An approach to modeling biofilm growth during the flocculation of suspended cohesive sediments
Shen, X.; Toorman, E.; Lee, B.J.; Fettweis, M. (2019). An approach to modeling biofilm growth during the flocculation of suspended cohesive sediments. JGR: Oceans 124(6): 4098-4116. https://dx.doi.org/10.1029/2018JC014493
In: Journal of Geophysical Research-Oceans. AMER GEOPHYSICAL UNION: Washington. ISSN 2169-9275; e-ISSN 2169-9291, more
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biofilm; flocculation; cohesive sediments; population balance equation;quadrature method of moments |
| Abstract |
The floc size distribution (FSD) is crucial to predict cohesive sediment dynamics in aquatic environments. Recently, increasing attention has been given to biofilm effects on the FSDs of suspended particles since the presence of biofilms on particle surfaces may lead to larger flocs and thus higher settling velocities. In this study, results from a settling column experiment conducted by Tang and Maggi (2018; ) under nutrient-free and biomass-free, nutrient-affected and biomass-free, and nutrient-affected and biomass-affected conditions, with different suspended sediment concentrations, shear rates, and nutrient concentrations, have been used to validate modeled FSDs that is based on the population balance equation solved by the quadrature method of moments. In addition to the processes of aggregation and breakage, the effects of biofilm are expressed in the growth term of the population balance equation. The logistic growth pattern is used to account for an increase in biomass, which is primarily controlled by the specific growth rate and the carrying capacity. In this study, the biofilm growth rate is assumed nutrient dependent, and the carrying capacity of floc size is hypothesized to be proportional to the Kolmogorov microscale. With eight size classes to interpret a simulated FSD, the predicted and observed FSDs exhibit a reasonable match for all nutrient-free and biomass-free, nutrient-affected and biomass-free, and nutrient-affected and biomass-affected conditions. This simplified bioflocculation model fills the gap between the simulations of the FSDs of cohesive sediments without and with biofilms and has the potential to be included in large-scale models in the future. Plain Language Summary In estuaries or adjacent coastal regions, the transport of suspended sediment is responsible for many environmental and engineering issues, for example, siltation and dredging in navigation channels and harbors, water quality, water clarity, pollutants transport, and ecosystem responses. Suspended sediment particles can flocculate and thus can form aggregates with size, shape, density, and settling velocity largely different from the building particles. A challenge to predict the particle behaviors originates from a lack of flocculation models that are able to address the variations in floc size distributions. The aim of this study is to develop a flocculation model that includes besides the classical aggregation and breakage driven by turbulence also a biological process, which is biofilm growth. The biofilm growth and its impact on flocculation and thus floc size are simulated in a similar way as the growth of microbes but with different growth rates. The model is validated with laboratory experiments that have shown that the sizes of flocs made solely with sediment particles largely increase when incubated microbes are present. This model provides a sound basis to simulate the behavior of natural particles (minerals, organic, and biological particles) and particles from human origin (plastics) in future environmental risk assessment studies. |
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