Abstract
Microbial growth in drinking water distribution systems (DWDS) depends on a great number of factors, and its control represents a great challenge for management of these engineering systems. The present case study assessed the influence that a pair of factors—water chlorination and flow velocity—had on the biofilms formed in a model DWDS in 626 days. The culturable bacteria number and biomass of the biofilms developed under the flow velocities of 0.3 m/s, 0.5 m/s, 0.7 m/s and 1 m/s were determined during three consecutively applied regimes of water chlorination to 0.05 mg/l (in 380 days), 0.42 mg/l (in 46 days) and 0.14 mg/l free chlorine (in 200 days). The results demonstrated that biofilm formation was a prolonged process directly depended on flow velocity at drinking water chlorination to 0.05 mg/l. The increase in the water chlorination to 0.42 mg/l chlorine resulted in both the reduction in culturable bacteria number and biomass removal, but the bacteria killing and the biofilm removal were distinct processes. The biocide action of chlorine was faster and more effective than its biomass removal effect. The chlorine decreasing from 0.42 to 0.14 mg/l resulted in increasing the biofilm HPC densities, although the biomass removal process was still continuing. The study carried out contributes for better understanding the biofilm behavior in DWDS and demonstrates that biofilm formation could be managed within a DWDS through operational decisions on parameters that can be changed and controlled as flow velocity and chlorination to safeguard drinking water quality.
Highlights
Drinking water is an oligotrophic environment where most of the microorganisms predominantly live attached to the surfaces in a complex, dynamic microbial assemblage called biofilm (Flemming 1991; LeChevallier et al 1987)
The microbial growth control in drinking water distribution systems (DWDS) represents a great challenge because the survival of microorganisms is based on interaction of many variables
The present case study quantifies the effect that water chlorination and flow velocity had on culturable bacteria number and biomass of the biofilms formed in a model DWDS in 626 days
Summary
Drinking water is an oligotrophic environment where most of the microorganisms predominantly live attached to the surfaces in a complex, dynamic microbial assemblage called biofilm (Flemming 1991; LeChevallier et al 1987). Biofilms are formed by microbial communities that are embedded in a self-produced matrix of extracellular polymer substances, such as polysaccharides, proteins and extracellular DNA that provides architecture and stability of Research Laboratory, The Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, 78 Nikola Gabrovsky str., 5000 Veliko Tarnovo, Bulgaria the biofilm (Flemming et al 2016; Lemos et al 2015) Most of their characteristic features such as social interactions, resource capture and enhanced survival of exposure to antimicrobials rely on the structural and functional properties of the biofilm matrix (Flemming et al 2016). Relevant bacteria and parasitic protozoa could attach to preexisting biofilms (Flemming et al 2002; Torvinen et al 2007), and soil-inhabiting filamentous
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