Besides wastewater and rainwater, the sewers in the Netherlands also transport a lot of water that is not supposed to be in the sewer. This water is called extraneous water and is the result of unwanted discharges in the sewer system. This could be groundwater infiltrating leaky sewer pipes, or pumped from construction sites directly into the sewer. Or it could be surface water flowing in from wrong illicit connections. In the Netherlands, all this extra water account for a quarter of all the influent that enters the treatment plants.

This thesis studies the effect of a reduction of extraneous water in the influent on the workings of the treatment plant. For this two cases are studied: the treatment plant of Dokhaven in Rotterdam and Willem Annapolder in Kapelle. Dokhaven is a high loaded treatment plant with an A-B configuration that treats the wastewater from the center of Rotterdam in which it is located. Willem Annapolder is a low loaded treatment plant with biological phosphor removal and pre-denitrification and treats the wastewater from different cities and municipalities in its surroundings connected by long pressure mains. Both cases are known to receive a lot of extraneous water in their influent.

Two models were made for each case study: one that simulates the dynamics of the influent concentrations and another that describes the water treatment processes in the treatment plant. After calibration, the models of the treatment plant were run with a different influent in which the extraneous water was reduced.

Previous studies have indicated that a reduction in extraneous water leads only to small changes in the effluent concentration, but due to the reduced flow leads to significant reduction in the effluent load. In the case of Dokhaven the models show similar results. In the case of Willem Annapolder, the effects of the pressure mains was also taken into account. This caused the resulting effluent ammonia concentration to increase when extraneous water was reduced. The reduction in clean extraneous water increases the dry weather concentration of the pollutants in the pressure mains. When a rain event occurs, this water with high concentration is pushed towards the treatment plant with increased flow, which causes increased peak loads at the start of every heavy rain event.
Although the overall nitrogen load on the effluent was also decreased at Willem Annapolder when the extraneous water was reduced, the pressure mains resulted in a much lower decrease than in the case of Dokhaven. This thesis thus shows that it is important to take into account the sewer system when evaluating the effects of reducing extraneous water.