The 14 Steps of Surface Water Treatment: How does it work?

Published on 16/05/2022 Last updated on 23/10/2023

Before water flows out of the tap, it goes through a complex purification process. This is vital, because water sources are increasingly polluted with health-harming chemicals. But is it enough? According to the research, some toxic substances are still present in our tap water.

Water companies in the Netherlands extract around 12.5 million cubic meters of water every year, 40% of which come from surface water. Water goes through several steps to rid it of unwanted substances, and this will depend on where the water is extracted, and which water company is involved. In this article, we take a closer look at how Waternet and WRK (N.V. Watertransportmaatschappij Rijn-Kennemerland) purify the water they extract. Both companies serve households and businesses in and around Amsterdam at the rate of around 90 million cubic meters of drinking water a year.  

Step 1: Extraction

WRK extracts and transports water for Waternet. Around two thirds of it comes from the Lek Canal, near Nieuwegein, which is part of the Rhine river catchment. Water quality monitoring stations are dotted along the Rhine between Switzerland and Nieuwegein. They inform Waternet whenever the river has been contaminated. If the quality of the water does not meet the legal standards, the extraction is limited and water sourced from deep underground is added. In some cases, the extraction is halted until the issue has been resolved.

Step 2: Coagulation

Once the water has been extracted, it is sent into large reservoirs where Waternet removes the largest pieces of dirt and debris. It does this via a process called coagulation. Coagulants such as ferric chloride are added to the water. They cause a chemical reaction that makes smaller impurities clump together into larger pieces that sink to the bottom of the water and can be more easily removed. This has to be done before the water can go through Amsterdam’s water pipe network. Coagulation removes around 70% of organic matter, as well as some bacteria and viruses, but further steps are needed before the water is pure enough to be sent to the tap.

Step 3: Sand filtration

The water is then passed through filters containing six layers of gravel and coarse sand, which remove more impurities. Bacteria in the filters remove substances such as ammonia. These filters are flushed with water that is then sent back to the Lek Canal. The filtered water is stored in two filtration containers before being transported to North Holland’s Provincial Water Supply Company (Provinciaal Waterleiding-bedrijf Noord-Holland) and Amsterdam’s dunes (the Amsterdamse Waterleidingduinen), which provide around 66% of Amsterdam’s drinking water.

Step 4: Transportation

The water is transported by WRK via 210 kilometers of pipes. These are made of concrete and PE plastic (polyethylene).

Step 5: Infiltration

The water arrives at two large storage containers near the Amsterdamse Waterleidingduinen. There, it slowly seeps from five large infiltration areas through the sand bed to the bottom of the dunes. As the water sinks down, the sand filters and breaks down bacteria, viruses and organic micro-pollutants.

Step 6: Extraction

When the water arrives at the bottom of the dune, it mixes with sedimentary water. This part of the system works with gravity. The water drops down to the lowest part of the dune: the Oranjekom. After around 90 days, four large pumps transport the water from the Oranjekom basin to water purification plants in Leiduin.

Step 7: Rapid sand filtration

The water is passed then through rapid sand filters, filled with six layers of gravel and coarse sand, where any remaining suspended particles in the water are removed.

Step 8: Ozonation

After passing through sand filters, the water arrives at large ozone containers, where ozonated air is bubbled through the water. Ozone is a highly reactive oxidant that destroys bacteria, viruses and metals as well as pesticides. Ozone can also improve the taste, color and smell of the water.

Step 9: Softening

The water is then descaled in softening reactors filled with calcite. Caustic soda is added, causing lime (the substance that causes water to be ‘hard’) to settle on the calcite. This reduces the water’s hardness, making the water better for our household appliances and preventing a buildup of limescale in water pipes.

Step 10: Carbon filtration

Waternet removes any remaining impurities from the water via an activated carbon filtration system. Activated carbon (also called activated charcoal) is carbon that has been processed to have very small pores. These pores trap the bacteria and particles, removing them from the water.

Cleaning the carbon filters regularly prevents any blockages. The water used for this process is returned to Step 1 of the water 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).

Step 11: Rinse Water Treatment

This is a secondary process and not really part of the drinking water treatment process. Rinse water is the water that has been used to clean fast sand filters and carbon filters. Waternet makes this rinse water suitable for reuse in the purification process by putting it through coagulation (adding ferric chloride to remove larger impurities).

Step 12: Slow sand filtration

The final water treatment step is slow sand filtration, where water moves through fine sand. This filter removes any particles that have been left by the carbon filters, while also trapping bacteria and pathogens.

Step 13: Storage

The water is now ready to flow through the tap. Waternet stores it in two pure water reservoirs that can hold 13,000 cubic meters of water.

Step 14: Transport and distribution

Waternet’s Leiduin site produces on average 180,000 cubic meters of water a day. This can increase to 240,000 cubic meters on a hot summer’s day. Around 70% of this flows through water pipes to Amsterdam, where it is distributed to other reservoirs or directly to households. The remaining 30% goes to the municipality of Heemstede and water companies PWN and Dunea.

Contaminants in our water

Despite this extensive purification process, contaminants are still found in our tap water. These substances – such as microplastics, glyphosate, lead and PFAS – end up in drinking water sources because of pollution. And many of them have been linked to health issues.

The Netherlands’ National Institute for Public Health and the Environment (RIVM) recently issued a worrying report about the condition of our drinking water sources. It found that toxic substances were present in 135 of the country’s 216 water extraction points.

Vewin, the Dutch national association of water companies, sent a letter to State Secretary Vivianne Heijnen (Infrastructure and Water Management) calling on the Netherlands to support a motion for the establishment of an intergovernmental panel on chemicals, waste and pollution, with a view to enacting a worldwide ban on chemicals such as PFAS. For years, Vewin has been campaigning for a ban or a far-reaching restriction on the use of persistent pollutants and substances that contaminate drinking water sources and endanger human and environmental health.

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