Bart Chadwick

Scale up considerations for sediment microbial fuel cells

Scale-up of sediment microbial fuel cells (SMFCs) is important to generating practical levels of power for undersea devices. Sustained operation of many sensors and communications systems require power in the range of 0.6 mW to 20 W. Small scale SMFC systems evaluated primarily in the laboratory indicate power densities for typical graphite plate anodes on the order of 10–50 mW m−2. However, previous work also suggests that SMFC power production may not scale directly with size. Here, we describe a combination of lab and field studies to evaluate scale up for carbon fabric anodes with a projected surface area ranging from 25 cm2 to 12 m2. The results indicate that power generation scales almost linearly with anode size up to about 1–2 m2 of projected surface area. Our model suggests that anodes larger than this can experience significant reduction in power density, confirming laboratory observations. These results suggest that the majority of losses along the anode surface occur closest to the electronics, where the amount of current passing along an anode is the greatest. A multi-anode approach is discussed for SMFCs, suggesting that scale-up can be achieved using segmented anode arrays.

Multiple Cathodic Reaction Mechanisms in Seawater Cathodic Biofilms Operating in Sediment Microbial Fuel Cells

In this study, multiple reaction mechanisms in cathodes of sediment microbial fuel cells (SMFCs) were characterized by using cyclic voltammetry and microelectrode measurements of dissolved oxygen and pH. The cathodes were acclimated in SMFCs with sediment and seawater from San Diego Bay. Two limiting current regions were observed with onset potentials of approximately +400 mVAg/AgCl for limiting current I and -120 mVAg/AgCl for limiting current II. The appearance of two catalytic waves suggests that multiple cathodic reaction mechanisms influence cathodic performance. Microscale oxygen concentration measurements showed a zero surface concentration at the electrode surface for limiting current II but not for limiting current I, which allowed us to distinguish limiting current II as the conventional oxygen reduction reaction and limiting current I as a currently unidentified cathodic reaction mechanism. Microscale pH measurements further confirmed these results.

System-On-Mud: Ultra-Low Power Oceanic Sensing Platform Powered by Small-Scale Benthic Microbial Fuel Cells

A self-sustainable sensing platform powered entirely by small-scale benthic microbial fuel cells (MFCs) for oceanic sensing applications is presented. An ultra-low power chip featuring an ARM Cortex-M0 processor, 3 kB of SRAM, and power management unit (PMU) is designed to consume 11 nW in sleep mode for perpetual sensing operation. The PMU includes a switched-capacitor DC/DC converter designed for efficient energy harvesting and step-down conversion for a wide range of input and output power. A small-scale MFC with 21.3 cm2 anode surface area was connected to the PMU to charge a thin-film battery of 1 mAh capacity. A 49.3-hour long-term experiment with 8-min sleep interval and 1-s wake-up time demonstrated the sustainability of system-on-mud concept. During sleep mode operation, the system charges the 4 V battery at 380 nA from the micro-MFC generating 5.4 μW of power, which allows up to 20 mA of active mode current with net energy neutrality.

Chip-on-mud: Ultra-low power ARM-based oceanic sensing system powered by small-scale benthic microbial fuel cells

An ARM-based sensing platform powered entirely by small-scale benthic microbial fuel cells (MFCs) for oceanic sensing applications is presented. The ultra-low power chip featuring an ARM Cortex-M0 processor, 3kB of SRAM, and power management unit (PMU) with energy harvesting from MFCs is designed to consume 11nW in sleep mode for perpetual sensing operation. A small-scale micro-MFC with 21.3cm2 anode surface area was connected to the on-chip PMU to charge a thin film battery of 1mAh capacity. A 49.3-hour long-term experiment with 8-min sleep interval and 1 sec wake-up time demonstrated the sustainability of chip-on-mud concept. During sleep mode, the system charges the 4V battery at 380nA from the micro-MFC generating 5.4μW of power, which can support up to 20mA of active mode current.

Pumping microbial fuel cells

Experimental data is presented comparing microbial fuel cell (MFC) power from buried (control) and chambered anodes exposed to slow flow pumping (2 mL/min). Results show that upon initial pumping (3 hrs), a robust upturn in MFC power from the chambered anodes was stimulated over several days, while a second pumping (4 hrs) appeared to resuscitate and sustain increased power for five more days. Analysis of energy gained (G) in the test setup vs. energy input (I) required for a commercial low power pump revealed a potential G/I ratio of 2.4. A pier side test was also conducted to demonstrate how tide-induced hydrostatic pressure changes could be used to pump an MFC chamber.

A framework for sea level rise vulnerability assessment for southwest U.S. military installations

We describe an analysis framework to determine military installation vulnerabilities under increases in local mean sea level as projected over the next century. The effort is in response to an increasing recognition of potential climate change ramifications for national security and recommendations that DoD conduct assessments of the impact on U.S. military installations of climate change. Results of the effort described here focus on development of a conceptual framework for sea level rise vulnerability assessment at coastal military installations in the southwest U.S. We introduce the vulnerability assessment in the context of a risk assessment paradigm that incorporates sources in the form of future sea level conditions, pathways of impact including inundation, flooding, erosion and intrusion, and a range of military installation specific receptors such as critical infrastructure and training areas. A unique aspect of the methodology is the capability to develop wave climate projections from GCM outputs and transform these to future wave conditions at specific coastal sites. Future sea level scenarios are considered in the context of installation sensitivity curves which reveal response thresholds specific to each installation, pathway and receptor. In the end, our goal is to provide a military-relevant framework for assessment of accelerated SLR vulnerability, and develop the best scientifically-based scenarios of waves, tides and storms and their implications for DoD installations in the southwestern U.S.

Performance of an in situ activated carbon treatment to reduce PCB availability in an active harbor

In situ amendment of surface sediment with activated carbon is a promising technique for reducing the availability of hydrophobic organic compounds in surface sediment. The present study evaluated the performance of a logistically challenging activated carbon placement in a high-energy hydrodynamic environment adjacent to and beneath a pier in an active military harbor. Measurements conducted preamendment and 10, 21, and 33 months (mo) postamendment using in situ exposures of benthic invertebrates and passive samplers indicated that the targeted 4% (by weight) addition of activated carbon (particle diameter ≤74 µm) in the uppermost 10 cm of surface sediment reduced polychlorinated biphenyl availability by an average (± standard deviation) of 81 ± 11% in the first 10 mo after amendment. The final monitoring event (33 mo after amendment) indicated an approximate 90 ± 6% reduction in availability, reflecting a slight increase in performance and showing the stability of the amendment. Benthic invertebrate census and sediment profile imagery did not indicate significant differences in benthic community ecological metrics among the preamendment and 3 postamendment monitoring events, supporting existing scientific literature that this approximate activated carbon dosage level does not significantly impair native benthic invertebrate communities. Recommendations for optimizing typical site-specific assessments of activated carbon performance are also discussed and include quantifying reductions in availability and confirming placement of activated carbon. Environ Toxicol Chem 2018;37:1767-1777. Published 2018 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

GPS drifter technologies for tracking and sampling stormwater plumes

Source assessment and control is critical challenge for investments in sediment cleanup as well as in the prevention of future contaminated sediment liabilities. The ubiquitous and non-point source nature of these sources make them difficult to effectively characterize and control while at the same time, regulatory pressure to reduce them and cleanup adjacent water bodies is increasing. Among the key challenges of stormwater compliance and characterization are: What are realistic exposure scenarios that are protective of the environment but not so conservative as to be impossible to achieve?; What is the fate and transport of the particulate fraction of these sources and their linkage to regional sediments that may be impacted by episodic discharges?; At what point are these sources sufficiently controlled to warrant large-scale investment in remediation of the adjacent sediments? Based on the requirements described above, the objective of this project is to demonstrate a family of technologies adapted from the oceanographic and environmental arenas that could significantly improve our ability to address contaminant source exposure, transport and fate challenges at coastal site in a relatively simple and cost effective way. Through the adaptation and integration of oceanographic drifter and scour cell techniques, we have developed three key technologies including: the Drifting Exposure System; the Drifting Particle Simulator; and, the Sediment Deposition Detector. These technologies provide a new set of capabilities that are highly applicable to characterizing the exposure, transport and fate of stormwater contaminant sources. Each of these technologies has undergone refinement and testing in the first year of the project and are now in the process of being demonstrated at field sites in San Diego Bay and Pearl Harbor