Wastewater treatment

In most industrialized countries, the maximum pollutant content of industrial wastewater before it is discharged into wastewater treatment plants is regulated by law for various substances and substance classes. Companies must comply with strict environmental regulations and are subject to official controls. This also applies to water pollution caused by surfactants, which are used across all industries — as foamers, detergents, wetting agents, dispersing agents, and for many other purposes. In the semiconductor industry, for example, surfactants in wastewater are a particular challenge; they must be completely removed during recirculation in the chip factory's water treatment system.

Tensiometric measurements of the surface tension, which is linked to the surfactant content, help companies to comply with legal limits or to ensure the quality of their own water treatment.

Checking the primary degradation based on the surface tension

All applications of surfactants are based directly or indirectly on mediating between water and hydrophobic substances and reducing the surface tension of the water or the interfacial tension. Due to this property, surfactants also attack the cell membranes of microorganisms, which play an important role in the treatment process in sewage treatment plants, and can kill them. Therefore, the primary degradation, during which the surface-active properties are lost, must already be well advanced before the wastewater can be fed into the sewage treatment plant. The degradation products (metabolites) that are no longer surface-active are completely degraded in the sewage treatment plant in the case of most approved surfactants.

As primary degradation and surface tension are directly related, the process can be monitored using tensiometric measurements. A mobile bubble pressure tensiometer can be used to check compliance with a limit value for water directly on site. Measurements on laboratory samples with a stationary force tensiometer using the ring method or the plate method are also common. Tensiometric methods do not replace the determination of concentrations of substances of concern, which are subject to their own limit values. However, they can considerably reduce the number of time-consuming and cost-intensive analyses.

The relationship between primary degradation and surface tension applies to every surfactant regardless of substance class and chemical structure. Tensiometry is therefore universally applicable as a test method for surfactants. In some cases, the concentration can even be correlated directly with the surface tension and determined quickly and easily using the bubble pressure method.

Content determination for the use of surfactants in sewage treatment processes

At first glance, it seems absurd to break down surfactants before feeding them into the sewage treatment plant and then add them again during the treatment process. However, the chemo-biological process in the activated sludge can actually be controlled and optimized through the targeted addition of suitable surfactants:

• The adsorption of surfactant molecules on the cell membrane, which would destroy the membrane and kill the bacteria if overdosed, increases the permeability for biomolecules in the moderated range and thus accelerates the degradation process.
• Surfactants reduce the bubble size of injected oxygen, which improves mixing and ensures that the gas remains in the tank for longer.
• Surfactants also reduce the droplet size of hydrophobic liquids, giving bacteria a larger surface to attack.
• Even the removal of heavy metals, which enter the wastewater during electroplating or semiconductor production, for example, is faster and more complete with the use of surfactants. 

However, the added surfactant is also degraded by bacteria and adsorption to the sludge also reduces the surfactant content in the aqueous phase. The surfactant must therefore be added at regular intervals. Compliance with a surface tension limit value can serve as a criterion for close monitoring – if the surface tension rises to this value, dosing is required. Depending on the concentration range, classic methods such as the ring method  or plate method or dynamic bubble pressure measurement can be used. The latter can also be carried out on a mobile basis using single-point measurements in a matter of seconds.

Quality testing in water treatment for the semiconductor industry

Water consumption is particularly high in the semiconductor industry. Chip factories generally maintain their own water cycle in order to recover the large quantities of ultrapure water (UPW), which is required for wafer production, from the wastewater. The removal of surfactants, which are used when polishing and cleaning the wafers, for example, also plays a major role here. The surface tension of the water changes even with very low levels of surfactant contamination and is therefore an important test criterion for assessing the quality of the water or identifying weak points in the complex filter system.