Rhizofiltration of urban effluent : microbial ecology and conceptual treatment mechanisms

[摘要] ENGLISH ABSTRACT: Polluted urban runoff is a challenge that is globally met by governing bodiesemploying best management practices (BMPs). One such BMP is rhizofiltration, anovel type of phytoremediation BMP designed to mimic riparian ecology, with thegoal of rapidly filtering large volumes of urban runoff before it enters rivers. Thephysical, chemical and biological mechanisms behind pollutant removal within arhizofiltration system however, are still largely unknown. The overall aim of this studywas therefore to assess the ability of a pilot scale rhizofiltration system to reduceconcentrations of the physico-chemical pollutants ammonium, chemical oxygendemand (COD), nitrate, phosphate, sulphate and suspended solids, as well asmicrobial indicators of faecal pollution, in simulated urban runoff. The faecalindicators included coliphages, faecal coliforms, potentially pathogenic yeasts (PPY)and Salmonella and/or Shigella. To achieve this study's aim a conceptual model wasfirst constructed to identify potential bacterial mechanisms of pollution removal andto estimate the effect of physico-chemical conditions on microbial communities withinthe rhizofilter medium. Then, the overall performance of the filter was measured withregard to its bioregeneration and sorption capacity for the abovementioned pollutants. Sorption equilibrium, for most of the pollutants in the simulated runoffpercolating through the filter, was reached within 45 minutes. Partial bioregenerationof the filter medium occurred within a week for ammonium, COD, phosphate andsulphate, as well as for the microbial pollutants. Evidence was subsequentlyobtained supporting the hypothesis that this regeneration is brought about bymicrobial activity, since metagenomic (16S rDNA high throughput sequencing) andphospholipid fatty acid (PLFA) analyses revealed the presence of viable dynamicmicrobial populations within the rhizofilter medium. Significant correlations between relative quantities of microbial operational taxonomic units (OTUs) and nutrientconcentrations were also uncovered. The rhizofilter plants selected for a microbialcommunity distinct from an unplanted control, however, this did not relate todifferences in filter performance. This phenomenon was ascribed to the rapidpercolation rate and design of the rhizofilter which maximizes aeration of the filtermedium. It was contended that these properties, combined with the composition ofthe simulated urban runoff, selected for functionally similar organisms. TheActinomycetales were the most abundant bacterial group in both the planted andunplanted filter media. However, the plants appeared to select for Mycobacteriaceaeand nitrifiers identified as the Nitrospiraceae. Among the transient OTUs in the filtermedia were taxa associated with the human gut, including the Campylobacteraceae,Moraxellaceae, Porphyromonadaceae and Prevotellaceae, while theEnterobacteriaceae containing faecal coliforms were below the detection limit of themetagenomic analysis. Strains of four Candida species consistently occurred in thesimulated urban runoff. The abundance of these PPY in the influent and effluent ofthe rhizofilter were affected by physico-chemical factors. Subsequent metagenomic analysis of the fungal community within the filter media revealed a low relativeabundance of candidal PPY. In short, the rhizofilter design and wastewatercomposition selected for copiotrophic aerobic microorganisms capable ofmineralizing potentially recalcitrant organic carbon and driving oxidative processessuch as nitrification whilst removing human microbial commensialists and pathogens.

[发布机构] Stellenbosch University