Date of Award

2024

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

First Advisor

Eric D. Roy

Abstract

Stormwater runoff from urban areas threatens water quality and ecosystems around the world. For freshwater ecosystems, phosphorus (P) is often a primary concern, as excess P loading can cause eutrophication, symptoms of which include harmful algal blooms. To mitigate these threats, stormwater infrastructure is often employed to reduce P loading. Sand filters are a type of stormwater infrastructure that primarily function to trap particulates and thereby reduce downstream sediment and P loads. However, sand filters typically exhibit a negligible capacity to retain dissolved P forms immediately available for uptake by primary producers, due to low P sorption capacity of sand. To target both particulate and dissolved P species, a P-sorbing material amendment can be added to sand filter media to increase P sorption capacity. This thesis examines the use of alum-based drinking water treatment residuals (DWTRs), a waste byproduct of drinking water treatment plants, to enhance P removal in sand filter media. The research included two phases: (1) a field study to determine stormwater P load reductions provided by DWTR-amended sand filters under real-world conditions and (2) life cycle analysis (LCA) to determine the net freshwater eutrophication benefits of traditional and DWTR-amended sand filters.

In the first phase of this research, two stormwater sand filters enhanced with DWTRs (3-5% of the sand layer by volume) were monitored from Fall 2022 to Spring 2024 in Chittenden County, VT. The composition of stormwater runoff received at both sites was markedly different, dominated by dissolved P at the residential site, and mostly particulate P at the more industrial site. Due to this difference in influent water quality, 99% of the total P removed at the residential sand filter was in the form of dissolved P, while 8% of the total P load removed at the industrial site was dissolved P. Because the removal of dissolved P by sand filters tends to be negligible, the dissolved P load reductions observed at both sites are likely attributed to the DWTRs. Overall, the two systems reduced total P loads by 65-84%. This field study indicates that including DWTRs in sand filter media is an effective way to couple both physical and chemical P removal mechanisms and thereby enhance water quality improvement performance.

In the second phase of this research, a life cycle tool, OpenLCA, was used to estimate the environmental benefits and burdens across the life cycle of the studied sand filters. Using design specifications and known material sources, the environmental impacts of material acquisition, transportation, construction, maintenance, and disposal were estimated using the ecoinvent v3.10 database, ReCiPe 2016 Midpoint(H) impact assessment, and Monte Carlo simulation (n=10,000 runs) to determine embodied global warming potential (CO2-eq), freshwater eutrophication potential (P-eq), and other environmental impacts. Simultaneously, P load reductions exhibited in the field study were projected to a 20-year lifetime to estimate the total P removed by DWTR-enhanced sand filters against hypothetical sand-only systems. Estimated payback periods (i.e., the time needed for on-site P load reductions to fully compensate for embodied P emissions) ranged from 6-10 years for DWTR-enhanced sand filters and 7-69 years for sand-only filters.

In conclusion, DWTR-amended sand filters demonstrated excellent stormwater P removal in the field and can substantially increase the net environmental benefits from an LCA perspective relative to traditional sand filters.

Language

en

Number of Pages

154 p.

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