How water quality is measured and protected
· 11 min read
Water is one of the most valuable resources for life on Earth. Yet industrial activity, agriculture, urbanisation and population growth continue to put pressure on rivers, lakes, groundwater and seas. How do scientists determine whether a body of water is healthy, and what technologies are used to keep it clean?
Every river, lake and wetland is a complex ecosystem with a certain capacity to break down organic matter, recycle nutrients and recover from disturbances. But that capacity is limited. When pollution, excessive water abstraction, habitat modification or climate change places too much pressure on an ecosystem, its natural balance can be disrupted.
Water used by households and industry normally enters sewer networks, treatment plants and eventually the wider water cycle again. One of the fundamental principles of sustainable water management is therefore straightforward: water returned to the environment should be treated sufficiently to avoid damaging the ecosystem receiving it.
This does not always happen.
Pollution comes in many forms. Plastic waste is perhaps the most visible example, but nutrients from agriculture, untreated or insufficiently treated wastewater, pesticides, pharmaceuticals, PFAS, heavy metals, microplastics and pathogens can all affect aquatic ecosystems.
Ocean acidification is another major concern, although it has a different primary cause. The global ocean absorbs a substantial share of the carbon dioxide emitted into the atmosphere. This alters seawater chemistry and reduces its pH. Average surface-ocean pH is now around 8.1, and the increase in hydrogen-ion concentration since pre-industrial times corresponds to roughly a 30% increase in acidity. Wastewater can aggravate water-quality problems locally, but rising atmospheric CO₂ is the principal driver of global ocean acidification.
Europe has made enormous progress in wastewater treatment and pollution control over recent decades, but many ecosystems remain under significant pressure.
According to the European Environment Agency, only around 37% of European surface water bodies achieved good or high ecological status in the reporting data assessed for its 2024 State of Water report, while only 29% achieved good chemical status. Agriculture is one of the most important sources of pressure because of nutrient and pesticide pollution, while urban wastewater, industrial chemicals, atmospheric deposition and modifications to rivers and wetlands also play important roles.
There is an interesting contrast between environmental water quality and the water people encounter recreationally. Europe performs significantly better when it comes to designated bathing waters. In 2024, more than 85% of almost 22,000 EU bathing-water sites were classified as excellent, and 96% met at least the minimum EU quality standard.
That does not mean the surrounding ecosystem is necessarily healthy. Bathing-water assessments focus primarily on microbiological contamination such as E. coli and intestinal enterococci, whereas ecological and chemical water assessments cover a much broader range of pressures.
Water degradation is also not caused exclusively by pollution. Climate change, drought, higher water temperatures, changes in river flow, wetland loss, sediment accumulation and artificial alterations such as dams and embankments can all affect the condition of aquatic ecosystems.
Water quality is traditionally assessed by collecting samples and analysing them in laboratories.
A monitoring programme may measure dozens or even hundreds of parameters depending on the type of water body and the suspected sources of pollution.
In the EU, the Water Framework Directive has provided the main regulatory framework for protecting rivers, lakes, transitional waters, coastal waters and groundwater since 2000. Instead of looking at only one pollutant, it assesses the overall condition of a water body.
For surface waters, this includes both ecological status and chemical status. Ecological assessments consider biological organisms, nutrients, physical and chemical conditions and changes to the structure of rivers and lakes. Chemical assessments compare concentrations of priority pollutants with legally defined environmental quality standards.
Monitoring is increasingly important because regulators are looking for a wider range of pollutants. New EU rules that entered into force in May 2026 strengthen monitoring requirements for substances including certain PFAS, pesticides and pharmaceuticals. For the first time, the updated framework also addresses microplastics and indicators of antimicrobial resistance.
Some of the most common instruments and methods include:
But chemical analysis has an inherent limitation: scientists need to know what they are looking for.
Thousands of different substances can potentially enter waterways. Testing every sample for every possible chemical would be extremely expensive and technically difficult. This is one reason why water-monitoring programmes increasingly combine conventional chemistry with biological and continuous-monitoring approaches.
Another technique is biomonitoring.
Instead of measuring only the concentration of individual chemicals, scientists examine how water affects organisms or how biological communities change over time.
Algae, bacteria, small crustaceans, molluscs, aquatic insects and fish can all act as bioindicators.
For example, a chemical may be present at such a low concentration that it is difficult to identify individually but still have measurable effects on aquatic organisms. Biological testing can therefore reveal overall toxicity that conventional chemical analysis might overlook.
Long-term ecological monitoring goes even further. The presence or absence of particular species can indicate whether a river has suffered from nutrient pollution, oxygen depletion or habitat degradation over many years.
Traditional sampling gives researchers a snapshot of water quality at a particular place and time. A pollution event occurring several days before or after the sample may therefore be missed.
Continuous sensors can help solve this problem.
Small monitoring units installed directly in rivers, reservoirs or treatment facilities can continuously measure parameters such as temperature, conductivity, pH, dissolved oxygen and turbidity. More advanced sensors are being developed to detect individual pollutants.
Europe's IBAIA project, funded with €4.7 million through Horizon Europe, is developing an integrated monitoring platform combining several sensor technologies. The system is being designed to detect or monitor microplastics, organic chemicals, nutrient salts, heavy metals, salinity and physicochemical parameters such as pH and oxygen.
The wider goal is to move from occasional laboratory measurements towards increasingly continuous and automated observation.
Such systems could detect pollution events much earlier, identify abnormal changes in water chemistry and allow authorities or treatment operators to respond before contamination spreads downstream.
Wastewater is one of the major pathways through which human activity can affect aquatic ecosystems.
After water is used in homes, offices or industrial facilities, it normally travels through sewer networks to wastewater treatment plants. Treatment removes contaminants before the water is discharged into rivers, lakes or the sea – or increasingly reused.
Modern treatment usually involves several stages.
Screens remove larger objects such as plastics, textiles and other debris.
Sand, grit and heavy particles are separated, while sedimentation tanks allow suspended solids to settle.
Microorganisms then break down biodegradable organic matter.
Aerobic treatment supplies oxygen to microorganisms, while anaerobic processes operate with little or no oxygen.
The result is a substantial reduction in organic pollution and the biological oxygen demand of the wastewater.
Additional processes may remove nutrients such as nitrogen and phosphorus.
This is particularly important because excessive nutrient concentrations can cause eutrophication – rapid algae growth followed by oxygen depletion that can create aquatic "dead zones."
Depending on where the treated water will go, it may then undergo disinfection using technologies such as ultraviolet light, ozone or chlorine.
More advanced treatment can also use activated carbon, membranes, advanced oxidation processes and other technologies to remove trace pollutants that survive conventional treatment.
The challenge is increasingly shifting from conventional sewage treatment towards contaminants that occur at very low concentrations but may still have environmental effects.
These include pharmaceuticals, cosmetic ingredients, pesticides and other persistent chemicals.
The EU's revised Urban Wastewater Treatment Directive therefore goes beyond conventional wastewater treatment. Among other requirements, it expands wastewater collection and treatment, requires additional nutrient removal where necessary and introduces quaternary treatment for micropollutants.
The directive also applies the extended producer responsibility principle to help finance the removal of certain micropollutants and requires wastewater treatment plants to move towards energy neutrality by 2045.
This represents an important change in philosophy: wastewater treatment plants are gradually evolving from facilities designed mainly to remove solids and organic matter into complex systems capable of dealing with nutrients, trace chemicals, health indicators and resource recovery.
Anaerobic wastewater treatment is particularly interesting because microorganisms process organic matter without oxygen and generate biogas containing methane.
Instead of simply consuming energy to clean water, treatment plants can therefore recover part of the energy contained in wastewater.
Anaerobic systems are especially useful for wastewater with high concentrations of biodegradable organic matter, including effluent from food processing, breweries, pulp and paper production and some agricultural operations.
The biogas can be burned for heat and electricity or upgraded into biomethane.
Smaller decentralised treatment systems are also being explored for individual buildings, neighbourhoods and industrial facilities. Their potential advantage is that wastewater can sometimes be treated and reused closer to where it is produced – for irrigation, industrial processes or other non-potable applications.
Several technologies could play a larger role as water-quality requirements become stricter.
Ion-exchange resins exchange unwanted dissolved ions for other ions attached to a synthetic resin. They are widely used for water softening but can also be designed to remove specific metals and other contaminants.
Adsorbents capture contaminants on their surface. Activated carbon is already widely used, while researchers are developing more selective materials capable of binding specific metals, organic chemicals or emerging pollutants.
Membrane technologies, including nanofiltration and reverse osmosis, can separate extremely small contaminants from water. Their main challenges are energy consumption, cost and the need to manage the concentrated waste stream left behind.
Advanced oxidation processes generate highly reactive chemical species capable of breaking down organic pollutants that are difficult to remove biologically.
Plasma treatment is another emerging approach. Electrical discharges can generate reactive compounds capable of destroying microorganisms and decomposing persistent organic molecules. Although promising, it remains less widely deployed than established biological, membrane and chemical treatment technologies.
Researchers are also developing new materials capable of capturing pollutants.
One example comes from scientists at the Baikov Institute of Metallurgy and Materials Science, who reported work in 2024 on modifying synthetic hydroxyapatite.
Hydroxyapatite is a calcium-phosphate material capable of forming porous structures and adsorbing contaminants. By itself, however, it has relatively limited catalytic activity.
The researchers incorporated molybdenum-containing compounds into the material, creating a powder intended to combine adsorption with improved catalytic properties.
Such materials could potentially be used to capture or break down contaminants in water, soil or industrial waste streams.
The concept is part of a much broader international research effort to develop more selective sorbents and catalysts for environmental remediation. The critical question for all such laboratory technologies is whether they can eventually be manufactured economically, regenerated safely and deployed at industrial scale.
Water-quality management is gradually undergoing the same transformation already seen in many other environmental fields.
Traditional laboratory testing will remain essential because it provides highly accurate measurements and can identify complex chemical compounds. But it is increasingly being complemented by biological indicators, automated sensors, satellite observations, data analytics and real-time monitoring networks.
At the same time, wastewater treatment is moving beyond simply making sewage safe enough for discharge. Treatment plants are increasingly expected to remove nutrients and micropollutants, recover energy and resources, enable water reuse and provide information about public and environmental health.
Europe illustrates both sides of the challenge particularly clearly. Decades of regulation and infrastructure investment have dramatically improved sanitation and bathing-water quality, yet the ecological and chemical condition of many rivers and lakes remains poor.
The next step is therefore not simply to build more treatment infrastructure. It is to understand pollution faster, detect a broader range of substances, prevent contaminants from entering water in the first place and manage entire river basins as connected ecosystems.
Clean water is ultimately not only a treatment problem. It is a question of how agriculture, cities, industry, energy systems and natural ecosystems interact across the entire water cycle.
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