The necessity to assess sewer systems and wastewater treatment plants (WWTPs) as integral parts of the wastewater system has been well known for several years and discussed in many conferences. Until recently, sewer systems and WWTPs were improved (or optimised) separately or independently, which resulted in suboptimal solutions. Nowadays, in The Netherlands as well as in other European countries, a trend can be recognised towards more integral solutions. Nevertheless, due to a lack of knowledge on the interactions between the sewer systems and the WWTPs the implementation of this way of thinking in practice takes a long time. This paper describes the results of two cases in which the interactions between sewerage and wastewater treatment are incorporated within the optimisation of a wastewater system. The first case illustrates the importance of taking the interactions into account, while the second case shows how to deal with the interactions within a wastewater system optimisation study. It is concluded that the combination of total wastewater system analysis, incorporating the interactions within the wastewater system, with efficient search algorithms is expected to be very valuable in future wastewater system optimisation studies.