The state of development of process modeling is assessed from the practitioner's point of view. The history of model development and application within a consulting firm is described. The utility of currently available models is portrayed for the commonly used biological treatment processes. The perspective given is one of assessing model usefulness in its most practical context, that of its utility in a design practice. Areas are identified where additional research and model development are needed.

The use of process and two-dimensional clarifier modelling to investigate the implications of upgrading Melbourne Water's Eastern Treatment Plant to a nitrification/denitrification process is discussed. Results indicate that the existing clarification capacity is sufficient for the increased solids loading arising from operation at a nitrifying sludge age but that the existing diffuser system may need to be replaced in order to achieve the required oxygen transfer rate. The impact of step feeding into unaerated zones to reduce the aeration tank suspended solids concentration and to achieve denitrification is discussed and the installation of baffles to improve the performance of peripheral feed clarifiers is suggested.