UC Berkeley

Global water scarcity will be a defining challenge of the coming decades, with 2.5 billion of the world’s population expected to live under water stress by 2050. Scarcity, and the accelerating factors of the food-energy-water nexus, population growth, and climate change will converge on the built environment, where buildings account for an estimated 25% of water use and 40% of energy use globally. Urban water re-use presents a promising opportunity to manage this crisis. However, current efforts to realize water reuse are generally capital-intensive, owing to centralization of recycling plants, or problematic for the food-energy-water nexus, as decentralized or building-scale plants employ recycling technologies that are energy-intensive. This dissertation presents my research to use sunlight to heat and treat domestic greywater in building facades. I propose a synergistic solar photocatalytic greywater recycling and solar thermal heating system in an envelope easily integrable into facades, enabling water re-use and energy savings at the building scale. To support this proposal, my dissertation explores greywater quality and properties, basic solar photocatalytic reactor design, and simultaneous solar photocatalytic reaction and solar thermal collection. Together, these research pursuits develop a foundation of knowledge to support the realization of new energy-offsetting and water-recycling systems at the building scale.

In Chapter 2, I describe research to understand the input, greywater, and its poorly-understand optical parameters that are essential to understand for any photodependent treatment process. I show that greywater absorbs heavily in the UVA/B range, a characteristic modeled by few currently-published synthetic greywater formulations, and that the absorbance is correlated to relatively high chemical oxygen demand (COD) and soluble COD. These results indicate that as UV-based methods for water recycling gain traction, careful consideration of the inherent absorbance of the greywater matrix must be accounted for in system design.

In Chapter 3, I explore the underlying treatment process proposed, solar photocatalysis via titanium dioxide, and characterize it for a simple inclined plate reactor system. I report that inclined plate photocatalytic reactors under simulated solar light show performance that is primarily governed by light intensity and light capture, rather than hydrodynamics and mass transfer, even when the two are coupled through system design parameters (e.g., inclination angle). This result is in agreement with previous literature on UV-driven inclined plate reactors, and offers new insight as to the operation of solar photocatalysis under low-UV and laminar or transitionary Reynolds number regimes.

In Chapter 4, I explore how the basics of the inclined plate reactor can be scaled up to a design relevant for building façades, and propose, fabricate, and characterize the Vertically Integrated Multistep (VIMS) reactor. Under real solar conditions in Berkeley, CA, I show that the VIMS system effectively decolorizes methylene blue in a westward-facing afternoon sun condition. Based on volumetric performance modeling, I suggest that the VIMS system can, at a 9m2-scale, treat up to 60L of greywater per day, although these results are substantially reduced under limited solar intensity, e.g., cloudy conditions.

In Chapter 5, I show how integration of solar photocatalytic systems, including VIMS and a simple flate-plate façade collector, can couple with simultaneous solar thermal gain to yield a multifunctional system. This research represents, to my knowledge, the first reported simultaneous solar photocatalytic water treatment and solar thermal gain system. Under simulated solar conditions, I experimentally demonstrate that ASHRAE measures of solar thermal collector efficiency and photocatalytic reaction performance are opposed to each other in multifunctional systems. However, given the relatively warm temperatures of greywater entering the system in real-world application, I suggest that significant promise exists for high solar photocatalyic treatment performance.

These results indicate that buildings and their facades present a promising platform to offset water scarcity through re-use, and point to further research in multifunctional façade-integrated systems to realize sustainability in urban energy and water.