Biofiltration-based “green technology” as a techno-ecological nature-based solution for drinking water treatment and climate change resilience

Abstract

While drinking water treatment process design is based on current and anticipated source water quality, changing climate makes it increasingly difficult to anticipate drinking water source quality and treatability. Although most climate change-exacerbated landscape disturbances can pose threats to drinking water security, wildfires can be especially concerning for drinking water treatment because they can episodically increase turbidity and alter dissolved organic matter (DOM)—key drivers of the design and optimization of drinking water treatment processes—in receiving source waters. Shifts in DOM concentration and character can be especially difficult to remove with conventional treatment technologies, as they can exert significant oxidant demand and increase disinfection by-product formation if not removed prior to disinfection. These impacts can challenge treatment plants beyond their design or operational capacity, ultimately resulting in increased infrastructure and operating costs, service disruptions or even service outages. Thus, they emphasize the need for new approaches to mitigate these threats. “Green” technologies or nature-based solutions (NBS) are increasingly proposed as climate change adaptation strategies for mitigating such threats. Perspectives on the factors that comprise green technology in the water industry are varied, however, and while biological filtration processes continue to emerge as some of the most promising green technologies in the drinking water industry, their reliability in responding to deteriorated or more variable source water quality after disturbances such as wildfires has not been investigated. Accordingly, the major goals of this research were to (1) develop a framework for characterizing green technologies relevant to the water industry and (2) evaluate biological filtration treatment technology resilience in buffering altered source water DOM after wildfire.

The green technology framework developed herein differentiates “greenness” by examining key attributes that may cause environmental impacts across technology life cycle through the lens of the environmental setting in which it is applied. It demonstrates that green technology used in the water industry can be described by four main attributes: natural-resource basis, energy consumption, waste production, and footprint. These attributes are closely linked and must be considered relative to the biophysical and human environments in which they are applied and the other technologies to which they are being compared. Biological filtration approaches emerged as key examples of green technologies in the drinking water treatment sector; however, case studies also underscored that operational control is often reduced as technology greenness increases.

Biological filtration treatment resilience in buffering elevated source water DOM after wildfire was also investigated. Elevated/altered post-fire DOM can be especially challenging to treat because it can be smaller and more aromatic after disturbance. More aromatic DOM is especially difficult to coagulate and may lead to greater formation of regulated disinfection by-products. Bench-scale biofiltration experiments were conducted using wildfire ash-amended source water (in duplicate at three levels: low, medium, and high ash content). Turbidity and DOM (measured as dissolved organic carbon [DOC]) were typically well-removed during periods of stable operation. These results indicated that the wildfire ash and associated DOM that it released to the water matrix did not reduce the DOM biodegradation capacity of the biofilters. DOM fractionation revealed that this was because low molecular weight neutrals (which are known to be readily biodegradable) and biopolymers fractions of DOM were reduced; however, humics were largely recalcitrant. Thus, this work provided a proof-of-concept demonstration that biological filtration may serve as a techno-ecological NBS for climate change adaptation. Notably, operational resilience may be compromised if the balance between readily removed and recalcitrant fractions of DOM change, as was observed when baseline source water quality fluctuated for brief periods during the investigation, underscoring the need to balance trade-offs of operational control and resilience to other types of source water quality change.