Microbial ecology and functional insights into contaminant bioattenuation in engineered shallow open water treatment wetlands

Abstract

Water resources in arid and semi-arid regions globally are experiencing increasing stress from population growth, climate change, and the increasing spread and recognition of contaminants that impair potential water reuse. The adoption of low energy, high volume systems such as the engineered wetlands may contribute to solving this challenge with sustainability advantages over more actively engineered and managed approaches. The objective of this dissertation is to better understand microbial colonization and biological processes involved in trace organic and nutrient attenuation in a novel shallow, open water wetland construct colonized by a benthic photosynthetic biomat receiving nitrified wastewater effluent. The microbial community found within the biomat at both the pilot (400 m2) and demonstration scale (7500 m2) was dominated by the diatom species Stauirsa construens var. venter and an assemblage of bacteria and archaea. This construct allowed for the simultaneous photolytic and biological attenuation of TOrCs offering more consistent and a smaller footprint when compared to vegetated wetlands. The pilot-scale system which received nitrified wastewater effluent (20 mg/L NO3) also demonstrated robust nitrate removal. Denitrification was the primary mode of removal with an aerial nitrate removal rate faster than 75% of constructed vegetated treatment wetlands. Interestingly, a combination of gene-specific studies coupled to inhibitor and kinetic assays suggested that anammox (anaerobic ammonium oxidation) could be responsible for 15% or more of the nitrate removal. In order to query how anammox could be present in an organic rich system with limited external input of nitrite and ammonium, the geochemical and molecular inquiry of the biomat was conducted three dimensionally to understand stratification and nitrate attenuation processes. Additional laboratory microcosms, where contributions of sulfur and nitrogen species could be controlled, were used to further query mechanistic insights from the field-scale system. This collection of results demonstrated that sulfide induced dissimilatory nitrate reduction to ammonium was responsible for nitrite and ammonium production, which in turn supported anammox organisms of the Brocadiaceae family. Due to the success of the pilot-scale system, a larger demonstration-scale system containing three parallel cells was constructed ~350 miles south where natural microbial community colonization and treatment performance were monitored from system establishment through almost 3 years of operation. Despite no form of active intervention in colonization, the shallow cells were dominated by the same species of diatoms. Analysis of phylogenetic 16S rRNA gene sequencing analysis revealed the establishment of an anaerobic community after summer growth and bacterial and archaeal community convergence to one that was highly similar to the established pilot-scale system. Overall the design of the open water unit process cell, notably the shallow water level (20-25 cm) and utilization of a liner to prevent emergent macrophytic growth, select for a similar microbial community and reproducible performance despite geographical separation and different influent properties. The biomat also achieved similar contaminant attenuation rates to those in the mature pilot-scale system. These findings help enable adoption of this system by water entities with a need to treat a variety of water contaminants at a reduced cost and in doing so increase access to otherwise impaired waters for beneficial reuse with gains for ecological and human health and well-being