The Imperative for Sustainable Urban Water Management
The imperative to make energy and resource consumption more sustainable is prompting a critical reconsideration of all human endeavors. Within urban water management, the drive to enhance sustainability is grounded in the recognition that water services consume a substantial amount of energy and that wastewater contains valuable resources, including water, heat, organic matter, and essential plant nutrients.
To make urban water systems more sustainable, a paradigm shift is needed. Among the proposed strategies, source separation coupled with anaerobic co-digestion appears to be an effective means of recovering energy, water, and nutrients. However, as existing centralized infrastructure that serves tens to hundreds of thousands of people is difficult to alter, and the technologies needed to realize this strategy are challenging to implement in single-family homes, we consider the scale of a city block as an optimal starting point.
Using a quantitative model of unit processes that simulate energy, water, and nutrient flows, we examine the technical and economic feasibility of a representative decentralized system, as well as its environmental impacts. To realize potential synergies associated with on-site use of the recovered resources, we complement the decentralized water system with vertical farming, photovoltaic energy generation, and rainwater harvesting.
Our analysis suggests that decentralized water systems can serve as a cornerstone of efforts to enhance resource efficiency and improve the resilience of cities.
Transitioning from Traditional to Decentralized Water Systems
The traditional approach to urban water management has focused on energy-intensive treatment processes that often dissipate the energy and nutrients contained in wastewater. Recognizing the limitations of these conventional methods in achieving environmental and economic sustainability goals, the urban water cycle must evolve.
Prior efforts to recover resources from wastewater have primarily focused on centralized treatment plants, which benefited from economies of scale but struggled to take advantage of the efficiencies that could be gained through the collection and recovery of separate waste streams rich in specific resources. Centralized resource recovery systems also had difficulty in taking advantage of the benefits of fit-for-purpose water treatment due to the need for expensive new pipe networks to distribute non-potable water.
The convergence of information technology (IT) and advancements in modular technologies, however, holds the potential to enable the safe operation of small-scale water treatment and water supply networks. These innovations can take advantage of source separation and its high resource recovery efficiency, together with the reuse of water of different qualities. They 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.
Unlocking the Benefits of Source Separation
Separation of wastewater into three different components – 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.
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 (e.g., by anaerobic digestion) is a proven means of recovering energy and water.
Anaerobic digestion, despite its lower energy conversion efficiency compared to the solids obtained by centralized systems (biosolids), offers several advantages:
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The minerals obtained from small-scale systems, such as P precipitates (Ca3(PO4)2(s)) and stabilized urine liquid concentrate, exhibit higher purity than conventional biosolids, making them more suitable as fertilizers with little additional treatment.
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The production of fertilizers in proximity to their point of use reduces costs associated with transportation and integration into a larger supply chain.
The environmental impact of both decentralized and centralized approaches was evaluated through a life-cycle analysis. The key findings include:
- Global Warming Potential (GWP) was reduced mainly due to the lower net energy usage of the anaerobic digestors in the decentralized system.
- Eutrophication Potential (EP) was substantially diminished for the decentralized system because more than 80% of the treated water was recycled, and the nutrients recovered reduced the nutrient-containing effluent discharged to surface water bodies.
Synergies for Enhanced Sustainability and Circularity
After assessing the 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.
Vertical Farming and Nutrient Recovery
The nutrients recovered by the decentralized treatment systems can serve as fertilizers used locally for landscaping purposes or the local cultivation of food. Using buildings as a means of producing fresh foods captures the imagination of the public and has the potential to generate revenue that could offset some of the costs of operating the distributed treatment system.
We considered commercially available modular, hydroponic systems capable of growing a variety of high-value crops, including 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 daily loadings of the main constituents (carbon, nitrogen, and phosphorus) in each source-separated stream and how these contribute to source recovery in the form of fertilizers that can be used in the automated vertical farms to produce a variety of valued products, including water content (green, vegetables, fruits, and mushrooms), were analyzed.
By efficiently using the recovered plant-essential nutrients, which are relatively inexpensive due to the low cost of obtaining fertilizers on the open market, the amount of the 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 (<5% daily adult intake) 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 (which roughly correspond to the vertical farming OpEx), the produce value could represent a potential payback mechanism for offsetting the costs associated with the decentralized system.
Therefore, although the inclusion of vertical farming would increase the initial investment by roughly 3.1 ± 0.3%, the operation and deployment costs could be offset in less than 8-13 years if the produce is valued at market prices, and it could support the economic feasibility of the decentralized system by providing an indirect revenue stream.
Integrated Photovoltaic Energy Generation
The use of decentralized energy resources (i.e., 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.
Energy consumption within the block was categorized into three categories based on demand: (1) the water recycling system, (2) the vertical farm, and (3) household energy consumption. The photovoltaic 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 to 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.
Comparing the Performance of Decentralized and Centralized Systems
A detailed cost and energy analysis 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.
The analysis of energy consumption indicates that both modes of providing water services require approximately 2 kWh m^-3. To enable a fair comparison of both systems, the centralized system incorporated direct/indirect potable reuse, ensuring both systems provide the same functionality – delivering drinking-quality water without developing new traditional supplies.
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 $1.8 m^-3 for the decentralized system before accounting for offsets from food production.
Much of the energy use for the centralized water system is associated with operating conventional wastewater treatment plants (60%), and advanced treatment plants are needed to prepare the water for potable reuse. On the other hand, grey water treatment and purification represent the largest energy demand for decentralized systems (50% of total use).
The anaerobic digester’s ability to recover energy and heat from brown water and food waste gives decentralized systems the potential to offset over 50% of their total energy demand, a substantial advantage that differentiates them from centralized systems.
The decentralized system’s appeal further increases when considering energy from photovoltaic systems (with potential for free energy after payback), especially in locations with high energy costs for water import and distribution. While centralized systems could also benefit from photovoltaics, the diverse range of potential solutions makes a detailed comparison beyond the scope of this article.
The Economic Viability of Decentralized Systems
The gross cost of these two systems is examined against the cost of water in representative cities. The results indicate that both modes of water provision have similar costs expressed through volumetric tariff rates relative to typical volumetric costs for several United States cities, such as Barcelona and Hong Kong. Costs in Lisbon, Santiago de Chile, and Rome are about 25% lower than our projected costs.
This comparison highlights a crucial aspect of water costs and prices: much of the water utilities’ actual costs are fixed (mainly asset costs, staffing, overhead costs, etc.) and, therefore, independent of the actual amount of water produced. Most utilities charge both a fixed cost (e.g., ‘service fee’ or ‘connection fee’) and a consumption-based variable cost (US$ m^-3), with fixed costs easily representing up to 75% of the total cost.
Further differences between the cost of water treatment and supply and the final price paid by consumers in different cities can also be attributed to a range of interconnected factors, including the need to recover costs of investments in water infrastructure, efforts to encourage water conservation, and the need to subsidize water users who are unable to pay the full costs of service.
In contrast, decentralized systems hold the potential to tap into two payback mechanisms powered by synergistic strategies. The first payback mechanism involves potential indirect savings generated by the availability and sale of locally grown, water-efficient, and organic produce. 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 second payback mechanism is the proposed photovoltaic system, which 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.
Conservative estimates (only comparing the price difference with that of direct potable reuse strategies) suggest a payback period of 10-14 years for the decentralized system. The concepts proposed in this study not only bring forth economic, environmental, and technical advantages but also hold the potential to captivate the public’s attention through their alignment with concepts associated with sustainability and the circular economy.
Overcoming Barriers to Widespread Adoption
While many innovative decentralized urban water solutions have been around for years, real implementation of these practices at scale remains challenging. Arguably, 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, where an approach that has an early lead in innovation acquires dominance in a market that restricts the advancement of other technologies, is often extremely difficult to overcome because sunk investment costs are large, and institutions that benefit from the status quo resist change.
Furthermore, an inability to fully account for the societal, economic, and environmental benefits of alternative approaches often thwarts efforts to break the lock-in effect. Similar to the early days of the adoption of rooftop-scale photovoltaic energy production, the lock-in effect coupled with incomplete accounting for the social and economic benefits may slow the rate of uptake of distributed water solutions.
However, aligning with a broader recognition that the existing system of centralized water supply and treatment is inconsistent with society’s sustainability goals and mirroring the eventual economic advantages of rooftop photovoltaics, decentralized water systems offer a compelling case for breaking the lock-in effect in urban water management, promising solutions to water and sanitation challenges while contributing to broader goals of resilience, reduction in pollution, and economic gain.
Several mechanisms through which the lock-in effect can be overcome include:
- Government Support: Subsidies that lower the costs of new approaches during their early phase of development, and ordinances that require the use of new technologies and mandate institutional reforms.
- 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.
- Public Awareness and Legitimacy: Demonstration projects that build familiarity with unfamiliar technologies and transparent management and regulation to cultivate strong trust.
- Regulatory Adaptation: Collaborative policies at local and national levels to create a supportive regulatory framework for innovative solutions.
As the needed changes may require institutional reforms and may result in higher costs for water services during the initial phase of implementation, it will be necessary to describe the broader value proposition beyond immediate benefits to users of the water system. Hybrid water systems potentially provide benefits to the entire community by enhancing the resilience and sustainability of water, food, and energy.
The adoption of hybrid water systems will also require a vision for system financing, design, and operation that convinces decision-makers that the transition will take place in a manner that is economically efficient, reliable, and protective of public health and the environment.
Conclusion
Decentralized water systems can serve as a cornerstone of efforts to enhance resource efficiency and improve the resilience of cities. By efficiently recovering water, energy, and nutrients from wastewater and leveraging synergies with vertical farming, photovoltaic energy generation, and rainwater harvesting, these systems offer a compelling path towards more sustainable urban water management.
While barriers to widespread adoption exist, primarily rooted in technological inertia and lock-in effects, the economic, environmental, and societal benefits of decentralized systems make a strong case for overcoming these challenges. Through targeted government support, research and development, public awareness campaigns, and regulatory adaptation, the transition to hybrid urban water systems can be facilitated, ultimately contributing to a more resilient and sustainable future for our cities.
