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In the 1960s and 1970s, the sewer and the purifier agreed: the Netherlands had to be drained. All the water from households and street gullies had to go down the pipe and to treatment. Mixed sewers were constructed at an admirable pace. Thus, Dutch municipalities managed to connect 99.6% of the Netherlands through the sewer system to a wastewater treatment plant of one of our water boards. This makes the Netherlands a global leader.
But perceptions have changed. For at least forty years we have been phasing out mixed sewers and building separate systems. The premise: clean rainwater does not have to go to treatment. Indeed, under the motto “the thicker the water, the better the purification process” you want rainwater precisely not to dilute. Moreover, we desperately need rainwater in soil and surface water, as a buffer in dry times. So you have to disconnect rainwater.
That requires systemic change in neighborhoods that we thought would last for many years to come. Many municipalities are working hard on this. Companies are developing innovative infiltration systems: we need to retain and infiltrate rainwater, we welcome every drop.
The rioleur and the purifier still agree on that rationale, but cracks are appearing in the collaboration. Others are also coming forward. For the water level manager is not always keen on large quantities of disconnected rainwater. Managers of polder pumping stations in the western Netherlands do not always want all that extra rainwater from the city. They believe that the city should not be allowed to pass on to the rural area.
And it gets even more complicated, because the seemingly logical principle “you don't have to treat clean water” is not always true in practice.
Rainwater gets dirty along the way
Stormwater runoff encounters everything in built-up areas: microplastics from car tires, bird droppings, leaves, cigarette butts and everything else that ends up on the street. On top of that, things go wrong in practice. One faulty connection, somewhere in a utility room on private property, can cause an undetected flow of wastewater into a storm sewer. Because of factors like these, stormwater is sometimes not so clean at all by the time it reaches surface water or infiltration facilities.
That “too dirty” rainwater is not what the water quality manager wants in surface water. It harms people and the environment. And surface water quality is already under great pressure in the Netherlands, partly due to pollution from industry and agriculture. We don't want dirt in soil and groundwater either, and rightly so. We desperately need clean groundwater for drinking water supply, industry and watering. The quality of our groundwater sources is also under pressure.
Double investing through too narrow a trade-off
If you look not only at quantity, but also at water quality, you can prevent disconnected rainwater from having to go to treatment at great expense. This unfortunately happens in practice: a neighborhood is disconnected at great expense, then the stormwater sewer turns out to have a negative effect on the receiving surface water, and then the outlet is still returned to the treatment plant.
That is double investment: first disconnecting and then returning it to the treatment plant anyway. Whereas in such a case it would have been more efficient to maintain and improve the mixed system.
How do we get out of this knot?
Fortunately, we can do a lot. It starts with prevention: how do we make water less dirty? Think about curbing unwanted pesticides for private use, or no longer allowing zinc gutters. And there's more.
In addition, there are increasing opportunities to pre-treat collected rainwater before it goes to surface water or soil. With new geotextiles, we can adsorb more and more pollution from the water. We can also profit by detecting faulty connections and by regularly emptying gullies. A lot of dirt accumulates there that sticks to sand. A logical place for targeted removal.
The bottom line is that we must not only look at the amount of “clean” stormwater that stays away from treatment, but also estimate how clean that water actually is. Then the consideration follows: can you improve the quality, and what is the best place to discharge or treat?
This requires an integral consideration that includes both quantity and quality. As a knowledge institution, we like to see that solutions are chosen based on facts, with a values-driven analysis that looks across funding and organizational boundaries for the most efficient and effective choice.
Compartmentalization gets in the way of that analysis
My impression is that such an analysis is hampered by compartmentalization in the water sector. Within water boards, between purifiers, water level managers and water quality managers. Within municipalities, where everyone has wishes for public space and the urban water manager does not always have the upper hand. And between water boards and municipalities, where the financial side also comes into play.
Because whoever receives the “not so clean stormwater” pays. In process costs, in construction costs, or in water quality and environmental costs. It would be good if the consideration can transcend those interests and we put the interest of the water chain as a whole at the center. Then you can make an informed choice for the best solution.
About the author: Hilde Niezen: director-director of the RIONED Foundation
Hilde Niezen is Managing Director of RIONED Foundation, the knowledge authority and umbrella organization for urban water management: wastewater, stormwater and groundwater in cities and towns. Since January 1, 2022, she leads RIONED and works on knowledge development, standards and research that helps municipalities and partners to organize their water tasks efficiently and feasibly.
RIONED Foundation is the umbrella organization for urban water management in the Netherlands. We are there for and by all relevant authorities and companies. Responding to new challenges and opportunities, we stand up for the importance of urban water management: taking good care of waste, rain and groundwater in cities and towns. We understand and support the professional world.
