Biological processes have a long history of use in drinking water treatment. In recent decades, rapid biological filtration has achieved increasing acceptance. In terms of organic carbon, the classical application of biofiltration has been for the removal of easily biodegradable material, along with the related objectives of reduction in disinfectant demand and byproduct formation. Additional objectives receiving emphasis more recently include the removal of trace contaminants and the use of biofiltration as a membrane pre-treatment to reduce fouling. Although various models for biofiltration have been developed, these are generally complicated and research-oriented, and no modeling approach or performance parameter has found general acceptance in practice.

The paper summarizes previous modeling approaches and illustrates the usefulness of a previously developed index of contact time, X*, referred to as dimensionless contact time. In addition to empty bed contact time, X* contains other factors which are important for biofiltration performance: the reactor surface area available for biofilm development and substrate biodegradability and diffusivity. This paper shows how X* can be simplified and also illustrates its application to the removal of trace contaminants (e.g. pharmaceuticals, endocrine disrupting substances or odorous compounds such as geosmin). A major section of the paper analyzes the requirements for biofiltration when used as a membrane pre-treatment to reduce fouling, and illustrates the applicability of X* for this use of biofiltration. Thus X* is presented as an overall unifying parameter. The last section of the paper introduces a new practically-oriented parameter, the Biofiltration Factor (BF). The use of this parameter, which is directly related to X*, is proposed for biofiltration design and operation, once some additional work has been undertaken.

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