This paper presents a simple theoretical approach to calculating of the retention time of water and/or wastewater in intermittently dosed filters. It assumes that a single dose of water is completely mixed with the previously stored water in the filter and that the field capacity of the granular medium is achieved before the next wastewater application. Retention time is weighted by the water volume stored within the filter. It is presented in a discrete form. The model was successfully validated on the basis of empirical data gathered by Schudel and Boller.
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