Decentralized water infrastructure typically refers to small and medium-sized water infrastructure that uses locally available water sources including gray water and stormwater run-off, and work independently or combined with conventional water infrastructure [20
]. Wastewater recycling, gray water, rainwater and storm harvesting infrastructure are generally considered to be decentralized water infrastructure, Low Impact Development (LID) techniques such as bio-retention facilities and permeable pavers can also be included. While sizes and scales matter, these are not the sole features that distinguish decentralized water infrastructures from conventional infrastructures. Rather, the important difference lies in the integrative functionality of decentralized infrastructures working across urban water management sectors which are traditionally compartmentalized. This feature enables the diversification of water supply options and extended internal water circulation within urban water systems through the pathways. The top half of Figure 3
represents a linear water flow achieved in the conventional centralized water system often described as the take, make, waste approach [23
]. The decentralized water system that uses alternative water sources is depicted in the bottom half of Figure 3
, which illustrates the circulation of water pathways within the urban water system.
4.1. Advantages of Decentralized Infrastructure
The perceived shortcomings of conventional water systems prompted substantial interest in more sustainable approaches to urban water delivery systems, which in turn led to the development of the SUWM paradigm. SUWM emphasizes the decentralization of urban water infrastructure as a fundamental physical requirement. The advantages of decentralized urban water systems address all of the major limitations of conventional centralized systems today, in that decentralized systems are integrated and sustainable, as well as resilient. Moreover, decentralized urban water technologies can complement existing centralized urban water systems. As additions or partial improvements to the original system, decentralized technologies introduce sustainability and resiliency capacity into the water system without requiring the wholesale replacement of existing infrastructure.
By increasing system interconnections, decentralized water systems are more integrated than centralized systems. Indeed, the key feature of urban water infrastructure under the SUWM paradigm is integrated functionality across urban water management sectors that are traditionally compartmentalized. While centralized systems operate on the premise of “water in, water out” (i.e., processing drinking water and wastewater sequentially), decentralized water systems utilize multiple water sources, feature greater path diversity, and extend internal water circulation, thus enabling more efficient resource usage overall. This integration underlies the sustainability and resiliency of decentralized water technologies.
Decentralized water systems’ efficient use of resources is a key reason they are more sustainable than conventional urban water systems [12
]. In particular, decentralized water infrastructure utilizes non-conventional water sources and fit-for-purpose water supplies. This is a response to the mismatch between the production and utilization of potable versus non-potable water through a centralized system. Only a small portion of potable water supplied through conventional water systems is actually used for potable purposes [12
], while approximately 40% of wastewater generated from single households can be reused as gray water for non-potable purposes [52
]. Stormwater is another potential source of usable water that is not leveraged in existing centralized systems but that can be incorporated into decentralized systems. Theoretically, the rainfall collected from one square kilometer of land in a city like Atlanta (where average precipitation totals 49.6 inches per year) can produce approximately 333 million gallons of water annually, which can support the annual water demands of 23,000 people, assuming they each use 50 gallons of water per day [46
]. Utilizing such alternative water sources in urban catchment areas, decentralized technology can contribute to reducing water demands from conventional water systems by 30% to 60% [22
]. Reducing the amount of water collected for centralized water systems increases environmental flows that are critical for restoring and maintaining the health of an ecosystem.
The water saving potential of decentralized technology also enhances the cost efficiency of water systems by reducing their energy consumption and minimizing their ecological footprint, both of which benefits also improve the sustainability of the water system overall [23
]. Cost–benefit analyses for decentralized technologies such as wastewater treatment, gray water, and rainwater harvesting have demonstrated positive economic by reducing energy demands for water treatments and transfer. Xue et al. [54
] found that on-site gray water treatment decreased system energy consumption by more than 50%. Such savings can be augmented by combining multiple decentralized technologies. For example, a water system with on-site gray water treatment and rainwater harvesting is estimated to consume about 25% of the total energy of a conventional water system [54
]. The cost-saving potential of decentralized technologies is even greater when one takes into account the ability of decentralized systems to mitigate peak water demand and thus reduce the need for capital investments to increase the capacity of existing treatment facilities [55
]. Decentralized systems also help minimize the ecological footprint of urban water systems through water resource recovery, which can also be extended to include nutrient recovery [20
], and utilization of sludge and water to generate electricity to power the system [57
Decentralized water systems also have the potential to enhance equity and environmental justice within the communities they serve. For example, consumers can benefit from the cost efficiencies outlined above if they translate into lower overall service prices. Additionally, because decentralized technology can be designed to supplement existing water systems, decentralized solutions can be implemented in targeted areas to correct historical imbalances in infrastructure investment and improve water service and water quality to specific underserved constituencies.
Combining decentralized water infrastructure with a centralized system can also increase the resiliency of that urban water system by reducing the vulnerability of the system to both shocks and gradual change. By design, decentralized urban water systems have greater capacity to cope and adapt: they can draw on a diversified portfolio of water sources, increase system buffer capacity by reducing potable water demands, and utilize multi-scale networks and pathways [17
]. Decentralized system components that are geographically dispersed and independently operated also provide safe-failure features by limiting impacts of system failures to smaller geographic areas and preventing a domino effect of failure among other system components [58
]. Furthermore, due to lower capital intensity (because of lower fixed costs) and shorter construction timelines, communities can deploy decentralized infrastructure more rapidly to respond to external disturbances, such as climate change and demographic variability, and with less operational risk [19
Decentralized infrastructure is not just adaptable, but also flexible. It can be low-tech, low-cost, and flexible in its service boundary [22
], enabling the infrastructure to respond to specific local institutional requirements and demand conditions [20
]. Decentralized water infrastructure can be adopted and managed at multiple scales, from the individual homeowner to the region [59
]. Thus, it can be a tool for local communities with to manage urban water problems and help to address localized concerns over water through innovative approaches based on synergies between local actors and local conditions. For example, local farmers in the town of Maldon (the Victorian Goldfields in Australia)—who previously suffered from limited water resources—gained access to additional local water sources through the installation of a water transport pipe linked to local mine operators—who previously charged for the pumping and containment of underground water in mine operations [22
]. Officials can also leverage the multi-scale nature of decentralized urban water systems and the corresponding expansion of water access to increase community engagement in water planning and enhance resident awareness of water-related issues.
The above advantages make decentralized infrastructures particularly compelling for addressing the problems of growing cities that otherwise must make large investments to expand existing centralized water facilities, secure more water resources, and remediate ecological impacts due to excessive water withdrawal [17
]. At the same time, these characteristics also respond to the needs of declining cities, by optimizing system scale to reduce operational costs for underutilized water facilities [17
]. The advantages also address environmental and social vulnerabilities particular to lower-income communities; therefore, decentralized water systems offer many potential benefits for society at large and for communities that are underserved by current infrastructure.
4.2. Impediments to Adoption
Despite the potential benefits of decentralized water systems, there are multiple barriers to their adoption. Most significantly, path dependencies related to both infrastructure technology and management structures favor existing systems over new ones. Because of the large fixed costs of existing centralized water systems, agencies often make maintenance and upgrade decisions based on those that will extend the useful life of the system [61
]. This approach favors tweaks to the existing system, rather than introducing new technology that reduces system costs in the long run [62
]. Although the original investments in centralized water systems are sunk costs, they create a “lock-in” effect for the original and “proven” technology, which only further biases investment decisions toward the status quo and makes it harder to adopt decentralized technology [61
Technological path dependency is not only a result of the perceived economic value of new technology, but also of the existing structure of actors and institutions that manage the system [25
]. The compartmentalization of the water sector across water supply, sewage, and stormwater functions has been embedded into the division of managerial responsibility for water service provision, operation, and maintenance [16
]. This fragmented administrative structure does not lend itself well to the management of an integrated system; were such infrastructure to be built, reorganization of roles and responsibilities would likely be necessary. Institutional actors may also be wary of increased task burdens or reporting standards, particularly given that the unproven nature of urban water system technology inherently comes with public health risks that require monitoring and potential intervention [33
]. For these reasons, there is a lack of institutional will to adopt decentralized water system technology [24
]. The lack of legislation, public acceptance, and community involvement in planning further weaken the political support for such change [64
Institutional decision-making criteria also do not consider the full range of costs and benefits of centralized and decentralized infrastructure. In addition to making investment decisions premised on the “sunk costs” discussed above, conventional cost-benefit analyses underestimate the social and ecological costs of centralized water systems [24
]. At the same time, decision makers may overlook many social and ecological benefits of decentralized water systems, such as increasing community water security and conserving ecological water flows, as these impacts are broad in scope and more difficult to quantify [66
]. A conventional cost-benefit analysis could even construe the water saving potential of decentralized technologies as a threat to the financial stability of the water department on the premise that decentralized systems would reduce service revenues. Additionally, the time horizon of the cost-benefit analysis is crucial. A short-term focus can indeed undermine the perceived cost-efficiency of decentralized water infrastructure, as the cost of the new infrastructure must be amortized over its useful life [12
Another perhaps counter-intuitive barrier to adoption of decentralized water infrastructure is the flexibility of the technology. In many respects, this flexibility is a relative benefit of decentralized water systems because it allows for custom, context-sensitive solutions; however, the flexibility also makes system design and management more complex. Decentralized urban water systems consist of overlapping facilities for water collection, storage, and distributions that occur over multiple spatial scales and duplicate water networks for potable and non-potable water [20
]. Engineers can also design decentralized components to link up with the existing centralized system so that the technology can complement the existing system, rather than require a total system overhaul. The result is the interconnection of diverse technologies and new patterns of interaction among even existing system components. Such complexity impedes the establishment of best practices that ensure successful application of the technology and promote its increased adoption and diffusion.
The effectiveness of decentralized systems is highly dependent on system configuration and specific contextual factors, including the system’s selected water sources, network scale, topology, and external subsystems [12
]. The untested interactions among system components can cause unintended long-term negative consequences that require action to ameliorate. For example, gray water systems can increase the concentration of pollutants in wastewater, which can lead to corrosion and soil deposits in the sewer pipes, which yield increased wastewater treatments costs [20
]. The tradeoff of flexibility is therefore increased complexity in implementation and management decisions, but with an appropriately broad and long-term consideration of the overall costs and benefits, the value proposition of decentralized water systems remains promising. See Table 1
for a brief summary of advantages of and impediments to decentralized water systems.