Current soil-waste-vent (SWV) system design guides aim to produce ventilated drainage networks for buildings which protect attached water trap seals from unwanted air pressure effects. Such effects may cause trap seal failure that are caused by airflow induction during discharge. The ventilation design guides are based on steady-state experiments utilizing cold clean water as a test media. However, most `grey' and `black' water sources are dosed with detergent, and are often warm, which significantly alters the behaviour when compared to clean water. Thus, current design standards provide only approximations of SWV system response. Results indicate that induced airflows in warm detergent-dosed water can be significantly higher than those measured in clean water systems, by a factor of more than 2. This paper quantifies the effect of detergents in SWV systems in terms of observed air entrainment rates and previously published research on the factors that contribute to air entrainment. Results of previous work are cast into dimensionless groupings suitable for inclusion into a mathematical simulation model based on a finite difference scheme. It utilizes the method of characteristics as a solution technique to simulate drainage system operation via the equations that define unsteady partially filled or full bore pipe flows and the boundary conditions represented by water traps and other common system components.