New Delft Water Walk
The Municipality of Delft is responsible for collecting wastewater. Delft’s old sewer system is a combined system. This means it transports both wastewater and rainwater together. It was designed to handle rainfall of up to 20 mm per hour. In case of more rain, the excess is discharged onto the surface waters via one or more of the many overflows in the Delft sewer system. This, of course, has unfavourable consequences for the quality of that surface water.
The Delft sewer system transports the wastewater to the Harnaschpolder Wastewater Treatment Plant, which is operated by The Water Board of Delfland. Wastewater from nearby municipalities is also treated there. The purified water is then discharged into the North Sea at Scheveningen (Houtrust). However, some of that water, the rainwater, was clean and did not had to be purified at all. A waste of effort and money.
Therefore, Delft has started separating the discharge of rainwater and wastewater. This separation is not yet complete, and it is not possible everywhere, for example, if there is insufficient space in the street. Delft currently has 120 km of combined sewer system, and of 113 km of wastewater sewer and 140 km of rainwater sewer. This concerns the so-called main sewer system, excluding all connections to homes, businesses, etc.
It seems very expensive to build and maintain such a dual sewer system. However, these double costs are far outweighed by the lower treatment costs and the improvement in the quality of the surface waters. Moreover, climate change does not only increase the total rainfall but also its peaks during heavy downpours. For example, the once-in-a-century peak rainfall is expected to increase from 58 mm per hour in 2020 to 70 mm per hour in 2050.
In addition to the construction of the dual sewer system, more and more buildings and properties are being disconnected from the rainwater drainage system. Local water storage facilities must then be created to temporarily store rainwater. You can see this at other stops along this walk.
Delft’s sewage system is connected to a large main sewer network, the wastewater of which is purified at the Harnaschpolder Wastewater Treatment Plant before being discharged into the North Sea.
We take it for granted that Delft has a sewer system. But some hundred years ago, we did not have one, and all wastewater was discharged into the canals. This became untenable during the 19th century. So, plans were made, but everything took a very long time. For example, in 1866, a report recommended filling in the Delft canals and installing closed sewers. Fortunately, that did not happen. It took another 60 years before the municipality seriously started making plans for a sewer system. It was not until 1928 that those plans were approved, and work began. Incidentally, the construction of our sewer system was not fully completed until 1953. Such a system has an average lifespan of about 60 to 80 years. Therefore, Delft is replacing many sewer systems for quite some time now. You have probably noticed this yourself.
