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The need to simulate the conditions of transport of the pollutants and the dissolved oxygen in theflux of sewage arises in the modelling of sewage biodegradation process in gravitational sewer system.In the properly kept sewers, sewage flow velocity is generally uniform, which allows the model describ-ing the pollutant transport to be simplified. The application of further simplifying assumptions, justifiedby the conditions inside the modelled object, allows us to get the credible results with the acceptableefforts. These simplifications can be considered, depending on the costs of data for model calibrationmeasurement and the numerical calculation costs. The present work is focused on finding the most con-venient description of transport and transformation of dissolved and suspended substances by the advec-tion–dispersion formulas, being one-dimensional description of sewage flow. The literature review con-cerning various treatment of longitudinal dispersion coefficient was presented. The influence of flowconditions in the modelled system on the dispersion coefficient was also shown. Finally, the matrix de-scription of pollutant fraction transport and biodegradation in gravitational sanitation conduit based onadvection–dispersion equation, along with the example of numerical procedure, was described.1. INTRODUCTIONThe need to simulate the conditions of transport of the polluting substances andthe dissolved oxygen in the flux of sewage occurs in modelling of sewage biodegra-
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