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Multiphase Modelling of Bacteria Removal in a CSO Stream
Citation:
Rowney, A. C., L. Pechacek, M. E. Hulley, T. O'CONNOR, AND R. I. FIELD. Multiphase Modelling of Bacteria Removal in a CSO Stream. Presented at EWRI/ASCE World Environmental & Water Resources Congress 2010, Challenges of Change, Providence, RI, May 16 - 20, 2010.
Impact/Purpose:
To inform the public.
Description:
Indicator bacteria are an important determinant of water quality in many water resources management situations. They are also one of the more complex phenomena to model and predict. Sources abound, the populations are dynamic and influenced by many factors, and mobility through the environment is the result of a wide range of phenomena including fluid behavior, water chemistry, species characteristics, predator/prey relationships and other factors. This paper describes research into factors affecting bacterial removal in the context of stormwater and waste water treatment processes, and develops a model of multiphase bacterial transport and removal in that environment. This model is specifically oriented towards removal processes in traditional wastewater treatment facilities, in which disinfection and sedimentation are the major removal processes, with the intent that it will eventually be an asset in the management of CSOs and related water resources problems. Disinfection is a function of the dose and duration with which a wastewater stream is exposed to a particular disinfectant, and gross empirical models and associated data addressing the behavior exist. However, models that are commonly in use tend to treat removal as an overall phenomenon, perhaps dependent on a simple (often first order) removal representation driven by a removal rate constant. In reality, the factors that govern the rate and completion of disinfection are numerous and include such things as adhesion to surfaces, embedding in a matrix, stress responses of the indicator organisms, and recovery of those organisms from their stressed state. All these factors work to reduce the degree of disinfection. The survival, re-growth, re-emergence of recovery of those organisms counters the intent and efficiency of disinfectant addition. Accounting for these realities explicitly offers a potential that disinfection practices can be adjusted to remove target organisms to better effect. This paper describes development of a model that represents bacteria behavior in two phases in a fluid stream, namely the fluid medium and the entrained solid material, including the removal of bacteria by sedimentation and disinfection, influenced by the movement of bacteria between the solid and fluid phases. The model proved to be mathematically tractable, and representative cases were simulated to demonstrate that optimal dose/removal responses can be derived using this approach. There appears to be a potential to design the sedimentation/disinfection balance in terms of net cost for specified removal effectiveness and residual content. Testing this model demonstrated its efficacy in solving problems of this type. However, it also shows that real data will be required on a case by case basis to develop sit-specific results. Some of the significant factors that are necessary to accomplish this level of analytical resolution are as yet unknown, i.e., the reversibility of bacterial adhesion to a substrate is questioned by some researchers. Therefore, the model is useful in prioritizing future research on basic processes as well as in exploring hypotheses as to preferred approaches to stormwater and wastewater treatment and disinfection. Recommendations are made for extending this work, both in terms of data and simulation components.
