Unit process open water (UPOW) wetlands are shallow engineered wetlands that have been naturally colonized by a photosynthetic, benthic biological mat which enables robust attenuation of nitrate, pathogens and trace organics. The purpose of this study was to evaluate system resilience to extreme events encountered during field deployment. Two disruption events, biomat dehydration and flooding, were evaluated. In the fall of 2018, a pilot scale UPOW wetland system was dewatered for nearly four months with samples taken prior to restarting flow. In February 2019, a demonstration scale UPOW wetland was flooded by an unseasonable high flow event resulting in the intrusion of particulate sediment into the biomat. Through the use of this field sampling and laboratory-based microcosms, the performance and microbial ecology of both rehydrated and post-flood UPOW wetlands were evaluated. Batch microcosms with pre and post-flood biomat were utilized to evaluate the impacts of the flood event while flow through microcosms were used to examine biomat rehydration. The batch microcosms demonstrated a decrease in bulk nitrate removal rates between pre and post-flood samples (0.51 +/- 0.086 day-1 vs 0.25 +/- 0.016 day-1), however the rates were similar when normalized to percent volatile solids (4.45 +/- 0.76 day-1 vs 4.11 +/- 0.27 day-1). In addition, post-flood microcosms demonstrated decreased biomat activity as quantified by pH and DO extremes and diel oscillations. Analysis of 16S rRNA showed the post-flood microbial community to be significantly different from that of pre-flood biomat (ADONIS p = 0.001). The rehydrated biomat demonstrated a higher degree of microbial and operational resilience with significant overlap in bulk nitrate attenuation rates and ecological fingerprint when compared with the fresh biomat. The ability of a UPOW wetland to attenuate nitrate at a limited rate following a flood event provides time for an operator to triage a damaged treatment train and schedule repairs. Additionally, the ability of the system to attenuate nitrate after rehydration demonstrates the opportunity for the design to be utilized in support of seasonal or surge flows, providing another use for the UPOW technology.

Resilience of photosynthetic biomat within a unit process open water wetland following extreme events

ACS Fall 2020 Virtual Meeting

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