Surfactants (short for "tensioactive substances") are chemical compounds that reduce the surface tension of liquids, especially water. They enable the mixing of hydrophilic (water-loving) and hydrophobic (water-repellent) substances, which makes them indispensable components in cleaning agents, emulsifiers and defoamers. Due to these properties, surfactants play a decisive role in various industries, for example in the food industry, textile processing, metal processing or the chemical industry.
However, surfactants are often undesirable components in water and wastewater technology, as they can lead to foaming problems, toxic effects on organisms and considerable difficulties in wastewater treatment. Therefore, a thorough understanding of their properties, effects and the processes to remove them from water is essential.
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Chemical structure and properties of surfactants
Surfactants have an amphiphilic structure, i.e. they consist of:
Hydrophilic head:
- This part is soluble in water and can consist of ions (anionic or cationic) or neutral groups, for example.
Hydrophobic tail:
- This part is water-repellent and usually consists of a hydrocarbon chain that dissolves well in fats and oils.
Due to this structure, surfactants act at the interface between water and oil (or air) and reduce the surface tension of the water. They allow fats and dirt particles to dissolve in water, which is why they are used in numerous industrial processes.
Classification of surfactants
Surfactants are divided into four main groups depending on the charge of their hydrophilic part:
Anionic surfactants:
- The hydrophilic part carries a negative charge.
- Most frequent representatives: Soaps, alkylbenzenesulfonates (ABS).
- Application: Industrial cleaning agents, emulsifiers in metal processing.
- Disadvantage: Anionic surfactants tend to foam and are often difficult to biodegrade.
Cationic surfactants:
- The hydrophilic part carries a positive charge.
- Common representatives: Quaternary ammonium compounds.
- Application: Disinfectant, corrosion protection in cooling water circuits.
- Disadvantage: Toxic to aquatic organisms, difficult to degrade.
Non-ionic surfactants:
- They contain no charge in the hydrophilic part.
- Frequent representatives: Fatty alcohol ethoxylates (FAE).
- Application: Defoamer in water technology, cleaning agent.
- Advantage: Low foaming, high biodegradability.
Amphoteric surfactants:
- These surfactants contain both positive and negative charges and change their charge depending on the pH value.
- Application: Special industrial cleaning agents and biologically sensitive processes.
Origin and input of surfactants into the water
Surfactants enter industrial wastewater treatment and the environment in different ways:
Industrial applications:
- Cleaning processes in the food and beverage industry.
- Degreasing and emulsion processes in metalworking.
- Washing and dyeing processes in the textile industry.
- Used in the chemical industry as emulsifiers, wetting agents or dispersants.
Oil and petrochemical industry:
- Surfactants are used as emulsifiers and wetting agents in enhanced oil recovery (EOR).
Paper and pulp industry:
- Use of surfactants for surface treatment and in defoamer formulations.
Surface treatment:
- Wastewater containing surfactants is produced in electroplating plants or paint shops through degreasing and rinsing.
Effects of surfactants in wastewater
Surfactants pose considerable challenges for water technology:
Foam formation:
- Anionic surfactants often cause stable foaming in sewage treatment plants or surface waters. This can inhibit oxygen uptake and disrupt biological processes.
Toxicity:
- Cationic surfactants in particular are toxic to aquatic organisms even in low concentrations. They damage cell membranes and have a negative impact on the ecosystem.
Bioaccumulation:
- Surfactants can accumulate in the environment and in living organisms and have long-term effects.
Disruption of wastewater treatment processes:
- Surfactants hinder flocculation processes, reduce the efficiency of filtration systems and disrupt biological degradation processes in sewage treatment plants.
Removal of surfactants in wastewater treatment
Various processes are used to remove surfactants from industrial wastewater:
1. biological treatment
- In the activated sludge process many surfactants are degraded by aerobic microorganisms. Non-ionic surfactants are the most biodegradable.
2. precipitation and flocculation
- Anionic surfactants can be bound by the addition of precipitants such as aluminum or iron (III) salts in CP systems and removed from the wastewater by flocculation.
3. adsorption on activated carbon
- Surfactants that are difficult to biodegrade can be replaced by granulated activated carbon (GAK) can be adsorbed. This process is effective for removing residual substances.
4. membrane process
- Reverse osmosis and nanofiltration enable the physical separation of surfactant molecules from wastewater.
Conclusion
Surfactants are indispensable components of many industrial processes, but at the same time represent a major challenge in industrial wastewater treatment. Due to their amphiphilic structure, they are difficult to remove and can disrupt biological processes and have toxic effects on aquatic ecosystems.
Through targeted biological processes, precipitation and flocculation, activated carbon adsorption or membrane processes can be used to effectively eliminate surfactants from wastewater. For industrial companies, it is crucial to treat surfactant-containing wastewater at an early stage in order to comply with legal limits and minimize environmental damage.
Responsible use of surfactants and the use of biodegradable alternatives also make an important contribution to sustainability and the protection of our water resources.
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