Research On The Attachment Of Faecal Coliforms To Suspended Sediments And The Exchange Of Faecal Coliforms Between Water And Bottom Sediments

Faecal coliforms are used as primary indicator bacteria in China. Bottom sediments harbor a much higher number of faecal coliforms and are therefore recognized as an important pollution source. The attachment of faecal coliforms to suspended sediments and the exchange between bottom sediments and water have significant impacts on water quality.In this research, physical experiments were conducted to study the equilibriumprocessof the attachment of faecal coliforms to suspended sediments. Then numerical modeling and theoretical analysis were employed to study the impacts of different exchange processes near bottom sediments on microbial water quality.The cell size of feacal coliforms were much larger than the pore size of suspended sediments, and this indicated that faecal coliforms can only attach to the external surface defined as the sediment surface excluding the internal surface of the pores. The attachment reachedequilibrium in 5~15 min and therefore can be supposed to be instantaneous in surface waters. The linear partition model was suitable in describing the attachmentand the partition coefficients k were in the range of 0.1376~0.9387L/g. Environmental factors were found influential on the attachment except for the water pH. The values of k slightly decreased with the increase of sediment concentration S, water temperature T and salinity SL, while they significantly increased with the decrease of particle size D and surface organic matter content OMC. Although the particle size had the greatest impacts on the attached amount per unitmass, it had little impacts on the attached amount per unit surface area. Based on this, a formula relating partition coefficient with particle size has been established. The experimental attached fractions fp ranged from 7.2% to 92%. The formula relating fp with k and S can be expressed as fp=k S/(1+k S) and the reliability of this formula was well verified by the experimental results.The impact of resuspension on the concentration of faecal coliforms in overlying water was much larger than that of settling, and this impact increased with the increase of the concentration of faecal coliforms in bottom sediments. Attaching to suspended sediment would significantly influence the transport and fate of faecal coliforms. The calculated results employing the constant attached fractions were probably 12.44% lower or 28.13% higher than those employing the constant partition coefficients. The calculated results employing the averaged partition coefficient was 14.97% lower or 33.74% higher than those employing the measured partition coefficient. The application of the numerica l model in an artificial flood case indicated that the model can be used to predict the process of resuspension and settling of faecal coliforms associated with particles. This also suggested that the impacts of resuspension and settling on microbial water quality were very significant but the duration of the pollution risk was limited during flood periods.Based on reported experimental investigations on the exchange between pore water in bottom sediments and overlying water, it was found that the values of the mass transfer coefficientε can be classified into two regionsaccording to the value of the shear Reynolds number Re*and the permeability Pelect number Pe. When Re*Pe6/5<2000, the value of ε was determined by hydraulic characteristics, while it was determined by bedform and the permeability of bottom sediments when Re*Pe6/5≥2000. The direct exchange process significantly increased the concentration of feacal coliforms in overlying water when ε≥8×10-8m/s. The application of the numerical model in a low flow case indicated that the model successfully captured the direct transfer of faecal coliforms from pore water to overlying water. This suggested that direct exchange may cause significant deterioration of microbial water quality during low flow periods and can has a relatively long time impact.