Investigation into next steps for engineered open water wetlands and saturated lignocellulose bioreactors as passive unit processes, An

Abstract

Engineered passive water treatment systems are generally less resource intensive, require no energy to operate, and have less start-up capital requirements than centralized systems. These systems include septic tanks, small-scale anaerobic digesters, wetlands, and lignocellulose bioreactors. This dissertation presents next steps in the use of shallow, engineered open water wetlands and lignocellulose bioreactors as passive treatment systems. First, we evaluate the potential resilience of open water (UPOW) wetlands to extreme disruptions of desiccation and flood. Using field samples in laboratory scale experiments, we show that UPOW wetlands, and the self-colonizing photosynthetic biomat within them, are operationally resilient to desiccation events but do not demonstrate the same resiliency to sediment intrusion from flooding. This requires active design measures to mitigate the potential for sediment intrusion due to a catastrophic flooding event. Additionally, we demonstrate the potential for using UPOW wetlands seasonally. Next, a system of bench-scale columns were loosely packed with a woodchip and alfalfa hay mixture and exposed to a consistent sulfate concentration and an experimental nitrate gradient that ranges from 0 to 40 mg/L-N nitrate to assess tradeoffs between nitrate and sulfate dominated respiratory processes on zinc and copper attenuation within lignocellulose bioreactors. The results suggest the biogeochemical gradients within these bioreactors impact the mechanism of metals immobilization, and consequently, the theoretical potential for remobilization due to environmental perturbations. Lastly, this work continued to investigate the response of these bioreactors to inherent variations in dominant microbial respiratory processes, and by extension redox potential, as a function of source water geochemistry and contact time. These results suggest that biogeochemical gradients can be established within lignocellulosic bioreactors by accounting for hierarchical respiration of multiple electron acceptors. These insights can be used to facilitate broader attenuation of both inorganic and organic water pollutants than could be achieved in a system dominated by just one respiratory regime.