The Kristalbad area: the multi-functional water machine between Hengelo and Enschede

Retention area and “water machine”

The city of Enschede is situated on a weir embankment. Hence, water from Enschede flows towards the lower-lying city of Hengelo (and subsequently on to Almelo). In the past, excess water from the surrounding area used to be discharged as quickly as possible via the Twente Canal. At that time, this sufficed. However, climate change entails increasingly frequent drier periods. For that reason, rapid discharge to lower-lying areas is not always the best option, as the natural environment and agricultural operations need to be supplied with water during droughts as well.

Urban waste water is discharged to the waste water purification plant in Enschede and subsequently flows to the Kristalbad area via the Elsbeek brook. During severe downpours, the Kristalbad area collects large volumes of water, in order to ensure that residents of Hengelo and lower-lying areas keep their feet dry. The Kristalbad area has a total storage capacity of 187 million litres of water, i.e., nearly 75 Olympic swimming pools full. Furthermore, in addition to the sewer system and several basins, the city features a subsurface peak water storage of another 40,000 m3.

In the event of excess pressure on the sewer system and the purification plant, unpurified water enters the Kristalbad area via a bypass around the purification plant (overflow). This happens several times a year. For example, during a 23mm downpour falling within a matter of a few hours, some 80,000 m3 of unpurified water will flow into the Kristalbad area. In such cases, the water machine function of the area will become even more important. The purifying process will remove some of the pollutants from the water before it continues on to the Elsbeek brook.

The water machine technology is based on a Swedish concept. The water that leaves the purification plant is clean (it meets all the standards), yet it is full of nutrients (phosphates, nitrogen, and silt particles), it has a low oxygen content, and contains little or no organic life. In the Kristalbad, the water is rendered biologically active and chemically balanced by means of light, air, plants, and good bacteria. The water is purified in three compartments featuring an ebb and flow system. Each compartment features a 20-hour cycle in which it fills with water, subsequently drains, and runs dry for a while. Especially during the latter phase, natural processes will degrade any substances deposited on the soil.

Before the Kristalbad area became fully operational (in anticipation of, among other things, the development of vegetation and the completion of the hydraulic system), a two-year monitoring of the outflowing water showed that the Kristalbad purification process nonetheless already had a positive impact on the water quality. At the time, the reduction rates of effluent pollution ranged between 14% and 92%. Yet the phosphate, nitrogen, ammonium, ammonia, copper, zinc, iron, and e-coli concentrations still did not meet the standards at that time. The district water board is currently assessing whether a new monitoring process needs to be launched.

Ever since 2016, the water quality and water volumes at the entrance and exit of the Kristalbad have been continuously measured. In addition, the frequency of sewer overflows to the Kristalbad and their impact on water quality are monitored. This provides the managers with insight into the performance of the water machine, and enables them to assess whether the water machine meets the requirements set for the design.

Ecological connecting zone

Both railway tracks and a bicycle highway run straight across the Kristalbad area. They constitute a significant component of the infrastructure between the three cities, but the area is also a “highway” for plants and animals. The Kristalbad also serves as an ecological connecting zone, and thus plays a pivotal role in the migration of flora and fauna.

Recreational activities

In addition to its functions as a retention area, “water machine”, and ecological connecting zone, the Kristalbad also serves as a recreational area in which people can stroll around, ride their bicycles, and enjoy nature.

Contact person

Hendrik Jan Teekens
Gemeente Enschede
hj.teekens@enschede.nl