This paper discusses the prediction of the mass-of-contaminant remaining in ballast water tanks as they are purged by fresh sea water or by sterilised discharge water. The purging is assumed to take place under steady-flow conditions. Further, the assumption is made that the process
involved is, in chemical engineering terms, a perfect- or well-mixed one. In view of the long residence-time likely, such a model is the most rational choice among the standard models used in mixing theory. However, a short discussion is given on possible ways of enhancing the mixing, of which
careful distribution of the flow from the inlet-piping design within a tank, would be helpful. Whether extensive and expensive computational fluid dynamic studies are justified for designing the necessary pipe-work is queried. A number of examples with interconnection between tanks are
worked through – the associated modelling and mathematics is straightforward. A result of some significance emerges; that judicious arrangements of tanks, in series and/or parallel, has the potential to make for large savings in time-of-operation and in energy expenditure. The arrangement
whereby sterilised outflow-water is used as the inflow, ie a closed system, may be particularly useful for short-haul vessels or those operating in lakes and rivers.