Drinking Water

What’s the usage of drinking water?
Besides the above mentioned usage, the following topics are also relevant as drinking water applications:

1. Body care and cleansing
2. Cleaning of goods intended to be in contact with food
3. Cleaning of goods intended to be in contact with the human body
4. All water in any food-production processing facility, undertaking for the manufacture, processing, preservation or marketing of products or substances intended for human consumption

According to the directive 98/83/EC (Article 3, 1.), drinking water excludes:

1. “natural mineral water […] in accordance to […] 80/777/EEC […]”
2. “waters which are medicinal products within […] 65/65/EEC […]”

Where are the threshold values for drinking water?
Threshold values were defined for a range of parameters. They are divided into microbiological, chemical, and indicator parameters. One of the most common ones are nitrate and chloride. According to the European directive 98/83/EC, the associated thresholds are 50 mg/l and 250 mg/l, respectively. The full lists can be visited under this link (ANNEX I, parts A to C).
Country specific guidelines are for example:

• Germany:TrinkwV
• UK:   WRAS/REG 31, 33
• Switzerland: SR 817.022.102

If thresholds are not met, drinking water is not allowed to be provided. The associated authorities/relevant bodies must then take remedial actions or put in place restrictions to its use.

Depending on geographical location and climate conditions primary sources of drinking water are groundwater (wells), surface water (lakes/rivers), and seawater (oceans).

Groundwater

Groundwater is formed when rainfall percolates through the soil and collects in underground reservoirs called aquifers. Groundwater is often considered relatively clean because it is naturally filtered through layers of soil and rock. However, treatment may still be necessary:

• Aeration: To oxidize iron or manganese present in the water.
• Filtration: Removal of particles, dissolved substances, and sometimes microorganisms.
• Disinfection: Usually done through chlorination or UV treatment to kill bacteria or viruses.

In some cases, groundwater can be contaminated by agricultural runoff or industrial pollutants. In these instances, more advanced treatments like activated carbon filters or reverse osmosis may be required.

Surface water

Surface water is sourced from lakes, rivers, and reservoirs and is a primary drinking water supply in areas with abundant rainfall and accessible water bodies. Surface water is often more polluted with organic matter, sediments, and microorganisms, necessitating more intensive treatment:

• Pre-Treatment: Large debris such as branches, leaves, and sediments are removed through screens and sedimentation tanks.
• Coagulation and Flocculation: Chemicals are added to bind smaller particles into larger clumps (flocs) that are easier to remove.
• Filtration: Sand or membrane filters are commonly used to remove remaining particles and microorganisms.
• Disinfection: Similar to groundwater, final disinfection is carried out through chlorination or UV light.

In regions with higher levels of pollutants, additional processes like ozonation or activated carbon treatment may be necessary to remove pesticides, hormones, or heavy metals.

Seawater

Seawater is the most abundant water source on the planet, but its high salt content makes it unsuitable for drinking without desalination. In water-scarce regions, such as the Middle East, parts of Africa, and coastal areas, desalination is increasingly used to meet water demands. Seawater must undergo desalination to remove the salt before it can be used as drinking water. The two most common methods are:

• Distillation: Seawater is heated until it evaporates, leaving the salt behind. The water vapor is then condensed back into liquid form. This process is energy-intensive and is typically used in regions with abundant cheap energy.
• Reverse Osmosis: Seawater is forced through a semi-permeable membrane that blocks salts and other impurities. This method is widely used and typically more energy-efficient than distillation.

After desalination, the water is often re-mineralized to add essential minerals removed during the process, followed by a final disinfection.

Pre-Treatment, Coagulation and Flocculation 

Large debris such as branches, leaves, and sediments are removed through screens and sedimentation tanks. To supplement the sedimentation coagulants (Fe or Al salts) and flocculants (polymers) are added. These two serve to bind smaller particles to form micro or macro flocs to accelerate the subsequent sedimentation. Typical measuring parameters are (measuring technique see below):

• Salt or polymer concentration
• Filling level
• Turbidity
• SAC
• Sludge level

Aeration 

Groundwater, being isolated from the atmosphere, can accumulate various dissolved gases like carbon dioxide (CO₂), hydrogen sulfide (H₂S), and methane (CH₄). These gases can affect water quality by altering its taste, odor, and corrosiveness. Aeration introduces oxygen into the water, allowing these gases to escape into the air.

Metals such as iron (Fe²⁺) and manganese (Mn²⁺) are often present in groundwater in dissolved forms. When the water is exposed to air (specifically oxygen), the metals oxidize. Once oxidized, these particles can be easily removed in subsequent filtration steps.

Several key parameters are typically measured to ensure the process is effective at removing gases and oxidizing dissolved metals:

• Dissolved Oxygen (DO)
• pH
• Temperature
• Turbidity
• Oxidation-Reduction Potential (ORP)

Filtration

The filtration process can be divided into filtration through a matrix (e.g. sand filter with or without activated carbon) and filtration on a surface (e.g. membrane filtration).

Sand filtration is used for the separation of solids as the raw water flows through the filter and the solids adhere and remain in the matrix. As the filter has to be rinsed regularly, often several filters are operated to clean the raw water on the other filters in the meantime. In addition, a biologically active layer builds up on the filter surface, which must be regularly removed. If an activated carbon layer is embedded, dissolved organic substances or impurities that are difficult to degrade are also removed. Monitoring parameters are (measurement technique see below):

• Turbidity
• Oxygen
• SAC

Depending on the pore size, membrane filtration is able to remove not only solids but also dissolved particles and also has a disinfecting effect. However, membrane filtration is not approved for disinfection at least in Germany. Depending on the pore size, a distinction is made between micro (0.1–10 µm), ultra (0.001–0.01 µm), nano filtration (0.0005–0.007 µm) and reverse osmosis (<0.001 µm). Monitoring parameters are:

• Particle/solid
• Hygienic parameters
• Turbidity

Disinfection

For disinfection, the addition of chlorine, chlorine dioxide, ozone (chemical disinfection) or UV treatment (physical disinfection) can be considered. In Germany, ozonation is not permitted as the sole disinfection method.

In the chemical processes, microorganisms are inactivated. The inactivation depends on the c*t values, i.e. on the concentration (c) to which the microorganisms are exposed over a certain time (t). The disinfection performance is limited by the presence of ammonium (formation of chloramines) and at higher turbidity levels (>1 NTU). Chemical oxidation of inorganic or organic components also takes place in all processes. For ozonation this is the primary objective. Monitoring parameters are (see below for measurement technology):

• Chlorine
• Chlorine dioxide
• O₃
• pH
• UV transmission
• Turbidity

Physical disinfection damages the DNA of microorganisms and thus deprives them of their ability to divide. The performance here depends on the UV dose and requires a low turbidity (<1 NTU). Chemical oxidation only takes place in the presence of H₂O₂.

Others (e.g. decarbonization, activated carbon treatment) 

Decarbonization, in the context of reducing calcium and magnesium ions in water, typically refers to a process aimed at reducing water hardness. Hard water contains high concentrations of dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. The decarbonization process for reducing calcium and magnesium ions is typically known as lime softening. It works by chemically precipitating the carbonate hardness from the water. After the chemical reactions, the precipitated calcium carbonate and magnesium hydroxide are removed from the water through sedimentation and filtration. Typical monitoring parameter are (measurement technology see below):

• pH
• Turbidity
• Total Dissolved Solids (TDS)
• Conductivity

Activated carbon removes dissolved organic and hardly degradable (micro) impurities. In addition, it also promotes biological degradation and provides protection in the event of short-term exposure shocks. The process can be carried out using its own filter bed, stored in other filters or integrated into the treatment process using powdered activated carbon. Depending on the raw water and rinsing processes, it can take one month or one year until the carbon is fully loaded. Afterwards the coal has to be regenerated or burned. Typical monitoring parameter is (measurement technology see below):

• SAK before and after the treatment stage

Desalination typically involves multiple treatment steps to remove salts, microorganisms, and other contaminants. The two most common desalination technologies are reverse osmosis (RO) and thermal distillation. For both processes sweater is pre- and post-treated. 

Pre-Treatment 

Coarse screens are used to prevent large debris such as seaweed, fish, and floating materials from entering the system. Next, suspended solids, organic matter, and other fine particulates that for example could foul the membranes in the reverse osmosis process, are being removed through sedimentation and subsequent fine filtration. When using RO antiscalants and antifoulants are being added to protect the reverse osmosis membranes.

Reverse Osmosis (RO) 

Reverse osmosis relies on high pressure to force seawater through semi-permeable membranes, separating the salts from the water. The water that passes through the membrane is called permeate. The remaining water, which contains a high concentration of salts, is called brine. Brine is typically 1.5 to 2 times saltier than seawater and must be properly managed to minimize environmental impacts when discharged back into the ocean. Typical monitoring parameter are (measurement technology see below):

• Turbidity
• Salinity and Conductivity
• Flow Rates
• pH
• Chlorine and Chlorine Residuals

Thermal Distillation 

Thermal distillation involves the use of heat and typically pressure to evaporate water, leaving salts and other impurities behind, followed by the condensation of the purified vapor into fresh water. Similar to RO the remaining concentrated brine has to be managed. The main types of thermal distillation used for drinking water generation are Multi-Stage Flash (MSF) distillation, Multi-Effect Distillation (MED), and Vapor Compression Distillation (VC).

In MSF seawater is heated to its boiling point (typically around 90–120 °C). The heated seawater is then introduced into a series of flash chambers where the pressure is progressively reduced. Due to the drop in pressure, some of the seawater "flashes" (evaporates) into steam without additional heating. The steam produced in each chamber rises, condenses and is collected as fresh water, ready for post-treatment.

In the first stage (or effect) in MED seawater is sprayed or fed onto heated surfaces causing it to evaporate into steam. The steam produced in the first stage is used to heat the seawater in the second stage, where it evaporates again at a lower pressure and temperature. This process is repeated across multiple stages, each at a lower temperature and pressure than the previous one. In each stage, the steam generated condenses on the heat transfer surfaces and is collected as fresh water.

In VC seawater is heated and evaporated into steam. The steam produced is compressed using a mechanical compressor. Compressing the steam increases its pressure and temperature, which makes it more efficient for heating and evaporating more seawater. The compressed steam condenses on heat transfer surfaces and is collected as fresh water.

Typical monitoring parameter in thermal distillation are (measurement technology see below):

• Temperature
• Flow Rates
• Salinity and Conductivity
• pH

Post-Treatment 

Post-treatment is necessary to stabilize the desalinated water and make it safe and palatable for human consumption. Process steps include a pH adjustment to a suitable value for drinking water between 7.0 and 8.5, remineralization to improve taste, stability, and health benefits, and disinfection using Chlorine or UV light.

How to monitor drinking water
Frequency and the extent of drinking water monitoring depends on the size of the supplied area and the monitored parameter. In general, larger drinking water plants have to be monitored more often. For example, in a supplied area of 500 m³/day, conductivity has to be monitored once a year. A detailed presentation for Germany can be found here. Independent of this mandatory minimum surveillance, waterworks are usually monitoring their water quality continuously. Besides the monitoring, these continuous measurements also enable a control of the single processes.
The most common analytical monitoring parameters are as follows (in alphabetical order):

• Chlorine
• Chlorine dioxide
• Conductivity
• Particles/solids
• pH
• ORP
• Oxygen
• Ozone
• Temperature
• Turbidity
• UV transmisson