Water and Wastewater Decentralization at Urban District-Level to Enhance Resiliency and Sustainability

Decentralized source separation systems for water, energy, and nutrient recovery are emerging as promising solutions to address the mounting challenges in urban water management, particularly as traditional centralized infrastructure faces increasing pressure from population growth, climate change, and resource constraints (Bhambhabi, 2025; Keller, 2024; Garrido-Baserba, 2024; Leigh, 2019; Capodaglio, 2020). Traditional water and wastewater infrastructures and the energy-intense activated sludge process are being questioned because of their environmental and economic burden and negligible resilience against climate change extreme events (Sedlak, 2023; van Loosdrecht & Brdjanovic, 2014; Roefs et al., 2017).

This study presents a comprehensive four-year investigation into the technical, economic, and environmental implications of implementing decentralized source separation systems for urban water management. The study confirms that increasing the recovery efficiencies of water, energy, and nutrients by combining decentralized source separation, anaerobic digestion, and filtration approaches holds the most potential for the global water system's sustainability, reliability, and resilience.

Methodological Approach.
The research methodology incorporated multiple complementary approaches: experimental pilot plants (2 CSTR for black water and 1 dual-reactor for grey water), advanced digital twin development (with real-time capabilities), process and cost modeling for techno-economic assessment, and a dedicated life cycle analysis in collaboration with top experts.

While decentralized source separation could take many forms and configurations, the following setup was chosen as a representative candidate for future systems. Two distinct experimental setups were developed and operated. The first consisted of thermophilic continuously stirred-tank reactors (CSTRs) for black water treatment, operating at 55°C with a daily flow rate of 200 mL, distributed across four feeding events at 6-hour intervals. The reactors were inoculated with thermophilically digested material from a full-scale plant and designed to examine various feeding regimes relevant to decentralized scenarios, including variations in food waste availability, urban density patterns, and fluctuating organic loading rates. The second setup featured a dual-compartment constructed wetland system for grey water treatment, integrating physical filtration mechanisms with phytoremediation processes. Comprehensive monitoring included key parameters such as pH, alkalinity, BOD5, COD, total nitrogen, total phosphorus, and surfactant concentrations at multiple sampling points throughout the treatment process.

These experimental systems were complemented by an advanced digital twin developed using SIMBA# modeling software. This twin incorporated real-time data from supervisory control systems, laboratory parameters, biogas production, and weather forecasting APIs. The digital twin facilitated comprehensive analysis across three critical domains: cost analysis (including monitoring requirements, sensor specifications, and infrastructure sizing), technical specifications (controlling strategies and operational parameters), and life cycle assessment. Additionally, it allowed for the evaluation of potential synergies with complementary technologies such as rainwater harvesting, solar photovoltaics, and vertical farming for nutrient recovery, with a specific focus on nitrogen and phosphorus recovery pathways.

Key Findings
This study confirmed the potential advantages of decentralized source separation approaches compared to conventional centralized systems. The economic analysis revealed that these integrated services demonstrated potential for more than 50% reduction in overall operational costs following a payback period of 5-10 years, varying by location and local utility rates. Space requirements were found to be manageable within urban contexts, with the core system requiring 7-9% of basement space at the city block scale, while integration with vertical farming would require an additional 15% of basement floor space. The system could require 85% less water from utilities while simultaneously improving resource recovery rates - showing a 50% increase in nitrogen recovery and a 90% increase in phosphorus recovery.

Environmental impact assessments revealed less than one-third of the conventional system's global warming potential and eutrophication impacts. The life cycle analysis demonstrated a 60% reduction in carbon footprint compared to centralized systems, primarily due to reduced energy consumption and improved resource recovery efficiency. The vertical farming integration showed particular promise, with the potential to utilize recovered nutrients for local food production, demonstrating a circular economy approach to urban resource management.

This research provides the most comprehensive techno-economic and environmental assessment of decentralized source separation systems to date. The findings demonstrate these systems' viability as sustainable alternatives to centralized infrastructure, with improved resource recovery rates, reduced environmental impacts, and significant economic benefits. These insights inform urban planners, environmental engineers, and policymakers on implementing decentralized water management systems and advancing circular economy principles in urban development.