed treatment

A conceptualized vision of the future in which the maximum sustainable reuse of treated wastewater is achieved is illustrated on Figure 7. As shown on Figure 7, both small and large reuse water volume opportunities are used. Because APT and AST are developing technologies, there is little guidance in the conventional engineering literature on the design and implementation of these systems. Although the depiction in Figure 7 is idealized, some of the other important challenges that must be overcome in achieving its implementation are considered below.

Insufficient supply of wastewater

Strange as it may seem, in some ideal locations for the implementation of reuse, there may not be sufficient wastewater flow in the collection system, due to reduced water consumption, to meet the need(s). This situation has already occurred in Southern California and in Athens. In reduced wastewater flow situations, it may be necessary to pump wastewater from another sewershed to meet the reuse demand. The need for inter sewershed pumping is illustrated on Figure 7 where wastewater from another sewershed is pumped to the satellite treatment plant.

Need for new infrastructure

Where beneficial reuse is to be made of purified wastewater for landscape irrigation at parks, greenbelts, and individual residences (see Figure 7) dual piping systems will be required under current health and safety regulations. Because of the cost associated with retrofitting existing residences, dual systems should be installed as part of the development of new residential areas (see Figure 8). A number of existing buildings in New York and San Fransico have been retrofitted with dual plumbing with highly treated effluent being used for toilet flushing and landscape irrigation. The inclusion of dual plumbing is becoming more common for new high-rise buildings (Chavan, et al., 2025).

In some areas, where construction issues and pumping costs to reach locations for indirect potable reuse including surface spreading, groundwater injection or reservoir augmentation, are not cost-effective or feasible, serious consideration should be given to direct potable reuse (Tchobanoglous et al., 2011, 2015). An AWTF, permitted as a drinking water facility, could be built adjacent to or near an upstream satellite treatment facility as illustrated on Figure 7. For example, the Orange County Water District Groundwater Replenishment System AWTF (Coordinates 33.691° N, 117.941° W) is built adjacent to the Orange County Sanitation Wastewater Treatment Plant (OCWD, 2026). Another example is the new AWTF being built adjacent to the NCRP in San Diego, as discussed previously.

Collection system issues

Reduced flow in portions of the wastewater collection system, downstream from the satellite facility, could lead to solids deposition and grease accumulations with the potential to form odors. To avoid low flow conditions, satellite treatment facilities should be located where there is sufficient flow to dilute any waste solids returned to the collection system for downstream treatment. In some cases, it may be necessary to augment the wastewater flowrate by pumping wastewater from another sewershed, as discussed previously. Also, it may be possible to reduce low flow impacts by modifying a portion of the collection system to achieve higher scour velocities.

Sludge management issues at centralized facilities

Downstream solids management typically includes gravity separation, thickening, and anaerobic digestion for energy recovery. To date sludge management from satellite WWTPs at centralized plants has not been an issue. The city of Los Angles operates two large satellite facilities, the Burbank plant built in 1966 and the Tillman plant built in 1984, while the Los Angeles County Sanitation Districts operates six satellite plants dating back to 1962. The only problem encountered recently is that wastewater flowrate to the Tillman plant has to be augmented, as discussed previously, to reach the desired water recycling goal. It is not anticipated that APT processes will pose a problem based on measured constituent concentrations. However, the influent flow to downstream centralized facilities may have elevated nutrient and total dissolved solids (TDS) content due to upstream conservation practices. The more concentrated wastewater flow increases potential odor generation and the rate of corrosion at the headworks. Changes in influent characteristics may also require special considerations for treatment and reuse.

Operational requirements

Operational issues for satellite treatment facilities are similar to those for conventional WWTFs, but less intensive because of the reduced number of unit operations. Finding qualified operators is a national problem. As noted previously, for the new technologies discussed in this paper, it is clear that AI (artificial intelligence) will play a larger role in their operation as well as in the operation of centralized treatment systems. For smaller satellite systems, a team of roving operators can be used to respond to any alarm conditions. Also, in emergency situations, flow diversion at satellite facilities can be implemented easily.

Regulatory issues

With the exception of DPR, most states and countries have regulations for the reuse applications identified in Table 1. At present, California is the first State in the United States to adopt regulations for DPR (State of California, 2024). Other DPR projects, such as Windhoek, Namibia, and Big Spring, Texas, operate under permits that are site specific. As the need to conserve water becomes more severe it is anticipated most States will develop DPR regulations using the California regulations as a guide.

Discussion

Water reuse must become an essential element of integrated water management for large metropolitan and urban areas. In planning for water reuse, the full range of potential applications must be considered. Because wastewater treatment plants have historically been located in relatively remote locations, the implementation of many of the reuse opportunities identified in this paper will require new infrastructure. If treated wastewater is to be used sustainably in the future, wastewater management infrastructure must be rethought to consider how satellite wastewater treatment facilities may be utilized. Implementation of advanced primary treatment (APT) and advanced secondary treatment (AST) technologies can be used in satellite treatment applications that will overcome past space and location limitations.

Footprint requirements for satellite treatment facilities employing the APT and AST technologies, as discussed in this paper are about 20%–25% of what is required for conventional WWTP facilities. The smaller required footprint makes it less challenging to locate satellite facilities. Within the metropolitan and urban environment they can be housed in warehouses, buildings that look like nearby residential developments (see Figure 2-a2), or buried under parks or recreational areas. The ability to treat wastewater will also not be an issue as the new APT and AST technologies have proven to provide reliable treatment performance. Further the existing APT and AST technologies will continue to improve in the future. New technologies will also be developed, which may further reduce footprint requirements.

Integration of the new technologies within existing wastewater collection system infrastructure will also involve challenges beyond siting and treatment effectiveness. To maximize some reuse opportunities may require the transfer of wastewater between sewersheds, especially if constant flow is to be maintained. Low flow conditions in portions of the centralized wastewater collection system due to the extraction and or interception of wastewater may cause localized problems which must be addressed.

In developing water management programs that are reliable, affordable, sustainable, and protective of public health, all feasible water reuse opportunities and strategies employing distributed satellite treatment must be given serious consideration.

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Footnotes

Edited by: Hao Chen, University of Vermont, United States

Reviewed by: Mahesh Balasaheb Chougule, DKTE Society’s Textile and Engineering Institute, India

Mark Elliott, The University of Alabama, United States