Mechanisms of microbial reduction and implications for design and operation of the biosand water filter Public Deposited

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  • March 21, 2019
  • Elliott, Mark Adams
    • Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering
  • The biosand water filter (BSF) is a promising point-of-use (POU) technology for household treatment of drinking water in developing countries. Daily batch operation without filter dewatering leads to holding of a large fraction of the filtered water for idle periods typically up to 24 hr. Despite success in implementation and reduction of diarrheal disease, the mechanisms of microbial reduction and the effects of intermittent operation are largely unknown. Previous studies of filter performance have not led to adequate understanding of the dynamic nature of BSF; treatment efficiency varies both during a single charge and over weeks of operation during the maturation process. Additionally, fecal indicator bacteria have been used almost exclusively to assess BSF performance. Therefore, the impacts of BSF operation and design on waterborne virus reductions were unknown prior to the research reported here. This research was aimed at overcoming these deficiencies in well-controlled laboratory experimentation. The main hypothesis was that idle time increases microbial reductions to produce a dynamic pattern over the filtration cycle (a few hours) and over repeated cycles as the filter matures. The specific objectives were to: (1) relate hydraulic condition to reductions of bacteria and viruses; (2) gain insight into the mechanisms of virus reduction; and (3) accordingly propose changes to design and operation. Reductions of bacteria and viruses increased over weeks of filter maturation. The highest reductions during a daily cycle were observed during filtration of the filter's pore volume, indicating a positive effect of idle period. A deep-bed maturation process referred to as "media aging" contributed to virus attenuation as shown from measurements at a depth of 30 cm during the idle period. The rate of virus attenuation was first-order and increased through 5 to 10 weeks of operation. An active microbial community appears to be responsible; suppression of microbial activity by addition of sodium azide eliminated virus attenuation during the idle period. Grazing and/or production of microbial exoproducts, including proteolytic enzymes, could be pathways. Modifications to design and operation of the BSF are proposed to take advantage of microbial attenuation processes during idle time.
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  • In Copyright
  • "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Environmental Sciences and Engineering."
  • DiGiano, Francis A.
Degree granting institution
  • University of North Carolina at Chapel Hill
Place of publication
  • Chapel Hill, NC
  • Open access

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