By Devrim Kaya, Allen P. Davis and Birthe V. Kjellerup
For more than 50 years, environmental scientists, natural resource managers and regulatory agencies have recognized urban stormwater runoff as an important source of contaminants of concern in waterways. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and heavy metals bind to stormwater particulate matter and settle into aquatic sediments, where they persist for decades, impact the health and behaviour of aquatic life, and accumulate up the food chain.
In one three-year project, researchers from the University of Maryland will design a modular stormwater treatment system ready for field testing, based on data collected at three test sites. The final design could be applicable at industry sites and in communities anywhere.
In the first phase of the project, the team will develop various inexpensive geomedia capable of capturing the contaminants. PCBs and PAHs adsorb strongly to particulate matter and media with high organic content, and dissolved copper forms pH-dependent surface complexes with organic and inorganic material. In fact, preliminary results from ongoing studies of PCBs in deposited stormwater sediment at the University of Maryland suggest that a large fraction of contaminants of concern could be removed by simply trapping particulate matter.
Possible sources for the geomedia, based on the findings of previous studies, include locally-sourced “standard” bioretention media and amendments such as yard waste, wood chips, activated carbon, biochar, aluminum, and/or iron oxide. The team will select media based on its high sorption capacity, rate of uptake, leaching potential, and ability to promote biodegradation of PCBs and PAHs and/or to immobilize copper.
The team will then develop a stormwater best management practice (BMP) that layers these geomedia in a “treatment train.” This approach will allow site managers to leverage multiple types of chemical interactions, including partitioning, filtration and adsorption, to capture a larger concentration of contaminants, without causing clogs that would require more frequent maintenance. With help from a passive sampling strategy that monitors reduction levels, site managers will be able to mix and match the layers to create the optimal system for a location’s contaminants and climate and to know when to replace modules as their effectiveness begins to wane.
The project will address several key research questions. Firstly, what are the fundamental processes that determine the concentrations and mass loadings of PCBs, PAHs and copper, distributed among different size fractions and organic content of the stormwater particulate matter? How will PCB congeners distribute among different stormwater particulate matter fractions? How can this be exploited for designing treatment systems?
The project will look at which types of geomedia will be able to most effectively remove PCBs, PAHs and copper from the stormwater to comply with varying discharge criteria, and at how these types of geomedia influence the biodegradation of PCBs and PAHs and immobilization of copper. Can amendments be introduced to the geomedia that can enhance the rate and extent of biodegradation? How can the geomedia be combined with particulate matter removal technologies in a treatment train approach to comprehensive stormwater treatment?
Thirdly, how can the selected geomedia, and possibly separately-collected particulate matter, be included into the development of sustainable operations for the on-site treatment of PCBs, PAHs and copper in stormwater runoff?
The project will also see if passive samplers can be applied in stormwater management systems to assess the efficiency of geomedia-based treatment solutions. Can the use of passive samplers lead to reduced maintenance concerns?
Finally, the project will examine if the answers to these questions can lead to scale up and implementation of low-cost, low-maintenance stormwater BMPs that target contaminants of concern.
The role of microbes
To boost the effectiveness and sustainability of the treatment train, the researchers will explore ways to seed the media with microbial communities capable of immobilizing copper and biodegrading PCBs and PAHs into harmless byproducts. And, it is here that the spatial reach of the project will prove most challenging. The team will need to create stable biofilms for regions with different rainfall patterns so bacterial communities are not washed away.
To do this, the researchers will first have to determine whether soil and weather conditions can be exploited to increase biodegradation of adsorbed PCBs and PAHs at all. Although this question has yet to be fully investigated, University of Maryland researchers have found PCB-degrading bacteria in the top media layers of a campus bioretention cell, suggesting it is possible for indigenous microbes to serve the role.
This project will also be among the first to examine how the accumulation of potentially toxic metals in a treatment system affects microbial activity.
Devrim Kaya, Allen P. Davis and Birthe V. Kjellerup are with the University of Maryland Department of Civil and Environmental Engineering. This article appears in ES&E Magazine’s June 2018 issue.