Unlocking the Potential of Decentralized Water Systems
A recognition that humanity’s energy and resource consumption is unsustainable is prompting a critical re-evaluation of all human endeavours. In urban water management, the desire to enhance sustainability is driven by the realization that the provision of water consumes large quantities of energy and that the wastewater produced by households, commercial activities, and industries contains valuable resources. Wastewater represents an underutilized source of water, energy, and plant-essential nutrients.
However, most urban water systems were designed to treat and dispose of wastewater in a manner that minimizes surface water pollution and risks to public health. To achieve these goals, energy-intensive treatment processes that often dissipate energy and nutrients contained in wastewater are used. Recognizing the limitations of traditional wastewater management approaches in achieving environmental and economic sustainability goals, the urban water cycle must change.
The Potential of Source Separation and Decentralized Systems
Most prior efforts to recover resources from wastewater have focused on resource recovery at large, centralized treatment plants. This approach benefitted from economies of scale and was relatively easy to integrate into the institutions responsible for urban water management, but it was unable to take advantage of the efficiencies that could be gained through the collection and recovery of separate waste streams that were rich in specific resources. Centralized resource recovery systems also struggled to take advantage of the benefits of fit-for-purpose water treatment because expensive new pipe networks were required to distribute non-potable water.
The convergence of information technology (IT) and modular technology advancements holds the potential to enable the safe operation of small-scale water treatment and water supply networks that can take advantage of source separation and its high resource recovery efficiency together with the reuse of water of different qualities. These innovations also have the potential to enable flexible and resilient hybrid or ‘off-grid’ small-scale systems, where citizens enjoy access to high-quality water in a manner that is less expensive, more sustainable, and less polluting than existing approaches.
Separation of wastewater into three different components, namely yellow water (urine), brown water (feces and flush water), and grey water (everything else), has the potential to reduce the costs of recovering water, energy, and nutrients relative to the conventional approach of treating the combined wastewater streams. Slowly but steadily, the number of initiatives taking advantage of this approach is increasing.
The Benefits of Decentralized Waste-to-Energy Solutions
Black water (combined brown and yellow water), which constitutes roughly 20% of the volume of household wastewater, contains about 90% of the carbon and nitrogen and 80% of the phosphorus discharged by households. Treating this resource-rich stream separately (for example, by anaerobic digestion) is a proven means of recovering energy and water. Anaerobic digestion, despite its lower energy conversion efficiency than the solids obtained by centralized systems (biosolids that contain about 4.6% and 2.3% N and P, respectively), offers several advantages:
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Higher Nutrient Concentration: Urine-derived fertilizers obtained from yellow water, such as P precipitates (Ca3(PO4)2(s)) and stabilized urine liquid concentrate, exhibit higher purity than conventional biosolids.
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Localized Nutrient Recovery: The production of fertilizers in proximity to their point of use reduces costs associated with transportation and integration of the nutrients into a larger fertilizer supply chain.
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Environmental Benefits: The environmental impact of decentralized systems was evaluated through a life-cycle analysis that considered 15 mid-point indicators within different European cities. Global warming potential (GWP) and eutrophication potential (EP) categories were examined in detail, revealing substantial reductions compared to centralized systems.
Enhancing Sustainability through Synergistic Strategies
After assessing water, energy, and plant-essential nutrient production within the decentralized system, we considered synergies that could enhance environmental sustainability, reduce costs, and promote circular economy practices. We focused on practices that reinforce the spread of renewable energy and contribute to local food production.
Vertical Farming Integration
The nutrients recovered by the decentralized treatment systems can serve as fertilizers used locally for landscaping purposes or the local cultivation of food. To address the potential for using recovered nutrients for the cultivation of high-value crops, we considered commercially available modular, hydroponic systems capable of growing tomatoes, lettuce, strawberries, spinach, and mushrooms.
The allocation of production area for each crop was determined with an optimization model that factors in capital and operational costs, marketable weight per plant, crop harvest cycle, consumer pricing, and per capita consumption. The relative quantities of these five crops were based on yearly average consumption, as reported by the United States Department of Agriculture.
By efficiently using the recovered plant-essential nutrients, the amount of these selected crops produced by the vertical farming operation would exceed the United States national average consumption for residents of the housing block. The production would be equivalent to a daily salad per resident containing roughly two cups of lettuce, half a cup of spinach, one medium tomato, a few mushrooms, and occasionally, a few strawberries. This low-calorie salad would contain vitamins, minerals, and fiber, which are essential for a healthy diet.
Assuming current market prices, the value of the theoretical daily (organic and locally grown) produce would offset the vertical farming investment and operational costs in approximately 10–12 years. After the payback period and also considering the revenue generated by selling surplus crops, the produce value could represent a potential payback mechanism for offsetting the costs associated with the decentralized system.
Integrating Renewable Energy Generation
The use of decentralized energy resources (small-scale power generation located in proximity to consumers) is gaining interest as an integral part of the transition towards renewable energy sources. This approach has the potential to satisfy about 20% of a country’s electricity demand while simultaneously enhancing grid reliability and resilience.
To evaluate possible synergies between the energy demand of the decentralized water system and the added capacity provided by decentralized energy resources, we considered a photovoltaic system integrated into the city block containing the decentralized water system. Representing diverse Köppen–Geiger climates, the cities of Barcelona, Toronto, Santiago de Chile, Hong Kong, and Miami were selected for analysis.
The installations are anticipated to offset an average of 12 ± 3% of the estimated total domestic electricity demand. The decentralized system and the vertical farm represent approximately 2.0 ± 0.7% and 2.4 ± 0.2% of the total annual energy demand of the considered development, respectively. Consequently, the integrated photovoltaic installation would reduce grid energy consumption by approximately 8–11% compared with a development lacking these features.
This allows for the payback of initial investment costs (approximately 4% of a new development cost) in 10–18 years, with the exact timeframe influenced by urban location. Following this payback period, the ongoing energy cost savings could then support offsetting the investment costs of both the vertical farm and decentralized approach.
Techno-Economic and Environmental Assessment
A detailed cost and energy analysis, which builds upon approaches used in previous studies, indicates that the performance of the decentralized system is similar to that of the centralized system. However, when factoring in cost offsets from food production, rainwater harvesting, and energy generation, the decentralized system becomes substantially more cost-effective, potentially reducing costs by half or more compared to the centralized system.
Similarly, considering energy produced through photovoltaics and food waste digestion, the decentralized system demonstrates a substantial decrease in grid energy consumption, potentially using half or less than the centralized system.
Cost Comparison
Considering the cost of potable water, wastewater treatment, water reuse, and sewer services for centralized treatment systems, the overall cost of water from the centralized system would be approximately US $2.2 m^-3, compared to US $1.8 m^-3 for the decentralized system before accounting for offsets from food production.
Much of the energy use for the centralized water system is associated with operating conventional wastewater treatment plants (60%), and advanced treatment plants are needed to prepare the water for potable reuse. On the other hand, grey water treatment and purification represent the largest energy demand for decentralized systems (50% of total use).
The anaerobic digester’s ability to recover energy and heat from brown water and food waste gives decentralized systems the potential to offset over 50% of their total energy demand. This substantial energy recovery advantage differentiates them from centralized systems.
The decentralized system’s appeal further increases when considering energy from photovoltaic systems (with potential for free energy after payback), especially in locations with high energy costs for water import and distribution.
Payback Mechanisms
The decentralized system’s initial costs are approximately 3.5% of the cost of a new building, and the initial cost of the photovoltaic infrastructure and the vertical farming account for approximately 4% and 3.5% of the total cost, respectively.
The proposed concept offers two indirect payback mechanisms that could deliver a return on investment in about 10–15 years:
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Nutrient Recovery and Vertical Farming: The efficient in situ recovery of nutrients holds the potential to bypass the competitive disadvantage associated with artificial fertilizer prices and price the recovered nutrients based on their ability to produce food valued at market prices. The theoretical value of nutrients (as produced crops) illustrates the potential for nutrient recovery to enhance the economic viability of these systems.
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Renewable Energy Generation: The proposed photovoltaic system has an estimated payback period in the range of 6–15 years, depending on location. This means that the energy would be at no cost after the payback period, further decreasing the payback period of the decentralized system.
The decentralized system also taps into another payback mechanism that holds the potential to reduce the payback period to less than 10–15 years: the ability to efficiently provide drinking water. Separating grey water allows for more efficient treatment with reverse osmosis, making it cheaper than treating mixed wastewater or even desalinated seawater.
When the same functionality is considered (providing drinking-quality water without developing new forms of traditional supplies), decentralized systems can cost about half as much as the centralized treatment approach. This highlights the economic value proposition of decentralized systems, especially in the context of increasing water scarcity and the growing costs of expanding centralized infrastructure.
Overcoming Barriers to Widespread Adoption
While the concepts proposed in this study bring forth economic, environmental, and technical advantages, the real implementation of these practices at scale remains challenging. Over the past two decades, architects, developers, and utilities have shown that it is feasible to construct buildings or neighborhoods that are capable of recovering and recycling a marked fraction of the water and nutrients that would otherwise be discharged as waste. However, translating demonstration projects into widespread adoption remains a substantial challenge.
The most challenging barrier is the lack of broad institutional support, which is primarily rooted in technological inertia reinforced by the institutions responsible for urban water management. The near absence of existing buildings with dual plumbing and the limited experience of builders and building operators with such building-scale water recycling systems, combined with concerns about public perception of the risks of exposure to unsafe water, are sometimes offered as reasons for a lack of support.
However, the larger problem of technology ‘lock-in’ probably explains much of the hesitancy about decentralized water systems. This phenomenon, which has been observed in numerous social-technical systems, often occurs when an approach that has an early lead in innovation acquires dominance in a market that restricts the advancement of other technologies.
Overcoming the lock-in effect requires several mechanisms, including:
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Supporting Iconic Demonstration Projects: Government can play an important role through subsidies that lower the costs of new approaches during their early phase of development, when costs are likely to be highest.
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Mandating Institutional Reforms: Governments can create ordinances that require the use of new technologies and mandate institutional reforms that assure smoother financing, permitting, and monitoring of new approaches.
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Documenting Financial and Social Benefits: Research and development efforts that document the financial and social benefits associated with the use or sale of recovered nutrients, lower water bills, reduced consumption of chemicals, lower energy requirements, and reduced waste disposal can help build political support, public interest, and institutional support.
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Leveraging Lessons from Successful Transitions: Learning from the successful adoption of solar power can guide the implementation of distributed water systems. As was the case for solar energy, the first installations could start with environmentally motivated early adopters, leading to cost reductions through innovation.
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Fostering Public Legitimacy: Achieving legitimacy is crucial for the adoption of new technologies. This often hinges on a user’s belief that the technology offers personal benefits, familiarity with how the technology works, and trust in the institutions managing it.
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Adapting Regulatory Frameworks: Successful transitions require the adaptation of institutional arrangements and new regulatory frameworks. Collaborative policies at local and national levels, driven by the efforts of utility and government leaders, will be crucial for the creation of a supportive regulatory environment.
By addressing these barriers and leveraging the multiple benefits of decentralized water systems, we can unlock the potential of these solutions to enhance the resilience and sustainability of our urban environments.
