To purify ground water and make it safe to drink, it is put through a long and technical cleaning process. This process is necessary because drinking water sources are becoming increasingly contaminated with man-made pollutants. Unfortunately, despite these efforts some toxic substances are still found in our tap water.
Water companies in the Netherlands extract around 12.5 million cubic meters of water every year, 60% of which comes from groundwater. This water goes through several processes to rid it of impurities and make it safe to drink. The processes depend on where the water is extracted. In this article, we take a closer look at Waternet’s groundwater purification process. The Amsterdam-based company is responsible for supplying 90 million cubic meters of water to the capital’s residents every year.
Since 1930, Waternet has extracted seepage water from the Bethunepolder, a reclaimed lake (seepage polder) in the Utrecht Noorderpark. The polder is part of a 6000 hectare area that is being reclaimed for nature conservation. Because of this, the water makes good raw material for drinking water. It is extracted, then led via the Waterleidingkanaal to the Waterleidingplas in Loenderveen.
The first step in purifying the water is called coagulation. Ferric chloride is added to the water; this causes a chemical reaction that makes small impurities and debris clump together and sink to the bottom. Coagulation removes around 70% of organic matter, heavy metals, bacteria and viruses. After this treatment, the water goes from brown to clear.
After much of the debris has been removed by coagulation, the water goes to the Waterleidingplas, a drinking water basin. Due to the wind, the water in the basin undergoes a natural cleaning process that breaks down ammonium, organic matter and bacteria. If the weather is cold, Waternet adds a little phosphoric acid to further break down ammonium. After three months, the water is pumped out and taken to the next step of the purification process.
Any remaining debris is removed as the water goes through 24 rapid filter tanks. These tanks contain six layers of gravel and coarse sand. Bacteria in the sand break down substances like ammonium. The filters are regularly rinsed and the rinse water is transported back to the coagulation plant.
An 11,3 kilometer pipeline transports this water to raw water distribution tanks in Weesperkarspel (Amsterdam-Zuidoost). From here, the water flows to the next step, the ozone tanks.
In these ozone containers, ozone gas is added to the water. Ozone is a highly oxidizing agent that breaks down organic substances (like pesticides) and eliminates pathogens, thereby improving the water’s quality. The ozone also improves the water’s smell, taste and color.
The water is then sent into softening reactors filled with calcite and caustic soda. The caustic soda causes limescale to settle on the calcite, and results in softer water. This is important for the longevity of household appliances, since limescale can damage heating elements [link to limescale article]. Softening also increases the water’s pH (acidity), which leads to fewer lead and copper residues in the water.
The water is now ready to be filtered through carbon. Activated carbon, which has very small pores, traps any remaining impurities from the water and breaks down bacteria.
Cleaning the carbon filters regularly helps to prevent any blockages. The water used to clean these filters is returned to the first step of the purification process. The carbon filter supplier regularly reactivates the carbon (this means the carbon is heated back up to 900 degrees Celsius – the temperature at which carbon becomes activated).
This step is a secondary process and not really part of drinking water purification. Rinse water is the water that has been used to clean (rinse) the rapid sand filters and carbon filters. Waternet makes this rinse water suitable for reuse with dynamic sand filters and by adding a small amount of ferric chloride to remove any larger impurities. Once cleaned, this water flows to the Waterleidingplas.
As part of the final purification step, the water flows through a filter made of gravel and fine sand. This removes any particles left in the water from the carbon filters and captures any remaining pathogens and bacteria.
The water is now ready to be sent to the tap. Waternet stores it in two water reservoirs that can hold 30,000 cubic meters of water. Storing the water is necessary because water is in high demand in the day but less so at night.
Waternet’s water pipe network is 2,000 kilometers long. The water is supplied via seven distribution pumps.
It takes a lot of work and precise processes to make water safe to drink. Despite these steps, some contaminants are still present in tap water. Most of these contaminants are present because of human activity, for example PFAS, glyphosate, microplastics, and lead. Long-term exposure to these chemicals is linked to health problems.
A recent report by the Netherlands’ National Institute for Public Health and the Environment (RIVM) states that 135 of the country’s 216 water extraction points show signs of contamination.
Vewin, the Dutch national association of water companies, is concerned. “We are running out of time. Measures to improve quality are absolutely necessary,” says chairman Peter van der Velden.
Where is the pollution coming from? According to Annemarie van Wezel, professor of environmental ecology at the University of Amsterdam, there are two main sources. “The pollution can be explained by substances flowing in the rivers from other locations. Another explanation is that our country has a lot of industry and agriculture in a relatively small area.”
The United Nations and European Union are discussing the possibility of banning harmful chemicals like PFAS. If this ban is put into action, this is a nice step. But it is a drop in the ocean given the hundreds of other chemicals (such as pharmaceutical residues and micro-pollutants) that are already in our environment. A real solution still seems a long way off.
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