At the Gammelkerbeek, in the east of Twente, Jos Jogems of the Vechtstream water board smells the sickly smell of the treated sewage water. The sewage treatment plant of Oldenzaal feeds the narrow, meandering stream with a stream of clear water via an aeration staircase. The sweet scent that rises above the cascades makes Jogems nostalgic. “Only when I started working for the water board did I understand that this smell belongs to sewage treatment plants.”

Although the Gammelkerbeek originates a few hundred meters further, just east of Oldenzaal, most of the water that flows through the stream comes from the sewage treatment plant. Without the water from the treatment plant, the stream – and the underlying nature and fields – would run dry. The treatment plant cleans approximately 800 cubic meters of wastewater from households and businesses in the area per hour.

But the water that the treatment plant discharges into the Gammelkerbeek is not clean enough to meet European water quality standards. WWTP Oldenzaal must remove ten times more nitrogen and phosphorus from the wastewater, says Jogems. “That will be quite a challenge. Everything is possible, but these are very expensive techniques and the question is whether the return outweighs the costs and the small environmental gain that you make.”

40 percent is not enough

However, the Vechtstream water board and the rest of the water managers in the Netherlands do not have much choice. In 2009, the Netherlands committed itself to strict European standards for water quality, with the aim of clean and biologically healthy waters. In addition to nutrient standards, the waters must meet dozens of criteria for biological, ecological and chemical water quality. After two postponements, approximately 750 Dutch surface waters must meet the standards in 2027. And they are strict: in 2021 no water was completely in order, data research by NRC. Water managers fear consequences that are comparable to the nitrogen crisis on a local scale. The strict standards have already led to permits being refused.

Nutrients – nitrogen and phosphorus – play an important role in the health of a water. They are decisive for good biodiversity, and are also part of the legal obligations. In too high concentrations, the nutrients lead to the proliferation of one or a few plant and animal species. For example algae, which suffocate other species because sunlight can no longer reach the bottom of ditches. Certain flora and fauna cannot survive in such waters.

Reducing the concentrations of nitrogen and phosphorus is difficult. The required concentrations of at least one of those two nutrients are still not achieved in about 40 percent of the waters. Also in 2027, according to an analysis of the forecasts of the water managers themselves, a quarter of the Dutch waters will still not meet the standards.

It is a complex problem, say water managers. After all, a large part of the emissions falls outside their direct influence. “Roughly speaking, there are three sources for nutrients,” says Maarten Nederlof, who is responsible for the water quality from the treatment plants at the Rijn en IJssel water board, among other things. “That is us, via the sewage treatment plants, that is agriculture – via the manure – and that is supply from abroad.”

Fifty years old

The water boards have little influence on the water policy of Germany or Belgium. And imposing restrictions on fertilization is, in their view, the responsibility of the national government. Water authorities mainly want to cooperate with the agricultural sector – they are also needed for water level management and the reduction of plant protection products and heavy metals that end up in the waters via animal feed.

The water boards, on the other hand, are directly at the controls of the water treatment plants. “Especially if a relatively large treatment plant discharges into a small and vulnerable body of water, it must be taken care of,” says Nederlof. “At Rijn and IJssel we are improving five.” A total of 66 interventions in the approximately 320 Dutch treatment plants are planned. They are all costly operations, which must be completed before 2027. Otherwise, the improvements will not count towards achieving the European agreements.

No water board has to improve as many sewage treatment plants as Vechtstromen, in the east of the country. The water board manages 23 treatment plants. Thirteen of these, like WWTP Oldenzaal, must be optimized before 2027. “That is relatively much compared to other water boards,” says responsible director Erik Lievers. “If a sewage treatment plant discharges into the Maas or the Rhine, you also have to meet requirements, but they are different if you discharge directly into a nature reserve.”

Fifty years later, the sewage treatment plants really need large-scale replacement and renovation

Eric Lievers water board Vechtstreams

What matters throughout the country is that most treatment plants were built in the 1970s, when the treatment of domestic wastewater became a legal requirement. Lievers: “Fifty years later, the sewage treatment plants really need large-scale replacement and renovation. The question then is: do you still have to purify with the existing techniques, or do you opt for other purification techniques?”

The waste water is currently cleaned in most treatment plants by an old and proven biological process in three ‘stages’, says Jogems, as he follows the route of the waste water through the treatment plant in Oldenzaal via steps and bridges.

Read also: The next crisis presents itself: the quality of Dutch waters is poor and hardly improving

Jogems: “The water is raised by a auger and then first flows along large rake grids – which remove large contaminants like baby wipes. It then passes through a sand filter and is then pumped underground into these purification lines.”

Jogems is now standing above a large, concrete basin in which thick brown wisps swirl through the water. It’s not faeces, but bacteria at work. Once they are ‘fed up’ with the contaminants, the bacteria sink to the bottom as sludge. Slowly rotating arms ensure that the sludge comes together at the deepest point of the basin and is then sucked away. The sludge is digested to produce biogas, which is used to generate energy for purification.


At least ten times lower

Today’s treatment plants remove about three-quarters of the phosphorus and nitrogen from the wastewater, says Jogems. If the Gammelkerbeek is to meet the new standard, the concentration for phosphate must be at least ten times lower. “That is a real challenge for us,” says Jogems. “The techniques are there. Not only are they very expensive to purchase and install, we also have to maintain them and keep them in use. It’s not like we put a factory behind it and then we’re done. We really need to provide tailor-made solutions for each sewage treatment plant.”

The water board is currently experimenting with a cloth filter. “We actually try to remove a whole load of phosphate from the secondary settling water via a thick carpet, just before the water enters the stream. You can also use sand filtration. They are all fourth stage techniques – an extra step in the purification. If you want to fit that into existing treatment, it will cost millions.”

Whether it will work? The first results of the fabric filter are encouraging, says Jogems. Nederlof: “The smelly, black locks from before the seventies are a thing of the past. Now we have to take the step to tackle the nutrients as well. You may wonder whether we will make it in time, but we are making really big steps.”

It was only recently that the Vechtstromen water board succeeded in obtaining financing – an estimated 60 million euros extra. On paper, it should now be possible to tackle all thirteen purifications before 2027, says director Lievers. “We depend on what the market can handle. You need the materials on time, the installers who will realize it.” Jogems: “There are only a few specialized contractors who can do this. We have this issue, the whole of the Netherlands has this issue, so everyone is also fishing in the same pond.”

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