Developing a Design Method for Blackwater Reuse in Urban Non-Sewered Sanitation Systems: Insights from the NEST Case Study

Abstract

Non-sewered sanitation systems have emerged as a valuable complement to sewered systems over the past two decades. Their importance is pronounced in urban areas of low- and middle-income countries, where they could prevent ~600,000 deaths annually by supplementing inadequate sanitation infrastructure. Additionally, in high-income countries, non-sewered systems are gaining traction due to sewer capacity limits and the growing emphasis on resource recovery.

Despite these advantages, the treatment and reclamation of blackwater (toilet wastewater) in non-sewered systems remains challenging. Approximately 20,000 liters/year of freshwater are used per person for toilet flushing, equivalent to an individual's drinking water needs for 15 years. Blackwater reclamation offers a promising solution to reduce freshwater extraction and prevent environmental pollution. However, research in this area has gained prominence recently, and there remains a gap in designing treatment trains. Current research focuses on individual technologies rather than proposing holistic frameworks that could aid urban planners and researchers in selecting technologies along the treatment train. Furthermore, the carbon footprint of complete non-sewered systems has not been quantified.

Addressing this, the thesis aims to: (1) develop a methodology to select technologies along the treatment train, (2) quantify the carbon footprint of a non-sewered system, and (3) assess the relevance of blackwater reuse in urban communities worldwide. The NEST building in Switzerland serves as a case study to demonstrate the application of the proposed approaches.

The methodology involves collecting data on blackwater quality and quantity, compiling a list of suitable technologies through literature review, and refining this list using pre-selection criteria. A pre-selection matrix groups technologies according to these criteria, assisting researchers in selecting context-specific technologies. Configured treatment trains are evaluated using 14 decision-making criteria to ensure social, technical, economic, and environmental sustainability. Applying this methodology, the most suitable treatment train for non-potable reuse (e.g., toilet flushing) includes a moving bed biofilm reactor, chemical precipitation, ultrafiltration, granular activated carbon, and UV disinfection. For indirect potable reuse (e.g., recharge of drinking water reservoirs), nanofiltration should be included.

Process designs are developed for the treatment trains using BioWin and used for the carbon footprint analysis. Indirect emissions from electricity consumption, particularly in the urine concentration unit, are identified as the largest contributors. Transitioning to renewable energy could significantly reduce the carbon footprint, emphasizing the need for comprehensive life cycle cost and benefit evaluations.

Finally, the thesis assesses the feasibility of applying the developed methods and results in urban communities across various income levels. For low-income countries, the selection of simple, robust technologies is emphasized due to frequent power cuts and high maintenance costs. Middle-income countries require a balance between efficiency and operational ease, while high-income countries benefit from resource recovery, necessitating legislative support for non-sewered systems.

In conclusion, this thesis establishes a framework for designing safe, reliable, and sustainable non-sewered sanitation systems for urban blackwater treatment and reuse, addressing a critical gap in current literature. It calls for pilot testing to validate these theoretical frameworks, advancing the operational knowledge of non-sewered sanitation to match the maturity level of traditional sewered systems.