Effect Of Water Velocity On Bulk Water Quality And Pipe Wall Biofilm Population In Drinking Water Distribution Systems

The biofilm in water supply network not only provides habitats for pathogenic bacteria and parasites,but also causes pipeline corrosion,resulting in deterioration of bulk water quality and threatening seriously the safety of water supply quality.At present,relevant research mainly focused on the effects of water network conditions on biofilm formation and ignored the key role of hydraulic conditions.The mechanism of how hydrodynamic conditions govern bulk water quality and biofilm population structure is not clear.This paper attempted to analyze the biofilm population structure and bulk water quality under different flow conditions,and to establish the relationships between biofilm population structure and bulk water quality,thus providing scientific basis and theoretical support for optimal operation of water supply network.Firstly,the water quality of supply network and biofilm formation under different steady-state flow conditions was investigated in this paper.The results showed that the flow rate significantly affected bulk water quality.At the flow rate of 0.5 m/s,water quality was the worst,associated with the turbidity(0.47±0.06 NTU),TOC(5.26±0.17 mg/L),total number of bacteria(4.79±0.02 log cell/mL)in bulk water and the total number of biofilm bacteria(5.48±0.06 log cell/cm~2)reached was the maximum,while the residual chlorine concentration(0.28±0.02 mg/L)reached the minimum.At different flow rates,the total number of biofilm bacteria was positively correlated with the TOC(R=0.94,P<0.001),total bacterial population(R=0.92,P<0.001),and negatively correlated with the residual chlorine(R=-0.68,P<0.05).Secondly,the effect of fluctuating velocities on water quality and biofilm formation was investigated in the simulated drinking water distribution system.The results showed that bulk water quality varied with fluctuating patterns.A significant decline in the fluctuation amplitude and an increase in fluctuation frequency of flow rates reduced the quality of bulk water.When the fluctuation amplitude and frequency of flow rates were small,the biofilm biomass was the highest(4.61±0.04 log cell/cm~2).The total bacterial count of biofilm is only positively correlated with the TOC(R=0.71,P<0.05)in the bulk water quality under various fluctuation patterns of flow rates.Finally,the relationships between biofilm community structure and bulk water quality under different flow velocity conditions were investigated.The results showed that,at the constant flow rate of 0.5 m/s,the community diversity of biofilm cells was the lowest.Increasing the frequency of flow fluctuations,the biofilm population structure showed higher diversity.Meanwhile,the relationship between community structure of biofilm cells and bulk water quality exhibted various trends under steady state and fluctuating hydraulic conditions.Under the hydraulic condition,the higher biofilm population diversity,the better water quality,while an opposite trend occurred under fluctuating hydraulic consitions.