Oasen drinking water: quality, hardness and price

Reading time: ~12 minutes

What is Oasen?

Oasen is the drinking water utility of the Groene Hart. It supplies tap water in the east of South Holland and the south-west of Utrecht, to places such as Gouda, Alphen aan den Rijn, Woerden, Gorinchem, Papendrecht, Ridderkerk and Zwijndrecht. As with every other water utility, you cannot choose Oasen yourself: where you live determines who supplies your water.

Its roots lie in the nineteenth century. In Gouda the water was so polluted at the time, partly because of cholera, that the municipality set up the Goudse Waterleidingmaatschappij on 3 December 1883. It started with 165 connections. After a series of mergers the company took the name Oasen on 1 December 2005, loosely translated as “bringer of life”. Today almost 300 people work there and the supply area covers more than 1,100 square kilometres.

That last figure explains why “how good is my tap water” never has a single answer. Oasen works with seven treatment plants and several well fields, so the composition differs from town to town. Exactly as with the water quality of Vitens and with the drinking water of WMD in Drenthe, the rule holds: the national average says little about your own tap.

Where does Oasen’s drinking water come from?

From the river Lek, but not directly. Oasen uses bank-filtered groundwater: river water that seeps into the soil along the banks and is filtered there naturally. From a depth of 15 to 100 metres, Oasen pumps that water back up.

That is an in-between form you see more often in the Netherlands and it matters for quality. Pure groundwater lies protected below ground and is therefore cleaner; river water is directly exposed to everything that happens upstream. Bank-filtered groundwater sits between the two: the passage through the soil already removes a great deal, but the origin remains the river. How those differences play out between regions is set out in the overview of the regional differences in tap water quality.

Oasen is outspoken about the pressure on that source. Climate change is making groundwater in the west of the Netherlands saltier, which is called salinisation, and in dry summers that effect is already visible. Population growth and industrial discharges come on top of that. The company’s reasoning is sober: what does not end up in the source does not have to be treated out of it either.

How does Oasen produce drinking water?

In two ways, and the difference between the two is the most interesting story in this whole article.

Conventional treatment

At most plants the pumped-up water goes through a series of steps: aeration, sand filters, softening, activated carbon filters and UV disinfection. Each step tackles something different. This method has been refined over decades and delivers water that comfortably meets the legal requirements, but it has one limit: salt and some other substances cannot be removed completely.

Membrane filtration (One Step Reverse Osmosis)

Oasen is converting its treatment plants step by step to One Step Reverse Osmosis, or OSRO for short. Instead of a series of steps there is then just one: the groundwater is forced under high pressure through a membrane that works like an extremely fine sieve. Only water molecules pass through. Salt, iron, medicine residues and bacteria are larger and stay behind.

That order is not a detail. Reverse osmosis is known for removing the useful minerals along with the contaminants, and that is one of the recurring objections in the discussion about whether reverse osmosis water is healthy or unhealthy. Oasen solves that by remineralising. A household reverse osmosis unit without a mineralisation step does not, and you can taste that difference.

The technique has a price too. With membrane filtration a residual stream of about 20 percent of the pumped-up groundwater is left over, which goes through the sewer to the wastewater treatment plant. Energy use per thousand litres roughly doubles. Against that, pellet softening and activated carbon are no longer needed, which Oasen says cuts its CO₂ emissions by 70 percent.

The switchover has been under way for a long time. The first pilot plant stood in Ridderkerk in 2005, in 2017 the Schuwacht plant in Lekkerkerk went half over to membranes, in early 2023 De Put in Nieuw-Lekkerland opened as the first one entirely on membrane filtration and in late 2024 De Hooge Boom in Kamerik followed. About a quarter of total production is now treated with membranes. The long-term goal is all of it.

How hard is Oasen’s water?

Soft, almost everywhere. Hardness in the Oasen area lies between 6.3 and 8.7 °dH, and apart from four places everything falls into the “soft” category. For comparison: Oasen counts water below 8 °dH as soft and above 12 °dH as hard, and the Dutch Drinking Water Act prescribes a lower limit of 5.6 °dH.

This is a selection from the table that Oasen publishes per town and updates every quarter.

One exception belongs with this: the drinking water that comes out of the tap in Zwijndrecht and Hendrik-Ido-Ambacht South comes from the Evides treatment plant, not from Oasen itself. That explains why those two places have their own row in both the hardness table and the PFAS table.

What does drinking water cost at Oasen in 2026?

Your water bill consists of a rate per cubic metre, taxes per cubic metre and a fixed amount per year. These are the figures Oasen publishes for 2026 and 2025.

Two things stand out. The total price per cubic metre rises by five cents, roughly three percent. And the standing charge stays the same at €114.07, which in a year of rising energy and material costs is not a given.

What do 1,000 litres of tap water cost at Oasen?

A cubic metre is exactly 1,000 litres, so the answer is in the table: €1.68 including taxes in 2026. Per litre that is roughly a sixth of a cent.

At a household consumption of around 100 cubic metres a year you end up at about €168 for water plus €114 in standing charges, so roughly €24 a month. The fact that the fixed part weighs almost as heavily as consumption is exactly why saving water delivers less than people expect. How that works out per water utility and per type of search is set out in the overview of what a cubic metre of water costs.

Is there PFAS in Oasen’s drinking water?

Yes, in very small quantities, and in every region well below both the legal standard and the stricter guideline value from the RIVM. Oasen publishes the measured values per region, and that is rare enough to mention.

The context: the European drinking water standard is 100 nanograms per litre for the sum of twenty PFAS. The RIVM, the Dutch National Institute for Public Health and the Environment, also advises a considerably stricter guideline value of 4.4 nanograms per litre in PFOA equivalents. That guideline value is advice, not law.

The figure that says the most here is the technology behind it. Where Oasen finds PFAS in the sources, activated carbon removes those substances largely, while the plants converted to membrane filtration remove them completely. That is the same order you see at home: a carbon filter catches a lot, a membrane catches everything. Which technique does what is set out per method at filtering PFAS and which technique works, and where PFAS has been measured in the Netherlands you can see on the PFAS map of the Netherlands.

There is one nuance that is missing from almost every conversation about PFAS and tap water. The RIVM states that people do not exceed the health-based limit value through tap water alone, but through food and tap water together. The guideline value of 4.4 nanograms has been calculated back so that two litres of water a day fills about 20 percent of the weekly PFAS budget; the remaining 80 percent is for food and other sources. So anyone who worries about PFAS and looks only at their tap is looking at the smallest part. More background is at water filter for PFAS.

What Oasen does not control: your own pipes

A drinking water utility’s responsibility stops at the water meter. Oasen carries out more than 94,000 water quality checks a year and tests for thousands of substances, which makes tap water better monitored than any other foodstuff in the Netherlands. But behind the meter your own installation begins, and there are three things there that no treatment plant solves for you.

Lead pipes. In homes built before 1960 they may still be in place. Small lead particles then dissolve into the water, and too much lead can harm your health. Replacement by a certified installer is the only real answer. What lead does and how you get it out is at filtering lead out of tap water.

Standing water. Water that sits in the pipe for a long time, for example after a holiday or in a long riser pipe in an apartment, picks up taste and substances from the pipe. The rule of thumb: let the tap run briefly until the water is cold. How long water keeps in a jug or a bottle is at how long you can keep tap water.

Boil water notice. After a burst main or after works, Oasen may temporarily be unable to guarantee quality in full. You then get a message to boil the water for 3 minutes. That is a different scenario from an outage with no water at all, which you can read more about at no tap water and what to do then.

Do you need a water filter in the Oasen area?

No. We would rather say that ourselves than have you read it somewhere else. Oasen’s drinking water comfortably meets all legal requirements, the PFAS values are below the strictest guideline value there is in every region, and the water is soft. A blog by a filter shop telling you this water is no good deserves your distrust.

Why people in the Groene Hart still filter comes down to three reasons, none of which is about safety.

• Taste: bank-filtered groundwater from the Lek tastes different from pure groundwater, and some people notice that. What filtered water is exactly and how it differs is at what filtered water exactly is.
• Your own home: old pipes, an apartment with a long riser pipe, or a house whose installation you simply do not know.
• The total: the RIVM advises bringing total PFAS intake over a lifetime down. You have little grip on your food, but you do on your tap.

There is one more argument that is actually stronger in this area than elsewhere, and it comes from Oasen itself. Their newest treatment plant takes everything out of the water with membrane filtration and then deliberately adds minerals, because water that is too bare is undesirable. That is exactly why we built the Mineral+ with five filter stages the way we did: purify first, then bring back calcium, magnesium and potassium via Swedish Mineral Rock™. A household reverse osmosis system without that last step skips the part Oasen is legally required to carry out. Which approach does what to your water is in the overview of what the different filter techniques do with groundwater.

Comparison: what the options deliver

The PureFilter Mineral+ costs €149.00 including the first filter. A replacement filter costs €65.00 and lasts about twelve months with 1 to 3 people, and about six months with 4 to 6 people. The duo with two cartridges costs €115.00. Converted, that is less than one cent per litre. If you would rather not think about it, the Filterplan sends the cartridges automatically and on time for €48.00 or €86.00 per delivery. How often replacement is needed is at how often you replace a water filter.

At PureFilter we see that customers from the Groene Hart rarely call about limescale, which is exactly what you expect at 6 to 8 °dH. They call about taste, about an old house in a village centre, or because they read something about PFAS. Installation takes 5 to 10 minutes: you do not need a plumber, one pair of water pump pliers is enough to connect it to the cold water pipe, and you change the cartridge later with the key supplied. If you are unsure about the place, the comparison of a water filter system on or under the worktop helps. An overview of our range for home use is in the webshop.

Looking up your own water composition

The figures in this article apply to the Oasen area as a whole, but your town is attached to one of the seven treatment plants and those differ from one another. Oasen publishes the full composition per town and updates it every quarter.

• The hardness and the full report per town are on the page about water hardness and composition.
• The PFAS measurements per region are at PFAS and drinking water sources.
• Submitting your meter reading, viewing your invoice or reporting a fault is done through Mijn Oasen; this article is not the place for that, but the Oasen customer service is.

The monitoring itself is strict and continuous. Oasen takes samples at the source, during treatment, in the water storage, in the distribution network and at customers’ homes, and has them analysed in the laboratory. Deviations are therefore spotted before they reach your tap.

Frequently asked questions

Conclusion

The Oasen drinking water in the Groene Hart comes from bank-filtered groundwater of the Lek, is soft almost everywhere and comfortably meets all legal requirements. PFAS stays below the strictest guideline value there is in every region, chlorine is not used, and with more than 94,000 quality checks a year this water is better monitored than any foodstuff in the supermarket.

The figures to remember: hardness between 6.3 and 8.7 °dH, €1.68 per cubic metre including taxes in 2026, €114.07 in standing charges per year, and PFAS values of 0.0 to 2.8 nanograms per litre against an RIVM guideline value of 4.4.

The most interesting part comes from Oasen itself. Their newest treatment plant takes everything out of the water with membranes and then deliberately puts calcium back, because bare water is undesirable. That is the same order we follow at kitchen scale, and it is the best argument against a reverse osmosis system that skips that last step. Anyone who filters in the Oasen area does so not because the water is bad, but because they want to decide the last few metres themselves, from water meter to glass.