The user can specify the underflow rate as a constant rate, a rate based on a timed schedule or as a percentage of an influent elements flow rate. The approach was to develop a semi-mechanistic model based on the processes occurring in the primary settling tanks. The solids entering a primary settling tank have a particular size distribution, which is a function of the influent TSS and the sewage collection system. These are referred to as primary particles. The remaining particles will have a settling velocity.
Primary particles can form flocs as a result of collisions with secondary particles.
Secondary particles can be broken by collisions with other secondary particles. The settling tank was modeled using a layered approach. It was assumed that ten layers would yield sufficient resolution. The influent flow can be distributed onto one or more layers, excluding the very top layer. In order to apply the model it is necessary to characterize the distribution of primary and secondary particles entering the settler.