Linking microbiology and performance of slow sand filters for wastewater treatment
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Slow Sand Filtration (SSF) is biologically mediated method of water treatment dependent upon the natural development of a complex microbial community within the filter-bed. Due to increasing water scarcity worldwide, the need for wastewater re-use is growing and the suitability of this environmentally friendly, adaptable, cheap and easy method for wastewater treatment needs to finally be determined. The aim of this thesis was to take advantage of the rapid progress of molecular microbial ecology techniques which now allow the characterisation such a complex community and to distinguish the microbes responsible for the biological contaminant removal. This knowledge would provide the platform to improve this centuries-old method of water treatment to meet the needs of the modern world. Through a highly replicated laboratory-scale study, the treatment performance of two SSF configurations, the Traditional SSF and the Manz SSF, were examined to determine the capacity for the secondary treatment of municipal wastewater. The microbial ecology of the filter-bed was determined through direct microscopy, Q-PCR, T-RFLP and MiSeq amplicon sequencing. Finally, sand from the surface layer, or "schmutzdecke", was used to prepare microcosms for a targeted DNA-SIP analysis of the mechanisms of 13C-labelled E. coli removal. Both configurations proved excellent at bacterial removal (>98%) as well as achieving varying levels of nutrient removal, all of which occurred predominantly within the sand surface, or schmutzdecke. Crucially, backwashing of the Manz SSF proved more effective at maintaining flow and produced a more biologically active schmutzdecke. The development of an indigenous microbial community within the filter-bed was associated with the increase in treatment performance. Backwashing of the Manz SSF was shown to prevent the excessive accumulation of larger metazoans such as nematodes and annelids in the schmutzdecke. DNA-SIP analysis demonstrated that the suppression of the dominant annelid Aeolosoma hemprichi population in the backwashed Manz SSF allowed other predatory eukaryotes to achieve significant 13C incorporation. Categorization of the microbial community based upon their association with the influent showed the potential to distinguish the indigenous microbes that were beneficial for overall SSF performance whilst the more direct, targeted approach demonstrated the effect that configuration had on the microbes involved in specific contaminant removal.