An Innovative Solution for 'First Flush' Cleansing in Katwijk

During a construction team meeting, an engineer from Royal Haskoning-DHV put forward an idea that not only provided a solution to the dilemma but also led to the development of a new filtration system that may prove beneficial for more infrastructure projects in the future. Jacco van Dijk remarked, “He had previously worked with the Rockflow rainwater buffer and infiltration system and knew about the filtering effect of the stone wool elements which are fundamental to the system. Could this be the solution?, he wondered aloud.”

Consequently, Daan Los, Advisor for Urban Climate Adaptation at Rockflow, was invited to join the project team comprising the South Holland province, contractor Boskalis, and engineering firm Royal Haskoning-DHV. Daan Los explained, “ROCKWOOL has been investigating the filtering properties of our stone wool for specific pollutants for some time. Besides laboratory tests, various field tests have been conducted where polluted runoff rainwater is treated by passing it through a stone wool filter. A testing site was established in Denmark in 2017, where samples have been taken from the incoming and outgoing water streams since then. When we were invited to contribute our ideas to the challenges surrounding the Tjalmaweg, it presented an opportunity to apply our knowledge in practice in the Netherlands.

With ongoing advancements in water quality, the subject of purification is becoming increasingly critical. The standards for rainwater infiltration or its discharge into surface waters are expected to become significantly stricter. Anticipating this, we are currently aligning mainly with German and Danish standards, where the legislation in this area is more advanced than in the Netherlands.

Test results reveal that Rockflow stone wool effectively captures all suspended particles larger than 45 µm in diameter. Particles smaller than 45 µm are also retained as 'filter cake' forms due to the accumulation of larger particles in the runoff rainwater. Additionally, dissolved substances such as heavy metals, phosphates, and polycyclic aromatic hydrocarbons (PAHs) are efficiently filtered out, with at least 50% removal efficiency.  

Jacco van Dijk remarked, “In the construction team, we pooled our diverse expertise and contributed additional knowledge. I presented the initiative to the municipality of Katwijk and my colleagues in the province. For instance, someone joined who would manage the pump chamber. Together, we devised a system. I found this collaboration extremely positive. Ultimately, it had to be a system that I could confidently sign off on and take responsibility for. Naturally, I also applied my practical understanding from a civil engineering perspective, but the specific expertise in filtering pollutants with stone wool was primarily with Rockflow. This includes the challenge of removing contaminants that remain within the stone wool elements themselves, engineering that process, and managing the flow of water through the system. You can't pump the entire 240 m³ into the elements in one go, as that would cause the manhole covers to rise. Therefore, we opted for pulsed pumping. Ten minutes on, ten minutes off for settling. In this way, each of us looked at it from our own knowledge and expertise until we were collectively convinced that it would work. It's about continually thinking in terms of solutions together.”

The objective is to filter 12,000 m³ of dirty water annually. A total of 6.6 hectares of draining area is directed to a 240 cubic metre settling chamber. This water is cleaned based on a 'first flush' discharge, with a maximum flow rate of 40 m³ per hour, following a pulsing pump regime of 10 minutes on, 10 minutes off. To create the filter surface, the channels in a Rockflow package of stone wool elements are used. From the receiving pit, the water flows through eight inlet pipes to eight inflow channels in the filter buffer. It then passes through the stone wool and exits through outflow channels to a discharge pit, eventually being released into surface water. A flow meter in an emergency overflow provides a signal when the maximum flow through the filter buffer is insufficient to handle the inflow from the first flush pump chamber due to dirt accumulation in the channels. The channels are then cleaned with a high-pressure hose, restoring their absorption capacity.

Regular Maintenance

The developed Rockflow filter system requires periodic maintenance. For this purpose, Rockflow will support Boskalis for the first ten years, gaining extensive knowledge about the system's functionality. Jacco van Dijk remarked, “How often would you need to take water samples? And how do the test results relate to our duty of care towards the Hoogheemraadschap regarding the permitted levels of substances per cubic metre? When do you perform minor maintenance (like cleaning the channels, ed.) and when should the elements be completely replaced? Also, what happens if oil is released in a car accident and flows away, and how do you handle that? This is the expertise we need to develop.” 

Sustainable Solution

The calculations are based on minor maintenance twice a year. After ten years, the point would be reached where the elements are so saturated with dissolved contaminants that they need to be replaced. Van Dijk states, “It has been agreed that Rockflow will take back the old stone wool elements after this major maintenance, recycle them, and process the contamination. This is completely controlled. Thus, it is truly a sustainable solution, fully in line with contemporary environmental consciousness.”